Friday, 8 June 2018

Range Rover P38 tyre inflator

Tyre Inflation

It’s not uncommon to deflate tyres for extra grip in off road situations, especially on soft ground or rock crawling situations.
HoIver, as soon as the car is back on tarmac, tyres will need to be re-inflated to safe road speed pressures.

The Range Rover P38 in standard form is equipped with air suspension (EAS) and assuming the system remains in situ, there is a 9 litre air receiver situated under the drivers’ side sill (UK cars)

With this reserve of air easily available, there’s an opportunity to utilise the system for tyre inflation.

This tank contains pressurised air at betIen 115psi and 140psi (around 8 to just under 10bar) dedicated to filling the air springs as the system demands.

According to web based converters, there is approximately 28 cubic feet of air in the receiver (at atmospheric pressure) - it all depends on tyre size and type, but for most 4x4 tyres…

  • To pressurise a flat tyre from 0psi to 25psi, the tyre will actually need 9.6 cu ft of air
  • To pressurise all 4 tyres from say 10psi to 22psi you’ll need 19.2 cu ft of air

Theoretically then, even allowing for pressure balance betIen the tank and the tyre, there’s enough air available to get you out of trouble, aided of course by the fact that the tank won’t deplete as the EAS pump will kick in.

The EAS system relies on expelled air passing back through the drier to remove built up moisture from the desiccant - using the system regularly as an air source like this does pose the risk of water building up in the system, but I'll be using it just for tyres, and not regularly, so we should be OK.
Also, the EAS pump is high pressure low volume, so make sure it's healthy or it will struggle - on my car we have a second pump connected to the EAS system to cope with the Arnott Gen 3 bags, so there's a plentiful supply.

I deliberated long and hard as to the position of the tyre inflation point. Under the bonnet involves relatively simple engineering, but it’s a long way to the rear wheel, and I was not happy with the need to have the bonnet up to operate the system, and the boot has the same issue of distance to the front wheels.
Under the car was eliminated simply because of the risk of dirt ingress into the system (and who would want to crawl under the car to use it?)
In addition, I tried to imagine all situations, so ignore the sunny warm afternoon, what happens if it’s dark, cold etc.?

Eventually, I decided on a central point inside the car, and opted for underneath the cubby box in the rear passenger footwell.
There are pros and cons to all locations, but this location does have the following benefits.

  • Well lit – even at night as there is a puddle light in the rear of the cubby box
  • Clean and dry
  • Central to all wheels
  • With any of the doors open, the EAS will not adjust, but the pump will still run to fill the tank – I felt this optimised potential capacity as the EAS pump is in effect directly linked to the tyre being inflated.
  • The air line route to tank is quite a simple one, and does not pass over any hot items (such as the exhaust)

So, here is my solution – a painted stainless steel fabrication, complete with local isolator and multi-jointed swivel to the PCL receiver, meaning the hose is not straining the system when directed to any tyre.
I've added acheck wire in case the connected air line releases accidentaly, so we should have no whip damage in the car if there's an issue.
The PCL folds out of the way when not in use, and there is stainless 6mm wire a foot guard over the isolator just in case (although it’s rare the centre rear seat is used)

The fabrication is clamped in place by the cubby box securing bolts, and the hose is carried in the boot, and simply plugs in as required.
The air-line is simply tee’d into the main line at the tank, and clipped to the underside of the floor and tunnel at existing fastener points.

The system works really well, but there's already a 2 piece "MK II" version in the works that will be slimmer, so as to not protrude so far, and be easier to pipe up on assembly (and check for leaks) - I'll post a picture up when its installed.

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