Coaxial chain and swingarm pivot

[andreis]

This is to follow from a thread of sebulba. I'm curious, why hasn't this caught on?

Coaxial chain and swingarm would give you constant chain slack. You can also leave a little slack to allow for chain rotation when the wheel travels, but this is not necessary.

I know bimota tried a bike with it and that husqvarna does this on some of its offroad bikes.

Are there any reasons against it? And before someone says that it's a case of "benefit vs cost", I see no cost at all to having the sprocket mounted on a slightly different axis in order to allow this (you need just one extra sprocket to make the transfer from the current location to another axis). So please show me where I'm wrong.

Cheers,
Andrei

[embee]

I'm afraid I think it is very much a case of cost/benefit.

Yes, technically it makes a very nice arrangement.

Practically it's much easier to do it like most bikes are arranged.

Bottom line is that you can "get away with" the usual simple arrangement, real world engineering is always about compromise, and this is a classic example.

[yorkie_chris]

Making the swingarm pivot would be very difficult, you'd probably need 2 adjusters as you couldn't have any "pinching" to take up tolerance in frame.

One extra sprocket? That is cost itself, plus cost of machinign all things for it to run on, maintenance of an extra sprocket and chain, etc etc.

Or if you put swingarm pivot forward, would also help with stability as you could fit more swingarm length into same wheelbase, but still you would make bike wider and more complex. This is a compromise I think I would stick with the standard layout for.

[Bibio]

changing sprockets would be a complete mare :-)

[mikerj]

Are there any reasons against it? And before someone says that it's a case of "benefit vs cost", I see no cost at all to having the sprocket mounted on a slightly different axis in order to allow this

The point being the "slightly different axis" happens to be the exact same axis as the swing arm pivot, so it involves quite a complex bit of engineering. Explain how you'd do this with no extra cost?

[Sid Squid]

There are some bikes that are so constructed, one that springs to mind is the Motodd Laverda.

An engineering solution to a problem that we just don't have. This, (like automatic tensioners for the chain), isn't about cost - it just isn't a problem.

[punyXpress]

Had a bike like that many moons ago.
The swinging arm was also a chaincase, so the chain ( duplex ) would NEVER wear out in its nice clean oil.
Aren't the relative heights of sprocket centres and swingarm pivot ' carefully calculated ' to get desired squat and lift when wringing & closing the throttle?
Or are they just made to fit?
. . approximately.