Figuring speed: old sprocket vs. new

[TheOtherRide]

I just got a 45-tooth sprocket to replace my 56. Since I'm not putting it on tonight, I was trying to reason out what kind of top speed I'll get out of it. So here goes...

45 teeth will be making the same revolution as 56 were.
My top speed with 56t is about 40 mph (without redlining).
The ratio of teeth is about .8 (45/56). This means that the new sprocket, with the chain moving at the same rate as it did with 56 teeth, will make the same revolution in .8 of the time that it did with 56, right?

So if I go 40mi/60min with 56, I should be able to go 40 mi/(60*.8 ), or 40mi/48min with 45.
Converting that to mph...
40mi/48min=x mi/60min
(60*40)/48=x=50mph (provided I have any clue what I'm writing about)

50mph won't win any races, but it'll get me to and around town. We'll see what really happens this weekend. :wink:

[ambassador]

Pauli,
Just make sure that you have a "chain breaker", I got my new 42t sprocket Mon and can't find my breaker anywhere and I had a good one, it'd break 600's.. Your going to need to shorten the chain for such a big drop... Just though I'd let ya know...

[Akonababe]

We have 45 tooth sprockets on our vivas and 60mph is no problem at about 8500rpm. It cruises really good at 50 to 55 and going down hill you we can get up to 68 to 70 but the bike isn't happy there. I weigh about 240 so you should be very happy with the 45 tooth sprocket.

[TheOtherRide]

Thanks, Ambassador, my RBM has one, but I need to make sure he can find it!

Akonababe, that's good to know. I haven't pushed my rpms above 7000 much. I think this sprocket will be just what I need; I didn't want to go overboard and kill it every time I tried to take off. :oops:

[IronFist]

Princess I bow to your bike knowledge, and your math. Can you do my homework for me? My math is far easier, with very few numbers.

I've got a 150cc, with a 46 tooth rear sprocket. The front sprocket is unknown. I can't even run the bike till the weekend when I get the plates, so I don't even know the top speed.

Here is my question.
1. If I drop to a 42 tooth rear from a 46 tooth rear sprocket, will 4 teeth make much difference?

I only weigh 123 pounds. When I sit on the bike the shocks don't move, I have to bounce up and down to move them. I probably don't need much torque

[liverchip]

It will make a bit of difference, but I would wait until you actually ride it a bit before you start changing out sprockets. As far as a previous posting in regards to the chain breaker necessity, I have always just used a grinder to remove the rivet head and remove the rest if it with a punch. Works like a charm.

[TheOtherRide]

Now if I only had a grinder...lol. Nah, we got one of those, too.

N2R, I was playing with the numbers and realized it's easier to figure out backwards from how I did it the first time.... 46/42=1.1 (This just means that if you had 2 identical bikes, in the same gear, at the same rpm, but one bike had a 46t and one had a 42t rear sprocket, the 42t would make 1.1 revolutions for every 1 revolution the 46t made).

So if you're in the same gear, at the same rpm, you would go 1.1 times faster (i.e., the rear tire would travel that much further) with the 42 than with the 46.

If your speed in 5th gear at 6000rpm is 50mph with the 46t, you would go about 50*1.1=55 with the smaller sprocket.

This is all based on me thinking I have any idea what I'm talking about! Plus, you're gonna lose torque as your sprockets get smaller.

I agree with Liverchip; you should ride the bike first, especially since you don't know what front sprocket you have. If you're bored, you can take the cover off & check it out. If it's a 17t, you might want to ride it as is for a while if you're a total newbie to riding like I am! I don't know how much difference there can be in gearing in these things either???

[justinkr58]

, hi again.
i have a 35 tooth on mine and it still gets up and goes,(i weigh 165)
i can start of from a stop and go up a 15 foot hill almost as steep as this /

sure it lost torque but i also lost all my high rpm vibrations and can cruse on side streets in 3rd.
i was woried to get the 35 but?? works great.

this weekend i am taking it to a gravel pit, ill try it ut there.

[ambassador]

There are more posts on this subject in the forums, Pauli excellent info, you go girl!!! 15t front and 42t rear will probably net you the most these bikes have to offer, ON-ROAD as far as speed goes... Read the forums, seriously, Pauli, you rule!!!

[IronFist]

Princess Rocks!
I will ride it As Is for a good while. I'm thinking about later, with the motor worked in. In Canada we have alot of secondary highways. 60mph would be speeding. 80kph is about 50mph. These roads are sweet for riding. Curves to carve. Again I only weigh about 3 cases of beer. I never want to do a wheely, or trail ride. But my bike is only a 150cc, so thats why I'm thinking of speed. Actually, it's my crusing speed I'm thinking of. For myself, 5 miles an hour for a 4 tooth drop is worth it, eventually. 35 tooth might be right too, I'm in no hurry. Does anyone know the size on the new Honda 120cc sport bike, crz??? 120. (I think it's called) It might be 6 gears and not applicable.

[TheOtherRide]

I was just pondering the chain length making a difference in all this, too. I'll have to shorten the chain a few links, meaning that the chain (lol, I just typed china instead of chain about 3 times before I got it right) will revolve faster around the sprockets, simply because the same point will cross the front sprocket more frequently than it did when the chain was longer...hmmm...so I'm thinking my estimate of gained speed is a little conservative, which matches up with what Akonababe said about 50-55 being an easy cruise for a 17/45 setup, rather than my 50mph.

I think I've officially proven that I'm a geek.

[IronFist]

Even better! N2R

[red2003]

Pauli, you figured it right the first time. Your over all gear ratio is what will determine the change in speed. My Roketa has a 14/46 setup, so 46 / 14 = a final drive ratio of 3.29. My Roketa cruises fine at 55 mph. 60 starts to get some vibration from higher engine RPM. I would say 65 - 70 is about the limit. If I changed out the rear sprocket to a 56, my final drive ratio would be 56 / 14 = 4. So that's a direct proportion problem like you solved before. At the same RPM my bike used to be doing 55, I'd now only be going 45. You'll be fine going the other way. Your math is good. Your Kung Fu, well that's yet to be seen .......... . BTW, I find a grinder to work better then a chain breaker anyway. Gets the same results and makes better sparks.

[madmaximum]

Hey Pauli, where did you get your sprocket from?

[TheOtherRide]

I got a JT 269-45 from a local motorcycle repair shop, one of those guys that doesn't have a big-4 name involved and still works out of an old garage. We stopped in one day to check 'em out & they were able to order it for me.

If they hadn't, I would've gone through Bruce!