Jackshafts?

[isiahstites]

Curious of the pros and cons of a jack shaft. I have never worked with one and don't know much when it comes to them. So any knowledge or info you may have regarding jack shafts I would be grateful for your input.

Scott

[JackD]

Do a search on here for 'jackshaft" and you will find several discussions on the subject to start.

[Dean Los Angeles]

No real mystery here. The motorcycle designer builds the engine to align with the rear sprocket. When you put a different engine, or two, and the sprockets can't possibly align, a jackshaft is a possibility. Two bearings, two sprockets (or belts) and a shaft, all big enough to handle the horsepower. The bearings can be inboard or outboard of the sprockets. If both sprockets are outboard on the same side, the brackets have to be able to handle the torque. The centerline of the shaft has to be considered when you factor in suspension movement.

The standard sprocket drive is through a keyway. You can shove a lot of power through one, but there are limits, and then a splined shaft becomes necessary. Keyed shafts can be bought off the shelf, splined shafts you're going to have to fabricate along with the sprockets. The extra gearing means extra drag, but also a way to get more possibilities on gear ratio.

The Fat Tire bunch uses a jackshaft to clear the huge rear tire:


This is what Kent's liner looks like:

[isiahstites]

Is there any advantage speed wise other than more gearing options??

Scott

[Sumner]

Scott I'm planning on putting one in my lakester to expand the gearing options with a Busa in it that runs slower than the 750 I have now.

To avoid a keyed or splined shaft I'll machine a carrier to go onto a shaft with one sprocket on one side of the carrier and another on the other side.  This way no power will have to be transmitted through the shaft from one sprocket to the other.  The sprockets will bolt on.

Soon I'm going to expand my spreadsheets for motorcycles to include the option of adding a jackshaft and corresponding sprockets:

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

If anything the jackshaft might hurt on speed as it will eat up a little more HP due to two more bearings in the drivetrain, but like in my case and probably Kent's it might be the only option to get the gearing you want.

c ya,

Sum

[1212FBGS]

not a good idea... just one more thing to break
kent

[fastesthonda_jim]

not a good idea... just one more thing to break
kent

Good advice from a PRO-fe-shun-l breaker.  I should know.

He broke my heart when he came back "up" on his tires...sniff

[1212FBGS]

hey, hey, now were talking chains and sprockets here not bodywork.... oh wait I've broken lots of those also...
kent

[panic]

Don't use it unless you have to - power loss, breakage, remember it has to be where you can check it for adjustment, lubrication, get the salt off, what will it wreck if it breaks, etc.
However: it allows the actual final drive chain to be located on a better vertical center (i.e. midway through the swing-arm travel) to reduce suspension bind on power application, so you can put the transmission where you want it rather than at the best height.
It allows really small gearing changes without taking the clutch apart or the rear wheel off.
It allows better sprocket tooth count on the transmission and rear wheel. By splitting the final ratio between 2 sets of sprockets you won't be faced with the problem of "they don't make one this size to fit my ____".
Allows use of prime number sprockets to maximize chain and sprocket life.
Allows adjustment of chain without moving the rear wheel: the jackshaft slides forward and back, and is screwed or shimmed up and down for tension; adjusts both chains together.
Not relevant for LSR, but avoids using sprockets too small for best life (smaller than 17) or too big for good chain speed (more than 60).