Todays thick question - Tyre widths

So what are the actual differences caused by tyre widths? I know the fatter
the tyre, the bigger the contact patch (in theory) - but does that hold true
through the whole range, or just when the bike is upright?

Is a narrow tyre (eg 150) easier to ride 'off' than a 190, or is the
potential lean the same, but with less contact area?

 

That'd depend entirely on the profile and lean angle. The lower the
profile, the flatter the curve. The higher the profile, the further you
can lean before you go off the edge, as the curve is steeper. It'll also
turn in more easily.
Width has very little to do with it, but the wider the tyre, the greater
the radius for a given profile, and there's a practical limiting factor
in there somewhere.

 

It tends to hold true throughout the range, but obviously tyre
shape/profile plays a part.

Potential lean is the same. Where you get the difference is in power
capability: a wider tyre will allow more power, full stop, whereas a
smaller tyre will simply spin.

So (for example) a 150 section tyre will be fine for something up to
maybe 70-80bhp, and you'll still be able to achieve dizzying angles of
lean, but put 120bhp through it and wind on the power too early, and you
will eat dirt.

 

My understanding was a wider section tyre allowed a larger contact patch;
useful for more powerful bikes. The problem here was wider section tyres
need to be leant over further to achieve the same amount of 'turn' as a
narrow sectioned tyre.

Running into ground clearance and other leaney type problems. Which is
probably why older, skinny tyred, bikes don't have to ridden as if you're
a deranged monkey to achieve the same corner speeds.

 

This also depends on the tyre profile, if you think about it.

Indeed.

And MZs can be persuaded to corner exceedingly quickly.

 

Yes, but as a comparator between the bulk of older and newer tyres there
is not as much variation of profile as there is section.

 

True.

 

Which in turn depends on the wheel width.

A 180 section tyre in a 5.5 inch rim presents a different profile to the
same tyre in a 6 inch rim.

 

More accurately, a 180/55 tyre will present the correct profile in
one size rim, and the wrong profile in another. It can only be a 55
in one of them, as the wrong-size rim will cause it to deform.

 

But doesn't a narrow tyre have more a greater weight to surface area ratio
and hence have the same grip as a wider tyre - which has a bigger contact
patch and therefore spreads the weight over a larger area, giving less grip
per square inch of contact?

 

They both have "some"

 

Rubber / asphalt friction does not work like that. One of the basic
reasons being that rubber is a very weak material. The schoolbook
formula for friction is only true for a very specific set of circumstances.

 

It's all about profiles, innit.

For a *very* simplistic answer, here's a picky...
http://www.baita.demon.co.uk/Stuff/Tyre-Widths.jpg

Let's say the wheel rim is 170 in all cases.

On the top row, the one on the left is fitted with a 150, 170 in the
middle, 190 on the right. The red bars show the relative contact
patches while upright - more contact on the 150's, less on the 190's.

Now crank the fuckers over to 45degs. On the bottom row you've got
less contact on the 150's and more on the 190's. The one with the
constant contact patch is the "correct" fitting 170's.

But you've got to ask yourself where you want the grip. When you're
upright, or cranked over?

HTH

 

HooDooWitch said...

I'm fitting a 150 rear into the GPZ500s for track use. Purely so I have
the maximum contact patch possible at full lean. In the US the race-boys
use 150 section D207GPs in their GPZ race series.

 

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FJ1100/1200s have a 150/80 rear tyre...