John Bonnett
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| posted on 22/12/10 at 06:39 PM |
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Benefit of lowering tyre pressure
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]
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Ben_Copeland
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| posted on 22/12/10 at 06:49 PM |
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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.
Ben
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eddie99
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| posted on 22/12/10 at 06:51 PM |
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Remember to pump them back up once you are on clear roads as you'll damage the tyres otherwise.
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tomgregory2000
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| posted on 22/12/10 at 07:15 PM |
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quote:
Originally posted by John Bonnett
The theory is that by dropping the pressure so that the sidewall flex and the treads open out.
Ummmm no, increasing the preasure will open out the tread but reduce your tyre footprint where as lowering the pressure gives you a muuuuch bigger
foot print
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RazMan
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| posted on 22/12/10 at 08:46 PM |
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Tom's right you know
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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gavin174
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| posted on 22/12/10 at 09:04 PM |
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you should increase the pressures..
gives a smaller contact area
look at the ice racers, more weight on a smaller area
http://www.essexkitcarclub.com
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ReMan
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| posted on 22/12/10 at 09:16 PM |
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quote:
Originally posted by RazMan
Tom's right you know
I would imagine that this is correct on tarmac
However on a softer surface of Mud/snow then the wider footprint of under inflation is filled in the middle with the traction medium and so offers an
advantage
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daviep
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| posted on 22/12/10 at 09:32 PM |
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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
Have you ever tried this with success in the real world.
My experience is that dropping tyres pressures does aid traction in winter conditions, I've done it many times.
Davie
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bi22le
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| posted on 22/12/10 at 09:41 PM |
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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.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
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craig1410
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| posted on 22/12/10 at 10:46 PM |
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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.
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cliftyhanger
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| posted on 22/12/10 at 11:10 PM |
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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
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craig1410
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| posted on 22/12/10 at 11:14 PM |
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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
Don't ice racers have metal studs in their tyres? If so then they will need pressure through smaller surface area to drive the studs into the
ice. On snow you want to sit on top because you only get stuck when you spin the wheels and form a small hole from which you can't escape. By
lowering pressures you make your tyres more compliant and reduce the tendency to dig in.
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DorsetStrider
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| posted on 22/12/10 at 11:34 PM |
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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.
Who the f**K tightened this up!
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craig1410
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| posted on 22/12/10 at 11:44 PM |
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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.
So why does your theory not work in practice? My colleague wasn't lying when he said he got completely and utterly stuck and then by lowering
the pressures he got home easily with no fuss. Btw, he is also an amateur rally navigator so is no mug and was driving an E-Class Merc with full
traction control etc. Rear wheel drive obviously but he's a very competent driver.
You say that the open tread pattern from higher pressures will "cut through the snow to the tarmac beneath" but this just doesn't
happen in practice and if you do cut through 3 inches of snow then all you are doing is making life MORE difficult by driving "uphill" all
the time. The whole point is to stay on top of the snow, however deep or shallow, to allow you to make progress.
Seriously, have you tried this in practice?
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DorsetStrider
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| posted on 22/12/10 at 11:57 PM |
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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?
Who the f**K tightened this up!
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Davegtst
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| posted on 23/12/10 at 12:02 AM |
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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]
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daviep
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| posted on 23/12/10 at 12:14 AM |
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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?
Regardless of whether you think it won't work it does, and in many different circumstances, but that's only my experience 
“A truly great library contains something in it to offend everyone.”
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craig1410
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| posted on 23/12/10 at 12:22 AM |
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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?
My "friend"? Not sure why you put that in quotes since I actually said my "colleague". Maybe you think he is my imaginary
friend. His name is Clive Black and I will happily give you his number if you want to give him a call and verify the story. Anyway, regardless of his
driving qualifications, it was the guy from the tyre shop who told him to drop his pressures, but what does a tyre shop owner know about tyres
eh???
My friends knowledge base was something along the lines of, he knew his tyres were spinning and no amount of subtlety on the throttle pedal (and a bit
of left foot braking for good measure) helped him make progress. He tried rocking back and forwards, tried manual gear selection (auto box) but
nothing was working. He dropped his pressure as per the suggestion and off he went home!
Sorry but empirical evidence beats theory and qualifications any day. The challenge for good theorists is to revise their theories to explain why the
real world works the way it does. It is not the job the the rest of us to pretend we didn't see the car drive out of the snow just because it
doesn't fit with some theory!
Yes, riding ON the snow is better than trying to carve a groove THROUGH the snow. Have you ever followed a car which left grooves in the compressed
snow/ice right through to the tarmac? The fact is that we all drive ON the snow whether we have fat tyres of skinny tyres. Skinny tyres are better
where the snow has just started falling or in melting slush but where the snow has already compressed into ice or where you have no prospect of
digging through to the tarmac then you are better staying on top of the snow.
Sorry if I am being argumentative but it has been a long, difficult week... Nothing personal.
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PSpirine
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| posted on 23/12/10 at 12:38 AM |
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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!
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craig1410
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| posted on 23/12/10 at 12:46 AM |
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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]
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Ninehigh
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| posted on 23/12/10 at 01:04 AM |
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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
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craig1410
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| posted on 23/12/10 at 01:12 AM |
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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
Maybe, but it's easier to let air out than to put it in if you are stuck.
I would definitely vouch for higher pressures in heavy surface water or slush as it helps to push the water/slush aside and prevents the water lifting
the tyre off the surface (aquaplaning). I thing the key thing is to keep an open mind and be prepared to try things that might otherwise seem
counterintuitive to some. 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.
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Ninehigh
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| posted on 23/12/10 at 01:48 AM |
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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
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craig1410
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| posted on 23/12/10 at 01:52 AM |
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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
Indeed!
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JoelP
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| posted on 23/12/10 at 07:55 AM |
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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.
Beware! Bourettes is binfectious.
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