# Wheel size doesnt matter?

Discussion in 'General Questions' started by darwin, May 31, 2011.

1. ### darwinWell-Known Member

Many on this forum have stated wheel size doesn't matter in speed or torque when using different motors on the same wheel size? WHY ARE THERE GEAR RATIOS? As in size of gears to gear a motor right to the aplication? my point is I think bigger wheels 700c will give you more torque ie. low end than say a 26in wheel..............responses?

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2. ### srdavoActive Member

Are you talking about Friction Drive?

3. ### motorpsychoActive Member

wrong, wrong, wrong!!!
A smaller rear wheel will give more low end torque.
a 44 tooth rear sprocket on a 20" wheel with a stock h.t. engine will give about 25 mph top speed.
throw a 41 tooth sprocket on that 20" wheel and you gan top end, up to around 30 mph.
put a 36 tooth on the 20" wheel and your top end speed will increase quite a bit.
but for every bit of top end speed that you gain, you will lose some of the bottom end torque.
if you put a 44 tooth sprocket on a 26" wheel with a stock h.t. engine, you should get right around 30 mph top speed. go to a smaller rear sprocket and the top speed will go up, low end torque will go down.
just like a drag car, tire size does affects the gear ratio big time. a shorter tire will give you more bottom end torque, where a taller tire will give you more top end speed without changing the gear ratio in the differential.
ever see a 4 wheel drive truck with 44 inch mudder tires on it?
if they leave the stock gear ratio in the differentials, the truck will lose a ton of bottom end torque.
so, to do it right, thay have to put lower gears (higher numerically) to re-gain the low end torque.
so lets say that a guy wants to run 44" tall mudder tires on a truck that originally came with 30" tall tires and stock differential gears.
lets assume that the gears in the differentails are 3:23, which is calculated to give the best low end torque and give good crusing speed while keeping the rpms fairly low on the highway (i'm talking pre-overdrive here).
so the guy throws on a set of 44 inch tall mudder tires. Now he has just decreased his 3:23 gear ratio to (for example) 2:73. this will give a lower crusing rpm on the highway, but the truck would have a hard time pulling itself out of a mudhole.
now, if he throws in a set of 4:88 gears (which would be a lower gear) the truck would now have a final gear ratio (for example) of probably 3:50, which would put close to what the gear ratio/ tire size was originally, but give it a little more low end torque.
a 3:23 gear(in m.b. terms) would be a smaller diamater rear sprocket, where a 4:88 gear would be a bigger rear sprocket.
the smaller the gear, the more top end speed you will get. the smaller the tire/wheel diamater, the more bottom end torque you will get. there are a lot of things to play with as far as sprocket sizes, wheel sizes and tire sizes.
I have 2 m.b.'s that are set up very similar. they both have 41 tooth rear sprockets and they both have 20" rear wheels.
one bike has a 21 inch tall tire and it tops out right around 30 mph. the other bike has a 24" tall tire and it tops out right around 35 mph.

tire size does play a major roll in gear ratios.

Last edited: May 31, 2011
4. ### Big RedActive Member

Wheel size.

Onehundred&ten% RIGHT.
Big Red.

5. ### loquinActive Member

And, the only time where wheel size makes no difference is on Friction Drives (as Dave implies.) With friction drives, the ONLY two variables in determining ground speed are engine (drive spindle) RPM and drive spindle diameter.

In other cases, gear ratios are used to reduce engine RPM to a more manageable wheel RPM - usually in the 300-400 RPM range for a 26 inch diameter wheel.

For a given wheel RPM, a larger wheel diameter has a correspondingly larger wheel circumference, and thus, a correspondingly greater distance traveled for each wheel rotation (and, a correspondingly greater ground speed.) However, the torque is reduced by exactly the same ratio of wheel diameters, when increasing wheel diameter.

Think about it... If you're tightening a nut on a bolt, and the the bolt is spinning , you put a wrench on the head of the bolt to keep it from turning. If you use a long wrench, it's dead easy to hold that bolt, but if you do it with a stubby wrench or rachet, it could be really hard to keep the bolt from turning. The stubby wrench is equivalent to hte small diameter wheel - the distance moved (speed) is less, but the torque is greater. The long wrench has more distance (speed) moved but less torque.

6. ### srdavoActive Member

Friction

Thank you, Lou.

Here's how we did it in the "old" days .....http://motoredbikes.com/showthread.php?t=4175