Changing sproket - Fuel Economy, Hypermiling, EcoModding News and Forum

saunders1313 · 08-08-2008, 11:45 AM

Let's pretend for a minute that I know nothing about sprokets and why or how it would be beneficial to install a larger one. Also I had heard that putting in a larger one would make the bike more difficult to drive, anyone on what becomes more difficult and why?

bikin' Ed · 08-08-2008, 12:13 PM

Sprocket, where I come from, is the toothed gear most forward on a bike. It is turned by some sort of motor or by the rider via the cranks on a bicycle. The larger the front sprocket, the harder it is to turn and the higher the speed at a given rpm.

If you are talking about the rear wheel sprocket, it is turned by the chain and rotates the wheel. The smaller it is, the harder it is to turn and the faster the wheel will turn, given the same rpm at the front sprocket.

dcb · 08-08-2008, 12:42 PM

FWIW, I went from a 47 tooth rear sprocket to a 37 tooth (I have a 15 tooth front sprocket FYI). It is a significant reduction in rpm for a given vehicle speed.

Now my 4th gear is like what my 6th gear used to be. It took a little getting used to when taking off in first, and when selecting gears for a given situation, but I like it and am getting good mpg results with the setup. It's all instinctive now.

When cruising I am at a lower RPM with more throttle opening (more efficient generally), and if I need the acceleration to get up a hill or something, I can always downshift.

saunders1313 · 08-08-2008, 01:02 PM

So if I make the rear sprocket smaller I will be able to go the same speed, but at lower rpms? So instead of say my 4th gear being good for 35mph to 50 mph it would shift to maybe 45 mph to 60 mph? Just trying to get a feel for this. How difficult was it to get used to and how hard is it to change to a smaller sprocket? Do I need to change anything else, or just the chain tension? (smaller sprocket would equal more slack in chain)

Gregte · 08-08-2008, 08:45 PM

Quote:

Originally Posted by saunders1313

So if I make the rear sprocket smaller I will be able to go the same speed, but at lower rpms? So instead of say my 4th gear being good for 35mph to 50 mph it would shift to maybe 45 mph to 60 mph? Just trying to get a feel for this. How difficult was it to get used to and how hard is it to change to a smaller sprocket? Do I need to change anything else, or just the chain tension? (smaller sprocket would equal more slack in chain)

You can figure out exactly what the speed difference will be with a different size sprocket, at a given RPM.

First you count the teeth on both of your current sprockets. Lets say it is 15T front and 45T rear. That is a ratio of 45:15 or 3:1.

Now lets say you change the rear to 40T. You now have a ratio of 40:15 or 2.666:1.

If you divide the old ratio by the new ratio you get 1.125. This means that your new sprocket ratio will make the bike go 1.125 times as fast at the same RPM as the old sprocket ratio.

If it used to go 35 mph at 3000 RPM now, with the new smaller rear sprocket, it will go 35 X 1.125 (39.4 mph) mph at 3000 RPM.

The difficulty of changing the sprockets is entirely dependent on what bike you are talking about. Back in the day, it was typically a one hour job on most bikes I had ever owned. The chain will almost certainly need to be shortened if your new sprocket is more than a tooth or so smaller.

Again, depending on your bike, it may be much simpler and cheaper to change the front sprocket. Just do the math as above to learn what size will make what difference.

dcb · 08-08-2008, 10:40 PM

I didn't have a larger front sprocket commonly available myself, so I looked around for the smallest rear sprocket that would fit my hub/chain size. I found JT Sprockets: Motorcycle & ATV Sprockets to be invaluable and cross referenced the materials there to something that would fit off a completely different bike.

Your 17 tooth front sprocket is already the largest commonly available (as was mine) but you could go down to 38 teeth (from 42) on the rear. Just shop/ebay around for a 1997-2000 Suzuki TL1000 S V/W/X/Y rear sprocket, or by number it is a 499.38

dcb · 08-08-2008, 10:53 PM

regarding the chain, I just kept the original chain and front sprocket and shortened it by eye with the new rear sprocket in place. They have special tools for breaking chains, but I managed with a c-clamp and a socket and some swearing (and a new master link)

Gregte · 08-09-2008, 11:59 AM

Quote:

Originally Posted by dcb

regarding the chain, I just kept the original chain and front sprocket and shortened it by eye with the new rear sprocket in place. They have special tools for breaking chains, but I managed with a c-clamp and a socket and some swearing (and a new master link)

My preferred way to shorten a chain was to grind the heads off of two pins with a grindstone. Then a small hammer and punch will easily push that pair of pins through. We had a chain breaking tool at the shop I worked at but I never used it. Grindstone worked better.

Also, if you do not have the option of changing the front sprocket, the math that I pointed out above can be simplified. Just consider that if you put on a rear sprocket that is 10 percent smaller, for instance, than your existing sprocket you will raise the speed of the bike 10 percent at a given RPM, that simple.