[Spridgets] Bentley-RR Trivia: was calculating speed

[Weslake1330 at aol.com]

Not really.  The wheel diameter is irrelevant because that's what  gearing is 
all about.
 
The engine has a max rpm and in an ideal world it makes it's max power at  
that rpm.  If you know the drag for the motorcycle you number crunch to  
calculate the top speed for the power it produces.  Lastly you gear the  motorcycle 
to produce that speed at that rpm.  On a motorbike you can  increase the drive 
sprocket size or you can reduce the wheel sprocket size to  change the overall 
gearing.  You can change the wheel size but you don't  need to because it's 
simpler to swap sprockets.
 
To say the wheel diameter effects top speed on a motorcycle and makes it go  
faster is like saying fitting 15inch wheels to a Spridget will make it go  
faster.  The fact is on a Spridget you fit a taller diff rather than  increase 
the wheel diameter.  Also on the Spridget neither fitting a 5  speed box or a 
taller diff increases the top speed unless it was undergeared in  the first 
place.  In fact a lot of power tuned Spridgets might have a  higher top speed in 
4th than 5th because using 4th at 110mph+ drops the rpm  better on the peak 
power than using 5th does.
 
W e s l a k e 1330
 
In a message dated 21/10/2008 19:58:57 GMT Daylight Time,  
oldsaabguy at comcast.net writes:

Maybe  this is a matter of semantics...

True, the motorcycle absolute top  speed is limited by drag and HP.

However, for a motorcycle with an top  speed limited by gearing or  
engine revs then a larger diameter tire  will allow for a higher top  
speed ALTHOUGH this tip speed may not be  the absolute top speed  
limited by drag.

For example a 20 hp  bike motor that will turn 6000 revs (and  
pretending that the tranny  is direct drive) will turn the tires 6000  
revs.  If those are  10 inch circumference tires then the bike will go  
speed x,(no, I'm  not going to calculate it out.) but if you have 20  
inch  circumference tires then the bike will go twice as fast *** IF  
***  it is not limited by drag and that the engine can turn the 20  
inch  circumference tires at 6000 revs.