Race car suspension Class

[drobbie]

Thanks for the quick reply Mike,
This is a modified asphalt car, tube chassis, three link rear. The car is 58% LS so the using the what you say, yes it is close to the discussions you mention. As for the Front Rc, that is a work in progress right. Off season is a great time to sit back an reflect on what you had and to make changes going forward. A little more about the car so maybe you could give me some hints. It is asymmetrical by one inch on the lowers (RF lower longer than LF) and the uppers are the same length. Equal height spindle and ball joints. 95 inch wheelbase and 63 inch tread width.
It was my concern the Rc was a little high not so much to the right being an issue. So maybe a little direction of getting corrected would jump start some ideas.
Let me know if you want more info

 

[Ranger Mike]

The 3 inch RC height is right on. You will not get any lower with the stock components you have to use. Try to move it to 3 inch offset from 5 inch. If you can move it to 4 inch you will be running what everyone else is running. 3 to 3.5 is ideal. Do you have suspension software to show RC moving in roll and droop? Do you run flat track or medium banked ( over 10 degrees)? do you know your camber build per inch of bump?
what spindles are you using?

 

[drobbie]

I have access to RC software so I when back an played around with some numbers based on the existing adjustments available on the car. The attached picture is what I came up with. I think it might be more in line with your recommendations. I use stock Mustang II spindles with tubular upper and lower controls arms. It also has a mustang R&P.
I did run it through 1.75 inches of dive and 1.5 degrees of roll and the Rc moved to .9 high and 4.5 to the right.
As for the tracks we run, I race in a touring series so it is anywhere from a small medium banked 1/4 mile to 5/8 high bank with long straights. There are also a couple of 1/3 mile sweeping turn tracks.
Let me know what you think.
-DR

 

[Ranger Mike]

Sounds like you have a pretty good start to a wining season. The software you have is great. You can dial in the front RC with it. You can always swap in a Chevelle spindle that is an inch taller if you have to, Ford pinto spindle may be same as you are running, not sure.
One thing to note- you have a whole range of different tracks from flat sweeping to serious high bank. I would put major effort into charting the correct camber curve for each track. Then set up car for each event with correct camber shim package. A completely flat rack would need negative 1.75 degree camber per inch camber build on the right front and positive camber on the left front 0.5 to 3/4 degree per inch gain . A 10 degree banked track needs neg. 1.5 degree per inch rt. side and left front 0.5 to 3/4 degree pos. camber per inch gain. A high bank track would take 1 to 1.25 degree per inch of bump rt. side and left front 3/4 to 1 degree per inch gain pos. But..use the pyrometer to dial these in. These are ball park settings. I am sure you know the correct springs for 1/2 mile and longer fast tracks. The time you spend in that cold garage this winter, mapping the camber build for each track will pay off big time. Serious race teams do this. Weekend warriors just swap in the bigger springs and change stagger and go race the high banks ( never taking the time to change the camber build). So their contact patch is off pretty much of the time. You will have the advantage of maximum contact patch matched to the track or darn close too it.
One final note- I would concentrate on smoothing things out aero wise on the tracks over 1/3 mile. Granted you can’t do anything about those ugly tires but you can get benefit with small changes to smooth out flow. I have spent days in the garage taping tuffs of yarn on the car and looking at the air flow. That hog barn fan really flows the air but its cooolllddd.

 

[Warriorracecars]

You lost me on this one Mike. The left side camber gain part. The left will go toward negative in dive so could you explain this a bit more? Also have you experimented with Jacking? As in extreme arm angles that utilize the force that is generated in the cornering phase to accomplish vertical loading on a given corner or tire?

 

[Ranger Mike]

When you are in a race car turning in at corner entry, you feel like you're being pushed toward the outside of the turn. Most of us refer to this as the centrifugal force. WRONG. There isn't any force pushing you outward. Centrifugal force is what physicists call a pseudo or a fictitious force, because it doesn't really exist. More specifically, in Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a "fictitious" force) observed in a non-inertialreference frame, and also the equal and opposite reaction to a centripetal force.
So is the centrifugal force isn't real, why do you feel like there's something pushing you out the right-side window when you make a high speed left turn? The answers lie in Newton's laws of motion. An object going straight will keep going straight unless a force makes it change speed, direction or both. When a driver is bombing down the straightaway and starts to turn, the centripetal force makes the car turn and, because he's buckled tightly into the car, he turns also. The force he feels is because his body is trying to keep going straight. The seat and shoulder straps, lap belt and sub straps tied to the car are all exerting a force on him toward the inside of the turn while he's trying to go straight. The net result is that the driver perceives a force to be acting outward, but it is actually acting inward.
Got it?
The force that makes a car turn is called the centripetal force. Centripetal literally means "toward the center". Imagine you had a rubber ball with a string attached to it. Whirl the ball over your head in a horizontal circle. What makes the ball go in a circle instead of flying away from you is the force the string exerts on the ball, which pulls the ball in a circle.

A race car doesn't have a string attached to make it go in a circle but it does have TIRES. The tires contact the pavement exerting force toward the center of the turn. Engineers talk about lateral force. The lateral force is perpendicular to the direction the car is going at any moment.

The size of the centripetal force is given by multiplying the mass of the car by the speed of the car squared, and then dividing by the radius of the turn.

F= MV2 / R where centripetal force equals the mass of the car, v is the speed of the car and r is the turn radius.

Without going into a lot of math , the faster you go, the more force you need to be able to turn. Tighter turns require more force. Just like Aerodynamics, the force isn't linearly dependent on the speed. If you double your speed, the force needed to turn goes up by a factor of four. If you triple your speed, the force increases by a factor of nine.

pls re-read post # 691 on page 34 on weight jacking.
When a race car goes into a turn three things can happen and two are bad.
1. Tires don’t have enough down force and will slip.
2. Tires have too much down force and will overheat the right front tire and eventually will slip.
3. The car completes phase one turn entry and enters mid turn phase two.

The key to this event is to keep maximum tire contact during the dive and roll. This is why the right front tire goes negative camber and the left front tire goes positive camber in the turn. We want both front tires to carry the same amount of load when turning. This is why we bias the car with left side weight. We purposely offset the weight up to 60% static when we place the car on the weight scales. We do this knowing that this need to be done to counter weight transfer during cornering.
WRONG! No “weight” is transferred. The tires react like weight was transferred but what we are really dealing with is FORCE as described above.
Back to the race car racing down the back straight at 90 MPH. When we go into turn entry phase one we change both speed and direction via the tires. The car wants to continue going straight. The suspension and tires are the only tools we have to deal with this force. During the turning event the body will roll to the right side in the typical left turn. It rolls through the front and rear Roll Centers (RC). Some of the momentum Force is scrubbed off by the coil springs ,ARB (sway bar) and dampers (shock absorbers) compressing and converting the force to heat. Once the body has taken a set the tires are left to deal with the rest of the force. If we look at the post # 691 on page 34, we see the force vectors of straight sideways lateral force shearing the tires and the right front tire contact patch countering the body roll force. If we have the front roll center located too far to the right side we start to lift the left front tire in a jacking effect. If the front roll center is located too far to the left there is not enough leverage angle to counter body roll and the force shears the tire contact patch. Ifin have the front roll center located properly, we have the maximum down force possible to stick the right front tire and provide maximum tire adhesion to counter the force and we beat he other race cars out there. Savvy?

 

[Warriorracecars]

wow awesome. thank you for spending so much time and thought on these posts. I will disagree on the movement of the lf from its static position though. Our cars begin statically with 4" ride height. Our cars are on bumpstops as well when they enter the first turn of the race (or tire warmup laps even) So we start our static camber somewhere around + 7 degrees (left front). the movement from this static setting is toward negative as the chassis moves into dive. So that's where I lose you as you state that te LF moves to positive. I know I'm missing something in your description just not sure what. Thanks RM

 

[moparx12]

Ranger Mike,I figured i'd join the fray here as i have been following these posts lately and have some questions i would like to pick your brain with. As for WarriorRacecars question, I also find myself confused about what you said regarding camber gain. Maybe it is simply terminology that I'm not used to, but i want to be clear and i have to assume you are saying the LF tire on a left turning racecar will LOSE positive camber (less tilt) - I just can't see the LF ever increasing in camber in a left turn, big springs/bar/otherwise. Again i am probably just misunderstanding you their but it definitely got me thinking about it. Please clarify if you don't mind.

Jacking force. I hear that word thrown around a lot and i am not sure i fully understand what it means. By reading your posts, i gather that your definition (and you are not alone) of jacking force, in summary, would be a force that has a tendency to unload an inside tire as a result of a lateral force imposed on its CG and and rotating about an imaginary (dynamic) neutral axis through a high roll center. Correct me if I'm wrong, but i used to think jacking force is what caused our FWD mini stock back in the day to lift its left rear tire off the ground on turn entry.

I think i understand it from that perspective. Where i get confused, and where i would like to get your opinion on, is the desire for this "jacking force" being built into our cars to give us some magical added force on the LF tire. I think it is a gross misuse of the word, because i just don't see how running the LF LCA inner pivot higher than the ball joint has any correlation at all to the classical "jacking force" explanation. What would you call it then? Why does it work? I have seen cars with these setups and they go into the corner often landing on the LF bump first (contrary to the "jacking" described above)... if setup the right way (or wrong way), we could actually generate a negative roll angle. I would venture to guess that a car with a high inner pivot as i described would also probably have a high RC. Can we look at what happens in this type of setup through the lens of roll center alone? RVD teaches us that all else equal, total lateral load transfer stays the same. In steady state cornering, it is the height of the CG, track width, and lateral acceleration that determine this. Why then, besides the obvious slight drop in CG and added aero downforce, does this type of setup seem to work for some of us?

And one last question for now, what is wrong with a little camber change in the corners? Does this upset the car? wouldn't we want both front tires to increase in "tilt" (if we could) with increasing lateral force?

Thanks!

 

[Ranger Mike]

Welcome
I grew up racing ( on the race team) Mopar starting with 1963 Max 426 cid Wedge. It was an NHRA SS/DA super stock. Was Mopar guy until the government bought them out. Now I am a Mercedes guy. But I digressed.

We want static negative camber on right front so when we go into a left hand turn the tire moves to the positive direction ( away from vehicle center line) in BUMP. On left front we want positive camber of about a degree and the tire will move one to two more positive degrees in droop (rebound) during turn entry.

Jacking Force - pls re-read post # 691 on page 34 on weight jacking.
When a race car goes into a turn three things can happen and two are bad.
1. Tires don’t have enough down force and will slip.
2. Tires have too much down force and will overheat the right front tire and eventually will slip.
3. The car completes phase one turn entry and enters mid turn phase two.

The key to this event is to keep maximum tire contact during the dive and roll. This is why the right front tire goes negative camber and the left front tire goes positive camber in the turn. We want both front tires to carry the same amount of load when turning. This is why we bias the car with left side weight. We purposely offset the weight up to 60% static when we place the car on the weight scales. We do this knowing that this need to be done to counter weight transfer during cornering.
WRONG! No “weight” is transferred. The tires react like weight was transferred but what we are really dealing with is FORCE as described above.
Back to the race car racing down the back straight at 90 MPH. When we go into turn entry phase one we change both speed and direction via the tires. The car wants to continue going straight. The suspension and tires are the only tools we have to deal with this force. During the turning event the body will roll to the right side in the typical left turn. It rolls through the front and rear Roll Centers (RC). Some of the momentum Force is scrubbed off by the coil springs ,ARB (sway bar) and dampers (shock absorbers) compressing and converting the force to heat. Once the body has taken a set the tires are left to deal with the rest of the force. If we look at the post # 691 on page 34, we see the force vectors of straight sideways lateral force shearing the tires and the right front tire contact patch countering the body roll force. If we have the front roll center located too far to the right side we start to lift the left front tire in a jacking effect. If the front roll center is located too far to the left there is not enough leverage angle to counter body roll and the force shears the tire contact patch. Ifin have the front roll center located properly, we have the maximum down force possible to stick the right front tire and provide maximum tire adhesion to counter the force and we beat he other race cars out there.
See page 12 post # 229 for rear weight jacking regarding FWD.

We do NOT want to build in weight jacking. We do want to manage the degree of weight jacking present in any given race car to maximum advantage as it does serve to kill off some lateral force. Hence the relocation of the front Roll Center height and lateral location.
I like to keep the upper and lower control arms “ level” as I hate Anti Dive and see no advantages. I feel the geometry of parallel movements about way any short term advantages of anti dive simply because it is a lot easier to focus and tune in the chassis. I think a tuned in suspension should just kiss the bum stops but not pancake on them as this makes things turn into go kart chassis time. I can not comment of setups you have seen,,,got no clue.

As you have said “ Can we look at what happens in this type of setup through the lens of roll center alone? RVD teaches us that all else equal, total lateral load transfer stays the same. In steady state cornering, it is the height of the CG, track width, and lateral acceleration that determine this. Why then, besides the obvious slight drop in CG and added aero downforce, does this type of setup seem to work for some of us? “
Who is RVD? See my post #811 above..there is no weight or load transfer...we do have to deal with FORCE during cornering..a terminology thing but communication is everything.

I think you really need to read the whole post from the start.

To win you need to get there first. Tires are everything. Maximum contact patch. That’s why camber is critical. The better you control camber the better the car handles. There is a reason Indy cars have about 1/2 degree camber build! To get max tire contact we use low roll centers ; less camber build. But this means a longer lever between the CG and RC so stiffer springs are needed with all other things being equal. Add to this the requirement to properly load the right front tire with down force to make the car stick in the turn. We do this with relocation of the front roll center and / or using aero down force. See my post on BIG BAR SOFT SPRING. The BBSS has the front RC located to the left of center to take away down force on the rt frt tire. So the front roll center has to be correct to take car of turn entry and the rear RC is used to hook things up on turn exit..my opinion.

 

[outlaw36]

Thank you Mike. to be more specific, Left side weight being equal, other than camber gain/loss dependent on control arm length, can you think of any other reason to favor one style over the other? Moving inner pivot mounting points left would require a bigger split in LCA lengths left to right as compared to moving chassis centerline. Or in other words,, moving the tires to the right by mounting points moving right, which would allow for more equal length of the LCA. As far as droop, these cars are Using considerable amounts of rebound in the front shocks. The chassis stays down throughout once reaching bumpstops. On the rear end I have seen two chassis builders , (senneker and port city)utilizing a modified "birdcage" style of trailing arm configuration. still 3 link but the senneker style utilizes trailing arm mounts that "float" with attatchement point at the axle being centerline rather then from a dropped position under axle tube. same with port city, but the mount for them is solid mounted with a heim. they are calling this "active" rear suspension. Curious about your thoughts.

Rich

I am also curious about these "active rear suspension", here is a Port City asphalt car.

 

[Ranger Mike]

Welcome 36, the active suspension deal has been out since late 1980s i think BMW and Mitsubishi had them in production vehicles. Hydraulic actuator raising or lowering trail arms to counter body roll during cornering adds another dimensions to handling. I think they are like traction control. Not permitted. ENGINE AND COMPONENTS MAY NOT BE REMOTELY ADJUSTABLE.

 

[moparx12]

This "active rear suspension" does not have any actuators/hydraulics or anything like that.

Feel free to correct me but this is how i see it:

The way port city has done it will allow the rear end to twist more under accel, which does a couple of things. First, it will shorten the wheelbase of the car under acceleration.

It will do that because the trailing arms are positioned poorly to react to the torque component of the rear end for this style. the trailing arm pivot(s) will move up as the car undergoes accel, until the forces balance out - not sure how long that will take but i would guess that the transient application of force to the rear tire patches would last longer than in a conventional 3-link. Depending on the tire and where you are at in terms of overloading it, this may help bite off the corner.

The fact that the wheelbase shortens also means that more FORCE is being transferred to the rear tire patches. just like a narrower track width causes more loading on the outside tires in a turn, shorter wheelbase will cause higher load on the rear tires under accel. There may be all kinds of other things affected by this "active" suspension but the transient part of the load applied to the rear tires as well as a dynamic change in wheelbase are the two that stand out to me the most.

 

[Ranger Mike]

Sounds like induced rear under steer or rear over steer depending on what side you want shorten. Without seeing it in person, sounds like it is doing the same thing as the spring loaded radius arm without the liability of the spring breaking. This is good. I used a spring loaded radius arm old late model and liked the fact that the spring could be tuned as track required. The 3rd link that is a torque link also can be adjusted. Are these gimmicks or genuine engineered better solutions? What ever floats your boat. I personally like a 3 Link with top torque link and i like adjusting the angles of the trail arms and 3rd Link and damper to tune it in. IF you have enough room to adjust it.
Anyway, thanks for taking the time to point out emerging technology. I am on way to Florida so will be out for a while.

 

[loganc_racer]

If we could go back to the front suspension geometry. In some stock vehicles, the axis of the frame mounting points of the upper and lower A-arms are not parallel to the center line of the car. They are also angled in opposite directions to each other. The upper not so much angle as the lower depending on caster/camber shims. What is the reason for this ? (Not referring to anti-dive)

 

[Ranger Mike]

Welcome Loganc,
i am not a manufacturing or automotive engineer so the design reasons for this are not known since i was not in the design review. I can take a guess. Automotive companies need to compromise ride quality with safety considerations. The anti dive feature is the reason we have differing mount point angles. Rather than run very stiff front springs to resist the forces of inertia and mechanical resistance that the brakes create thru the front suspension,
we mount the upper and lower control ares at different angles from " level" as viewed front the side of the car. The Instant Centers thus created will cause the same effect as higher spring rates thru the braking force on the front suspension and the chassis will resist dive. This will handle the "weight" transfer to the front . and still enable soft springs for a pleasant ride.
On most chassis, the lower control arms are level front to back as viewed from the side. The uppers usually have about 2 degrees of angle front to back, higher in the front. This is called anti-dive. Now look at the vehicle centerline front to rear and the A-arm chassis mount points ( top view). No way are they parallel. The lower A-arm front mount is typically directly inside of the lower ball joint. ifin you drew a line between the lower ball joints (left and right), the front mount of the Lower A-arm fall almost on that line. The rear mount kinematically controls lower joint longitudinal movement during wheel travel. It contributes to caster gain and is also critical for comfort over bumps without using stiffer springs and harming the ride comfort. Compromise.

The same can be applied to the Upper A-arm. Draw your imaginary line between the upper ball joints and look for how the Upper A-arm relates. There are several thoughts on Upper A-arm mounting that ultimately control caster gain. You need find what adapts best for the 2D veiw of the suspension. It could be the front mount position or center of the A-arm.

 

[loganc_racer]

Welcome Loganc,
i am not a manufacturing or automotive engineer so the design reasons for this are not known since i was not in the design review. I can take a guess. Automotive companies need to compromise ride quality with safety considerations. The anti dive feature is the reason we have differing mount point angles. Rather than run very stiff front springs to resist the forces of inertia and mechanical resistance that the brakes create thru the front suspension,
we mount the upper and lower control ares at different angles from " level" as viewed front the side of the car. The Instant Centers thus created will cause the same effect as higher spring rates thru the braking force on the front suspension and the chassis will resist dive. This will handle the "weight" transfer to the front . and still enable soft springs for a pleasant ride.
On most chassis, the lower control arms are level front to back as viewed from the side. The uppers usually have about 2 degrees of angle front to back, higher in the front. This is called anti-dive. Now look at the vehicle centerline front to rear and the A-arm chassis mount points ( top view). No way are they parallel. The lower A-arm front mount is typically directly inside of the lower ball joint. ifin you drew a line between the lower ball joints (left and right), the front mount of the Lower A-arm fall almost on that line. The rear mount kinematically controls lower joint longitudinal movement during wheel travel. It contributes to caster gain and is also critical for comfort over bumps without using stiffer springs and harming the ride comfort. Compromise.

The same can be applied to the Upper A-arm. Draw your imaginary line between the upper ball joints and look for how the Upper A-arm relates. There are several thoughts on Upper A-arm mounting that ultimately control caster gain. You need find what adapts best for the 2D veiw of the suspension. It could be the front mount position or center of the A-arm.

Thanks Ranger Mike. So to make sure I'm on the same page, top view looking down on the car, the angle that is there controls caster gain, positive or negative depending on if the wheel is in bump or droop ? I will review the previous info and research how this can benefit/not benefit my 86 chevy monte carlo street stock. By the way to determine roll center which mounting points do you use when in this situation the pivot points don't line up with each other? do you use a point that intersects the ball joint perpendicular arm pivot axis? Oh and by the way, real race cars have fenders! LOL. Thanks.

 

[Ranger Mike]

longanc,
Correct. if you look at it you have two a-arms swinging thru different arcs and you get slight movement of ball joint that will ad or take away caster. Slight amount but it is still there.

The Bad news is that you can not do anything about this due to the track rules. The good news is everyone else is in the same boat. Because you have differing mount points that will not be in line you need pretty good 3-D software like Suspension Analyzer that figures all this for you.You need to go on line and buy “Street Stock Chassis Technology “ from Steve Smith or telephone 714-639-7681 and buy a copy. He goes into real good detail on starting out with street stock chassis and improves the camber curve and bump steer.

Again due to the rules you will have odd Roll Center Migration because you have to use stock components and this really limits how the RC can be adjusted. You may have the front RC at rest , with offset to the left side. It should migrate to the center under roll and dive. This is where the software becomes such a critical tool. You can play with ball joint height, swap out spindles with differing heights, etc.. to get as good a set up as possible.
I found this good article on street stock Roll Centers a while ago

http://www.onedirt.com/tech-stories/suspension/finding-your-center-finding-your-front-and-rear-roll-center/

 

[loganc_racer]

Thanx Ranger Mike

 

[John LNZ]

Hi Mike
Happy New Year to you from NZ. A couple of questions for you please. I can't seem to find any information on how to find anti-squat in a typical sprint car rear end layout - radius rod on top, torsion arm on bottom. In our situation we use a lift bar instead of torque tube. Can you provide any information on this please? Also, in some of the earlier posts you are talking about the position laterally of the top 3rd link being in the centre of mass. Do you think this would be applicable with a lift bar? Thanks

 

[Ranger Mike]

John, thanks for the post. I have zero knowledge of sprint car set up as described. Do not know about lift bar. Any speculation could make you race to the rear so I can not comment. There are areas that I am not up on and Sprint and Super Modifieds, F1 cars are a couple. Wish you well in 2015 racing. Sorry
rm

 

[loganc_racer]

Ranger Mike and Others,
In reference to posts 819-822.
I have the street stock book by Steve Smith, and went over the link you supplied, every time I read it I get a better understanding. This forum has been even more helpful in understanding what's going on. In past weeks I have been collecting data for my 1986 Monte carlo Street Stock, this data II will share at a later time as I have not got all the data that I think you folks will want to know. While looking through this info either the forum or the book, I have seen talk of rear steer and rear bump steer. I have built a bump steer rig and have data for the front. How should I proceed in checking the rear bump? Should the start point be with all 4 lower control points of equal height (arms level). Should I do each side with the opposite side at one height, or put right in bump and left in droop, or both. When I get done collecting data I will have a boat load of questions. Attached
are photos of a couple tools I made
Thanx Loganc
.

 

[Ranger Mike]

Good job Loganc
I recommend measuring bump steer and rear steer at ride height. With the race car topped off with fuel, sand bags or substitute weight in the drivers seat for the driver, all tires at race setting tire pressure wise, measure the length from the floor to center of wheel hub on all four wheels. This is your ride height wheel center setting. Next you have to remove the springs, set the chassis so that the wheel hubs meet the ride height spec. go thru the bump steer process on the front. You should have pretty good idea of the process on the front by now. You need to correct any bump steer caused by the steering linkage.
On the rear we need to find out what is happening to the wheel base length during cornering. You need to attach the dumb end of a tape measure to the front hub so wheel base may be measured at the rear hub. On the right rear at ride height measure the wheel base. See what happens when the right rear goes in bump one inch ,2 inch , three inches of bump. On left rear go through the same process but measure droop at one inch, 2 , 3 inch etc.. 4 inch if you think you have this.
This will tell you if you have induced rear steer ( either under steer or over steer). I personally do not like to run either and want it as neutral as possible but your being limited to what GM did in 1986 on the Monte Carlo..it is what it is.

While it is winter, pull out the rear end and check to make sure it is not BENT. Another thing is to make heavy duty brackets that will measure shock travel on the left side. The rubber grommets are ok for bump travel but you need to know droop travel as well. Ideally you want the left side drooping 3 inch and right side 3 inch bump..Ifin everything else is working like it should.

 

[Ranger Mike]

Actually, you can add 10 inch to each ride height setting and place the chassis on jack stands and measure bump steer, if you run out of room under the car.

 

[Jared0020]

Ranger Mike need some help. We bought a car last year it's a street stock 75 Chevelle cut down to 108" we had a buddy check front and rear RC. Here is what we came up with front RC went from 1" right static and 7" left dynamic. And rear was 20" and 21 " in travel. The rear package tray has been raised 2" and the car runs at 5" ride height. What can we do to get the rear RC down. Thanks

 

[Ranger Mike]

Welcome Jared, always nice to see new talent arriving in the new year. I recommend you spend the $$ and buy Street Stock Chassis Technology by Steve Smith. It is the one thing you can do to develop a plan to win. The rear roll centers are what they are as rules will not permit any changes in t he mounting of the trailing arms.
Look at the attached photo. You could change the top angle of the trail arms to lower the intersection point at risk of tech inspection catching you...yeah right...You can do major handling improvement by correcting the front roll centers. This book also tells about different spindle heights and ball joints. You need to get the front RC to the right side by 3 inch and stay there. This will load the rt ft tire enough to make the car turn in without pushing.
Buy the software for chassis analysis as mentioned and benchmark the car. Then you can have some advantage with those hard tires you have to run.
Its a lot of work but its what it takes to win...

 

[Jared0020]

We can move the points in the rear. We won many raves with this car but it's always been on the lose side and not much side and forward bite because the RC is so high on the back. I've been trying to find am article where it can tell me what happens when you move your points up or down which way the RC goes.

 

[loganc_racer]

Ranger Mike and Others,
In reference to posts 819-822, 826-828
Ok I have finally got what may be enough data to get some good conversation going here. 1/2 mile paved track, 5-10 degrees banking ( i have not measured but that's the ballpark). It is similar to Martinsville, NASCAR Sprint Cup teams actually practice at this track. My car is a Street Stock 1986 Monte Carlo, Stock chassis suspension mounting points front and rear, stock lowers, UB Machine tube uppers per rules on front, aftermarket longer ball joints lowers + .750" uppers +.900" stock spindles. Stock lower and uppers on rear, Currie stock replacement 9 inch housing with stock lower points, due to physical size of a 9 inch housing the upper points are higher than stock but legal to the rules. Hoosier Commanche tires. Springs1100rf 1000lf ss100 non adjustable shocks both sides front, 1 1/4 SS sway bar. Last ran 250 rr and lr with afco shocks 1035 on lr 1030 rr have been all over with rear spring combinations on both sides from 175 to 275 heavier let and right. Car has never had a problem with corner entry except when i put too much brake in the rear, at apex it pushes and i can't get in the throttle as soon as I think I should be able to. If I loosen it up for apex I get loose coming out, it is manageable but again its a trade off. it is such a sharp edge it teeters on, but if the great cars are off a little I have a feature winner. Seems that I have enough motor, and am assuming I am a decent driver. Front roll center is 3 1/2" above ground and 1 3/4" to right side. Left instant center is 27 5/8" outside of right side ball joint, right side instant center is 23 13/16" outside of let side ball joint. Rear roll center is 17 1/4" above ground. No ackerman in 45 degrees of steering input. Right side front toes out about 5/16" in 3" of bump. Left toes out less than 1/16" in droop. ( bump measurements are at 20 3/4" ) I have since moved things around and now have right side to less than 3/16" out. At the rear I had 5/8" roll understeer with 3" bump at RR and 3" droop at LR, with the right side moving to a shorter wheel base by 3/8" and the left side going longer by 1/4". I have since moved things around (raised rear ride height) and now have improve the situation to roll oversteer of 3/16". Because I am only one man and I don't know how to operate the dumb end and smart end of a tape measure at the same time (LOL) and stuff is in the way, I need to explain how I checked rear steer. I set up a laser beam behind the rear axle and measured from axle hub to laser on each side. So now because I don't have ackerman do I need to induce/simulate ackerman by way of bump steer? or does the change from roll understeer to roll over steer do the trick? I don't think that raising the rear ride height by 3/4" has significantly changed the rear roll center, in my head I figure it is lower by a very small amount and that fixing the rear roll steer trumps the roll center in this case ? The rear axle is straight but I am not sure how to determine If it is square in the car or what to reference it to, and the fact that I have to use stock mounting can I do any thing about it Have I completely failed to understand this class LOL? I also talked to a former track champ in this class who including his sons have about 3 or 4 championships gave me some tips that kinda suprised me because some of the thought follows the theory I have learned here but some not. (no they are not in this class anymore, they moved up LOL) OK we will go with this for now, I got to go to bed and I spent an hour typing/pecking this out.
Thanx for any and all comments,
Logan

 

[Ranger Mike]

I would say we have a track champion in the making. Here is my take.

Things get hairy on a 1/2 mile track. We run Winchester and Salem speedways around here and that’s some High Bank FAST... action. The speeds on the hi bank can kill you. Your low bank track will be a little slower but you can still get into trouble. So my number one concern is lap belts and good roll cage. Make sure the UVs have not taken their toll on the belts and I would change them out after three years on general principles. Roll cage...make sure its safe.

I like the fact that you have mastered the rear end roll steer and can add or take away some induced rear steer. Also the fact that you legally changed out the rear mount location may add to lowering the rear Roll center. See post #830 above. I am not too worried about the rear end as this is the area we address once we take care of phase one and two.

You are running those hard spec tires. I fear the front Roll Center will migrate too far to the left on turn entry. The RC height is ok but offset need work. We need to know what the front RC is doing and when.

There are numerous options to get it to stay offset to the right side to plant the right front tire.

Once this is done we address phase two. You are pushing at apex because the lack of Ackermann. Read page 33 post #646 and #674. We need induced Ackermann to make the front track properly. Here i will get in trouble. I think depending on the track you need 1 to 5 degrees difference. Right now without it it is pushing. This screws up apex turn in. You want the Ackermann to turn the car at apex and not push so you can get on the throttle immediately. Now in your case, 1/2 mile wide sweeper, the wider the turns the less Ackerman required at the left front. You can use bump steer to add toe and you can relocate the drag link mount. You can bend the spindle arms but do not do this to one side only. It will mess up the geometry. The lack of the 1 - 5 degrees is hurting you. I like very little static toe and keeping the bump steer to the minimum and I like ackermann to help turn the car. You can get a combination of all of these.
On 1/2 mile tracks and above less is generally needed. Just like with rear stagger, too much Ackerman will make the car loose on turn exit or will cause premature tire wear. Too much Ackerman can over heat the left front so that it will not perform on the long run. The amount your run depends on your set up and the track. Some tracks like more and others less.

Most short tracks (oval tracks) have a corner radius of 150'+/-. For a race car that has a 108" wheel base and 60" width, the inside tire would need to turn 3.4 degrees and the outside tire would need to turn 3.3 degrees to have Ackerman Steering. In other words, the inside tire needs to turn 1/10th of a degree more than the outside tire in order to fulfill the Ackerman requirement for this car and corner.

However, at racing speeds, tires develop what is known as slip angle. Despite the name "slip angle" it has nothing to do with slipping or sliding. Instead, it describes the flexing or twisting of the tire's contact patch. It's not unusual for racing tires to develop 6 +/- degrees of slip angle before they loose traction. DOT (street) tires can develop as much as 10 degrees of slip angle before they loose traction.

Due to the magnitude of flex (slip angle) in the tires, that tenth of a degree steering difference mentioned earlier is negligible. One could conclude when it comes to tuning your race car, conventional (stock) Ackerman would not be a concern. I personally do not buy this. But you can decide what is best...

Spring are real close. I assume 1/16 toe out and you have camber build handled. You are real close tot the winning combination but front RC and Ackermann should
be optimum.

see pic from Steve Smith Paved Track Stock Car Technology

 

[loganc_racer]

I forgot some data:
Static caster/camber settings:
LF caster 1 1/2 degree positive, camber 1 1/2 degree positive
RF caster 4 degee positive, camber 3 1/2 degree negative.
Toe: I have been any where from 1/4" into 1/4" out. I shoot for 1/16" into 1/8" out, hoping to be zero. I do use toe plates. ( I know ) This by the way is my most dreaded job because even with new tires on new wheels I feel like there is way to much room for error, and then throw in slip angle, I begin to think "does it really matter" ! Give me some ideas please.
During bump steer check:
RF gains negative camber in bump, 1/2 degee per inch of bump total 1 1/2 degree in 3 inches bump.
RF also gains positive caster in bump. I did not spec it out per bump inch, but will if you think I should.
LF gains positive camber, 1/4 degree in 1st inch of droop, then 3/4 degree more between the 1st and 2nd inch,
1 1/4 degree total by 3rd inch of droop. LF gains positive camber.
You are referring to roll center migration as phase one? I will need to research that more to get in my head what I need to do.
Phase 2 ackerman?
I see that you prefer minimum bump, and toe with required amount of ackerman. By my calculations, 1/16" toe out doesn't do much for ackerman, and please correct me if I am wrong, to induce just 3 degees of ackerman, you would need, @ 20 3/4" diameter, 9/16" of bump steer, (or combinatin of bump, droop, and toe) so I get that Is it safe to bend the spindle arms, if so can you give me advice, are they cast iron or cast steel?
Are you not worried about the rear end because I actually got rid of the roll understeer, or because the roll center migration and ackeman are more important even if I had the 5/8" rear roll understeer?
Thanks for the reminder about seat belts, its actually time, and based on rpm, gear, and tire size we are near 90mph on the straights.
Ive got relatives in southern Indiana a niece in Salem and cousins north of Salem, near Brownstown.
Thank you very much, Life got in the way of my racing career, LOL, and now that I am in my 50's, have time, and money, this really speeds up my learning curve.
Loganc

 

[Ranger Mike]

Yes..Life does get in the way...so does ex wife, bill collectors, certain government agencies...
back to reality...
I am not concerned about rear steer simply because you have a good grasp of it and can change it as you require. We can not change the rear RC much if any so the next best is to use ballast or rear steer in phase three. I do not like static toe over 1/8 inch because it cuts down the straightaway speed. Thats why i like ackermann.
I do not have street stock specs on 1/2 mile track so let me ask a few racers.
mean time find out what the front RC is doing in dive and roll please. Its just snowed hugely here and getting into the race car hauler for my notes aint going to happen soon..

 

[Ranger Mike]

good general reading on front end geometry...

http://sports.espn.go.com/rpm/nascar/icons/news/story?id=3377931
bottom line is ..ackermann is dynamic toe out for left front...you can get it many different ways ...

 

[loganc_racer]

Interesting article Mike. The caster part of the article for some reason confuses me the way it is written.
Loganc

 

[Ranger Mike]

im heading t o Germany for a week so don't have my notes. What specifically is not clear on caster?

 

[loganc_racer]

No hurry here, so enjoy your trip, but I quote this from the article
Understanding the Complexities of Front End Geometry, By Bill Borden " If you move the upper pick-up point forward of the vertical centerline that runs through the two points where the tire, wheel and spindle attach to the arms extending from the frame then you are creating positive caster. Move it back and you create negative caster." I just thought it was the other way around, unless his "forward" reference does not mean to the front of the car, or maybe I really don't understand caster at all.
If its a misprint, then cool, I think I got it, if not I need to re-think stuff. Either way, I still need to grasp what is happening with the caster split to help the car turn left, is it strictly a feel thing to the driver, or does it enhance traction causing the car to turn better as well.
no hurry for an answer,
Loganc

 

[Ranger Mike]

Log, i would have to agree..i think it is a typo...the line drawn between the upper and lower ball joints is caster. it it hits the ground in front of the tire contact patch then it is positive caster. Just like a bicycle front fork, caster self centers. The line from the bike frame neck to the fork axle hits the pavement in front of the bike tire. The bicycle that got wrecked and has a bent front fork is very wobbly and will not self center because the line from the frame neck to the axle is almost the same as the tire contact patch.

The more distance between the caster line on the pavement to the tire contact patch the more difficult it is to turn the steering wheel. Thats why you need power steering when you run over 4 degrees caster or you end up with arms like Popeye the Sailor. Now if we have 4 degrees caster on the right front and 2 degrees on the left front we have caster split. As soon as we go into a turn ( left hand turn) the easier the car will steer as the resistance is less on the left side.

One more thing about caster. it build camber linearly. For every degree caster, as you turn the front wheel X degrees you add X amount of camber.
Caster also helps weight jack the car (adds wedge). Which brings us to Steering Inclination Angle (SIA). i am writing this in a hotel room without note but ifin i remember correct...SIA or Spindle Inclination Angle ( same thing) takes away camber and is non linear. You loose 2 degrees camber at 30 degrees wheel turn but 5 degrees at 45 degrees..not sure of the exact number but i know it is not linear. Typically you run 8 degree spindles but you can buy 12 degree spindles now. The idea is the jack some wedge into the set up without running a lot of static wedge.
So you get a lot of things to wrap your brain around like static toe out..sweet...and the static camber has a beautiful camber curve over 3 inch bump..until you crank the wheel 30 degrees and now have ackermann (dynamic toe out) and added in more camber via caster but..the SIA is removing camber...so now what?? read this link..super good stuff on front end setup

http://ismasupers.com/downloads/tech-talk/Tech-02%20Suspension%20Geometry%20relations%204.pdf
Now if you have very restictive track rules...and you knew how to make the car handle better in the turn than the other door slammers..legally...and non detectably ...why would you not become expert on these things that don't cost you a thing except time?I recommend you google SIA and study up on it. My notes are back in the car hauler and the lock is froze but i am in Germany and the beer is Zer Gut!