As a regular competitor in trials I shouldn't be surprised. After all, before attempting a section we drop the tyre pressures to the minimum
permissible limit.
Our lane has been impassable due to compacted snow over the last few days. At 40psi in my Hilux tyres it wouldn't look at it. I could get no grip
at all. By dropping the pressures to about 15psi I put it in gear and just drove up as if it were a dry road. Absolutely amazing.
The theory is that by dropping the pressure so that the sidewall flex and the treads open out.
Thought I'd pass this on in case it may be of help.
John
[Edited on 22/12/10 by John Bonnett]
[Edited on 22/12/10 by John Bonnett]
Works for me too... RWD Merc Vito absolutely useless in the snow and ice. Tyres usually around 45-50 on the back.. Dropped it to 20psi and its been ok since.
Remember to pump them back up once you are on clear roads as you'll damage the tyres otherwise.
quote:
Originally posted by John Bonnett
The theory is that by dropping the pressure so that the sidewall flex and the treads open out.
Tom's right you know
you should increase the pressures..
gives a smaller contact area
look at the ice racers, more weight on a smaller area
quote:
Originally posted by RazMan
Tom's right you know
![]()
quote:
Originally posted by gavin174
you should increase the pressures..
gives a smaller contact area
look at the ice racers, more weight on a smaller area
I agree with the low pressure idea. Common practice in sking resorts.
Just a thought thought . . .
I went to my work christmas meal on saturday. We left about 4 hours of heavy snow approx 4" deep.
We had to go out to Detling (kent) which is on a hill. Our choice of car for 4 adults?
A pug 106 R plate!
We thought that hi pressure over small area may cut through the ice, and it did. We got there and back no issues!!
Happy day.
Yes, lowering pressures can definitely help in snow. There was another thread recently where someone disagreed with this suggestion and I
couldn't be bothered arguing. A colleague of mine got stuck recently and the owner of a alloy wheel and tyre came out to make this suggestion
which allowed him to go on his way with no more fuss.
There is no question about it- it does work in practice whether it can be rationalised by theory or not.
Indeed, back to the trials thing, they are people who do this competitevly, and you would be regarded as a complete idiot if you pumped your tyres up
for an event, no matter what the theory is. Best listen to experience
quote:
Originally posted by gavin174
you should increase the pressures..
gives a smaller contact area
look at the ice racers, more weight on a smaller area
Yes ice racers have metal studes in their tyres...
However decreasing the pressure in snow is exactly the wrong thing to do. As it closes up the tread pattern and causes you to float on the surface.
Increasing the pressure opens out the tread pattern, decreases the deflection and as a result you have a much better chance of cutting through the
snow to the tarmac beneath and maintaining traction. It will also open out the tread pattern helping the tyre shed any ice and snow it's picked
up.
The caviat to this is when you are in REALLY deep snow (inside the artic circle) where the chances of finding the road surface beneath is practically
zero whatever you do.
quote:
Originally posted by DorsetStrider
Yes ice racers have metal studes in their tyres...
However decreasing the pressure in snow is exactly the wrong thing to do. As it closes up the tread pattern and causes you to float on the surface.
Increasing the pressure opens out the tread pattern, decreases the deflection and as a result you have a much better chance of cutting through the snow to the tarmac beneath and maintaining traction. It will also open out the tread pattern helping the tyre shed any ice and snow it's picked up.
The caviat to this is when you are in REALLY deep snow (inside the artic circle) where the chances of finding the road surface beneath is practically zero whatever you do.
Yes I have tried this is practise. For many years.
As for your "friend" without knowing all the variables (and there are many) it is impossible to say why it worked in that 1 particular
instance.
I don't know what your friends experience or knowledge base is. But let me give you some of mine... I am a member of the IMI (institute of motor
industries), An instructor of motor vehicle technology at the local college, I've done a bit (not much admittedly) of rally driving.
I can't be bothered arguing this point any further... but I ask you to just think about what happens to the tyre by increaing or decreasing the
pressure. In your own post you say that you are "riding on the surface of the snow" how is that in any way going to help traction?
As I said in my earlier post, yes you would want to stay on the surface if the snow is so deep that the vehicle would dig in and ground itself if you
didn't... where in the UK is the snow that deep?
I've been lowering the presure in my tyres all week to get out of trouble. I got completly stuck on a patch of ice on a slight incline the other
night. Dropped the presures and just drove off. Luckily my van has a compressor on board so i just pumped them up again and off i went. I
haven't tried increasing them yet, maybe next time i get stuck.
[Edited on 23/12/10 by Davegtst]
quote:
Originally posted by DorsetStrider
Yes I have tried this is practise. For many years.
As for your "friend" without knowing all the variables (and there are many) it is impossible to say why it worked in that 1 particular instance.
I don't know what your friends experience or knowledge base is. But let me give you some of mine... I am a member of the IMI (institute of motor industries), An instructor of motor vehicle technology at the local college, I've done a bit (not much admittedly) of rally driving.
I can't be bothered arguing this point any further... but I ask you to just think about what happens to the tyre by increaing or decreasing the pressure. In your own post you say that you are "riding on the surface of the snow" how is that in any way going to help traction?
As I said in my earlier post, yes you would want to stay on the surface if the snow is so deep that the vehicle would dig in and ground itself if you didn't... where in the UK is the snow that deep?
quote:
Originally posted by DorsetStrider
Yes I have tried this is practise. For many years.
As for your "friend" without knowing all the variables (and there are many) it is impossible to say why it worked in that 1 particular instance.
I don't know what your friends experience or knowledge base is. But let me give you some of mine... I am a member of the IMI (institute of motor industries), An instructor of motor vehicle technology at the local college, I've done a bit (not much admittedly) of rally driving.
I can't be bothered arguing this point any further... but I ask you to just think about what happens to the tyre by increaing or decreasing the pressure. In your own post you say that you are "riding on the surface of the snow" how is that in any way going to help traction?
As I said in my earlier post, yes you would want to stay on the surface if the snow is so deep that the vehicle would dig in and ground itself if you didn't... where in the UK is the snow that deep?
We had around 8-9" of snow here... no chance of digging through the snow to asphalt (the car would just beach on the sand), so out goes the
"higher pressures" idea on skinny tyres to dig through.
I couldn't get up a hill for 10 minutes (tried everything) - now I'm on 15psi and haven't had a problem yet. It means I have to avoid
about 5 miles of main road on the way to work as I wouldn't want to drive at 50mph on a cleared road on that pressure, but I've yet to get
stuck.
And no, I'm not in a 4x4 and being an idiot, I've got 8 miles of this to deal with each way on the way to work:
http://picasaweb.google.com/lh/photo/9ZzNRIOqbr6yQOBnsowpUQ?feat=directlink
Empirical +1 for low pressures. And in weather like this, I'll take practicality over theory any day!
Now that's proper snow! Never mind lowering tyre pressure to drive in the snow, you'll need tennis racquets on your feet just to get to the
car!!
We only had 6 inches or so but it was certainly enough to grind the country to a halt a couple of weeks ago. I actually stayed at work until 8pm that
particular Monday and then drove home without any trouble. Some of my colleagues left early and took up to 10 hours to drive 15 miles but myself and a
few other guys had a meal at a nearby restaurant after a full day's work and then went home after the worst of the traffic had cleared.
I have to admit though that there was a point when I thought I was sleeping on the office floor...
[Edited on 23/12/2010 by craig1410]
Maybe both sides work, if you lower the pressure the contact patch widens a bit and helps to sit on the snow whereas if you raise them the contact
patch narrows cutting through the snow.
Both work just as well in different ways
quote:
Originally posted by Ninehigh
Maybe both sides work, if you lower the pressure the contact patch widens a bit and helps to sit on the snow whereas if you raise them the contact patch narrows cutting through the snow.
Both work just as well in different ways
quote:
Originally posted by craig1410
Once you get home, you can sit all night in front of a warm fire and wonder why increasing or decreasing the tyre pressure allowed you to complete your journey.
quote:
Originally posted by Ninehigh
quote:
Originally posted by craig1410
Once you get home, you can sit all night in front of a warm fire and wonder why increasing or decreasing the tyre pressure allowed you to complete your journey.
Witchcraft![]()
the conecpt of 'cutting through the ice/snow' is nonsense, once its been compacted you wouldnt get through it with a hammer and chisel, nevermind a rubber tyre. I've never been stuck on fresh snow, only on the bumpy ice that lasts for days.
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
On snow under inflation causes additional flexing which tends to make the tread grooves self-clearing.
Well whatever the arguements, the facts are it works for me. So does putting weight over the drive wheels too.
+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.
Another for low pressures!
Also used to do trials.
quote:
Originally posted by bitsilly
Another for low pressures!
Also used to do trials.
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]
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
look at these massive 4x4s that drive in the arctic, they lower the tyre pressures to something like 5psi
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.
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