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Discussion Starter #1
Hey All,
It's my first time on tools/garage/paddock, but thought I might get a better response to this question here rather than at the fireblade forum.

I'm calculating gearing and road speed for a YPVS (RZ350 over the pond) project; Would you calculte the rolling diameter of a wheel as the full dimensional diameter (18" wheel=457.2mm 140/60 tyre=60% of 140mm=84mm, so 457.2+(84x2)= 625.2mm diameter) OR do you take into consideration the deflection of the tyre at the contact patch?

It may be too small to be worth considering, or maybe at speed, when the tyre is expanding due to the centrifugal forces, it's at it's full dimensional size. For reference I calcuted the standard bike, with standard tyres to be at 120mph at 9500rpm in top, which seems to me to be pretty much right.

Any input would be appreciated.
 

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Actually the deflection doesnt affect it!

Why...
the tyre is compressed and forms a contact patch, so yes the "radius" is now smaller. But its also no longer a circle and hence 2pi*r doesnt apply.
The important thing is the circumference - that is the distance the tyre will move in one rotation (ignoring slip)
And the circumference of tyres is relatively fixed by construction (mainly by belts). So you can calc it from 2pi*r when its a circle. But it will keep pretty much the same value even if you push down on it.

Think of it this way - get a rubber band and make it a circle. Measure it. Thats the length of the band. Now flatten one side - still the same length!

I said above "ignoring slip" but at high speeds this can be a big factor. Your tyre speed and actual speed could differ by a couple of percent due to wheel slip. Full throttle, top speed - you are effectively doing a mini-burnout.
 

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Discussion Starter #3
Yeah, come to think of it i've read about tyre slip before, but more with cornering. Interesting about the tyre circumference, considering it more like a tank track that has to roll it's full length rather than a decreased rolling raduis.
Cheers dicknose (cool name:) )
 

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Yeah, come to think of it i've read about tyre slip before, but more with cornering.
Anytime your tyre is "working" you have slip.
So that includes cornering, braking, accelerating and even applying power to keep a speed (obviously higher speed needs more power = more slip)
If you are applying a force from the tyre in a direction, then you also have slip in that direction.
This is not the same as a lockup and slide. But there is always a small percentage of slide while getting traction!


Interesting about the tyre circumference, considering it more like a tank track that has to roll it's full length rather than a decreased rolling raduis.
Yes thats pretty much how to think of it!

Lots of people get hung up on the "radius changes", but that is also the reason you cant use the circle rule.
And more tyres (both car and bike) are designed to keep roughly a constant circumference even when they "contact patch"
There is some change - and some "detect flat" systems use that.
But its a lot smaller change than you would think just looking at the change in radius.

Yeah Im a geek!
Got degrees in engineering and mathematics and done a couple of years of physics.
 

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Did a quick google and found a discussion on another forum.
Interesting thing is to look at the initial graph
View topic - Must a tire slip to generate force? - F1technical.net
You will see that force(or torque) goes up with slip - ie more traction requires more slip!
But once past a point that depends on tyre+surface and you cant get any more traction. Push harder and you overcome traction and start to get more slip. Worse - you are now getting into a slide/burnout in that you start getting less traction for more slip!

I think most people understand the concept of sliding = less grip.
What is confusing is that the opposite holds true at times, for normal traction, more slip=more grip!
 

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Discussion Starter #6
Wow! A discussion going from tyre slip to particle physics, I'll keep an eye on that forum.
Isn't the slip that you are talking about pretty small and concerning road tyres, you're average joe would be unaware of it, but racers search for it?

Tyres(track oriented) do seem to hold on to more traction even with a slide these days, or at least thats what it feels like.
 

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You should be completely unaware of the slip while you have "traction"
The slip percentage is very small. Unless you are a computer with sensors and can check say front to rear wheel speed you wouldnt even notice it.

Note - if you are accelerating in a straight line the rear wheel will be going faster than your road speed. The front tyre is slightly resisting your motion and in theory would be going slightly slower (the road is push it around, while the rear tyre is pushing the road past!)
When you brake its the reverse - the front tyre is now the one with the big slip difference.
 
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