Thread: Narrow tyres V Wide tyres

winner.

Originally Posted by HSVRangie

winner.

What do you mean?

Originally Posted by mrapocalypse

1. Scuse me JDNSW, but why would a skinny tyre have a longer footprint and a fat tyre have a necessarily shorter footprint when the have the same outside diameter?

If you have a tyre with a 32 inch diameter, length ways they will have the same footprint, cross ways a fat will have more surface area covered, or is there some piece of tyre science I am missing here?

2 Now one big issue with FATS is unsprung weight, which i am sure is something the rally cars mentioned earlier would be taking seriously!

3 And when it comes to "Cutting down through the loose surface material", that's a little random isn't it? I would think that a big, fat mud pattern tyre that grabs a huge handful of the surface, gets grip and stays above the sludge, dust, loose gravel is better than a half submerged skinny tyre! Or is there some piece of tyre technology I am missing here too?

1. With a load on the wheel, the round tyre flattens at the point of contact. The area of the flattened bit depends only on the tyre pressure and the weight on the wheel (ignoring sidewall stiffness, which is negligible for any ordinary tyre), since the air pressure in the tyre has to balance the weight on the wheel. Since the area is constant, the wider tyre has a shorter flattened section. The overall diameter of the tyre makes no difference either, although it does affect the angle between the unloaded part of the tread and the flattened bit, which is likely to be important climbing over obstacles, although the difference is not really significant for realistic choices in diameter. The really big effect of increased diameter is increased ground clearance and approach/departure angles, and even small improvements here can make a big difference.

2. Yes, unsprung weight is important although probably more on rough roads than either offroad or an smooth roads. However, just because the tyre is wider does not necessarily mean it is heavier, although the tyre/wheel combination is very likely to be heavier, as a lot of the mass in a typical tyre/wheel is in the rim, and goes up with width. Similarly, a lot of the mass of the tyre is in the belt, and this goes up directly as the width. But changing to wider tyres often includes going from steel to alloy wheels, which may be lighter than the narrower steel wheels (although not necessarily).

3. Depends entirely on conditions - no good getting a big bite on the surface loose or slippery stuff if it just slides over the stuff below it when you want to climb a hill or go round a corner. The main factor here is the tyre tread rather than the width. If the problem is that it is just soft, and you want to avoid sinking, the main criterion is tyre pressure - and the critical factor here is how low a pressure you can get away with. In theory since the tyre flexes less for a wider tyre, everything else being equal, you should be able to run a lower pressure in the wider tyre, and be better off with it, but only in the pressure range below the one you can run on the narrow tyre. And everything else rarely is equal.

John

Cooper 245x70x16 ATs. Run at the correct pressure for the conditions do me just fine & they are legal in Qld too.

Originally Posted by JDNSW

Unless someone has repealed the laws of physics, the area touching the ground will be given by the weight on the wheel divided by the tyre pressure - for any normal tyre, the sidewalls provide little support.

We all know that P=F/A, but the sidewall DOES make a difference. I remember some experinemts someone did a while back, there was up to 30% variation in the size of the contact patch for different tyres at the same pressure on the same vehicle.

I probably missed it in the first 6 pages or so but I can only store so much info at once ,it seems to me though that those who want to debate the virtues of skinny verses fat have missed a vital point, ie at what stage does a skinny stop being a skinny & become a fat. Sorry to those who don't want to debate that, please pass on by.
To my mind & I stress this is largely theoretical, 7.50x16s are skinny, as are 235/85r16s, 255/85r16s, 205/80r16s etc. getting into the 75 aspect ratio tyres & I believe that you're running into a grey area & at 70s then you're prbably into tyres that behave much the same as the so called flotation tyres ie 31x10.5x15s,33x11.5x15s,35x12.5x15s etc have an aspect rastio of respectively approx, 73%, 76% & 78%. Without the experience of having tried a heap of different tyres, it strikes me that there will be an ideal ratio between the length of footprint & area of contact verses the bow wave that a tyre sets up. I also feel that the footprint of a tyre lengthens to a greater extent the taller the side wall is, which is why when discussing skinny verses wide its important to discuss real skinnys rather than maybe ones. Taller sides walls are likely to get longer for the same pressure drop than are lower ratio tyres & at the same time retain a smaller bow wave. We also don't seem to have taken into account what affect heavier side walls have, ie the 6 ply rated walls on flotation tyres verses the 10 ply rated ones on LT tyres, which I'm sure also muddies the waters a bit. If you take into account all this & then the huge variety of conditions we're asking tyres to perform in the it becomes almost impossible to quantify. Perhaps McRover was right to simply ask what do you run & how do you find them, I might also have added what construction are they & what conditions were you running them in, remembering no tyre can possibly be eveything to everybody. Sooo, wanna start all over again?
Steve.
PS I'm running 245/75r16sLTs in a semi mud tread, they're fine for most of what I do but then thats nothing too extreme as the handbrake won't allow it. When I get my ideal LR it will be a "fender or similar & run 235/85r16s, coz I think they'll do OK, so there.

I ran 235/85/16's Cooper ST's on my RR Classic. The main reason I made this decision was because it was a 4 door with only a 2" spring lift and I didn't want to (couldn't afford) a body lift, and didn't want to cut into the rear doors for clearance on full articulation. The other 2 vehicles that I was going 4wdriving with at the time were a dual cab hilux running 31/10.5/15 Hankook MT's and an 80 series running 285/75/16 BFG MT's. I surprised myself at how well my tyres went on muddy fire trails compared to the wider tyres of my mates cars. I would often do it a bit easier in the mud compared to the others, but not significantly. The other big surprise came when I moved to WA from Vic and thought I would have problems on the sand because of my skinny tyres and aggressive tread pattern (for an AT). I found that when my tyre pressures were dropped to around 16psi, there wasn't many places I couldn't go on the soft stuff, and I did it easier than others I was travelling with that were running standard size tyres on various vehicles. Even though I was running skinny tyres, because of the extra sidewall height, the contact area lengthened considerably still giving me much better flotation than expected.

Originally Posted by HSVRangie

winner.

here as well