Tyres and Rims - "Is that our wheel?"

Rims

1600's come out stock with 13" rims. This is simply not enough clearance off the ground for offroading. For this reason we've gone and used some offset Rover rims. These are 15", allow the use of some of the more common tyre seconds and give an extra inch height over the standard rim. Obviously, there's a trade-off with the car's centre of gravity rising, however this really can't be helped. The offset was necessary due to the larger tyre diameter. They simply wouldn't fit inside the rear guards (even after they'd been flared).

Go for steel rims. Alloy rims may be lighter and stronger, however steel rims don't break/shatter. We've had many instances where an alloy rim would have been destroyed, where-as the steel rim was simply buckled. Nothing that a rubber mallet wouldn't fix... Of course if you're interested in winning then alloys are the go. Just be prepared to bend/break some.

Tyre Pressures

You should take all of the following with a large grain of salt... Tyre pressures depend on how fast you want to run, the risks you are willing to take (generally rolling a tyre off the bead), the tyre profile, the tyre sidewall construction, the use of beadlocks, the course condition, how the tyres were mounted onto the rims, etc.

24 psi - soft sand/gravel, relatively smooth
28 psi - hard gravel/rock, rough patches
32 psi - really rough stuff (anthills, boulders, quarries, cliffs - buggy country!)

Now, tyre pressures are one of those things (and there are a couple!) that I just don't really have a good grasp of. Sure, if you run with a lower pressure, you'll generally get more tyre in contact with the ground. However with offroading you run the risk of repeatedly knocking the tyres off the rims. Raising the pressure decreases this problem, but reduces the traction. It really depends on the conditions of the course. If it's nice and smooth/sandy then lower the pressures a bit, otherwise I would recommend keeping them up. There's nothing worse than popping a bead and losing 10 minutes changing the damn tyre.

Tyres

This picture is of an RE46R Potenza 185x70R15 medium grade rally tyre.

This tyre is a good all-round tyre for offroad conditions. As a course may contain lengthy sections of mud, sand, gravel and rock it is nearly impossible to select any one type of tyre over another. Generally, if the tyre has an open, deep tread pattern then it will be suitable.

This particular tyre is quite worn. The majority of wear occurs at the edges of the tread, as this is where most friction is developed. As the tread wears, the cornering ability of the car will degrade, however straight line speed is not really affected.

This picture is of an Michelin medium grade rally tyre.

This tyre is also quite worn, however we haven't the bucks to purchase a new set at the moment.

One thing to note, whilst talking about tyres/rims. These things are held on by nuts that can come loose. I know we should get a checklist together, but not checking that these nuts are tight is one of the banes of my life. It's more than annoying to remember that you haven't tightened them when one of the wheels falls off and takes off down the road without you...