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Disc Brake Issue

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Disc Brake Issue

Old 05-28-21, 10:49 AM
  #1  
LS05
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Disc Brake Issue

I have a a Trek Checkpoint ALR5. It is equipped w/hydraulic disc brakes, Shimano 105 to be exact.
My problem occurs when riding, I spot an unavoidable bump or pothole and I apply the brakes, There is a rather violent vibration in the front brake after hitting the bump/pothole. It's as if the front brake is braking and immediately fading. The vibration continues until i release the front brake. When I engage the front brake again, on smooth road, front braking is normal. I do not have the same problem w/the rear brake.
I am a large rider. I'm 6'4", weighing 200 lbs. I've read that cycling disc brakes are susceptible to fade for large riders, but haven't come across anything like what I'm experiencing. My LBS has checked the front end integrity of the bike with no negative results.
Any thoughts would be appreciated.
Thank you
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Old 05-28-21, 11:12 AM
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The go-to answer for brake hysteresis issues is a glazed or inconsistant spot on the rotor. You may try to scrub the rotor with aluminum oxide sandpaper. A few hundred grit ought to be enough to knock off the contaminants that have glued themselves on.

Another answer is the rotor itself may have worn inconsistantly due to the various cut-outs providing differing surface contact areas. The least surface area parts (either side of the hole in the braking surface) would have worn more than the areas that contact the whole pad. In this case a new rotor (& maybe even pads) would be well worth looking in to.

A good measurement of rotor thickness would tell you a lot.

As a heavier & "confident" rider (200lb) that tends to push things, I'd like to suggest scintered/metallic brake pads. They may be a bit noisier in certain circumstances, but they do tolerate higher heat loads much better than the organic resin type. They also tend to be more fade resistant & tend to not deposit themselves on to the rotor so much.
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Old 05-28-21, 11:26 AM
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Also check headset adjustment.
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Old 05-28-21, 12:27 PM
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Thank you both...Will ask my wrench to address your suggestions.
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Old 05-28-21, 06:52 PM
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Originally Posted by base2 View Post
The go-to answer for brake hysteresis issues is a glazed or inconsistant spot
Agree. How is this hysteresis, though?
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Old 05-28-21, 07:18 PM
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Originally Posted by LS05 View Post
I have a a Trek Checkpoint ALR5. It is equipped w/hydraulic disc brakes, Shimano 105 to be exact.
My problem occurs when riding, I spot an unavoidable bump or pothole and I apply the brakes, There is a rather violent vibration in the front brake after hitting the bump/pothole. It's as if the front brake is braking and immediately fading. The vibration continues until i release the front brake. When I engage the front brake again, on smooth road, front braking is normal. I do not have the same problem w/the rear brake.
I am a large rider. I'm 6'4", weighing 200 lbs. I've read that cycling disc brakes are susceptible to fade for large riders, but haven't come across anything like what I'm experiencing. My LBS has checked the front end integrity of the bike with no negative results.
Any thoughts would be appreciated.
Thank you
I hope you're not actually holding the brake yet when the wheel makes hard contact with the bottom of the pot hole or opposite lip of the pot hole. You should be braking hard upon reflex when you first see the obstacle or pot hole and then in the split second before solid contact with the obstacle release the brake so the tire can roll over or through said obstacle. The way everything goes back to normal after your incident makes me think its got to do with the headset, but that's just my opinion.
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Old 05-28-21, 08:27 PM
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Originally Posted by MoAlpha View Post
Agree. How is this hysteresis, though?
https://en.m.wikipedia.org/wiki/Hysteresis
Specificly, it is elastic hysteresis. It's under the "In mechanics" heading. The gist of it is the current state has a direct relationship to the previous state & a feedback loop of some sort is formed. Most people tend to associate it with magnetism or electronics, but the principal is found in many systems. This one example being the friction/force acting on the rotor at a given moment vs the elasticity of the fork. The area in between the curves being the sum of the change of velocity of bike, the bike rider, & some nominal heat gain in the rotor, pads, & elsewhere in the system.

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Old 05-29-21, 05:10 AM
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Originally Posted by base2 View Post
https://en.m.wikipedia.org/wiki/Hysteresis
Specificly, it is elastic hysteresis. It's under the "In mechanics" heading. The gist of it is the current state has a direct relationship to the previous state & a feedback loop of some sort is formed. Most people tend to associate it with magnetism or electronics, but the principal is found in many systems. This one example being the friction/force acting on the rotor at a given moment vs the elasticity of the fork. The area in between the curves being the sum of the change of velocity of bike, the bike rider, & some nominal heat gain in the rotor, pads, & elsewhere in the system.
Thanks. I deal with hysteresis in neurophysiological systems from time to time, but never thought of it as an amplifier of fork vibration.
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Old 05-29-21, 06:52 AM
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Originally Posted by MoAlpha View Post
Thanks. I deal with hysteresis in neurophysiological systems from time to time, but never thought of it as an amplifier of fork vibration.
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Old 05-29-21, 03:48 PM
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Originally Posted by LS05 View Post
I have a a Trek Checkpoint ALR5. It is equipped w/hydraulic disc brakes, Shimano 105 to be exact.
My problem occurs when riding, I spot an unavoidable bump or pothole and I apply the brakes, There is a rather violent vibration in the front brake after hitting the bump/pothole. It's as if the front brake is braking and immediately fading. The vibration continues until i release the front brake. When I engage the front brake again, on smooth road, front braking is normal. I do not have the same problem w/the rear brake.
I am a large rider. I'm 6'4", weighing 200 lbs. I've read that cycling disc brakes are susceptible to fade for large riders, but haven't come across anything like what I'm experiencing. My LBS has checked the front end integrity of the bike with no negative results.
Any thoughts would be appreciated.
Thank you
I'm not sure why you think this vibration is related to brake fade? Brake fade is a loss of braking power due to the build up of excessive heat. Braking for a pot-hole is not going to cause brake fade!

It would also help to know what is actually vibrating? Is it the brake rotor itself or the bike forks? Obviously if something is slightly loose (caliper, rotor, headset, wheel hub etc) that could cause vibration under braking. But you can also get vibration from one of the components resonating. For example I had an old mtb that used to suffer from a really bad rear frame vibration under heavy braking. It was simply down to the resonant frequency of the chainstays, along with worn suspension pivot bearings. Fitted new pivot bearings and the vibration magically disappeared.

The fact that you don't have this problem on smooth roads does point toward something being loose or worn with play. Loose/worn headset would be my first guess. Play in your front wheel hub second. Probably also worth tossing in a new set of front brake pads to see if that makes any difference. Make sure the rotor is clean, running true and not glazed too.
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Old 05-31-21, 06:42 PM
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Seems to me it’s likely fork flex. Bigger rider, hard braking, flex in the fork will result in some flex that will be sort of a stuttering that’s consistent with what the OP describes.

Take away, try to brake more smoothly,and if you get some stuttering with hard braking don’t worry about it.
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Old 05-31-21, 07:53 PM
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Originally Posted by base2 View Post
I hope you don't mind if I borrow your slogan (My lights are obscenely bright because drivers are dim.) I seem to find myself in need of it. I'll return it when I'm done with it, thanks,
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Old 06-02-21, 07:53 AM
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Originally Posted by Bigbus View Post
I hope you don't mind if I borrow your slogan (My lights are obscenely bright because drivers are dim.) I seem to find myself in need of it. I'll return it when I'm done with it, thanks,
Anytime. If I need it back I'll know where to go.
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My lights are obscenly bright because drivers are dim.

I shouldn't have to "make myself more visible;" Drivers should just stop running people over.
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