Race car suspension Class

[marshalljr]

I reposted this by mistake, however on dirt, increasing jbar angle tends to tighten corner entry and center but can hurt forward bite on exit. A compromise is needed to optimize corner speed and momentum onto the straightaway. I only increase angle for very slick conditions.

 

[tom jennings]

hi -- I'm replying to an old post, as i make my way through this fantastic forum... i can't thank you all enough, concise, dense, courteous, wide-ranging. and especially Ranger Mike... on page 12 (2011), working to the present!

Squaring chassis set up

for cars with lower ball joints in front, i worked up (probably, re-discovered, in ignorance) a nice trick. i chopped the head off a Zerk fitting, inserted a cotter pin (loop end outside), slightly bent the legs to retain, clipped short, and screwed into the bottom of the ball joint. a hand side effect is that the cotter pin neatly holds a tape measure or string.

 

[Racer Dean]

Hello All

My name is Dean and have spent the last 3 weeks reading this thread from start to now and am extremely impressed with the content, ideas and the level of knowledge shown by all. Thoroughly enjoyed and thanks.

But I'd like to ask for some advice if possible,
I race a non wing Sprint car on dirt in Australia and wasn't completely happy with running the same set up as everyone else. (Torsion bar, Z link)
I've changed my suspension to a full coil car using a 4 link setup.
After reading this thread, I'm starting to realize the more I learn about racecars, the more questions get raised.
What I'm after is any ideas on a good suspension software that can work with roll centers using solid front axles.
I just can't see how I can raise or lower the roll center in the front?
To me it's always going to be in the center of the axle
Any help would be appreciated

Dean

 

[Ranger Mike]

Welcome Racer Dean, and thanks for the kind words..
I called Kevin at
PerformanceTrends.com
He said they are working on adding solid front axles to Suspension Analyzer. Right now the only thing is the Circle Track Analyzer. Front axle is assumed to be located by a panhard bar.Kevin Gertgen
Performance Trends
contact him at feedback@performancetrends.com

Actually I prefer the Circle Track analyzer even though it is 2D. it is quicker to put input in since you only measure t he distance from center line and height from ground. The more sophisticated Suspension Analyzer requires you to measure the distance from the front of the car for each data point.
plus it is cheaper!
click on link below, i think they have downloadable trial version
performancetrends.com/Circle_Track_Analyzer.htm

 

[Fiveothis]

Ranger Mike, a huge thank you to you and all the other contributors on this thread. I just spent a week reading ALL 55 pages. I understand almost all of the info but have a few questions.

First when talking about roll centers and jacking effect, is there a different thought process when setting roll center location between a car with independent suspension versus straight axle? Obviously from reading these 55 pages you believe in low roll centers that are offset 3 inches to the right, is that the same for a straight axle all things being equal between 2 cars except the obvious suspension design change.

Second question when you talking about setting the roll center 3 inches right, what about a car that has the suspension offset to the right 6 inches or more overall to increase left side weight? Is the center-line still the center of the frame or is it center of the contact patches??

 

[Ranger Mike]

Thank you for taking the time to wade thru 55 pages. Nice to know the effort was worth it.
We offset the front Roll center to add down force tot the right front tire when turning left. This is easy to do when you have control arm suspension and the chassis software. You can get the same benefit on beam axle or straight axle if you have coil over shock set up and panhard bar. If you have the traditional mono spring set up like the Ford Model T used, I am not sure you can offset.
Regarding the Roll Center offset and left side weight offset- They are not related and should be treated separately. Left side weight bias means the weight is shifted when you have the racer car setup on weight scales and the left front and left rear wheel weights are 55 to 60% more than the right side. It is a static placement. Wheel back spacing can be changed to help this. Typically you are mounting the engine offset to the left, fuel cell, battery and any ballast. Roll center offset requires the race car dynamics of chassis roll to add down force on the right front tire to turn the car, and proper static weight placement to cause the right rear and left rear tires to hook up and drive out of the turn.

I always scribe a center line on the chassis for reference. Using the tire contact patch is ok if you do not plan on swapping out different back space wheels. I got to many other things to worry about at the track so use chassis center line for all datum. Consistency is the key point here.

 

[Fiveothis]

OK Mike. Yes let's suppose a vehicle with straight beam axle, coil over shocks, and Panhard bar. Obviously the roll center height is centerered between the 2 rod ends but how do you go about offsetting the roll center left to right? Is it as simple as moving the location of the Panhard bar location? I was thinking in Carroll Smiths books he says with a beam axle the location of the Panhard bar left to right doesn't matter because the car "feels" the roll center left to right half way between the top mounts of the coilovers?

As far as the offset chassis and centerline the offset of the wheels would never change. But if I use the frame as centerline versus using the tire contact patch centerline the difference between the 2 is over 6 inches. So again should I not use tire contact patch centerline?

 

[Ranger Mike]

The roll center on a solid axle is the place the panhard bar crosses the axle center line, be it front or rear axle. you have unlimited means to design and build the best combo for your set up. If you plan on using contact patch center line, great. Just remember the offset wheels are the same every time.

 

[moosetallone]

Mike
On a dirt late model left side mount jbar, what's your thoughts on running soft right rear spring with spring rubbers and a lower jbar angle to help with loose off condition
Though being softer spring will allow the car to roll over easier and plant the tire with the spring rubber, keep the angle lower on the jbar to keep from pogo sticking the rear of the car up on the jbar.

53% left 53% rear

 

[Ranger Mike]

FiveO the RC is the intersection point of the panahard bar and axle. The example happens to be the p-bars center point as well. I agree with the software.

Welcome Moose..i am not a fan of spring rubbers as they wear out or fall out and are a variable to figure out when handling suddenly goes away. How much rear roll steer are you running now? What are the angles for the lower bars now? If you want more body roll soften the ARB (sway bar). To add side bite and bite off corners try lowering the panhard bar and add a little angle. You can take out the right side wheel spacer and add more tot the left rear wheel spacer too.

 

[Fiveothis]

OK Ranger Mike I got it now. Is there a certain amount of angle in degrees I could baseline a setup with? I have read and understand the broom handle theory and read many times your example of 3 to 4 inch offset to the right for roll center location. But for my application with the major amount of suspension/chassis offset it would seem to me and by looking at the picture of the performance trends software I need a roller center farther to the right than the typical stock car examples to get the needed angle.

 

[Ranger Mike]

I got not degree for your set up as there are too many variables. I will asy too much offset will lift the left front tire, overload the right front tire. use the pyrometer to adjust. If you have a wing you will add down force as well. all these add up so don't go big too early..gradual changes are better.

 

[Fiveothis]

OK Ranger Mike. Let me throw you another curve ball. Let's say we have 2 identical cars with the only difference being front roll center location. Everything else exactly the same. Car 1 has the front roll center 3 inches above ground, 3 inches right of centerline and that gives a force angle from right front contact patch to roll center of 20 degrees. Car 2 has roll center 6 inches above ground, 3 inches left of centerline, BUT the force angle from the right front contact patch to roll center is still the same 20 degrees. Will these 2 cars handle the same and will they have the same amount of download onto the right front tire?

 

[Ranger Mike]

Assume on this example , both cars have a track width of 66 inches and the engine is located at vehicle center line and cam shaft is 16 inch height. We use this as approximation of the Center of Gravity (CG).
Car 1 has a distance of the RC to Cg of a little over 13 inches ( 16” –3” = 13”, plus the 3 inch offset ..I did not do the triangulation but it is a little longer than 13”.
Car 2 has a shorter distance of 16” – 6” = 10 inch and change. This means the lever is shorter and has less mechanical advantage.
Now add to this, the fact that the RC offset to the right means 55% of the vehicle weight will rotate and plant on the right front tire. ( 66/2 = 33 center line and add 3 inch offset. 33 +3= 36...36/66 = 55%). Compare this to the RC offset to the left by 3 inch. Now you have only 45% of the weight rolling onto the right front using a shorter lever arm. .The left offset RC setup means the car will push going in and be loose off the turn. Forget about the jacking effect as you need to stick the right front tire first to cause the pole vault jacking effect.

 

[Fiveothis]

Got it Mike great explanation.

The only thing I can't wrap my head around is why the RF needs additional load to help turn the car. It would seem for maximum traction you would want to keep the roll center left and keep the LF tire loaded more since it does less work

 

[Ranger Mike]

like everything in life it is a compromise. Too much down force on the right front will make the left front tire lift, too little and the rt front skids instead of sticks. Ultimately you want all four tire carrying equal load. Thats why you use a tire pyrometer.

 

[marshalljr]

Then you either need a shock with more travel or you need to limit the travel via spring rate changes. Using a chain won't cure the problem, just keeps the shock in one piece.

As soon as any part of the suspension binds or locks or hits the end of a stop, the spring rate on the wheel effectively goes sky-high and that tire loses traction. If it is a solid axle like yours, it also changes the spring rate on the other wheel the same way, again causing a loss of traction.

By using a chain or a short shock on the left rear, you're making the right rear act like it has a soft spring when that car is flat and a hard spring when it hits the stop as it roll. This can change the handling rather abruptly with only a small change in roll angle and make the car a handful.

Get the chassis movement/roll under control like Mike is suggesting.

 

[Warriorracecars]

Assume on this example , both cars have a track width of 66 inches and the engine is located at vehicle center line and cam shaft is 16 inch height. We use this as approximation of the Center of Gravity (CG).
Car 1 has a distance of the RC to Cg of a little over 13 inches ( 16” –3” = 13”, plus the 3 inch offset ..I did not do the triangulation but it is a little longer than 13”.
Car 2 has a shorter distance of 16” – 6” = 10 inch and change. This means the lever is shorter and has less mechanical advantage.
Now add to this, the fact that the RC offset to the right means 55% of the vehicle weight will rotate and plant on the right front tire. ( 66/2 = 33 center line and add 3 inch offset. 33 +3= 36...36/66 = 55%). Compare this to the RC offset to the left by 3 inch. Now you have only 45% of the weight rolling onto the right front using a shorter lever arm. .The left offset RC setup means the car will push going in and be loose off the turn. Forget about the jacking effect as you need to stick the right front tire first to cause the pole vault jacking effect.

I wish this worked on my track. I can say that it does not. Actually creates a tight center. This is a very flat 3/8 almost round track. coldest tire on the car is the lf with 51% cross after loading 1 1/8 bar. I will be moving the mc to the left maybe 3" and see what happens. Mike when you move the mc to the right, do you notice any difference in the ability for the right to "get down"? The reason I ask is that with my car. I have equal springs left and right at the moment. Without any sway bar involvement the right corner is MUCH harder to compress in the shop and shows on the track.

 

[Ranger Mike]

are you still using bump stops?
and you are loading the ARB as well?

 

[Warriorracecars]

yes bump stops and 2-3 rounds in swaybar

 

[Ranger Mike]

warracecars, my advise and opinion does not apply if you are running on bump stops and adding in preload on the ARB. Both of these are counter to my entire thinking on making a race car suspension handle during cornering. good luck on your effort.

 

[Warriorracecars]

no problem Thanks Mike.

 

[Tyredone]

Ranger Mike: Thank you for advice - I bought and read Steve Smith's books. I now have a couple of questions about the car, where building for paved flat track with left and right corners:
1. Bump and Rebound travel - how much either way?
2. The third link pick up point on the diff? ... In your comment (Pg 38 / 749) you gave an example: "i.e. if car has 60 inch track width and 53% left side weight then 60 x .53 = 31.8 inch and 3rd link should be mounted 31.8 inch to the left of the RIGHT TIRE CENTERLINE." ... And in Steve's book he also suggests: "If a car has a 60-inch rear track width, and a 58% left weight percentage, the calculation is: 60 x .58 = 34.8. So, the centre of the weight mass at the rear is located 34.8 inces to the left of the center of the right rear tire." ... Please forgive my ignorance - I know my track width, but are you able to explain how to work out the left side weight so I can calculate as per examples above? (Sorry, I'm not the sharpest tool in the shed.)
Appreciate your help :)

 

[Ranger Mike]

Ranger Mike: Thank you for advice - I bought and read Steve Smith's books. I now have a couple of questions about the car, where building for paved flat track with left and right corners:
1. Bump and Rebound travel - how much either way?
2. The third link pick up point on the diff? ... In your comment (Pg 38 / 749) you gave an example: "i.e. if car has 60 inch track width and 53% left side weight then 60 x .53 = 31.8 inch and 3rd link should be mounted 31.8 inch to the left of the RIGHT TIRE CENTERLINE." ... And in Steve's book he also suggests: "If a car has a 60-inch rear track width, and a 58% left weight percentage, the calculation is: 60 x .58 = 34.8. So, the centre of the weight mass at the rear is located 34.8 inces to the left of the center of the right rear tire." ... Please forgive my ignorance - I know my track width, but are you able to explain how to work out the left side weight so I can calculate as per examples above? (Sorry, I'm not the sharpest tool in the shed.)
Appreciate your help :)

hello
tyredone
i am in chicago and away from my notes. suspension travel? what type car is it? a formula car will have a lot less travel, a street stock a lot more..
what is your question on left side weight. both calculations are correct. track width is distance from tire center line to tire center line. The percent left side weight is as you noted. the amount will be determined by local track rules. if you are hung up on the center line versus center of the tire (one single point) don be...use tire center line.
glad to have you stop by on this post..
rm

 

[Rajesh Shirsagar]

The suspension of an auto structures the basic interface between the distinctive components that work together to deliver its execution. Suspension is the thing that outfits the force of the force unit, the downforce made by the wings and the grasp of the tires, and permits every one of them to be joined adequately and deciphered into a quick on-track bundle.

Taking after the restriction on PC controlled "dynamic" suspension in the 1990s, the majority of the Formula One auto's suspension capacities must be done without electronic mediation.

 

[Ranger Mike]

ifin it ain't in the rule book..its legal!

 

[Jared0020]

If I gain 50 HP what kind of pick up am I looking for in laptimes on a half mife track. Is there a ratio for that. Thanks

 

[Ranger Mike]

Thats a pretty open question. If your car weighs 2500 lbs. 50hp will add a bunch of speed. If you race a 3700 lbs. door flapper, it will help but not as noticable. there are just too many variables. It will definitely help. I am sure there are advanced computer models you can buy to figure this. My guess is on 1/2 mile track you may cut 0.200 second per lap.

 

[drobbie]

I recently saw this picture posted on a social media site. In trying to educate myself more I was wondering you'd give a few thoughts on what causes this and what to do to possibly correct it. I have some suspicions on what is causing it but I'd love to get more input to create a full rounded education (and possibly help others).

 

[Ranger Mike]

Beautiful!
A classic case of the front roll center being located too far to the right of center and possibly too far above ground level. This set up is really quick during qualifying but excessively loads the heck out of the right rear tire under acceleration. Great for short term qualifying but will burn off tires too quick in the feature. This set up on this particular track with this compound tire has created too much leverage and will lift the left front tire. The right front tire is really gripping and lifting because you have too much down force on the right front tire. You are racing a three legged stool. Need to get more heat on the left front tire.

Now here is the problem. Unless you know where your front roll center is both horizontal and vertical, any track corrections is going to be chasing your tail. If you do have the front roll center located around 3 to 3.5 inch high and 3 inch to the right of center line, then you can correct the above situation by swapping springs and fine tuning the shocks. This is why the garage time to bench mark the car and map all the pivot points is so critical. Get it real close in the comfort of the garage before you do a lot of head scratching at the track.

 

[drobbie]

Ranger Mike, I had someone else say the RC was too far to the left. Can you get into a little more detail about the placement and how it would jack up the LF like this. Lots of cars in this area seem to have the same issue.

 

[Ranger Mike]

i wrote a whole bunch on this subject
post 229 on page 12
post 251 page 12
post 691 on page 35
post 811 on page 41
post 879 page 44

in my opinion there i s only one reason to have the front RC offset to the left. to kill off down force on the right front tire since you got too much down force because of aero. but i could be dead wrong. i just got to go with what i know.

 

[Ranger Mike]

BIG BAR SOFT SPRING

A lot of you fellows have been emailing me on the current trend of Big Bar Soft Spring (BBSS) set up. I will attempt to explain it as far as I know about it.
The traditional set up runs something like this. You calculate the total amount of spring rate required to counter the momentum generated during the cornering process. (ok weight transfer if you want to stick with it!)
This total spring rate is then splint up into three equal amounts to be handled by the two springs at each wheel and the Anti Roll Bar (ARB ) ok sway far if you must! The traditional set up also supposes sticky gummy tires with a lot of grip and assumption that you will use body roll to down load the right front tire for required grip.
Two things have evolved to digress from this traditional pacakge. Sanctioning organizations have mandated spec tires as a cost saving (hha hah - in racing! righttt) and late model bodies are getting really sleek and aero dynamically active. Don’t try this with a 1972 Monte Carlo body as it is as aero as a brick.

One more thing – BBSS seem to work better flat tracks or under 5 to 8 degree banking. Banked tracks compress the suspension due to the higher speed and require additional suspension travel. Banked tracks keep the nose close to the ground throughout the turn. The ARB does less work on banked tracks as cars compress into the banking where as on flat tracks the body roll is much more in effect. We get more aero advantage on the high banks because the track let's you run faster into the turns.

The BBSS wants to keep the car body down as low as possible to block off any air from getting under the car. Air under the car creates lift. Nose down means more airflow over the entire body and more
down force on all four tires. We get the body down by using as soft a spring pacakge as possible and not bottoming the car during cornering. The ARB is maxed out to prevent anybody roll. The roll center will not migrate as much as the traditional method because we have minimal body roll. This also means we get not down force on the right front tire via body roll. We do get down force from Aero dynamics of the sleeker car bodies used these days. Depending on the amount of aero down force generated, we have to back off the roll center offset and even park it left of the center line to balance the amount of down force on the right front tire.
If we do the math, the spring rate we take away from the total spring rate package with the softer springs is compensate for by adding in a bigger ARB to total out the same rate. It is not quite this simple as we have to add in more spring rate because we have more aero down force than before, thus more grip which means higher spring rate but you get the idea.
With the large ARB, the left front is held down during cornering and we have more air flow (grip). This includes air flow over the side of the car during cornering. Think of those outlaw Modified’s with the huge clear Plexiglas panels mounted along the cars centerline. Insane but really adds to cornering effect.

This BBSS packager also means stiffer dampers (shocks) and stiffer rear springs.
We need a stiffer rebound- softer compression shock kill off any lift on the left front tire. We want the front tied down after the nose settles after corner braking .The stiffer rebound controls oscillation.
Don’t forget, we are no longer driving that big old whale that rolls over during cornering. We have minimized suspension travel and thus increased steering wheel response.
With the front end tied down we have a few other benefits. The nose won’t raise up and the rear won’t squat when we nail the throttle. Lowering the tail kills off any down force by the rear spoiler since the spoiler is lower and out of the main stream of air flowing over the car right when you need traction.

BBSS use stiffer rear springs to keep the spoiler in the air stream thus adding forward bite. A stiffer RR spring keeps the LF tire planted instead of trying to lift and carry thru the turn.
We want the Left Front to drop under braking. The stiffer rebound shock holds the nose down ( along with the stiffer right rear spring and ARB) We keep the nose down on entry and throughout the turn.
Anti squat means you may need to run more split in the panhard bar to get an equal amount of rear steer as the stiffer springs coupled with the stiffer sway bar create less roll. Changing trailing arm angle as
needed will help rear steer the car through the center of the turn. You have a lot less suspension travel front and rear with this set up and this means a lot less roll steer as we had before.
the rear linkages needs to be based on less travel. This set up uses a lot more RR spring rate than LR .

Since we are heavy on the downward travel side of things on the front steering geometry ,we have to re-think camber curve. Forget about measuring camber at static ride height. With this set up as soon as you get to speed the car is pancaked down for the remainder of the race since shocks with huge amounts of rebound hold the car down. By now you should know the shock travel for maximum bump ( middle of the corner). This is where you need to measure the camber, at the center of the turn.
This is where we need to set the proper camber to gain maximum tire contact patch to help the car turn. Too much RF camber and the inside edge will not hold ,too little and we start to scrub.
AT Mid Turn ride height, shoot for 4.5 degrees of negative camber at the RF.. The left front starting setting would be 3 degrees positive with the car at mid turn ride height.

The critical thing to remember is that this set up works best if you start with a traditional set up Neutral car. This includes the roll center at around 3 inch above ground and offset to the right side 2.5 to 3 inch.
You have to do the mat hand calculate the actual wheel rates and current ARB rate and motion rate. You have to do the math on the rear Wheel rate. Then you have to estimate 250-300# front springs a huge 500 to 1000# ARB, left rear spring with 150-200# and right rear 400 to 500# spring. I have seen these vary all over the map. You have got to deal with the cornering momentum (weight transfer – I hate that term) with the proper spring ARB package and this includes the roll center placement. You still want it off set the Roll Center to plant the right front tire regardless of the small about of body roll. We are dealing with linkages here and the Center of Gravity still acts on the Roll Center even though we do not have a 3 to 4 inch travel. We still need to plant the tire and turn the car.

Ackermann effect is very detrimental to the BBSS setup because the LF corner is forced down and a lot of the front load is carried by this tire. Both front tires must track along their proper arcs, tangent to the curve and perpendicular to the radius. This comes out to around 0.100 inch of added toe on the left front tire for a small 1/4 mile track. For 1/2 mile long track 0.040 inch of added toe of increased LF steering angle.

The Big Bar Soft Spring Setup is not for the novice racer. To switch from a traditional fast running neutral set up to the BBSS is difficult enough. You have to know how to calculate spring rates, wheel rates , motion rates ARB motion rates, roll couple. As a minimum you need the computer software package. You must know about damper (shocks) and the effect on motion transfer. Anti squat and Cross weight (wedge) factors in too.To start out racing the BBSS right out of the gate is a very difficult task. It will take a minimum of a season racing and this includes tune and test, collection of track data cataloging of set up changes, and at track changes. Not for the amateur racer. Add to this the fact that this set up has been popular for 10 years and has minimal info on it...what does that tell you?

ifin you insist on going BBSS right out of the box..good luck with that!
rm

 

[mikey531]

Has anyone tried using two springs on the front of an asphalt modified? In my simple mind if I wanted a 500 lb spring on right front could I use a 800 and 200? that would give me my 500 rate and the 200 would react quicker than a 500 for weight transfer? I was thinking about doing this to both left and right front.

 

[Ranger Mike]

i am sure it has been tried before. The big hurdle is adding un-sprung weight. I think the rate of "weight transfer" i hate this term...is dialed in with dampers (shocks).