Pad Knockback Issue??
Pad Knockback Issue?? I've been trying to diagnose a strange issue I've been having with my brakes.
When I'm driving quickly and hit the brakes, the brake pedal with occassionally drop a couple of inches upon application, and then return to the top on the next brake application. It seems to happen most commonly after going around a turn at speed. The other 95% of the time though its fine and grabs right at the top of the travel of the pedal.
Front brakes are ALL new, rears are in good condition. Lines are good, and the master holds pressure like a champ. The entire system has been bled, fluid is ATE Super Blue DOT4 Racing fluid.
My only idea is that the side-to-side play in the rear axles is causing pad knockback after going around a turn hard. Since the rotor is bolted to the axle hub and the caliper is mounted to the axle tube itself, any transverse movement in the axle will cause the rotor to move side to side within the caliper. This could possibly cause the rotor to push the piston back into its bore slightly, resulting in extra long pedal travel on the next brake application. I checked the play in the axles, and they do move in and out a noticeable amount.
Has anyone ever heard of this?? And if so, is there a solution??
When I'm driving quickly and hit the brakes, the brake pedal with occassionally drop a couple of inches upon application, and then return to the top on the next brake application. It seems to happen most commonly after going around a turn at speed. The other 95% of the time though its fine and grabs right at the top of the travel of the pedal.
Front brakes are ALL new, rears are in good condition. Lines are good, and the master holds pressure like a champ. The entire system has been bled, fluid is ATE Super Blue DOT4 Racing fluid.
My only idea is that the side-to-side play in the rear axles is causing pad knockback after going around a turn hard. Since the rotor is bolted to the axle hub and the caliper is mounted to the axle tube itself, any transverse movement in the axle will cause the rotor to move side to side within the caliper. This could possibly cause the rotor to push the piston back into its bore slightly, resulting in extra long pedal travel on the next brake application. I checked the play in the axles, and they do move in and out a noticeable amount.
Has anyone ever heard of this?? And if so, is there a solution??
Yes this is possible. It is more likely from the front hubs. As the hubs wear you get a lot of play. I also believe it's possible from the rear, as I think I have the same problem after a diff replacement recently.
That's exactly what it is.
You also need to know that no matter how much you attempt to make the axle not "move", there will always be some deflection present, and in solid rear axles, the transmission of lateral forces is significantly more efficient than CV mounted axle designs.
The only thing you can do to your rear axle in addition to removing differential deflection is install a bearing kit to reduce the amount of movement, but remember... there's quite a bit of lateral forces going through any axle upon turning, and the more cornering grip you can achieve the worse it's likely going to get.
As Chris mentioned, front hubs are another place to look at.
If you still have that CTS-V kit up front, then depending on piston size, that also could be a contributor as well when there's enough deflection to knock those pistons back, not to mention that ALL solid calipers have opposing pistons which significantly increase the possiblity for piston retraction. Also, larger rotor diameters even with the same mass as the OE versions will certainly increase this phenomena significantly. Again, heavily related to MOI.
Bear in mind that almost all of the fancy GT, prototype, as well as open-wheel racers have knockback springs installed in their calipers. In fact, I haven't heard of a single high-end professional race team that refrains from using them when running solid calipers. The only drawback to them is that they're going to put added drag on brake rotors, therefore they're not practical for everyday driving.
You also need to know that no matter how much you attempt to make the axle not "move", there will always be some deflection present, and in solid rear axles, the transmission of lateral forces is significantly more efficient than CV mounted axle designs.
The only thing you can do to your rear axle in addition to removing differential deflection is install a bearing kit to reduce the amount of movement, but remember... there's quite a bit of lateral forces going through any axle upon turning, and the more cornering grip you can achieve the worse it's likely going to get.
As Chris mentioned, front hubs are another place to look at.
If you still have that CTS-V kit up front, then depending on piston size, that also could be a contributor as well when there's enough deflection to knock those pistons back, not to mention that ALL solid calipers have opposing pistons which significantly increase the possiblity for piston retraction. Also, larger rotor diameters even with the same mass as the OE versions will certainly increase this phenomena significantly. Again, heavily related to MOI.
Bear in mind that almost all of the fancy GT, prototype, as well as open-wheel racers have knockback springs installed in their calipers. In fact, I haven't heard of a single high-end professional race team that refrains from using them when running solid calipers. The only drawback to them is that they're going to put added drag on brake rotors, therefore they're not practical for everyday driving.
Last edited by Foxxtron; Apr 11, 2010 at 08:39 AM.
I ended up finding the problem, the driver's side cone-clutch in the Auburn is completely shot causing excessive lateral play in the left axle shaft. The passenger side is nice and tight, the driver side moves a good 1/4" in and out.
Time for a new differential.
Time for a new differential.
