No pressure.

25 09 2015

A recent mtbr thread (I know, I know…) got me thinking…

Here is my input on a thread about ‘plus’ tyres…

“So I got to thinking about this a bit more. I dont know too much about the physics, but I do know I like the combination of a higher volume tyre at lower pressure for my relatively low speed, reasonable tech riding on a hardtail or rigid bike.

Why is that?

Considerations for tyres (and/or wheels):

Rolling resistance
Puncture resistance
Rim durability

My thoughts:

a) Lower pressure = more comfort given the same casing construction

b) A bigger volume tyre feels similar to a small volume tyre at less pressure (pounds per square inch…I guess it is self explanatory) given similar casing

c) Less aggressive tread gives less rolling resistance given the same casing construction and size (I don’t know how true, but for rolling resistance of pick up truck tyres, 60-70% is often associated with the tread design)

d) Increased hysteresis can lead to increased rolling resistance (‘tacky’ DH tyres anyone?)

e) Increased hysteresis gives more traction (‘tacky’ DH tyres anyone?)

f) Less tyre pressure is unlikely to make you slower (increase rolling reistance) given the range that is realistic for bikes. It is also, probably, unlikely to make you faster if considered in isolation

g) Bigger volume tyres create a bigger foot print in general, though pressure and casing design affects this

h) Bigger foot print is associated with more traction

i) Lower pressure tyres can lead to pinch flats with tubes, and rim strikes

j) Casing design can be beefed up (specifically on the sidewall) to mitigate this, but then the tyre becomes less supple – see hysteresis

k) Casing design can reduce or promote sidewall wear and piercing type puncture risk

So: a bigger tyre volume, with a lower pressure, and less aggressive tread might provide similar or better traction than a smaller, narrower tyre.

It might roll at the same speed (or perhaps faster if the tread allows) and it might be more comfortable.

If it is made well – with good quality casing – it might not wear fast or be at risk from punctures and it might not be too heavy.

Given the whole light strong cheap triumvirate, it is unlikely to be inexpensive.

Depending on how you load the tyre this may be a *good thing*. If it is more likely to flop around on the rim (too narrow a rim, or just high bulbosity) and is used in a high speed/direction changing terrain, it might feel more vague. This is probably why enduro racers/DHers are using no more than 2.5” tyres I suspect.

I guess that, in a nutshell, is why I like ’em?

Now – I am no expert – this was purely typed as a vague self assessment. I’d love to hear if anyone else has any thoughts on all this.”