Gearing, calculating new top speed

This is not a question. Im bored and Id figure id share this equation with all of you. I came up with it, so I have no source to back it up. Just seems like simple math and seems to be accurate.

This equation assumes your motor will be hitting the exact same top rpm as you previously were. If your gearing for a higher top speed, this equation assumes your motor has enough torque to deal with the new gears.

Actually, its 2 equations. For the first one, you will need to know how many teeth on your front and rear sprockets, and you will need to know your current top speed.

This equation is for people who dont want to do calculations where you have to know the gear ratio of your transmission and wheels, and engine rpm.

Equation 1:

S = current top speed

F = current front sprocket tooth count

R = current rear sprocket tooth count

X is used in equation 2

X = S / ( F / R )

Equation 2:

F = new front tooth count

R = new rear tooth count

X = whatever you got from equation 1

S = new top speed

S = ( F / R ) * X

I have a Moby with stock 11 / 54 gearing, and was averaging 40mph top cruising speed. I ordered a 13 tooth front gear, and using this equation, have calculated a 47mph top cruising speed. Once my Moby is together, we will see how well this equation works.

Or, if anyone else wants to test the equation, and share the results, that would be great too.


Re: Gearing, calculating new top speed

it wont work that great. You leave out way to many Dependants. total weight, HP, rolling resistance. It works for theoretical but i don't think a sprocket will give you 7mph and if it does it will kill your bottom end.

Re: Gearing, calculating new top speed

its all mechanical so on paper its possible. what you need to do to be truely accurate is come up with another equation to figure what amount of torque is required to pull each gear, then create a hp/weight chart to correspond to your gearing forumla. i expect it in a week or so :-D

Re: Gearing, calculating new top speed

I agree, that gearing for a higher top speed, this equation will probably be optimistic, and the higher top speed you are gearing for, the more optimistic the equation will be.

I am personally gearing primarily to run at lower rpms. My engine would be revved really high at 40mph, so I am gearing so that at 40 my engine will be revving more modestly. And whatever more top speed I get, great! I thought with a pretty modest gear change that I should be getting close to that 7mph, but what do you think? Like 5mph?

I have a 70cc, and I think my compression is about as high as it can be (theres a pit on my piston which looks like its there from hitting the spark plug). My point, my bike is torquey as it is, I can sacrifice some bottom end, but hopefully the new sprocket wont "kill" my bottom end. If it does, Ill be switching back to an 11 or seeing if I can find a 12.

That said, this equation can be used very well for those few, if any, people out there who actually want to gear for more torque. In fact, I bet this equation would be a bit pessimistic in such the occasion (engine may be able to rev a little higher than before).

Re: Gearing, calculating new top speed

lol yeah thatd be great, but this equation is more for people wanting to do modest gear changes, and for just getting a quick but educated estimate on a new top speed.

Its going to be most useful on motors with more torque, in other words, bikes with big bore kits.

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