907
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| posted on 23/12/10 at 08:07 AM |
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Those pics of the cross section of tyres are all very well but surely a tyres footprint is two dimensional.
If under inflated it's the "front to back" that changes dramatically I would have thought.
Welcome back to the site JB. 
Cheers,
Paul G
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britishtrident
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| posted on 23/12/10 at 08:37 AM |
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On snow under inflation causes additional flexing which tends to make the tread grooves self-clearing.
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Ben_Copeland
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| posted on 23/12/10 at 08:50 AM |
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Well whatever the arguements, the facts are it works for me. So does putting weight over the drive wheels too.
Ben
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FASTdan
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| posted on 23/12/10 at 08:54 AM |
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+1 for low pressure. Worked well on several occasions for me in my otherwise useless mondeo. The cutting through the snow really doesn't stand
up in real world situations - the skinny tyre cars that appear successful are usually light and nimble which imo is a more important factor.
My Xr2 goes anywhere in snow despite the wide tyres, especially with pressures lowered.
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bitsilly
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| posted on 23/12/10 at 09:17 AM |
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Another for low pressures!
Also used to do trials.
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MikeRJ
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| posted on 23/12/10 at 09:50 AM |
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quote:
Originally posted by bitsilly
Another for low pressures!
Also used to do trials.
Are trials normally held on snow then? 
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John Bonnett
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| posted on 23/12/10 at 10:09 AM |
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I didn't mean to start a heated debate, only to pass on my experience of what worked in a practical situation.
Mike sometimes trials do sometimes take place in snow yes, and in every case, mud, snow and polished rock, the technique is the same; let the tyres
down to the lowest pressure allowed by the regs. NTF cars even have the covers screwed to the rims to prevent them slipping round and pulling out the
tyre valves at ultra low pressures.
There may be some discussion on why it works but I don't think the cross section of the tyres in various forms of inflation shown on a previous
post is valiid for mud and snow. The fact that the treads do self clear under lower pressures indicate that the treads probably do open out rather
that close up.
As far as pumping the tyres up for more grip is concerned Dorset Strider, what pressures are used on skidpan cars and for doing doughnuts? I'll
certainly not be pumping my tyres up on the forthcoming Exeter but it might cause a bit of amusement among the other competitors if I did.
I shall retire at this point and wish you all a very happy Christmas.
John
[Edited on 23/12/10 by John Bonnett]
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indykid
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| posted on 23/12/10 at 10:43 AM |
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as has been stated, lower pressures do open out the tread pattern, at the front and back of the contact patch. the section diagram is pretty
worthless
i imagine the mechanism is akin to opening the tread, grabbing a bit of snow, compacting it as you drive over it, then opening up to release and
clear. the smallest radius a tyre can adopt on a flat surface is at the front and back of the contact patch. flexing the tyre to a smaller radius
opens the tread pattern. lower pressures = bigger contact patch = more flex in the carcase, so the smaller the radius.
on fresh snow, where there's little resistance to shear, high pressures might better help compact the snow so the tyres can grip something, but
with the weight of a car it's not much of an issue.
skinny tyres are good for lateral grip, which is pretty important in snow/ice rallies, but it's for digging in, not for cutting down to the road
surface.
in slush, high pressures or narrow tyres would clear it more effectively
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blakep82
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| posted on 23/12/10 at 11:59 AM |
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look at these massive 4x4s that drive in the arctic, they lower the tyre pressures to something like 5psi
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FazerBob
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| posted on 23/12/10 at 03:05 PM |
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I've used the low pressure trick in the past and it does work. One thing to remember though is that your tyres will be under-inflated when you
get onto clear roads. This will cause the tyre to over-heat, and could cause premature failure.
Also remember that in the event of an accident your insurance company can limit or refuse payout as your vehicle is not legally roadworthy with
underinflated tyres.
Bob
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40inches
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| posted on 23/12/10 at 03:57 PM |
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Two weeks ago tried to get up my drive with 30psi, the tyres did indeed cut through the snow to make contact with the drive, only trouble was, the
snow was 6inches deep in front and behind the tyre, cue tyre smoke and black rubber patch on drive, let tyres down to almost flat and simply drove up
drive and parked, simples
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