From: dingbat@codesmth.demon.co.uk (Andy Dingley)
Newsgroups: rec.autos.4x4
Subject: Re: I"M A WHIMP!!
Date: Thu, 18 Apr 1996 15:08:35 GMT

The moving finger of Snowy <rsinha@superlink.net> having written:

>drive in the soft stuff 

I don't get to drive on sandy beaches much, because ours are usually
waterlogged. 

The dunes benind the beach are off-limits to vehicles, owing to
colonies of Natterjack toads. The Natterjack is a rare "4WD" toad. As
it lives in soft sand dunes, it moves around by crawling on all four
legs, not jumping on the back two.


Your problem is excess ground pressure. 

Pressure = all-up weight / (area of a tyre's contact patch * number of
tyres)

Easiest one to start with is vehicle weight. If you're planning a day
out on sand, take the hardtop, big toolbox, 3 spare tyres, generator
set, extra fuel tank and general junk out.

Now the number of tyres. A 6x6 conversion is probably excessive, but
shouldn't be ruled out altogether  8-)  You'll often see farm tractors
with double tyres though - a second rim & tyre bolted alongside the
other.

How can you increase the size of the contact patch ?  The most obvious
is to fit wider or larger tyres, but a less aggressive tread patten
may also help.

Lowering the pressure of your tyres allows the lower part of the tyre
to compress, increasing the contact patch. A theoretical tyre that was
just like an inner tube would have a contact area inversely
proportional to the internal pressure, and a ground pressure equal to
the internal pressure. Dropping it from 30 to 15psi would double the
size of the contact patch and reduce the ground pressure from 30 to
15psi.

Real tyres aren't like this. As well as carrying the vehicle weight on
the air in the tyres, they also carry a portion of this force in the
tyre sidewall. You can only let a tyre down to the extent where it's
still working as a pneumatic tyre, if an appreciable force is going
through the sidewall, it's no longer working in the designed manner.
Many modern off-road tyres are so stiff that their pressure can't be
lowered very much before they start to run on their sidewalls.

You should only lower tyre pressures while the tyre is still working
as a pneumatic tyre and the contact patch is increasing. Stop when the
sidewalls start to buckle, or the tread patch is no longer flat.

A "flat" tyre is one in which you've let so much air out that the
sidewall squashes completely. This is no longer working as a pneumatic
tyre. Pressure has become irrelevant and the continual bending in the
sidewalls will destroy the tyre's internal reinforcement. You should
never let a tyre down far enough that it squashes into this state.
100 yards of towing a vehicle on a tyre in this state is enough to
destroy the tyre for all future use.


Tubeless tyres are held onto their rims by internal air pressure
holding a "bead" along the edge of the tyre into a channel inside the
rim. Drop the air pressure too much, the bead clamping force becomes
too small and the tyre pulls off the rim.

You can refit tyres to rims in the middle of nowhere. You don't want
to do this, and you certainly don't want to do this if you haven't
practiced putting tyres back onto rims in the comfort of your garage
first. I've seen film of Icelanders driving on glaciers who inflate
tyres back onto rims by using aerosol cans of lighter fuel. It's a
neat party trick, but I've no idea how you do it.

>i also undertand that i have to let ALOT of air 
>out of the tires.. BUT to around what PSI? 

Don't let _any_ air out of the tyres if you need to drive back on them
in that state. Driving on tarmac with softened tyres will break them.
You should either have a compressor or air bottle with you, or two
sets of rims & tyres.

Don't let tubed tyres down. The tube will tend to walk around inside
the tyre. This will either pull the valve inside, or pull it off
altogether.

Don't let tyres down unless you have a rim with a good bead. You're
likely to pull the tyre off the bead anyway, but some alloys have a
weak bead design that has real trouble keeping them on.

Don't let "extra-heavy-duty off-road rock-proof super-mangler" tyres
down. Soft tyres will flex a lot more as they turn and on a tyre
design with considerably re-inforced tread or sidewalls, this can
damage the internal construction. 

Sand tyres are lightweight, with light & flexible construction. _Real_
sand tyres have minimal tread patterns designed to pass over the sand
- sink in and you dig them out. The more of the tyre tread patch which
is rubber, not tread pattern, also helps the contact patch. Chunky
tread patterns will tend to disturb the sand and ride _in_ it, beneath
the surface. They sink in until the sand botoms out on the bottom of
the tread pattern, not just the top of the blocks.  It's not so bad if
you have the engine torque to drive it like a paddle steamer, and good
fun too, but it's not a good way of getting from A to B.

I wouldn't recommend tyres to you, because I don't know what you can
get locally. Look for something with a dense tread pattern like
Deseert Duellers or BFG All Terrains, lightweight flexible sidewalls,
and a relatively broad size for your rims. Don't go over size on the
rims. This is a bad idea for any tyre, but if you know they're going
to be run soft, then you'll have much more trouble with beads pulling
off the rims.

--
Andy Dingley                          dingbat@codesmth.demon.co.uk

"Cut the second act and the child's throat"
  - Noel Coward, on seeing the young Bonnie Langford on stage