LocostBuilders - powered by XMB

Benefit of lowering tyre pressure

John Bonnett - 22/12/10 at 06:39 PM

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]

Ben_Copeland - 22/12/10 at 06:49 PM

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.

eddie99 - 22/12/10 at 06:51 PM

Remember to pump them back up once you are on clear roads as you'll damage the tyres otherwise.

tomgregory2000 - 22/12/10 at 07:15 PM

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

RazMan - 22/12/10 at 08:46 PM

Tom's right you know

gavin174 - 22/12/10 at 09:04 PM

you should increase the pressures..

gives a smaller contact area

look at the ice racers, more weight on a smaller area

ReMan - 22/12/10 at 09:16 PM

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

daviep - 22/12/10 at 09:32 PM

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

bi22le - 22/12/10 at 09:41 PM

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.

craig1410 - 22/12/10 at 10:46 PM

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.

cliftyhanger - 22/12/10 at 11:10 PM

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

craig1410 - 22/12/10 at 11:14 PM

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.

DorsetStrider - 22/12/10 at 11:34 PM

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.

craig1410 - 22/12/10 at 11:44 PM

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?

DorsetStrider - 22/12/10 at 11:57 PM

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?

Davegtst - 23/12/10 at 12:02 AM

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]

daviep - 23/12/10 at 12:14 AM

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

craig1410 - 23/12/10 at 12:22 AM

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.

PSpirine - 23/12/10 at 12:38 AM

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!

craig1410 - 23/12/10 at 12:46 AM

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]

Ninehigh - 23/12/10 at 01:04 AM

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

craig1410 - 23/12/10 at 01:12 AM

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.

Ninehigh - 23/12/10 at 01:48 AM

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

craig1410 - 23/12/10 at 01:52 AM

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!

JoelP - 23/12/10 at 07:55 AM

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.

907 - 23/12/10 at 08:07 AM

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

britishtrident - 23/12/10 at 08:37 AM

On snow under inflation causes additional flexing which tends to make the tread grooves self-clearing.

Ben_Copeland - 23/12/10 at 08:50 AM

Well whatever the arguements, the facts are it works for me. So does putting weight over the drive wheels too.

FASTdan - 23/12/10 at 08:54 AM

+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.

bitsilly - 23/12/10 at 09:17 AM

Another for low pressures!
Also used to do trials.

MikeRJ - 23/12/10 at 09:50 AM

quote:
Originally posted by bitsilly
Another for low pressures!
Also used to do trials.

Are trials normally held on snow then?

John Bonnett - 23/12/10 at 10:09 AM

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]

indykid - 23/12/10 at 10:43 AM

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

blakep82 - 23/12/10 at 11:59 AM

look at these massive 4x4s that drive in the arctic, they lower the tyre pressures to something like 5psi

FazerBob - 23/12/10 at 03:05 PM

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.

40inches - 23/12/10 at 03:57 PM

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