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suspensions not level

suspensions not level

(OP)
Hello, I am an aeronautical engineer by trade but I have an issue on a subject that goes beyond my field of expertise, and therefore I’m here to ask help to the experts!

I have recently purchased an off-road vehicle (Australian Ford Everest - right hand drive) and shortly after decided to install a 50mm lift suspension kit. Prior to installation, I have measured the height of the wheels to the top of the wheel arches and measured that the left side was 15 mm lower than the right at both front and left wheels. After the installation of the new springs and shocks the difference was unchanged, so I deduced that the cause was not related to the springs having asymmetric stiffness/length. I have had the car weighed with 4 load cells at each wheel and the difference between wheels was minimal, 15 kg heavier on the left at the front and 3 kg heavier on the right at the back, so it seems to be reasonably balanced. The fuel tank is on the left, but empty or full tank seems to make hardly any difference on the height. I have disconnected the swaybar links at all 4 corners and it made no difference either; I was surprised to find out that there was no load at all on any of the links, as if the swaybars were set up by design considering the lean on the left………

I have also measured that there is a difference in the distance between the lower wishbones to the bump stops to make sure that the height difference was not caused by the body not sitting level on the ladder frame.

My main concerns are that the right front wheel, being higher, is actually above the height increase legally allowed in Australia without requiring re-certification, and also the CV joints on that same side might be constantly working at angles beyond acceptable values for safe constant operations and could wear much sooner than expected.

I now cannot think of any possible explanation other than the frame having been built asymmetrically. I really would like to understand what the possible causes of the lean are to work out the best solution to level the car.

Thanks in anticipation!

RE: suspensions not level

I recently had a similar problem where one side of my car would dip and grind whenever it would accelerate. The shocks and rear wheel bearings had to be replaced. My suggestion might be that your whole wheel assemblies need to be looked at.

RE: suspensions not level

(OP)
It is a brand new car and, as stated, all the shocks have been replaced with the lift kit.

RE: suspensions not level

If it's brand new then why are you worried about it? I stated that you should check the wheel assemblies.

RE: suspensions not level

plazma's advice is bizarre in context.

First up, the tolerance on fender gaps is at least +/-10mm. On a body on frame car, probably more. 15 kg in corner weights is nothing. Notice that you have a warp weighting problem of 9 kg. That is actually pretty good. If we guess that the wheelrates are 40N/mm, or 4 kg/mm, you can see that the different wheel loads is not enough to explain much of the difference in fender gaps.

"I have also measured that there is a difference in the distance between the lower wishbones to the bump stops " is unclear to me, are you saying you found the same difference as the difference in fender gap at each wheel? That's odd, the motion ratio of the front bump stop is very small, and I can't even remember where the rear bump stop is, I thought it was in the shock.

One old trick is to release each bolt in the suspension arms slightly while the car is sat on the ground (and then retighten). This releases the torsional preload in the bushings. Hacks often tighten the bolts while the vehicle is on a 2 post hoist, leading to all sorts of funny conditions when the car is back on the ground.

I've attached a file which simplifies these sorts of conversations




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: suspensions not level

(OP)
Hi Greg, thanks for your post.

The new springs are 12kgf/mm at the front with a ratio of approximately 0.58 to the wheels, and the rear ones are 6-8 off/mm progressive.

The difference in gap at the bump stops is probably less than 10 mm, it'a difficult to measure accurately because of the position and both surfaces not being flat. By eye I would say that the ratio is around 0.5 to the wheel.

The guy that installed the lift kit tightened the suspensions and swaybars bushings with the car on its wheels in front of my eyes, after sending me for a short drive to make the springs settle.

RE: suspensions not level

"The guy that installed the lift kit tightened the suspensions and swaybars bushings with the car on its wheels in front of my eyes, after sending me for a short drive to make the springs settle."

good

I know measuring the bump stop clearance at the front is difficult, from memory the motion ratio is only 0.3 and access is awful. There's a whole list of things that cause ride height variation, I don't think you can do much about them. If you are otherwise happy with the car just put spacers on the spring seats or grind a bit off the ends.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: suspensions not level

I would guess that adding a driver would reduce the L/R height difference by 5 or 10 mm (maybe less with the stiffer springs).

je suis charlie

RE: suspensions not level

(OP)
Thanks guys.

Increasing/reducing the length of the springs by means of spacers was the obvious solution but I wanted to make sure that it was also the best solution, and I was not going to put a plaster on a completely different issue. Luckily both front coils have a 10 mm spacer at the moment so I can reduce the right front strut length by 10mm fairly easily. Greg, you seem very familiar with my car suspensions, do you happen to know what the exact motion ratio of the front struts is, so I can calculate accurately the correct change in spacer thickness required to achieve the desired height at the wheel?

The lift kit installer suggested that to correct the problem I could just shorten the front right strut, and a different suspension garage told me instead that they can correct it by installing a spacer on the rear left wheel coil spring, but in my opinion this would increase considerably the warp (I’ve learned a new technical term! smile ), which doesn’t sound like a good idea, am I right? Therefore I am more incline towards having both things done at the same time to keep the weight distribution uniform. What is an acceptable warp in kg’s?

The other potential issue I can foresee is that there was no load on the swaybars links with the car leaning on one side, so I am wondering if when the height is adjusted to be level, the swaybars will have some preload on them in the level attitude; or is 15 mm (7.5 on each side) within the play of the linkages and the swaybars link ends will fit easily into their holes without preload?

Out of engineer's curiosity: what are all other things that cause ride height variation, as you mention? The only other causes for the asymmetry i can think of are build tolerances in the position of the suspensions mounts welded to the frame, or on the functional dimensions of the linkage arms, and hence, as you suggested, there is nothing I can do about it........

Gruntguru, I did measure the difference with a 95kg driver onboard and surprisingly was just a couple of mm smaller than with an empty car. Same with a full/empty 80 litre tank of fuel. I was expecting much greater change but that’s what I measured!

RE: suspensions not level

MR for the front spring is 57%

9 kg of warp is no big deal for a passenger car.

Causes of ride height variation are obviously a whole bunch of tolerance stack ups dimensionally, shock absorber gas preload, spring length/rate variation, assembly issues, bushing preloads and probably several more. Then you've got cg in plan view.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: suspensions not level

(OP)
My main concern is if I have to work on both front and rear to level the car and keep the weight warp low at the same time, or can I just work on the front struts collars (i.e. removing 5 or 10 mm collar at the front right without affecting too much the weight distribution?

RE: suspensions not level

(OP)
What would be a maximum acceptable level of warp?

Also, I am not sure I understand why I cannot have both: if I put spacers at both right wheels it will level the car and also keep the weight distribution balance, would't it?

The issue is that the mechanic is not sure if the seat of the rear coil springs is deep enough to take safely a spacer, so I might have to work on the front struts only, hence the question about how much warp I can introduce before having to worry about it.

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