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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.
Just finished your October column in "Racecar Engineering" and your comment about using inertia damping to damp tires got me to thinking specifically regarding drag racing. Tire "hop" is one of the major contributors to loss of traction in the upper drag racing classes – Pro Stock, Fuel Funny Car and Top Fuel. Watching slow motion video of this phenomenon you can see the tire begin to deform and then become unstable as it starts to go into tire hop. Would it be possible to reduce or possibly even eliminate this by using a properly designed and applied inertia damping device?
They do have minimum weights: I think that Top Fuel is 2350 and Funny Car is around the same and I would estimate that each of them probably has at least 70% of that weight on the rear wheels. Now I can tell you that if an inertia type damper worked, the NHRA may possibly require these cars to have them as it could have a great effect on the "show". They don't like races that involve tire smoke. Also there might be a positive related to the engine life, as once the tires start to spin the engine load goes down, cylinders go out from no load and then major engine damage can and does occur. I also think that most of the cars could easily find a place for some extra weight if the inerter actually worked, as many have ballast.
I do think this might have possibilities.
First, I should point out that I do not consider myself a real expert on oscillatory phenomena. I do know a bit, however. Readers are welcome to comment, and enlighten me further.
I don’t think inertia damping can entirely prevent tire shake. I think it might reduce the severity, but chances are that a car will still need to avoid the occurrence of significant tire shake to get a good run. However, inertia damping could possibly win its user a race, in a case where both cars get tire shake.
I have not had the opportunity to study tire shake videographically, but my understanding is that it is a combination of two kinds of oscillation, provoked by stick/slip effects at the contact patch. The tire takes a bite, wraps up, breaks loose and unwraps, then repeats the process many times in quick
succession. This subjects the car to both vertical and longitudinal oscillatory excitation, and often leads into complete “up in smoke” breakaway.
The cars in question generally have locked axles, so the tires tend to stick and slip in-phase, but I can imagine them in some cases oscillating out-of-phase. That would produce oscillatory roll and yaw excitation in addition to ride and thrust oscillation.
We can’t necessarily prevent the basic stick/slip action at the contact patch with a damper, but we can to some extent damp the resulting oscillation of the vehicle.
Some of the vehicles in question have rear suspension, and some are rigid. They are all subject to tire shake. Where there is rear suspension, we have the possibility of mounting an inertia damper on the axle as well as on the frame.
Digging a bit into the question of how inertia dampers work, and what it takes to optimize them, one quickly discovers that the term “inertia damper” seems to have attained buzzword of the hour status: it is presently being used to describe almost any kind of damping device that incorporates any form of inertia-sensitive element. In mountain biking, dampers and forks with acceleration-sensitive valves are being sold as inertia dampers. We have the Cambridge/McLaren/Penske “inerter” or “J-damper” which spins a little flywheel when the suspension moves (to what purpose I cannot imagine, since we otherwise go to great lengths to minimize unsprung component inertia). Evidently there is even some fictional device in Star Trek called an inertia damper. I suspect this has much to do with the buzzword of the hour effect.
In the current context, what I mean by an inertia damper is a device that uses a mass that moves in opposition, or at least accelerates in opposition, to the mass to be damped. This achieves damping by destructive reinforcement of oscillations, or by friction, or by some combination of these.
Probably the most familiar form of such dampers in cars would be found in crankshaft torsional dampers, often erroneously called harmonic balancers. There are two basic forms of these, and they offer some insight into the possibilities and limitations of such devices.
The most common form of torsional damper, sometimes called an elastomeric damper, has an inertia ring around a hub, with a layer of rubber in between. The rubber serves as a torsional spring. The ring then has a rotational natural frequency. This is chosen to cancel the natural frequency of the crank. The idea is to get the ring trying to accelerate one way while the crank snout is trying to accelerate the other way. The forces then oppose each other and the acceleration of the snout is diminished. Additionally, the rubber has some internal hysteresis, so there is some frictional damping as well.
One drawback to this approach is that for best results the damper has to be tuned to the rest of the combination. In a stock engine, the combination is known, so this can be accomplished, although there may still be modes of vibration that remain undamped. In modified engines, the torsional natural frequencies can be different than stock. When we lighten the rods, pistons, and counterweights, we raise the natural frequency. Sometimes rods used for racing are heavier than stock, for more strength. That, and the corresponding counterweighting, lowers the natural frequency. When we drill the rod journals, we raise the natural frequency. When we increase the stroke or use smaller rod bearings, we lower the natural frequency. If we use undercut counterweights to reduce the overall mass of the crank, we lower the natural frequency. If we use small-radius counterweights with tungsten in them to reduce rotational inertia, we raise the natural frequency. These variabilities make it hard to optimize a general-purpose elastomeric damper, or any kind of damper that uses springing and cancellation by choice of frequency, for aftermarket applications.
The other kind of torsional damper is the frictional type. These can use Coulomb friction (sliding friction) or viscous friction. In either case, there is an inertia ring completely enclosed in the body of the damper, and free to rotate within it, constrained either by spring-loaded friction surfaces or by silicone damping fluid in the clearance between the inertia ring and its housing. Frictional dampers generally will damp any oscillation, regardless of frequency. There is no worry about constructive reinforcement (resonance) at non-optimal excitation frequencies. The damping mass has no natural frequency, as there is no springing. The unit is non-oscillatory.
Applying the same principles to a linear damper on a car frame, we could use a mass working against a spring, with or without a hydraulic or coulomb friction damper. If we use just a sprung mass, the system will be highly frequency-sensitive. If the mass is to move vertically with respect to the frame, to damp z-axis oscillation, it will have to be supported on some form of springing, and will therefore have some natural frequency, unless it’s overdamped (damping ratio >1, or damping coefficient greater than critical, making the system non-oscillatory). However, the springing can be made very soft, and hydraulic damping can be provided that will make the system overdamped. The damper will then suppress all likely excitation frequencies.
One possible physical arrangement might be a mass on an arm, held up by a coilover, with a very soft spring that would require a spring compressor to install. One of these on each side of the car would damp both roll and ride oscillation.
For longitudinal or x-axis damping, we could have a mass on an arm that hangs straight down at static condition, and swings backward and forward in response to longitudinal accelerations. This could be used with just a hydraulic damper, and no springing. As with the z-axis damper, we might use one of these on each side of the car, and get damping of yaw oscillation.
Such dampers would become more effective as we increase the mass of the inertia weights.
Finally, we might take advantage of the fact that tire shake involves a rotational oscillation as well as a vertical and longitudinal one. We might put a torsional damper at the hub of the wheel.
We might also borrow from dirt Late Model technology: react axle torque with a torque arm, and use a coilover for the drop link. There are other ways to damp axle housing rotation as well, such as horizontal shocks.
Again, I would not hope that any of these ideas would eliminate tire shake. However, they might control it to some degree, and might in some cases be enough to win a race if your competitor also gets tire shake.