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Issues with Setting Ride Height on 350Z with BC Racing DS Coilovers

Motorsport Wheel Alignment Fundamentals

Relevant Module: Practical Skills > Adjusting Ride Height Using Coil Over Suspension

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Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

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Hi All,

I’m having issues setting the ride height on my 350Z with BC racing DS coilovers, these are the rear true coilover conversion, so its integrated spring rather than the original separated design.

I have also replaced mostly all suspension arms to fully adjustable, all new bushes and ball joints etc.

So Iv watch both suspension and alignment courses. This is my first time doing this in-depth suspension setup.

BC manual says to adjust the ride height from the bottom mount, contrary from what Iv learnt in the HPA courses being that this should be adjusted at the spring perch.

But here’s what I’m not understanding. If I need to lower the spring perch, I will not have any preload in the spring (2-3mm from BC) and it won’t be captive during full droop / jacked up. Obviously don’t want this situation, and since I’m setting at stock ride height, there shouldn’t be any concerns with wheels making contact with fender guard. Hence Iv still opted to adjust at the lower mount.

With that, Iv set all coilovers to the same heights left and right. Issue I’m having is, I’m seeing a 14mm difference in ride height between left and right on the front. Only a 4mm difference in the rear left to right.

Iv checked and double checked the coilovers are exactly the same length, checked that nothing is obviously bent or out of place. The front lower arms are still the stock arms.

Moving on, I can adjust the bottom mounts to get the car level and at the stock ride height. But once the car is jacked up and full droop, there is obviously a large difference that one wheel hangs lower due to that 14mm difference adjusted from the lower mount. And I’m not really liking it lol

Just to add, I also checked with other 350Z owners that have installed these same coilovers, they had also reported seeing similar differences in left and right with equal setting of lower mounts and adjusted level from lower mounts.

Any quick feedback here is greatly appreciated, as I’m right in the middle of this now. Is that difference in jack up droop normal and I’m just being anal, or something else at play here.

When you are measuring the height difference are the anti-roll bars disconnected?

If the gas pressure is adjustable / measurable on the dampers -- are they the same from side to side? What about tire pressures?

Double check that all springs are seated correctly, especially since you indicated they were loose in full-droop.

Do you have a driver equivalent weight in the driver's seat?

Yeah, front right side ARB end link was disconnected throughout the adjustments. ARBs are still stock with new bushings and adjustable end links set to stock length.

Can’t measure the pressure of the dampers, but they are all set to the 1st lowest setting for valving. All tyre pressures set prior to any measuring and adjusting.

I set the spring perch back to the original 2-3mm preload position when received out of box. I had marked the exact location so this is back to where it was and spring is properly seated. At the moment I’m basically back to the beginning and the heights are set equal again from side to side.

Don’t have any weight in the driver seat or car at all, as I just trying to set this up to stock settings for now.

Interestingly enough, before I did all the suspension upgrades and new coilovers - I had taken the original ride height measurements. This showed that the front right was siting 13mm lower compared to the left. Reckoned this was caused by worn and blown out shocks, coils, bushes etc. However, since replacing everything I am still seeing the same 13mm lower on the front right side when all equally adjusted. Should maybe add here that the car is RHD.

Have also noted another issue with my wheelbase measurements. Stock measurements should be 2650mm. And I’m measuring right side of 2658mm and left 2655mm. I had replaced the rear traction arms with a GKtech adjustable arms. Although I measured and set these to the stock arm lengths, suspecting this is causing the difference. Do I just need to lengthen/shorten these to get them equal and back to stock wheelbase? Didn’t replace any other arms that would affect wheelbase.

Thanks!

First, as David said, you must, MUST, MUST(!) disconnect at least one link of each anti-roll bar (ARB, AKA "swaybar") to prevent across-axle interference.

Second, the correct adjustment on these is a bit controversial...

The traditional damper has a single threaded adjustment, so one may need several lengths of springs and/or "anti-rattle" (AKA, clearance, take-up, or otherwise termed light spring) to get the correct spring rate, ride height, and suspension travel. If the spring is the wrong length, and/or the wrong rate, you may find you need too much pre-load or a helper sping to prevent the spring unseating. There are other options, but normal practice is just to adjust the spring seat for the required ride height.

You don't have that type, yours has two separate threaded adjusters - the intent is one is used to adjust for the spring, and the other, lower one, is used to adjust the ride height. The correct procedure, according to the various manufacturers, and I agree with, is with the damper assemblyextended, the SPRING seat is adjusted to just take up the clearance between the spring and the seat, then turned a set number of turns, or distance, to hold the spring in place with minimum pre-load. This gives the maximum spring travel, and works with a range of spring lengths. THEN the ride height should be set by using the lower threaded adjustment, which is screwed into the bottom part which is usually either a stub axle housing or a damper mounting point -

The controversy is that some believe the ride height should be adjusted on the latter type using the spring seat adjustment - this is not what the manufacturers, nor I, recommend, because it will limit the droop available for the suspension. HOWEVER, there are times when limiting droop is an advantage (eg. aero'), and if you're SURE, or want to experiment, try it.

Excellent point, David, I hadn't thought of the gas pressure, but that can be significant. Tyre pressures and tyre diameter can make a big difference, especially on a stiffly sprung car. It's also the vehicle is checked as run, not just driver weight, but fuel (usually a half tank is used) and other fluids, etc, in place don't forget a driver's 'ice box'or whatever, if used.

I would expect the un-even droop to be down to incorrectly adjusted ride-height adjustment and/or ARB pre-loading from a twist in it. Some vehicles have bump stops/limiters on droop, usually under the top wishbone if used, that may be a little bit different in settings.

Hi Gord,

Thanks for also replying. I had replied to David’s questions above for more info.

When the lower mounts are adjusted equal, there is no issue with the droop, both side are equal when jacked up. But once the car is lowered onto the ground, the front left sits 14mm higher.

Then, If I adjust the lower mounts and level out the car - that’s when I’m seeing the big difference in droop once car is jacked back up.

Just an update for anyone interested:

Front of the car has been levelled with adjusting the bottom mounts, but the coilovers now have a 18mm difference left to right.

like I said, car is level now and that difference is only noticeable while jacked up and can see the droop. Don’t know how I feel about it, just seems weird to me…

And to get the ARB end link reinstalled, I had to jack up the drooping wheel to the same height as the other to get it to line up. SMH This has turned into a PITA!

Hmmm, do you know the history of the vehicle - it may have had a crash, or been jumped heavily, and the chassis and it's mounting points may be tweaked, throwing everything out a little. I'd suggest checking the ground to chassis/suspension hard points and doing some checking of diagonals, to check for twist.

I haven't worked on them, and these are generic suggestions, mind. Oh, you may find a read of this of use - https://my350z.com/forum/brakes-and-suspension/482664-suspension-101-a.html - or you may not?

Hey Gord,

I don’t know the history, had it for about a year, hardly driven in my ownership. But it’s pretty immaculate underneath. No visible signs of any damage or that any nuts/bolts etc have been disturbed. It’s real clean.

Starting to think something is indeed bent. The fact that this difference in front ride height left to right was apparent with the stock suspension, makes me believe this was a pre-existing issue since I still have the same problem now. I just had the front end completely disassembled for the bushing replacements and didn’t notice anything obviously bent/damaged.

However, what I have found is a crack in one of the front wheels. And what definitely appears to be a wheel curb rash repair.

So to crack a wheel, it must have hit a curb pretty hard, which is again indicating something could be bent.

The front stock lower control arms are forged aluminium, I’m hoping these are bent now.

I just replaced the FUCA’s with some adjustable ones. Need to have a closer look at the spindles.

I guess I need to pull it all apart and check everything again. Any tips on how to check for bent arms and chassis mounting points? Like I said, there is nothing obvious.

I'll get back to you, but there are some guides on-line - looks for things like "squaring a chassis", "Checking chassis alignment" - or substitute "frame" for "chassis". It should bring up a number of videos and guides.

Slowly been going back and forth with checking this.

I reckon something is bent on the front left corner. Been measuring everything I could, and still can’t put a finger on what is bent.

Chassis and frame to ground, while car is on the floor, is all true.

Measured the LCA and compression arm and nothing appears bent.

Haven’t really been able to measure the knuckle, but wheel to bottom of the ball joint, and wheel to the top of the upper arm mount is the same both sides.

Upper arms are new and adjustable. But during install of these, I noted that the front left was much more difficult to install in the frame. It was a much tighter squeeze compared to the right. There is also a creaking sound coming from this location while bouncing the front suspension.

From measurements, the front left wheel is sitting 5mm further back than the front right.

This is also the same corner wheel which is cracked from, a guess of, an impact.

A couple other tell tail signs that are pointing to the same corner - there is a rub through on the inner arch where the wheel obviously had contacted it while on full lock at some point in the past.

Another one is that while jacking up only the front from the cradle - the front left corner will always lift before the right. Even when both coilovers are set equal.

I reckon FL corner has been push backwards and upwards. But which component(s) that are bent I’m still unaware.

FML, this has turned into a disaster…

That's a bummer, but the good news is if it is the suspension component(s) it's a LOT easier and cheaper to remedy than the chassis/hard points.

The bad news is it's a rather complex suspension design, from I've been able to find out. Normally it's be a relatively simple job of comparing the sides to each other, carefully measuring them as opposites.

Back in the day, it was a reasonably easy job to check the KPI - King Pin Inclination - when the caster and camber were set. If something was bent, it'd show up as a difference, but not sure if that would apply here - I would expect so but...

If you look carefully, you may find evidence of fine cracks, but in the end, might be a case of looking for a s/hand corner from a known-good write-off. that may be a good idea, anyway, as you don't know if there's a crack slowly getting worse and ready to fail.

On that, have a very careful check of all the chassis hard-points for cracks or distortion, that might be the result of an impact - better to fix anything now than have it fail later.

Forgot, besides general guides for checking, you may be able to find some OEM measurement points, that are used for checking repaired vehicles for straightness. You may find more success on some of the 350Z specific forums, and I expect there are a couple of clubs, too?

Got the car back together and roughly aligned before going to the alignment shop.

This was the result of my own DIY alignment. Must say I was pretty pleased with the result for a first try. Alignment tech said it was good as is, and no further adjustments have been made.

But, I’m a bit worried of that 2deg/min difference on the KPI.

Should I be concerned about this difference? Is it confirming there is still the possibility of a bent component?

If so, I’m assuming that focus should be on the front left as the maximum OEM spec is 5.5deg/min. Bear in mind I have more positive caster compared to OEM spec.

All comments and help welcomed and greatly appreciated. Thanks!

Attached Files

I'd say there's definitely something bent there - the caster and KPI are measured between two points, as I think I said earlier.

These points are normally the top mount of the strut, or stub axle, and the bottom joint in the wishbone, or track control arm, the strut, or stub axle, is attached to.

That vehicle uses a virtual bottom joint, but the principle is the same - and any variation in the values means one, or both, points it's passing through are in the wrong place, ie bent somewhere.

I forgot to mention it before, but while it's most likely the lower component(s) took the damage, it's possible they were replaced and the problen lies with the chassis mounting point at the strut's top.

Have a careful look over it for any sign of crinkling, cracking, or other irregularites - both at the top and at the bottom.

Hey Gord,

One last thing to bug you with.

I contacted Z1 (manufactures of the FUCA’s) they are saying I need to adjust the caster the same for both sides. Which obviously makes sense. Although, I previously refrained from adjusting the chassis side heim joint mounts as their instructions stated to keep these equal. To adjust caster, these would need to be set unequal.

However, I’m presuming that instruction to keep equal - is for their rubber mount variant, as to not twist them. I don’t believe this applies to the heim joint variant that I have.

Anyway, plan is to dial in the caster adjusting the FUCA’s. Just to recap, please see attached the current alignment specs.

As you can see, caster currently has a 0.78deg/min (1.3deg decimal) difference left to right.

My question is, once caster is adjusted the same - would you expect to see the KPI/SAI track closer together? Rather than the 2.34deg/min I’m seeing now.

Hoping caster is the issue here, or am I clutching at straws here…

Thanks

Attached Files

As I said, the vehicle uses a complex linkage, which I'm not familiar with, so I'd have to say 'try it and see' if the KPI is improved, as the caster should be the same both sides - with uneven amounts the vehicle will tend to pull one way or the other. Awhile there are times when unven castor is useful, such as on ovals and heavily cambered roads, normal practice is to keep it even side to side, just as with all the other geometry.

I suspect not, but ...

Did you check the chassis points, including the top mounts are square?

A little surprised, disappointed even, that some of the chaps who have worked on these hasn't chipped in - we would both learn from their input.

Hey Gord,

Yeah Iv since checked everything is square. I found a body repair manual for this car, see attached pic. Have measured everything I could, with the engine in the way, and everything checks out square.

With this, Iv also found out that my ride height is actually perfectly level while measuring from the chassis to the ground. The way I measured it before was from ground to the fender lip as instructed by HPA courses. Turns out my front right fender has been in a bender and was throwing ride height measurements off.

Would strongly suggest HPA include that ride heights should be measured and verified from the chassis to ground rather than the fenders.

So that’s no longer a concern for now, I’ll pull the fenders off at some point and inspect, probably throw on some carbon fibre ones while I’m at it.

I’m also disappointed nobody else has replied, as I even know the 350Z was a HPA shop car. So I’m sure these guys would be very knowledgeable on this…

Attached Files

WOW! - I never would have guessed that you were measuring the chassis ride height using bodywork. For me (prototype sport racer bodywork) -- body work is just rules compliance, I would NEVER consider that to be fixed (in an accurate manor) relative to the chassis.

Yeah exactly, totally makes sense. Don’t know why I didn’t figure that out sooner…

The HPA course info and instructions should really be changed to disregard the fender/body measurements for ride height.

In my case, while the fender had a prang throwing my ride height measurements off, Iv been running around checking for something bent - when it’s been the fender the whole time…

Just need to sort out this front caster difference and hope the KPI tracks the same.

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