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      02-11-2010, 10:37 PM   #278
AP Racing - Chris_B
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Drives: E34 540i/6, others
Join Date: Jan 2009
Location: SoCal

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Originally Posted by swamp2 View Post
Chris: How about a more detailed description of "knock back"? What is a design causes it, what does it really feel like. What is really happening with the various wheel and brake components. What designs reduce or eliminate it and how does that work? Thanks in advance.
Simplified version (?): Disc brakes perform best when the pads rub lightly on the rotors while the car is moving. This produces almost zero drag (but not exactly zero!) and allows inputs from the driver (pressing on the pedal) to have the most immediate action. As soon as the master cylinder piston starts moving, pressure is built in the brake circuits. This pressure causes the pistons to push the pad's friction material up against the rotor to slow the car. In an ideal world, no work or motion is lost when the driver starts to apply the brakes.

In reality, a whole bunch of stuff is going on to reduce brake system effectiveness and feedback to the driver. The object of the game is to mitigate all these nuisances so the driver doesn't notice the deck is stacked slightly against him.

An issue that has significant ill effects is pad knockback. This is when some action or another has pushed the pads back away from the rotor. Since there is no residual pressure in the brake lines (nor do we want any), the pads will stay away -- or "knocked back" -- until the driver pushes the pedal again. Only now, the first part of the now very soft pedal travel gets used up moving enough fluid to bring the pads back in contact with the rotor. Once in contact, line pressure will begin building to make the brakes work again. It is a very uneasy feeling to suddenly have a soft pedal when it was all good just a moment ago. If there are no mechanisms in place to bring the pads back in contact with the rotor, the pedal problem reveals it's ugly head.

Major causes of pad knockback:

1. Wheel bearing flex: Under heavy cornering, all wheel bearings flex. Some just a little and some way too much! Since the rotor is attached to the hub/spindle assembly, the rotor goes where the inner race of the bearings go. The caliper is fixed to the upright, which stays in line with the outer races of the bearings. When the bearings flex in a left hand turn, the rotors will tilt to the right with respect to the upright. This pushes the pads on the right (front left inner and front right outer) back into the caliper slightly. A right turn does the same to the opposite pads.

2. Cornering G-force: In some professional formulas I've been involved in that use significant downforce, cornering loads are often well over 3g's. If a pad weighs a 1/2 pound, that is now an equivalent of 1-1/2+ pounds pushing the pistons back into the caliper (plus the damned bearing flex!).

3. Vibration and/or shock: In some rally or off-road vehicles, the violent nature of the road surface can jar the pads back into the calipers.

4. Rotor runout: If the rotor is starting to wear unevenly or is no longer in near-perfect alignment with the hat or hat section, the friction surfaces can start to slightly knock the pads away from the rotor.

5. Rotor coning: When some rotors are very hot and run at very high speeds, they will expand in a non-uniform manner. They will start to "cone" around the hat section. This is one of the primary reasons all real race cars use floating, 2-piece rotors. On the street, there are more options to fix this, such as the strap drive system.


1. Design and develop components that reduce or eliminate pad knockback. They may cost a little more, but well worth it. AP has been doing this for years by experimenting with seals, seal grooves, anti-knockback springs behind the pistons and other stuff. Larger, stiffer bearings are a huge expense for most vehicles. Even so, some of the WRX (5-100mm lug pattern) guys move up to the STI hubs (5-114.3mm lugs). The larger, stiffer STI bearings are worth the hassle. Audi A4, Lexus IS300 and late-model Pontiac GTO are more examples of less than ideal wheel bearings.

2. Tap the pedal with the left foot after cornering heavily and before you intend to use the brakes again. This is fun to watch on the track when you are behind someone battling this condition. Well, you might not be behind them for long as most passing occurs under braking.

I vote for solution #1.

OK, this wasn't such a short answer, but I hope it helps. I'm out for a 3-day weekend, so keep on motorin'! May you all enjoy yours, especially if you are working your way closer towards your next deeper brake marker...4...3...2...1(!).


Last edited by AP Racing - Chris_B; 02-11-2010 at 10:40 PM. Reason: Clarity?