Author Topic: Tyre pressure - Loaded or unloaded?  (Read 1589 times)

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Andym535

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Tyre pressure - Loaded or unloaded?
« on: July 19, 2019, 11:47:07 am »
Maybe this has been discussed before. I could Google the answer or even go out the the garage and check, but here goes....

Does the load on the wheel have any impact on the tyre pressure?

If you were to check the pressure of the rear tyre with the bike on the centre stand and the wheel not touching the ground, would it be higher, lower or the same if you measured the tyre pressure with the bike on the side stand and the rear wheel on the ground? If weight does change the pressure, and common sense suggests it should, then should you set the pressure with someone sitting on the bike?

When it stops raining I might have to go and find out the answer experimentally.

Offline raesewell

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Re: Tyre pressure - Loaded or unloaded?
« Reply #1 on: July 19, 2019, 11:53:31 am »
I have tried this before, it's the same.  ;)

Offline richtea

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Re: Tyre pressure - Loaded or unloaded?
« Reply #2 on: July 19, 2019, 01:14:12 pm »
Physics! I've heard of it.

Pressure is related to volume and temperature:
- decrease the volume and the pressure goes up
- increase the temperature and the pressue goes up

And vice versa, of course.

So, assuming the tyre doesn't decrease in volume when the tyre is loaded (and the temperature is the same) then the pressure won't change.

In real world use if you put the bike down and load the tyre, the tyre changes shape at the touch point, but the loss of volume where you squashed it down is made up for by the tyre bulging out elsewhere - keeping the overall volume the same.

If you drop the bike down heavily, then there may be a temporary localised pressure increase, whilst the tyre flexes itself into the new position/shape. But that's only very temporary, like a ripple of air inside the tyre.

A real physicist will be along soon to correct me...  ::)

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Re: Tyre pressure - Loaded or unloaded?
« Reply #3 on: July 19, 2019, 06:20:40 pm »
OK - just guess work and very possibly rubbish but, I  think some of your squash under load is incorrect.  A tyre does not "bulge" more if you over inflate it. As such, I don't think the "loss of volume" due to the tyre deforming when it's on the ground is made up by expansion elsewhere.  I think the pressure does increase but it's such an insignificant increase it's not really noticable.  Either way, differences in temperature, inaccuracies in the measuring equipment and barometric pressure differences all make the difference in pressure when checking on or off the center stand irrelevant.
Correct rear brake use is scientifically proven to shorten stopping distances in EVERY road situation.

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Offline richtea

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Re: Tyre pressure - Loaded or unloaded?
« Reply #4 on: July 19, 2019, 06:56:50 pm »
https://physics.stackexchange.com/questions/132892/does-car-tire-pressure-change-with-weight-of-car-load
The answer with 23 votes seems about right to me, i.e.:
With the numbers I used above, the fractional volume change is only 0.03%

And another, but for bicycles (potentially a more pronounced effect of a fat bloke on a skinny tyre, in my case  :-[):
https://bicycles.stackexchange.com/questions/21227/how-much-does-rider-weight-increase-tire-pressure
...we're still looking at a volume change of 10-5

Offline Matt

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Re: Tyre pressure - Loaded or unloaded?
« Reply #5 on: July 19, 2019, 09:13:11 pm »
Yeah my tapeworm infested gut tells me it's buggered up more with them degrees Celsius.

And me forgetting to pump it up.

But I want Costas to come and remind me how low he runs his pressures :D. I'm heading that way!
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Brian c. 2019

Andym535

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Re: Tyre pressure - Loaded or unloaded?
« Reply #6 on: July 19, 2019, 10:17:35 pm »
I think that it doesn't change much if your within a fairly narrow range of normal load and pressures (and temperatures), but would be noticeable for, say, a very under inflated tyre or very overloaded bike. Tyre size and construction must also come into it so I expect something like a Suzuki Van Van might see more change than a K with super stiff Variant A Angel GT IIs.

I'll be really disappointed if it turns out to be an urban myth that you can burst a vehicle's tyres by overloading it!

Offline richtea

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Re: Tyre pressure - Loaded or unloaded?
« Reply #7 on: July 19, 2019, 10:31:42 pm »
I'll be really disappointed if it turns out to be an urban myth that you can burst a vehicle's tyres by overloading it!

Andy, you've got exactly the pneumatic kit to try it!
I'll bring my camera and safety glasses, you pump up one of your old tyres.

But before we start where's the nearest hospital?

Andym535

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Re: Tyre pressure - Loaded or unloaded?
« Reply #8 on: July 19, 2019, 11:11:12 pm »
I'll be really disappointed if it turns out to be an urban myth that you can burst a vehicle's tyres by overloading it!

Andy, you've got exactly the pneumatic kit to try it!
I'll bring my camera and safety glasses, you pump up one of your old tyres.

But before we start where's the nearest hospital?

Hmmm... I put some new wheels on the car this week. The originals were getting a bit corroded after 230,000 miles of neglect, so I have some spare wheels and tyres 'in stock'. You probably noticed the 20 tonne shop press in my garage too. I can see one of us winning Warwickshire's first Darwin Award if we pursue this though.

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Re: Tyre pressure - Loaded or unloaded?
« Reply #9 on: July 20, 2019, 08:56:49 pm »
You can certainly burst a tyre by running it overloaded and underinflated at high speed, the verges and central reservations of the countries motorway system will attest to that.

Offline richtea

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Re: Tyre pressure - Loaded or unloaded?
« Reply #10 on: July 20, 2019, 11:47:59 pm »
You can certainly burst a tyre by running it overloaded and underinflated at high speed, the verges and central reservations of the countries motorway system will attest to that.

That would be increased heat through over-flexing which creates the additional pressure, not a decrease in volume. Stop confusing them, Brian!

Offline Phmode

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Re: Tyre pressure - Loaded or unloaded?
« Reply #11 on: July 21, 2019, 05:02:19 pm »
It's in my job description mate  :-*