Break Pulsing and Bar Shake - Not the Rotors?

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Break Pulsing and Bar Shake - Not the Rotors?

Postby mjmorrisva » Wed Mar 13, 2019 3:18 pm

Friends, I managed to get my new motorcycle lift, tools, and such to start working on the bike. I was able to get the carbs balanced enough to take the bike out for her maiden voyage (to get inspected and registered). The only issue I had was the front break has noticeable pulsing and some bar shake when you apply the lever. This happens at any speed but especially when downshifting and slowing to a stop. With 36,000 miles on the bike I didnt think the rotors were warped but I checked anyway. After inspecting them carefully with a high-quality precision straight edge I can find absolutely zero warping. The front break pads look like they are almost new with plenty of pad left. Front rim spins with a slight touch of the pads....exactly as I would expect.

Any thoughts on what to diagnose next? Anyone have a similar problem? I'm thinking maybe the wheel balancers are not properly applied as the tires are brand new and maybe they didnt balance them appropriately...but then again that would typically show up in constant riding conditions...no?

Your thoughts and advice please,
Matthew
1982 R100RT
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Re: Break Pulsing and Bar Shake - Not the Rotors?

Postby VaSteve » Wed Mar 13, 2019 9:17 pm

I agree that if the wheel was unbalanced, you'd feel it all the time.

When you touch the lever, is it hard or gentle?

(The follow pertains to my experience in a track/race car):

What it sounds like is that you have uneven pad deposits on the rotor. Pads transfer some material to the rotor and if not distributed evenly around it feels like that. The solution in my car, is to drive it hard and get the rotors HOT after some hard braking. That ensures an even transfer of material. Might not be the same for a motorcycle and no place to drive it like a nut to do that.

Or a pad has glazed up. Sounds like this bike is new (to you?)

Maybe take the pads out and scuff them on the pavement and/or hit the rotors with a clean scotchbrite pad and/or some brake cleaner.
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Re: Break Pulsing and Bar Shake - Not the Rotors?

Postby Georgeinva » Thu Mar 14, 2019 12:12 pm

Matthew,
If you can see a maximum runout of 0.2mm (0.0079") using a straight edge your eyes are way better than mine. You need to measure the runout using a dial indicator.
Image
Given that most people do not have the required tools take VaSteve's advice. Start with a emery cloth and scuff the rotor from the axle out, not round rand round. Remove the pads and scuff them with emery cloth an a known flat surface. Clean everything with brake clean, make sure not to get any on the paint or plastics. Let it dry and reassemble. Take it easy on the pads for 5-10 miles then ride it like you stole it. Several hard stops from 60 or so should do. Make sure not to drag the brake and check every other stop for changes in color. You do not want to overheat the rotors.
Good luck, George
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Re: Break Pulsing and Bar Shake - Not the Rotors?

Postby Henri2 » Tue Mar 26, 2019 8:40 am

Maybe it also has to do with a weight from balancing the front tyre.
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