Sprocket Talk

[bigdano711]

It has recently come to my attention that a 14/33 (.424) combo might work better than the often recommended 17/45 (.378). The figures in decimal are direct representations of the ratio...it helps my brain to see it that way. To me, it just looks taller, like going with 17/40 (.425)


What I'm trying to understand is what the YouTuber is trying to explain, but either I'm missing it or what he's saying simply does not compute. I was hoping to get you guys' expert opinion on the matter. He does explain that a similar Honda from the factory comes with the 14/33 combo, which I find intriguing.

[TominMO]

When he is saying a 233 gear ration for the 14/33 sprockets, he means 1:2.33. (Actually it is 2.35 though.) Rather than dividing 14 x 33 as you are doing, he is dividing 33 x 14. Same difference, just a different way of doing it.

You are correct, it is taller, but that seems to work for him and how he rides on the highway.

[Sport Rider]

I'm curious to understand if there is value to one or the other approach. seems like from an overall gear ratio perspective they would both achieve similar speed/RPM numbers. But I wonder if the larger sprocket in the back on a 17/40 would provide better torque to the rear wheel under load. I have absolutely no clue. hmmmmm....

[ProDigit]

A taller front will reduce the torque on the chain, and therefore allow the chain to last longer.

It'll also reduce engine vibrations through the frame, acts a bit like a flywheel, not the sprocket itself, but the sprocket driving that chain.

Generally speaking, there are only 2 forces exerted on the chain, 1 from the engine to the wheels in terms of thrust and acceleration,
And 2, from the wheel to the engine in terms of engine braking.
The former being much larger than the latter.

In terms of rotating mass, usually the mass of sprockets don't significantly contribute or detract from acceleration; if you were for instance to compare a steel 34t to an aluminum 34t.
But if you're counting in the realm of milliseconds, you might as well get the smallest sprocket setup you can get, and using an aluminum rear sprocket, to reduce weight.

In terms of reliability, using taller sprockets will usually result in longer lasting drive train.

Ideally, for longevity and mpg reasons, you'll keep the stock rear sprocket, and go up on the front one to a 17, 18, or 19tooth, or more if you can.
Ideally you'd be increasing the front sprocket in the same amount as you decrease the rear sprocket, to the point where both sprockets are nearing equal size (if gear ratio wasn't an issue).
For instance a 15/45 will have a lot more sprocket and chain wear than a 20/40 or 20/35 setup.

In terms of what gear ratio is ideal for touring or even top speed, you'll want to have at least 4th or 5th (or with a 6spd 5th or 6th) gear as a top speed gear, and if you can, use the final gear as a true overdrive gear where you can't reach top speed, but usually can still be able to reach a respectable speed for downtown, resulting in extended mpg.

The ideal engine rpm to ride in at any time, is the rpm where the torque of the engine is highest, if you need it. Otherwise it's the lowest rpm your engine can push, without lugging.

On small bikes, they usually tune it to ride at the hp peak, which quite often is near the redline. But this is quite inefficient.
A 50cc doing 45mph at 8k rpm may use just about the same fuel as a 250 doing the same at ~3-3.5k rpm.

The absolute worst you can do, is put a small front and a large rear sprocket on. It'll be bad in terms of mpg, sprocket and chain reliability, engine reliability, cush drive and bearings,and the reliability of the wheels and tires (especially if you are running spokes).