Thread: Big Tyres: Pros and cons

Or to put it even more simply, how can a wider tyre not have a wider footprint?

Originally Posted by Rick122

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...

Its really not that hard to comprehend, especially if you actually sat down with a whole heap of tyres and looked at them, as in actually really looked at them. The only anomoly is that the 7.50 they have used has a broad tread face, and to be honest they are out there and open pattern bias ply tyres are like that as they square off the edges - The Simex which are also bias actually flare the tread blocks out to a treadface. When I look at those contact patches at the lower pressures and compare the amount of space between the patch and the top/bottom of the page, there looks to me to be a significant difference in the patch length, they are not similar at all - it highlights there that the wide tyre does have a longer contact patch. Actually the wide tyre goes off the end of the page!

Originally Posted by DiscoMick

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?.

That's basically correct. The additional sidewall high will allow the tyre to be aired down further and increase the length of the footprint.

The other thing is that sometimes as the tyre profile drops, the treadface increases, so to take an extreme, if you compare the treadface of a 255/85-16 and a 255/60--18, you will probably find the 18's tread face is (eg) 35mm wider. This is because the width measurement is taken through the bag of the tyre, not at the treadface.

But that said, without measuring, I'd expect a 235/85-16 will put down a much larger contact patch than a 235/70-16.

Edit didn't see this thread was actually ten pages long, silly app took me too the start

Originally Posted by Slunnie

Its really not that hard to comprehend, especially if you actually sat down with a whole heap of tyres and looked at them, as in actually really looked at them. The only anomoly is that the 7.50 they have used has a broad tread face, and to be honest they are out there and open pattern bias ply tyres are like that as they square off the edges - The Simex which are also bias actually flare the tread blocks out to a treadface. When I look at those contact patches at the lower pressures and compare the amount of space between the patch and the top/bottom of the page, there looks to me to be a significant difference in the patch length, they are not similar at all - it highlights there that the wide tyre does have a longer contact patch. Actually the wide tyre goes off the end of the page!

I think you really should read the article again. So let me reply with quotes from the article and then summarize.
"As you should be able to see, there is little difference in footprint until you reach 10 psi."
Then he goes on to say "How could it be possible for a narrow tyre with 1950's technology on a split rim to have an almost identical footprint to a*modern wide tubeless tyre in a size renowned for floatation and 4WD ability ? bit of a surprise eh?".
And then, "I was interested to see how the sidewall flexed in both tyre constructions. I then let the pressures down to 15 psi (below), a pressure that can be needed every now and again when the conditions are wet & soft. Another time for lower pressure is climbing in steep country and also climbing very stony ground."
So he let the pressures down to 15psi, not 10 as in the first picture.
"When you look at the footprint pictures and then the above image of the Bias sidewall, you'd have to wonder why you'd use Steel Belted Radials at all in tough country with sharp bits that you have to drive over. Yes, I do realise that 15 psi is too low for normal work, but it was a good way to illustrate the difference in sidewall constructions."

So to summarize:
A) the first picture you posted shows virtually no difference in contact patch between the two tyres at 20psi, a pressure you would likely to go down to in soft sand etc.
At 15psi pressure the radial tyre would be at great risk of getting staked through the side wall and was too low a pressure for normal off road work. You can clearly see that in the pictures in the article, the sidewall is almost on the ground! Imagine what it would have liked like at 10psi!
C) So 10psi is the only pressure where the contact patch was bigger yet at that pressure it would clearly not be wise to run it that low. So you wouldn't run that tyre at 10psi, so it shouldn't be used as the basis for an argument saying that proves it's better than the "quite close to being indestructible" MRF77.

Also, I am certain that if he had chosen a modern radial tyre with a high profile to compare eg. A 235/85r16, instead of a 14ply bias ply tyre, it would have been a clear win for the tall and narrow in contact patch and we wouldn't be having this argument.
I think I might do my own contact patch tests for comparison. Can you even buy bias ply tyres anymore? Wonder when this article was written.

On modern land rover alloys, as low as 10 to 12psi is quite safe
That's if you're actually going 'off road' as opposed to merely touring.

Sent from my HTC One using AULRO mobile app

A 235/85 will bag the same as a 265/75, have the same length contact patch assuming a comparable carcass, but will have a narrower contact patch, but do your testing.

Originally Posted by Disco Muppet

On modern land rover alloys, as low as 10 to 12psi is quite safe
That's if you're actually going 'off road' as opposed to merely touring.

Sent from my HTC One using AULRO mobile app

Yep, I've spent a week at 10.5psi on sand without any problems. Great bead retainers... unless you're trying to change a tyre!