Wide Vs Skinny Mud Tyres

[toughnut]

There was a comparo in one of the 4x4 mags last year I think. It wasn't between tires, it was about tire pressure. They were using skinny tires and found that when you deflated the tire it made the contact patch a lot longer rather than wider. I'm pretty sure they compared it with a wider tire and found that the longer patch gave more grip than the wider contact patch. I can't be assed looking for it but I'm sure that someone else here will remember it.

[HotFourOk]

Well, thats correct thinking.. the longer contact patch has more rubber perpedicular to the direction of motion... hence it should resist slipping more.

This is comparing tyre heights though...

Although, in this case, both tyres are the same height, which would give them the same length of contact patch, it's just that the wider tyres patch would be wider.

[Shadow]

I also dont understand the surface pressures argument. If a tyre is inflated to 32PSI (POUNDS PER SQUARE INCH) it has exactly the same pressure over every square inch. Thats what 32PSI means.

The fact is, A wider tyre has more grip because it has more surface area with which to engage the friction coefficient of the surface it is on.

So the 305 has 7% wider contact patch, and the 285 is putting 7% more pressure on the contact patch it has. This is how the thinner tyre digs into the mud or through the loose gravel easier - there is proportionally more weight on the contact area of the tyre.

At 32psi (Pounds per Square Inch) a tyre loaded with 750KG of Weight needs (750KG in pounds = 1650pounds, ==> 52.5square inches) of contact patch.

Which means a 33x12.5" tyre will have a contact patch of 4.2"x12.5", a 33x7.50" tyre will have a contact patch of 7x7.5". (not allowing for the extra width due to sidewall bagging, or the load taken directly by the sidewall), which again means the surface pressure is exactly the same.

[-Scott-]

I also dont understand the surface pressures argument. If a tyre is inflated to 32PSI (POUNDS PER SQUARE INCH) it has exactly the same pressure over every square inch. Thats what 32PSI means.

The fact is, A wider tyre has more grip because it has more surface area with which to engage the friction coefficient of the surface it is on.

So the 305 has 7% wider contact patch, and the 285 is putting 7% more pressure on the contact patch it has. This is how the thinner tyre digs into the mud or through the loose gravel easier - there is proportionally more weight on the contact area of the tyre.

At 32psi (Pounds per Square Inch) a tyre loaded with 750KG of Weight needs (750KG in pounds = 1650pounds, ==> 52.5square inches) of contact patch.

Which means a 33x12.5" tyre will have a contact patch of 4.2"x12.5", a 33x7.50" tyre will have a contact patch of 7x7.5". (not allowing for the extra width due to sidewall bagging, or the load taken directly by the sidewall), which again means the surface pressure is exactly the same.

What he said!

[toughnut]

I'm going to have to find this article to back this up, I can see it. But one more thing that the article went into is that the side wall of the tire didn't bag up enough to add a significant portion of contact area in the form of width. Now to spend the next 8 hours looking through mags to find this.

[Beastmavster]

So you believe that somehow the skinny tyre somehow deforms to allow a 7" long strip tread touch the ground to support the weight?

I dont believe that to be the case at all, I'm sorry.

Using your example with the skinny tyre, if the tyres were aired down to 16psi, then the contact patch would be 14" long?

If down to 12psi, then over 18" of tyre is on the ground? The tyre is completely flat on the ground for more than the width of the rim?


The problem with this, that shows that it is ludicrous is that the tyre would then basically be complely flat like a piece of bread at the bottom.

[-Scott-]

So you believe that somehow the skinny tyre somehow deforms to allow a 7" long strip tread touch the ground to support the weight?

I dont believe that to be the case at all, I'm sorry.

Using your example with the skinny tyre, if the tyres were aired down to 16psi, then the contact patch would be 14" long?

If down to 12psi, then over 18" of tyre is on the ground? The tyre is completely flat on the ground for more than the width of the rim?


The problem with this, that shows that it is ludicrous is that the tyre would then basically be complely flat like a piece of bread at the bottom.

No. I'd guess that as the pressure reduces that far the tyre will bag, and sidewalls come into play. Don't forget he's talking 750kg on one tyre, 1500kg per axle, or 3tonne vehicle if weight is evenly distributed - a 3 tonne 4by running 7.50 tyres at 16psi? They'll bag.

We should stop guessing, and somebody do some testing.

Scott

[Beastmavster]

I'm going to have to find this article to back this up, I can see it. But one more thing that the article went into is that the side wall of the tire didn't bag up enough to add a significant portion of contact area in the form of width. Now to spend the next 8 hours looking through mags to find this.

That was in a review by 4wd monthly if I recall. The extra contact patch from airing down mainly came from the extended length, not the extra width of additional sidewall bagging.

But I dont recall there being anything like 7" of tread length touching the ground at once at 32psi and 14" at 16psi.

From memory there was a 33" buckshot mudder and a 7.50R16 in the test, some tyre black and some paper for the tyre prints.

[sudso]

I read that that article and test.

The contact patch with the skinnier tyre when deflated had about the same percentage increase as the contact patch of the wider one but the wider tyre's contact area wasn't as long as the skinny one.

They did the 2 sets of tyres on the same vehicle and the deflated tyres didnt make the contact patch any wider on either tyres, just longer, although the sidewalls do bell out giving the appearance of a wider contact patch.

They likened the increased "footprint" length as having the same traction effect as tracks on a caterpiller or army tank.

In the outback just about all locals stick to cheesecutters

[Gwagensteve]

They likened the increased "footprint" length as having the same traction effect as tracks on a caterpiller or army tank.

Bingo!

As I have said, in every field except tarmac, tyres width to height fits into a ratio that provides useful balance between drivability and floatation. In Paris Dakar, they run heavy cars with massive power over far bigger dunes than we ever run in australia on 235 85 16's. Only the yanks (who have never been competitive over there) run on 12.5s.

It in only on very soft sand that unlimited floatation is an asset. In every other condition, some gound pressure is required to create traction. (even paddles in sand work a bit like this though)

Years ago, Dad and I ran two hiluxes that were identical. One ran 31 10.5 BFG Muddies, the other 235 85 16 muddies. both tyres offered about the same footprint area.

The narrower tyre was much more drivable, on the same day, on the same car.

Scale up the width/height ratio and a 235 85 16 pretty much turns into a 37 12.5 (give or take a bit)

I am not bagging wide tyres, (hell, I run 12.5's) but height, not width, is where it is at.

[HotFourOk]

I am not bagging wide tyres, (hell, I run 12.5's) but height, not width, is where it is at.

So... considering this topic is choosing from tyres which are the SAME height... do you go skinny or wider??

Most people are comparing tyres which are a larger diameter to wider tyres which are a smaller diameter... which isnt the case here.
It's fairly common that a larger overall diameter tyre will peform better, within reason, that's why everyone upgrades.

But, if you have two tyres exactly the same height, do you go wider or not?

[Beastmavster]

Well, the argument was really started about 33x12.5x15 versus 285/75/16 versus 305/70/16.


In the scheme of things the difference of about 15mm between the 33"s (smallest) and the 285's (biggest) is almost totally irrelevant. It's only a variation of about 1.5%.


Having not tested the Patrol on all 3 combinations, I can't truly say. And I'm not going to buy 3 different sets of tyres to find out.

I'd think back to back testing of different vehicles with different tyres is going to show more about how much tread depth is left, who the better driver is, who gets 2" off line etc. Almost impossible to do real world tests on this.

[toughnut]

I ran both 285/75/16 BFG AT's and 305/80/16 Baja Claws. Two completely different tires but when I went up to the cape I used the AT's. The run a lot better on the sand compared to the Claws and there is a lot of almost normal roads. The thing the AT's were crap at was mud. I looovvvvee lockers.

[sudso]

narrower tire of same diameter as a widey is not good.

But a taller narrow tire better than short wide one.

For the same contact area, long narrow works better than short fat.

I bet bald tyres would probably work good on sand

christover

I've found that road tyre tread patterns work best in sand because the tread blocks are so close together, they dont bite in and dig down like a muddy etc, they float better and yes, balding muddies work better in soft sand for that reason.

As for the contact patch when deflated debate, I dont think it matters how wide the tyre is, I believe it's the diameter>width and sidewall height ratio that matters.

my2c

[Mick_n_Sal]

What are the advantages and disadvantages of the 285/75/16 over the 305/70/16.

Given their close sizing, I'd look at other issues - are they the same ply rating? do they have the same number of sidwall plys? do you need to fit flares for one of them? will they fit your existing rims? which would be easier to replace in a more remote area as it is a more common size?

[Beastmavster]

285/75/16 is a FAR more common size than the 305/70/16. For remote locations that's going to matter.... but then again if you follow that argument to it's logical conclusion you'd just use 7.50R16 for any remote touring.

[crankycruiser]

I reckon it depends on wat sorta mud ya playing in...

If its that marshy crap then ya want fat tyres.. better floatation.. and for clay i reckon that skinnier would be better..

[sudso]

I reckon it depends on wat sorta mud ya playing in...

If its that marshy crap then ya want fat tyres.. better floatation.. and for clay i reckon that skinnier would be better..

But dont ya want the tyres to bite down to the harder stuff rather than float?

[fnqcairns]

Used to own a 2wd dune buggy, no way is skinnier better in sand and overall skinnier is better in mud.

cheers fnq

[-Scott-]

285/75/16 is a FAR more common size than the 305/70/16. For remote locations that's going to matter.... but then again if you follow that argument to it's logical conclusion you'd just use 7.50R16 for any remote touring.

http://www.outerlimits4x4.com/PHP_Modul ... 8&start=20

[Beastmavster]

But dont ya want the tyres to bite down to the harder stuff rather than float?

Depends on how deep down the hard stuff is

[toughnut]

I doesn't matter what you do. The only way to not have to compromise is the have buku amounts of cash and only drive on one surface for each tire. Take me for answer. My biggest compromise (and most others) is cash. Silverstone are kind enough to let me drive on their tires for free. They may not be the best in all situations and I've had a few people laugh in my face when I tell them what I drive on. But I've changed a few opinions as well when they see how well they work for a reletively small tire. They are amazing in the mud and on rocks and general wheeling. They just dig too well to drive nicely on the sand. Not to say they don't still work better than most.

[Aquarangie]

I'm much a skinny tyre advocate. I am currently running 31x10.5's and even I think that is a bit wide for me. I used to have 235/85R16 Goodyear MT/R's and found they would grip better on clay surfaces as mentioned and generally are easier to fiit to my Rangie (no 'butchering' of guards, etc..

I was at Levuka on the long weekend and watched with intrest the blokes with wide tyres having a few probblems getting grip. I dunno if they they had muds or alls, but found I was able to maintain traction much better than say a 12.5 width. Only 2 inch diffrence, but it was plainly obbvious that wide isn't the 'be all to end all'.

Plus I don't like the big wide look myself as much as I dislike huge spring lifts


Trav

[ludacris]

[quote="Shadow"]How can a skinny tyre work better on sand?

A 7.50inch tyre bagged would be a max contact patch of about 14" (and its not a flat contact patch). A 14" bogger has that without bagging, and sand driving is all about surface area!. The tread pattern is also important but we arent talking tread pattern, were talking width (assume a 7.5" bogger aswell)

I also dont understand the surface pressures argument. If a tyre is inflated to 32PSI (POUNDS PER SQUARE INCH) it has exactly the same pressure over every square inch. Thats what 32PSI means.

The fact is, A wider tyre has more grip because it has more surface area with which to engage the friction coefficient of the surface it is on.

The only time I can think a narrower tyre will produce more grip is when the narrow tyre cuts through the slop and hits firm ground. Otherwise the fatter tyre wins.[/quote]

The fatter tyre always wins because even if the skinny tyre cuts through the slop it ends up stuck on its diffs quicker. The only reason I have lost forward motion is when my diffs have bottomed out.

LudaCris

[ISUZUROVER]

I also dont understand the surface pressures argument. If a tyre is inflated to 32PSI (POUNDS PER SQUARE INCH) it has exactly the same pressure over every square inch. Thats what 32PSI means.

Cris, if you have a 2t 4x4 - assuming perfect weight distribution it will have 500kg on each wheel. Ground Pressure (or Surface Pressure) is equal to the force (mass) divided by the surface area (P=F/A). So a tyre with a larger contact patch on the ground will have a lower surface/ground pressure.

IME - In some cases more ground pressure seems to be a good thing, in other cases not. Also IME, as others have said also) it is better to have a larger diameter skinnier tyre, than a smaller diameter wider tyre (assuming both have the same contact patch area).

[Rangie ute on 38'']

when in doubt put tractor tyres on , ihavent seen to many of them bogged

[Shadow]

I also dont understand the surface pressures argument. If a tyre is inflated to 32PSI (POUNDS PER SQUARE INCH) it has exactly the same pressure over every square inch. Thats what 32PSI means.

Cris, if you have a 2t 4x4 - assuming perfect weight distribution it will have 500kg on each wheel. Ground Pressure (or Surface Pressure) is equal to the force (mass) divided by the surface area (P=F/A). So a tyre with a larger contact patch on the ground will have a lower surface/ground pressure.

IME - In some cases more ground pressure seems to be a good thing, in other cases not. Also IME, as others have said also) it is better to have a larger diameter skinnier tyre, than a smaller diameter wider tyre (assuming both have the same contact patch area).

But a 2t 4x4 on 2" wide tyres running 32psi will have the same contact area as a 2t 4wd on 24" wide tyres running 32psi. The contact area is a constraint of the tyre pressure. (if we neglect tyre wall resistance)

The bow wave argument sounds very logically and i can understand this argument, aswell as the argument which says thinner tyres can cut threough the mud better, but if your argument is that sometimes more ground pressure helps, and thats why skinnier tyres are better, just increase your tyre pressure!.

[Suspension Stuff]

To put it simpler, if you have a wheelbarrow tyre fitted to your 4wd pumped up at 32PSI, it will have more pressure per square inch on the ground then a large tyre per square inch on the ground.

There has got to be a perfect width. If you build a tyre 3m wide your 4wd just wouldn't have the grunt to push it through the sand, so at some point there a tyre that is too wide for the sand.

Each tyre has advantages and disadvantages the trick is to minimise the disadvantages and maximise the advantages for the stuff I plan to drive.

Also if I choose a wider tyre it may limit some flex and or cause me to have to chop guards.

[Shadow]

To put it simpler, if you have a wheelbarrow tyre fitted to your 4wd pumped up at 32PSI, it will have more pressure per square inch on the ground then a large tyre per square inch on the ground.

no it wont

the wheelbarrow tyre will have 32Pounds of pressure per square inch. Same as a 400" tyre.

The problem is that the wheelbarrow tyre would probably be sitting on its rims at 32PSI because it cannot create enough contact area with the ground with which to transmit the force(weight).


No matter what size tyre you have, if you have it inflated to 32PSI, it will have 32PSI of contact pressure.

A wheelbarrow tyre, lets say 12"x4", if inflated to 32PSI and bearing a load of 500KG will have a contact patch of (500KG = 1100Pounds) ==> 34.375 square inches. That means that lets say it bags to 6" wide, it will have a contact patch of 6" by 5.73".

A 33x12.5" tyre inflated to 32PSI bearing a load of 500KG will have a contact patch of exactly the same, 34.375 square inches, but its contact patch will be 12.5" x 2.75".

[Suspension Stuff]

Yes it will. Divide 750kg 1/4 of your 4wd by the contact patch area.