# speed is of the essence

Discussion in 'Frame Mounted Engines' started by revelstone, Jun 27, 2008.

1. ### revelstoneMember

not really. just a quick question is 4700 rpm normal for these bikes? what kind of gearing would i need to make 4700 rpm turn into 20 mph on 26 inch bike? sincerely hope this is in the right place. if i goofed, Please, move me to right place.

2. ### eljefinoMember

Some have run 6000 so 4700 sounds like a nice cruise.

You could do the math yourself if you know the internal gearbox ratio; it's "approx" 4:1 on the HT motor.

3. ### BSAGuest

This question is really relevant to the drivetrain so I'm moving the thread to frame mount drivetrain section.

BSA

4. ### fastboy9Member

not sure if this will help you out but there are loads of online calculators that will give you the rpm, like this one: http://www.thedirtforum.com/rpmcalc.htm.

Using that formula and re arranging it will give you an equation to work it out.

mph = (rpm * cir) / (gear * final * 88)

where rpm = engine rpm
cir = tire cicumference, in feet
gear = gear ratio of your bike
final = final drive ratio of your bike
88 = combines several conversion factors

sooo... for a 26 inch wheel the tire circumference would be pi x d = 207.372 cm

(4700 x 207.372) thats the first bit of the equation.

But I'm afraid its far too early in the morning and I've got a hangover so you will have to do the rest!!

5. ### revelstoneMember

trying to determine the cc of an unknown engine. can someone see if my math is right?
radius of cyliner is 2.65 cm, compression stroke (after port closes) is 2.3 so
3.14159 X 2.65 X 2.65 X 2.3 = 50.742 OR maybe there is something about figuring cc of an engine i don't know. hey it could happen ! ?

6. ### revelstoneMember

whoops

there is no internal gearbox. :???::shock: any ideas ??

7. ### fastboy9Member

I think he just means the big gear turning the little one, can somebody help me out on the definitions? I think its the drive gear and the primary gear. What I mean is when you take the clutch cover off you have a straight cut gearing, and that is at a ratio of 4:1