Gear Ratio Question...

[MIG1773]

Ok, so I just bought a Murray Hard Drive with the stock 6.0 hp Tecumseh, jackshaft, and 16" tires. The plan is to use it to race on a homemade dirt track with a moderate uphill/downhill grade on the straightaways. I have looked over all of the ratio formulas, but dont really know what is practical, so here is my question... If I want to keep the stock tire size, what would be an optimal gear ratio for good torque and decent top speed (25 to 30)? Also, with 16" tires do I actually need the Jackshaft or can I just go directly from the clutch to the wheel sprocket without burning up my clutch? if so, again, what would be a good ratio? By the way, I am 6'2" and weight about 210 lbs, if that matters... Thanks so much for your help.

 

[OzFab]

6:1 is optimal, maybe out to 7:1 considering the tyres & weight...

Can you post a close-up of the clutch/jackshaft assembly?

 

[KingsSprockets]

HI,

I can probably help you out but in order to do so I would need to know:

1) The number of teeth on the engine sprocket.
2) The number of teeth on the jack shaft sprocket that the engine is connected to.
3) The number of teeth on the OTHER sprocket that is ON the jack shaft.
4) The number of teeth on the sprocket that is connected to the the wheel or axle that drives the rear wheel.
5) The current Top speed of your go Kart.
6) The rear tire diameter (overall height from top to bottom)

If I can get this info I can help you re-gear your kart.

Generally speaking, if your Kart has a jack shaft set up it is to REDUCE gearing to increase engine torque to the rear axle.....bypassing this reduction will most likely result in an over geared Kart (i.e. gear ratio that is too high)....this would result in poor acceleration and burned out clutches (over gearing would be like trying to start out a car in 3rd gear from a dead stop....can be done but hard on clutches and bad for acceleration).

Anyways, if you can get me the info I can probably figure out some gearing that would help you meet your goals.

Hope this helps you.

Andrew

 

[MIG1773]

I just got the kart so I dont know about top speed. The jackshaft is also being repaired, so I cant count the teeth at the moment... i guess my question is will a 6:1 or 7:1 ratio work with 16" tires without burning up my clutch?

 

[Doc Sprocket]

If you're racing, you're likely to put your foot down and keep it there. You MAY then be okay to go as high as 6:1, but you'd have a better hole shot and more punch coming out of tighter corners, down closer to 7:1. A lot of this will depend on your track, and what you need to achieve.

 

[MIG1773]

If I go with something in that range will the jackshaft still be necessary or will I be ok without it?

 

[Doc Sprocket]

Probably. As you go deeper (towards 7:1) you start getting into some pretty large axle sprockets. As an example- if you have a 12T clutch, you'd need a 84T axle sprocket to achieve 7:1. There are benefits and liabilities involved. No jackshaft means you're going to be pushing your luck for ground clearance, and really running out of room to gear down further if need be. However- you'll only have one chain run, less weight, less drag. Using a jackshaft allows you to use smaller and cheaper sprockets, leaves you with a limitless reduction range, and fine-tuning your ratio more flexible.

 

[MIG1773]

Will a gear ratio acheived with a jackshaft cause less clutch wear than the same ratio without a jackshaft? for example a 12 tooth clutch to a 75 tooth sprocket is a 6.25:1 ratio... A 12 tooth clutch to a 25 tooth jack in to a 12 tooth jack out to a 36 tooth sprocket is also a 6.25:1 but uses a jackshaft to get there.... in these examples, with the jackshaft version put less strain (wear) on the clutch as opposed to the direct version, or is it all determined by the gear ratio?

 

[landuse]

Will a gear ratio acheived with a jackshaft cause less clutch wear than the same ratio without a jackshaft? for example a 12 tooth clutch to a 75 tooth sprocket is a 6.25:1 ratio... A 12 tooth clutch to a 25 tooth jack in to a 12 tooth jack out to a 36 tooth sprocket is also a 6.25:1 but uses a jackshaft to get there.... in these examples, with the jackshaft version put less strain (wear) on the clutch as opposed to the direct version, or is it all determined by the gear ratio?

No, there will be no difference in strain/wear on the clutch. The same amount of torque will be applied