As described in the blog about the switches of the MAN HX60, it is a very capable off road truck with many gadgets that can bring you far. Not using these gadgets correctly will get you more stuck and / or even worse: will break stuff.
There are still some universal off roading rules that will get you a long way, even without using all the gadgets.
Most effective tool in the shed
The most effective (and cheap) tool in your off road shed is a very simple one: letting air out of your tires.
A fully pumped up tire is great for on road use, but off road a hard tire will not be able to ‘follow’ the surface that you are driving on. The point of contact from the tire to the surface below is also very small, thus wheel spin is eminent. As hard tires are not able to adjust to the surface, they tend to dig into the surface instead of rolling (floating) over it.
By lowering your tire pressure you create a longer foot print (barely wider as many people like to think). The difference between a hard and really soft tire can be around a 300% longer footprint! This gives a lot more traction and flotation.
Further advantages are a way less harsh ride (suspension in the tires) and can help reduce drivetrain stress (tires can deform).
Negatives: it is a bit of work and takes time letting air out and in again, especially with huge 14.00R20 tires. Furthermore an increased risk of sidewall penetration, running a tire of the rim and a (significant) decrease of ground clearance. With our huge 14.00R20 tires that is not the biggest issue though.
Tire pressures we like to use
Our Mountain Yacht is around 15,5 tonnes travel ready (7,25 tonnes front axle and 8,25 tonnes on the rear axle). This means loaded with 780 liters of diesel, 800 liters of water, clothes, E-MTB’s, two spare tires, recovery materials, spare parts, food and so on.
For highway – and general road use – we set the tire pressure at 8 bar for the rear and 7,8 bar front tires. We did some testing with tire pressures and up to now this suits us best.
Driving off road is a different cup of tea. For easy paths for relative short periods of time we do not alter the tire pressure, it is not necessary. When the terrain gets tougher, we sometimes choose to not alter the tire pressure until we start digging in (still having the tire pressure tool in our shed). And sometimes we lower in advance so we can tackle (almost) anything that comes with confidence. It depends on the situation and the type of terrein we expect to encounter.
Rocky roads: we air down to around 4 bar. These type of roads are dangerous for the tires. Especially for the side walls, lower pressure makes the tires more vulnerable for sidewall punctures. In riverbeds with smooth rocks the tire pressure can be lower when needed.
Trails – mild mud: we also air down to around 4 bar. The tires can follow the surface pretty good, slightly higher speeds can be maintained and not much risk of running a tire of the rim.
Heavy mud – ruts: down to 3 bar, when needed even down to 2,5 bar. Be careful not to run the tire of the rim by excessive wheel spinning. When things get really extreme – slippery sideslopes and so on – we will mount the snow chains for extra grip. This is common practice in the forestry industry.
Fresh snow: on the highway or main roads we maintain 8 bar. On winding mountain roads we like to air down to around 4 bar. In really extreme conditions even lower to create as much footprint as possible. Note: as of this winter we will have 14.00R20 dedicated snow tires with snowflake. They should be way better, especially on hard packed snow and icy roads. We also have 4 snow chains.
Sand: there are several types of sand, the softer it gets, the lower we go. We usually start out around 4 bar, in really soft sand we lower as much as 2 bar when necessary, but usually 2,5 bar does the job well.
ATTENTION: tire manufacturers are not so happy when you drive on low tire pressure. It can crack the sidewall or you can run the tire of the rim. We are not saying that you should go below the pressure that a tire producer advises, we just do because the results on soft surfaces are just way better. We have been doing it with every off road tire we ever had, up to now without problems. You really need to pay attention that you will not drive too fast on soft tires or have excessive wheel spin. On rocky surfaces: look out for sharp rocks that can cut the sidewall of your tire.
Beadlocks will make sure that tires will not run off the rim. We might purchase them in the near future as our new set of winter tires will be on the current rims. We are looking at the Hutchinson beadlocks. Much lower tire pressures are possible when using beadlocks. The downsite is that they are rather expensive and it is difficult to swap a tire in the field (as I understood), opposed to the rims we have now. I used to have hutchinson rims on my Land Rover Defender competition truck and they never let me down and made it possible to drive as low as 0,4 bar. So, still some thinking to do here.
There are several ways of airing down. A central tire inflation system is the most ‘luxurious’ and frankly the easiest way of doing it. We do not have a CIS installed.
For deflation we just use old fashioned tire deflators that are set to 2,5 bar. Just screw them on the valves and they will air down to 2,5 bar. When I want my tires at (let’s say) 4 bar, I just check regularly by hand. Simple, but it works for me as I have been doing it this way for over thirty years.
Pumping up the tires
Again, a central tire inflation system is the most convenient way to go. I have never had it before, so I do not miss it (yet).
The MAN HX60 has air outlets on both sides. This means that you can pump up two tires at the same time. To do this quicker it is wise to set the engine at 1000 or even 1200 rpm, this way a lot more air is created bij the HX, thus shortening the time for pumping up the tires to 8 bar. I manually check when it reaches 8 bar.
Recently a German friend of us (Markus) created a portable central tire inflation system in a box with a pressure regulator and 4 outlets that go on the valves of the tires. It can be used for deflation or inflation and it retrieves the air from the trailer outlet (maximum 8 bar). We build one for ourselves but have not come around to test it properly yet, first test proved that it functioned though. Will elaborate on this in the future.
We hope to have given some insight in what tire pressure can do for the off road capabilities of your truck. For us, these tire pressures function well, for lighter – or heavier vehicles – probably other pressures apply. It is just something you will have to find out for yourself. When you have different experiences or something to add, please drop a comment as we are all here to learn from each other.