does the rolling circumfences / perimeter of a tire change with the tire pressure?

[Willem Winch]

I thought this is a quite tricky question and it took me a while and some pondering to find the right answer respectively the right explanation (well, I think it is the right anwer... you might not agree...)... have an own go (before you look up the answer on my website) ......

[jsinton]

Of course is does.

[Willem Winch]

did you measure it?

[RBURGOSS]

Of course it does, but to practical matters, what is important is the "static loaded radius" which in term, rules the actual working circumference.

Probably, at an unloaded state, the tyres may keep the overall diameter about the same if inflated from 10 to 10 lbs, but when loaded with car's weight, the actual radius (center to ground) will change depending on the tyres pressure. How much it changes? Depends on tyre carcass build or course, but again, I doubt it difference between practical tyre pressures (25 to 35 lbs) will be significant or even measurable by standard means.

[veilside1]

yea it does, but not in any significantly measurable way. and obviously the build of the tire as well as aspect ratio will play a large part in this as well. obviously a low pro with a 1.5in sidewall will deflect less than an 80 series SUV tire with a 6in sidewall.

[Willem Winch]

I have LT 265/75-R16 Pirelli Scorpions ATR, which do not fit in the spare well anymore... when I assessed whether it is possible to get them somehow into this narrow spot, I start measuring the diameter, easy to do with a cord: just roll it around the tire and mark it where the coils met, then measure from mark to mark... there you have the perimeter and the equation 2 x R x Pi gives you the radius respectively the diameter....I did it for 0 psi, I did it for 40 psi ...

There is no!!! measurable difference in perimeter or diameter, at least not with this methodology, and this methodology is accurate in order of plus/minus 1 mm... ... o.k. these are tough LT tires, but on the other side 0 - 40 psi is also quite a pressure range...

Robert,
the static load "radius" isn't a radius anymore if the contact surface becomes a flat spot: here the equation 2 x R x Pi is obvious not valid ...

...some more pondering - with some answers (? - you might not aggree) here: 4x4 article 7 - Tires Part 3

[RBURGOSS]

Originally Posted by Willem Winch

Robert,
the static load "radius" isn't a radius anymore if the contact surface becomes a flat spot: here the equation 2 x R x Pi is obvious not valid

Ha... let the games begin! This is gonna get interesting.

Agreed: Static loaded radius is no more "radius" when the contact spot becomes flat, but (this is a big BUT), depending on the tyre pressure, that flat spot area may be smaller or larger. At higher pressures, the flat area will be smaller and FARTHER FROM THE CENTER OF THE TYRE at exactly perpendicular position from the tyre's rotational center.

But then, its change is so small that I guess it could be measured at very long distances (tyre turning and floor marks method), probably making a difference of very few cms over 1 kilometer length.

[Willem Winch]

Originally Posted by RBURGOSS

...Static loaded radius is no more "radius" when the contact spot becomes flat, but (this is a big BUT), depending on the tyre pressure, that flat spot area may be smaller or larger. At higher pressures, the flat area will be smaller and FARTHER FROM THE CENTER OF THE TYRE at exactly perpendicular position from the tyre's rotational center.

...no doubt about this ...
but when a higher pressure (e.g. 40 psi vs 0 psi as I measured in my testing) doesn't change the tire diameter (for the wheel just lying on the ground so no flat spot) the perimeter is still 2 Pi R and this length, the rolling circumferences, will not change also if the tire is on the car and the car is heavily loaded as the steel plies in the tires tread doesn't allow longitudinal flex. While depending on the pressure the size of the flat spot changes, but all what it does is that it changes the way how to calculate the perimeter now - the equations change, not the perimeter............ ...

[ernwilson@msn.com]

why dont you tell us somat use full instead of useless info. lol.

[Willem Winch]

well, everyone is entitled to his own opinion about what is useful and what is not so useful - my apology that I couldn't fullfill your expectations ...

actually IMHO for vehicles that are equipped with LSDs, lockers, 4WD etc. it is quite usefull to understand what causes wind-up and how to avoid it to minimize wear and tear and repair costs...