Disc Brake Squeal vs Feeler Gauges and Shim Stock

Disc brakes are fickle things. They howl and squeal some times, they rub occasionally and are a pain when you swap between different wheels. I have had a lot of trouble with the brakes on my bike. They actually do the “stopping” bit quite well, but they’ve been noisy and fickle to adjust.

Finally, finally … I have them fixed and dialled in, but it took some Serious Engineering to get to this point.

Previously, I had just adjusted the callipers by the traditional method of squeezing the brakes whilst tightening up the calliper bolts. However, for some reason this was quite fickle on this bike (SRAM callipers). Sometimes the disc would still rub, and even when I got that to stop, the brakes would squeal .. really badly … in wet conditions.

Obvious things first: sand the pads, clean the rotors with isopropyl alcohol. Still squealing.

Next try new pads and lightly sanding the rotors AND cleaning them again with IPA. Then do a super careful bed-in process. Still squealing.

Take out the pads, pump the pistons in and give them a good clean with brake fluid, then work them in and out a few times to make sure they’re moving evenly and not being lopsided. Still squealing.

Next, try switching from sintered pads to organic pads. Small improvement, but still squealing.

What causes squealing? It’s when the pads stick-then-slip rapidly. In my case, at about 600Hz. And it turns out my spokes also resonate at about 600Hz, which makes things very loud. The pads shouldn’t really be stick-slipping. It can be caused by contamination. Or possibly by pads pressing at a slight angle?

I went back to align the callipers again. With the pads out, I aligned the centre-line of the calliper body with the rotor really carefully with a magnifying lens before tightening the bolt. Guess what – still squealing.

Next I deployed the big guns. The gap in the calliper body which the rotor runs through is 4mm (measured using a 4mm hex key) and the rotor is 2mm thick (measured using callipers). So there should be a 1mm gap on each side of the rotor – at both the ‘top’ and ‘bottom’ ends of the calliper. I took some feeler gauges, which are normally used for measuring valve clearances on motorbikes, and stacked them to get 1mm and used that to measure the gap either side the disc (with the pads out) to set the calliper position before tightening the bolts. What I found was that the calliper shifted slightly but forceably in one direction every time as I torqued up the bolts. This wasn’t very noticeable visually, but was obvious using the feeler gauges. After realising this, I loosened the bolts and readjusted until I managed to get the bolts torqued up with an almost exactly same 1mm gap on either side, at both “ends” of the calliper.

And the moment of truth: pads back in, and … the squeal has gone! I did a damp cyclocross race a few days later, no squealing at all. Result A victory for high-precision measurement.

Buoyed by this success, I also decided to tackle to problem of switching between wheelsets. Generally, the discs end up in slightly different positions on each wheel – so although front-wheel-A might run perfectly with no rubbing, front-wheel-B will rub. I’ve always just lived with this in the past when switching my one bike between road and cx wheels But I realised that I could shim behind the rotor on one of the wheels to get it to match exactly with the other wheel. Years ago, I had bought some plastic shim stock from RS, which is plastic of a precise/known thickness. With the ‘rubbing’ wheel in the frame, I cut small pieces of 0.5mm and 0.35mm plastic out to use as feeler gauges to measure the “gap” on the non-rubbing side of the rotor. The 0.5mm didn’t fit but the 0.35mm did, so I decided I needed to move the rotor by half of that, or about 0.2mm. The space behind disc rotors is a circle with id=34mm and od=38mm, so after a bit of time with a set of compasses and some scissors, I had a custom shim. Once reassembled, the previously rubbing rotor was running perfectly – and now I can flip between my cx wheels and road wheels without ever having to faff around with brakes.