Jack Shafts...why? Pros and Cons

[The Uker of oz]

Ok....another "newbie" question here....

Jack Shafts.....why would I need / want one?

I'm building a scratch built frame in kind of a "scooter" style....and I'm laying out the drive line and motor and thinking "should I build this one with a jack shaft drive system?" Then I go into problem solving mode, and I'm thinking about what the advantages would be of having one.

As I'm looking at it...the advantages I see are:
gear ratios: you can play with different drive ratios right?
motor mounting position: seems to me you could better "center" the motor by not having to have the big offset needed to line up the shafts/sprockets.

So what else are they good for? Why would I want or not want it?

Interested in what people have to say about the matter....
Thanks in advance,
Todd
"the Uker of OZ"

 

[ScooterBoy]

I think you answered the question already.....

To me the big ones are gear ratio & engine placement.....

I would definitely make provision for a JS on the bike, whether you end up using it or not.

 

[MiniBike Paul]

There are 2 kinds of JS's - welded on the bike, which also solves the problem of putting the chain on the right side if that is necessary or desired, and the backing plate mounted to the engine type which really only serves the gear ratio issue.

I run the latter, the Pro-Mod unit, on all my riders and racers; I feel that lower, changable, gear ratios are #1 priority!! Engine to wheel gearing is fine if top end is all you care about . . . . but for me acceleration is way more important - funny, this is what started my my first fight with Supernova a long time ago - when I said the same thing about acceleration and then went on that by removing the governer and doing valve springs and gaining RPM you could have the best of both worlds!! Nothing has changed!

 

[KB2ROCKET]

pros =
easy engine placement
shorter chain runs
makes it easy to run a wide wheel/tire
you can run a smaller rear sprocket on smaller wheel/tires by splitting the ratio and in some cases ratio changes can be easyer by changeing the jack shaft sprocket
wider possible ratio range adjustments
.
CONS=
they take up more room
and require a little more power to operate

 

[The Uker of oz]

Ok.....good info! So I got the main things right at least...was going down the right path.

I didn't think about the requiring more power thing....a little more stuff to turn. I do have a smaller motor for this, it's 2.5 to 3.5 hp Tec (Reo Raider....seen recently in my other threads....) so that might be a consideration.

I was somewhat concerned with space and what adding a jack shaft would do for that. I'm trying to keep this thing on the small side, getting the motor as close to the rear wheel as I can so I can have a floor board in front of the motor like a scooter.

Might have to start a thread in the "Projects" section to document the build.

Todd
"the Uker of OZ'

 

[Mac]

Two chains make the chance of the chain popping off less likely.

 

[The Uker of oz]

MAC.....
I'm gonna stick my neck out here.....

Why does two make it less likely to pop a chain off? Seems counterintuitive...seems like more chains, more problems. Please explain.....

(now it is getting later in my work day.....so maybe that's the reason I can't wrap my head around this one.....)
\
Todd

 

[MiniBike Paul]

Long chains flop all over the place, short don't, right Mac

Hey Mac, PM me a $$ amt.

 

[The Uker of oz]

Ok....that makes some sense....no floppy chain = good:thumbsup:

 

[Txpowdercoater]

1.ability to change the drive to other side of rim, like circle dirt racing, all left turns means left side gear on rim would always be digging in while carving, so move the drive to right side and lean like a mofo.
2.ability to change gear sets very quickly and easily on either side of shaft for an almost infinite range of gearing.
3.shortens chains
prolly other reasons, like on my bike, I also have the disc brake and hyd caliper on the jackshaft, no room by rim.
but that pretty much sums it up for me.

 

[tin solider3]

There are 2 kinds of JS's - welded on the bike, which also solves the problem of putting the chain on the right side if that is necessary or desired, and the backing plate mounted to the engine type which really only serves the gear ratio issue.

I run the latter, the Pro-Mod unit, on all my riders and racers; I feel that lower, changable, gear ratios are #1 priority!! Engine to wheel gearing is fine if top end is all you care about . . . . but for me acceleration is way more important - funny, this is what started my my first fight with Supernova a long time ago - when I said the same thing about acceleration and then went on that by removing the governer and doing valve springs and gaining RPM you could have the best of both worlds!! Nothing has changed!

There is a third category of jackshaft that isn't very common. It slides under the engine and sits on the plate, no welding involved. It acts like the welded jackshaft. This is the jackshaft I use, BMI sells them. You can get them cheaper on eBay though.

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[PS122]

There is a third category of jackshaft that isn't very common. It slides under the engine and sits on the plate, no welding involved. It acts like the welded jackshaft. This is the jackshaft I use, BMI sells them. You can get them cheaper on eBay though.

Sent from my DROIDX using Tapatalk

Like this??: Clutch Mount with Jackshaft (03191), www.bmikarts.com : BMI Karts & Supplies : Go Karts Parts : Racing Go Kart Parts : GoKarts : Versailles, Ohio : USA : Indoor Track : Tips

 

[tin solider3]

Yea just like that

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[TomH]

how does that thing work, both sprockets on the same side or what?

 

[Mac]

how does that thing work, both sprockets on the same side or what?

Either way.

 

[The Uker of oz]

Woah.....that is really cool! Surprised it's not more popular.....seems like a pretty simple way to do it. Plus you could pull it out and use it on something else if you're the type that likes to keep mixing and matching and trying new things. Interesting.... might have to go that way:thumbsup:

Todd

 

[tin solider3]

I think this setup is an easier way to go. Here's a few with a clutch mounted on the jackshaft.

 

[anthonymaez]

that's a dope setup^^^

 

[Dave_In_Florida]

It seems to me a jackshaft mounted clutch allows you to change the clutch's engagement rpm (related to wheel speed) based on the gearing of the primary side (engine to jackshaft). Does this make sense or am I missing something?