Suspension-tuning the Jeep Grand Cherokee ZJ

When I first came over to Chrysler in 1989, it was specifically to take charge of ride, handling and suspension development for the ZJ (the original Jeep Grand Cherokee). I took a tuning direction totally different from where it had been going, bringing the European style of ride and handling — with smaller stabilizer bars (sway bars, or anti-roll bars, as we call them in Europe), and the dampers (shock absorbers) tuned to control the roll and body altitude of the vehicle.

1994 Jeep Grand Cherokee

When I first came over, it was in October. We went down on a tuning trip to Chrysler Arizona Proving Grounds (APG). I was just getting my feet under me. I’d just had a child; I’d just changed countries; I’d just changed companies, and I was in the middle of a Michigan winter which I wasn’t used to. And in February, we went down to Arizona.

John McNeely started at Chrysler in 1978 as an engineering supervisor, and retired in 2007 in that position (moving from the Proving Grounds to Jeep Vehicle Dynamics). Today, he is a Battalion Chief at a Tennessee fire department.

Engineer and successful racer Dennis Moothart died at the age of 57 in 2006; he was then Chief Executive Engineer for Chassis Engineering for Chrysler.

Northwestern graduate Harris Grether was a Delco engineer for 46 years, passing away in California in 2012.

The guy in charge of the development of Jeeps at the time was Dennis (“Denny”) Moothart. John McNeely was the workshop supervisor, who, unbeknownst to me, had been promised the job by Denny Moothart. I just walked in there oblivious to that fact and only found out about it some years later. “This is a new job; I’m going to start doing it,” I thought.  John made it very difficult for me and I didn’t know why. I didn’t know he was being difficult. I just thought this was the way things worked. I can now see why that was and can somewhat understand it now.

Anyway, we went down to Arizona at the end of February. The shock absorbers had been tuned with elastomer foam/rubber shock absorber mounts, which gave no force buildup but were very good for isolation, which was a German trait of tuning, but not the way you want to tune vehicles, based on my experience at Lotus.

We were down there for two weeks, and we had a Delco shock engineer and  representative down there, a very old suspension and dynamic tuners/shock absorber guy named Harris Grether. A lovely man, a real nice guy. He was very old, and he was used to the Buick way of tuning vehicles with open bleed settings, the car just wallowing over the road on its damping, and controlling the roll on the stabilizer bar.

jeep Grand Cherokee 1995

I could not get him to put high force figures into the dampers at low speed. I couldn’t get him to put what we call zero bleeds or quarter bleeds, and I was even making these components to go into the shock absorbers in their ride trailer at APG.

John McNeely, unbeknownst to me, was filming this to take back up to his boss, Denny Moothart, to say, “Hey, this Sharp guy you’ve employed is a complete wanker.” I suspected something was up, but I felt it was unusual to do filming like that, especially as Jeep was sensitive of having any record of suspension or handling work which the lawyers could have pounced on in any litigation. Some of the technicians were giving me subtle hints, which I only realized years later. I remember going back to my hotel room and literally sweating on how I could get the team, both Jeep and Delco, to do what I asked.

We were particularly tuning the Consumers Union lane-change maneuver, and I couldn’t get this thing to go through there properly. It was flopping all over the place, and I just couldn’t get Harris to put the damping settings I wanted in there. It was a big handful even at about 55 miles an hour.

orvis editionI didn’t know why John was filming all this, but he was. So on the last day of tuning I said to Harris, “Right, that’s it, Harris. I’m just going to insist that you’re going to put this zero bleed in there.” He was a lovely older gentleman and I did not want to upset him, which is usually not like me as I can be a bit of a dick at times when it comes to getting the tuning right.

He said, “It won’t ride; it’ll be horrible. Nobody will like it.”

I said, “Just do it,” and he did it. I went through the Consumers Union lane change, and I think I went to about 60 mph perfectly controllable the first time, then I went through at 65, then I went through at 70, then I went through at 75. Big change to the balance, handling and confidence feel of the vehicle. Not knocking down cones, but totally controllable. I could’ve gone even faster. I could’ve gone probably 90. (well probably a bit of a stretch, but I felt I could give it a try and not end up on the roof).

I think it opened a lot of the US guys’ eyes up to the tuning potential that the shock absorbers have, which is not a characteristic that American style tuning generally used. There’s a reason for that, because of pothole noise and poor road service conditions, while generally in Europe you have better road service conditions.

The 1993 Grand Cherokee was best in class (including Ford Explorer, Chevy Blazer, Nissan Pathfinder, and Toyota 4Runner) in acceleration, braking, wet braking, and cornering.

John’s jaw dropped. He was very upset. I could’ve actually gone through a lot quicker, and it would’ve been dangerous, but  [at these speeds] the car was planted on the road and was perfectly safe.

We came home and brought the test vehicles back up to Denny Moothart, who to be fair was open minded, and he drove it and loved it. He couldn’t understand why there weren’t huge stabilizer bars on it though. I think Denny had a 33 millimeter front stabilizer bar and I took it down to 26 or 25mm. I think we ended up at 26mm, which is a huge difference in the cross-sectional area on the stiffness.

That was my first experience of coming to the States, not realizing I was being set up. So there I thought, “Aha, that’s interesting.” When I eventually discovered what went on, by one of the workshop guys who told me some years later, I was more than a little pissed.

bob lutzThe car was shown to the senior management. They all loved it, too. I think Bob Lutz came out with a statement, saying this is the first time he’s felt into an American vehicle with European tuning.

So that was sort of quite a feather in my cap, I think. That was quite good. … I’m going to be rather bombastic and boastful here, but we tuned the pig’s ear into a silk purse. It had a Haltenberger steering linkage, one of the only things you could do with a live axle, and not the best steering arrangement by any means.

Live axles are just not the way to go, but we did the best we could and we actually did make a pretty good vehicle out of it. It was softly sprung. And it did roll more. It was purposefully meant to roll more, which Denny Moothart had to gradually grudgingly accept. He wanted like a dead flat car, and he couldn’t do that and have acceptable off-road handling, but also on freeways when you’re on a freeway turn ramp and there’s a bump in it, there was no compliance and the car just shot all over the road.

When I first came over and drove the XJ (original Cherokee), I went around an off-road ramp, hit a bump in it or a pothole and I nearly shot through the armco barrier because it lost grip from the tires straight away, because it overloaded the contact patch with its heavy stabilizer bar. I thought, “Oh, this is not good.” Or some more blunt Anglo Saxon expletives.

1989 jeep technology

Wear or improper tuning in the Cherokee suspension can cause extreme shimmy or “the death wobble.” See details.

That’s why I brought in the softer compliance in roll suspension tuning. The XJ, Cherokee, was very flat, so when it went in to roll, it built up corner force very quickly. But all the stored energy in the spring and particularly the stabilizer bar, as soon as you released that corner energy, it shot the body back . . . it doubled the roll-in rate, meaning as you roll into the corner, and then subsequently  roll out of the corner once start taking the steering off thus releasing that stored energy in the springs and roll bars, the roll rate into and back out of the cornering maneuver should be identical or as close as can be acheived. It was about twice as quick releasing that energy, which is very disturbing for a driver.

quadra-link suspension

By tuning the low-speed and especially the rebound side of the dampers, we were able to control the body roll in and the body roll out of the cornering maneuver to be symmetrical in its rate of roll. That’s what really improved the handling, because it was then a known quantity to the customer and the driver and the client.

Francois CastaingFrom that I got a call from Craig Winn about a month or two later to go meet and have lunch with Francois Castaing. I went into a little of a diatribe into suspension tuning and what’s wrong and how it should be done. He came back to me a little bit tongue-in-cheek, a little bit sort of challenging saying, “You’re telling me how to do suspension tuning and blah, blah?” Because he had been a Formula One guy like myself.

I also brought European-style silica tires, which were much better for wet grip, to the ZJ. I was focusing a lot more on wet grip for the Jeep. The car was fantastic. I’ve got to say for an SUV, it was the very first SUV to perform like this.

On the ZJ, I wanted to pick one tire to do all the tuning work on so I didn’t have to fuss with other tires to tune all the other mechanical components on . We really tuned one Goodyear tire. It was a small tire [P215/75R15], the smallest tire in the range of ZJ when we launched it but perceived to be the high volume tire, and it looked a little odd in the wheel house but surprisingly it was the fastest, it rode the best, it was the best handling tire for grip even than the bigger tires. It was just because it had a lot more work on it and matched the suspension better.

Then after a year of it being in production, Goodyear said, “We don’t like this tire. I think it was called the Invicta. It’s an old tire pattern/mold design. We’re going to change it.”

The car really, really liked that tire. And the car was never as good after it was put on other tires.

1999 Jeep Grand CherokeesIt’s funny how tires have a big effect. With a small tire, it allowed great articulation into the wheel house and it was great off-road as well and it was brilliant in the snow as well, which is not typically the case with a silica tire. But the tire tread design was complimentary to it. I’ve never driven a tire that was actually as good on a vehicle as that.

But I think it was a function of how much time was spent on it and tuning it, because we did a lot of work with the belt angles and the flipper design and the tire and all the various 20-odd components that go into a tire like tread design.

When the car landed in England for the first time with the same tires and the tuning I put on it, Land Rover were shocked. They fell over. The first day they had a chance to drive one, they got onto Goodyear and said we want those tires, which were the tires I’d co-developed which were a lot more wet grip. The Grand Cherokee was much, much quicker than a Land Rover on road in the wet English road conditions. It had a lot better manners, shall we say.

Coming up: Making Grand Cherokees; the V8 Option; and the European Grand Cherokee

Chrysler History and Other Bios • Jeep Grand Cherokee ZJJeep Cherokee XJ

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