200,000 k's BT50

Albynsw

Well-Known Member
So where does the excess tyreface go when you "reduce your radius'
When you drop your pressures you are effectively altering the centre point of the tyre circle but the face of the tyre ( circumference) is still the same, it just has one flat face where it touches the ground
 

mikehzz

Well-Known Member
Hey, I remember it now. All tyres have a flat bit that is in contact with the ground, they aren't full circles. The effective radius and therefore effective circumference of the tyre is the smaller imaginary circle you can draw inside the tyre that has that flat bit as its tangent. The flatter the tyre, the longer the flat section and closer to the centre making the imaginary circle smaller. The tyre rotates at the speed of the imaginary circle.
 

mikehzz

Well-Known Member
For the tyre to even have a flat bit, the rubber has to compress and become deformed. The flat bit is what used to be the arc but squashed flat. The distance of the arc is greater than the length of the flat bit, therefore the circumference has to be smaller. I had to switch on my reserve brain cells to write that. :)
 

mikehzz

Well-Known Member
If you really think about it, it becomes clear why tyres will delaminate if run at low pressure. To fit the part of the tyre that used to be on the outer circumference in to the flat section in contact with the ground would really require a lot of mangling. The steel belts would either be pushed sideways or buckle in towards the centre as they can't really compress like the rubber can.
 

hiluxdriver

Well-Known Member
Think I have it worked out, and am starting to think that I was wrong. Distance from the hub to the contact patch seems irrelevent when the circumference of the tyre doesn't change. I guess if you could inflate the tyre so much it stretched it may have a marginal difference, but generally the distance around it stays the same. That's how I see it.
 

mikehzz

Well-Known Member
Think I have it worked out, and am starting to think that I was wrong. Distance from the hub to the contact patch seems irrelevent when the circumference of the tyre doesn't change. I guess if you could inflate the tyre so much it stretched it may have a marginal difference, but generally the distance around it stays the same. That's how I see it.
I think you wil find that the circumference does change due to the deformation. Otherwise those indirect tpms systems wouldn't work. They are specifically designed to pick up differences in wheel rotation.
 

mikehzz

Well-Known Member
To demonstrate, take a square bit of rubber and stretch it in one direction, the size in the other direction gets correspondingly smaller. You cant have rubber stretch at all without it contracting somewhere else.
 

cam04

Well-Known Member
You are describing the 'effective rolling radius' of a tyre which is different to the circumference and that is where the confusion is. Circumference of steel belts in tyres doesn't change. The effective rolling radius goes up and down with load and pressure and that will make the tyre complete differing revolutions per kilometre. The VW system needs up to 25% difference in pressure over time before it will register a fault - it is hardly instantaneous.
 

mikehzz

Well-Known Member
You are describing the 'effective rolling radius' of a tyre which is different to the circumference and that is where the confusion is. Circumference of steel belts in tyres doesn't change. The effective rolling radius goes up and down with load and pressure and that will make the tyre complete differing revolutions per kilometre. The VW system needs up to 25% difference in pressure over time before it will register a fault - it is hardly instantaneous.
What they are saying is correct. If the circumference of the tyre doesn't change, then the revolutions per kilometre has to be the same irrespective of the radius at different points on the tyre. Because the revolutions do change, I have to assume that the circumference does change and that the change is linked to what happens to the tyre when it goes flat and starts bulging and deforming.
 

mikehzz

Well-Known Member
Another point, you can't change the radius without affecting the circumference. Forces are at work and something has to give.
 

mikehzz

Well-Known Member
A simplified version of what's happening.

CCF13102017.jpg
 

Mr Rum

4x4 Earth Legend
Hmmm...

E5672B20-29D2-427D-B273-54449B344CE8.jpeg


Edit: If it can, I must be missing something.

Edit MkII: Assuming it’s correct, the post below explains what I was missing.
 
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cam04

Well-Known Member
Another point, you can't change the radius without affecting the circumference. Forces are at work and something has to give.
Actually you can, that's why it's called an effective radius, not actual radius. The larger the contact patch (via load or low pressure or both) the higher the slip rate, meaning the tread is deforming and grabbing the tractive surface and increasing the rolling resistance of the tyre. The circumference doesn't change, but the distance travelled per revolution does due to slippage against the tractive surface.
The only proper way to explain it is mathematically. See attached.

http://www.google.com.au/url?sa=t&r...Dynamics.pdf&usg=AOvVaw1WNqOuaiZLEMZe8HYHUjhv

Page 3 may also shed some light on why a slightly lower pressure (sometimes) decreases rolling resistance and increases efficiency.
Fat cyclists (Clydesdales) like me (115 k.g.) are much better off riding a 28mm tyre at 90 psi than we are riding a 23mm tyre at 120 psi for that very reason.
 
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Albynsw

Well-Known Member
So the rolling diametre is constant by measurement but the slip rate on the road surface with load or deflation varies.?

Part of the confusion is thinking of a tyre as a true circle when in fact it has a flat face be it minor when at high pressure but still theoretically there or exaggerated at low pressure but the rolling circumference is constant

I think the mods should ban Mikehzz for starting this
 

mikehzz

Well-Known Member
I dont think Mr Rum's far prettier picture is correct because tyres a specifically designed not to bulge outwards as illustrated on each end of his flat bit. They are far more likely to bulge inwards in the flat bit. Also if the actual tread patch in contact with the ground stretches wider in any way it will force the circumference to shrink. That's mikehzz's first law of rubber stuff. :)
 

Mr Rum

4x4 Earth Legend
I dont think Mr Rum's far prettier picture is correct because tyres a specifically designed not to bulge outwards as illustrated on each end of his flat bit.
That was really just for illustrative purposes, as I didn't have an answer for where "z" went.

The document cam04 shared explains how it can "appear" to vanish though, and it makes perfect sense to me now.
"z" is still there (it has to be), and it's still equal to "z", however it's only going to have travelled "z - slip".

Or, "The circumference doesn't change, but the distance travelled per revolution does"

The change in circumference isn't real, and is merely an illusion.:)
 
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