For solid axle race cars, brake induced wheel hop has been an issue since racing began. For today's production based and GT purpose built race cars, one wheel hop event not only insures an off track event but can also cause transmission and rear end failures. Prolonged wheel hop has even been shown to cause crankshaft failures and engine block cracks. The dilemma is always the same. How do I get as much braking contribution from the rear of the car without accidentally getting the destructive wheel hop?

Many racers accidentally develop themselves into wheel hop. Adjustments aimed at getting more traction off the corner and aggressive driving can often exaggerate the issue. Tuning changes that include softer springs, compression and rebound damping changes, higher levels of anti-geometry, stiffer bushings and tire pressure changes can all reveal wheel hop. Once wheel hop is discovered the only solution often seems to be reducing the amount of rear brake bias until it goes away. Even then a rapid brake application or a wet race track often sends the driver a reminder that the problem is never far away.

In engineering terms wheel hop is the excitation of the unsprung suspension system at or near its natural frequency, typically in the 9 – 11 Hz range. Translated this means that the rear axle and suspension bounce between the tires and the chassis 9 to 11 times per second. Think of the tire like a basketball that will bounce with any abrupt impact. Sitting on top of that basketball is a 300 lb weight and that is connected to the chassis by a spring. This is the system we are trying to keep stable. The physics of the problem are completely dependent on the weight of the suspension, the spring rates, the tire stiffnesses and the amount of shock damping present. Making the axle and suspension very light to reduce the impact on the basketball like tire or stiffening the suspension springs a great deal so nothing can move all begin to address the problem. A great deal of shock damping in the lower speed range can help if soft suspension springs are present.

While the physics of the suspension are constant, the suspension still needs some type of force to start the vibration. Any system of springs and masses will just sit there until excited by some input. The key is understanding what excites wheel hop
Consider a typical braking event. The driver applies the brakes as late and as hard as he or she can entering the corner. As the brakes are applied the rear of the car rises; a result of the weight transfer to the front of the vehicle. Not only do the rear tires unload, the suspension also rotates into rebound about some point defined by the suspension links. Rotation about this point causes the tire to move forward as the wheel drops. But since the driver is already braking at the limit of the tire's ability, that additional forward movement of the tire causes it to slide. The sudden rear tire slide causes the axle to unwind and the rear of the car to settle a bit. The tire moves rearward as the suspension settles and regains traction. The cycle starts all over. This happens quickly. The changing force is the anvil that starts the system vibrating. The driver feels the resultant suspension hop and the axle wind up changes due to brake torque variation.

It would be hard to give a credible explanation of a suspension instant center in only a few paragraphs. Instead I suggest you just realize that all those rear suspension links act to force the tire to rotate about a point in space somewhere forward of the rear axle.
The suspension can be simplified to look like a single long link, attached to the axle and defining an arc that the suspension follows as it moves in jounce and rebound motion. If the single effective link is short, the arc is tighter with more movement in the forward direction. If the attachment to the chassis side is higher, the arc changes to again allow more movement in the forward and reward direction with suspension movement.

 Most racers have learned that increasing the angle of the upper links of their rear suspensions changes the tire loading under acceleration and can be used to promote more traction. Increasing the upward angle of the lower control arms can do the same thing. In both cases the results are effective suspension link lengths that are shorter or attached higher on the chassis. While they can help acceleration, the changes also promote more brake induced hop.

Suspension Types
Torque Arm suspensions, both the production Camaro type and aftermarket designs, have short effective suspension link lengths. The suspension rotates about the torque arm attachment to the chassis near the rear of the transmission. It is very difficult to keep these suspensions from brake hopping. Keeping the attachment point low and mounting it in rubber can help but still represents a compromise in performance.

3-link and 4-link designs offer the most adjustability in terms of the effective suspension link length. The angle of the upper control arms can be changed in most designs allowing the effective link attachment point to be nearly at the front bumper – nearly twice as long as a torque arm. These designs also offer a level of adjustability for different ride heights and desired levels of rear steer that simply can not be accomplished with Torque Arm designs.
 
Our Recommendation
First and foremost, keeping the suspension as light as possible always helps reduce wheel hop. Any amount of mass in the axle and suspension acts like an anvil on the tire. Use the lightest suspension and axle parts available. Keep the wheels and brakes light.

If you do not have an adjustable rear brake bias system, install one. Proportioning valves are inexpensive and very effective. No matter how good your suspension is, some track condition will cause wheel hop. Reducing the rear brake capability is the only driver adjustable response to hop. Reducing the amount of rear brake during the race as the fuel load changes is the best remedy for accidental wheel hop when using all of the car's braking capability.
I strongly suggest you avoid torque arm suspensions. They are simple, very durable and they do promote good acceleration traction. For drag racing and street rod cars, this is a very good choice. For road racing, the short effective suspension link length always requires less rear brake than you might otherwise be able to use. It is possible to minimize wheel hop with a torque arm by sloping the lower control arms downward. This helps but also insures a great deal of roll understeer making a balanced car hard to achieve. Mounting the arm in a sliding bushing much like the production Camaro also helps but only slightly. I still contend that fitted with a properly designed 3-link or 4-link suspension, any torque arm car will be able to use more rear brake and will ultimately be faster.

With 3-link and 4-link suspension there are 2 key parameters to keep in mind. First, the lower control arms want to be nearly level. More than 5 degrees of upward angle can create hop. This configuration also induces a great deal of roll steer so it is generally not acceptable for other reasons as well. Second, upper arm angles need to be kept small. For most cars with short control arm lengths, this angle is no more than 10 – 12 degrees downward. The complete answer involves some suspension modeling including the control arm lengths, pivot distances from the axle, tire diameters, plan view angles and more.

Even with proper design and instant center locations it is still possible to induce wheel hop in some cases with a rapid brake application. When you get any hint of wheel hop there are a number of tuning changes that help.

Lowering the tire air pressures can help but there is a practical limit to this based on tire construction and handling.

Adding rebound damping in the low speed range can reduce wheel hop. Having different levels of damping on each rear wheel is also effective limited by other handling concerns.

It is possible to add compliance and damping to the upper links of a 3 or 4-link design with good advantages for acceleration traction and hop.this controls axle wrap up and sudden tire loading.

Using rear brake pads that do not have a high initial bite can tip the scales on wheel hop. This is a common practice.

Speed Solutions Engineering has rear suspension designs for Camaros and Mustangs. Some now include compliant links for improved acceleration and hop control. We also do custom designs for any GT, Road Race or Oval Track application.

Any questions about kits, parts, installations or services are welcome. Contact Bishop by phone at  866-867-8324 or e-mail bishop@bishopsales.com and we will respond promptly. If you're in the neighborhood, then drop by the shop at 1130 West 15th Street North Vancouver BC and we will gladly give you a tour of our facilities, but beware – there may be a Monster and ferocious horsepower beasts in our shop when you visit! We really love to share the fun with other performance enthusiasts who want to take their cars to the next level of performance, appearance and handling!

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