The skinny bike

Road disc brakes are pretty great, in terms of stopping power, modulation and consistency. However, the very tight tolerances between pads and disc can be very frustrating to set up without rub, and it only takes a very slight ding to the rotor to have a constant noise that can be tricky to remedy. So – a thought experiment. Why are the pads so close to the rotor? In order to get the most mechanical leverage – big movement at the lever, small movement at the pads. Why do we need that much leverage? Because the rotors are very small, so to be able to slow that much larger rotating mass of the wheel, we have to squeeze the rotor tightly. Therefore if the rotor was bigger, we could have a caliper with the pads further away, and this would be fine as we wouldn’t need as much mechanical advantage. To take this to the extreme example – what if the rotor was the size of the rim: 622mm in diameter? In looking at this problem, it became apparent that it would be possible to dispense with a separate rotor and use the rim of the wheel as the braking surface. Testing of different materials for the braking surface and the pads concluded that a machined aluminium surface and standard pads provides fantastic dry weather braking. Not awesome in the wet, but for those conditions having the braking surface further away from the wet tire/rim in the form of a separate, smaller rotor makes the best sense.

With the rotor being almost five times bigger, the brake pads can be several millimeters clear of the braking surface without sacrificing power, making brake rub nonexistent even if the wheel goes a little out of true. It also turns out that a well designed mechanical caliper can provide all the power and modulation required – no need for the usual hydraulics, it turns out a steel cable will do this job admirably! Testing also showed that a 32mm tire can be comfortably fitted under the caliper.

So I decided to build a bike around this concept. And I discovered another advantage to this radical braking system – by moving the caliper mounting from the end of the fork leg to the fork crown, the fork legs do not receive any asymmetrical loading, and can be built to have compliance for comfort that will not be activated under braking forces. There was also no need for the regular oversize through-axle – instead the wheel can be mounted totally securely with open dropouts and a cam-actuated quick release lever that allows for the wheel to be removed in seconds without tools. And with no hydraulics, there are no concerns about needing to block the pads from excessive movement whilst the rotor is removed. AND it is even easy to add cable splitters or other forms of cable quick release if the bars need to be removed for travel.

I am very excited about this revolutionary bicycle, and look forward to testing it. Look at all the gains – the same braking power, modulation and control, but with less weight, easier to remove wheels, no hydraulics, more compliant fork. No rotor rubbing noises! Yes a couple of downsides – not ideal in the wet, can’t use the best shaped aerodynamic (carbon) rims. Could this oversize rotor concept be the next big thing in cycling?

<Removes tongue from check; awaits onslaught>

The bike I built around this concept allowed me to try a few other ideas out too. I wanted to try undersizing the tubing a little from what I usually use to see how that felt for ride quality. I continued my gradual adjustment with my personal bikes of lengthening the toptube and shortening the stem. Funnily enough, I had the bike fully designed when Open released their new road bike, and they implemented many of the same ideas, albeit in carbon with disc brakes – 25.4mm seatmast, offset seattube, 72.5deg headtube. I have built with an ‘upside down’ headset setup for many years – having a one-piece stem/steerer makes for a very clean appearance. On this bike I machined the fork crown to include the pinch bolts so there was nothing to braze on, and the extra thickness of the crown spaces the legs for the tire clearance. Keeping with the undersized theme, I had an old 26.0 handlebar so built the stem to match. There is a 1” steerer, but with the headset shimmed to fit into a 1-1/8” headtube – this just gives room for the gear cable stops to be inset into the headtube for the internal routing. At the rear, the offset seattube with a subtle bit of shaping in the middle clears the 32mm tires with a chainstay length of 404mm.

I have grown use to having extra bottle capacity, so added bosses on the rear of the seatmast, but brazed them internally so it maintains clean lines when the cage isn’t installed. The seatmast cap uses the same pinch bolt system as the fork crown, allowing for fine tuning of the saddle height.

The parts are a mixture of mostly used components I had in the shop. I always admired the original Sweet Wings cranks back in the ‘90s – so far ahead of their time with a two-piece construction, oversize spindle and external bearing. So I jumped on the chance when I came across a used set for sale. I revamped an old set of EE brakes with new bushings, and cleaned up the used Campy Record 11spd shifters and derailleurs. I had a set of DT 240 hubs that had been unlaced, so picked up a locally made set of Astral rims to go with them.

A side part of this project was curiosity about the weight of a bicycle mostly made of metal (there is a carbon handlebar, saddle rails and some carbon in the shifters and derailleurs). With a steel frame, fork, stem and crank, and aluminium wheels with those big tires, complete with bottle cages and pedals, the bike comes in at 15.5lbs. Pretty happy with that.

I haven’t had a chance to ride it extensively yet, but an initial couple of rides tell me it feels plenty stiff enough, and rides, well, like a bike….. Once I get to really push it hard on some descents I’ll get a better sense of the characteristics.

9 Comments

  1. Amazing. This newfangled brake standard is gonna catch on big time, I can feel it! XD Lovely work as usual.

  2. Love the bicycle, Rob. That fork is truly a thing of beauty, and the simplicity of the bike is so refreshing. The clearance between the rear tire and seattube looks exceedingly small with those tires. Is this not an issue? Always a pleasure admiring your work.

    • Yes the clearance is tight, but plenty for a dry-weather bicycle! Thanks for the kind words.
      Rob.

  3. impressive! beautiful! thank you (happy to see your creations)!

  4. That is just about the most beautiful bike I’ve seen in a long time and in a string of amazing bikes that you have built. Gotta agree on the Sweet Wings – love the girder spider legs!

  5. Rob, many thanks for writing all this!
    The re-revolution of the rim brake…

  6. Rotors with tires attached.
    nice. simple. lighter. more powerful. less likely to overheat. Better handling at the fork,and thus the steering is improved.
    I think there’s a real future in a disc brake that has a tire attached.
    I’ve been seeing this revolutionary brake design succeeding just about everywhere.
    I also wonder about the alleged limitation of wet weather riding.
    For one thing, my worry is more that my tires won’t grip, not that my brakes can’t slow the bike.
    For another, the Everesting Record was set in the rain, on a rim-disc bike.
    81 or 82 descents of a wet road at 80 kph or greater, and then slamming on the brakes to make a u-turn to go uphill.
    Here are more of my thoughts on the topic http://jralong.com/2020/08/03/radical-brake-design-takes-everesting-record-and-strade-bianche-in-one-week/

  7. Question on the derailleur cable routing — is that done with a full internal tube for the cable housing, or is that a cable housing stop with just the cable running internally? How was the routing handled at the bottom bracket?

    • Hi David,
      The housing stops at the headtube, then bare cables run the rest of the way. There is a small opening where the downtube joins the BB for the cables to exit.
      thanks, Rob.

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