Advice on multi-stage gearing

[SWINEMOTHER]

Alright, so been lurkin here for a while but have never had need to post until now.

Currently working on a project that will require multi-staged gearing. Background on the project:
Found an old e-ton 50 out in a pile of scrap while renovating my shop. Engine and transmission were worthless, but the frame and tires were in great condition. Decided to pull the engine and attempt to pop an extra predator 212 I had laying around in there. Due to the drive rotation of the engine and the side the rear wheel sprocket is on, I opted to run my drive chain back and transfer it across via a jackshaft. The problem arose when i realized that I would be fielding a theoretical 40+ mph machine into the hands of a 9 year old if I used the 10>10>10>28 gearing. In order that's cvt drive>jackshaft input>jackshaft output>rear wheel. The tires are 16in. Played around with the various ratios a bit and sort of settled on 8>12>8>36.

This, if I have the math right, would allow me to run the engine up close to the normal 3600ish rpm, reducing the rpm of the jackshaft to 2400rpm, which in turn would give me roughly 25 mph at full throttle through the 8t to 36t ratio. Moving the throttle stop in to derate to 3000 shaves another 4ish mph off. Potentially might further reduce rpm limit to 2500 to sit just under 18 theoretical mph.

Real question is is there any detriment to multi-staging such large gear reductions? sprocket damage, etc.

Thoughts and comments appreciated.

 

[Kartorbust]

One thing to note is chain length. Short chains will build up more heat than a longer chain, which could wear the sprockets out faster.

 

[SWINEMOTHER]

That's a good point. Though, the jackshaft will be sitting where the old drive was located and the engine is as far forward as possible, making the lengths relatively the same to what they were before.

 

[anickode]

One consideration (although of minimal concern on a kart) is the strength of the chain. The second chain in the reduction will be under more strain than the first one. Unlikely that you'll exceed the tensile strength of a chain on any go kart, but it is something to consider.

Hypothetical situation here:

Say you want a 10:1 reduction on an engine that produces 10 ft/lb torque, and you aim to do that with a 5:1 and a 2:1.

Clutch is 10 teeth and let's say for ease of calculation, 2" diameter. The engine will exert 120lb of tension in the chain. This in turn applies 120lb of tension to a 20 tooth 4" sprocket, resulting in 20 ft/lb torque. Jackshaft transmits this to a second 2" 10t, applying 240lb of tension to the chain. The 10" 50 tooth final sprocket turns that 240lb of force into 100 ft/lb torque on the axle.

If you reversed those ratios and did the 5:1 first, you'd end up with 50 ft/lb at the jackshaft, putting 600lbs of tension on the second chain to yield 100 ft/lbs at the axle.

Again, well within the working limits of most chains used on go karts, but chain wear will be a bigger factor in the second chain. The increase in strain will be dependent on your sprocket arrangement. The principle also applies to the sprockets themselves. With (as exampled above) the 5:1 first, you would need a stronger jackshaft (thus a stronger shaft key as well) to handle the load than with the 2:1 first.

The main time this whole thing comes into play is when people put big engines on karts with big wheels, and want the smallest axle sprocket possible to maximize ground clearance.

 

[SWINEMOTHER]

So perhaps I should use heavier chain. I was planning on using #420 as it were.

 

[anickode]

So perhaps I should use heavier chain. I was planning on using #420 as it were.

That'll be fine for the arrangement you have proposed.