Schlumpf Belt Drive System

Schlumpf Innovations — the German Swiss outfit that sells that ├╝ber-cool 2 speed “Speed Drive” crank — announced an “Advanced Belt Drive System” last week at Eurobike. This modular system fits onto the rear hub to simplify conversion of a bike design to a belt drive system, especially when used in conjunction with Schlumpf’s geared cranks.

Schlumpf Advanced Belt Drive System w/ 27 tooth cog mounted on fixed gear hub

According to Schlumpf, some problems with bicycle belt drive systems include high tension required to prevent slipping, and low tolerance for misalignment.

High tension is rough on your bottom bracket and rear hub bearings and can result in reduced performance and life for those moving parts. The overly tight belt keeps the belt from slipping, but it also increase wear on bearing surfaces. If the belt is too slack, though, it has a tendency to slip. I’ve cranked up a hill on a maladjusted belt drive bike, and slipping when you’re out of the saddle can be oath inducing.

The other challenge is chainline, or rather beltline. I haven’t noticed this issue before, but Schlumpf claims design tolerances must be much tighter for belt drive bikes because belts have less tolerance for misalignment than chains. Belts are more prone to breakage and wear when misaligned, says Schlumpf.

Schlumpf’s solves the tension problem by giving you an extra large cog, so the belt has more teeth to grab onto by the hub. You have a new problem, however: a ridiculously low gear.
Schlumpf Belt Drive System: 16T cog vs 26T cog

To solve that problem, you need Schlumpf’s planetary gear crank. The Schlumpf Speed Drive multiplies the effective size of your chainring by 1.65. The High Speed Drive gives you a 2.5X ratio. This means your 42 tooth chainring on a Schlumpf High Speed Drive acts like a 105 tooth chainring. With 26 teeth on Schlumpf’s rear belt drive cog, that’s an effective 70 gear inches with the Speed Drive, or 106 gear inches with the High Speed Drive.

So to save the bearings on your $30 bottom bracket, you just need to purchase a $500 Speed Drive. Or you can buy an extra large chainring, which aren’t quite as expensive (but still pricey).

I’ve tried Schlumpf’s Speed Drive on a couple of folding bikes and really like them. The application works for folding bikes because their small wheel size necessitates larger gearing (or faster pedaling). You shift gears by popping your heel against a button in the middle of the crank. As of last year, they work with coaster brakes and fixed gear bikes as well. That means you can have a two speed fixed gear bike.

Alan @ Ecovelo scooped me on this news. Charlie @ WIRED claims this gadget can bring belt drive to any bike but that’s not strictly true: I think Schlumpf claims to simplify the design process with their modular system, but you still need a break apart real triangle to get the belt into place.


  1. OR- You could just run a chain drive, eliminating the special sized cog, alignment issues, chances for bearing failure/belt slippage, and run any cog/chain ring sizes you want.

    You know, I think sometimes that if belt drive had come first, everyone would be dumping the system for chain drive systems if they were developed later.

    Chain drive systems get a lot of negative criticism from the belt drive proponents, but are their complaints really problems? I don’t think so.

  2. Actually, Schlumpf is Swiss. The big concept with his take on the belt drive is using a bigger tooth pitch. The small pitch on most belt drives is what necessitates the high tension to prevent slipping; with bigger teeth, slipping is less of an issue, so belt tension can be lower. But the bigger pitch also necessitates bigger cogs. The reason the rear cog is proportionately larger is to get a better wrap angle.

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