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.
STIFF REBOUND DAMPING
I am working for a dirt late model stock car in the midwest. Recently I have been listening to some pretty good racers outside our area and they started "tieing down" their front end and right rear with excessive rebound. They are talking in the range of 200 - 300 pounds at 6 in/sec (depending on track condition and spring rate in that corner). One of the drivers I talked to said it gives the car a very comfortable, predictable feel. I was always taught that excessive rebound can take grip out of the tire. I know many NASCAR teams are now coil binding and tieing down their front end, mainly I think for aero advantage. Is there a better path to follow to get the front end down besides excessive rebound or is the advantage of this worth the loss in grip through excessive damping? I can see there is a trade off dilemma here. I would also like to here your opinion on linear vs. digressive shock valving; I listened to both sides of this and I tend to think digressive seems to make more sense.
We could say that most of the time, any damping at all takes grip out of the tire, and so does any spring rate greater than zero. That is, ideally we'd like the tire to go over bumps with no change in load whatsoever. This is not possible in the real world, for a variety of reasons, but the basic idea is that we'd like the suspension to be as compliant as possible, from the standpoint of keeping the tires in best contact with the road.
On the other hand, from the standpoint of aerodynamic control and camber control, we want a go-kart: no compliance at all.
All suspension settings are compromises between these considerations.
Additionally, in some racing classes, people do funny things to work the rules. In stock car racing, including all NASCAR classes, there is a ground clearance rule. We want the car to have enough static ride height to pass tech, and still go through the turns with the front valance barely clear of the ground. Once the valance is down there, we'd like it to go up and down as little as possible, so it can stay as close to the road as possible without scraping. That means we want the front suspension to
compress easily, especially when cornering, and then go solid when it reaches the desired ride height. This is terrible for riding bumps, but if the track is smooth and air speed is high, it works.
Now, does this work when air speed is lower, and the track is rough? Probably not. Of course, this depends on just how much slower and rougher the conditions are. Dirt Late Models do go fast enough to generate serious aero forces. However, even with the setups we have come to think of as conventional for these cars, the lower edges of the body, along the right side and the right front, commonly dig into the track on bumps and are designed as sacrificial parts: we expect them to get torn up. We make them out of tough plastic, and make them easy to replace.
I question whether it makes any sense to try to get the car to ride lower when it's already tearing up bodywork against the track.
For stock car shocks, 200 pounds at 6 in/sec in extension is not really extreme. 300 is definitely stiff. Whether either of these implies a hold-down valving depends on the corresponding value in compression. The relationship between anti-extension (extension or rebound damping) force and anti-compression (compression or bump damping) force, at a given absolute velocity (e.g. 6 in/sec) is often described as the control ratio at that velocity. Conventionally, the anti-extension or rebound damping force is taken as the numerator. A control ratio of 1 is a true 50/50 shock. 1.5 is a popular control ratio for most purposes. Anything over 3.0 is considered a definite hold-down valving. Anything less than 1.0 is an easy-up or hold-up valving.
Extension damping unloads the tire whenever the suspension has an extension velocity. It is widely recognized that this reduces grip on the downhill side of a convex bump. It is less widely recognized that compression damping also does this. This is so because when the suspension resists compression more forcibly, the sprung mass acquires a greater upward velocity on the uphill side of the bump. When the wheel passes over the crest of the bump, the sprung mass is still moving upward, and accelerating downward. The downward acceleration effectively reduces the weight of the sprung mass, giving the spring less to react against as it tries to push the wheel down. The upward displacement of the sprung mass translates to less force at the spring, also reducing the force pushing the wheel down.
A high control ratio does tend to improve ride quality. Or, more properly, soft compression damping improves ride quality more than soft extension damping does. Soft compression damping also tends to extend the life of the suspension components, and indeed many other components. Theoretically, this might let us build things lighter.
For readers less familiar with shock terminology, "digressive" valving properly means that the force increases with absolute velocity at a decreasing rate. Stated another way, the force vs. velocity curve is concave toward the velocity axis of the graph (conventionally the horizontal or x axis): in a conventional plot, the compression traces are concave down and the extension traces are concave up.
Some shocks sold as "digressive" actually are only digressive in the upper velocity ranges. In Bilsteins, "digressive" valvings are ones using the "digressive" piston design. The main feature of this design is that it accommodates discs with notches at their periphery, which create bleeds. The bleeds affect the entire curve, but most of all they affect the very lowest velocities. At these low velocities, they actually give the curve a progressive characteristic: concave away from the velocity axis. The force vs. absolute velocity curve has a pointy "nose" on it. The damping is soft at very low velocities. The greater the bleed area, and the greater the preload on the stack, the further up the velocity range this region of progressive damping extends.
Conversely, in Bilsteins at least, the valvings with "linear" pistons are either near-linear or digressive throughout the velocity range. So the terminology can be confusing.
Anyway, is soft damping at very low velocities desirable? It depends to some extent on the driver, and to some extent on how much sliding or mechanical friction there is in the suspension. Mechanical friction, also called Coulomb friction, adds Coulomb damping to the system: damping that is largely independent of velocity, and therefore highly digressive at low velocities. Having the hydraulic damping progressive at low velocities tends to compensate for this. Racing suspensions, with lots of fresh, tight rod ends and spherical bearings, have considerable Coulomb damping. Struts have considerable friction as well. Rubber bushings have little or no Coulomb damping.
And is true digressiveness at higher velocities desirable? Most experience suggests so. We can say for sure that mid-to-high-velocity digressiveness allows a more controlled feel with less harshness when hitting curbs or big bumps.