Tyre Pressure Question

[mattrsa]

thanks for all the replies guys

I ran the tyres on the same pressure as standard. I think when doing this I noticed is said the tyres should not exceed 60psi which I was obviously no where near. The van felt and drove (350 miles) fine, if anything it actually felt better than on the standard rims and tyres.

Matt

[AdrianC]

One theory I've seen elsewhere on the web is to look at the max pressure on the tyre, and the max load. Then calculate the _actual_ load for that end of your van, as a %age of that max load. Then apply that %age to the max pressure.

I have no idea if it's valid, but it's gotta be worth a try. This is one of those times when an IR thermometer might help - a good run, and check temps are even across the width. If the pressure's too high, then the centre of the tyre will be hotter. If it's too low, the edges of the tread will be hotter. Leave it long enough, and wear will mirror that.

One little question... These 185 fronts. What width is the rim? Because I rather suspect that 185s are going to be a bit narrow for 16" Merc rims.

[mattrsa]

not 100% on the width but I will check tonight. The guy I bought them from had them on his van prior to selling them to me so I just assumed they were correct and since I have had them on the van has driven well.

Regards
Matt

[CJH]

One theory I've seen elsewhere on the web is to look at the max pressure on the tyre, and the max load. Then calculate the _actual_ load for that end of your van, as a %age of that max load. Then apply that %age to the max pressure.

That's virtually the basis of the ETRTO spreadsheet I linked to above, except that the ratio of axle load to the tyre's maximum load is raised to the power of 1.25, which means you end up with a slightly lower proportion of the tyre's maximum pressure.

I got my van back at the weekend, and I eventually remembered to check the max load and max pressure on the sidewalls of my tyres. I've got Super2000's on the front and Vanpro's on the back, and they're both rated at 850kg and 65psi (max). Using the speadsheet, and the 'Normal payload' axle loads of 1100kg front and 1300kg rear, then the spreadsheet gives 38psi front and 47psi rear. That's not a million miles from the recommended 37 and 54 respectively.

Using the 'Higher payload' axle loads of 1200 and 1400, it gives 42 and 51.

So it looks like that formula gets you pretty close to a sensible pressure. If you can't be bothered with the spreadsheet, just put this in your calculator:

Tyre pressure = Max tyre pressure x (max axle load/(2 x max tyre load))^1.25

(the '2' is because there are 2 wheels on each axle)