Chasing front brake siezing/tight

Technical and Repair Discussions

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PostPosted: Mon Jun 26, 2017 8:39 am
I've picked up a shimmmy/snake in the front brakes/wheels.
At the end of session, the front brakes/rotor/hub are hot hot compared to other cars
When I jack up the front, the front wheels drag heavily, and only spin about a 1/4 turn when you spin them.
Ive changed or rebuild everything at least once.
Everything from different caliper brackets to new calipers, pins, bushings.
The only thing I haven't changed is the upright/hub.

It's not hydraulic, as when I pop the bleeder, nothing comes out.

Any areas of investigation as next steps would be appreciated...
Scott Cypher
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PostPosted: Mon Jun 26, 2017 10:06 am
I didn't see mention of rotors; what's their condition? Check the play on the master cylinder rod to make sure there's some slack, as that could contribute to brake drag. Normally, I would expect shake/shimmy to be bearing related or wheel imbalance. Is there any play in the bearings (there should be no noticeable movement if you elevate the wheel and wiggle it vertically and horizontally)? Finally, check the balance bar to make sure that it remains level between the front and rear master cylinders. I've seen instances of the rod lengths differing enough in length to cause one side of the adjuster to hang up, altering front-to-rear brake balance. I asssume when you talk about rebuilding the calipers that you've replaced the o-rings and dust boots, and inspected and cleaned the pistons, put in fresh, high quality fluid and bled front and rears properly. When's the last time the master cylinders were rebuild?
Bob Breton - SRF 51 - San Francisco Region

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PostPosted: Mon Jun 26, 2017 12:13 pm
Thanks, Bob

Master cylinders rebuilt; currently 3 sessions (not weekends) on them
Rotors new last time out this weekend; got worse
No tension in push rods
Balance bar even
Rebuilt caliper with new SS piston and rings.
No dust boots as they always burn off
Flush and fill new fluid 3 sessions ago

One wheel bearing has noticeable play;
CSR who looked at it wasn't concerned nor think it was part of the drag

None of these appear to affect the drag; its heavy whether hot or cold.
I can find nothing in the linkage or hydraulics as to cause,
So I'm assuming it's in the caliper/bracket/pins

The dust boot is interesting;
I don't run them because they always melt and hear that was common.
What do others do on this point?
I could see that as a cause for stuck piston.
Scott Cypher
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PostPosted: Mon Jun 26, 2017 12:48 pm
On the caliper pins/bushing, are they the correct size? There are different size pins and bushings out there, and a mismatch can happen where a smaller pin is used with a larger bushing.

How's the brake bias setting? Too much front could be a source of overheating.

Lastly, tires. I've had Hoosiers that seem to pick up some strange vibrations, almost like a failing wheel bearing or rubber pickup, especially under heavy sustained loads. Not consistent from set-to-set. Very sensitive to tire balance as well.
Bob Breton - SRF 51 - San Francisco Region

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PostPosted: Mon Jun 26, 2017 3:55 pm
Scott & Bob,
would Brake Fluid cause this type of issue?
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PostPosted: Mon Jun 26, 2017 4:13 pm
No, I can't think how fluid would cause it.
I've had this same issue (but worse) on multiple occasions. The cause was me hopping in a car that wasn't mine and adjusting the push rods so that the brake pedal is higher (further above the gas pedal). I'm a rev-matcher so I prefer the brake pedal positioned in this manner but on more than one occasion I've raised it a couple of turns of the push rods and the front brakes start to drag really badly. One time the only way I could fix it before coming in (only took one pace lap) was to pull back on the brake pedal with my foot after depressing it. I could feel the car jump forward if I did so under acceleration.

As a test, I'd try that. Go out, brake the car hard, then accelerate and pull back on the pedal with your foot and see if you can tell a difference.
If they are dragging badly on track, once you get in you may have so much heat in them that pulling the pedal back may not solve the problem at that point.
Hope you find it!
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PostPosted: Mon Jun 26, 2017 5:34 pm
here is an idea, but unlikely: The current pads have slightly thinner friction material than the original (Hawk) pads. If for some reason you have the old pads they will drag for a few sessions, until you wear them a little.

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PostPosted: Tue Jun 27, 2017 3:07 am
I'll try the pedal pull back, Denny, thanks
Caliper pins/bushings brand new, matched set. Very little slop , don't know what the tolerance on the fit should be.
Pads are not new, so thickness isn't an issue
Bob, I've had similar with the tires, but it tends to go away after some use. I just think the shimmy is a red herring; it's either a symptom of the brakes binding, or it's unrelated.

The overall heat is interesting; is there a point they get so hot it doesn't matter, they will always drag? I ask because even after the front end cools off completely, they still drag/bind. They never seen to "let go"


What about:
-Bent pad backings
-lube on pins/bushings
Scott Cypher
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PostPosted: Tue Jun 27, 2017 8:37 am
As an experiment I would suggest switching the front and rear calipers. if the problem persists in the front then you know it's one of the other components if the problem moves to the back then you have isolated the calipers being the problem
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PostPosted: Fri Jul 28, 2017 9:30 am
Found it. One caliper had a wider space between the pad and the piston; it must have come from a different make of Renault with a thicker rotor. Very odd. It sat back farther on the pins and the piston would get slightly cocked, and would not return into the caliper on its own. Changing it out solved it.
Scott Cypher
SRF #688 G2
SRF3 #577 G3
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