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UCI legal? That depends…

  • By Aaron Hersh and Nick Legan
  • Published Jan. 18, 2012
  • Updated Jan. 20, 2012 at 1:10 PM EDT
The top tube is broad and includes an accessory mount.Photo: Aaron Hersh

Stopping a fast bike

Cervélo teamed up with Magura to create a hydraulic rim brake for the P5. Cables are out. Magura, a premier MTB brake manufacturer and the sole supplier of all BMW motorcycle brakes, designed a hydraulic stopping system that is compatible with all road wheels and mounts to any standard basebar. Amazingly, these hydraulic brakes are lighter than most cable brake systems.

Anyone who has ridden a time trial bike with extremely tight or circuitous brake cable routing is all too aware that a brake with twisted housing lacks stopping power and lags before reopening after braking. Once a few drops of sports drink leak into the housing, braking performance become even worse. Hydraulic brakes solve all of those problems. Although bikes with straight and simplistic brake routing certainly reduce the problems with cable-actuated brakes, a single ride aboard a mountain bike with hydraulic disc brakes makes the potential of hydraulic brakes immediately obvious. Replace the mountain disc brakes with rim brakes and you’ve got yourself a slick cable-free ride.

Cervélo’s P4 rear brake, one of the first truly integrated calipers, suffers from those issues and the Canadian company wanted a better solution for the P5. Rather than starting from scratch to design a hydraulic brake themselves, they approached Magura and suggested the two companies work together to create a hydraulic brake for the P5. Magura handled the hydraulic mechanism and Cervélo integrated the brake into the bike.

Cable braking systems use pulling force to squeeze the rim but hydraulic systems only push, they can’t pull. Hydraulic systems function like this: A plunger is pushed into the hydraulic line running through the basebar when the rider squeezes the brake lever and forces fluid (the RT 8TT uses mineral oil) through the line. This fluid doesn’t compress and a twisted line doesn’t impair its movement, so the hydraulic line transmits all the pressure to the far end of the system, which actuates the brake.

This fundamental change from pulling to pushing forced Magura to design a unique road caliper that functions with a hydraulic system. The Magura RT 8TT brake caliper has a piston that sits beneath two brake arms that actuate on pivot points above the piston. The piston drives upward and forces the upper segment of the arms outward to the sides, which rotates the brake arms and forces the lower portion to squeeze inward toward the rim.

So in reality, the Magura system is a hydraulic/mechanical hybrid, unlike disc brakes. Magura claims the RT 8TT can create 44% more force than the nearest competitor listed in their presentation. On the road, it feels substantially more powerful and responsive than a cable-actuated brake. Perhaps the biggest difference in braking feel is the way stopping power ramps up with only a small increase in the pressure on the lever. Instead of putting a death grip on the brake lever to max out the brake caliper’s stopping force, the RT 8TT requires only a subtly tighter squeeze on the lever.

Although it would certainly be easier to flip ass over teakettle with these brakes than with cable brakes, stopping power isn’t excessive or jerky. These brakes are not, however, as powerful as hydraulic disc brakes on mountain bikes, but then you wouldn’t want them to be.

The Magura brakes are plenty strong and offer a supremely consistent, reliable connection with the brake caliper. They are a fantastic improvement over any other hidden or integrated brake. Don’t be surprised if every high-end time trial bike comes with hydraulic calipers within a few years.

In addition to their stopping performance, the RT 8TT is remarkably easy to install and use. The calipers attach to any standard brake mount with a standard brake bolt. Their width can be adjusted to fit rim between 17mm and 28mm wide by turning an easily accessible bolt with a 2.5mm hexagonal wrench. This range of rim widths is wide enough to cover every wheel we are aware of. Magura doesn’t yet produce brake shoes (the part that holds the pad) or pads, but the brake arms take any standard brake shoe. A set of Shimano or SRAM brake shoes will mount to the RT 8TT and brake pad angle can be adjusted to match any wheel.

In addition to the width adjustment, the brakes have a quick release switch on the lever for easy wheel removal. Should the rider forget to reengage the brakes before jumping on the bike, the brake will automatically reengage itself.

Magura says their complete braking system—calipers, levers, and the connection between the two—weighs 495 grams and “one of the lightest” cable systems weighs 519 grams. They explain that a meter of housing and cable weighs roughly 70 grams, but an equal length of their hydraulic line and fluid weighs only 50 grams.

Shimano Dura-Ace Di2 riders will have to sacrifice their brake-mounted shifters or splice an additional shifter into their system to retain the ability to shift while riding with hands on the basebar. Other than price, the inability to use these Di2 shifters is about the only drawback we can find to these brakes.

Magura doesn’t yet produce a splitter that would allow for a 3rd brake lever that could be installed on the aerobar extensions.

The brakes are only available with aero brake levers for now, but Magura product manager Stefan Pahl says, with a smirk on his face, that they will offer road-compatible versions “not too far in the future.” Magura will also release a cheaper version called the RT 6TT that is functionally identical to the RT 8TT but heavier.

The brake lines must be bled periodically to remove any air that makes its way into the line, but these brakes should require less service than cable brakes. They must be centered using the brake bolt only.

The RT 8TT will come exclusively on the Cervélo P5 until April 1st. On that day, Cervélo dealers will receive components for aftermarket sale at the kingly price of $750 a set, without pads or brake shoes. Dealers that do not carry Cervélo will have access in June and other bike companies will be able to spec the brakes as part of their complete builds in 2013.

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