Can someone elighten me further on the whole twin-wheel debate? I'm taking a twin rear wheel van to africa and hadn't anticipated that being a problem.
like nick, I assumed that twin rear wheels would be preferable in mud and sand as they would spread the load over a wider area.

Can any one explain the following:

• The mechanics of why they are worse? (so i can attempt to adapt my driving style)
• How effective / dangerous is letting the tyres down? Are we talking 'not ideal but effective' or 'heading for a blow-out in a couple of miles??'.
• How often one should check for trapped stones (once a day or more often?)
• Whats the best way of getting a twin wheel with no difflock over sand and mud? (i.e. tyre inflation/entry speed/use of sand ladders/snow chains)

Cheers!!!

I got stuck with twin wheels all the time! I met people who had taken one of the two wheels off. if you have two then you have to make two tracks in the sand or mud. I never drove on desert just bad roads I also did not let the tyres down. if you get big stones stuck in your tyres you will hear it, just check them when you can its a good idea to check around the vehicle before you drive off in the morning to make sure all is well.

Graeme

Twin tyres correctly inflated are no better/no worse than a super single tyre ON A BITUMEN ROAD and good graded dirt roads. The super single may have a slight advantage for fuel economy being a few kilos lighter. If a super single fails you have nothing left to drive on and are forced to change a tyre wherever it blows, with duals you can limp to a safe spot - hard ground or whatever. Dual tyres still seem to be the choice of road train operators in Australia, which would indicate that super singles dont perform well in that environment.

Hard dirt roads with a lot of small rocks that can lodge between tyres is the real killer for dual tyres, you can end up with an unrepairable side wall hole in a few kms. Low inflation will also cause the side walls to flex and touch and you end up with a tyre fire, which on a van is usually catastrophic as there is insufficient space to throw the burning tyre (trucks just keep driving until the tyre burns off the rim). Dry powder FEs wont do anything - you need lots of water to put out a tyre fire. Check the tyres whenever the road surface changes and you can increase your speed, its possible to check tyres and get a blow out within minutes. Tyre temperature is a good indicator of something being wrong - use the back of your hand.

Deeply rutted dirt roads that only have one tyre supported also cause touching problems. You can also get tyres jammed in ruts.

I never lower tyre inflation, I got a lecture from some Isuzu engineers who built 4x4 & 6x6 trucks about how lowering tyre pressures actually increases rolling resistance by increasing the approach angle over sand/snow (I'll get flamed for this). They'd actually conducted tests so I am more inclined to follow their views than the aftermarket tyre salesman. The critical thing is to use tyres appropriate to the vehicle and the road conditions - its always a compromise. Having the largest diameter sand tyres that will fit your vehicle will be atrocious on bitumen roads, having mud lug tyres will be hard on sand, etc. etc.

IN Sand:


The leading single tyre pushes a small amount of sand infront of itself .. and compacts that which it rolls over ... let us say drag = A


The next tyre in line with the first .. does a similar job to the first tyre .. but because of the compaction of the first tyre has less sand to push .. and less compaction takes place .. drag = A-B

If you run a third tyre along side the second tyre (on the same axel) it has the same drag as teh first tyre .. drag = A

If you run a third tyre behind the second tyre (a third axel) then you have even less drag drag = A-2B say as an aproximation. In the past (1950s?) Leyland Badggers were configured this way .. and prefered for the Birdsville Mail run (lots of sand hills with a heavy load) for this reason.


You cannot do anything in your driving style to change this...
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Letting both tyres down on a single axel twin wheel .. may lead to the sidewalls of each tyre rubbing against the other one .. so don't do it for long distances .. nor do it at speed...

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Regards Frank Warner
motorcycles BMW R80 G/S 1981, BMW K11LT 1993, BMW K75 G/S