Wednesday, December 23, 2009

Specialized Shiv Concept Tandem

Specialized just revealed a super-cool prototype (it's been called a concept bike, but this seems very functional, nothing concept about it): a tandem version of the Shiv.

Wow! Photos stolen from BikeRadar:


Interesting phasing on those cranks. One of the cool aspects of this bike is the shaft drive:


Super-aero, clearly. But at what cost?

Any drivetrain has losses. Losses are generally considered to be proportional to transmitted power, although it's been experimentally shown that for a bicycle transmission efficiency is higher at higher chain tension than at lower chain tension. This is fairly trivial: aspects of drivetrain loss are not necessarily proportional to transmitted power, but rather proportional to chain motion. In any case, a fixed gear drivetrain, such as on the timing chain on a tandem, is quite efficent. For example, 98-99% is typically claimed.

An alternate for the timing chain on a tandem is the Gates belt drive. This also claims to be as efficient. Here's a friend's super-sweet custom Calfee with a belt:


So what about shaft drives? I looked up some references on shaft drive efficiency. Granted, historical numbers may not reflect the state of the art system used by Specialized! And of course efficiency may depend on drivetrain load with a shaft, as it does with a chain.

Here's the efficiency coefficients:So is it worth the increased power loss, assuming these numbers are valid? Suppose you lose 3% of captain power to drivetrain loss. The CdA of the captain may be comparable to that of a single bike: let's say 0.25 meters2. That's super-aero. If 90% of power is from wind resistance, then to offset a 3% loss in drivetrain efficiency from the captain, CdA needs to be decreased by 3% / 90% = 3.3%. This requires a reduction in CdA of 82.5 cm2. If Cd = 0.8 for drivetrain components, this requires a reduction in cross sectional area of 103 cm2. a 54 tooth chainring has a diameter of 27 inches / π = 21.8 cm. The width of a chain is around 8 mm This makes for an effective cross-sectional area of 17.5 cm2.

So it seems it's not even close. But of course my numbers are pulled out of the blue: I'm not counting the wind resistance from the length dimension of the chain, for example, and the timing chain is quite long. So maybe it's a wash.

Additionally, there's the weight factor. Shaft drives are supposedly heavier. But then again I don't know Specialized's design...

I'd love to see some real numbers. If the drivetrain efficiency loss could be kept to 1% or less, things become a lot more interesting.

4 comments:

specialist said...

It is very cool, but it seems it is made from wood and filler, so there might not even be a shaft inside there:
http://forum.slowtwitch.com/cgi-bin/gforum.cgi?post=2627363;search_string=%20awesome%20model%20from%20the%20specialized%20design%20master%2C%20;guest=82053205&t=search_engine#2627363

djconnel said...

Ah! We've been duped :). So it is "concept" after all.

Alison Chaiken said...

For a tandem it seems to me that you'd have to account for losses due to the effort riders would have to make to pedal together. In other words, Rider A's best cadence on some arbitrary terrain wouldn't necessarily be the same as Rider B's. Rider B may be a steady pedaler but Rider A may naturally be more jerky and may induce chain oscillation, etc. A tandem team would naturally develop a common riding style after long hours together in the saddle, but I would guess that some physiological inefficiency would remain. I've never ridden a tandem, so maybe it's not true?

djconnel said...

I previously commented on Brian and Janet, using a very shoddy analysis. A somewhat less shoddy analysis suggests they were 10% less power together than the two were capable of separately. But would this be solved with a shaft drive? I'm not sure. But then, like you, I've not ridden a tandem (things to do in 2010).