Quote:
Originally Posted by Aussie Mike
OK, there is a bit of science to it but this is my understanding.
It depends on the load rating of the tyre and the weight at each corner.
The pressure numbers on the tyre placard usually depend on the load applied to the tyres and not so much the handling or comfort. It's a safety number to maintain the load rating.
On the side of the tyre there will be a load rating number and a speed rating letter.
For example on the side of a Kumho KU36 in 315/35/17 there is the number 102W
The load rating is 102 (850KG per tyre) and the max Speed rating W (270Km/h)
Most of the tyre manufacturers will publish the data for their tyres. Bob Jane has a handy table on their web page toi interpret the numbers.
load-index-speed-symbol
The load rating is a maximum load but it is dependent on pressure. So to know the pressure you need to run will depend on how much load you have on your tyre.
For example on the rear I have the 315/35/17 tyres. And say my car weighs 1100KG and has a 52% rear bias so that means 572KG on the rear wheels or 286KG per tyre.
I haven't been able to find the load tables for Kumho but Toyo have them on their site
LoadInflation_Table
If you go to the 2nd to last page it has the standard load inflation tables.
For 102 rated tyre at 36PSI the rating is 1874 pounds (850KG)
At 22 PSI the load is 1246 pounds or 565 KG. Still well above the actual load you are applying to the tyre.
So as long as you have enough pressure to maintain your load rating you can tune the pressures for comfort or cornering.
Hope this helps
Cheers |
FYI, both the load index and the Speed index are set by the Tyre & Rubber Manufacturers Association, in conjunction with tyre manufacturers.
It is
the International Standard & manufacturers have to submit construction & materials data for each tyre they make to the Assoc. for verification etc to say that the tyre is manufactured to meet the load & speed indexes. Therefore the load & speed ratings from all manufactures are the same.
Having been a rubber grub for more years than I want to remember, tyre pressures are most always up for debate........kinda !
Vehicle manufactures set their recommended pressures primarily for ride comfort & then for understeer.
Why understeer you ask ? Well the idea is that when the average punter arrives at the corner too fast the driver stands on the brakes, turns the wheel and the car plows straight ahead slowing the vehicle down & reducing impact
Hmmmm !
Why ride comfort (while I'm at it) When buying a new car, the buyer might spend 1-2hrs at the dealership looking at the car and doing the deal-however-
you (on avg.) spend only about 5-8 mins actually driving it. When you do get to drive it, you are usually taken by the sales person on a well travelled route used by said sales person that is not only well known but has few pot holes and a smooth surface. They get out at some point and you drive back to the showroom again by a nice smooth route as well.
THAT is the impression that they want you to have of the car-it's smooth & quite and has a lovely ride-not rocket science just good salesmenship
Will continue Tyres 101 tomorrow
Rog