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Topic-icon 82 R100RT Break Pulsing Badly

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4 months 3 days ago #4578 by mjmorrisva
82 R100RT Break Pulsing Badly was created by mjmorrisva
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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4 months 2 days ago - 4 months 2 days ago #4580 by Wobbly
Replied by Wobbly on topic 82 R100RT Break Pulsing Badly
IMHO you'll need a dial indicator to detect warped rotors, which your issue certainly sounds like. Tire balance is more of an "at speed" issue, with or without brakes applied. You don't need "high mileage" to encounter a warped rotor because that has more to do with the "normalization" of the rotor metal before the disk assembly is even made.

The only suggestion I can offer is to change the fork oil. Otherwise, borrow a dial indicator and indicator mount.

Don't hide 'em, Ride 'em !!
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Last edit: 4 months 2 days ago by Wobbly.

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4 months 2 days ago #4581 by 8166
Replied by 8166 on topic 82 R100RT Break Pulsing Badly
I don't recall seeing any history on the bike; has it been sitting long? Or ridden in the rain and put away wet? I suspect there's a spot of corrosion, contamination, or hardness on one/both rotors that is causing a variation in coefficient of friction as the rotor spins. You might get lucky and clean it off with repeated hard applications of the brakes (not breaks), but more than likely you'll need to really go after them with an orbital sander and some 100 grit paper. Before you do that, though, use a good solvent to clean off whatever is on there, otherwise you stand a chance of driving the contaminants deeper into the material. Use solvent again after the sanding. Then take the pads out and take about 0.06" off the surface using a disc or belt sander. Try not to breathe the dust. Once that is all done, reassemble and pretend you've got new pads and rotors. Apply the brakes gently for short intervals, giving them plenty of time to cool down before repeating. City riding with lots of stops and starts is perfect for this. Once the pads start to grip better, gradually increase the intensity of braking, but still give 'em plenty of time to cool between applications. If during this process the pulsing comes back, you might have a hard spot, which the sanding will do nothing to help. At that point it's time for new rotors, unfortunately.

FWIW, I once purchased a used K75S from a BMW dealer. It had about 30K miles on it, and he said it had new rotors on it. It was obvious the pads hadn't seated yet, and when they did, the brakes started pulsing. I did the sanding thing, and the pulsing came back again. A friend suggested I switch to Ferodo pads, and the problem went away and stayed away. I haven't used BMW pads since.

8166 Scot Marburger, This email address is being protected from spambots. You need JavaScript enabled to view it.
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4 months 2 days ago #4582 by mjmorrisva
Replied by mjmorrisva on topic 82 R100RT Break Pulsing Badly
Thank you sir, I plan to replace the fork oil anyway along with ALL the other fluids, plugs, air filter, etc. Then take the bike out and ride it for a few months to shake it out.

Cheers,
M

1982 R100RT

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4 months 2 days ago #4583 by mjmorrisva
Replied by mjmorrisva on topic 82 R100RT Break Pulsing Badly
Scott, thank you for the reply. The bike is "new" to me as a Christmas present from my wife. She saw me admiring it on-line and got it as a surprise. Not the first time she's bought me a BMW bike. At any rate, the bike is a one-owner all original from Maine with 36,000 miles. She got it from Max BMW where it sat for a bit because they had it priced to high. Unfortunately, it seems the original owner who took very good care of her passed and I dont have the bike's records. Ugh.

I plan to try new pads, bleed the breaks, replace fork oil before attacking the rotors.

Cheers,
M

1982 R100RT

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4 months 29 minutes ago - 4 months 25 minutes ago #4588 by Wobbly
Replied by Wobbly on topic 82 R100RT Break Pulsing Badly
By all means try Scot's suggestions first. If for no other reason than that brake rotors, even third party, are NOT cheap !

If it does come to rotor replacement I can tell you that the floating versions from EBC are highly desirable over the stock BMW units, and probably a good bit cheaper.

One final suggestion. The tire air pressures suggested in your manual are no good for tires made within the last 15 years. I've found that with modern name brand tires something closer to 35psi front/ 33psi rear works much better.

Hope this helps.

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Last edit: 4 months 25 minutes ago by Wobbly.

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