Need gear ratio help?

[Dave S.]

I just completed rebuilding an old yerf dog. It now has . 6.5 Honda clone, 18 in rear tires, live axle, 10T cent.clutch, 54 tooth sprocket. I increased the rear tire size from 13 to 18 and used the same size sprockets. That is my problem. After running it for about 3 min. with very poor results the clutch was smoking. So i have done a lot of reading on-line this morning and i know the tire size is the issue. My question is what size clutch sprocket and what size axle sprocket do i need to make these tires work?

I have seem similar posts and was unable to get my answer reading them. If you need to know if i want high end or low end speed, do i want on road or off road, or related info..
This is for my 11 year old son. It is his first powered toy. It does not have to be a rocket, i just want to set it up so it is reliable. Preferably without a torque converter. He will mainly be on grass with occasional cruise down the street.

 

[Doc Sprocket]

This can be worked out mathematically, pretty easily. Figure out how many percent larger the new tires are, and increase rear sprocket tooth count by the same percentage.
My quick calc came to 74.52, so get a 72T if you can, or around that range. Err on the larger side.

 

[Don Qui Racing]

if the clutch is still good then just go to a higher number on your rear gear. 54 is just to tall of a gear. i was told once that for every tooth you add or take away is a 2 to 3 mph difference. so start in the mid 60's. then i would suggest getting a comet torgue converter in the fututre. will save clutches and still have low end torgue and good top speed.

 

[Dave S.]

Thanks for the help I do appreciate it. As I read your banner about stupid people I'm afraid to ask this next question. I went on ebay and looked at several 72 tooth sprockets. If i increase the sprocket to 72 teeth and change the chain and clutch to accommodate a #35 chain will that solve my problem? I thought I would have to increase the diameter of my rear sprocket, because I increased the diameter of the rear tires. The 72T sprockets are 8 3/4 inches in diameter, that is the same diameter as my 54T #41 chain sprocket. Will the increased number of teeth actually make a difference on the same size sprocket?

 

[Dave S.]

If i get a 72 tooth its the same size sprocket but needs a #35 chain. If i get a 60 its bigger in diameter and takes the same #41 chain.

 

[Doc Sprocket]

Your thinking along the right lines- reducing the chain pitch will reduce the sprocket size, so YES, you can buy more clearance that way. Doing it this way requires new chain and clutch of course, but might save money and complexity in the long run if it lets you avoid a jackshaft.

 

[Doc Sprocket]

Remember a cardinal rule- Sprocket diameter does not matter (for gearing)! Tooth count does.

 

[Don Qui Racing]

Remember a cardinal rule- Sprocket diameter does not matter (for gearing)! Tooth count does.