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T APLUS 22
Forums Junkie

Location:
Melbourne
Registered:
October 2003

Re: right tyre pressures for 235/45/17's?

Mon, 05 January 2004 19:28

Scorpion wrote on Mon, 05 January 2004 22:19

T APLUS 22 - In my humble maths training, to be a ratio, AR has to have a numerator and denominator. So yes, Aspect Ratio relates to the tyre width but as the percentage ratio of the height to the width. So a 245/45 tyre has a nominal tyre width of 245mm and a nominal height off the rim of 245mm x 45% = 110.25mm. I say nominal because actual height depends on tread depth which obviously changes during the life of the tyre.

Also, in my humble physics and/or maths training, lowering the AR shouldn't reduce the area over which the force of the car's weight is spread as in using a lower profile tyre one normally uses a larger rim to maintain the overall diameter of the wheel.
In fact, one would expect the surface area to increase because of the wider tyre profile. Too wide and you can easily aquaplane on a wet road because not enough force is being applied over the contact patch to push the tread through the water layer on the road to make contact with the road surface - and water gets trapped as it can't escape quick enough.

The correct tyre pressure will be determined by experience with a given tyre on a given car as other factors such as tyre wall strength come into account in how much air is needed to resist bottoming the rim.

Sorry to be a killjoy but it pays to get to know your car and experiment with pressures above and below the 38psi suggested in previous posts. Watch how far around the edges that the tyres scrub on the road with the different pressures and driving styles.

Scorpion
well, quite humble. if you'd check with your local dealer it is called a ratio... however no-one said anything about being a constant ratio. Its simply a wording of AR to describe the size of the tyre. A larger width on the tyre will actually change the overal height of an inlflated tyre, even if the profile remains the same.

And as for the second comment, well you might want to read my post again, I was talking about the total surface area of the tyre... not the area keeping contact with the ground. If you don't understand what I mean, then pm me and I'll go into more detail. So, there was no killjoy, just a miss-informed post in some respects, and if you aren't sure what your talking about when trying to disprove someone else, then maybe its best not to try and put holes in others statements.

[Updated on: Tue, 06 January 2004 08:28]

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