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When I apply the front brake I can feel a "shudder" or "pulse" through the front wheel, fork and handlebars. It's faster when I'm travelling faster and slower when I'm moving slower.

I assume there's something wrong with the rim, causing the brake pads to catch (rim braking, not discs). But I've checked the rim carefully and can't see anything. There isn't any ledges and I can't see that parts of the rim are wider than other parts (although I haven't checked with calipers or anything since I don't have any). It's happened since the wheels were new. They're Charge Dish wheels. I've used multiple different brake pads and they have all had this problem (to varying degrees).

How can I diagnose and fix this problem?

Updates based on comments:

  • The bike is an SE Lager, about 18 months old. I'm running Conti GP4000S 23mm tyres.
  • I've spun the wheel and eyeballed the rim/pad clearance. I can't see any noticeable bulge, but I'm assuming that's what's causing the problem.
  • It's a large frame (I'm 6'3")

Thanks for the suggestions. If there is a bulge in the rim can I fix it, or do I need to replace it?

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1  
I guess you must angle the pads such that the back/tail/heel of the pad touches slightly before the front. –  ChrisW Sep 28 '11 at 2:53
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Have you simply lifted the wheel off the ground, spun it, and checked whether it remains equally spaced between the brake blocks as it spins? (Check on both sides, as there may be a wide spot in the rim. Also check that the wheel doesn't move up and down relative to the blocks as it spins.) –  Daniel R Hicks Sep 28 '11 at 11:18
    
What kind of frame/fork and what is the size of the bike? In addition to a loose headset, I have seen this kind of thing happen on some older steel frames and sometimes on larger frame sizes. –  user2491 Sep 28 '11 at 11:41
    
Remember to check for in/out as well as side-to-side. Moving in/out changes the apparent width of the rim, and also, if the rim moves "in" too much the tire can rub against the brake pads. –  Daniel R Hicks Sep 29 '11 at 1:20

6 Answers 6

up vote 4 down vote accepted

By your description, it totally seems to be some slight irregularity on rim surface, be it a bent spot, some fluctuation around valve-hole or seam, or some slight variation on total width.

I had the exact problem you described three times: after hitting a pothole too hard once, and after my bike fell off a moving car's bike rack thereafter.

Both times, it was some rim irregularity, and it was on the same rim. First time, I kept riding and the very rim wearing out with braking eventually solved the problem. Second time, I had to put a new rim, because the "pulse" was very intense and dangerous (fork flexing and all).

The third time I had this problem, I had a defective rim which eventually cracked on the side, and I felt a progressive worsening of this "pulse".

AND HERE GOES THE SOLUTION!

I noticed this irregularity caused an assymmetric MONLIGHT REFLECTION because I was riding at night, leaned the bike to the side, and was marevelled by the beautiful circle of moonlight on the shiny breaking surface of the rim. But there was a bent spot, pretty much invisible to direct observation.

So, I propose you go with your bike close to a shadowed wall on a very sunny day, and use the braking surface of the rim to reflect sunlight on this shadowed wall, optionally letting the wheel turn slowly, and repeating the procedure with the other side of the rim.

For sure, if there is a bent spot, the slightest one, you will see it.

You can increase the distance to the wall to make the effect more obvious, and you should expect some natural assymmetry. The "abnormal" thing to watch would be a very localized spot of aberrant light distrtibution.

EDIT: suppose you found the micro-bent spot, a good way to wear it away is to put not-so-expensive brake pads, go to your favourite muddy downhill track in a wet day (preferrably one where the weels completely sink in the mud), and use the brakes mercyless. Your rim will begin to shine!! (pun intended ;o)

Hope it helps!

(well, you could always turn the bike upside down, spin the wheel, close your eyes, and literally feel variations on rim surface with your bare fingers. It is not so sensitive as the sunlight method. Do it with caution!)

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Very interesting! The rims are matt so not super reflective but I'll definitely try this on the next sunny day. Apart from letting the brakes wear down any bulge, is there any way of fixing it? Or should I just replace the rim? –  Mac Sep 29 '11 at 0:56
    
You can also hold something (a pencil is good) against the fork (or tied to it with a rubber band) as you spin the wheel, with the tip of the pencil as close as you can make it without touching the wheel. This lets you see small variations in spacing. –  Daniel R Hicks Sep 29 '11 at 2:54
    
@Mac: I used to tap irregularities down using a thick wood stick and a hammer, but usually it does not work so well. If you are patient, you can use sandpaper, but you said your rims are MATTE! Look at my edit... –  heltonbiker Sep 29 '11 at 13:42

As Jason S said it could be your headset. Engage the front brake while standing by your bike and try to move your bike forwards and backwards. Do you feel play in the front end then? If so it's likely to be the headset.

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The pulse varies "speed" along with the bike, so it seems it is a periodical pulse unrelated to the act of braking, but to wheel spin WHILE braking. –  heltonbiker Sep 28 '11 at 14:08
    
This is definitely worth a check before toe-ing in the brakes and it does get worse at speed if loose headset-related. –  ʍǝɥʇɐɯ Sep 29 '11 at 0:34
    
@JasonS they're rim brakes. No suspension. –  Mac Sep 29 '11 at 1:06
    
I tried as you suggested and don't see any play in the headset. –  Mac Sep 29 '11 at 1:06
    
@JasonS I could put my old wheels back on - good idea. It does seem to be one shudder/pulse per revolution. Any suggestions on what to do if it is the rim? Any way to repair it? –  Mac Sep 29 '11 at 5:38

My home made trueing stand consists of the bikes frame and nylon wire ties (zip tie). By attaching the wire tie to the fork or the seat stay it can be trimmed so it is about 1/4 inch longer than the gap between the wheel and frame. If you rotate the tie you can get it to just touch the wheel. Rotate the wheel and check the gap or deflection of the wire tie as the wheel rotates,if you set the tie so it is at the edge of the rim you can also check for an out of round condition.Of course trueing the wheel gets more involved.

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  1. Check that the wheels are true.
  2. Like ChrisW mentioned, angle the pad slightly to make sure the brake pad doesn't touch the wheel all at once to have more leeway in the rim.
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Do you have any ideas how to check for true wheels? I've eyeballed them and they look okay, but I figure I must be missing something. I have set the angle as you've described. –  Mac Sep 29 '11 at 1:05
    
@Mac: You can easily build an improvised trueing stand. Raise the bike a bit so the wheel runs free, spin the wheel and hold something pointy close to the rim. Watch how much the distance between rim and pointy thing changes. If you feel sciency, magnetize your rim and use a coil to measure the change in magnetic field for extra fun. –  thiton Sep 29 '11 at 6:37

Some general things to check for:

  • Wheels are true
  • Wheel brake surfaces are clean (I use some automotive brake cleaner on a rag and wipe them down if I accidentally get oil on them)
  • Brake pads have some pad left (make sure they aren't too worn)
  • Brake pads are angled slightly as they contact the rim

Sometimes I've also taken a metal file and rake it over the brake pad surface a bit to scuff them up. Not sure if that would help your situation though.

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Good points. They're new pads and I've checked the other stuff you mention. I'm not sure how to check for 'absolute' trueness. I don't have a trueing stand. Maybe it's a trip to the LBS. –  Mac Sep 29 '11 at 1:06

Another potential explanation is the oscillating change in brake cable tension during braking.

This problem is more common on big frames. I'm 6'4" and I've experienced this on several bikes. Cantilever brakes can exaggerate this issue.

When you brake your fork is bending backwards in response to the force being placed on it. This can cause the cable tension to slacken slightly and the brake caliper to open causing a slip. This in turn allows the fork to return to normal and cable tension increases. Then this repeats.

On one bike I owned changing from a Avid shorty 6 to a wide Kore brake eliminated this problem. Using a more aggressive toe-in on the pads can help too.

Since your bike has a caliper brake you may want to make sure that your brake housing ferrels are still in good shape. Basically anything that might introduce slack into the braking system or subvert the compression resistance of the housing should be checked.

Good Luck!

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