If the car has sat nav installed and available to the car's computer, as many do now, the speed across the ground could be compared to the wheel rotation. That could detect all 4 wheels being equally underinflated. But, that's 2018, not 2001.
Apologies to Lonnie Donegan. :-) Brian
You must be in the sales side of speedometers if you think a 1% change difficult to detect these days.
From my distant past of mechanics of motion the centre of rotation of a wheel is its point of contact with the ground and the rotating radius/diameter will be less the more the tyre is squashed. Beyond that and trying to find my books I cannot clarify other than it worked in getting my degree.
Before some of you roll your eyes think about it, if the centre of rotation was anywhere else the tyre would be scrubbing. The point of contact is instantaneously stationary.
My limited experience isn't with car tyres. Now, the circumference doesn't have to change, only the effective rolling circumference (see down-thread). (An ellipse, if not too far from circular, has v. liitle change of circumference, IIRC, so view a softish tyre as a one-sided ellipse and there's even less change).To me, the rolling circumference is simply that which is calculated from the 'radius' at the point under load, so if the axle is 10% lower the rolling circumference is
10% less.On my bikes I measured rolling circumference (it's difficult to measure radius to the ground of a loaded tyre as being sure that the bike is at 90 deg. to ground whilst loading the bars... For example figures only, a 5-bar tyre might be 212 cm unladen and about 210
- 211 with ~30 kg on the bars. A 6.5 - 7-bar tyre showed no measurable difference. I haven't measured lower pressure tyres.
The difference at 5 bar is
I think we're at cross-purposes here. Obviously, as you say, when a tyre is flat, the axle is nearer the ground. But is it reasonable to regard that as the radius of the tyre? If you simplify the shape and call it an ellipse, then you have two radii; quite how many radii would be needed to describe a tyre with a flat on one side, I wouldn't like to speculate. But the circumference, perhaps perimeter would be a better word, won't have changed significantly. It'll just become distorted, i.e. no longer circular.
But that assumes there is a weight pressing down on the tyre, which I grant you, would be the case for a loaded tyre on a vehicle. But I was thinking of an unloaded tyre; does the circumference change between under-inflated and fully- or even over-inflated? When I were a lad in the days when tyres had inner tubes, we used to go to our local garage and get old tubes that were no longer serviceable, patch them where necessary, inflate then and use them as super-sized rubber rings for taking down to the beach (and probably getting blown out to sea!). As they were inflated, the radius and circumference most certainly did increase; they blew up like a balloon. But put them into a tyre and there'd be no significant change in the circumference as they inflated and deflated. The structure of the tyre wouldn't allow that to happen.
Likewise, I suggest that the circumference (perimeter if you prefer) of a modern tubeless tyre doesn't change significantly as it inflates or deflates.
Every full rotation of the hub must correspond to a full rotation of the perimeter, regardless of the state of inflation of the tyre, otherwise serious slippage will be occurring between the tyre and the rim, which would result in friction heating and fairly rapid failure. So as far as speedometers and odometers are concerned, state of inflation won't make a significant difference.
(What amounts to 'significant' as I've used it here, I'm not sure; it depends on the pressure difference being considered between under and fully inflated, and the elasticity of the structures within the treaded surface of the tyre, amongst other things, but others in this thread have mentioned figures of around 1% for the stretching of the perimeter as the tyre is inflated. In this context, I would regard that as not significant).
So the short answer to your question is "yes".
Which you have just agreed with by claiming there is a small change.
Can't see that being too difficult in itself - especially as you probably have input from other sensors and know the steering angle input and so can assess when you are driving straight and not under high acceleration etc.
Type pressure monitoring will need to be more sensitive to rate of change than absolute difference since unequal tyre wear would otherwise be flagged.
I just love watching Turnip arguing that a system that exists, is in production and works is an "urban myth". What next, Flat Earth?
Yes. You are effectively making a triangle with the base being the flat part of the tyre. And that base is going to be a shorter length than a similar triangle where that base is a curve.
In other words, the radius at the point of contact to the road determines the gearing. Whatever happens to the rest of the tyre is immaterial.
Oh dear. Oh dear oh dear.
You just dont get it, do you?
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It has no meaning.
Pins/ angels/ head of/ count.
Well unless the tyre is slipping on the rim or in the road, there is no way anyuthing else can.
And those who actually measured
No, the world is full of stupid people pretending to be clever.
The 'effective rolling radius' is the circumference divided by 2 PI.
Trying to make a squashed dougnut into a circle doesnt really fit though.
Excatly. And yet those that say that the radius has altered by 20% and yet the circumferemce hasn't altered at all, have only one way to make the RPM rise by the amount they say the radius has rteduced. Introduce tyre scrub.
It's like climate change all over again. Facts dont fit the theory, so introduce 'feeedback'
Pretty irrelevant. The centre of rotation is actually on a planet in a far distant galaxy.
I refer you to :
No, That is pretty much wot he said
Not if you have any understanding of mechanics. The whole tread, up to and including a caterpillar track, goes round once per revolution of the track or tyre.
THAT is what determines the RPM/speed relations ship.
What happens with tyre pressures is quite clear. The tread stretches slightly under higher pressures. How much will be a function of the tyre construction. And this is what the sensors rely on.
Since no wheel is circular using radious as a concept is plain wrong. At best you can calcualate a '*radius it would be if it were round*,' from the actual circumference.
For it to be any other way the tyre must slip on the rim or on the road, substantially.
No the urban myth says that since there is say a 10% change in what peooplle think is 'the radius', therefore the RPM will be 10% slower.
Why would there be monitors on steering angle?
Well teh way it aseems to work is that one wheel will overotate with respect to its diagonal consisetntly.
But not by very much.
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Oh dear. What I was arguing was that the reason the systemn works is not what people here believe.
If you told me that essence of angel was what was in antibiotics and they fought the demons in the pus, it doesnt mean that I disagree that antibiotics cure infections, when I tell you you are talking bollocks.
But basic logic is another thing that has passed you by it seems.
For the stability control system.
But once they're a standard part of the car you can use feed the data to other systems, such as overlaying the curves on the reversing camera display, or knowing when you're travelling straight and therefore the TPMS doesn't have to worry about different wheel speeds caused by the action of a differential.
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