Pad Knockback -- For road racers

Slow old poop · Jul 27, 2005

I seem to be suffering from pad knockback. This occurs when the car goes through a corner at high Gs, causing the front spindles, knuckles, hub and rotor to distort, thus pushing the pistons away from the pad. When braking for the next corner, it requires two pumps of the brake pedal just to get the pistons back into position so I can brake hard.

You can see a full technical description of this phenomenon at http://www.stoptech.com/whitepapers/knockback.htm, "Pad Knockback
A Common Racing Phenomenon"

This is an inconvenience at best, and a lap time killer at the worst, because, instead of standing on the brakes, I have to waste a second or so getting the brakes back. Bummer, dude.

We thought at first the high boost was overcoming the vacuum booster check valve, but inputs from folks on this list convinced me that isn't the case. It isn't boiled fluid, because we changed out the fluid to Motul and bled the brakes religiously. Naw, it sure seems to be pad knockback. Everything described in the technical note fits my situation exactly.

The previous owner replaced the left front hub and wheel bearing, and it helped the problem, so I plan to replace the right front.

Questions:
Are there any racing parts I should be installing? Such as high-strength hubs?
Do the hubs from a 3000GT fit? (They are about twice the size of Eclipse hubs.)
What else should I replace besides the hub and wheel bearing?
Anybody else had this problem? What did you do to cure it?

Rich/slow old poop
1990 Eclipse

bloc · Jul 27, 2005

anything can be fabricated with enough know-how and money.

if retrofitting stronger hubs from another car doesnt work out, see about getting a stock one and bringing it to a job-type machine shop and seeing what they think. there are alloys of steel out there that are worlds stiffer than the stock material likely used on the hubs of these cars. it wont be cheap though. from what i know of lathe and mill work, you'd be looking at a few hours of machine time and quite a bit of cost in tooling, and that wont come cheap. many smaller shops wont even have the tools on hand to cut the splines inside the hub and it would need to be outsourced.

as greg mentioned, he doesnt have problems with pad knockback, even though his setup is similar to yours, and he's likely matching or exceeding your cornering forces. one difference to consider is that if your overall tire/wheel height is larger, then you will have more leverage at a given cornering load to deform the hub to the point of pad knockback. real world though, and this is probably too small of a difference to consider.

you said it's a lap time killer at worst. i'd argue it's a safety issue at worst.

wortdog · Jul 29, 2005

You might as well replace all of the wheel bearings, front and rear. I've never noticed a knockback problem myself on the track, even though both of us have giant front rotors that amplify knockback.

And if you can't solve the problem completely, tap the brakes very slightly on the straightaway, it won't cost you much time at all and you should be able to brake much harder when you come up on the corner.

Dauntless · Aug 1, 2005

Slow old poop said:
I seem to be suffering from pad knockback. This occurs when the car goes through a corner at high Gs, causing the front spindles, knuckles, hub and rotor to distort, thus pushing the pistons away from the pad. When braking for the next corner, it requires two pumps of the brake pedal just to get the pistons back into position so I can brake hard.

You can see a full technical description of this phenomenon at http://www.stoptech.com/whitepapers/knockback.htm, "Pad Knockback
A Common Racing Phenomenon"

This is an inconvenience at best, and a lap time killer at the worst, because, instead of standing on the brakes, I have to waste a second or so getting the brakes back. Bummer, dude.

We thought at first the high boost was overcoming the vacuum booster check valve, but inputs from folks on this list convinced me that isn't the case. It isn't boiled fluid, because we changed out the fluid to Motul and bled the brakes religiously. Naw, it sure seems to be pad knockback. Everything described in the technical note fits my situation exactly.

The previous owner replaced the left front hub and wheel bearing, and it helped the problem, so I plan to replace the right front.

Questions:
Are there any racing parts I should be installing? Such as high-strength hubs?
Do the hubs from a 3000GT fit? (They are about twice the size of Eclipse hubs.)
What else should I replace besides the hub and wheel bearing?
Anybody else had this problem? What did you do to cure it?

Rich/slow old poop
1990 Eclipse

I think that before I went to the trouble and expense of aftermarket hubs that I would ensure that what I have is up to spec, especially in light of Greg's success running the stock setup. He is making similar power to you and has no knockback with the stock hubs, so I suggest sending him a PM or email asking for details about his car's preparation. IMO knockback is a compliance issue, so to me it stands to reason that eliminating compliance in the system is a key to eliminating the problem. Maybe it's as simple as fresh wheel bearings and suspension bushings.

PS - Tom and Greg...you guys blew it by not coming by to say hello at the San Jose Grand Prix. We had sponsorship from Gordon-Biersch Brewing Company, which included 15 cases a day of cold beer to give away to fans and friends. Man, we floated the place!

Greg Collier · Aug 1, 2005

Rich
As I said previously the hubs are as old as the car but the wheel bearings and suspension bushings have been replaced and I did weld the ball joints in. The motor was making so much torque I threw one out while on the track (pretty scary)! I did get a little front brake shutter on the Stop Techs when they were first installed, but re-bleeding the system took care of that.

Dauntless said:
PS - Tom and Greg...you guys blew it by not coming by to say hello at the San Jose Grand Prix. We had sponsorship from Gordon-Biersch Brewing Company, which included 15 cases a day of cold beer to give away to fans and friends. Man, we floated the place!

Stan
Wasn't able to make it to the San Jose Grand "Rally" Prix but caught a little of it on TV. Man that was a tight track with a nice little jump that must have played havoc on everyone's suspension. I heard the USTCC boys crashed a couple of cars, but they had a ball :thumb:

The beer would've been GREAT!!!!

Greg

terefic181 · Aug 8, 2005

Dauntless said:
Dauntless said:
...you guys blew it by not coming by to say hello at the San Jose Grand Prix. We had sponsorship from Gordon-Biersch Brewing Company, which included 15 cases a day of cold beer to give away to fans and friends...
Click to expand...

CRAP!!!
You mean I missed out on FREE beer?

Anyway, sorry I wasn't able to make it. At least, I was able to see a little of the San Jose racing on SPEED.

Tom

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Pad Knockback -- For road racers

Slow old poop

bloc

wortdog

Dauntless

Greg Collier

terefic181

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