Vehicle Clearances

Experienced drivers will know all about this.

You may have been out driving along a forest road and come across a sign that says "4WD Track Only", or "4WD Dry Weather Track" or words to that effect. Such a track will normally involve some steeper climbs or descents and normally these steeper sections will have humps formed every so often.

In this picture you can see that there is not much clearance under the vehicle.

This hump is on an overall downhill slope. When it rains, water will flow down the track causing ruts to form if the water is not drained away from the track. In the above, water flowing down from the left is stopped by the hump and drains off to the side (towards the camera from behind the rear wheels).

Generally speaking, the steeper the hill the height of the hump increases. The Defender shown (stock standard suspension) has good clearance and for maintained fire trails and forest roads has rarely scraped over these drainage humps (also called spoon drains).

This image is looking down a slope which has been eroded by water flowing unchecked (inadequate humps/spoon drains present) straight down the track forming the rather large rut. In this case the climb could be done fairly easily by staying with both wheels to the left of the rut (in the picture), or by a wheel either side of the rut. ( As this rut continued for some distance, I walked up first and decided where I needed to cross over the rut).

These ruts deteriorate quickly with rain.

The presence of humps/drains also requires good clearances at the front and rear of the vehicle. The difference clearances are normally given as angles - measured in degrees as per the following diagram showing approach angle, ramp over angle and departure angles.

All these angles can be increased by raising the suspension or giving the vehicle a lift. A small lift of 50-65mm will make a noticeable difference to the terrain that can be crossed without a dramatic impact on driving performance on sealed roads.

The addition of a bull bar sticking out the front will decrease the approach angle - especially if the front springs are not replaced to compensate for the extra weight well in front of the front wheels.

The approach angle of this Land Rover Discovery with a (non-winch type) bull bar, is quite different from the Defender that I prefer to drive. At first I would sometimes scrap the bull bar on the track exiting from a creek bed or drain. To overcome this required slowing down over these obstacles.