Are You Operating Your Tyres Beyond Their Design Limits?

Wednesday, Feb 09, 2011 at 10:31

Member - Rob D (NSW)

(Forum post now posted as a Blog at the request of Members)

To my surprise I found that I had been operating my existing tyres well beyond their design limits and I am lucky that I was not stranded on the Canning Stock Route with two de-laminated rear tyres.

I recently had to decide to either replace, or upgrade the worn 265/75R16D (‘D’ = 8 ply rating) 4WD tyres on my Landcruiser 100 series (I changed the original 17 inch alloy rims for 16 inch steel rims). I had not been all that happy with these tyres on the rear of my vehicle as they got very hot when deflated to the pressures needed for sand or gravel road driving. To be fair, I must confess that I have dual wheel carriers on the rear tow bar, a long range fuel tank and Black Widow drawers all of which put a very heavy load on the rear end even before I pack for the trip and attach the Ultimate Camping Trailer. I weighed the vehicle on a public weigh bridge and was surprised at it tipping the scales at 3570 kg without the long range tank filled and carrying very little load (Ultimate Camping Trailer not connected).

Whilst attending a sand driving course, a well respected 4WD driving instructor told me that he thought that my existing tyres were bulging too much at the pressures needed for sand driving and suggested that I should change the existing 265/75R16D tyres to the larger 285/75R16D tyres. I bounced this idea off three different 4WD tyre supplier ‘experts’ who disagreed and told me that the 265/75R16E (‘E’ = 10 ply rating) had a higher maximum load rating than the 285/75R16D and that I should use those; I pointed out that the 265’E maximum load rating was at 80psi and the 285’D maximum load rating was at 65 psi, and further that I never intended to drive off-road with either 80 or 65 psi in my tyres. When I asked about load ratings for their tyres at realistic off-road tyre pressures such as 20psi, 30psi, 40psi and so on, none could provide me with the required information, so I decided to research the problem myself.

I spent about 12 days searching the internet and consolidating material to fully understand the issues. There is too much information to post on this site so I have posted it on my own website in bite size pieces. http://www.netspeed.com.au/rob.dobson/
It contains information on:
• Tyre load vs inflation pressure vs speed for 265/75R16 and 285/75R16 tyres.
• Tables containing load vs inflation pressures for most LT tyres.
• How much can load be increased if speed is reduced?
• At what temperature do tyres become permanently damaged?
• How long can you keep tyres before they deteriorate?
• And lots more including links to other useful websites.
I hope you find it useful.
If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob
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