Rotational mass at such a small radius might as well just be static mass.

You get to take out a few links of the chain for a few more grams

Question: fully hiding the chain? Partially? Only the big chainring? What's your gut feeling?

The problem of fully hiding the the chain is that you lose a lot of flexibility to shape the chainstay. Traditional chainstays are not deep enough and if you reduce too much cog size (something that isn't always possible due to the IGH), you start losing efficiency. Assymetrical chainstays can also be good from a structural POV but, to know for real the aerodynamic advantadge, you have to do a pretty accurate CFD analysis with moving legs

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http://cds-0.blogspot.com

Quote:"So, to wrap it up:
- We have a 8.5w gain by getting rid of the FD, RD, Cassette, 2nd chainrig. Potentially a bit more if we hide the chain
- There's a 2% decreases in drivetrain efficiency dur to increase friction in internal hubs. That's 5w at 250w
- Weight: The Alfine Di2 is 1600g - but we get rid of the RD (270g ultegra Di2), FD (162g), Cassette (208g) - 2nd chain rig (150g). So about a 900g of weight penality, which is a rotational mass. "


Looking at the Alfine, it seems like the weight can be reduced by incorporating lighter materials. I think the hubs are heavier than what they could be is because of the market in which these hubs are intended for. Plus, I thought I saw some internal hubs that can incorporate a coaster brake. I would think that this means you can ditch the rear brake caliper, cable, and lever on the base bar. Come to think of it, you could get rid of the chainring shifter up front too. Both of which would add to the weight savings and possible aero benefit.

Update

Richard at Wheelbuilder.com says the Shimano hub comes in 32 or 36 hole. Zipp 808 comes in a 24 hole. He says they have a way of lacing the 36 hole hub to the 24 hole rim, impressive cost $1645 built.

Anyone got access to a wind tunnel to compare this set up ( with a wheel cover) to a standard zipp 808 with a cassette and front and rear Ds?

Heck, rotational mass of wheels even further out on the radius might as well just be considered as static mass...especially in context of the total bike + rider mass and it's inertia.

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http://bikeblather.blogspot.com/

Reviving an old topic.
I found this discussion very interesting indeed.

Why do you have to use a chainring & chain to drive the rear hub?

The cranks could be adapted to use a shaft drive and pinion gear at the rear hub.
The shaft drive & pinion gear can be fully enclosed within the BB & Chainstay........
There would be nothing sticking out in the wind...........super aero.
Might be worth a lot more than 8.5 watts.

Is there a UCI ruling which does not allow shaft drives & fully enclosed drivetrains?

True, enclosing the drive system would hide from wind. However the simple roller bicycle chain is extreamly efficient at transferring rotational movement. Somewhere on ST the loss was approximetaly quantified (depends on chain and how clean/lubed, etc). But we are talking very very small loss.

Shaft drive systems (they make this for bikes now) use gears (helical I believe) to both change rotational axix 90 deg, and translate the power from cranks to rear wheel. Gears use a tooth profile (called involute) to transition force from tooth to tooth. This pressure/force vector is not tangential to the round gear (or dam near tangential on cog/chain wheel system). That (plus friction between gear teeth) is more loss than gained by aero advantage.

Look at the most competitive motorcycle racing. All (I believe) still use a chain rather than go to a BMW type shaft drive.

As for UCI, not sure. I am a Tri geek and don't follow what UCI thinks a bike should look like.

driveshaft would add even more frictional losses in the drivetrain.

pretty much all of these schemes fall apart because everything is less efficient than a chain/cog direct drive.

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mfrassica
You made a few good points against shaft drives on bicycles, especially the additional frictional losses.

However, in motorcycle racing - frictional losses is not the dominant factor in their preferred use of the chain drive.
In a motor driven shaft & pinion drive while accelerating or putting the power down, there is a tendency for the shaft to 'climb' the pinion gear at the rear wheel.
This is bad for handling at high speeds............and really bad out of corners.
I do know quite a few 'bikers' so to speak.

Frictional losses and weigh issues aside, it is an interesting idea.
I would love to see someone prototype a purposely designed TT bike with a fully enclosed shaft drive drivetrain, DI2 Internal hub with a wheel cover and then test this in CFD (or better yet in the WT).
Only way to really find out if this would be indeed faster than a conventional design.

Good discussion here.

Not really, the design would have at least 5% more power loss due to friction. No way you are getting a 5% reduction in drag in total drag, since the bike itself makes up less than 15% of the total drag anyway.

I built one of these in 2008 after SRAM bought Zipp. We suddenly had access to all sorts of things like internal gear hubs that we hadn't before, so we worked an I-motion9 into a super9. Unfortunately the friction more than offsets the aero drag...but it really, really looks fanatastic! I had visions of a SRAM Hammerschmidt crank with 9 speed internal hub in the disc...so sweet, but when you do the math (and road testing) you see that the friction losses almost can't be overcome without some dramatic new technologies.

Over the years, quite a few of our US national TT records were set on fixed gear TT rigs as beyond the aero advantages you have better drivetrain efficiency with the highly controlled chainline and lack of pulleys, etc..for flat TT's the math makes a very compelling argument in this direction!

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I liked "Electronic shifting gives you more freedom to enjoy your desires."

I've often wondered if this would be worthwhile for a very flat Ironman course... you know, for us watt weenies.

 

I don't like the idea of having a specific wheel married to a specific bike b/c of proprietary shifters, chain, etc.

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Ah, Ok that makes sense about the 'climbing' of the rear wheel on a motorcycle. However, this 'climbing' is the result of this non-tangential force I am talking about. On bikes/motorcycles this angle made up from both the involute of the gear tooth profile AND the helical tooth angle used to smoothly mesh teeth so transition is smooth.

For non-racing bicycles, shafts make a lot of sense. In racing where we are focused friction forces measured down to the grams, it does not. Heck, I saw a ST thread talking that once the 11T cog is used, it creates more friction (due to small diameter) than larger cogs, and therefore should not be used.

A well maintained chain is crazy efficient, just look at the track bikes. They look at every last spot of friction to the point they remove bearing seals and other stuff. They, and the hour record setters have all used chains. now that I think of it, even HPV's use chains because of the low friction losses.

From what I've seen, there are shaft drive, (timing) belts, old 10 mm pitch chain, yet the roller chain is still dominant even in the world of electronic shifting. To be honest, with the more speeds on the cassette (now 11 speed) we are almost making a constant velocity drive system. I have been waiting to see if the OEM's try to do away with the chain all together and replace with some other means. The closest think I could think of that would be close is some sort of ball & chain similar to what is used on our lamps for a pull chain. With modern fibers like Kevlar, we can get small, light strings capable of handling tremendous tensile force.

Anyone else have some novel ideas out there???