MOPED ARMY

i think you'd be best off working the whole problem backwards. Start w/ your tire/wheel combos circumferences and see what effect it has on RPM at the speed you want

example : a 17x2.25 gazelle is 23inches tall(diameter) so its 6.018 feet per revolution. it seems like a lot of math but once you put it all down on paper its easy to see

What crackstar said. Do some calculations, it's actually not that complicated.

Let's say you have a wheel that is 17" and has a 1" tyre on top of it.

That means the diameter is 457.2mm (=2R). 2PIR = 23.141592654*228.6mm = 1436.34mm per revolution covered (That's 1.43634 meter per revolution)

Now do the same with the wheel you're replacing it with, and you can see the difference and calculate what gear you will need.

ok we are getting there. but i still dont know the whole equation. i need a longer equation.

like rpm x wheel diameter x gearing = ????mph anyone know how to do that? i think i might need to know main gear teeth too.

Copied from wiki :P

http://www.mopedarmy.com/wiki/Calculating_timing

Spoel-/uitlaattiming = Transfer-/Exhausttiming

Timing (In degrees)

Portheight (in mm)

Deckheight (in mm)

Stroke (43mm standard)

Con. rod length (90mm standard)

Inlaattiming = intake timing

Timing (in degrees)

Intakeheight (in mm)

Pistonlength (At the longest point, in mm)

Deck (in mm)

Stroke (43mm standard)

Con. rod length (90mm standard)

Poortbreedte = Port width

True width (in mm)

Portmap width (in mm)

Bore (in mm)

Snelheid, toeren = Speed, RPM

Speed (In KM/H)

RPM

Front sprocket (In number of teeth)

Rear sprocket (in number of teeth)

Internal gearing (Leave it the way it is for Puch)

Tire radius (In meters. The length that the tire rolls for one revolution, if you catch my drift)

Informatie = Information

Place the radio button at the variable that's unknown. (The variable you want to calculate)

Well, you can do that pretty easily with the Puch calculator.

It isn't as complicated as it looks. You just fill in the gearing, the speed and the wheel circumference and press "Bereken". Then check the radio box on the gear you want to change and change the wheel diameter to the wheel diameter you will be using.

what i want to know is how big of a effect tire width will have on top speed. less tire on the road = less friction right?

put it on and play with it simple

Harold, my bike has MB5 wheels, 18", and i run 16 x 36 gears. Doggy off the line but it goes low to mid 50's with my 210 pound ass on it. tempted to put a 17t on front and have a speed run with the GPS. I used a 420 chain with the stock puch sprocket with no issues. I didn't even have to JB weld the sprocket on, lol.

You have to make some assumptions before you make the calculations.

Assume 10,000rpm

Then figure reductions thru the gears in the gearbox

then knowing the drive sprocket speed determine the reduction (or multiplication) between the drive sprocket and the rear wheel sprocket.

This will give you the rear wheel rpm. Calculate the circumference of the rear wheel tire and multiply it times the rpms it is going. (remember to convert the wheel circumference to feet and decimal feet.)

Divide this out to determine speed over the ground in miles per hour.

Then by using different sprocket ratios you can figure out the over the ground speed with the same engine rpm.

You will have to discount wind resistance as the speed changes and whether the engine can pull the speed desired.