Ok… I need some help on my new bike, when you put on the front brakes it has a pronounced vibration. Enough that when you apply the brake it makes you lose all confidence
This is what has been done so far…The headset has been tightened, taken out and re installed to make sure it was correct. There is no play in the headset that we can (bike mechanic) find. The wheels are Bontrager Race X lites and have been checked and checked with no obvious issues. We even tried 3 other kinds of wheels but the vibration is still there. The carbon fork shows no stress fracture either. The bike is solid as any bike I have been on except for when the front brake is applied. The bike has the new 7800 Dura Ace Brake calipers that have been checked over and over to make sure they are tight on the fork. Someone mentioned for me to replace the caliper completely to see if the caliper itself is bad. The guys at the bike shop are thinking it is the fork vibrating, but the manufacturer says no way and that they have never had this be the issue. Any help you could give would be great!
last thing i would check is that the front of the brake pads are touching before the the rear of the pad. This done wrong (might?) cause a vibration. Otherwise I have not idea. Try new pads.
First off, let me acknowledge your poise and grace for being courteous enough to not reveal the brand of the bike. Since this may be any number of problems possibly unrelated to the brand, you were gracious and fair by not saying what brand it is. Good for you. A lot of us could take a lesson from you in forum etiquette. Well done Sir (Ma’am).
Well, I look at things like this from a checklist perspective and it sounds like you did also, so good for you and your bike shop- who sounds like they know what they are doing. For the benefit of other posters, this is how I would begin the diagnosis: Is the front wheel firmly clamped and centered in the fork dropouts? Once clamped as high as possible in the dropouts, is the wheel straight in the fork with equal distance between the wheel and the fork blades on each side? Is the quick release skewer functional? Is the rake and trail of the fork appropriate for the frame according to the builder? When the front brake is applied and the front tire is on the ground, and the bike “rocked” gently forward, is there any perception of movement at the head tube/fork steer tube junction? When the above test is performed does it produce excess visible fore/aft deflection of the fork? Does it produce “clicking” or “creeking” sounds? If the front end of the bike is gently dropped down onto the front tire from about 2 inches off the ground- gently mind you- does it produce any unusual noise? Does anything sound loose? Is the down tube of the frame, at the 6 O’Clock position, dimpled, rumpled or deformed in any way? Be sure to inspect the length of the underside of the down tube. The frame could have been subtley damaged if the shipping carton were dropped on its front in transit. Is the frame, welds or lugs dented, dimpled or deformed? As viewed from above, is the hole in the fork where the brake caliper bolts to the fork perpendicular to the crown of the fork? Removing the caliper and inserting a medium sized Phillips screwdriver into the hole will act as a gauge to verify the hole is straight and, therefore, the brake is straight also when mounted. As another poster correctly mentioned, is the “toe” angle of the brake pads correct, with the leading edge of the pad contacting the rim just slightly prior to the trailing edge?
If this examination/trouble shooting checklist produces nothing irregular then you may just have one of those frustrating circumstances where a conspiracy of subtle, not easily detected things are creating an odd vibration that can’t easily be elminated. some times the way the material in the fork responds to energy and the way the frame responds are different enough that it produces some difficult to explain phenomenon. A solution to this frustrating mystery could be a new fork with a beefier crown/blade assembly.
To a degree, this is the argument *for *integrated headset and forks. When the front end is made together as a unit it works together as a unit- a unified, optimized, integrated front end that works well and has no mysterious problems.
At any rate, good luck with this. I have had this before and, I am sorry to tell you, I replaced the frame it freaked me out so bad. Do not for one minute feel “whimpy” about bailing on the frame. If it were a Formula 1 race car (and it is the bicycle equivalent) it would be scrapped in a second if it couldn’t be diagnosed and fixed.
Thanks everyone for your suggestions. A quick update! It did end up being a problem with the front brake. Not sure if this is true or not, but the Shimano rep confirmed your thoughts when talking with my bike mechanic. They replaced the pads made a couple of adjustments and it is flawless now. The Shimano rep said that more than likely no one cleaned the new rims and what little oil residue that was left contaminated the pads. Again not sure if this is true but what the heck, it is fixed.
“If this examination/trouble shooting checklist produces nothing irregular then you may just have one of those frustrating circumstances where a conspiracy of subtle, not easily detected things are creating an odd vibration that can’t easily be elminated. some times the way the material in the fork responds to energy and the way the frame responds are different enough that it produces some difficult to explain phenomenon. A solution to this frustrating mystery could be a new fork with a beefier crown/blade assembly.”
Although the culprit has already been identified…
These mysterious phenomenon you are referring to, Tom, are often resonant frequencies…every material and structure has a resonant frequency…while the designed product may not have resonated…variances in mfg could potentially create a situation where the part, or combination of parts set up a resonant freq (or less likely, a harmonic of the resonant freq) that interferred with the proper function of the bike. Of course, this would happen at a certain envelope of speed and/or braking…usually these things are additive such that no one part is fully responsible for the situation, thereby making them, as you say, difficult to diagnose…but changing one piece in the chain, say the fork, or wheel, or even frame, cures the problem…but not for any of the visibly obvious reasons…
Our resident structural engineers should feel free to poke my probably imprecise description here…I’m drawing on decade and a half old physics knowledge…but Tom’s F1 reference made me remember a recent discussion of a racing team having to scrap a particular body because it had peculiar resonant issues that just happened to coincide with a particular race speed range.
A little preventive stuff to reduce the chance of this happening again… careful when lubing the chain, a little bit will grease the rim and brake pads again.
Windex and a clean 3M type abrasive (non-metallic and no sandpaper type stuff) pad to scrub the rims and brake pads periodically. This should go a long ways towards preventing a repeat.
Jay
hmmmm…actually it may have been the Armor All Tire Foam I sprayed on my tires to make them shine that did me in! Kind of hard to control the wind when you are spraying away!
