Sprocket Size

[Fake_name]

Hi,
I have a couple of mini bikes that need a lot of work, and both are on 145/70-6 tires. I bought a couple 60 tooth sprockets (I am using 420 chain) and they give pretty much no ground clearance. Is going down to something like a 50 tooth worthwhile? I have heard that a 40 can be aggressive on the clutch and will lose a good amount of torque. Torque is more important than speed to me

 

[Minimichael]

There are calculators out there. You'll need the height of your wheel, too (I'm guessing 14"?). You might also consider reducing the teeth at the clutch, then calculating your ratio for a 50 tooth. You got me wondering now though wouldn't #35 chain sprockets be smaller? I've done 420 chain on everything too, but that could help you.

 

[Minimichael]

I use this one-
https://www.eccarburetors.com/jackshaft-calculator.html

You'll probably want at least 5.5 to 7 range.

 

[Jim Donovan]

You got plenty of clearance with the tire size you are using. They are 14" diameter. A 60 tooth sprocket is 9.8" in diameter and you add 1/2" for chain height so you have 10 1/2". Keep the tires with at least 10 pounds of air and you will not be bottoming out unless you are jumping rocks.

A 40 tooth would give you speed but you would have to push it to get going and it will not get you up a hill because all it would do is slip and fry the clutch.

 

[Fake_name]

This is what it looks like with the 60 tooth (yes, the tire is aired up), a 35 chain would have definitely helped this, I am brand new to minibikes and read that a 420 chain is better, so i got that and a clutch for 420 chain. At this point I am towards the top of my budget, but idk if it would be better to downsize the sprocket or get a 15 inch tire (even then I wouldnt have a lot of clearance)

 

[Fake_name]

My main goal is to be able to turn left without scraping. An added bonus would be if I can clear the grass in the yard so it doesn't get stuck all in the chain

 

[sparkwizard]

I would use the advice from @Jim Donovan , above and look for a rear sprocket about 9 or 10 inches in diameter. I suspect that would be a 50 tooth. You can remove a few links from your chain and that should solve the problem. I'll bet that bike climbs like a mountain goat with that 60 on it.

 

[Jim Donovan]

The good news with the #420 is that it is roller chain, #35 chain is not and it is beating the sh_t out of the sprockets with each revolution and it is only half a strong as the #420 chain. granted you have more gear selections with the #35 chain but given a choice I would always go with roller chain rather than continuous link chain like the #219 or #35. just an old guys opinion.

 

[Fake_name]

The consensus seems to be that i need to go to a 50 tooth. Does anyone have experience with a 420 chain 50 tooth on 14" tires? I really don't want to get them only to find out I still don't have enough clearance

 

[cl350rr]

This is a 10 inch tire on a 4 inch rim with a 60 tooth #35 sprocket. It gives about 1" of ground clearance on the chain with the tire fully inflated and no wear. Hope that helps.

 

[sparkwizard]

If your tires are 14 inches tall...you need a sprocket with enough clearance to not scrub the ground.
Measure the diameter of your sprocket. Subtract the the amount you think you would need to clear the ground in your turns.
When looking at sprockets online, they often list the diameter. What is the tooth count on your clutch?

 

[Fake_name]

My clutch is a 10 tooth

 

[cl350rr]

I run 35 roller chain on my go kart, and minis, never had a bad wear problem with it or breakage.

 

[Havasu Dave]

He's already bought his clutch, chain and is at the end of what he wants to spend. Without going in to rider weight, and horsepower, which are not mentioned here, the choice is either a smaller sprocket, or larger diameter tire.

The other thing is the "tooth" discussion- it is not valid to speak to different chain wheel sizes and tooth numbers, since the discussion is chain wheel diameters.

Example: A new to minis guy or gal reads on the internet that he needs a 60 tooth sprocket. Check. Now he/she reads that a #41 chain is superior. Check. So it stands to reason that he/she needs a 60 tooth sprocket at 41- series size.

Oops! A #35 60t is 7 1/4" while a #41 60t is 9 3/4"!

A #41 45 tooth sprocket will be slightly larger than the #35 60t sprocket. So the answer is, yes, a 50 tooth #41 sprocket would be better. A 46 would be OK too, given most of the assumed parameters with power and weight. It's all about chain wheel diameters.

 

[JimN]

Looks like a 196 or 212 on a frame that used to have a 97cc engine on it and probably weighs about 80 to 90 lbs. I would think that a 50 tooth sprocket on that setup would provide a decent amount of get up and go, even with a 200lb rider.

 

[sparkwizard]

That would be a 5:1 ratio. That should give you good all around power and speed, along with the clearance you need.

 

[Minimichael]

But anything below 5 is gonna suck.

 

[DaddyJohn]

Stock size on your MM80 would have been a #35 75T paired with an 11T clutch. If upgrading to a 212cc engine, #35 60-65 is good paired with a 10T clutch. So that’s about 7:1 gearing with the weak engine, or about 6:1 with the much more powerful engine. I’ve never had issue with the stock #35 size chain, but if moving to 420, I personally would still shoot for that 6:1 ratio.