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Research Topic: Reducing 4WD Off-road Tyre Pressures

By Ian Mander, 29 September 2005, minor updates 14 October 2005, 27 February & 21 June 2006, 5 January, 8 August & 9 November 2007, and 29 May 2009.

Question: Is down-pressuring when I go off-road all it's cracked up to be?

Answer: The short answer is possibly, but it depends a lot on the surface being driven on. Down-pressuring on soft sand offers the greatest advantage. Driving in snow or rough terrain at speed possibly the least advantage, and it may actually be disadvantageous or even dangerous.

I know people who let their tyres down whenever they leave asphalt. That's actually not really necessary and there are good reasons why you probably shouldn't let your tyres down unless you really need to. However, when you do need that extra grip, it can be a very useful (and easy) way of getting it.

Misinformation

In a recent magazine article I read the following:

Decreasing the tyre pressure increases the traction on soft surfaces, reduces the resistance to obstacles, improves fuel economy, provides a softer and smoother ride, enabling the driver to be safer and more stable during off-road conditions and leads to an all round better performance for the tyre.

There is more than a bit of misinformation in that paragraph, and further investigation shows the ideas come from the marketing guff of a company that sells products to easily deflate tyres to a set pressure. Let's have a look at those claims one at a time.

Increases the traction on soft surfaces. True. This is because more of the tyre width and especially "length" is in good contact with the ground surface. In other words, the footprint of the tyre is bigger. Great. That's why people do it, although it's not always needed.

Reduces the resistance to obstacles. True. If they're talking about the tyre deforming around a rock instead of bouncing off it, then sure. But of course that can also increase the chance of a puncture or sidewall damage so it's not necessarily a good thing. Anything more than a crawl and you might actually be better off at full pressure.

Improves fuel economy. Not true. Decreasing tyre pressure increases rolling resistance and so decreases fuel economy. The only situation why this claim might be true is if the greater traction in an off road situation prevented lots of unwanted or unnecessary wheel spin (which would be wasting fuel), such as a long drive in soft sand. If a properly inflated tyre provides enough traction – so there's no wheel spin – then the smaller the tyre's footprint the better your fuel economy will be. Racing bicycles have very narrow tyres to minimise rolling resistance; their tyres give them enough traction for the surface they are ridden on.

Provides a softer and smoother ride. True – unless you start hitting your rims on obstacles your tyres would normally have bounced off. Even if you don't hit things quite that hard, the softer ride comes at the expense of greater wear on the outer edges of the tyre and particularly the sidewall. It also means that vehicle speed needs to be kept down so the tyres don't overheat because of excessive sidewall flexing.

Enabl[es] the driver to be safer and more stable during off-road conditions. Not true. As far as safety goes, greater traction should mean less sliding around – generally a good thing, especially since it'll mean fewer dings in one's vehicle. However, lower pressure in a vehicle's tyres means less vehicle stability because of greater sidewall flex. Steering performance is also decreased. Dropping the tyre pressure too much risks pulling the tyre off the rim if sudden sharp steering manoeuvres occur – a potentially very dangerous situation (sadly mont of the photos are no longer there). And embarrassing.

Update: I've taken another look at this claim and it could be read as being rolled into the previous one, if you'll pardon the expression. The claim thus becomes that the softer and smoother ride (due to down-pressuring) is the thing that enables the driver to be safer and more stable during off-road conditions. Ooo, boy. Or perhaps all of the previous "benefits" are the cause.

Leads to an all round better performance for the tyre. Not true. A tyre's performance in different areas can each be measured separately, but tyres perform best overall when they are inflated to at least the manufacturer's specification. (That's why they specify a pressure!) Running a tyre at low pressure will increase one particular aspect of its performance (traction) at the expense of others, such as steering, handling, durability etc. Tyres steer better when they are hard, not soft. This is because with less flex of the sidewall (when the tyre is hard) the tyre responds better to changes in the orientation of the wheel rim. Emphasising one aspect of a tyre's performance to the decided detriment of other aspects may be desirable, especially considering the lower speeds involved with some off road driving, but it depends very much on the sort of off road driving being done.

So only 50% of the article's claims are true, and only one is true and doesn't have serious caveats (Latin for "warning").

Advantages

• Extra grip in many conditions – particularly soft sand. I've travelled at 100 km/hr on hard sand in various vehicles, so obviously down-pressuring isn't for all sandy situations, but I've also travelled in soft sand with hard tyres and found how fuel-thirsty that is (in both petrol and diesel vehicles).
• Can provide a more comfortable ride.

Disadvantages

• Decreases ground clearance. (A simple trade-off against the extra traction.)
• Sidewall flex. Too little pressure in the tyres means the sidewalls will get much more of a workout than they would get with normal pressure. Real damage can be done. This is particularly relevant when driving over objects like curbs or small rocks and logs (especially at speed), or simply driving on road at higher speeds. The extra flexing of the sidewalls also causes the tyres to heat up which can also help damage the tyres – the sidewalls get softer, for example.
• Increases the chance of a puncture by wrapping around an obstacle instead of bouncing off it.
• Extra tyre wear on outer edges. This is possibly not a major consideration because of the amount of time spent off road compared to on road.
• Running a tyre at too low a pressure can also cause the following problems:
  • Loose bead. The tyre needs a reasonable amount of pressure in it to keep it "stuck" to the wheel rim. Without enough pressure you might find you're spinning the wheel rims but the tyres aren't moving. (Expect a lot of heat to be generated really quickly.)
  • The tyre might lose pressure and deflate completely.
  • Sudden steering changes or tyre deflation could cause the tyre to come off the rim.

Conclusion

Reducing tyre pressure can be very useful for getting extra traction, especially in soft sandy conditions. However, there are a few things to observe. For typical off roading don't go under 1.25 bar (about 18 psi), or consider a higher minimum pressure than that if your vehicle is really heavy. Drive slowly and steer gently until reinflated.

I highly recommend having your own reinflation equipment – the local petrol station may not be open by the time you finish your off road trip, and it may be an undesirable distance from the end of the off road section of your trip. Holding up traffic on a main road by driving slowly to the petrol station is also inconsiderate.

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