Standing starts are hard on a car, and there are a few NASA classes that begin races that way. A standing start is even harder on a car if it suffers from wheel hop, which occurs when a tire loses and gains traction in rapid succession.
Wheel hop also can occur on corner-exit in slower-speed turns where a car is in a low gear and torque multiplication is high. To find out more about what wheel hop is, why it occurs and how to cure the problem, we caught up with Pete Epple, a marketing technician with BMR Suspension, a company that specializes in suspension upgrades for early and late model domestic cars.
“Wheel hop can be caused by a variety of things, from the wrong shocks to a better surface than the tire can handle or more power than the tire can handle, but the biggest thing that we really see is bushing deflection within the suspension system,” Epple said. “In an IRS car, you would see cradle bushing deflection, differential bushing deflection, and it creates large amounts of movement within the system that causes the tire to lose traction and then gain traction and lose traction and gain traction.”
Bushing deflection sets in motion an oscillation that occurs throughout the entire rear suspension. The condition is somewhat common on cars with independent rear suspensions, but it can occur on cars with live axles, too, particularly those that use coil springs and control arms.
From the factory, bushings are made of rubber, often with voids within the material, which allow for significant deflection. Rubber is great for alleviating noise, vibration and harshness, but falls short of the requirements for track use and racing. The rapid and severe torsional loading and unloading during wheel hop can wreak havoc on a drivetrain.
“Through all of our testing and all my previous experience, I’ve seen wheel hop break everything from the actual differential housing to axles to ring and pinions to even breaking the welds on control arms,” Epple said. “It can be extremely violent.”
Regardless of whether your car has a live axle or independent rear suspension, the fixes are largely the same: replacing the soft rubber bushings with either polyurethane bushings or spherical bearings. The rub, of course, is a less-refined ride and more road noise transmitted to the interior.
For street applications, polyurethane bushings are probably enough to get the job done. For track cars and racecars, Epple recommends spherical bearings because they provide superior articulation and zero deflection. For HPDE cars driven to and from the track, BMR also has “in-between” models of control arms that have a spherical bearing on end that attaches to the axle and a polyurethane bushing for the end that fastens to the chassis.
“The benefit there is you get some of the cushion of a polyurethane and then more articulation that you would want from a bearing or a rod end that you don’t get with polyurethane. It gives you a good combination of both,” Epple said. “It’s definitely going to be noisier than a control arm that has polyurethane bushings on both ends, but it won’t be anywhere as noisy as one with spherical bearings on both ends. It’s a really good middle road for that.”
These principles apply to more than just the applications for which BMR Suspension makes parts. If your Spec E30 or your Honda Challenge car exhibits wheel hop during standing starts, you’ve got a problem with bushing deflection. Of course the rule book comes into play in terms of whether you can use polyurethane or spherical bearings, but a wheel-hop condition should tell you where to look on your suspension system.
“The more of that soft rubber you can get rid of and minimize that deflection,” Epple said, “be it with a bearing for a hardcore handling application or a polyurethane or like an elastomer for more of a street application, anytime you get rid of that deflection, or at least minimize it, you’re going to minimize the potential for wheel hop.”
BMR Suspension mounted cameras under a 2015 Mustang to see what happens during hard launches. It gives you a good look at suspension bushing deflection that results in wheel hop.