Shudder at high speed braking with Wilwood

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Shudder at high speed braking with Wilwood

Postby avera01 » Mon Dec 09, 2019 3:28 pm

Pretty much since my last auto-cross training drive, I have developed a shudder when lightly braking at high speed; say to gently reduce speeds from 80 mph to 70/60 mph. Upon pressing the brake pedal I would feel in both the feet and steering wheel a rapid vibration/shudder plus sound. The car brakes straight, no pull and at lower speeds say under 50 mph, the shudder is hardly noticeable.

My first reaction was warped rotors since the car is a weekend driver with just two auto-cross training events since the 4 piston Dynapro was installed with their 12.19" 2-piece rotor about 2 years ago and now with approx 11000 miles of street use.

I did a runout test noting that the left side had a .004" variance while the right side .002". According to Wilwood, anything under .007" is not noticeable. They claim their rotors are true within .001". Contacting GWR, I was advised to perform an aggressive bedding (using BP-10 pads) and if that does not work, cut the rotors. The aggressive bedding helped just a little. Wilwood further stated not to cut the rotors and to check the suspension system followed by the caliper.

I am not a fan of cutting rotors as it is a reduction of thermal capacity and more probable loss of stopping power if the rotors are not cut properly (based on GM training I took years ago). I rather replace rotors. Plus finding a quality shop to cut the 2-piece rotors might be time consuming and costly. Now everything I have read indicates that today's rotors don't generally warp and the culprit is excessive pad buildup. Checking the rotor by touch did not help as it felt fairly smooth.

With further reading, on the Internet, I found the suggestion of using a racing pad to clean out the excess pad material. So I decided to give it a go by using Hawk DTC 60 and driving the car for approx 20 miles using light braking followed with just a few quick slow downs braking from 60 down to 40; but not to heat up the pads.

Then I replaced the pads (after a cool down period) with the BP-10 and re-bedded them. The vibration is gone, however the rotor have taken a bit of hit with those race pads. I was kind of expecting some wear. You can feel several fine grooves cut into the rotor from those pads, and with only 20 miles of street driving, those pads generated a whole lot of dust.

The beauty of this process is the easy exchange of the pads on the Dynapros, although for future consideration I would drive the car perhaps only 10 miles to clean the rotors. If there was any warping or the runout, its probably been removed in the process.

The brakes feel smooth and sure once again. Perhaps next time, I'll just replace the rotors, but I will keep the Hawk pads for a quick solution. I was prepared to ditch the Wilwood 2-piece rotors and purchased from GWR the bracket to utilize stock size rotors. But in the end I prefer the larger size rotors. The brackets are now part of my backup plan as I want to keep my ND for a long time.

I want to thanks GWR for their time on the phone and assistance is this matter.
avera01
 
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Re: Shudder at high speed braking with Wilwood

Postby jboemler » Mon Dec 09, 2019 4:09 pm

I've had good luck just sanding the rotors and rebedding. What many describe as "warping" is really just uneven deposits of pad material. While a harsher pad is one way to get the deposits off (and probably add others...), sanding (with a drill-mounted sanding disk) is much simpler.
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Re: Shudder at high speed braking with Wilwood

Postby Brian » Tue Dec 10, 2019 6:14 am

Ditto what Jim said, got a high spot of pad deposits and that can be flat sanded off.
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www.good-win-racing.com
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Re: Shudder at high speed braking with Wilwood

Postby avera01 » Wed Dec 11, 2019 5:17 am

jboemler wrote:I've had good luck just sanding the rotors and rebedding. What many describe as "warping" is really just uneven deposits of pad material. While a harsher pad is one way to get the deposits off (and probably add others...), sanding (with a drill-mounted sanding disk) is much simpler.


Hi Jim,

Is there a particular type of sand paper and grade that you use? I have seen an ad on the Internet for a kit which includes a round rubber sanding pad for the drill, sort sand paper and brake cleaning fluid.

Andrew
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Re: Shudder at high speed braking with Wilwood

Postby jboemler » Wed Dec 11, 2019 11:35 am

I'd use fairly fine paper, maybe 240 or greater, but you don't have to go crazy with the finish -- I certainly wouldn't bother buying a special kit. Just go around the entire surface, trying to be fairly even about it, and when you see bright metal show up (which is almost immediately), you're done. This isn't a "sand and finish" job, it's just a convenient way to brush stuff off a hard surface.
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