Disco tyre diameters, Mix/Match - essential ?

[Boydie]

Okay, as some of you may be aware I'm going to be driving down the Canning Stock route from Halls Creek to Wiluna in Western Australia riding shotgun for a mate who intends cycling the 1800 odd kilometers, of sand dunes, corrugations and salt lakes in a planned 10 days or less. He intends cycling at least 10 hours a day so lil' old me will be pottering along some 2 k's behind him at around 19 kph.  I am not allowed to give assistance merely to carry his spares and tools. He has to carry his own food, water bedding tent etc.

WA has received an extraordinary and unseasonable amount of rain this year and the spinifex has grown.  Now, for those of you who haven't encountered spinifex imagine a nasty vindictive vegetable porcupine rooted deep into the soil. It may be a form of tussock grass but trust me it can and in my case the sharp stalks have punctured my tyres both in the side walls and right through the tread. The problem is that the stalks are so thin that external detection of the leak with soapy water is almost impossible, the only solution is fully immersing the wheel in a tank of water - and tanks of water are rare in the desert.   The solution is to remove the case and fit a new one, I carry a pneumatic bead breaker and tyre levers and although I'd rather get someone else to do it, - because its bloody hard exhausting work - I can and have replaced several tyre cases.

So, okay I will be taking around 10 wheel/tyres with me, Mounted on my six rims will be 6 Bridgestone Desert Duellers - these are 235/70R16. I also have 4 Maxxis 225/75R16.   My question to the forum is this; the rolling diameter of the Bridgestones is a theoretical 29" (if they were brand new with 100% tread, which they are not, they all have around 7mm of tread depth to the limit bar) the Maxxis are a theoretical 29 1/4" - again if they were brand new with full tread, again they are not, they also have around 7mm of tread to the limit bar.

So, can I use the Bridgestones and the Maxxis together on the same axle with reasonable impunity with only 1/4" (6mm) diameter difference bearing in mind that if I get a puncture in the desert they will be running around 16 psi - do I have to use the same tyre brand/size on the same axle or can I mix-match as and if I get a puncture ? 

[garrycol]

Using a standard tyre diameter calculator, I get a difference of 8.5mm in diameter - this is less than half the thread depth of a good AT/MT tyre.  I would not drive any great distance on bitumen but I doubt there will be any issue driving offroad with that mix - if there was to be an  issue the wheel will just slip in the sand or when it goes over a bump to equalize anything stress wise out.  Obviously you will keep the mix the same for as long as possible - I assume you have 4 Bridgestones on the car, with two spares and the 4 Maxxis are in addition to these.  Not much room left in the Disco.

You will probably have more of an issue with fire with the spinifex getting caught up around the exhaust but I think you will be OK tyre wise - though I would certainly be stocking up on tyre repair strings and using those for the less damaging repairs.

Good luck with it - 180km per day in the sand and dunes on a push bike - Hmmm optimists at work there :-)

 

Garry

 

 

Edited June 2, 2017 by garrycol

[FridgeFreezer]

The issue with mismatched tyres will be the extra work done by the gears in the diffs (axle & centre), a long run at speed on asphalt could generate extra heat.

[Boydie]

Thanks for that input.  

I'll have the 6 Bridgestones on rims, one spare on the rear door, one spare rim and tyre on the bonnet,  the 4 Maxxis cases will be on the roof rack.  I wont be running any Maxxis / Bridgestone combinations on bitumen - only on sand and then only if I run  out of Bridgestones and I'm not able to repair the leaks.  I'm even tempted to try some puncture moose as a quick leak fix as I'll be tossing all ten cases on my return and having six new Bridgestones fitted.  

I've used moose in the past with some success but it makes a mess on the inside of the rims that has to be scrubbed off before a new case can be fitted otherwise getting the new tyre correctly balanced is a nightmare.  

As for grass fires, I dont think that will be an issue, the last time I went off road over the Gibson Desert going south from Mulan to the Gary Highway via Lake McKay and following camel tracks over the sand dunes all the way my EGT at the turbo never went over 280*C the engine simply wasn't having to work that hard, I dont think from memory it ever went above 10psi boost, another reason to select a diesel for desert touring but in any case I always make a point of getting under to remove any grass from around the underside at every stop as well as making sure I park up on clear ground. A bigger issue is keeping it out of the radiator and intercooler!