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Comment by pgreenwood | Hacker News Reader

If this drivetrain achieves its claims I will be very impressed. Shaft drive designs have been around for as long as chain drives; and they have never been as efficient. So mush is lost in the torsion of the shaft. A 2% increase in efficiency is an absolutely massive gain. The cycling industry is littered with flashy looking innovations that don't live up to the hype. The basic design of a diamond frame and chain drive has not been improved upon for over 100 years.

But every now and then there are innovations that acually take hold. For example, the slant parallelogram derailleur, hydraulic disk brakes, suspension (for mountain bikes), and recently narrow-wide front chain rings. We shall see how well this drivetrain goes.

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Shaft torsion does not lose you much energy, it mostly acts as a torsion spring: whatever you put in you will get back. The shaft will only warm up a tiny little bit from the amount of energy that it will lose during a single cycle.

Suspension does lose you a lot of energy, which is why you'll never see it on road bikes.

Springs are terrible when driven by biological "pistons". Applying a certain amount of force is much harder in some phases of the pedal stroke cycle than in others and springs would make it impossible to go easy when you want/need to.

It is a very hard spring, not something squishy like front fork suspension.

The biggest downside would be eventual wear of the driveshaft, the losses will be very low.

Note that almost every part of your bike acts as a spring in that sense, the frame flexes a bit when you pedal, as do the cranks and the shaft. Even the spokes in the rear wheel act as springs transmitting the force from the hub to the rim (which is one reason why they are oriented the way they are, that way they pull the rim along rather than that the spoke gets bent, the spoke is stronger in that direction).

Everything acts as a spring, and performance bicycle engineering goes to great lengths to make them as as hard as possible. Frame flex under pedaling load is measured, optimized against and a driver of buying decisions. Replacing the chain with something more springy? Good luck in that market.

> Everything acts as a spring, and performance bicycle engineering goes to great lengths to make them as as hard as possible.

Indeed. So if you use a shaft to drive the rear wheel that would definitely be part of the equation, I note they are using a hollow carbon fibre tube, which in that particular dimension is likely not ideal for the application unless it is given some more cross section. Even so, it is an interesting development.

> Replacing the chain with something more springy?

Chains stretch quite a bit, you'd be surprised.

"Chain stretch" is usually slang for chain wear. The chain gets longer as the rolling elements wear down so people call it "stretch" even though the metal isn't stretching. The actual stretch under load is very small.

I'm aware of the difference, but thank you anyway.

You can see the effect for yourself if you lock your rear hub and proceed to push down on the pedal (you can see it because the pedal is a nice long indicator effectively multiplying the distance the chain stretches).

A bit nicer setup is a micrometer at the end of a fixed section of chain with a weight attached.

Chain does stretch. About 1.5 mm under full load.

That's why you want chains with solid pins and solid plates.

What you are talking about is chain elongation as a result of wear, essentially the accumulation of slop in the bushings the pins go through.

Yeah, shaft drive bicycles ("chainless") were marketed as the "next big thing" around 1898–99 by big companies, like Pope Manufacturing Company's Columbia brand, as they were coming down from the Bike Boom of the 1890s.

The important thing with bicycles, compared with motorized transportation, is that the "engine" is still the same now as it was then, so everything is very limited by weight and efficiency. We wouldn't be able to practically use most motorcycle and automobile improvements that have come since the Model T if we were stuck with the Model T's engine power.

And FWIW, I ride a motorcycle with a shaft drive. The fastest motorcycles all use chains. Many now use tooth belts, but I think they may be limited in the power they can transmit.

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