Thread: Feed Back on 17" Tyres

Originally Posted by chuck

If narrower tyres work better off road then why do all the highly modified
4x4's run wide tyres?

Regards

Chuck

Because they run very tall tyres, like 35s or 37s and you need a reasonable width for that sort of diameter. Look at the Defender, other similar offroaders, they're on the likes of 235/85/16. I know of one very successful offroader driver, multiple champ, who runs the narrowest 35s he can.

Originally Posted by chuck

If narrower tyres work better off road then why do all the highly modified
4x4's run wide tyres?

Regards

Chuck

In competition, you run at very low psi - all other things being equal, the wider tyre will mold to the surface (rocks etc) better, as it has a greater internal volume. And of course, the greater width can span cracks/holes/ruts better.

Cheers,

Gordon

Threads that start discussing tyres tend to be very long, especially those that cover the relative merits of narrow vs wide.

Some points -- a wide tyre has the same contact patch as a narrow one, all else being equal, it's just a different shape. Narrow tyres have a longer contact patch, shorter ones wider. Both are oval shaped not rectangular.

The contact patch is a function not of width, but of tyre volume, weight and tyre pressure. However, the tyre construction has much to do with how a tyre's contact patch changes as these factors change.

A wide tyre spans a wider area than a narrow one, which is why they aren't so good in the wet; say a 285 has to move an area 285mm wide of water out of the way for maybe 140mm of tread length, whereas a 235 cuts a path only 235mm through the wet and its long contact patch, maybe 180mm then has a relatively dry road to work with. That principle applies offroad too in the wet as it often is. Wide tyres also run more risk, offroad, of not sitting flat to the terrain. Not an issue onroad, where width is an advantage (in the dry) for various reasons like tyre slip, penumatic trail, steering response which is why sports cars have wide tyres. And the longer contact patch of a narrow offroad means if you are driving over say a stone the tyre can bend over it like a mini tank-track because it's long (like the spanning point, but the other way around). Wide ones can't do that as well; you could argue they could do much the same laterally, but tyres are designed to work with ALL their tread on the ground, not part of the tread of the ground. Hence my general point about narrow, tall tyres being good offroad. But like everything offroad there will be the odd occasion where wide is better. The wider tyre will have a greater internal volume than a narrow one, but to achieve the same contact patch you'd air down further so it would be the same in the end. The bigger the volume of the tyre, the more you have to air down to get the flexibility and reduce loss of traction through bouncing. Those Icelandic 4X4s run virtually no psi but their tyres don't look flat as there's such a huge volume of air there, same as a smaller tyre but it's not compressed into a 245/70/17 or similar.

There's lots more to say but in reality the differences between D3 tyre widths aren't massive and offroad the driving technique, quality of tyre and pressure etc will make the difference, not whether you're running 235 or 275.

Comp trucks are a bit different to D3s as they run beadlocks on very large tyres which usually have incredibly stiff sidewalls (they're no fun to remove off rims), and the tread patterns they use are very aggressive.

A touchy subject! Personally, i find that the D3 works much better in soft sand on wider tyres, better on road handling as well, better in mud (some types) and much better on rocks.

Narrow tyres work better in snow (generally speaking, but again, depends on type of snow) and aquaplane later.

Sure there's somebody out there who will feel the oposite is true.

P.S. I don't see why a narrow tyre, of same compound, diameter and sidewall height as a wide tyre should have a longer contact patch.

A tyre's contact patch is only ever as wide as its tread width. Any bulge is only on the sidewalls. Therefore a wide tyre has a wider contact patch. *

Given a contact patch is a function of weight, construction and pressure if a tyre already has a wide patch then those forces don't result in a CP as long as if it were narrower, it just doesn't elongate as much. This can be measured. A long contact patch is not great for on-road driving which is one reason why sports cars don't go in for narrow tyres.

Two tyres of the "same compound, diameter and sidewall height" but one narrow, one wide are not the same, because the wider tyre will have a greater volume of air. This means for a given vehicle weight it should be run at a lower pressure, and that in itself can give benefits offroad as the tyre bounces less (but there are disadvantages too). However, it's a disadvantage onroad because you want high pressures for high speed road work -- and that's why low-profile wides, not high-profile wides, are common for sporties as these tyres have a relatively low volume of air thus are run at high pressure, but have the on-road advantages of a wide tyre.

The direct comparsion would be two tyres of equal diameter and construction, but different widths, and of equal internal volume. This would require the narrower tyre to have a smaller rim diameter.

It's also clear what a compromise tyres are!

* assuming it's flat to the ground, and the pressure plays a part too, at really high pressures for example there the contact patch doesn't extend to the sides of the tyre etc etc....