Technical FAQ: Disc brake wear, fork twist, and more
Disc brake wear
You may have written about this previously because disc brakes have been around for quite a while, but I thought I’d ask because I can’t be the only one who has had this problem. When racing in wet and sandy or gritty conditions, I noticed that my disc brake pads wear extremely fast, which results in nonfunctional brakes for extensive portions of the race.
I recently did a rainy, 36-mile gravel race with pads that had less than 200 miles on them. Twenty miles into the race, my brakes were useless. I had not locked up my brakes or done any hard braking up to the point when they stopped working. I also had this problem a few months ago in a snowy and sandy ’cross race, during which my brakes were shot after the first 15 minutes. There was a lot of hard braking in that race, so it did not worry me as much under those circumstances. In both cases, I had to resort to some flintstoning to keep from careening off the course. Needless to say, riding 16 miles or half of a ’cross race without brakes is a harrowing experience.
Is this happening due to poor braking technique on my part (this is my first bike with discs) or is this something that is inherent to disc brakes? If so, is there anything I can do to prevent the brake pads from wearing so quickly? I also noticed that everyone riding cantilevers could slow themselves down while I could not, unless I put a foot down. Do cantilevers work better in these conditions and if so, why is the industry pushing disc brakes so hard, when they seem, in my limited experience, to have a very high rate of pad wear and essentially total failure in adverse conditions? While discs seem to offer superior braking when they work, the potential for total failure seems to offset any benefit that they may have when they’re actually working. I think this would be, or should be, a major concern for companies that are putting these brakes on their bikes. Or am I just doing something wrong?
It has everything to do with the type of brakes you have and the rotors you are using; it probably has very little to do with your braking technique.
As the pads wear on cable-actuated disc brakes like yours, they become ever further from the rotor until you tighten the cable again. [Ed. note: Paul confirmed in a separate email exchange that these are cable-actuated disc brakes.] You could keep tightening the cable whenever your brakes cease to work, but this is not possible on the fly on a road or cyclocross bike, because the only barrel adjuster on the cable is at the caliper. Mountain bike brake levers for cable brakes have barrel adjusters on them, so a rider could keep tightening these down while riding in the conditions you describe and keep having brakes until the pads grind through and are metal-on-metal (pad-backing plates against the rotors).
Conversely, hydraulic disc brakes are self-centering, so as the pads wear, their spacing to the rotor stays constant. This is because, as the pads become thinner, the pistons in the caliper continue to advance further out in their cylinders, and additional hydraulic fluid from the reservoir at the lever fills in behind them.
The piston moves forward further with each brake application because the pad is being ground down by grit on the rotor, yet it retracts the same amount each time; thus, it won’t return to the same depth in the cylinder. A square-cross-section o-ring seal around the waist of the piston that sits in a groove in the cylinder wall twists within that groove as the piston advances. That “square seal” retracts the pad by then returning to its original shape, thus bringing the piston back with it, but always the same distance. Obviously, the pads can still wear out with hydraulic brakes until you’re hitting their backing plates, but you’ll have brakes right up until the time when you start scraping your rotor metal-on-metal.
In addition to using hydraulic brakes in the conditions you describe, you should also use a solid rotor without holes in it. Grit dragged around and around by the holes in your rotors is grinding your pads down.
And yes, in those conditions, cantilever brakes will be better because the stuff is too thin to clog them up. But in sticky mud, cantilevers will get clogged up and experience high pad wear, while riders with disc brakes will be smiling, as their brakes will be working great and wearing slowly.
Front wheel twist
The other day I was checking a headset on a road bike with disc brakes to see if it was loose by applying the front brake and rocking the bike back and forth. While doing that, I noticed the front wheel “twist” in the fork a few millimeters toward the rotor side when the wheel/bike had forward pressure on it (wheel was fully secured and has a thru-axle). It got me to thinking, how much would it shift at speed and under a full load, and to what effect does it have on the handling and tracking of the bike? Have there been any tests or reviews done on this? It makes sense that would happen, given the calipers and rotors aren’t centered on the wheels, therefore pulling on the wheels at different amounts from the left vs. the right side of the hub. Any insight and thoughts?
Reply from FSA:
What JR is seeing is the compliance (“twist”) of all the front-end components on his bike. You may recall when mountain bikes started using disc brakes this was also very easy to see, as the fork legs, steer tubes, wheels, hubs (axles) and most importantly the quick release skewers were all designed for rim brake applications. Since that time, all of these components have become much, much more robust and stiff.
Now that we are putting disc brakes on road bikes, we are again able to easily see the “twist” in the front end of the bike. To compound the situation, road bike forks and wheels are, in many cases, optimized for light weight, making the twist even more obvious. Additionally, road bike tires are able to generate higher traction on pavement than knobby mountain bike tires on loose dirt can (aside from MTB tires on slickrock). This again amplifies the “twist.” This all changes how the front of the bike will track, and the result is a decrease in performance and predictability.
I have not seen any published testing or studies on this subject. However, bicycle and component manufactures are well aware of these circumstances. Many of the abovementioned components have and are already moving toward reducing the “twist.” When executed properly, the thru-axle (12mm or 15mm) greatly reduces this affect. Fork design and larger steer tubes also contribute. Wheels with higher spoke counts and/or higher and more balanced builds play a part.
— Joel Richardson
FSA disc-brake product manager
Feedback from last week’s column
I’m the new marketing director at Cane Creek Cycling Components. I wanted to provide a quick clarification. In last week’s Technical FAQ, Lennard Zinn suggests eeBrakes to a reader as a solution to tire clearance issues with dual-pivot brakes on his bike.
In his response, Lennard refers to an older version of the eeBrake with an eccentric mounting bolt. Newer versions of the brakes do not use this type of bolt — however, the brakes may still work for the reader.
Please see the clarification from eeBrake creator Craig Edwards below:
The eeBrake has a unique arm design in which the tire clearance does not change with the rim width adjustment. The old version of the eeBrake used the eccentric mounting bolt. Newer versions do not. The new versions of the eeBrake and the old one on the high eccentric bolt setting have about the same tire clearance.
— Luke Bukoski
Director of Marketing
Cane Creek Cycling Components
The eeBrake I photographed and have used for years is indeed the original version, before Cane Creek bought the rights to them. I contacted Craig Edwards, and asked him this question, “So does that mean that you have increased the length of the oval hole on each brake arm to slide the pad up and down further, since you don’t have the adjustability at the center bolt?”
Edwards replied, “Yes, the newer eeBrakes have all of the pad height adjustment in the pad holder slot. The clearance dimension for the centerline of the mounting bolt to the underside of the eeBrake bridge is very close to the same on all eeBrakes. Note: this assumes the clearance measurement is taken in the max. Clearance position (high position) for the eccentric-bolt eeBrakes. With the eccentric bolt in the low position, this version of the eeBrake has less tire clearance than the other settings and eeBrake versions.”
As I said in my original answer, I believe that an eeBrake might fix the reader’s question. The front arm of his front brake caliper was dragging on his tire, and I believe that the eeBrake, be it the new version or the old version with the eccentric center bolt flipped up, will have more clearance than his and many other dual-pivot brakes, as their front arms tend to sit lower than the section of the brake surrounding the brake center bolt.
If your suggestions for adjusting the front brake doesn’t work, perhaps he could add a spacer (a few millimeters) between the front brake and the frame. The brake hole is most likely not parallel to the ground, so moving the brake out (or back) a couple millimeters could increase the tire clearance.
Assuming the brake bolt is long enough to accommodate a spacer (and there isn’t too much flex), this could be a really easy fix.
Those of us who like to run 28mm tires on traditional road frames have been wrestling with the question of tires clearing a fork only to rub against the brakes for years.
There is variance across models, but when the brake mount is lower, Shimano’s traditional brakes have a meaningful millimeter or two more clearance when compared to the other brands. As one with more vintage inclinations but who prefers dual pivots on mountainous descents, I have found this to be true with both the 6500 and 6600 brakes. I have heard later models are even better.
On one bike, switching to Shimano brakes (from Tektro) was the difference between running a 28mm Durano cleanly and rubbing the top of the tire every time I got out of saddle to sprint or climb. Before venturing into more expensive territory, I would start there.