Technical FAQ: Brake Dragging on the tire
Diagnosing front brake rub
I just got a new(ish) race frame and was moving my mix of Campagnolo Chorus over to it. The vertical tire/fork gap is very small on this frame — about 2-3mm with Conti 4000S2 25s. To get myself fit on the frame (steerer length, etc.), I put the wheels on, and, lo and behold, the tire was rubbing on the top/inside of the front dual pivot brake (2012 Chorus), as if the caliper bolt was set too low. The rear brake was not an issue.
It does not look like the brake hole is misaligned. Are there other brake options that would work with my Campy levers and not sit so far below the mount point?
Is it the front or rear arm of the front brake caliper that is rubbing on the tire?
It’s the front arm at first when the caliper is loosened to fit the front wheel. When the brake is depressed, the shape is somewhat between a U and a V and does not allow much room for a wide tire.
By the way, I now recall my teammate has a similar issue with his Trek Madone (circa 2010/2011) and 25mm tires on the front (Especially Conti GP4000s, which we all know are a little wider than other brands). If I recall, he has SRAM, though.
With a standard dual-pivot brake, there may be no way to avoid this, but I do have some things to try. I do think there is one non-standard brake that might work, though.
I measured the Campagnolo Record front brake calipers on two of my bikes, one the current style with the split front arm (like your 2012s must be), and the other the old style with a solid front arm. Both of them seem to measure about 7mm from the center of the brake center bolt to the underside of the rear arm. I also measured a couple Shimano brakes and got about the same result. My guess, without doing exhaustive measurements of various brakes, is that the amount of material on the brake arms surrounding the center bolt is not going to vary much from brand to brand and model to model.
That said, on both the Campy and Shimano brakes, the front arm hangs lower than the rear arm. It appears that if the tire is just barely clearing the front arm, you could make the front arm scrape on the tire by opening the brake further (releasing more cable). In other words, as you pull the lever, the front arm lifts higher away from the tire. Since the front arm is the problem, I would have told you that tightening your brake cable to reduce clearance from pad to rim would alleviate your problem. However, you mention that the consequent narrower “between a U and a V” shape would rub a wider tire, and I assume you mean that would affect your tire. If that’s not the case and you can get clearance with more cable pull, you could accomplish this setting with thinner brake pads (old ones or new ones cut down or ground down on a belt sander), a narrower rim, and/or running the brake with less free play in the lever before the pads hit the rim.
If those ideas don’t work, one option might be the superlight eeBrake. A unique feature of an eeBrake is that the brake bolt ends in an offset cam. In order not to greatly alter brake leverage with pad position, there is minimal up/down adjustability of the pads on the arms; you adjust brake pad height largely by rotating the cam either up or down. The caliper clamps onto the cam with a pinch bolt. This means you can raise your entire brake caliper relative to the tire by rotating the cam up. With the cam rotated up to lift the caliper maximally, I measure the distance from the center of the bolt to the bottom edge of the caliper to be about the same 7mm as the Campy and Shimano dual-pivot brakes that I measured. However, unlike standard dual-pivot brakes, there is no front arm on an eeBrake that goes up and down with the pull of the cable, and there is no variation of the brake’s height with changes in rim width. My guess is that if only the front arm is dragging on your tire, an eeBrake will fix your problem. It’s pricey, but it’s half the weight of your brake, stops as well or better than any standard dual-pivot brake, and costs a lot less than a new fork, which would have been your other option besides using a smaller tire.
Feedback on last week’s column
I read your Tech FAQ about the science of tire pressure. I’d just like to point out that except for being geometrically related to area, tire volume is irrelevant. Area is what determines hoop stress, and it’s that tension in the sidewalls that holds the rim off the ground and the cross-sectional radius of a tire that determines the compression vs. force of a tire against the ground.
I see a lot about the benefits of high-volume tires, but it’s not the volume that’s relevant. It’s the fact that the tires have greater area and width.
I wondered one other thing in regards to hoop stress and my mountain bike rim that split down the middle. When I look at my road bike tire, the tire comes off the rim wall at kind of a 30-degree angle at most, so I feel like the bulk of the stress on the rim is down low in the bead hook area. But the mountain bike tire, being so wide relative to the rim, comes off at kind of 60 degrees or more. So beyond the hoop stress, there is an almost orthogonal force from the tire wall pulling across the top of the rim wall. That is trying to split the rim apart, which happened to mountain bike rim. I wonder if that, more than the hoop stress, is what did in my rim.