# Solid verses pneumatic

#### Ajax Bay

##### Guru
tyres are designed not to stretch significantly
Correct. They are designed to stretch a small amount.
Imagine the tyre laid out as a tube: 2100mm long and (say) 25mm diameter (x-area = 491mm^2): Total volume 1031x10^3mm^3.
Now hypothesise your 'one inch' depression model removes one inch (25mm x 2/3 the tyre area so about 8.2x10^3mm^3).
The displaced air would require the rest of the tyre's diameter to 'stretch' by 4 one hundredths of a mm: that's not a stretch of the imagination is it? And the pressure would stay essentially the same - or specifically for this discussion compared to "1% "(Andrew's assessment - see quote I'm suggesting is awry below) negligible (rough calculation 0.3%).
Now a rise of 3 degrees Celsius would raise the pressure by 1%, and I'm sure we've all been on rides where the max/min is 3+ times that.
Pneumatic tyres work because there is a single air chamber that distributes any change in pressure round the whole wheel. Take a tyre, and squeeze an inch of it down to the rim, and the volume of the air chamber, and hence the pressure, will only change by about 1%.

C R

#### GuyBoden

##### Veteran
I tried the cheap Chinese solid tyres on my training roller bike, they were too difficult to get started due to the unbelievably high rolling resistance.

They are also difficult to get unto your wheels, unless you warm them up in the oven for 5-10 mins.

#### lazybloke

##### Lying in the grass looking at the clouds
And that 'modelling' (as you put it) will determine that the pressure in a pneumatic tyre (cycle or motor) experiences a negligible increase when a load is applied. Do not trouble the gauge: any change will be less than the instrument error.
Don't be a lazybloke; think about it.
'Modelling' as I put it? Do you have a problem with the scientific process?

Yes, I agree with you that the pressure increase is negligible, or at least is difficult to measure.
Perhaps the right gauge could measure it if the variables (tyre dimension, initial pressure and weight of rider) were carefully selected to maximise the effect.

Anyway, all I'm here to do is recognise that the laws of thermodynamics prevent elasticity being perfect. This can be easily demonstrated with tyres - the deformation of riding them actually warms them; inflating them causes energy to be lost in the pump connectors, and of course the effort increases as you inflate a tyre near to it's target pressure.

If there was perfect elasticity in air, tyres & inner tubes, the following would happen.
No heat generated when inflating,
Tyres would keep growing larger when pumped; the tyre pressure would always immediately settle at current atmospheric pressure.
Tyres wouldn't warm up
Tyres wouldn't grip!

Conclusion: Because elasticity is imperfect, applying weight to a tyre MUST cause some small increase in tyre pressure.

##### Über Member
8 pages so far and growing on a dead topic. We all know solid tyres are crap compared to pneumatic. Not worth any more words

#### Alex321

##### Über Member
8 pages so far and growing on a dead topic. We all know solid tyres are crap compared to pneumatic. Not worth any more words
So why add a post two days after the previous one, basically saying nothing?

##### Über Member
So why add a post two days after the previous one, basically saying nothing?
Because I wanted to...and I was saying something which was solid tyres are crap compared to pneumatic and saw no reason for 8 pages of posts about such an open and closed issue. Problem?

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#### Alex321

##### Über Member
Because I wanted to...and I was saying something which was solid tyres are crap compared to pneumatic and saw no reason for 8 pages of posts about such an open and closed issue. Problem?
Not a problem as such. I just have difficulty understanding why somebody would make that post, complaining about a thread growing, when it had stopped doing so until you posted.