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-   -   An evaluative tyre setup for a MM540 DP (https://www.team-bhp.com/forum/4x4-vehicles/47291-evaluative-tyre-setup-mm540-dp.html)

Hi guys,

My used '89 MM540 DP 4x4 came with some junk roadside tyres. The first step was to change the tyres as the footprint was important to me.


Got a set of 7.50 x 16 NDMS for my jeep. As I've heard from fellow jeepers that the 7.50 x 16 is great in any environment for the jeep.


Then started using the jeep for my factory use, which included a weekly run of almost 300 kms on pure tarmac road. The road presence of a jeep is superb with everyone and anyone giving way on the road. A load of 2 barrels filled with Diesel / oil made the jeep suspension soak up bumps like a car.

The major drawback came in from braking. Though it would not do speeds in excess of 60 kmph [self imposed limit], when an idiot runs across the road, it becomes imperitive to brake hard and the jeep / tyres fail to do justice to that situation.

Hence


This is my jeep with my used 215/75 R15 Pirelli Scorpion tyres on gypsy rims. This was just an experimental setup to explore the braking and handling of the jeep especially on tarmac.

Observations of the new avatar on my jeep. Take off and accelaration were better as the effective OD was smaller than the pervious setup. In gear accelaration was also brisk and made the jeep enjoyable to drive on the road.

The downside came in the form of a very hard steering. [These tyres were also excessively worn out and not uniform]

But braking had improved considerably and surprisingly the body roll increased too with broader tyres.

Drove the jeep in this guise for 2 weeks before changing back to NDMS setup.

Just thought of sharing this information with TBHP!

I did something similar and got the same body role effect.
It was not because the tyres were broader, it was because these particular tyres were taller and the sidewalls were not strong enough for the weight of the vehicle. This allowed a lot of role on the sidewalls. Subsequently went with Michelins, Goodrich and Nitto tyres and put a little more air in them. The ride dramatically flattened out and it handled beautifully. Sidewalls were my problem.

Dan, by switching from 7.5x16 to 215/75R15, his OD went from 31 inches to 27.8 inches. Can't be the affect of taller tyres. Actually I want switch from my current 235/75R15 to 7.5x16, so this info is both good and bad.

I was thinking body roll will increase with 7.5x16, so this is good news.

Right now I am happy with the braking performance, don't want to screw it up. So this is also bad news.

In my case the problem was not the height of the tyre it was the weakness, non-rigidity, of the sidewall. Changed to other brands, same size, and the problem went away. Over flexing sidewalls feel just like body roll.

I guess that explains the lousy handling I have with my Jeep. I'll overinflate it a little and see whether it improves.

What are the AT tyres that have tough/rigid sidewalls, that is an useful thing in off-roading. One of the GV tyres has sidewall damage after off-roading.

I have owned seven 4x4 vehicles (one at a time, I am not a tycoon) and driven several others. They ALL were a little quirky in the handling department for the usual reasons. A couple were downright dangerous until you made friends with them and got used to their limits. I did a quick flick move once in a Ford F150 4x4 at 85 kph to avoid a big crate of veg in the road. It was 100 meters before that acursed weapon of mass destruction stopped swaying and counter compressing on its suspension enough to let me think I might live thru its gyrations. Funny how time stands still at moments like that. I can still remember the expressions of sheer horror on the faces of people in oncoming cars as I careened past them. Tyres never did not eliminate all my handling problems but I like Michelin. They are rounder right from the factory, more in balance. BFGoodrich has good sidewalls and may not be a great tyre but they are not so bad, either, IMHO. I'd still buy them.

Interesting experiment, headers :)
Thanks for sharing it !

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I guess this would be due to the offset being different in the jeep and Gypsy wheels. Though the mounting PCD is the same, the offsets are different. From a visual comparison, I feel the Gypsy has a more positive offset compared to the jeeps. Hence the harder steering.

As DirtyDan indicated, this could be due to the softer sidewalls in the Pirelli tyres; also being 75 aspect ratio and softer radials, they would tend to flex a lot (thus leading to body roll), compared to the NDMS.

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BT, i guess the offsets do NOT change with a change of wheels. But the broader tyres as well as the sidewall effect is the reason, I'd think. Also, if the gypsies have a more positive offset, then the steering will become harder. One of the reasons IMO for the good steering [within limits] of a jeep is the negative camber associated with it!

@Samurai, good that i had good news for you? Howz your overheating issue sorted? I'm all ears for it.

@DD: you seem to have done a lot of antiques on road - A little OT: I've had a similar experience on road in my Tata 207.

Picked up a brand new vehicle in 2005 and was running it in and hunting for a driver. The usual 120 kms journey to my factory was uneventful until i realised the disadvantages of a goods carrier vehicle and why so many accidents happen on road.

While doing a sedate 80kph on no load condition, there came across a dog that ran right into my path - A sudden change of direction as well as a stomp on the brake had me going sideways for around 5 odd meters and then came the correction with the PS. The vehicle took a little over 20 odd metres to straighten itself. It was just snaking its way and corrections had to be done until the vehicle regained composure. It seemed like a left, right, left right flick until eternity. Thankfully, the speed too dropped to around 50kph and the vehicle was not loaded, else there might have been an amplification to the issue and god knows whether it would have been controllable.

The 207 i realised is not to be driven that that speed after all:Frustrati

Hi Vikram,

AFAIK there are no 7.50X16 NDMS/NDCC tyres available in INDIA.

What you have is 7.00X16 JKTyre "Super Traction" i.e NDMS.

Regards,

Arka
PS - (+/- 1 inch)
6.00X16 = 28"
7.00X15 = 29"
7.00X16 = 30"
7.50X16 = 31"

headers, "offset" is directly dependent on the construction of the wheel rims : this can definitely change with a change of wheel rims. When we use Gypsy rims (which have a positive offset) in a MM540 where the steering has been designed for negative offset rims, the steering will become harder.

.

Thanks EX670C, you seem to know my jeep better than I do :)

BT, I beg to disagree on your POV. Can you explain a little more in detail. Yes we are shifting the centres, but that need not necessarily increase the effort if the toe-in is adjusted correctly!

headers, I was just stating some technical facts there : nothing to do with my POV :).

I am not too much of an expert in these things, but from what I know : the effects of substantial wheel offset changes cannot be offset ( :) ) by changes in "toe".

When we replace the MM540 wheel rims with those of a Gypsy, we are in fact increasing the front track width.

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Hi Headers, I think BlueThunder is right.

No expert here myself, but if I understand things right - the king-pin angle (caster ?), along with the offset, causes the tire to be lowered (i.e. the vehicle is raised) as the wheel's turned resulting in hardness of steering - toe-in would not be able to compensate for large changes in offset.

O.T: The pics of your jeep, were they taken at the Eldam's-Road-TTK-Road signal, underneath the mini-flyover, near the Home-Style building ? :)

Guys, from what I've read - With negative camber (the usual case in most vehicles) the steering effect is lightened but traction is at a compromise, whereas with positive camber that reverse is true. There are only a few sports cars in the world that offer positive camber as OE. All F1 cars have positive camber.

Also there is substantial increase in tyre wear with positive camber. With Toe-out, the steering will feel vague and result in a drag to either side while driving.

I will take a closer look at the gypsy rims and the 540 rims to appreciate the differences in offset. But they will should hardly matter in making the steering heavy. A good wheel alignment with a proper adjustment / corrections of the Camber, toe-in will result in a good steering. (I did not do it as it was just an evaluation, but one can have broad tyres and a great steering with suspension adjustments) in a MM540

BTW, its nice to have you guys coming out with different ideas, it makes us understand our "so called" mods better!

Here's the thing about rim offset.
Positive offset would "narrow down" your track.
Negative offset would "widen" your track.
It is NOT the other way round.