bres dad

Hello. This has been an annoyance but seems to be getting worse. I have an '84 Paramount than when braking (moderate to heavy) the front wheel, it tends to shudder, similar to a warped disc brake. The bike has RX100 calipers & newer (bought last winter brand new) Vuelta rims. The headset is tight (checked by the LBS), front rim is true, caliper is not loose & brake pads are tight and aligned, hitting about mid rim. I did replace the pads but the shudder was happening before I replaced the pads (the old pads needed replaced anyway). I even went as far as to put a different front wheel from another bike on it and experienced the same thing. I also grabbed the caliper arms and wiggled them to see if one of the arms or something internal on the caliper might be loose but if it is, I can't feel it (it feels the same as if I wiggle the rear or a brake on another bike, including another one with the same RX100 calipers.) Its getting pretty damn annoying. Any ideas?

hkhailey

When you properly aligned the pads, did you happen to adjust the toe in? I use a third hand tool and two pennies, one under the rear edge of each pad before I tighten the pads. I had an 84 Myata that I had to use two pennies on each side to get the toe in to where it wouldn't shudder and squeal.

Toe in is also the problem when the brakes are noisy and squeal. This may or may not fix your problem but it's something to try.

SquidPuppet

Have you checked the headset to be sure it's properly adjusted? What you are describing is a classic symptom.

bres dad

I'll double check the toe in however the problem existed before I swapped the brake pads.

I thought the headset seemed fine (no obvious play, no movement if I were to press the front wheel against a wall something similar) and the LBS checked it while the bike was in for something unrelated.

FBinNY 

The shudder is a harmonic effect similar to how you can make a wine glass sing by rubbing your finger around the rim at the right speed an pressure.

It needs two conditions to happen.

1- a variation in traction force, aka grip/slip which changes the loads on the various parts.
2- something that moves in response to the braking force, and can move to a stop, then slip back to the otiginal position. The movement can be at the headset, play in the arms, flex in tha arms that allow twisting with the forward motion of the rims, or even flex in the fork blades which allow the whole fork to flex back, then release and spring forward.

Many carbon forks are a bit flexy and can cause shudder even if all is dialed in, but save blaming the fork for last.

TEST for and locate any play, by applying the brake as you might when it shudders, and pushing the bike forward and down (to prevent the wheel from skidding). If you get any feeling of grip slip, or movement to a stop, find where it is by putting your fingers on the lower headset, the brake arms, and also feeling for twisting at the shoe.

One common cause of shudder is brake arm twist, especially with new shoes. As the rim is moving forward it wants to take the shoes with it, so they exert a twisting force on the brake arms. At higher braking force, the arm forces the shoe flat, but in the mid range, the shoe can twist momentarilly lifting the toe, and digging the heel in with an effect like new chalk on a chalkboard. We toe brakes in slightly to offset the twist, and prevent the heel from biting.

Ironically, grippier shoes are more prone to shuddering because the forward force at the rim;'s surface is greater realtive to the squeezing force keeping the shoe flat. But I'm not suggesting that you oil the shoes to quiet them.

Generally (not always) shuddering improves as the shoes brak in , because the heel corner breaks down. If they don't improve, or you don't want to wait, you can reduce shuddering by filing s "ski tip" ramp on the back corner of the shoe. You want to go about 1/16" deep and angle in over 1/4" or so. This eliminates the heel's ability to dig in, but will also reduce the shoe's effectiveness in the rain, so consider it a last resort solution.

BTW- it you have a carbon fork, and all parts are solidly tight, your options are limited, and you might want to use the rear brake as a descent control brake at nmoderate forces, and save the front for hard stops.

bres dad

I'll resume troubleshooting this tomorrow. Here are a few notes:

1. The bike does have new brake shoes (F & R) but the this problem existed before I changed pads. I'll check the toe in on it. I thought I toed them in but I could be wrong.
2. The headset was brand new when I built the bike up last winter (this problem started a few months ago). When I checked it, I hand tightened it then maybe 1/8 turn more (I don't remember exactly) so it would turn freely but not too tight. The LBS checked this when I took it in to get new handle bar tape (the shop owner of one of the two bike shops in town is a master at wrapping bar tape. I usually do my own but I hate trying to install Fizik tape so I took it in) and checked it. I can take it back or to the other shop across town and let them take a peek at it worst case.
3. I used the same brake calipers on another bike and didn't have trouble, but I can swap out the front caliper with a newer (off my 2013 Trek) and see what that does. The caliper visually doesn't have any bent or loose parts.
4. The fork is a steel fork, original to the frame set

FBinNY 

One thing you can try is acetone (real acetone, not nail polish remover) on a rag to do a quick wipe and remove any film on the brake tracks. Often variations in brake traction because of oil or wax films on the rims are the initial inputs that lead o resonance.

My commuter develops terrible squeal, with a bit of shudder the morning after riding in the rain. Fortunately the system is very rigid, so whatever causes it wears off quickly enough. If you have a garden hose, and a flat place to ride, you can soap up the rims with a concentrated dish detergent solution (not straight soap) and ride in circles dragging the brakes to burnish off any film. Then rinse complete and ride it dragging the brake a bit to dry it off. Then road test and see if anything changed.