Friction drive: Tire diameter, does it matter?

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LaLongueCarabine

New Member
Aug 15, 2011
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United States
My brother has a 700C fixie and it's tire is much bigger than mine. Our motors are the same, both of us has the same expansion chamber exhaust, and both of us run stock carbs on stock jets, and he goes a full 4 mph faster than I can. I believe it's the bigger tire, at the same rpm his bike rolls a few inches further, and oh yeah we both sport 36T rear sprockets. My next build is a 4 stroke.

So my question is: If you use a friction drive, would a bigger tire lower your drive ratio and limit your speed? I know a bigger drive rollers give you higher speeds, but does the tire work against you is it's bigger?

Thatsdax even has an 1.5" drive roller that is supposed to get you to 35mph, but if get my own fixie wont my top speed be slower?

cvlt1
 
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DTFuqua

New Member
Jul 16, 2012
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Florida
On a friction drive, the tire diameter has no effect on speed differences. Maybe because of drag or resistance but not just because of the diameter or circumference.
 
tooljunkie

tooljunkie

Member
Apr 4, 2012
663
5
16
Manitoba,Canada
imagine if the friction drive wheel is on the ground pushing you.the bicycle wheel is the same thing,as it contacts the ground.
bigger friction wheeel +faster but less power.
 
Otero

Otero

Member
Feb 1, 2010
782
17
18
wa
The thing is the bigger the tire the lower your ratio, which would appear to be
slower, but because of the limited amount of torque these engines have, in some
cases you need a lower ratio to achieve full top end rpm. .... Or maybe your brother's
engine just runs better than yours.

.trk
 
S

swaq muffin

New Member
Oct 22, 2011
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Chapel Hill, North Carolina, United States
i have always thought that tire diameter ONLY in a friction drive doesn't affect speed. Because no matter how big the tire, one rotation of the spindle will push the bike exactly the circumference of the spindle no matter what tire size (with perfect traction). so, even though there may be less wheel rpm's the engine will still have a similar engine to mph ratio and will retain the same speed. only changing the spindle diameter will affect that. there was a good video explaining this but i can't seem to find it.
 
Otero

Otero

Member
Feb 1, 2010
782
17
18
wa
True, spindle size,tire size, and tire type all have an effect on performance. The
more drag,(friction), the engine has to cope with, the more torque or leverage
necessary to compensate. Smaller spindle = more leverage. Same spindle,
slightly larger wheel O.D., slightly more leverage. If the torque on both engines
and spindles are equal, the engine with the slightly larger wheel will not have to
work quite as hard as the lesser wheel.
When I changed the sheave on my belt drive from 15 1/2" to 19 1/2" to get more
low end I found I was getting a bit more top end as well. Since the engine didn't
have to work as hard, I gained appreciably more top end rpm and acceleration.
 
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