Does tyre size (diameter) influence friction drive speed/gearing

landuse

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Hi there

I was wondering about the above. I had a read about this in the sticky in the friction drive section, but it still didn't make sense to me. This is what the sticky says:

"Wait a Minute! Doesn't tire size play a factor in calculating the top speed? With friction drives, No, it does not. With all other drive types, tire size does matter. Here's why tire size doesn't matter with a friction drive: The speed of the tire rotation in RPM is related to bike speed, but, it is really the speed of the circumference of the tire, in miles per hour, which is directly related to bike speed. After all, the tire is in physical contact with the road, and if there is no slippage, the tire circumference speed and the bike speed HAVE to be equal. Likewise, if there is no slippage, the tire circumference speed and the roller circumference speed MUST be equal, because THEY are in direct contact. Think of it this way - Essentially, the tire is just a transfer roller (or idler wheel,) between the drive roller and the road... A smaller tire would spin faster than a larger tire, but, since the circumference of the smaller tire is proportionally less, (by exactly the same ratio as the tire diameter, and the RPM increase,) there is no difference in bike speed."

Surely the tyre diameter has something to do with gearing using friction drive, and not only the roller diameter?
 
Only if the bike weighs more. Other than that, no, tire diameter means nothing. The bike can only move as fast as the surface speed of the roller.


*edit* I'll try to describe it a little bit better than him.

I assume you're thinking about how gearing works, right? Well what you've forgot to add into the equation is that when you change your gearing, you haven't changed tire size. Now, if you would have changed your tire size the same % as you've changed your gearing, the bikes speed will stay the same. That's how it works with friction-drives. The tire is the gear so every time you change the tire, the gearing changes with it, cancelling each other out.
 
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What about this statement that I got from someone (who knows a lot) off another forum:

while the roller's surface-speed IS the same as the vehicle's surface-speed, the fact's irrelevant to the math that's used to calculate ratio and speed.

whether chain, belt, or friction, (assuming the same tire in all cases) the overal reduction-ratio from engine-crank to rear-hub is all that matters...

the thing that can confuse, until you look more closely, is that with friction drive the power is transferred along the tire's circumference, so the equation appears different in our mind.

but simply put, the math's exactly the same...with friction-drive, tire-diameter factors in twice...once as "rear sprocket size" and once as "tire diameter." obviously, the roller is the "front sprocket." on the calculator, use inches in diameter (of roller & tire) as number of teeth, move the decimal point (the same number of places for both values) to use whole numbers if necessary.

note: a 26" bicycle rim is 22", add the tire size (1.75, 2.0, 2.25, etc) twice to get actual tire-diameter.

with a small 2-stroke and a single-speed, the best bet is to pursue it's potential for high-rpm's...to a certain point, a lower gear will result in a faster vehicle than if you ask the engine to power it's way through a higher gear. it's my experience that a 32cc 2-stroke can pull a 26" (x 2.0") bicycle best at about 20-22:1 overall-reduction. with a friction-drive, that very-roughly translates to a 1.125" to 1.250" roller.
 
What about it? It's a somewhat wordy and convoluted way of stating tire size doesn't matter.
 
For direct friction drive, the size of the tire will not effect the final gear ratio at all, only the size of the drive spindle does.

If you read this carefully Lou explains the physics clearly in the sticky with the following:

Doesn't tire size play a factor in calculating the top speed? With friction drives, No, it does not. With all other drive types, tire size does matter. Here's why tire size doesn't matter with a friction drive: The speed of the tire rotation in RPM is related to bike speed, but, it is really the speed of the circumference of the tire, in miles per hour, which is directly related to bike speed. After all, the tire is in physical contact with the road, and if there is no slippage, the tire circumference speed and the bike speed HAVE to be equal. Likewise, if there is no slippage, the tire circumference speed and the roller circumference speed MUST be equal, because THEY are in direct contact. Think of it this way - Essentially, the tire is just a transfer roller (or idler wheel,) between the drive roller and the road... A smaller tire would spin faster than a larger tire, but, since the circumference of the smaller tire is proportionally less, (by exactly the same ratio as the tire diameter, and the RPM increase,) there is no difference in bike speed.
.
 
make a bike with a 1" wheel and direct drive it from the engine :)
 
Here's another way to think about it:

Suppose you had a 1 inch drive roller, and two different wheels you're thinking about using. A 26 inch wheel, and a 13 inch wheel.

The 13 inch wheel would spin exactly twice as fast as the 26 inch wheel, BUT, since each tire rotation takes you half as far as the 26 inch wheel, it has to spin around twice to go the same distance... So, at a given engine RPM, the bike would travel at exactly the same speed with either wheel (all other things being equal)

So, long story short... the tire diameter has NOTHING to do with the speed that the bike will go, with a friction drive. The wheel is nothing except a transfer roller. The ONLY two factors in calculating the potential speed of a FD are (1)RPM and (2)roller diameter.
 
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