High Speed Front Brake Pulsation

[AzBob_imp]

Before I get a chance to tear the front brake apart, was wondering if anyone had any ideas:

Noticed yesterday coming home the front brake pulsates at speeds > 50mph under moderate to heavy braking. Bike has only 6k miles on it. I was thinking that one of the rotors might be slightly warped, but I don't see how. I plan on taking a look at both rotors and pads when I get a chance this weekend.

 

[the Ferret]

Are you sure its the brakes? How is the front tire?

 

[Motogeezer1949_imp]

I had a bad set of rotors. Waranteed. They are kind of thin and get hot easily, especially in the mountains. I use both brakes a lot now rather than mostly the front to try to keep them cooler.

 

[grover_imp]

I had a bad set of rotors. Waranteed. They are kind of thin and get hot easily, especially in the mountains. I use both brakes a lot now rather than mostly the front to try to keep them cooler.

^this. That sounds like warped brake rotors

 

[Rocky_imp]

I had this pulsating problem (very rapid grab release, grab release) with the front brake on my Bonneville last year during braking. Braking from a high speed was machinegun like and very annoying.
I changed the rotor and it was gone.
Very strange for it to happen on a low mileage machine that has never been abused, but it did.
No such problem with the CB.

 

[dave]

I've replaced two front brake rotors on my VFR800 over the past ten years that suddenly, and for no reason warped. My bike exhibited the same symptoms you describe. Out of curiosity, was your front tire replaced recently? The mechanic could have damaged the rotor when replacing the tire.

 

[Rocky_imp]

I've replaced two front brake rotors on my VFR800 over the past ten years that suddenly, and for no reason warped. My bike exhibited the same symptoms you describe. Out of curiosity, was your front tire replaced recently? The mechanic could have damaged the rotor when replacing the tire.

No, the tire had not been changed.
While I was at it I took the opportunity to change the rotor with an EBC kit which included a new rotor and brake pads.
It much improved the braking so in a way I'm glad it happened.

 

[immgunn_imp]

They sell a dial indicator / vise grip tool for measuring rotor run-out. Maybe a parts store that lends tools has one. YouTube will show you how to use it. A micrometer is needed for thickness variation.

 

[Tim_in_AZ_imp]

Might just be brake pad deposits on the rotors.

 

[Pterodactyl_imp]

They sell a dial indicator / vise grip tool for measuring rotor run-out. Maybe a parts store that lends tools has one. YouTube will show you how to use it. A micrometer is needed for thickness variation.

A runout check is good advice. A basic check, to see if there is any runout, can be done by clamping a wire, or other object, in touch with the rotor and then rotating the wheel. Any significant runout can be then observed by eye. To measure that runout you will need a dial indicator. A reasonable quality dial indicator is not expensive. Runout limits are described in the service manual. I don't have my manual by my side but will post the runout limit if you need.

Cheers.
Minimum rotor thickness is 3mm. I measured mine last week.

 

[AzBob_imp]

Are you sure its the brakes? How is the front tire?

Good call, sir. My tires were embarrassingly low on air. I had noticed a fall-off in handling, and some squirly behavior and inability to hold a turn during high-speed (70+mph) sweepers. To be honest, I've gained some weight, and I thought maybe I had to re-adjust the preload. Fixed the pressure last night, rode in this morning, everything is working exactly as it should, no pulsation, set the lean and bike stays there, and no more wallowing in turns that I noticed had started. This bike appears to be very sensitive to tire pressure.

 

[ClassicVW_imp]

Most bikes ARE sensitive to tire pressure being off. It's the rider who does or doesn't pick up on it.

 

[AzBob_imp]

Most bikes ARE sensitive to tire pressure being off. It's the rider who does or doesn't pick up on it.

My CBR, with a much more sophisticated suspension, doesn't react in nearly the same way as the CB with regard to changes in tire pressure. Usually, if the tires get aired down on that bike, you jut get better grip. With the CB, it seems like unless the tires are within a few pounds of the recommended pressures, braking is affected, weird suspension stuff starts happening, etc. I dunno, I only have these two bikes for comparison.

 

[kvdv_imp]

Sorry to resurrect this zombie thread, but I believe I'm experiencing the same thing as the OP. My front tire pressure is spot-on at all times (I check prior to every ride), and the front brakes are definitely rhythmically pulsing in direct correlation with the speed of the bike, as described in the original post here. Before I run off and order a new set of front discs, can anyone comment on the possibility / advisability of just getting whatever warping exists machined out at a good brake shop? Sorry if it's a dumb question, I know that back in the day I had car brakes services that way, but I'm not sure if it's wise or even possible with these discs. No problem if I need to buy new replacements, but I was just curious about other options...thanks all!

 

[Lord Popgun]

First some specs. Front wheel runout is 0.020mm (.008in) both radial and axial.

Front disc thickness is 4.5mm +- 0.1mm (0.18 +-0.004in). Service limit 3.5mm (0.14in).

The book says to replace the disk if it is below specs on thickness.

If warped, it says to check the wheel bearings for excessive play and replace disk if bearings are normal.

Me personally--I'd put a dial indicator on the wheel and the disk and check for runout, check the wheel bearings, then replace the disk(s). They are pretty thin. Having said that, slightly resurfacing them might be ok. Hopefully someone more knowledgeable about this will chime in.

My car disks are pretty thin, and the forum for that said resurfacing doesn't work and they will warp right back, but the dealer said it does work and they have been good for a year with pulsing gone.
I also read, maybe on the forum here, that material transfer from the pads will cause pulsing. You might want to get some Scotchbrite and clean up the rotors and see if that fixes it.

 

[Ulvetanna_imp]

Check tire pressure before every ride. Every day if necessary. Or don't but be aware based on this thread that you are asking for trouble some day. A tire can pick up an invisible puncture, like a staple, that can't be seen and only detected by noting a pressure decrease on the gage. Sometimes the bead seal is not complete, and tires will leak slowly over time, or the valve core is not tight. You get out on the freeway, that tire will overheat and perhaps fail. Hit the high-speed turns, it's just something you don't need and five minutes of your time will help make sure you don't have these issues.

Toward the high range is better for street riding, can't really go wrong with the recommended 36F/42R if you weigh more than 180 pounds.

Just communicated with Dunlop regarding track pressures and was informed that they now make these recommendations based on a 180-pound rider, not 160 as it used to be. We are getting bigger and heavier, Dunlop figured it out. Used to be 32F/30R, now they're saying 32F/32R. I can't use those numbers, no traction or temperature in the tire, I run 29F/28R but use an infrared thermometer to make sure temps are where they should be.

It IS a science and tires are a very important system on your bike, in my opinion, the MOST important as they communicate directly with the road.

I don't wash my bikes often but I check tire pressure and inspect the tires visually before every ride, every time. Chain tension gets checked weekly.