What is your tyre rolling radius?

**windsock** · 13th May 2010, 07:50 AM

OK, I've been searching through a lot of threads, both here and elsewhere) seeking an idea of what peoples tyre rolling radii are relative to the actual stated tyre size.

I haven't found any consolidated info so I thought it'd be interesting to have information here on other peoples vehicles covering info like:

Vehicle type (e.g. 110 trayback)
Vehicle weight (e.g. 2000kg)
Tyre make, model and size (e.g. Dunlop Grippers 750R16)
Normal road pressures (e.g. 38psi - if you want to provide off-road pressures too that'd be a bonus but also provide off-road rolling radius )
Tyre tread information (New=15mm, now=5mm)
Inflated diameter (e.g. measured from one side to the axle centre horizontally - 395mm)
Rolling radius (e.g. measured from the axle centre to the ground on level ground - 370mm)

From all this info we can learn what make and size tyres give what kind of ground clearence for what type of vehicle weight etc. I am having difficulty finding such information at the moment. If I go with different makes and sizes, any rough estimate could end up being an expensive dissapointment depending on side wall strength and other things. I found I couldn't rely on tyre dealers as they only want to impress for the sale.

My truck info is posted in the example above which means while I purchased a tyre about 31.5inch diameter when new, after so many kms, I am actually running around on the rolling radius equivalent of a 29in tyre

(no wonder the salisbury is dragging!)

Cheers

Phil

PS, pictures of your land rovers would be good too

**Jim621** · 13th May 2010, 05:23 PM

Hi Phil,

Im not sure how your rolling radius can go from 31.5 inch to 29inch....that would imply to wear down 1.25 inches of tread!!??

What is your reason for asking the question?

Steve

**windsock** · 13th May 2010, 06:11 PM

Originally Posted by Jim621

Hi Phil,

Im not sure how your rolling radius can go from 31.5 inch to 29inch....that would imply to wear down 1.25 inches of tread!!??

What is your reason for asking the question?

Steve

Rolling radius comprises of sag in the side wall of the tyre as it sits under normal conditions. It differs from overall tyre diameter. A lot of discussion about tyres revolves around overall size and whether it'll fit or rub or whatever. I want to know what peoples actual rolling radius is, in other words, how much does their tyre size, truck weight and pressures etc effect their actual diameter as seen by the truck while driving.

My reference to the start diameter of my 750R16LT's and ending with 29" includes tyre bagging at normal driving conditions and wear and tear. I started with what I thought was a 31 inch tyre and now after a year and a bit of driving and thinking about the next set of tyres, I am interested in what tyres give what kind of rolling radius under what conditions. I am thinking about this alot more now than I did when I got the 750's.

I am interested in what the rolling radius is of for example 255/85/16s (or 235/85/16's or 265/75/16's etc) under a normal truck under normal useage, not laying flat on the dealers floor with manufacturer spec's being quoted if you know what I mean.

Not often discussed as I have found from searching without luck.

Cheers,

Phil

**JDNSW** · 13th May 2010, 07:29 PM

There are two separate considerations. Rolling radius, as you describe it, depends on both tyre diameter and the amount it deflects under load (depends on pressure tyre dimensions and sidewall stiffness), determines actual ground clearance and vehicle height, and also, together with tyre width and wheel offset, interference with wheel arches.

This is not the same as the effective radius for determining gearing - for any practical tyre pressure and at least for radial tyres, this radius is half the tyre tread circumference divided by pi and is unaffected by pressure, being determined only by belt length and tread depth.

Now some actual measurements - from flat concrete to hub centre.

Landrover 110 County, Bridgestone 604V 7.50R x 16, 35psi F&R, unloaded.

Front, half worn, 360mm, 365mm.

Rear, done only 5,000km 365mm, new, 375mm (pressure may be a bit high)

Landrover 2a 109 ute Steeltrek 7.50Rx16, 90% tread, 30psi F&R, unloaded

All tyres 370 - 375mm

John

**rick130** · 13th May 2010, 07:50 PM

Ok I'll play

'98 130 CC

2660kg last time it went over a weigh bridge, and that weight wouldn't vary too much from week to week.

255/85/19 Maxxis Big Horn 762's.

Inflated circumference when new @ 31psi = 2650mm = 844mm unloaded diameter = 422mm unloaded radius. (running a tape around a tyre and measuring its circumference is a much more accurate way of measuring its diameter than trying to measure across the tyre)

Generally run 40psi all round on road, so probably would have met the claimed 846mm diameter at my normal pressures. 26psi on extended periods of gravel/dirt roads.

Current tyre diameter/radius/loaded radius = ??? as there isn't a level piece of concrete within cooee.
Approx. 19mm taken off the bottom of the Sals to reduce plough/anchor effect.

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