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: email@example.com Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
COIL-BIND AND BUMP STOP SETUPS IN STOCK CARS
1) When Ron Hornadayís Camping World (Craftsman) Truck broke a swaybar arm at one of the races this season, the left side of the truck was noticeably higher than before. This could indicate a setup that included a highly preloaded, very-stiff front swaybar. Is there an advantage to a highly preloaded swaybar? Is it done to increase LF spring loading for a LF coilbind setup?
2) With today's front soft-spring Late Model setups, some racers use front bump stops for the main purpose of keeping the chassis from bottoming out on the track (using a stiff front bar), while other racers are hard onto their front bump stops as their primary suspension setup (using a smaller front bar, using various methods for swaybar preload). A lot of these setups seem to be trial and error, without knowing whatís really physically happening. Thereís not much information in the literature discussing the vehicle dynamics, load transfer, tuning methodology, etc. for bump stops systems. Seems itís not well understood why it sometimes works as well as does, nor what happens during a race with these setups as the tire-to-track friction coefficient changes, or the racing groove changes forcing the use of a different bank angle. Whatís your explanation of the physics behind the use of bump stop setups: a) for the urethane rubbers; b) for spring coilbind systems.
The primary purpose of these setups in stock cars is to work the ground clearance rules. It has been a longstanding tradition that stock car tech inspection includes a requirement that the car pass over a barrier that serves as a ground clearance gauge. This comes from the days when stock cars were much closer to stock than they are now, and there was a desire to have the racing vehicles bear some resemblance to actual road cars.
Without the ground clearance rules, there would be no reason to use bump stop setups.
One could argue that sanctioning bodies should simply abandon the ground clearance rules, or just set the requirement very low. This would definitely simplify setup, and probably lead to closer competition. However, there is also a case for leaving the rules as they are, to make computer-aided simulation and modeling so difficult that most racers will have to rely on brain-aided engineering instead.
The exact height of the ground clearance gauge varies among classes and sanctioning bodies, but it is generally considerably higher than we want to run the car on the track, particularly at the front. We would like the valance or splitter to barely clear the track, at all times. If we can't get that condition at all times, we would at least like to obtain it in the turns.
The idea, then, is to have a car that is very soft in ride at the front, but hits some form of stop when it gets to the height we would actually like it to run. We then use some combination of aerodynamic downforce, hold-down shock valving, and turn banking to keep the suspension compressed to the stops, at least when it counts most.
We thus have a car whose wheel loads and wheel rates are dramatically different when it's down on the stops than when we set it up in the shop. We can set the car at the on-the-stops ride heights in the shop, and see what cambers and alignment settings we get, but we can't faithfully reproduce the aerodynamic loadings, banking loadings, and damper forces. If we are a well-funded team with very good engineers and adequate wind tunnel time, we can simulate the loadings at a particular speed, turn radius, and banking angle. We can use the calculated or recorded aero forces and x, y, and z accelerations as inputs for kinematics and compliance testing, and get fairly realistic wheel loading measurements, provided we also do a decent job of estimating tire ground plane force distribution. Even this doesn't fully capture the damper forces, but it's useful, especially for comparing effects of changes everywhere but the dampers.
However, if we are a Saturday-night Late Model team, we generally won't have any budget for wind tunnel or K&C testing. We will be relying on qualitative understanding of the car's dynamics, and dialing the car in by degrees using that qualitative understanding. Fortunately, we will also generally be running one track repeatedly, or a small number of tracks, and will also be allowed free test-and-tune days and relatively cheap track rental during the week, and there will be no rule limiting how much we can test. That means we can do lot with qualitative understanding, if it's reasonably good qualitative understanding.
It is useful to think of the car on the bump stops as if it were a static setting, with some added loads involved which we don't precisely know.
Returning to the question posed first, yes I would say the observation described does imply a preloaded front sway bar, assuming nothing else got broken or damaged or moved. And the questioner iscorrect in supposing that this has an effect on the behavior of a coil-bind setup.
For a given set of static ride heights and wheel loads, a setup with a preloaded bar will have the left front spring compressed more at static, and the right front spring less compressed. This means that as the car comes down to coil bind, the left spring will hit coil bind first, or at least earlier than it otherwise would. The car will have a more de-wedged condition at the point of right-side coil bind than it otherwise would. This should translate to a freer car.
If the front bar is quite stiff, it is possible that the car might enter and exit the turns with only the left front actually coil-bound, and the bar holding the right front up. This would allow the car to have more roll compliance in the front end than it would if coil-bound on both sides, making it turn better in the middle. It would also be relatively left-stiff in pitch, which helps it put power down on exit.
We should note that the same will normally not apply to a bump rubber setup, where the springs do not coil-bind. The left spring will be compressed more at static, just as in the coil-bind case, but the shock will not. Therefore, the bump rubber will be no closer to bottoming out than it would be without bar preload. If we want a comparable effect with bump rubbers, we need to lengthen or pack the left rubber.
As to the pros and cons of stiff rubbers versus soft ones, that's a bit like the tradeoff between stiff and soft setups generally. Stiff ones give better control of aerodynamics but poorer absorption of bumps, and very high elastic roll resistance at the front, which often leads to a mid-turn push.
One sometimes hears it said that the front of a bump-stop car has to be stiff in roll to keep the left front corner down and the splitter or valance the proper distance from the track on both sides. Actually, it is only necessary that the car as a whole have adequate overall roll resistance. We can keep the front of the body from rolling using the rear suspension. The car doesn't have a swivel in the middle. The front needs to be soft, and then stiff beyond a certain displacement, in ride, but it doesn't need to have any more stiffness in roll than is required to meet the ride requirement. The front/rear roll stiffness distribution can be whatever it needs to be to make the car turn in the middle and come off strong.
Am I suggesting a rear anti-roll bar? Yes, if the rules allow it. It is allowed in the upper NASCAR divisions, and on Late Models by many sanctioning bodies. However, NASCAR does not allow it on Late Models. When a rear anti-roll bar is not permitted, one has to get roll resistance with the springs and the Panhard bar. Doing it with the springs has the advantage of keeping the tail a bit higher, which adds aerodynamic downforce. It has the disadvantage of making the rear end stiff in ride, hurting ability to ride bumps.
On tracks that are banked steeply enough to make the left rear compress in the turns, it is common to use a right-stiff spring split at the rear. This does have the advantage of de-wedging the car as it compresses on the banking, but it has the disadvantage of making the car right-stiff in pitch. That makes it harder to get the car tight enough on exit, when it's free enough in the middle. In some cases, however, this approach may be indicated, as a way of getting desired wheel loads mid-turn, without a ridiculously stiff wheel rate in ride at the rear.