Tyres and Rockcrawling

[JP Mark]

I've been thinking about this for a while and since watching Petes Wrangler at TT I'm starting to think that a very large tyre on a Wrangler can be a bit of a legacy. Jeeps are just too light to apply enough contact pressure with such a large tyre (38 and above). I did winchbreak the other day in 33s and 15 psi and went fairly well (although tyre was still spinning on the rim at that pressure). Additionally, I think everyone has seen a stocker do amazingly well in comparison to vehicles running lifts and bigger tyres. I think the factors in this are a much higher contact pressure at the tyre (smaller tyres and the fact that many newbies dont lower their pressures as much as most) and a lower C of G. Another factor (highlighted to me by TwinStix) is that suspensions may be too flexy, with wheels dropping into crevices rather than hanging for a while until the car moves forward and the wheel makes contact again. Also the body roll on some of these flexy Jeeps is getting ridiculous.
What are your opinions on this??? All the mods in the world aren't worth a damn if the rubber doesn't grip the surface. Sure, a larger rolling diameter has huge advantages but there must be a balance between this and contact pressure (and tyre compound as well I suppose).
So what am I thinking is the way to go - skinny 35-37 inch tyres(no wider than 12"), 8-14psi, adjustable sway bars front and rear (Curries), and portal axles to give clearance and keep C of G low (this is the bit that is too expensive though).
Time for a bit of discussion on the Tech page!

[Wooders]

Mark,

I still have long term dreams of making the CJ5 into a competion rig....
Now my thought on this rig are.
1. Keep it simple (& cheap). This also add the benefit of easier rego etc...
2. Leaf spings with moderate flex - I'm talking say equivelinet of 800-900 on 20deg (maybe less)
3. Vertually zero lift. Cut the guards plenty to let a say 33" tyre work. At most 1" Body lift.
4. PLENTY pf skid plates underneath so it can slide over stuff with less damage, and also less likely to hang up.
5. Add a Kline (or 2nd tcase) for deep gears, but without destroying the mild range reduction. This would also be allowed via smaller lift.
6. etc....
There's more but it's just kicking about in my head right now as I've other priorities....

[FireTruck]

For a good indication, check out what they do in the USA... they have been rockcrawling HARD for a hell of alot longer than we have here.

And 8-15 psi?? Wow, they are some high pressures. Would be ok with BFG's or maybe MTR's, but with a swamper etc you have to go way lower than that...

I agree with the suspension flex in as far as too much can be too much! Have a look at the Currie FireAnt in action... runs 10" coil overs (with high spring rates), antirock's front and rear. Not the biggest flex monster you have ever seen by a long shot! It is WAY lighter than a standard TJ (it is narrowed, tubed, and runs alumiminium skins), and runs 37" MTR's at low pressures.

I am going for some extra width this time around - from what I have seen it can be an advantage. 38.5x14.5 SX's for me... will see how they fare.

Back on the USA trends... trend now is BIG horsepower, LIGHT rigs (mostly custom tube frames with skins), and LOTS of skinny peddle.

Oh, and did I mention LOTS OF $$$$!!!!

[JP Mark]

8-15 is pretty high considering what we are all used to and you need next to nothing in a swamper before it even looks like flexing but that is my point - take one very large swamper at 2-4 psi and let it bag over an obstacle, huge contact patch but low contact pressure per unit area. Given weight of the vehicle is constant, a skinnier tyre or one at a higher pressure would have a greater contact pressure per unit area. Of course total contact pressure between the 2 examples is the same, just applied differently but this is the crux of my question high contact pressure over a small area Vs light contact over a large area? This would vary between rubber compounds as well, with different tyres letting go and losing traction at different points. i'm also assuming that the additional weight of larger tyres is not enough to increase traction. Difficult to prove unless you swap a heap of tyres onto the one vehicle to test the theory.

With your US example though Shane, do they need extra horsepower as vehicles become lighter so as to compensate for lack of traction at the tyre?
No right answer, just interesting discussion.

[Jeff]

Yes we've all watched a stocker drive over some tricky stuff without pause, but what about the times when said stocker has gotten jammed, you've let his tyres down & then he goes straight over?

More rubber on the rock equals more traction. At what point does it stop being help & start being hinderence?

My god I'm tired, I'm typing like yoda!

[Jimmyb]

well i have 31" ats, yeha i know small but consider the money that needs to be spent to get a xj in the air. anyways last weekend, rock crawling as you may have read i went for a sideways slide off a rock with my tyres full of mud into another rock and bend the crossmember. problem in this situation was? well the at's didnt shed the mud from the lugs at all so i may as well have been running slicks. i was only down @ 20psi, lower would have helped yes!

2nd scenario. watched a modifed zuk running 31" swampers over another rock crawl section. now he reckoned that they were as tall as a standard bfg 33". to be debated. anyways he wasnt down very low at all on pressure, maybe only 15 and they were new, so they werent bagging out at all, now with a vehicle of that weight, i would myself be running as low as i could, you wont spin the tyre on the rim running stock gearing and a 1.3 ltr engine, you might slide a tyre off sideways but hell, you dont have the weight there, so basically he was howling the tyres over the rocks, not getting good contact and after he got them wet, it got even worse.

[Wooders]

WAY WAY WAY too much pressure in scenario #2...