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Old 09-11-11, 10:03 AM   #1
WL Weller
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Front Wheel Vibration

I purchased a Giant Escape 1 early this summer. It has aluminum linear pull brakes. Recently on a very steep decline, I applied the brakes and the front wheel started to vibrate tremendously, almost jumping up and down. I took it to my LBS they fixed it under warranty and replaced the front brakes with a different type. They said they were having some troubles with the Giant brakes. The problem went away for a while, but has now returned, although not as severe as before. I wondering if it could be poor quality pads. The rim is in good shape. I have put about 650 miles on this bike. Any thoughts?
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Old 09-11-11, 11:00 AM   #2
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check the cones ,they might be lose , also check for a lose headset . the axle may be broken too .
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Old 09-11-11, 01:04 PM   #3
WL Weller
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The LBS said they checked for those things. I'm going to bring it in next week and have them take a second look.
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Old 09-11-11, 01:31 PM   #4
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Assuming that the headset and hub are tight ant there's no free play anywhere, you have to look at what's happening at the brake shoes and rim.

The primary cause of braking vibration is shoe rotation. The moving rim pushes the shoes forward. Since the shoe is attached back from the point of contact, this creates a torque with the shoe trying to rotate toe out/heel in. This causes the heel to dig in and increase braking force, until it reaches a point that it slips back and starts the process anew.

You generally get the worst vibration at mid range braking force, where there's enough force to create shoe twist, and not enough pressure to force it flat. Perversely, better shoes make the problem worst because they have a high ratio of friction to braking pressure. Other things that make it worse are lack of rigidity in the brake arms, brake mounting studs or fork blades, or variations in the rim surface, or brake shoes lacking adequate toe-in.

Start with the easiest first. Wipe the rims down with alcohol (methanol or fuel grade alcohol, not rubbing alcohol) to clean off any film. In extreme cases, I trap pieces of Scotchbrite under the shoes and ride about 100 yards in a parking lot to sand the sufaces clean. Then check the toe-in. The right amount will cause the shoe to rotate flat when you push the bike forward against moderate brake force. You can also file a ski tip angle at the heel of the shoes so they can't dig in.

So far you haven't spent any money, but if that isn't enough the next step is to use long tailed shoes like Kool Stops, to increase the resistance to twist. After that it gets more difficult. You want to eliminate any slop between the brake arms and mounting studs. This isn't easy, but sometimes a shim washer helps, or possibly filing the stud a hair shorter (absolute last resort because it's irreversible). On some brakes I've found that using thick grease between the bushing and the stud or brake arm helps a bit.

Lastly there things that are beyond your control, like fork rigidity and the all important distance from stud to rim, which decides how forward force on the shoe translate to arm twist. One of the reasons we're seeing so much of this problem, is that the fork width has increased to accommodate wider tires, with no compensating increase in component strength. If you ever crash and bend this fork, look for a replacement with the studs offset to the inside.
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