Technical Q&A with Lennard Zinn: How to stop cyclocross brake chatter

Published September 8, 2010 05:40PM

Editor’s note: Because of the number of requests, we are repeating Lennard’s column from last fall about brake chatter on cyclocross bikes. Enjoy.

Dear Lennard,
I read with great interest your response to a question a few years ago to someone who had written you about a bad brake shimmy problem they’d developed with a ’cross bike and cantilever brakes.

Though he was using a steel fork, I’m having very similar, and incredibly severe, brake stuttering myself, and I’m riding a full carbon WoundUp ’cross fork with Avid cantis.

It gets so harsh that the entire front end can end up bouncing across the pavement. It feels like the pulse you get in the brake pedal of a car equipped with anti-lock brakes. It’s very difficult to slow the bike down in a straight line.

I’m scared to death to try and use any front brake while cornering, for fear the front wheel would bounce out from underneath me. Would that brake booster you mentioned help relieve my problem? Beyond that, what would the next step be in trying to eliminate or reduce the severity of the problem?
Eric

Dear Eric,
Your question is a recurring theme in emails I’ve received over the years about ’cross brake chatter. I’ve selected your question to answer not only because you state the severity of the problem so well that we can all visualize your terror, but also because it reveals that I answered that question back in 2004 so poorly.

Now, five years down the road, I’m much better equipped to answer it because I have been racing ’cross myself this season and have encountered the exact problem you wrote about. It took me all season to fix it completely, although I improved on it enough along the way to be able to race that bike without problems in dry conditions by altering my braking style, and in wet sand and mud, the problem went away.

Other than dabbling in a race here and there over the past 29 years, my last ’cross race that I actually trained for and worked to get my bike set up for before this season was ’cross Nationals in Colorado Springs in December, 1980 (or was it January 1981?). I do remember having shudder problems back then, but they seemed minor relative to what I was experiencing this year.

My red bike in Photo 1, which has an Easton EC90X full carbon fork, had a terrible shudder, and I know other riders with the same issue with that fork. But it is not just the Easton fork and the WoundUp that you mention; people have written me with the same complaint with Reynolds, Cannondale, and Winwood carbon forks, steel forks, and proprietary carbon forks that come with frames of the same brand.

In my case, my nearly-identical second (the black one in the pictures) bike, equipped with an Alpha Q ’cross fork, has no shudder, other than when I really try to make it shudder by slamming on the front brake on dry pavement at over 40mph. (Editor’s Note: Please do not try to replicate Mr. Zinn’s experiment.)

At the beginning of the season, both bikes otherwise had exactly the same magnesium frame, same IRD front cantilever brakes, with stock (long) pads with threaded posts, and both had the same pad toe-in or about 2mm end to end. I interchanged wheels between bikes, and the Easton shuddered with all of my wheels while the Alpha Q did it with none of them. I got a number of sets of carbon wheels, so I switched the front pads on both bikes to yellow Swisstop pads, and the story remained the same.

My first improvement was to call Sean Coffey, brand manager at Easton, who races ’cross and wrenches for a team as well. He suggested shortening my brake pads, an idea he had gotten by looking at the Richard Sachs team bikes, which he said had incredibly tiny pads.

I cut my pads and holders as short as I could from the back end, and, combined with a lot of toe-in, reduced the chatter enough that I could race on it without feeling like I was putting myself or other riders in danger.

I should add that tightening the headset until it was on the verge of binding did make a tiny improvement as well.

Why does shudder occur?

Brake shudder is widespread because it’s built into the design of almost all ’cross bikes; it’s inherent to the design of a center-pull cantilever brake. To understand the reason why it happens and why reduced pad size, lots of toe-in, and a tight headset help take a look at the chart titled “Brake Shudder in cantilever brakes.”

As the brake is applied, the ground applies a force directed backward on the tire as shown, causing the fork to flex backward. Problem is, the brake cable is fixed at one end at the brake caliper and at the other end at the cable stop above the headset (as you can see in my case, at a cable hanger attached to a bolt on the stem face plate).

Think “bow and arrow” and imagine the fork between the cantilever bosses and the top of the headset is like the bow, and the cable is like the string. As the fork flexes back due to braking, the cable tightens like the string in the bow, because its two ends – the cable hanger and the brake calipers, have moved further apart. So even though you may have pulled the brake lever carefully enough to modulate it properly, as soon as the pad slows the wheel down, the fork flexes back and tightens the cable, which in turn pulls the pads harder against the rim. This in turn flexes the fork back further, which tightens the cable more, which pulls the pads harder against the rim, and so on.

Eventually, something has to give: Either the tire must slip on the ground, the rider must go over the handlebars, or the pads must break free from the rim. It is the latter that creates the shudder, the pads bind and release, bind and release, each time allowing the fork to flex back and forth and the tire to roll and stop, roll and stop. This is why the problem goes away in mud and wet sand, because the pad can break free smoothly. It is also why smaller pads with more toe-in help.

If the headset is loose, the problem is greater, because the length change between the brake posts and the cable stop atop the headset is greater as the fork moves back when the brake is applied.

Now you can understand why the advent of suspension forks with the cable stop attached to the brake arch bridging between the fork legs improved braking performance on mountain bikes with center-pull cantilever brakes – the arch with the cable stop took the steering tube’s flex out of the equation. And you can see why my answer from five years ago of a brake booster would not have worked unless the booster had a cable stop on it.

Almost certain to eliminate shudder is to affix the cable stop at the fork crown. The fork could still change length a bit between the crown and the brake bosses, but the length is very short, and there are two fork legs, both generally wider in a fore/aft dimension than the steering tube, so the fork flex would probably not ever be an issue. You can see three versions of a fork-crown-mounted cable hanger in the photo gallery.

Using one of these hangers will solve many riders’ shudder issues, but it won’t help those with full carbon forks. The problem with all of these hangers is that they require a hole drilled through the fork crown. This is not a problem with a steel fork or a carbon fork with an aluminum crown like the WoundUp, but I imagine your warranty would be void if you were to drill a hole through your crown on a full carbon fork.

You can see from the photos taken in the snow that I solved the problem on my red bike by using a front V-brake (a.k.a. “linear-pull cantilever brake”). The V-brake completely takes fork flex out of the equation, but you probably know that you can’t just run a standard V-brake with a road brake lever because it won’t pull enough cable and it will have too much leverage. I replaced my front brake lever (I run a single chainring with double chain guards, so I don’t have a dual-control lever on the left) with a Cane Creek Drop V lever, which pulls more cable and has less leverage than most road levers. Another option is to use short V-brakes designed for use with road levers.

Close inspection of the picture will reveal that I have installed SRAM road pad holders with yellow Swisstop road pads to reduce the area of pad contact to improve brake modulation. It still stops very fast and squeals on carbon rims, but there is no shudder, and brake performance is perfect (and silent) on aluminum rims.

The other way to eliminate the problem and still use the traditional ’cross center-pull cantilever setup is to get a fork that is very stiff fore-aft. As you can see below in the letters from Jochim Aerts, owner and founder of Ridley, perhaps the most sought-after ’cross bike today, and Edwin Bull of Van Dessel, this is the method that they have employed.
Lennard

From Ridley

There are three things to avoid this:
1. Most in important is the frontal fork stiffness . We saw this problem on our bikes in the past. Therefore we design already with the Python fork an oversize crown area.

Back then it was the best fork in the market, but still in some bad conditions the shudder came even on this fork. Therefore, we developed the new Oryx fork. With the 1-1/2” crown we could make the frontal stiffness 60 percent more than the Python cross fork.

2. Pad position under a small angle (“toe-in”): Always look that the brake pads touch the rim first at the front of the pad .

This is a short explanation the solution of that problem and the reason why we changed three models to 1-1/2-inch forks. Only the Crossbow (price-point bike) is still with the 1-1/8-inch steerer, but here we build a standard alloy/carbon fork that is heavier so that also this user doesn’t face the shudder problem.
Jochim Aerts
Ridley Cycles

From Van Dessel

In regards to ’cross bike fork shudder: In the past we used forks from several manufacturers on our cyclocross bikes. We tried most that were available to us, and were generally let down by the performance – i.e.: shudder.

We had been experimenting and testing 1-1/2-inch tapered forks on our road bikes with great results. The tests on the tapered road forks went so well that we immediately saw the need to translate this upper crown stiffness to ’cross bikes. After meeting with representatives from MOST major aftermarket/OE fork manufacturers, and receiving ZERO interest in 1-1/8-inch/1-1/2-inch tapered forks, we set out to develop our own.

The results is that after developing both road and ’cross 1-1/2-in tapered forks we have been using the 1-1/2-inch forks on our road bikes since 2007 and last July we started using our own tapered 1-1/8-inch/1-1/2-inch carbon ’cross fork on our Gin & Trombones production bikes.

We have been extremely happy with the results – especially in the cyclocross application. While fork shudder has not 100 percent been eliminated in all conditions (wet with fresh tubular glue on the brake surface), we could not be happier with how the forks handles and performs, and in most conditions fork shudder has become something we remember once having to deal with. The difference over the 1-1/2-inch forks and what we were using previously is like night and day – and these were the “top of the line” ’cross forks used on the vast majority of high end production ’cross bikes.

In ’cross, the 1-1/2-inch tapered fork provides extra stiffness between the steerer and the brake pivots, which has greatly reduced shudder – and greatly increased control. The lower part of the fork is much ‘softer’ allowing for flex to absorb rough terrain.