Wheel shake under braking on track

[uberchris]

I have started encountering an odd braking issue on track. After a couple laps, when I brake, I get a fairly violent shake in the steering wheel. It feels exactly like a warped rotor, but only happens once the brakes get hot (and never happens on the street). My setup is:

• DTC-60 pads
• Stock rotors (already replaced, issue still occurs)
• Stock calipers
• Fresh Castrol SRF fluid

Other stuff, if it matters:

• PS4S tires, 275/35r18 on apex 9.5et35 wheels
• Koni yellows, stock springs
• Turner front bar
• Stock everything else, but all bushings/ball joints < 40k miles old

I'm really struggling to figure this one out. The track I'm running at does have a higher top speed than the other tracks I've been at, but the brakes don't fade; the wheel just starts shaking. Here's a session FWIW: https://www.youtube.com/watch?v=Kh_8D6XaNps

Seems like the best bet is inconsistent pad transfer or something suspension related? I always do a cooldown lap, and I never set my parking brake in the paddock. Got any ideas?

Here's a couple pictures of the rotors:

[Nate047]

First thing I always think of with questions like this is pad material transfer. Did you try doing a couple of hard stops where ABS activates to see if you can get rid of it?

[uberchris]

First thing I always think of with questions like this is pad material transfer. Did you try doing a couple of hard stops where ABS activates to see if you can get rid of it?

Oh yeah. I replaced the rotors and bedded the pads before the last trackday and the issue still returned.

[Estoril]

What does the pad surface look like?

[Maxima SE]

Oh yeah. I replaced the rotors and bedded the pads before the last trackday and the issue still returned.

Did you use the old pads with new rotors? I have read its best to replace both so that the surfaces are as smooth as possible and the old uneven pad surface does not cut into the rotor surface. Not sure if this is actually what happens in real life but the theory makes sense.

[Nate047]

Yea I would also just add that if there is a circumstance which caused the deposits in the first place, it could happen again with new pads and rotors. Like if the pads are sitting a little too far out inside the calipers and lightly touching a hot rotor surface, you might not notice that visually.

I assume you are never using the parking brake when the rotors are hot also, that's a common known contributing factor.

[Slideways]

I'd guess pad deposits first, then wheel bearings.

[uberchris]

What does the pad surface look like?

When i changed the rotors the pads looked fine. No big chunks missing or anything.

Did you use the old pads with new rotors? I have read its best to replace both so that the surfaces are as smooth as possible and the old uneven pad surface does not cut into the rotor surface. Not sure if this is actually what happens in real life but the theory makes sense.

Yeah that would probably be ideal, but as you can see there is a lot of meat left in these pads. I assume bedding them should be enough to take care of that?

Yea I would also just add that if there is a circumstance which caused the deposits in the first place, it could happen again with new pads and rotors. Like if the pads are sitting a little too far out inside the calipers and lightly touching a hot rotor surface, you might not notice that visually.

I assume you are never using the parking brake when the rotors are hot also, that's a common known contributing factor.

Yeah definitely not using the parking brake at events.

I'd guess pad deposits first, then wheel bearings.

I'm just not sure what might be causing uneven pad deposits. Everything looks ok.

I'll check the bearings. Usually when those act up for me they make noise, though.

[bimmerfan08]

What bedding procedure did you use with the DTC-60 pads?

[Estoril]

I'll just note that you seemed to do nothing to warm up brakes on the out-lap.

[uberchris]

What bedding procedure did you use with the DTC-60 pads?

Just the one off the website: https://www.hawkperformance.com/how-to
Although “near race speed” isn't really possible on streets near me.

I'll just note that you seemed to do nothing to warm up brakes on the out-lap.

True. I usually just give the turn 3 brake zone a little extra space on the first lap out. What do you suggest?

[Estoril]

Just the one off the website: https://www.hawkperformance.com/how-to
Although “near race speed” isn't really possible on streets near me.



True. I usually just give the turn 3 brake zone a little extra space on the first lap out. What do you suggest?

I'm not a Hawk guy and don't know the habits/MOT of that pad. But Lap 1 is always 'warm up tires and pads' for most of us. First lap, first cold braking event at 120 mph seems like an assumption of full traction and braking performance on cold tires and pads. YMMV.

[mrgizmo04]

If you swap to street pads and hammer the brakes on an offramp/coming to a light and you can't replicate the shaking then we can prob rule out suspension/etc.

What most folks forget on the outlap is the brakes. You tend to go much slower than normal basically not using the brakes (except maybe a bit on super tight turns), so your bearings, fluids, tires to a certain extent get warmed up while your brakes don't. You need to do a few deliberate brake checks to help get the pads up to temp. DTC pads need quite a bit of progressive heat in them, and spiking them from cold on a super heavy braking zone smears them, which results in problems.

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[cobra]

This might sound simple but... make sure your lug nuts are FULLY torqued. Re-check them.

[jayjaya29]

Common issue. You have to brake harder to "push through" the vibration. Its basically uneven deposits. Brake late and harder for a session.