The Mark Ortiz Automotive
Presented free of charge as a service
to the Motorsports Community
Reproduction for sale subject to restrictions. Please inquire for details.
Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
MORE/LESS ANTISQUAT – OR WHAT? – TO FIX WHEELSPIN/REAR TIRE WEAR
I race asphalt super late models. The car is good on the short run but I'm having a big problem keeping the rear tires under me during long runs, although it seems decent on restarts when the tires cool. When I check the tires after the race, the rear tires end up almost 3/4" smaller in circumference than the fronts when they started the race approximately the same size. I'm not sure if I'm burning the rear tires up because I am:
(A) overloading them (I used to have so much antisquat that the rear suspension lifted on acceleration, keeping the CG higher, and transferring more weight... I've since backed off on this until now where the car squats a little bit - I have a camera in the RR to see all this)
(B) not loading them enough (because suspension squat drops the CG and so less weight is transferred) and this lack of load is causing wheelspin in the long run. Is there any way I can figure out which scenario is going on?
I've thought a lot about it and here's how I break it down:
(To fix A) Benefits of less antisquat (more rear suspension travel) for forward bite:
1. more reverse rake in panhard bar (more load transferred to LR off of the RR)
2. lower rear roll center (decreasing rear roll stiffness)
3. more rear steer to the left (based on where I have my trailing arm angles)
4. and possibly a longer response time in weight transfer (since the weight must go through the springs rather than instantaneously be transferred to the tires through the suspension links, letting the tires transfer from mostly cornering to mostly accelerating force without going outside the traction circle)
(To fix B) Benefits of more antisquat for forward bite:
1. maintaining or increasing rear chassis height to get higher CG and more weight transfer to back tires...usually good for bite.
What do you think? I know trial and error is sometimes best but I'm away at school most of the year so my chances to race and try things are rare. It may also be helpful to know that I run a spring-loaded third link at about 10° angle (1200lb. spring, preloaded ~12 turns) and the car is 2750 lbs. and 51% front weight.
First of all, anti-squat does not have a big effect on rear tire loading. To see just how small the effect is, let’s do some “back of the napkin” numbers.
Suppose we have a 108” wheelbase car where, at 0.8g forward acceleration and with 100% anti-squat (no rear suspension ride deflection), the c.g. height is 15”. The rearward load transfer is then 0.8 (15/108) of the car’s weight, or 11.1%. If the car had 49% rear statically, it will then have 60.1% rear dynamically.
Now suppose we add enough anti-squat so the rear lifts about an inch under the same power application, which will raise the c.g. about half an inch. Rearward load transfer becomes 0.8 (15.5/108) of the car’s weight, or 11.5%. That gives us 60.5% rear dynamically – not much change. Or, if the rear squats an inch, we have 0.8 (14.5/108), or 10.7%, and 59.7% rear dynamically.
So there is an effect, but the difference between hardly any anti-squat and a really large amount (in pavement car terms anyway) is less than a percentage point of dynamic rear percent.
Just for grins, let’s also consider a dirt Late Model that jacks the rear end up 3” under power, has the same 108” wheelbase, 55% rear statically, and a c.g. height under power with 100% anti-squat of 17”. If the dirt is really tacky, maybe this car can also attain 0.8g forward acceleration. With no anti-squat, rearward load transfer is 0.8 (17/108), or 12.6%, for a dynamic rear percentage of 67.6%. With 3” of jacking, the c.g. height is around 18.5”. Rearward load transfer at 0.8g forward is then 0.8 (18.5/108) of the car’s weight, or 13.7%, for a dynamic rear percentage of 68.7%. That’s three inches of rear jacking, delivering just over one more percentage point of dynamic rear, on unusually tacky dirt. Again, enough difference to be worth a little, but not enough to make a difference between burning up the rear tires with wheelspin, and not.
The questioner doesn’t mention what kind of Super Late Model this is. I know it’s a pavement car because there is no class by that name for dirt cars. However, there are a number of sanctioning bodies that have such classes, and their rules vary. At the nearest short track to me, Concord (NC) Motorsports Park, the rules are NASCAR’s. There are three Late Model classes: Limited Late Model, Late Model, and Super Late Model. They all look similar, but the engine rules are different, and maybe the tire rules as well. The Super Late Models are a touring series and don’t run there
every week. The speed difference between them and the rest is dramatic – easily visible from the stands. Their events are also much longer: typically the Limited Late Model feature is 30 or 35 laps, the Late Model feature 50 or 60 laps, and the Super Late Model feature 100 or even 150 laps, on a half-mile tri-oval.
Then there are other sanctioning bodies, mainly in the Midwest, that have Super Late Model classes with much looser rules, which allow, among other things, three-link rear ends (NASCAR requires truck arms) and bigger rear spoilers. The questioner indicates he is running a 3-link, so I know he’s not running NASCAR.
Conversation with fellow competitors should quickly reveal whether the amount of rear tire wear and heating described here is actually abnormal in this class. ¾” less circumference at the end of the race translates to about 1/8” more tread thickness worn off the rears than the fronts. For a powerful car on a short track, over a fairly long event, that may not be anomalous.
From the standpoint of theory, there are a number of things that may make the rear tires wear faster than the fronts, even if the car is not loose and even if the static rear percentage is not greater than the front. Wheelspin is one possibility, and if the driver experiences wheelspin, then we know that’s a factor. However, suppose the car has considerably more aerodynamic downforce at the rear than at the front. That will tighten it (add understeer) beyond what we would expect from the static weight balance. We can tune the setup to compensate for that, mainly by statically and/or dynamically de-wedging the car, or adding rear tire stagger. However, when the rear tires are helped by the aero balance and then loaded more unequally to make up for that, they are working harder than the front tires, and will heat and wear more, even ignoring the work they have to do propelling the car.
Ideally, in such a situation we’d add front downforce, but the bodywork rules may prevent that. Pavement Late Model rules generally allow only a valance at the front, with varying requirements for front overhang and static ground clearance, but a fairly large spoiler at the rear. It may actually be that in such a case it would be possible to create a setup that would run at its own limit for a longer time without taxing the rear tires so heavily, by reducing rear downforce and adjusting the setup to re-balance the car. However, chances are such a setup would have lower limits, i.e. be slower – so it makes more sense to tune for better speed and accept the fact that the rear tires will go away more than the fronts on a long run.
I mentioned stagger. That definitely enters into this, especially when running a spool. (NASCAR Late Models use lockers, but spools are allowed, and common, elsewhere.) Most spool setups use more stagger than theoretically necessary for the turn radius. Even the theoretically correct stagger for the turn radius causes the rear tires to fight each other down the straightaways: the left one drags and the right one has to slip more and provide all the propulsion. That will definitely heat and wear the rear tires. The front tires may also have some added work compensating for the yaw moment the stagger induces, but generally stagger will not add as much workload for the fronts as for the rears.
So one way to help rear tire life, and consistency on long runs, is to minimize rear stagger, especially with a spool. This does have a price, however. A setup with ample stagger is easier to drive because it has more controllable rear breakaway. A setup with minimal rear stagger is twitchier at the point of rear breakaway. So to some extent there is a tradeoff between short-run driveability and consistency over long runs.
Notwithstanding all of these complexities, one thing jumps out at me from the question. I am wondering why the questioner is running 51% front, if there is a wheelspin problem. Late Models typically have quite a bit of ballast, which makes it possible to get more rear percentage. If the track has tight turns and long straights, the car should be faster with more static rear percentage. If the track is really bowl-shaped, then maybe not. But if wheelspin is an issue, that suggests that putting power down out of turns and down straightaways is an issue. More static rear percentage will help that, much more than anti-squat.