Bike pulses while applying the front brake. I'm unsure what the cause is

Question

Once per rotation of the front wheel, when braking, the bike pulses under me. The brakes are calliper rim brakes.

1. Swapping wheels out fixes the problem - so it is definitely wheel related
2. The wheel is true
3. I can't see or feel anything different on the brake track.
4. I have tried cleaning the brake track with degreaser and rubbing alcohol with no luck
5. I have also sanded the brake track to try to remove anything that could be on there.
6. I have measured the rim all the way round - exactly 19mm rim width the whole way around.
7. The pulsing does not occur when the brake goes over the rim joint.

And it's still happening.

Any suggestions?

Answer 1

Measure the rim in a more accurate way for starters. Even 0.1mm can cause a change in the braking power.
Also, with the bike resting on its handles and seat, flipped over, ask someone to lightly hold down the front brake while you slowly turn the wheel and figure out exactly where the trouble spot is and how big it is. If there is no visible difference, and you don't have the means to measure the rim more accurately, then it probably is the width of the rim being uneven. On the other hand if you find that it does not occur at the same spot every time, maybe the problem is elsewhere.
If you can, see if the width of the rim changes radially, fatter near the tire and thinner near the spokes. If the difference in this width isn't constant, it could be causing the problem

Answer 2

If those wheels have significant mileage on them, you should worry that the rim wall (and braking surface) has become too thin and is bending under the pressure of the pads. Remove the tyre and measure the sidewall thickness with calipers all around the wheel. If part of it is measurably thinner than the rest, or if any part is below 1mm, it might be advisable to discard the wheel before the sidewall gives way and causes a bad crash.

The short video and forum thread might be useful.

Answer 3

I had this occur, and it was a monster of my own doing. The first thing I would look at is where you sanded the area that you assumed was somehow unevenly being grabbed by the brakes. I sanded a rim once, and I was careful to boot, but it created this incredible grabbing that was far worse than it was before.

The next thing I would look at would be the alignment of the rim brake calipers. be sure the mounting nut is secure and tight,and that the brake pads sit roughly equidistant from the rim.

When I true a performance road wheel, I settle for a single mm of lateral play. Not one of my customers has complained nor can I tell the difference. I doubt that it is an alignment issue. Pro mechanics go for 0.5mm lateral play, but hey...there professional racing mechanics.

And one more thing to check is that there isn't a loose spoke. Wheels are very strong and can survive a loose spoke. That spoke will compress excessively and loosen moreso at the top/bottom of the wheel rotation. This will create a temporary warp in the rim, and will also occur very close to the brake pads contact point on the rim. I had a set of Zipps that did that and it drove me nuts till I caught it during an inspection.

Hope it helps. Good luck!

Edit: Another weak point on a rim is where the valve stem protrudes. Does it occur there?

Answer 4

You need to toe the brake pads in. The pads are hitting the rim unevenly and causing vibration. I have had to do this many times on my CX bike. The pads need to hit from the front of the pad first, this reduces the chance of vibration occurring. Also using Swissstop pads will help greatly.