Big Tyres: Pros and cons

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Originally Posted by DiscoMick

I think we're mixing two issues here - tyre size and clearance.
Obviously a taller tyre gives greater clearance so lets concede that one.
On traction, say in mud, the narrower tyre should sink further so more likely to find the bottom and get traction. The wide tyre shouldn't sink as much so less likely to find traction and more likely to spin.
On sand, you want to float, not sink. A wider tyre shoud float more than a narrower tyre.
Another issue is tread. A more aggressive tyre is more likely to dig down. Thats good in mud and bad on sand.
A taller tyre will have a longer contact patch. A wider tyre will have a wider contact patch and have to push more in front of it.
Is that right?

Sent from my GT-P5210 using AULRO mobile app

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I agree with everything you have written there.

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Originally Posted by Slunnie

This picture below immediately falsifies the article in that link.

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Just saying something didn't make it true Slunnie. If you're going to make a statement like that you are going to actually show why it falsifies the article. As pointed out earlier, the 265 tyre is roughly 40% wider than the 750 tyre so the same should show on the paper, yes? However, the tyre widths in that picture look exactly the same, so I could say that article is completely false - based on that one fact. Anyway, happy trails.:)

So maybe the best tyres for the two situations of mud and sand are very different extremes:
Mud- tall, narrow with a chunky tread to bite down
Sand - wide with a mild tread to float, not dig down.
So, what's the best compromise for overall use, assuming we only have one set of tyres? Maybe a tall narrow tyre to sink into mud with a mild tread so it doesn't dig holes in sand and is quiet on the bitumen?
What do you think?


Incidentally, it was interesting on our Simpson crossing trip last year that the trip leader Dave, who is very experienced and works for ARB, chose wide Cooper AT2s with a mild tread on his Patrol trayback for the sand dunes.
He went over dunes that others, including some with more aggressive tyres, got stuck on. Interestingly, Dave was strong on lowering tyre pressures to avoid getting stuck. He said he thought snatch straps and winches were only last resorts when all else failed. As a result of his guidance, our group made the whole crossing without a single use of a snatch or winch, which compared with other groups who used snatches and winches multiple times. Having the right tyres and pressures was the key, not big lifts and lots of expensive gear.
So, that was on sand. Mud might be a different story.

Wow, quite few posts of tedium now.
Maybe some broader considerations could have the discussion move along.

Aircraft operating regularly into unsealed or soft surfaces use wider and larger diameter tyres.
Uh oh, wider and taller??
They are often referred to as high flotation gear which can have several tonnes happily move around on surfaces that would crumble under lighter machines.

So tall skinny guys are wrong " cutting into mud sand etc" because we don't want that resistance reducing take off performance or causing the aircraft to become bogged.
Wide tyre guys are wrong too
"Wide tyres bag out more" so that's also resistance that we don't want reducing take off performance and requiring longer rwys.

So why are they used?

Because they are far better than what comes std according to the intended purpose of the equipment they are fitted to and allow the equipment and operators to do things they couldn't do normally without the mod.

It is a fact that these tyres do less damage to the very fragile( by comparison to tracks) surface that they are used on.
While they may not be asked to provide traction under power, they are required to provide under brakes.

I know what I have fitted to my vehicles suits my needs and is overkill for regular use, but so is the vehicle. Most of the population don't even need a 4WD.

It is good to know that whichever is fitted to my vehicle that I will generally be considered a "big tyre ******" or "skinny tyre never goes off road" guy.

And the most irrelevant post bringing aircraft into an offroad discussion goes to...TIC
I'd also say your end statement is opposite to the norm. It should be "Big tyred, more concerned with looks than performance, predominantly seen driving in the suburbs" and "tall and narrow tyres more concerned with off road performance than looks, predominantly seen in the outback doing their job" haha.
I don't know how relevant this is now but to quote "The Rain Forest Challenge and The Trophy challenge have all been won by the aggressive Simex Trekker tyre (35x11.00).*" by all regards a very tall and narrow tyre.
Also nearly all Camel Trophy cars used 700R16. They did alright with the tall and narrow.
Anyway, have a great day people!

Hilarious thread! ...Ok, when Defenders can fly or drive on water maybe I'll consider wider tyres.

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Originally Posted by Rick122

Just saying something didn't make it true Slunnie. If you're going to make a statement like that you are going to actually show why it falsifies the article. As pointed out earlier, the 265 tyre is roughly 40% wider than the 750 tyre so the same should show on the paper, yes? However, the tyre widths in that picture look exactly the same, so I could say that article is completely false - based on that one fact. Anyway, happy trails.:)

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Sorry, I just thought that after reading the link that it was pretty blatantly obvious and didn't realise I had to spell it out. :D

Here is the premise that the thread hinges on.

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So contact patch is dependent on force and pressure and has no regard for dimensions. What this means is at the same pressure, a narrower tyre will deform more than a wider tyre to achieve the same contact patch. Any sized tyre will deform just the right amount to achieve the contact patch described by the formula above. Lets substitute some values to see how it works.

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And this picture is what shows that this premise is incorrect. You'll notice that at a given pressure the contact patches are not compensatory as the article states. Actually the wide tyre has a notably longer contact patch than the skinny tyre in addition to the extra width. The article is working off the premise that a skinny tyre will have the same contact patch as a wide tyre because it elongates more, yet the testing shown in the picture actually disproves this and the contact patch is significantly shorter an narrower than the wide tyre then it falsifies the article.

The picture below actually supports pretty strongly reasons at a technical level why a wide tyre should be much better on sand.... oh sorry, because there is a much greater contact patch and so increased ability to float and drive.

As an aside, I'm not sure what the measurements of those tyres widths are, but 7.50's are typically about 165-170mm wide at the treadface (I support this with my tape measure and a heap of 7.50's at home - Interestingly 235.85's measure at about 190mm) which is unlike the one used and I haven't seen any 235/85's with a wider tread face than a 265/75.

https://www.aulro.com/afvb/images/im...015/06/915.jpg

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Originally Posted by Slunnie

Actually the wide tyre has a notably longer contact patch than the skinny tyre in addition to the extra width. The article is working off the premise that a skinny tyre will have the same contact patch as a wide tyre because it elongates more, yet the testing shown in the picture actually disproves this and the contact patch is significantly shorter an narrower than the wide tyre then it falsifies the article.

As an aside, I'm not sure what the measurements of those tyres widths are, but 7.50's are typically about 165-170mm wide at the treadface (I support this with my tape measure and a heap of 7.50's at home - Interestingly 235.85's measure at about 190mm) which is unlike the one used and I haven't seen any 235/85's with a wider tread face than a 265/75.

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Actually it's this picture which I would be disputing for the above reasons. There is no way that 265 tread pattern is 40% larger than the one below it, yet that is what it should show. So I dispute the whole article. And yet you keep dragging it out as proof.
There are many more articles out there on the virtues of a tall and narrow tyre, yet you have provided only 1 article which is obviously flawed. All that picture shows is that when aired down, the tread contact patch is almost the same, yet by your reasoning, the wide tyre should be much larger.
Yeah I'm pretty much done with it...

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Originally Posted by Slunnie

Sorry, I just thought that after reading the link that it was pretty blatantly obvious and didn't realise I had to spell it out. :D

Here is the premise that the thread hinges on.



And this picture is what shows that this premise is incorrect. You'll notice that at a given pressure the contact patches are not compensatory as the article states. Actually the wide tyre has a notably longer contact patch than the skinny tyre in addition to the extra width. The article is working off the premise that a skinny tyre will have the same contact patch as a wide tyre because it elongates more, yet the testing shown in the picture actually disproves this and the contact patch is significantly shorter an narrower than the wide tyre then it falsifies the article.

The picture below actually supports pretty strongly reasons at a technical level why a wide tyre should be much better on sand.... oh sorry, because there is a much greater contact patch and so increased ability to float and drive.

As an aside, I'm not sure what the measurements of those tyres widths are, but 7.50's are typically about 165-170mm wide at the treadface (I support this with my tape measure and a heap of 7.50's at home - Interestingly 235.85's measure at about 190mm) which is unlike the one used and I haven't seen any 235/85's with a wider tread face than a 265/75.

https://www.aulro.com/afvb/images/im...015/06/915.jpg

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So (and I'm not pretending to be an expert here, just interested) if we take two tyres which are both the same e.g. 235 and 16, but one is 70 and one is 85 (e.g. original D1 vs. Defender), and both tyres have the same tread, then isn't it right that the 85 should have a bigger (longer) contact patch, while the width would be the same?


If a tyre's contact patch is longer, but not wider, then isn't the theory that that tyre would be better in mud, but not better on sand, because in mud you want to hit the bottom but on sand you want to stay on the top?
Or have I got that all wrong?


I ask because I'm just not convinced that, in most situations, its worth spending a lot of extra money on bigger, wider tyres.

Thats just wrong, whoever did those ink prints had just made images perceiving what he thought they should be, down below 20psi the contact pattern in the centre of the tyre would be very light, but its still as dark as at 50psi, at 10psi the centre would hardly be in contact with the ground at all , especially on a hard flat surface - such as you would need to make those prints. And at low pressures the outside of the tread pattern should be much darker as its pressing in harder, theory ok but execution has tripped him up. (of course on sand or mud the centre is still in good contact - but the prints weren't done on sand or mud)

Yes, the higher profile tyre will deform more and the contact patch should lengthen more. Regardless what that picture shows....;)
Personally, I think that narrow tyre must have had very tough side walls, so didn't deform as much as it should have.