why so few jackshaft frames?

bluechair84

Anyone have any idea why there are so few jack shaft frames these days? It seems there were quite a few on long travel bikes (I think I've seen modified Stinkys), and frames with gearboxes seem to be using them more like the Zerode. What was it the killed them off in the end?

supersonic

Not sure what you mean! What is a jack shaft frame?

bluechair84

Ah, one of those pulleys which redirects the tension side of the chain to another point, typically the pivot location. I found out today its called a jackshaft - but maybe that's an Americanism (eitherway, it's an amazing name).

supersonic

heavy and complicated - and no bugger seems to know how they work ;-)

http://www.ridemonkey.com/forums/f19/an ... on-228831/

bluechair84

Hmm, that made for some interesting and slightly mind melting reading. But much of it is a discussion of pulleys where there is only one chain, redirected. I'm wondering about those which have two chains like this:

I don't think that this design induces much pro or anti-squat (edit-- I was thinking about the shaft being located within the main pivot, which it isn't on the Zerode so they still need a chain tensioner). Do they warrant any merit on their own?

supersonic

The kinematics should be the same, whether a dual chain or single 'idler'. I'll have to put more thinking into it!

Northwind

It can do interesting things as far as chain growth etc- emulate a concentric pivot in some ways, if you make the middle point float, and it's got to have an effect on pedal feedback (sort of like moving the pivot in relation to the cranks)

TBH I don't know if it's just that there are other ways to get the results they want, or if it's that it's just unpopular- a guy I know had the GT DH bike with the secondary drive thing and everyone without exception went "Oh, that's cool. Looks heavy!" so that's probably hard to sell...

rockmonkeysc

K9 use a similar set up on their DH bike. Rides nice enough and isn't overly heavy. I certainly didn't notice anything I didn't like when I rode one.

bluechair84

Good picture. Effectively, that setup would give you huge amounts of pro-squat as the torque around the pulley is far greater than the cassette. Theory says that would be a pig to pedal. But if the pivot location is at the pulley, there is no anti or pro squat (I think).
I'm imagining a high singlepivot, maybe like an old Patriot with a single chain runing from the chainset to the pivot location, and then an entierly different chain from pivot location to cassette. There would be no kick-back, squat, whatever, the rear suspension would be entierly unhindered by the chain. It isn't a new idea, I just can't find a picture of a bike with this set up.
In use, the chain serves to stabalise bob under pedalling loads thorugh anti-squat. But I'm surprised that there isn't anyone using this set up anymore.

nicklouse

the reason many Box bikes came out



and other things

bluechair84

Yeah, that's the kind of thing. I just found this diagram


So, excluding the gearbox, what were the suspension characteristics like? Any idea Nic?

nicklouse

like the single pivot design they are.

ilovedirt

I thought it was called an idler, and I thought the main reason was to get rid of chain growth, so you don't have that dead feeling in the travel when you're trying to pedal? I'm no engineer though, and I've never ridden a bike that had one.

gt-arrowhead

Does anyone have a link to read about these kind of bikes? It seems pretty interesting and i would like to know how they work.

Cheers.

bluechair84

ilovedirt wrote:

I thought it was called an idler, and I thought the main reason was to get rid of chain growth, so you don't have that dead feeling in the travel when you're trying to pedal? I'm no engineer though, and I've never ridden a bike that had one.

Yes, I think you're right. The design seems great for descending as there will be no kick back, but of course, most designs use the chain to control pedal induced bob. Even the Zerode has chain growth...

ilovedirt

bluechair84 wrote:

ilovedirt wrote:

I thought it was called an idler, and I thought the main reason was to get rid of chain growth, so you don't have that dead feeling in the travel when you're trying to pedal? I'm no engineer though, and I've never ridden a bike that had one.

Yes, I think you're right. The design seems great for descending as there will be no kick back, but of course, most designs use the chain to control pedal induced bob. Even the Zerode has chain growth...

Yes, but the zerode wasn't designed in that way, it has the chain set up like that because the gearbox is in an alfine hub inside the frame. The Zerode has nothing to do with what you're talking about.

bluechair84

Course it does - the alfine takes the place of the jackshaft in the bikes Nicklouse posted. The difference is that they pivot around the jackshaft, the Zerode doesn't. So, Zerode chose to incorporate chain growth into the design when they probably could have built the pivot location at the alfine which would eliminate it.
One thing I have noticed since this started is that jackshaft seems to be sued exclusively by those bikes designed to go off cliffs. Ain't no squat forces required in freefall - and certainly not during landing! Is chain growth then more important than suspension 'plushness' (edit -- to all other bikes that aren't exclusively for hucking)?

ilovedirt

I think it was more that the zerode was designed around the gearbox, and they didn't feel that chain growth was much of an issue.