Vehicle modification: Tyres

[XJ Junkie]

In relation to this thread, here are some thoughts on tyres

http://www.4x4ag.co.za/forum/viewtopic.php?t=23

Tyres

One of the common beliefs with tyres is that a tyre that lasts many miles is a good tyre. But this does really depend on the vehicles philosophy & the owners perspective.

Because tyres that last long doesn’t necessarily mean that they actually perform well in terms of grip. You can’t have both longevity & performance.
Any tyre manufacturer can easily double the amount of mileage that their tyres can achieve, without too much fuss. But they realize that there will be a compromise in terms of a reduction in performance.
My current set of ATs are on 96kkm, which I’m a little annoyed about because it seems to indicate that I bought rubbish tyres, although my tyre guy reckons that vehicle weight also plays a role. A light vehicle on heavier tyres will prolong their life somewhat. My Bighorns were at 50% after 15kkm. Now that’s a good tyre!

Then we get to the issue of the belief that a bigger tyre is better than a smaller tyre, but bigger is not always better, especially when fitting tyre that are dramatically bigger than stock.

There’s a number of aspects that need to be considered.

A few years back, we took some visitors to AD in their Wranglers. 2 Rubicon’s & 1 Hi-Sport. All of them were supercharged & had 37 inch tyres. None of these Jeeps could outperform our regulars & I mainly blame their tyres. Sure, we are all driving God’s with vast experience which also played a role. Unlike the Rubicon’s, the Hi-Sport in the group had a D30 front diff, which eventually shat the bed.

Ever tried to lift a 37 inch tire? They are damn heavy. Now imagine that weight x4. The impact of bigger tyres on rotational mass is massive. All that energy generated doesn’t dissipate. So it puts enormous strain on driveline components, which end up breaking quicker than a D3’s Crank. One of the most common breakages we see on IFS vehicles are there little front CV driveshafts. They simply can’t handle that weight of a much larger tyre.

Then differential internals are also a common thing to fail. On SFA vehicles, the axles bend if the aren’t braces & reinforced. I’ve personally bent 3 axles. The first one I bent looked like the vehicle was smiling like D3 owner with a warranty claim approved.

The next important thing is gearing. Bigger tyres result in a negative change in the gearing. The bigger the tyre, the more the performance/power will suffer in the absence of re-geared differentials. Hopefully Apocalypse will share his expertise in this regard, especially in relation to the reduction in force that’s transferred to the ground.

Sprung mass vs grip. Everything below the spring is what’s referred to as unsprung mass, namely solid axle diffs, wheels & tyres. Spring mass is everything above the springs. With bigger tyres, it’s not just the weight that gets greater, but also the thickness of the rubber, especially the sidewalls. So in order for the tyre to be pushed to the ground, the sprung mass of the vehicle needs to be sufficient in order to compress the sidewalk of the tyre when the suspension loads up. If it can’t do this, then the footprint of the tyre is obviously worse off. This is probably the area that is the most negative aspect of oversized tyres.

Then of course there is cost implications. The other day I was reading on another forum where a guy thought he was building a monster performer JKU for dune driving. It’s diffs we’re stock & I don’t think the gearing was done, yet it has 37’s on. Before that vehicle breaks, I suspects it’s going to get it’s arse handed to it by a couple of Isuzu KBs as well as a few Jimmy’s.

So in addition to the huge cost of the tyres, a lot more money has to be spent on the vehicle, just to accommodate the wrong tyres.

Sand dune driving can be brutal by nature & it’s where incorrectly modified vehicles can get exposed. For sand driving in particular 33’s are about the max that should be fitted to the majority of SFA vehicles. Heavier vehicles like JKUs, Cruisers & Patrols can get away with 35’s if done correctly.
But recently I saw a double locked Patrol struggling to keep its wheels spinning. You need a lot of engine power to keep big tyres turning & even more power when the lockers are fired.

Stock Bakkies & bakkie based SUVs with IFS shouldn’t be going more than 31/32 inch tyres. If you see a bakkie with 33’s & stock CV driveshafts still intact, then that guy didn’t need 33’s in the first place.

The only real advantage of bigger tyres is a bit more height & width, provided that the sprung mass can compress those sidewalls.

[Michael]

Stock Bakkies & bakkie based SUVs with IFS shouldn’t be going more than 31/32 inch tyres.

So you're basically saying stick with stock tyre size? (If my maths is correct, a 265/70/17 is 31.6")

[XJ Junkie]

Stock Bakkies & bakkie based SUVs with IFS shouldn’t be going more than 31/32 inch tyres.

So you're basically saying stick with stock tyre size? (If my maths is correct, a 265/70/17 is 31.6")

That’s a big stock tyre! It’s nearly 32’. I personally wouldn’t but I suppose you could go a bit more.

[Mad Manny]

Ideally, you should not go up more than an inch.

So if your stock is a 32" (265/65 17 which is just under 32") don't go more than 33" if you want to avoid the pitfalls XJJ is referring to.

[Michael]

I find it interesting that the difference between a 265/65/17 and a 265/70/17 is a whole inch (30.6" & 31.6" respectively)

[Mad Manny]

I find it interesting that the difference between a 265/65/17 and a 265/70/17 is a whole inch (30.6" & 31.6" respectively)

Well, from 70% to 75% is an increase of 5% points in profile.
So it's an extra 5% of 265, which is 13.25mm.

So, on the diameter, you'll measure the profile twice, so you'll measure through an additional 26.5mm (13.25 twice),
26.5mm is just (just) over an inch...

[XJ Junkie]

So a 5% increase in diameter would be the same percentage loss in force

[ThysleRoux]

[Snip]So in order for the tyre to be pushed to the ground, the sprung mass of the vehicle needs to be sufficient in order to compress the sidewalk of the tyre when the suspension loads up. If it can’t do this, then the footprint of the tyre is obviously worse off. This is probably the area that is the most negative aspect [/Snip]

I remember a Zook SJ with a set of Safari Pitbull recaps at one of the AD fun events deflating to close to zero pressure and the sidewalls did not bulge at all. Debead waiting to happen......

[ThysleRoux]

Neil, I agree that tyres that last longer generally dont have the same grip as softer, faster wearing tyres, but there are exeptions.

The Kook MTs I had on RooiTjiep gave fantastic mileage - close to 95 k km when the one ate a sidewall after a puncture - and the grip was excellent in all conditions - sand, mud, rocks, gravel roads, as well as dry and wet tarmac.

[dawidloubser]

I find it so interesting how the real men that did the Camel Trophy in the 80s and 90s rode on really skinny tyres:
https://youtu.be/UGWp8R-IqoE?t=19m24s

[KurtG]

I have BFG KO2’s on my Pajero. I knew what I got myself into when buying them, tough but low grip especially when on wet tar.

However, for my use so far they have been the right choice because some of the trails I have done would probably have caused some damage to SUV spec tyres.

It was a personal choice matched to my needs so I’m happy with the choice.

[XJ Junkie]

I find it so interesting how the real men that did the Camel Trophy in the 80s and 90s rode on really skinny tyres:
https://youtu.be/UGWp8R-IqoE?t=19m24s

Those are proper mud tyres. The thinking behind them is that they cut through the soft top layer of mud in order to reach the harder mud underneath.

[Apocalypse]

Tyre width is quite an interesting one.

Drag is exponential to tyre width - if you double the width of a tyre you need 4 times the effort to move whatever it’s moving.

It very much depends whether you are after flotation or displacement of the material under the tyre.

Wide tyres simply don’t Allow for water or mud to move out from under them.

You’ll notice that even supercars no longer run those eighties 400mm tyres anymore - and if you’ve ever driven on massively wide road tyres in the wet you’ll know why . Skinny tyres are always better when there is a solid surface below the slippery stuff - snow over tar water on tar, shallow/ surface mud.

Plus it takes way less force to displace the hazards . Those old defenders and d1s weren’t exactly known for their massive torque and power figures...

Loose sand, deep mud , you are far better off with huge wide footprint .

[Ricof4e]

Tyre width is quite an interesting one.

Drag is exponential to tyre width - if you double the width of a tyre you need 4 times the effort to move whatever it’s moving.

It very much depends whether you are after flotation or displacement of the material under the tyre.

Wide tyres simply don’t Allow for water or mud to move out from under them.

You’ll notice that even supercars no longer run those eighties 400mm tyres anymore - and if you’ve ever driven on massively wide road tyres in the wet you’ll know why . Skinny tyres are always better when there is a solid surface below the slippery stuff - snow over tar water on tar, shallow/ surface mud.

Plus it takes way less force to displace the hazards . Those old defenders and d1s weren’t exactly known for their massive torque and power figures...

Loose sand, deep mud , you are far better off with huge wide footprint .

A very valid point Apoc.

What role does skinny-ish tyres play (with a high side wall) in loose sand? Like these tyres a LC comes out as standard with? I have read somewhere that in loose sand a high tyre wall is better than a wide track?

[Apocalypse]

A high side wall means you can deflate more.

It’s all about pressure. As you deflate the tyre you apply less pressure to the surface which means you bet more flotation, and a bigger footprint.

The downside is that the sidewalk collapses to form part of the footprint .that means it flexes as you drive. Which causes heat build up in the side wall and delamination of the layers, and tyre failure.

I personally never deflate below 75% of side wall height measured from hard level ground to rim