Ride quality & spring rates with Fortune Auto coilovers

[IamFODI]

A post in another thread generated some solid discussion, so it's getting its own thread.

Started here:

Curious what happened next...
I'm unhappy with my ride quality and my gut is the front is oversprung (8k(448lbin) which is mild by some but my understanding is stock ZCP is 3k(167lbin)...but my issue could just as easily be improper pre-load, so I've got some investigation to do.

First response:

Can you tell whether the unpleasantness you're feeling is from bump stop hits? If it is, then the problem is a lack of travel, and lessening preload or swapping in a softer spring would make things worse. See if you can figure that out first. If you can't tell by the seat of your pants, ask Fortune Auto for specs on damper travel and bump stop size -- basically the kinds of numbers I posted in the post you responded to.

As an aside: Given how differently my car rides from stock, I can see how a front spring rate like yours might feel stiff without some fairly sophisticated damping. The Öhlins spring rates (which I'm running) are only ~20% stiffer than stock in the rear but ~100% stiffer up front, and that was enough to add some significant firmness in some situations. Your front rate is 30% higher than mine. You might want to feel out your options for swapping in a different set of dampers, if not a different coilover kit altogether. Or at least having your dampers maintained and revalved. FWIW.

The responses that followed are now posts 2-9 below.

[ultimatemj]

Thanks for the feedback.
I think you are right, the next step is to check the pre-load, as too little could have me riding the bumpstops early and too much would be "oversprung".
Do you know where I can find any ride height measurements (hub center fender opening)? Having bought the car with coilovers already installed, I'm curious about stockM3 & ZCP heights.
And I did submit a request form to Fortune-auto...

Cheers.

[IamFODI]

Thanks for the feedback.
I think you are right, the next step is to check the pre-load, as too little could have me riding the bumpstops early and too much would be "oversprung".

Too little preload definitely could make you well acquainted with your bump stops.

Too much won’t feel oversprung until the spring force is greater than your corner weight. You know you’re there if your suspension doesn’t compress at all when the car is on the ground. That’d be an insane amount of preload though.

Do you know where I can find any ride height measurements (hub center fender opening)? Having bought the car with coilovers already installed, I'm curious about stockM3 & ZCP heights.

From TIS:


“Normal position” means with all fluids, 68 kg on each front seat, 14 kg centered in the trunk, and a full fuel tank.

[ultimatemj]

Thanks for digging that up!

I reviewed the install this weekend and the preload looks correct (1/4" on a 7" spring)

But the ride height was "slammed" (when compared to BMW specs).

Lower Rim Flange to fender was 570mm. E92M3 ZCP (comp) spec is 600mm, so I raised the damper body 1"...probably going to lower it back down to 595mm.


Strangely, it did improve the ride quality some...I'm surprised because that should have had little to no impact on spring load and when bumpstop travel begins. Must have something to do with shifting weight reward.

The old school thing that I find odd is when trying to push down on the car it's really hard to move it...the spring force makes it hard to compress. Yet these are the suggested street spring rates, the race are 10k fronts.

Napkin math....
- 8k (448lb-in)/(25.4mm/in)=17.64lb/mm spring rate
- 1/4" preload = 112lb
- 0.96 spring motion ratio
- 116 mm total FA damper stroke travel
- 25 mm bump stop length, of which 20 mm is usable?
- Damper stroke until bumpstop 116-25 = 91mm
- 880 lbs sprung corner weight

880 [sprung corner weight]-112[preload] = 768lb resting load
768/ 17.64 [spring rate] / .96 [motion ratio] = 45.4mm suspension compression with car resting on the ground
91mm-45mm = 46mm damper travel until bumpstop
46*17.64 = 811.4 lbs load to compress damper to the bumpstop

I am curious how to compare the BMW ZCP struts, because from what I've read they use 167lb-in springs and rely on the bumpstop for a progressive rate...initially adding 100lb-in and ramping up to 700lb-in. So spring rate goes from 167 to 267 to 867. And I think the stock springs are also 8" length, if that matters.

The other thing I'm curious about is droop...jacking the car so both front wheels are in the air, I'm only getting ~2"...wondering if that is same as stock.

[IamFODI]

I've read that stock droop is ~100mm / 4".

[ultimatemj]

Hmm, that's interesting...the article also says "the front and the rear suspensions are compressed by 4 inches (100mm) at the wheel"

Your calculation above "880 [sprung corner weight] / 13.5 [spring rate] / .96 [motion ratio] - 5 [preload] = 62.9 mm suspension compression with car on the ground"
And my calculation "880 [sprung corner weight]-112[preload] = 768lb resting load. 768/ 17.64 [spring rate] / .96 [motion ratio] = 45.4mm suspension compression"

Stock 169lb-in/25.4 = 6.65 spring rate, and there seems to be around 35mm of preload (232.75lb)...hence the need for spring compressors to take apart.
880-232.75= 647.25 647.25 / 6.65 [spring rate] / .96 [motion ratio] = 101mm suspension compression.
The stock damper are also have 150mm of travel (versus 116 of my coilovers), which (only) leaves 50mm of travel upwards...which is what you have vs 70mm upward travel in my set up.

Unless I'm missing something, it seems I need to swap to a lower spring rate.

[IamFODI]

Hmm, that's interesting...the article also says "the front and the rear suspensions are compressed by 4 inches (100mm) at the wheel"

Yep. That's the same as droop, no?

Looks like I missed the "at the wheel" part when I read it. Derp. But it'd be similar at the damper.

Either way, stock has a ton more droop than either of our setups.

The stock damper are also have 150mm of travel (versus 116 of my coilovers), which (only) leaves 50mm of travel upwards...which is what you have vs 70mm upward travel in my set up.

Your setup seems to hew closer to the rule-of-thumb of 60/40 bump/droop travel for track setups. Mine is closer to the stock ratio, though still much more bump-biased.

[ultimatemj]

How is your ride quality? In particular on "rough roads"?

Napkin math (presuming 880lb corner driver weight) points me towards reducing my front springs from 8k to 6k, as I'd have 10% more bump travel than stock and just get to the other side of the bump/droop ratio.

But there are 3 factors that I can't get any specific information about...and I don't want to trade one problem for another.
1) gas charge - shock body volume and PSI are needed to plot the resulting force at a given stroke. This is adjustable on some dampers, not on most.
1a) if my dampers have "low pressure", then a 6k might not be enough spring as I get deeper into the travel
2) bumpstop characteristics - There is one chart out there for the stock bump stop where someone measured the force at a given compression and found it to be "bi-modal" (100lb-in for 25mm, transitioning to 400b-in, and 700lb-in after that). As well as "range of compression before effectively becoming solid".
3) damping sophistication - slow speed/short travel damping vs high speed/long travel damping.

All 3 of those can dramatically change the calculations/results, but I guess I'll start with "relativity".

Thoughts?

[IamFODI]

How is your ride quality? In particular on "rough roads"?

Ride quality

Napkin math (presuming 880lb corner driver weight) points me towards reducing my front springs from 8k to 6k, as I'd have 10% more bump travel than stock and just get to the other side of the bump/droop ratio.

But there are 3 factors that I can't get any specific information about...and I don't want to trade one problem for another.
1) gas charge - shock body volume and PSI are needed to plot the resulting force at a given stroke. This is adjustable on some dampers, not on most.
1a) if my dampers have "low pressure", then a 6k might not be enough spring as I get deeper into the travel
2) bumpstop characteristics - There is one chart out there for the stock bump stop where someone measured the force at a given compression and found it to be "bi-modal" (100lb-in for 25mm, transitioning to 400b-in, and 700lb-in after that). As well as "range of compression before effectively becoming solid".
3) damping sophistication - slow speed/short travel damping vs high speed/long travel damping.

All 3 of those can dramatically change the calculations/results, but I guess I'll start with "relativity".

Thoughts?

My main thought is that we are definitely veering into territory where I'm not qualified to comment. 🤣

Assuming you have 12k rear springs as you mentioned here, dropping the front down to 6k would put you at similar spring rates to what I have. But as you said, there are enough other variables in the mix that it's impossible for mortals like us to say what the effect would be.

If your spreadsheet math is correct -- which, again, I'm not qualified to evaluate -- it looks like your 6k scenario would net softer spring force than stock until bump stop engagement. That doesn't seem like a good idea to me, especially given near-stock bump travel and much more abrupt bump stop engagement. You could end up with great compliance, but I'd bet it'd be punctuated by unpleasant bangs whenever you hit the bump stops. OTOH, all of this could probably be addressed just by adding preload. FWIW.