Race car suspension Class

[Ranger Mike]

All race cars have spring compression, even Indy cars. If they did not they would be big go karts. You need at least an Inch even on the hairiest BBSS set ups. Two to three inch is ideal, over 3 inch is for dirt set ups. If the roll center stays close to the origin by 2 to 3 inch over first 2 inch of compression, you are taking advantage of 90% of the tire down loading.

When the frame starts hitting the track, increase ride height 1/4" until it stops.

 

[Ranger Mike]

C10 ,where is the 3rd link or top link located (attachment point to the chassis, not the rear end.) on the rear end? Mark the center lines of both rear tires. Now tell me distance the 3rd link is from each center line.

 

[Savem c10]

C10 ,where is the 3rd link or top link located (attachment point to the chassis, not the rear end.) on the rear end? Mark the center lines of both rear tires. Now tell me distance the 3rd link is from each center line.

Morning RM yes I’ve read your literature on this and our track width is 64 inches and our left side weight is 57% so we do 64×.57 which equals 35.91. I had to do some specific modifications to the third link area on the CHASSIS which I’ve done and it’s exactly right where the third link needs to be and we’ve actually run the car one time with this way and I will tell you, it makes a significant difference on handling. I’ve got it I’ve gotta get back to work. We’ll chat later. Thank you RM.

 

[Ranger Mike]

Morning RM yes I’ve read your literature on this and our track width is 64 inches and our left side weight is 57% so we do 64×.57 which equals 35.91. I had to do some specific modifications to the third link area on the CHASSIS which I’ve done and it’s exactly right where the third link needs to be and we’ve actually run the car one time with this way and I will tell you, it makes a significant difference on handling. I’ve got it I’ve gotta get back to work. We’ll chat later. Thank you RM.

Excellent work! good job!!

 

[Cape15]

Hello Everyone, I am so glad to have found this forum in my search to making my asphalt super late model faster! I have recently come across information that some of my competitors are putting lead on their lower LF control arm and wrapping lead around the rear end housing to help with weight staying on those tires throughout the corner. Has anyone tried this? and is this a viable theory? More curious than anything. We have a no bump stop rule and no ride height rule 2700 lbs. tight 1/4 mile flat track with 10" American Racers and 66" track width. Haven't played with roll centers in the past but thanks to this forum will definitely fool around with it this year.

 

[Ranger Mike]

Welcome Cape
read post 562 on page 17 about Polar Moment. In my opinion, making a race car back into a go kart is stupid.
This is what not properly using the Suspension makes the car as well as Bump Stops...why have springs??
Many post here on this as well. That has to be one tight paperclip track you race on or else it is the Mount Lawn baseball Diamond! Anyway, figure out your roll center and set up car from there. let the monkey see monkey do crowd waste their time on trends and gimmicks, you study the basics and then get the checker.
Look at the current trend of having the front roll center to the left of center line.
It is acting thru a lever that is 11 degrees to the right front tire patch. Think it won't jack up the car??
darn sure will. The proper location is on the right of center line acting at 5 degree angle. Will still scrub off some cornering momentum but not lift the left side of the car. Adding lead to the left side a-arm is a bandaid not a cure.

 

[Cape15]

Thank you for the timely response. I've never heard of Mt Lawn until today but it does seem eerily similar to my track of Seekonk! I will most certainly will be implementing this new understanding to my car as well as other knowledge I have gotten from reading through prior posts. Thank you.

 

[Cape15]

I have been reading a lot from Bob Bolles publications and he mentions a lot about matching front and rear roll angles, not the roll centers themselves front and rear but the actual chassis roll together so the chassis wants to roll the same going through the corners. I may have missed this, but I have looked some thoroughly through this thread and haven't found this concept discussed much on here and was curious if that theory holds true. And if so, how can I find out how much the front and rear is rolling on my chassis as I know shock travel indicators aren't the most accurate and I def. don't have the money for a data acquisition system, if I have missed it, my mistake I honestly tried looking!

Also, in a prior post it was mentioned that for an asphalt car it is best to start at 52% Rear percentage. The car I have now came with a 17 gal. fuel tank. We run 40 lap - 50 lap races, so I was thinking if it was better to maybe put an 8 gal. fuel cell in that I have already have and take that 55lbs difference and put it lower onto the frame rail just behind the rear axle so I can maintain the higher rear percentage but have it lower in the car. Would this sound like a good idea if I can keep 52% rear? Or should I measure out the polar moment that RM mentioned and place the extra weight there, but lose the 52% Rear? Any suggestions would be appreciated. Thank you.

 

[Ranger Mike]

good suspension software will show roll. Shock indicators are good devices to feed back at the track.
do not change the weight % until you know what it is now. Do you have scales?

 

[Cape15]

Yes I do. We ran last year around 50.5% Rear with a no ride height jacking force based setup that we were told most everyone else ran . 600 LF 250 RF 600 RR and 150 LR with a 1 5/8in. Diameter sway bar. Around 56% cross. and 56.8% left. Never paid attention to roll centers. Was competitive one night with good pace but turns out we were way light when we were on track scales and once I added the extra weight we slowed right down the last races of the year. So I am hoping to change everything this year and start fresh

 

[Ranger Mike]

Go to the smaller fuel cell. You are very close on left side 58%. I would try to mount weight close to polar moment but shift back to try to get 51.5 rear weight. Perfect for 1/4 mile track. Cross wt between 54-57.5%
do not get hung up on target numbers. everything in racing is a compromise. You get it to within 0.5% of target, close enuff. Example - 58% left side weight is 1566 on 2700 lbs. car, 57.5% is 1552 or 14 lbs.


read post 1660 on page 48 on 3rd link location. Let me know your 3rd link location.
You do not need a data acquisition system. fabricate shock travel indicators for each shock that measure bump and droop.
if you want to win, buy the software and find the roll center location.

 

[Cape15]

So my rear tread width is 65 1/2 (max allowed is 66") and my left weight percentage is 56.8% as mentioned above (max allowed under rules is 57%). Multiplying them comes out to 37 1/4" I measured that over from RR tire centerline and my current third link (solid bar) is an extra 3/4" farther to the LR tire, so I can remount it closer to the 37 1/4" mark. As described in your previous post. I will change to smaller tank also and move around the lead. Also looking into changing the lower links to level, parallel to centerline and making them longer.

 

[Ranger Mike]

65.5" times 58% = 38"
i would wait until you can actually get the 58% left side before final 3rd link change. Yes even 3/4 inch on those hard spec tires will make a big difference when you step on the gas.

goood job!

 

[Cape15]

On a prior post it was briefly discussed but what would be the ideal engine mounting location on an unequal lowers chassis in conjunction with Roll Center location if I have longer RF lower control arm than LF, should I place the engine in the center of chassis and then have the 3" RC to the right or should i move the engine as far over to the Left as rules allow and keep the RC location at 3" right or change that so the engine's weight doesn't have as big effect as the chassis goes through the corners?

 

[Ranger Mike]

Ideally we want the maximum left side weight we can get. Do this first then worry about Rc location. We need accurate left side weight % , this is critical. We can check the rear 3rd link location and correct.

Next, we want to use an offset roll center to put more downforce on the right front tire to add traction. On asphalt we want 3" offset to right , on dirt 4" offset.

 

[Cape15]

RM-
I am trying to select a good starting spring package for my asphalt late model and have read your post about selecting starting spring rates post #19. I'm surely not the smartest person at figuring things out so I am a bit lost on calculating the wheel rates and converting it to spring rates. How does one figure out the G-force going through the corners at their home track? And wouldn't it change as we get the car faster through the corners thus changing the G-force and needing a different set of springs? When I first got this car it had 275s arcoss the front and 150 LR and 225RR with a 1 1/8th front sway bar, do you think this would be a good starting point for a tight 1/4 mile, 7deg. banking track and my car weighs 2750 w/driver. Any info. would be great!

 

[Cape15]

And we aren't allowed to run bump stops so I'd assume we would have to run a more conventional spring setup!

 

[Ranger Mike]

welcome cape
first look at the index on page 1. note under spring calculation i have post 19 page 1 and post 589 page 17 going in detail how you can measure Gs in your turn.
assuming the 1 1/8 inch anti roll bar is solid and 36" long with 12" arms you have 220# spring rate
typical spring package for your weight car is 330 LF, 325 RT FT, 175 lft rear, 150 rt rear, if you run a frt roll center 2 1/2 to 3 inch high with 3 " offset to the right

please read post 1660 on page 48 and tell me the location of your 3rd link. i suspect the 225 rt rear spring is on the car because the 3rd link is not properly located.

 

[Cape15]

This is the same car as before that I posted about in post #1692 and did the math for the 3rd link location and was off 3/4" and I've now changed it's location to correct it. My 1 1/8th sway bar is hollow however, would that change spring rates that I need?

 

[Ranger Mike]

this confirms the heavier rt rear spring on the old setup. Was to compensate for the lft rear getting more load due to 3rd link offset.
the hollow sway bars are very sensitive to wall thickness. what is the wall thickness of the arb and what is the arm length and overall bar length?

 

[Cape15]

this confirms the heavier rt rear spring on the old setup. Was to compensate for the lft rear getting more load due to 3rd link offset.
the hollow sway bars are very sensitive to wall thickness. what is the wall thickness of the arb and what is the arm length and overall bar length?

The bar is a one piece 45 1/2 inches wide center to center 14" arms and 3/4in inside diameter 1 1/8 in outside diameter so the thickness would be 3/8in.

 

[Ranger Mike]

bar is 145# at maximum 14" arm length
170# at 13" arm length
202# at 12" arm length
222# at ideal length of 11 1/2"

at 11 Inch arm length bar has spring rate of 243# , write all these setting down so you can tune at the track.

https://www.gtsparkplugs.com/Sway-Bar-Calculator.html

once the Roll center located and ARB and spring package it close to ideal we tune with the front ARB and rear panhard bar to fine tune. The car (on paper) is set up optimally. Now the real world influences like track banking, track conditions can be adjusted for. The other racers will have to fight flakey 3rd link problems ( too much left rear load has to be compensated with stiffer right rear springs and does not really fix the problem), lack of right front down force because they got no ideal body roll to add downforce. You have taken the time to fix camber build, bump steer, rear roll steer, maximum left side weight. car is as good as anyone can make it. Now you are fighting the weather and the track.

 

[Ranger Mike]

Rear axle camber bending on stock rear ends.
You can bolt on camber snouts to the quick change rear ends and get up to 2 1/2 ° camber. Now here is a trick to bend the stock Ford rear end and gain 1/2 ° camber. This is about all the stock splines and gears will take but adds significant performance when running hard spec tires. Note the tire contact patch on the diagram.


'Chassis Engineering' by Herb Adams came to mind where he describes camber and toe adjustments on a Ford 9 inch by means of heat shrinking. So to gain negative camber one would heat the top of the tube which will initially expand and then contract on cooling more than it expanded on heating causing permanent deformation. There is even a number (just found the book). Mr. Adams says that if you heat a 6 inch section of the tube until it is red hot, it will shrink for about a 1/2 degree bend.
On Ford 9 inch rear ends consider the adding of supports to keep the tubes from flexing.

You can run up to about 1/2 degree of negative camber in a Ford 8.8, Ford 9 inch non floater, and a Chevy 12 bolt by bending up the rear axle tubes with a little heat from a torch. A sharp axle guy can take the end off the tube and reweld it at a very slight different place higher on the tube than before and with a little angle to it. I have seen a Ford 9 in a Camaro Stock class that ran for 8 years with a little more than 1/2 degree of negative camber and the axles looked like new. The inner splines can take that much mis-alignment and there can be that much just in production tolerances. SCCA allow that much negative in American Sedan and American Iron and most guys put that much into the rear ends without wearing them out at all. To get more than that in a floater or non-floater you need to have the axle splines machined in a slight barrel shape which allows them to flex inside the splines at a larger angle. It doesn't take much.

 

[Savem c10]

RM!! I gotta tell ya we went to practice this afternoon and made few small adjustments to the roll center moving it more to right and wow what a difference 5 to 6 degrees can make on handling. I have studied a lot and want to thank you for teaching me and others on this forum . Wish you could have seen where we started with this car and where we were today . I will post Rms recommend set up if anyone is interested this would be for a 104.5 inch wheel base perimeter chassis at 57 LS and 2800 total weight. Rm thanks again hopefully tomorrow we carry the checkers .

 

[Ranger Mike]

Thank you Savem,

Glad you changed the front roll center from the left side to the right side offset and right height!

Kind words always appreciated. All I do is post what is in the books I reference and add a little clarity. You racers do all the work. the garage set up is half the battle. track tuning and driving is the other. Thanks again.
rm

ps don't forget to bring a cardboard ( for the trophy)

 

[Savem c10]

Hahaha you got it RM . We’re heading to the track early. I enjoy the education and have a never give up attitude. We will be successful …. I’ll report back with cardboard or without LOL. Have good day.

 

[hallpa00]

Great forum - thanks to very one who has shared…
Here’s my situation
Running in a pro truck series on 1/2 mile high bank asphalt.
2850 lb min weight on metric base chassis
57 % left
What is unusual is the chassis and body lead to very low rear weight - 44.3% in my case
We run around 57% cross. I have modified the front roll center and I think it is ok.
Springs are LF 550, RF 650, LR 200, RR 180, front antiroll bar is estimated at 632.
Our track experience is tight in, loose off. My sense is this:
Rear metric roll center is high and we can’t do much to change it. Also - rear weight is low so effective overturning moment is low. Shocks show little travel at rear (1”) compared to front (2-3”). My thought is to stiffen the rear to take more load and offset what the RF is seeing.
My question is this: do I only need to increase the RR and how far should I expect to take it? Is there a rule of thumb for rear roll stiffness versus front roll stiffness and how would that vary with a low rear weight car?
Many thanks for any insights.

 

[Ranger Mike]

Hall
welcome. my first question is what is the rear suspension? 3 Link? what is panhard bar setup? What rear roll steer do you have? what is rear track width? what is 3rd link location relative to the right rear tire centerline?
Strictly by the number you need to get at least 48" rear weight. 2850 lbs. total weight time 48% = 1368 #


you now have 44.3% rear weight or 1262 # so 106 # difference.


You could take 106# off the front and put it in the rear. But, Duke Southard wrote his book, Short Track Chassis set-up and had this nugget of information.


You do not need to go crazy on frame modification to get 48% rear weight. Note where the battery, fuel cell and ballast weight is now and do some calculations.

 

[hallpa00]

RM
Thanks for the info.
Class rules dictate the use of the stock metric 4 link rear suspension, although we do use a Ford 9 inch differential. The battery is located between the driver and the differential and we have a large fuel cell behind the differential as well as 60 lb of lead ballast. I’m slowly replacing heavier body panels with lighter plastic on the front to allow more rear weight percentage. I’ll do more calculations to consider other moves (alternator, battery further aft, etc.)
Rear springs are located on top of the axle tubes about 20” outboard of the center section. Rear track width is 63”. We target the lower rear links to be flat on the right and slightly angled down to the front on the left (about 1/2” lower front to back) on the left to give us a slight roll steer on the rear (all stock links on the rear). There is not much I an do to affect rear roll center except to change ride height which also affects the roll steer of course. In general, does it make sense to stiffen the rear? I’ll get another shot at the track in 2 weeks so trying to plan our changes.

 

[Ranger Mike]

i need tire temps on all 4 tires on a good run. outside middle and inside on each tire. do you know the location of the front roll center?

 

[drobbie]

Reading through the last series of exchanges, you have given a very simplistic look at the car setup (not that the remaining details don't also matter). But I have a quick question; it appears many of the recommendations you give are for roll centers, and spring rates are for cars that have travel 3-4 inches of suspension travel. My question is, what compensations or changes when you have a car that only travels 1-1/2 inches because of a 2-1/2-inch ride height and uses wider tires? Weight is roughly the same.

 

[Ranger Mike]

I owned a Formula Continental car raced in SCCA. Was like an Indy car. The car had rocker arm suspension so we had wheel travel of 2 inches and shock travel of 1 inch. Ride height was 1 3/4 inch to 2 inches, road course dependent. The trend back then was to use Bump Stops and even droop limiters. The idea being to have minimal camber build. Nonsense. All they did was turn a very expensive race car into a giant GO Kart. We took these bump stops off and threw away the droop limiter. We were Division Champions for 3 or four years because we let the SUSEND-SION do its thing. Suspends the Mass centroid via 4 springs that counter momentum of this mass trying to Roll over when cornering.
These cars were built in England and got shipped to the USA with tall hard skinny tires. We Yanks immediately bolted on spongy fat smaller diameter tires and went fast...to a point. The Roll centers were still for the taller tires. Only a few racers figured this out and milled down the uprights (spindles) to have correct geometry that would make the cars take off! And they did.
Drobbie, even the BBSS cars have some degree of body roll , slight as it may be but Roll Center placement is critical to have. Do not confuse shock travel with wheel travel as they are not the same. On paved track cars ideal shock travel is 1 1/2 to 2 inch right front and 2 to 2 1/4" on right rear. The ideal Wheel travel is 3 inch right front and 2 inch right rear.

 

[drobbie]

Thanks, Ranger Mike. You have confirmed a theory I have based on the studying I have done over the years. This gives me the direction I needed to make the car handle correctly. BTW, I have been working on Roll Center all winter and made the necessary changes to get it as optimum as possible. Thanks for all your great guidance.

 

[hallpa00]

Regarding the Pro Truck discussion...
My front roll center is 2.0 inches above ground and about 1 inch to the right of center. I use the Performance Trends software to tune. Most of the movement of the roll center is down as the front end droops in entry to the corner. My estimate of the CG height is 18 inches.
Here are some recent tire temperatures:

LF RF
Out Mid In In Mid Out
162 155 152 177 171 170
Average = 156.3 Average = 172.7

LR RR
Out Mid In In Mid Out
160 156 152 181 175 162
Average = 156 Average = 172.7

What I deduced from the temperatures is that I need some small camber adjustments but the average temperatures at each wheel suggest that the balance is close.

The track geometry indicates we need 2 inches of stagger and I run pretty close to that as much as possible.

One other area of concern I have is that the RF tire compresses about 3 inches whereas the LF only travels about 2 inches (based on math using the shock travel indicators). The rear only shows about 1.5" of travel (of course this is only measuring compression so I don't know about extension). I was considering crafting some shock travel indicators to measure both directions.

I mentioned the tight in/loose off behavior based on driver feedback (and from helmet video it seems to be accurate). We run nitrogen in the tires and as the race goes on the car gets looser and looser probably due to heat in the RR tire.

My last thought is that I can't be sure that one of the rear shocks is hitting an extension limit during cornering - I should be able to check this with the indicators mentioned above. Could that cause our behavior?

We run middle of the pack now - if I can find .1-.2 seconds per lap we would be top 5 in a field of 20+.

 

[Ranger Mike]

Hallpro welcome
absolutely cobble up trave indicators on all four corners. These are a must. Yes do check for chassis bind and make sure you do not have any.
Initial look at tire temps say the ft and rear left side need more positive camber. add 1/2 degree +
Rt ft to rt rear temp Ave is too close, car on verge of oversteer
rt rear should be 10° cooler than rt ft. so you need more cross weight as the lft rear to rt ft difference too big at 16°
try to get to 6° to 10° spread
do not change more than one thing at a time or things get real muddy. check chassis, put travel indicators on and change the camber on left side as you really need to put these tire to work.

Then run it , take tire temps, add cross weight a little at a time. make small change one at a time and you will gain every time. may be real small but 0.1 sec adds up!!