Race car suspension Class

[THORPE]

That's some ingenuity right there! Older fella use to tell me that's what rules were made for, to be broken. Lol. If'n u ain't cheatin u ain't winnin right?

Our rules are pretty open which gets me trying all sorts of sneaky tricks lol

 

[THORPE]

Raced saturday and had good results. Won the b main but the a main ended up being a wreck fest and I landed upside down.

I had my front roll center as low as possible. I had to add a little weight to the front to balance out the chassis so I placed it a little higher than the original CG on the center of my shock tower.

The rear RC was a little higher than the front.

Ran equal R &L springs front and rear. Just one rate stiffer in rear.
I tried it out and it was pretty good.
I tightened the droop limiters to just a little more than ride Height and man what a difference! Nearly put the car on rails turning in.

Next I tried sway bars front and rear and stuck it even more!

I don't completely understand why this all helped so much but I am much happier with the car.

My car was def one of the fastest that night.

I see room for improvement with my dampening being a little too soft now and I was running a torsen diff in the center which I have found to be too inconsistent on these scale cars. Next weekend will be locked in the center as I have had great results from that before.

 

[THORPE]

I have a question on springs too.

There are progressive rate springs offered for my car. They are basically a thinner wire with less coils overall.

Could this be helpful on oval or am I better off with as constant rate as possible?
Seems like it could help with straight line traction but not sure about how much it would help weight transfer and tire loading.

 

[Ranger Mike]

congratulations on coming up with a question that's going to make to old guy dig thru the car hauler to find notes o n springs..I looked at this about ten years ago when we raced min stocks..my initial opinion is that the mounting space dictates use of progressive springs...I will post once I dig into this matter..and..good job on the chassis set up..sounds like you are getting it dialed in

 

[THORPE]

Thanks!

I owe lots of credit to you and this thread.

About the springs. Luckily I am not limited on space or mounting location at all.

don't go through too much trouble looking but thanks for everything!

 

[Ranger Mike]

Thanks Thorpe..it is nice to be well thought of but I am only speeding up what you would have found out eventually..

Progressive rate coil springs are the ultimate in coil spring design. The principle is simple: soft comfortable ride when cruising and firm when cornering and traction is needed.
Most coil springs have rates that are constant; if it takes 400 pounds to compress the spring the first one inch, another 400 pounds will compress it the second inch, and so on. With a progressive rate coil, the initial couple of inches of compression requires less rate than the remainder of compression. This improves your "Sunday drive" ride quality, yet rate increase when you're in "Banzi mode". With progressive rate coils the first few coil wraps are more closely spaced than the remaining coils, or the entire spring is slightly conical (cone-like) in shape. This is fine for off the street / strip car , road vehicles and motorcycles. You need the extra spring rate when you snag that 250 pound barhog at 2 A.M. at the local watering hole..( 2s at 10 are 10s at 2..ugghhh).

To my knowledge Progressive Springs are used in my type of racing on pull bars in a typical 3 link rear suspension set up. Most all springs used in racing are linear. we can go into more detail about this but the whole deal with springs is to handle the transferred weight as the car goes around the track.
The options at the track during tune and test day is to find the correct spring package for that particular track with the typical temperature as race day...a lot of work as anyone can tell you..The main thing you want is consistency. In mechanical terms this is repeatability of the chassis when it goes through its motions and reproducibility by the drive to take the best race line while driving on the track during this drill. If you can focus on these two and dial in the chassis your chances of winning are far better than the seat of the pants effort most teams use. More on the above later but the question was progressive springs.
The current hot set up is to go with Stacking Springs , if the set up permits the long dual spring combination.


Basically, the system works on the principle that two springs working in series share the overall applied deflection. That is a fancy way of saying that when you stack two springs, the combined spring rate drops. With our system, we can lock out one of the two springs during the shock's deflection. Once this happens, any additional deflection is only moving one spring, so the effective spring rate increases to the active spring's rate.

By picking and choosing the two springs in the assembly, we can give you a fairly broad range of spring rates and rate combinations. The advantage of the adjustable locking ring is that it let's you select where in the shock's displacement you want the transition to take place. This gives you far more flexibility than dedicated progressive springs.The down side is that the spring stacks can get fairly long. But if you have room for the package, this gives you the chance to have a variable rate setup that you can tune using off-the-shelf springs. In other words, you get more flexibility at lower cost.
I got the PDF from Hypercoil .. best spring manufacturer in my opinion..used them for years

rm

 

[Ranger Mike]

Shock tuning

Steve09
Penske shocks have a remote canister that has a valve similar to a car tires valve. The canister also has 6 compressions settings..1 being full soft, 6 being full firm. The Shock boy has rebound settings that adjust the rebound firmness as well.
Our open wheel formula car has these shocks. We sent them in for rebuild. These shocks have to be rebuilt yearly because the steel shim packs will fatigue after so many cycles and the shock oil will break down and foam to the point of not dampening as designed after a period of time ( number of cycles). When we replaced these shocks we had the car set up close to the optimum ride height but the rear of the car was tail happy ( loose, over steering).
We were encountering Roll Snap. This was a condition where the driver when into the turn and the car would roll to the point that the rear tires would get to the point that traction would be lost and he had to drive it like is was on a dirt track and get up on the wheel, sprint car style. To correct this the options were to tune the shocks, lower the rear roll center, change the anti roll bar (sway bar),change the springs. Since we had raced this car for the last 5 years on this same track, we knew the springs were correct. The sway bag change was easy but we were too close to optimum handling to go that drastic. Shock tuning was the best option.

We dropped the rear end ( adjusted the ride height by lowering the car via the coil spring adjuster.) This meant that we lowered the rear roll center. The rear roll center was about 3 inches above the pavement. The Rear Center of gravity was about 12 inches above the pavement. When we lowered the car, the distance between the RC and CG was reduced and thus the moment arm was also reduced. This means the lever between the two was shorter and consequently the weight that is transferred during cornering had less force to act on the rear spring of the outside wheel.
Because the shock adjuster was backed off ( unscrewed the coil spring four turns) the spring rate pre load was reduced,
the car handled a lot better and the rear end was not breaking loose quite as much during cornering. This is where things get to mind warp..the spring rate was reduced but not to the point that the spring rate was less than required to handle the force of the transferred weight. These nitrogen shocks are normally run at 125 psi. I bumper the psi up to 170 psi. this effectively added 25 pounds to the spring rate. the car handled a lot better but still was on the ragged edge of roll snap. I moved the compression setting from 3 to 4 making the compression rate on the shock more firm..car ran better but still a tad edgy. Set the compression to 5, setting 6 being full firm. car was on rails..really dialed in.
If we could not bring in the shocks the next step was to pump in another 25 psi and dial the shock back to setting 1 ( full soft) and begin the process anew. The shocks canister can take an max of 300 PSI but i hate running this much as it is at max of the capacity. if we could not bring in the car with 200 psi i would have switched out the anti roll bar to a stiffer one and dialed back the shock psi to 125 and setting 1 (full soft) on compression.

This is hope WE dial in THIS car and may not work for everyone..but it works for me..hope it helps a little..

PHOTO IS SHOCK WITHOUT THE COIL SPRING IN PLACE

 

[steve09]

hey mike
I've been nursing a sore back for a week now,but I'm beter now those shocks are the
same as the set we have.plus the cansters have springs inside for less cavatation and
we change the oil in are shocks after every big race.we only run about 80 psi in the
cylinders,and my dads was under the weather also but we won are 8th stright in the
limited late model,that two spring design sounds like us, were running bump stops on
the shocks keeping the front springs soft and down on the bumps,and no bottoming out
and a lot of rebound in the shocks keeping it setting on the nose.but i didn't no that more
pressure in the shocks may be needed what's your opinon on that.

 

[THORPE]

Thanks for the info.

I like the stacked spring setup idea. I was able to replicate it even on my car with spare parts I had laying around.
I have a 6 lb and a 8lb spring combo with the collar stoping compression of the 6# and then using only the 8# just a bit before bottoming of the chassis side to side.

I haven't gotten a chance to try it out on the track yet but hope to soon. I have my notes ready to go back to the constant rate setup if I can't gets things to work the way I want them too.

 

[Ranger Mike]

Bump Stops

sorry to hear about the back problem,,i am going on a cruise with two ladies that have helped in t he pits and hope to suffer a similar ailment soon!

We ran a zero droop limiter on the formula car when we first started sorting it out. I never like it. We found that a good shock package would give you this effect internally if you had the right shim package. We have been thru the bump stop route too..
In the old days the rubber bump stop was a little more than a cushion to keep vital suspension parts from crunching when the car bottomed out. The spring rate went to the moon when the travel zeroed out. Koni came out with the cellasto polyurethane progressive bump stop and this is the hot set up. the is little job is mounted on the shock piston rod and is totally progressive due to the properties of the used. The length and therefore location on thew wheel travel curve where they come into play can be adjusted by cutting the cone or adding to the total length and the progression and total resistance can be varied by grinding either the OD or the length and angle of the cone. Never cut the cone off as you are back to the old days with zero progression and a hard bump stop.
80 psi seems low to me. I like 125 starting psi as you are in the middle of the shock pressure operating range. Changing out the shock oil that often seems a bit much and i would consult Penske on this.

One more thing..if you are NOT hitting the bump stops your are running too stiff a spring for your set up. One nice thing about the bump stop is the ability to be used as a travel indicator by sliding it up the piston shaft before hot lapping.

 

[Ranger Mike]

Shock tune and test

you need to dedicate a tune and test track day to play with them. Start out with a full soft set up and find out what a wet dish rag feels like. then go to full hard and rattle the drivers teeth. You will find out that neither extreme is good. Devote the rest of the day to playing with the shocks systematically and learn the transient responses. Basically you will find out that up to a point where the shocks make the suspension too stiff, increasing front bump reduces corner entry understeer ( push) until the suspension gets too soft that the laden corner falls over, reducing rear bump reduces corner over steer. too much droop at either end will cause break away at the end either by hanging the unladen wheel up in the air, or reducing tire compliance. Too little rebound adjustment results in a floating or oscillation car. Before you go home run a few laps with one front shock full soft first in bump then in rebound and finally both. repeat this with the rear. your driver will know when he has a dead shock and will help sort things out a lot quicker.
Attached from Koni ..a very good shock manufacturer

 

[bob1esq]

Incorrect. Read herb Andrews book, Chassis Engineering. What you fail to recognize is that the more "weight" that is transferred to the outside tire means that more "weight" is transferred off the inside tire. As more weight is transferred to a tire its traction modulus decreases. Thus, by transferring more " weight" to the outside tire, you are actually losing traction. The reason a low roll center can sometimes give more traction on that end of the car is because it causes the weight to transfer more slowly, over a longer curve, thus causing less instantaneous downward force to be placed on the tire. You would get the similar results with softer springs and shocks. Too much roll can also make your geometry go whacko, which causes a loss in traction.

 

[Ranger Mike]

Thanks for the nice post..let us look at it in detail.

Incorrect. Read herb Andrews book, Chassis Engineering.

I did.. had to take liberties with the photo to prove bona fides.

What you fail to recognize is that the more "weight" that is transferred to the outside tire means that more "weight" is transferred off the inside tire.

The only way to increase the amount of weight transferred is to physically move weight around on the car chassis..Move ballast, move a battery or fuel cell. You must also remember that the left front and left rear are both losing traction as is the right rear tire when entering a left hand turn. It all is going to the right front.

As more weight is transferred to a tire its traction modulus decreases.

Not necessarily true as there is a certain amount of weight required to plant the right front tire and assist it in turning.

Imagine that you have a car tire inflated and in good order. Place it on the garage floor..stand it up as it would be mounted on the car. Now place both of your hands on the tires outside diameter as if you were pulling it off the wheel lugs and move it toward you..,,moves pretty easy..right..??

Now go get the neighbors fat kid and have him sit on the tire ..try to move it toward you...scrubs a lot tuffer ..right..you have just applied down force on the tire and up to a certain point it will help the tire stick better than without the down force..
.
Thus, by transferring more " weight" to the outside tire, you are actually losing traction

when we transfer too much weight we start to push,,because we lose the traction (coefficient of friction)


The reason a low roll center can sometimes give more traction on that end of the car is because it causes the weight to transfer more slowly, over a longer curve, thus causing less instantaneous downward force to be placed on the tire.

Rate of transfer is strictly a function of the shock absorber and to some extent, the springs.

You would get the similar results with softer springs and shocks. Too much roll can also make your geometry go whacko, which causes a loss in traction.
been there done that..

The bottom line is this. In the post above we did not increase the amount of weight transferred. The only way to increase the amount of weight transferred is to physically move weight around on the car chassis..Move ballast, move a battery or fuel cell.

We were dealing with the exact amount of sprung weight before and after the chassis and shock adjustments.

A simplified version of what we did is as follows: We adjusted the length of the lever or moment arm that is between the Rear Center of Gravity and the Rear Roll Center. We shortened it up so the same WEIGHT moved through a shorter lever to impart less FORCE on the out side wheel. This meant that the out side wheel had less of a sheering effect on the tire and thus reduced the roll snap felt by the driver. Bumping up the shock pressure added to firmness of the shock and reduced the rate of load transfer to the wheel as well.


What really happens during cornering is a lot more complex.It involves refuting the myth of the kinematic roll center and Instant centers described above. It requires understanding Force Application Points, determining Force Based Roll Centers in 3D..
really gets foggy and hard to understand but I am working on this..the above explanation is the simplest to understand...and should suffice at this time..

bob1esq ,, excellent post and made me get off the couch and start digging into the weeds on this one..
rm

 

[THORPE]

Question for fun,

What if you were able to set the CG lower than the RC?
Seems like it would lean into the turn as force pushed the CG out.

Also seems like it would be easier to equally weight all 4 tires through the turn.

 

[Ranger Mike]

Thorpe ...I'm having a tuff enuff time figuring out what the heck the RC is doing when its located where is should be!

seriously..i have been working on a paper about RC and CG and what it all really means in the big picture..

all i can say is you need only one ball for football, baseball, basketball and golf , no so in racin

 

[the dr.]

I want to apoligize for my absence the past few months, BUT... first I want to thank Ranger Mike and all the others who offered help and advice to us, last night at North Georgia speedway, MMG Racing with driver Glenn Grimsley took the 2011 points title! I could not of done without the help and guidance of this site, and the advice of Ranger Mike. I have taken a car that was pretty fast and turned it into a freakin rocket ship... we have blistered them all year. got our first win all the while racing against cars with cheated up motors... while we are to the book, just proves you ain't got to cheat to win! thanks guys!

the dr.
crew chief of the MMG Racing car 64
2011 Econo hobby track champion
North georgia speedway!

 

[Ranger Mike]

CONGRATULATIONS DOCTOR
It is always good to hear such news,,
thanks for the kind words..
any contributions I made to your efforts were only to dilute the time line as I am sure you would have got there without my meager assistance. I just helped things along a little faster..
I need your help with this post in the future so please stay tuned

RM

 

[Ranger Mike]

myth of weight transfer

Weight Transfer - completely wrong , but we are stuck with it!

When you scale the race car you are measuring the wheel weight on each tire. Given a certain car weight, there is a certain amount of mechanical downforce applied to each tire and this downforce impacts the grip potential of the tire. While a car is braking, accelerating, or cornering, the effective mechanical downforce on, and therefore the grip of, the tires is constantly changing. These changes are referred to as "weight transfer." Of course, the weight of the car isn't changing, or moving about the car, but the forces on the tire contact patches are changing due to inertia and momentum. If you could put a set of scales under the tires when cornering , you would see what appears to be a constant changing of the weight at each tire, hence the name. Flat out wrong name , but its been out there for decades so we are stuck with it,

Anytime the car's direction changes through braking, accelerating, or cornering, each tire will experience a gain or loss of mechanical downforce. This weight transfer has significant impact on traction. Unfortunately for us, the net sum of the traction of the four tires does not stay equal. What is lost from the unloaded tires is not entirely transferred to the loaded tires. Overall there is a loss in traction. If too much weight is transferred to a particular tire, traction is totally lost as the tire is overloaded and grip goes away. There is no grip and the tire washes out. No front grip and the car pushes ( understeers). No rear grip and the car is loose and oversteers.

Because of this fact we need to minimize weight transfer. It cannot be eliminated, but it can be reduced. The amount of weight transfer is dictated by the vehicle's weight, location of the center of gravity, wheelbase, and track, and the amount of force applied during braking, accelerating, and cornering.

Weight transfer is a function of the vehicle's weight and the forces acting on that weight. Reduce that weight, and you reduce the product the of those forces imparted on the tires.

The vehicles Center of Gravity is starting point or origin of the "weight" transfered. This " weight" is multiplied by dynamic forces. The higher the CG point is, the greater the effect of the forces. Reduce the CG height reduces the product of the forces and vehicles " weight" transfered. One more factor is involved. The longer the wheelbase and wider the vehicle track in relation to the height of the center of gravity, the more resistance the car has to weight transfer. They behave as counteracting lateral levers to the vertical lever of the center of gravity point.

Now that we know what is being " transfered" we must understand where it is going. Where " weight" is being tranfered is related to the static weight distribution of the car, the roll couple distribution of the car, the height of the roll center of the car, and the slope of the roll center in relation to the ground plane.

I am working on Roll Centers and recommend you look at the attached PDF by Mitchell

 

[Ranger Mike]

Roll Center heights and offsets and why

Roll couple distribution is the relative roll stiffness between the front and rear of the car, and the left and right of the car. The front and rear roll centers are points that are designated by the vehicles mechanical suspension linkages. When we connect them with a line we have a roll axis. It is not necessarily parallel to the ground. Weight distribution, and roll coupling distribution can create a roll point at the front of the car which is lower to the ground that the roll point of the rear of the car. This creates a sloped line. The angle of this line has influence on how much weight is transferred, and where it goes.



Kinematic Roll Center. The most commonly used definition is the geometric (or kinematic) roll center; the Society of Automotive Engineers uses a force-based definition.

The location of the geometric roll center is solely dictated by the suspension geometry, and can be found using principles of the instant center of rotation. The SAE's definition of the force based roll center is, "The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll". from wikepedia..

The lateral location of the roll center is typically at the center-line of the vehicle when the suspension on the left and right sides of the car are mirror images of each other as in road course cars..not for the circle track folks.

The significance of the roll center can only be appreciated when the vehicle's center of mass is also considered. If there is a difference between the position of the center of mass and the roll center a moment arm is created. When the vehicle experiences angular acceleration due to cornering, the size of the moment arm, and angle of this arm, combined with the stiffness of the springs and anti-roll bars (anti-sway bars in some parts of the world), dictates how much the vehicle will roll. This has other effects too, such as dynamic load transfer.

If you remember nothing else about the roll center, remember this -
High RC = means more Camber gain per inch of suspension travel
Low RC = means Less Camber gain per inch of suspension travel

Let us consider 4 RC Heights:

1) Roll center height ( RCH) = center of gravity (CG)
No pivoting here, it means that there is no roll. Its like trying to spin a door applying force in the hinge. The car is turning, lateral force is applied, but there is no roll. This makes the car as hard as a brick bat since we arte not using the spring/shocks. BAD setup.

2) RCH between CG and the ground. Depending the percentage of that height you distribute how much force goes through the wishbones and how much through the spring/shocks. The range between 15% and 30% of RCH compared to CG is the most common place to locate the RC HEIGHT. Most of the race cars I've seen have front RCs within 1 inch of ground and the rear RC slightly higher - this is mainly to get front and rear roll in phase - basically making the rear load transfer happen a little faster than the front to compensate for the later development of slip angle on the rear axle vs. the front. A full blown super late model round track car with fabricated front clip has a 1.5 inch RC, the "stock clip" late models have 2.125" RC ( both are for 13 to 18 degree banked paved tracks).

3) RCH = ground height. All the lateral forces passes from the chassis to the tires via springs/shocks. You are not passing any force through the wishbones in a pure lateral load condition.

4) RCH below ground. More force than what's actually transferred passes through the spring/shocks, so that the wishbones are loaded under "a negative" force. This means outer top wishbone for example is not under compression, but under traction.
This is the case of Sedan Road racing cars that have to maintain the suspension geometry from the original street car when you reduce their ride height, there you have to find the best compromise between what you gain from aero and reduced CG height and what you loose for poor suspension geometry. Here you don't have jacking, but the contrary. Also it is the case of heavily "tuned" street cars. The Ford Falcons used to have the front roll center below ground, not because of a choice, but because that's where it lays when you lower the car with the wishbones pick up points that the rules stated. That was later "corrected" and now they are all above the ground.

Formula 1 Crespi XXV Formula Renault chassis. Front RCH -1 inch (that's underground ) Rear RCH - 1/2 inch (13mm) (underground). These cars are in a whole different ball game. The suspension of a Formula car has push/pull rod mechanism with springs attached between the rockers & the vehicle ("corner" springs) & a spring attached across the rockers to allow symmetrical movement, but to resist differential rocker movement , the sway bar (anti-roll bar). It is also possible to attach a spring between the rockers to allow differential movement, but to resist symmetrical movement (heave/pitch spring). Many F1 vehicles use all three types of spring at both ends of the car. Way too advanced for this post or this author for that matter so I think
Bob Hahn summed it up best.

From his notes " In more laymen terms, I believe you are asking about using migrating roll centers to tune your platform.

I've looked at your sketches and descriptions and see you are still using "above ground" roll centers (RC). The detriment is the further inside the RC travels the more roll is induced, and geometry is less effective.

We've been using migrating roll centers to tune suspensions for 15 years...way before force based software became available. We found locating static RC just below the ground, and migrating to ground level, on center with the outside tire to be most effective.

You'll find weight doesn't transfer to the outside, but actually to the inside via kinematics, ie causing the inside tire to load and the outside tire to unload...slightly. Yes sprung mass is still loading the outside tire but total wheel rate is less than conventional design.

This in turn not only allows more total grip, but also absorbs steering input disruption much easier, thus upsetting the vehicle less on turn in. This allows a deeper turn in, and quicker response manipulation in the long run.

Again, for the laymen, imagine, for each instance, a solid rod connecting each tire contact patch to the RC, and another solid rod from the RC to the CG. Imagine the cg as an extremely dense cannon ball, and the RC as the anchor point.

With the RC above ground, and traveling to the inside, there is a "pulling " force countering the CG. Gravity pulls the sprung CG down, with nothing to support it but springs, the platform rolls.

As the action proceeds, the RC "pulls" against the contact patches, at a decreasing angle, but extended arm which does cause a leverage effect, which in turn accelerates weight transfer, due to "lack of resistance".

The angle of "anti-force" or "anti-percentage" is paramount to weight transfer side to side, as it is under braking and accelerating in a longitudinal fashion.

When the RC is located below ground level, the applicable forces ALL happen in reverse. The CG is above ground the contact patches are "between" the CG and RC, in a 2 dimensional view, which should be understood before looking at a top view and visualizing 3 dimensionally.

With the RC below ground level, and migrating to the outside, the forces from CG to RC are in compression, In order for the cannon ball to move outwards, it must travel upwards due to the angle of the anchor point, the RC. Consequently, gravity as it is, the CG down force locates on the inside tire instead. Anchoring the RC at outside tire patch optimizes anti forces. Locating RC outside the outside tire patch, and above ground, actually causes the platform to reverse roll.

In more simple terms, if the RC is above ground, less movement is better. When static RC is located below ground, locating the RC under the outside tire patch under load is optimum.

Migrating roll centers add another dimension to setting up your platform, and in fact, considered "voodoo" by all but the upper levels of racing, who can afford the engineers, software programs, and testing and thus rarely used in the lower ranks. Most people simply lower the vehicle as low as the rules allow, and go with what they have. I've even seen vehicles raised, (by top teams)in order to prevent RC movement.

BTW, if you really want to upset the car during a "steady state turn", locate the RC just above ground in static, and have it below ground under load. Also, If you're not running below ground level RCs with your FWD vehicle...you're backing up!

...just my humble opinion...

Hope this has helped, or at least stirred the pot a little

Bob Hahn

Some day I hope to be near a mart as this guy..I'm still in 2D.


You can also change a cars response characteristics with roll center position, you are really tuning your elastic to geometric weight transfer ratio by where you position the rc.

A car with the rc closer to the cg will have higher % of geometric weight transfer that occurs, geometric weight transfer is instantaneous so you can have a car that points well but doesn't have as much mechanical grip compared to a car with a lower roll center which will have better mechanical grip (using your springs more) but poorer response (but is really dependant on tire side wall stiffness and spring and shock rates)

Jluetjen describes what happens when you have a Roll Center too high and it flips over the CG.


1) Roll centers are not static. As was mentioned earlier, they can move around (in 2 dimensions) quite a bit depending on where the car is in its roll and bump travel. Having a roll couple that suddenly doubles or triples in size can cause an awfully weird handling car, especially if the roll-couple were to suddenly reverse so that the Cg was below the roll center!

2) Suspensions with high roll centers are prone to jacking. The classic example is the rear suspension of a early Triumph Spitfire (see pic). Basically, because the roll center is relatively high in relation to the contact patch, there is a tendency for the chassis to be jacked up and over the contact patch. Not a big deal on 50's sports cars with non-sticky 50 series tires, but this can be a very big deal with the high loads induced by today's super-sticky rubber. If the roll center goes over the Cg, the chassis will actually "jack down" and start to roll to the inside of the turn.




Roll Center location should be placed at the vehicle center line for road course cars turning left and right.
On circle track cars turning left, RC location relative to the center line gives you a jacking force which is a component of the vertical load. If you have a jacking force of a certain amount the spring carries proportionately less load therefore deflects less - if you want soft springs for grip, but need to maintain ride height for aero reasons, I'm sure a little bit of jacking force could be useful. With "reasonable" RC heights, the net jacking forces wind up being pretty small. BUT..the location of the RC relative to the vehicle centerline can provide a huge jacking force to the point you start to carry the left front wheel when accelerating out of the corners. Most asphalt cars have the RC three inches to the right of center line. Dirt cars run 4 inch to the right RC. Imagine a point on the outside edge of the right front tire to pavement contact point. . Think of a pole vaulter sticking the pole in the ground and that is the anchor point. If we draw a line from this point to the RC we have a moment arm. When we enter a left hand turn the car tire sticks and thus the car tries to move up through this RC. Mean while a whole lot of transferred weight is coming from the left rear, and right rear and even the left front and counter acts this force. It all happens pretty quick. The total steady-state load on the car can only come from gravity and aero. That's just basic physics. Normally, the sprung mass is just supported by the springs. When the tires generate lateral force, the sprung mass is either pulled or pushed through the links. In order to reach equilibrium again, the springs have to either take up more or less load, relative to how much the tires are gripping.

A car with RC to the left of center line will push going in and be loose coming of because there is not a lot of down force on the right front to stick the tire and there will not be enough lift on the left front to plant the right read tire coming out of the turn. I had a hard time figuring this out until I read the attached paper by Mitchell. His analogy on how this pole vaulters thing worked is to use a shop floor push broom ( the kind with the long handle ending in the Tee shaped row of bristles). If you pick the handle up from the shop floor about 3 inches and push it glides pretty easy. If you raise the broom handle to shoulder level and push it, it takes some effort. RC on left side car does not have enough angle to do the job.

Boring technical summary


The lateral load transfer distribution (front vs. rear, how much each gets) is a big influence on chassis balance.
For the most part, total load transfer is a function of CGH and track width.
Of the total load transfer, a portion of it (proportional to distance from CG to roll axis) is a rolling moment, and is taken up by the springs and bars.
The remaining portion (proportional to distance from roll axis to the ground) is non-rolling overturning moment, and is taken up and split by the roll axis inclination.
The relative amount of front spring and bar, to rear spring and bar, decides how the rolling moment is split as load transfer to the front and rear suspensions
The roll axis inclination decides how the non-rolling moment is split as load transfer to the front and rear suspensions
The slope of the line from the contact patch to the RC indicates the proportion of jacking load on the sprung mass, to cornering force of the tire.
Roll axis inclination is not static, it will change when the car pitches, yaws in the turn. And Rc moves all over the place during cornering, acceleration, braking..it is squirrelly as hell..

The bottom line is that you should know where your Roll Center is and know how it effects your car at each track your racing. Toady's computer programs like Performance Trends circle track analyzer or Suspension Analyzer are as important as the tire pyrometer in today's racing..You absolutely must use it. and don't forget..its all about tires, Tires, TIRES...

High RC = means more Camber gain per inch of suspension travel
Low RC = means Less Camber gain per inch of suspension travel

 

[steve09]

hey mike
sorry i have been away for awile,we ran are super late model and got to much front
grip get in the coner and breaking trachion off,are shocks are real high on front rebound
over 1000 pounds,and not enough weight transfer to the rear the front is planted to the
ground.i lost the number you gave for the shock guy could i get it off you and give him
a call.thanks for the help

 

[Ranger Mike]

steve..private message sent..

 

[autodoctor911]

two upper ball joints?

hi, I am new to this forum. I have read some of the posts, and I am impressed with the knowledge available on this forum, as well as the professional demeanor. I am an autocrosser who has run various stock based sports cars and sedans with various levels of preparation. sometimes maybe a little more than allowed as far as modifying suspension points. I am interested in getting into SCCA GTA racing in SOWDiv. I will probably get an old perimeter chassis oval track car and rebuid it for the purpose. I have read the rules, and although the purpose clause warns against doing things innovative, the actual rules in place are not very limiting. They dictate that steel spindles designed for racing purposes must be used. they limit wheel offset to Rims must be 15”x10” steel stock car rims of a one-piece construction specifically designed for racing. Wheel offset must be a minimum of 3.00 inches and a maximum of 7.00 inches (i.e. - zero-scrub front suspension is not allowed).
I assume that they mean backspacing(7"offset would leave the center of the wheel 2" outside the rim). If a "zero-scrub" suspension is not allowed, does that mean that they are going to start measuring scrub radii. what is the minimum scrub?

anyways, I want to design a front suspension from scratch. I would like as close to the same geometry as a GT1 type car as possible, and to do this within the rules.

I was thinking that an upright with a low spindle to allow the ball joint to fit inside the rotor, and to keep the steering axis inclination ner zero, use two ball joints on top of the upright attatched to two separate upper control arms, with spherical inner joints as well, to allow the steering axis to be outboard of the upper ball joints at straight ahead. this would allow a high caster and trail, while minimizing jacking effect of caster while also minimizing the negative camber effect of caster on the inside wheel, and maximizing it on the outside wheel. the main benefit though is the zero kingpin inclination with minimum scrub radius.

This is either a stupid idea, or a great one. I don't know which, but it's got to be one or the other.
has this been tried in stockcar racing already? was it banned immediately, as I would expect? can the software you use model this?

 

[Ranger Mike]

welcome and thanks on behalf of the contributors, for the kind words
My dealings with Tech at SCCA has not been..warm and fuzzy... the annual tech usually goes ok but any deviation from what has been approved will raise all kinds of eyebrows..
you are racing for a plastic $13 trophy..but..just because there is no big dollars..does not mean the scrutiny is not there. This type of racing relies on the "Gentleman's agreement" to race properly. It cost $ 300 to protest a competitor and your protest must be exactly detailed for one singular offense...i.e. too much cubic inch displacement..so any time these racing doctors and dentist see any deviation that my offer a slight advantage, the poor SCCA tech guy is deluged with ...inquiries...Example..we ran Moon hub caps that were help on by 4 Grade 8 allen button head 8-32 bolts. The competitors had a cow.. finally at the end of the race session the regional Tech guy told us we needed to get approval from the National SCCA tech guys to race them. I called these guys and bottom line was ..no hub caps permitted because they might ..come off...it says it in rules so it must be true...

Your design ideas have been tried before and worked. I saw a similar set up a few years back when we were running outlaw super late models. It is highly doubtful SCCA would pass the non OEM suspension at initial car inspection.

My advice is to get the Performance Trend software and a set of car scales. Bench mark the stock suspension, use the software to find the roll centers and change the stock components to improve chamber gain and relocate the Roll Centers..word has it ( we would not know since we don't cheat) that you can significantly modify the spindles, control arms etc and no one would ever know by looking at them...
is this cheating?

Smokey Yunick once said " if its not barred in the rule book, its not cheating"

My opinion is Racing Doctors and Dentists can buy new tires ($880 a set) every race and run them with significant advantage,,,legally...we can't..the Racing Gods take care of this because these weekend warriors can't DRIVE a nail with a sledge hammer if they had to...so we have to use the 'rules..or lack of" to our advantage..just don't get caught..
rm

 

[autodoctor911]

from what I have heard, outlaw late models with coilover and rack&pinion steering have already been run,and although the guys with the "asa" type cars don't like it, they have been able to run with a 100 lb handicap for being 65" track and 46.5" overall hieght compared to the "asa" 62" and 49" hieght. I would like to run the taller, narrower, lighter setup, with the coilovers and rack, but I think that would raise some eyebrows. as far as I can tell, there is nothing in the rules to prevent using a pushrod suspension, just if the shocks have more than one way adjustment, you must add 50 lbs. I should get a suspension setup from a daytona prototype, and adjust for the smaller wheels and higher weight. I don't think I will push it that far though.

 

[autodoctor911]

here is the suspension rules:
IV.
A. Springs are open.
Suspension/Shock Absorber Specifications
B. The steering wheel must be mechanically coupled to the front wheels and activate only those wheels (no “steer by wire” or “four-wheel steering”). Power assist is allowed and may be driven off the differential.
C. A collapsible steering column, either by layout design or column construction, is required.
D. Front lower control arms must be made of steel. Upper control arms, strut arms and upper pivot shafts may be aluminum.
E. Spindles must be steel and designed for racing applications.
F. Independent front suspension with articulated upper and lower control arms is mandatory.
G. Major steering components including steering arms, tie rods, idlers, etc., must be fabricated from approved ferrous or non-ferrous alloys. All heim joints must be of aircraft quality.
H. Sway (anti-roll) bars must be made of steel. Heim joints are allowed to be attached to the lower control arm(s) and/or rear end. Driver adjustable sway bars are not allowed.
I. The longitudinal linking system for the rear of the chassis may not exceed four locations and may not include a “torque tube” of any design. Spring-loaded and/or cushioned (torque absorbing) links are permitted.
J. Either a panhard bar or Watts link may be used to locate the rear axle laterally.
K. Independent rear suspensions are not allowed.
L. As long as it has no remote reservoir, any single-adjustable shock absorber may be used with no weight penalty. If even one shock absorber is multi-adjustable or has a remote canister, a fifty (50) pound weight penalty is assessed.

 

[autodoctor911]

pretty open if you ask me.

 

[Ranger Mike]

yea..looks prime for some inventive fabrication..

you aren't a doctor are you..or did i step on it with crack about the racing doctors and dentists??

 

[autodoctor911]

no, I fix cars for a living

 

[the dr.]

Sorry I have been away from here for a while we have been busy getting things lined up for next year, but I want to ask a couple questions, I am being pretty careful with how I ask it since my "secret site" has been found out...lol some of my closest competitors have found some of my post here, but anyway...as everyone who has read my post may know we run our J-bar ( panhard bar) on the right side on our 3 link set up, where most run it on the left, around here racers refer to the difference in the two rear wheels as "bite" ( I.E. how much bite you got in the car? ) well most of the guys around here are heavy on the left rear but we are heavy on the right rear, is this due to the j-bar mounting point? I seem to have a problem wrapping my head around that, our right rear weight is 709 with driver in car and 677 on the left rear 948 on left front and 554 of right front, I may regret posting those numbers but...lol it gives us 48% rear and 56% left side... and the car is AWESOME fast.handles like a dream, right or not I ain't changing a thing as long as its this good, we are looking for some updates on things in the off season. we have found that when we get the car to roll over on the right rear the drive off the corner is unreal, if I look at the travel indicator after a heat or feature and see 5" of travel I know it was good and the driver says so when he gets out but if we don't have the travel of at least 5" it don't feel good to driver, and lap times are not as good. my second question is.. IF I was going to change springs where should I start? we have toyed with the idea of doing some extreme stuff in the off season to try and find a new setup that will work as good as what we got, we race on a very high bank 3rd mile clay oval..
thanks
the dr.

 

[the dr.]

yea..looks prime for some inventive fabrication..

you aren't a doctor are you..or did i step on it with crack about the racing doctors and dentists??

I bout spit out my coffee when I read that...we have a racing Dr here to... bout the same way as you described...

the dr.

 

[autodoctor911]

well, it looks like the reason you are heavier on the right rear is the low weight on right front. The panhard link should not affect the weight distrubution in static. looks like what you are running is a lot of wedge "out". If your setup is good but you don't allways get enough travel, I guess you should consider going with a softer rate all around. I don't know much about racin on dirt though. Are you near opposite lock when accellerating out of the turns? how do you straighten out after sliding through the turn? Is it just let off the gas, staighten the wheel and get back on it? I am curious as to how the physics work in a dirt car. If you were on the track by yourself, or out in front of everybody, wouldn't it be faster to take the turns without sliding the rear? you know, just like on a road course, slow in, fast out.

 

[the dr.]

well, it looks like the reason you are heavier on the right rear is the low weight on right front. The panhard link should not affect the weight distrubution in static. looks like what you are running is a lot of wedge "out". If your setup is good but you don't allways get enough travel, I guess you should consider going with a softer rate all around. I don't know much about racin on dirt though. Are you near opposite lock when accellerating out of the turns? how do you straighten out after sliding through the turn? Is it just let off the gas, staighten the wheel and get back on it? I am curious as to how the physics work in a dirt car. If you were on the track by yourself, or out in front of everybody, wouldn't it be faster to take the turns without sliding the rear? you know, just like on a road course, slow in, fast out.

well we are never full lock one way or the other, and we use the stagger to help rotate the car i guess you could say, as far as to how we get car going straight, well coming off the turns it just goes straight...lol and we are out of the throttle very little, give me a few and I will send you a video link of our car..in action from the drivers view...

the dr.

 

[the dr.]

the first part of this is hot laps where we were just making sure everything was working not really racing, if you watch the 10 car he only hit us 3 times, once resulted in a flat and we still came back to run 2nd... could of won if we hadn't of had to change a tire... hope this helps explain some of the things you asked...

the dr.

 

[autodoctor911]

hey mike, I was just reading your posts from sep. 27, and the concepts discussed in the second post seem to be invalid, referenced to the mitchell paper you attatched in the first post. I agree with the paper, the kinematic roll center has little to do with how force is distributed, especially with reguard to it's location laterally. The illustration with the cannon ball and the solid levers makes little sense. make that none. there are no forces applied at a roll center for a leverage to be applied against the contact patch.

 

[autodoctor911]

it looks like everyone is trying to go too deep before braking and losing speed driving sideways, instead of accellerating through the turn. looks like if you had a good handling(balanced), you could brake earlier, get the car set into the turn, acellerate through with a nice drift with only minimal corrections to the steering, and you would carry a lot more speed exiting the turn, and as a result down the straightaway. I guess I don't get dirt racing very well. the cars must go faster sideways than they do pointed straight ahead?

Is your driver missing an arm? I never saw his right hand on the wheel.