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Bright future for disc brakes fades briefly under a coating of Verona mud

  • By Lennard Zinn
  • Published Jan. 12, 2013
  • Updated Jan. 16, 2013 at 3:56 PM EDT

VERONA, Wisconsin (VN) — The bright future of disc brakes in cyclocross faded briefly under a patina of watery mud at the USA Cycling Cyclocross National Championships, as early competitors found themselves brakeless after a lap or two of sloppy racing going into the weekend.

The rapidly changing conditions in Verona reportedly meant fewer braking issues as the mud thickened on Saturday.

And temperatures in the teens may sound the all clear for Sunday’s elite racing, as the course is expected to be frozen solid.

But for many an earlier rider who banked on discs for his or her championship events, the unexpected problems proved a very cold shower indeed.

Two laps, no brakes

John Bliss of Team Kappius got a wake-up call before his championship race when his Avid BB7 brakes quit working after the first two laps of the 40-plus non-championship race on Wednesday.

Bliss said he had been using organic pads — “the same ones that I’d used all season” — but found himself forced to scour area bike shops in search of sintered metallic pads, which offer greater durability at the expense of initial braking bite in dry conditions, shorter break-in, and louder braking noise.

He came up empty-handed until one shop was willing to pull the sintered pads off of a tandem on floor display. Problem solved?

Nope.

At the end of the second lap in the 55-59 race on Thursday, Bliss said, he could pull his brake levers all the way back to the handlebar without slowing the bike. The course had become even muddier and slicker, on top of solid ice, since the previous day’s race.

A brief disc-brake primer

Cable-actuated brakes constitute the vast majority of disc brakes used in cyclocross, due primarily to the absence of hydraulic master cylinders built into road dual-control levers until recently.

The models most commonly used in cyclocross are the Avid BB7, followed by the Hayes CX-5.

Disc-brake pads are very thin, on the order of the thickness of a nickel when new, and their spacing from the rotor is very close. And cable-actuated disc brakes generally fade sooner with pad wear than do hydraulic ones.

There are two reasons for this. The most important is that they are not “self-adjusting” like hydraulic brakes. As the pads wear, there is no system to automatically either take up additional cable slack or reposition the pads closer to the rotor.

The other reason is that most cable-actuated disc brakes have only one moving pad.

The brake cable pulls a rotating arm that drives the outboard pad inward by means of ball bearings rolling in curved, tapering tracks. The moving pad pushes the rotor over until it contacts the stationary inboard pad.

When the lever is released, the outboard pad returns by means of a spring, and the rotor springs back to running straight up and down between the pads. This means that the outboard pad hits the rotor sooner and thus wears faster than the inboard one, and it also means that the loss of pad material results in a faster loss of braking than if the cable were pulling both pads equally toward the rotor.

As the pads wear, the user can simply turn the adjustment knobs on each side of the caliper clockwise to move the pads closer to the rotor. This can be done until the pad thickness has reached critical limits, but it can only be done while the bike is stopped; the rider can’t adjust braking on the fly.

Hydraulics self-adjust

A hydraulic disc brake, on the other hand, will continue to offer braking as the pads wear until the pad material is gone.

Hydraulic brakes generally have moving pads on both sides of the rotor and are self-adjusting. The pads wear evenly and have double the movement because the same amount of lever movement results in both pads moving as far as the single outboard pad moves in a cable-actuated setup.

As the pads wear, the brake automatically compensates by taking more fluid from the reservoir at the master cylinder to fill the extra internal volume created by the pistons moving further out in their cylinders to get the rapidly thinning pads to contact the rotor. This continues as the pads wear down, and fade generally does not happen in cold, wet conditions until the pad material is completely worn off of the pad backing plates.

Pad materials

Of course, the pad material is still thin, meaning that riders could still lose their brakes over the course of a race in these conditions. There have been plenty of muddy and slushy mountain-bike downhills in which racers have gone through their pads in the course of a two-minute run, even with top-of-the-line hydraulic brakes with four pistons, huge rotors, and bigger pads with far more surface area than road disc pads.

The three general types of disc-brake pads are:

• Organic (a.k.a., “resin”) pads made of tough, heat-resistant synthetic fibers like Kevlar or Twaron (both para-aramid fibers) bonded together with a petrochemical resin.

• Sintered metal (a.k.a., “sintered,” “metallic,” or “sintered metallic”) pads, which are metallic particles (generally copper alloy) fused together under heat and pressure; they may or may not have some other ingredients as well.

• Semi-metallic pads made of steel fibers mixed with other fibers in a ratio of up to 50-50. These are less common in bicycle applications than in motorcycling.

Change of pads, same result

David Weber of Team Kappius installed Jagwire Extreme sintered metallic pads specifically for his 45-49 race on Friday.

“They didn’t help,” he said, noting that he had to take a pit bike on every lap, since the pads on his Avid BB7s had worn enough that he could pull his levers all of the way back to the handlebar with no effect.

His crew screwed in his adjuster knobs during each bike’s sojourn in the pit, so he had good braking initially on the subsequent lap, but braking effectiveness would be gone again by the end of the lap.

The braking issues probably cost him a top-10 result, since he finished in 11th at the same time as the 10th-place finisher and lost at least 20 seconds over the course of the race by pitting four times in a five-lap race.

Weber said he loved the performance of his BB7s during the Colorado cyclocross season, which was a dry one.

“They work better than my road bike brakes,” he said. “I used to be amazed at how much better my road bike brakes worked than my cantilevers when cyclocross season ended, but now when I switch between my road and cyclocross bikes, I’m surprised at how bad my road brakes feel!”

No adjustment on the fly

Weber is not ready by any means to give up on discs, because he has felt they’ve been an advantage all season — until this race. Still, he bemoans the absence of an adjuster at the handlebar with which the rider could take up cable slack by turning an adjuster knob a click or two.

There is an argument for why both Avid and Hayes recommend that pad wear be taken up with the adjuster knobs rather than by tightening the cable, since it changes where the ball bearings are in their tracks when braking commences.

But when the alternative is no brakes at all, there is reason to wish for the fix Weber seeks.

Weber does note that a long Torx T25 driver “is worth its weight in gold here.” That’s because the BB7’s wheel-side pad-position-adjustment knob is very hard to turn when reaching through the spokes by hand, but there is a Torx T25 hole in its center to facilitate adjustment by means of that tool.

Fan turned foe

“I’ve suddenly become a disc-brake hater!” says Michael Robson of Moots, who has reviewed cyclocross disc brakes all season for VeloNews.com.

After taking a number of practice laps on Friday in anticipation of his 40-44 race Saturday and having no brakes left at the end of each lap, he planned to ride his cantilever-equipped bike and relegate his BB7-equipped machine to the pit.

“I totally see now why the Euros don’t use them,” he says. “Cranking brakes down every lap – hell, no!”

As conditions changed on Saturday, however, Robson had walked that back, saying his discs were not getting as wet and worked better during his 40-44 race.

Mechanic Chandler Snyder of Snyder Cycling Service, who has been working on many a distraught rider’s disc-equipped bike on Thursday and Friday, says hydraulic brakes would have helped, but added that “the pads are wearing so fast here that they would not last through the race either.”

In anticipation of the weekend

The course has been muddy and snowy since the start of racing on Wednesday, with the firm snow cover steadily disappearing amid rising temperatures and a quarter-inch of rain between the end of racing Thursday and the beginning of racing Friday.

From Wednesday through Friday, the course developed an ever-deeper layer of mud atop solid permafrost. The mud Thursday and Friday was thin enough that it created a thin coat all over everything, including disc-brake rotors, and dyed riders a uniform brown from head to toe.

The changing weather conditions seem likely to freeze the course in time for Sunday’s elite and collegiate racing, negating the disc-brake problems experienced by competitors during the week.

Meanwhile, Saturday’s racers — junior and under-23 men’s races, collegiate women, and masters men for three categories from 30 to 44 years of age — were greeted with the kind of thick mud like overcooked split pea soup that hangs all over the bike, rather than the thin, wet mud the consistency of minestrone that was the order of Thursday and Friday.

Saturday’s mud added pounds to the bike within a lap but was mostly thrown away from the center of the wheels. So riders with rim brakes carried a lot more mud dangling from their stoppers, those with mini V-brakes had wheels that barely turned due to the tight mud clearance, and those with disc brakes had mostly clean brakes that worked well.

 

 

 

FILED UNDER: Bikes and Tech / Cyclocross / News TAGS: / / / /

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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