Affect of replacing only front or rear sprocket to achieve a specific final reduction

[Lkhangee]

Hi
I have replaced a 125 c zs157fmi with a 200cc water cooled 163ML class engine.

This replacement engine has all its torque at low rpms and its performance curve flats out at 7k rpms.

My stock bike had 15/38 sprocket set mated to 428 chain.
Now I need to replace these sprockets I have the option to use 17/38 which will set final reduction to 2.23 or use 15/33 which will set final reduction ratio to 2.2.

What is recommended? Bigger front sprocket or small rear sprocket?

Or is it just the final reduction ratio and doesn't matter? And doesn't relates to the torque and horsepower curves of the engine?
Regards

[culcune]

This is just my opinion not based on engineering, but I would change the countersprocket to a 17 and leave the 38 in the back. Convenience is my reasoning because it saves you from dealing with the rear wheel, and should save you from having to shorten the chain.

That being said, 38 is very high, so going with a 17 might make your bike lag. I had an enduro with a 196cc 163 FML Zongshen engine; it came stock with 17 countersprocket and a 56 rear. I hated commuting on it, so had a custom made 40 rear, and the bike lagged in fifth--I had to get up to speed and generally remain in 4th. I enjoyed it much better on my commute, but thinking back, should have gone with a 44. The front came stock with a 17, so there was no point playing with that. I am of the belief that you might want to leave yours at 15 with the 38.

[Megadan]

I agree with trying the front sprocket swap. Much easier to do, and it will give you pretty much the same basic ratio without having to fiddle with adjusting chain lengths etc.

The ideal final ratio greatly relates to the power curve of the engine, but also the weight of the bike, the rider, other gear or equipment etc. I went through a few different sprocket setups that found the right balance for me, the power curve of the engine, and my use of the bike.

One thing you could do is also purchase a 16 tooth front. This will give you a good inbetween ratio option from what you currently have and what the 17 tooth will give you, just in case you find the 17/38 setup may be a little too tall for you.

[david3921]

Based on what I've read here on this site is to go as large as you can on the front as it's less stress on the chain. Of course the use of the bike dictates the size of the sprockets. The other thing that I've read (and I think it was Spud that wrote it) is that the teeth count on both sprockets should not be both even nor both odd. Something to do making sure the chain teeth don't engage at the same place, at the same time, on both sprockets.

[Megadan]

The hunting tooth concept is a lot more complex than that, and has little to do with the teeth counts being odd or even. It is actually a relationship between the front and rear sprocket teeth counts combined with the number of links in the chain. For example, the 17 front and 43 rear sprockets I used to run with a 128 link chain was one of the most ideal setups you could encounter for the "ultimate" in chain and sprocket life.

If you want to avoid all of the math, you can use this site. https://www.gearingcommander.com/

You will either have to find a bike in their database that used your engine to load some of the specs, or simply type in what you need into the required fields. But at the bottom of the page is a chart that shows all of the sprocket teeth and chain link to teeth contact figures to help you figure out what combinations to avoid.

[david3921]

Oops, you're right Dan. I forgot to add the comment about the chain.

[wlfpck]

Check this post for Honda Groms. Not the same thing as yours... but similar enough to give you some idea.

http://www.hondagrom.net/forums/13-g...need-know.html