Spring rate theory and application regarding our abnormal chassis.

[JoshS]

If you have coilovers on the rear you cannot go by our rear settings.

Let's be specific, because this is going to confuse people. BobKid: If you have relocated your rear springs to be concentric with your shocks, then you shouldn't go by these numbers. But if your "coilovers" are in the standard spring position but simply have an adjustable spring perch (a lot of people use the term this way, despite the fact that the coil isn't "over" anything), then yes, your rear rates make little sense.

[Bobkid]

If you have coilovers on the rear you cannot go by our rear settings.

I do, but wasn't sure how those rates would compare since the lever arm acting on those springs would obviously be different.

[rf900rkw]

From memory, the stock E30 motion ratio is 0.67 for the spring, 1.2 for the shock. This will give your 225 a wheel rate of 325. This would be equivalent to somewhere around a 700# spring on a normal setup.

[Bobkid]

From memory, the stock E30 motion ratio is 0.67 for the spring, 1.2 for the shock. This will give your 225 a wheel rate of 325. This would be equivalent to somewhere around a 700# spring on a normal setup.

Thanks Randy

[trkshoe]

Unless you got a cage tied in to the rear upper shock mounts... your car will be on the floor shortly... we're here to help, so take a pic and post it.

After reading this, I will contribute by saying there are a few things that are not considered.
1 - stock spring rates cant be compared to aftermarket, stock is a progressive spring, while most aftermarket springs are linear.
2 - the reason for the discrepancy front front to rear is partially contributed by tire size... we are 50/50 cars with a staggered setup.
3 - TC loves soft setups and selling you more springs as the driver improves (nothing wrong with that... if anything it's good driver training)
4 - rear sway is a good thing in our cars, however, for more thrashing movements the rear bar IS the reason why the rear end failure occurs
5 - here are my recommendations for the coupe with proper shocks (street only 400/450; street/autox/hpde 500/600; race 600/700 for bumpy tracks; more for east coast tracks)

[rf900rkw]

(2) so you're saying that because of less grip in the front, we need to run an insane forward bias on the suspension frequency to further kill the front?

(3) Agreed. What TCK sells will be "safe",; the car won't snap spin due to ham-footed driver inputs. It will always hit the tree nose first.

[trkshoe]

Logic there would be we need more grip in the front and hence higher compression/rebound rates in the front.
If you compare shock numbers, front to rear, that would support the need for higher spring rates.

[rf900rkw]

You are mixing steady state (springs, bars) and transient (shocks) forces, but no matter. Generally higher rates mean less grip. Want the car to rotate/point better? Stiffen the rear rebound,

[kornfeld]

I installed the TCK SA kit with 400# springs front and rear, and have put a few miles on it. I have rebound set to 1/4 turn from full soft on all four corners.

Overall, I like how it rides very much, and my feel of the car aligns with what others have posted (it feels like it corners flatter and is more responsive, yet it also feels more compliant when going over bumps and such).

However (and this is no surprise based on the discussion in this thread) the rear is very soft. Like, marshmallow soft. Which isn't the end of the world, because I would rather have it too soft than too firm; however, it feels like the rear suspension is able to compress way too much in certain situations. Specifically, if you imagine an abrupt dip in the road, like frequently forms when you transition from concrete to asphalt. A relatively abrupt, tiny trough forms in the road at those transitions, and as you move from one surface to the other, the car drops quite a bit and makes a "whooomp" sound before leveling out again and continuing on. It feels like I might be hitting the bumpstops in rear under these types of circumstances. I don't feel or hear anything actually impacting anything else, but it just feels like the rear suspension is able to compress too much.

Which leads me to wonder if stiffer springs will alleviate this issue. This type of movement in the suspension feels like it would fall somewhere between hitting a sharp pot hole and going through a long turn in regard to how abruptly it loads the suspension, and I'm not sure if a stiffer spring would alleviate this issue, or if it is more affected by the compression stiffness of the shock (which I don't have control over).

Thoughts? Would a stiffer spring help? How much of a difference is there between 400# springs and 500# springs?

[jbrannon7]

I think, if you are running 400 in the front you should be running 550 in the rear.

I installed the TCK SA kit with 400# springs front and rear, and have put a few miles on it. I have rebound set to 1/4 turn from full soft on all four corners.

Overall, I like how it rides very much, and my feel of the car aligns with what others have posted (it feels like it corners flatter and is more responsive, yet it also feels more compliant when going over bumps and such).

However (and this is no surprise based on the discussion in this thread) the rear is very soft. Like, marshmallow soft. Which isn't the end of the world, because I would rather have it too soft than too firm; however, it feels like the rear suspension is able to compress way too much in certain situations. Specifically, if you imagine an abrupt dip in the road, like frequently forms when you transition from concrete to asphalt. A relatively abrupt, tiny trough forms in the road at those transitions, and as you move from one surface to the other, the car drops quite a bit and makes a "whooomp" sound before leveling out again and continuing on. It feels like I might be hitting the bumpstops in rear under these types of circumstances. I don't feel or hear anything actually impacting anything else, but it just feels like the rear suspension is able to compress too much.

Which leads me to wonder if stiffer springs will alleviate this issue. This type of movement in the suspension feels like it would fall somewhere between hitting a sharp pot hole and going through a long turn in regard to how abruptly it loads the suspension, and I'm not sure if a stiffer spring would alleviate this issue, or if it is more affected by the compression stiffness of the shock (which I don't have control over).

Thoughts? Would a stiffer spring help? How much of a difference is there between 400# springs and 500# springs?

[rf900rkw]

I'm running 375/550.

[kornfeld]

Maybe my post was too long....


Will stiffer rear springs reduce the tendency of the rear suspension to compress excessively when going across troughs that exist at transitions between road surfaces? These transitions aren't as sharp as hitting a pot hole, but aren't exactly long duration events either.


It seems to me on the surface that stiffer springs will help in this regard--but I'm still trying to wrap my head around the interaction between the springs, the high speed valving, the low speed valving, and the impact of rebound and compression adjustments. I've read quite a few posts that have made me realize these things aren't quite as simple as one would hope.

[rf900rkw]

Prehaps my answer was too short. I thought it was obvious.Yes. Springs support the car. But shocks delay the transfer of energy into the springs. Thus sharp jolts are the domain of shocks, long compressions are the domain of springs. You are describing the classic symptom of being undersprung for the travel you are allowing based on ride height.

[kornfeld]

OK that makes sense. Like I said, I'm still trying to understand all of this correctly....so taking this from your post:

...shocks delay the transfer of energy into the springs.

I'm trying to flesh that out in my head. Does this all align with what you're saying there?

-The settings on the shocks primarily dictate how events of high intensity but relatively low amounts of suspension travel feel (like hitting the edge of a pot hole, expansion joint, etc). So, a very steep (potentially vertical) edge in the road, but only usually an inch or two in height at most. So the spring may not end up being involved very much or at all because the car is already jolted upward by the time the pot hole has passed. In other words, the shocks delay the transfer of energy to the point that they basically absorb all of the impact by the time the impact is already over and done with. (The energy is eventually still transferred to the spring of course, because as you say, the spring supports the car.)

-On the other hand, the spring rate primarily dictates how events that are low intensity and longer duration feel. So when I go through one of the troughs in the road, the shock delays the transfer of energy into the springs, but the transition is long enough that the car is still traveling upward or downward and the suspension still has more to compress or rebound by the time the spring has fully taken the load.

Does that sound right? Or am I still mixed up on something?

Thanks for the info you've posted around on the forums.

[Tim95M3]

Yes, stiffer spring rates will fix your issue. My dad's M roadster has TCK with 400/400 and it also has the "marshmallow" feeling you describe. Meanwhile I have GC 450/650 on mine and it's perfect. Much more confidence-inspiring. At least for me.

[trkshoe]

Want the car to rotate/point better? Stiffen the rear rebound,

That is absolutely wrong Randy... stiffer rebound means the shock is allowing the wheel/tire combo to decompress more rapidly, thus greater grip.

Best,
Patrick

[rf900rkw]

That is absolutely wrong Randy... stiffer rebound means the shock is allowing the wheel/tire combo to decompress more rapidly, thus greater grip.

Best,
Patrick

Sorry, but that backwards. More rebound will cause the weight to come off the tire quicker leading to less grip until the suspension restabilizes. This will allow a neutral car to tighten it's line simply by lifting from the throttle a bit.

- - - Updated - - -

Kornfeld--

First thing to understand is the shock, well, a proper shock, has many sets of valves that open progressively depending on the shaft speed. Normally we break them down into four characteristics, low speed and high speed for bump (compression) and rebound (extension). The high speed valving is generally what controls bumps and potholes. The high speed compression controls the unsprung weight of the suspension, the high speed rebound controls the spring. The low speed valving controls the rate that the suspension moves during weight transfer.

When the forces on the car change, we'll use cornering as an example, weight transfers via two paths. The first is via the mechanical lever of the suspension. This is transfer is instant and linear to load. The second is through the springs, both suspension and swaybar. This second way takes time, as it requires the chassis to move. And this is where the low speed shock comes into play. The load taken by the shock is instant, and as the suspension moves it is transfered into the springs. This gives a tuning method for the time the chassis is changing attitude, the dynamic state. The low speed shock valving only has effect while the chassis is changing state.

Springs and bars control steady state.

Low speed shock valving controls dynamic state

High speed shock valving controls ride.


Most single adjustable shocks, such as the Koni's used in the TCK kits, adjust the low speed valving only. This is a tuning device. It is NOT a ride comfort adjustment I will not speak for the elcheapo shiny anodized chinese stuff so favored by the "coilover crowd", as I have no experience with it and no one (not owners, vendors nor importers) can/will answer such questions.

I got a bit to far off there. I was about to get into roll centers, antidive/squat and other linkage concern that... uh... anyway. Long whoops like you're talking about (I go thorugh something similar ever day) are the realm of the springs. You could try to prop it up with shock, if you had a triple/quad adjustable shock, but that would be akin to putting a bigger buffer on a slow computer. You just delay when it bottoms out. Getting through a dip transition like that speaks volumes of both spring rate and front rear rate balance. The whoop I hit is the transition from s bridge to a concrete highway on a causway built on a bed of swamp peat. Most cars end up doing a fore/aft pitch pogostick for and ten twelve cycles. My car does two cycles. The first has some fore/aft, obviously. But it decays to a linear even up/down for the second cycle. *This*, the matching of suspension frequencies, is the difference between a Buick and a BMW. Damn, off on a tangent again...

[kornfeld]

I think I'm going to have to chew on all of that for a while.

Based on what you wrote, I was thinking that there would be some relationship that could be graphed....like if you laid out the motion of mass on either side of the spring as time vs displacement, the maximum amount of displacement would primarily be governed by the spring, and the slope of the line during transitions would be governed by the damper. (I'm not sure if the details there are actually true, but I started trying to picture things in that way.) So I searched around for info on that, and came across these pages:


http://forum.wscc.co.uk/forum/topic/...ots-explained/

http://forum.wscc.co.uk/forum/topic/...rs-for-my-car/


Does that all look accurate?

[rf900rkw]

Yes, decent articles.

The second post touches on suspension frequency. That is a subject that is brought up elsewhere in this thread, if I recall. 400/400 will give frequencies of 2.2hz front, 1.5hz rear, to use his terms, or 145cpm vs 95cpm that I'm used to. This goes back to my constant question of why people run such a high front rate.

[kornfeld]

Updating this...as I mentioned above, I started off with 400F/400R springs, and while it handled well, the rear felt too soft to me when going through dips at highway speeds.

I installed 500lb springs in rear yesterday morning and took it for a drive, and it was definitely a big step in the right direction. My experience lines up with what others have pointed out---these stiffer springs didn't actually cause the ride to be any more crashy or jarring over expansion joints or potholes, but it did help quite a bit with going through dips. If anything, I might think a 550 or 600 pound spring would be perfect. I'm going to stick with this for a while though and see how I like it.


And if anyone wants to buy a mildly used set of TC Kline 400lb rear shorty springs, let me know....

[SunzOut]

Went from 450/500 to 450/600 today (TC DA coil-overs/shocks) and really liked the change for Autocross. Was trying to get rid of the bad case of "pogoing". On longer corners the back end would tend to load/unload repeatedly making it real hard to get on the power.

The stiffer rear springs seemed to have reduced this. Maybe even removed it.

Did not feel like I had any less rear grip, which was a worry. May even try disconnecting the sway bar again

[saber63]

I'm just wanting to poke in on this one...

I went with some 450-500 tck DA setup. And it never felt right, I got to looking, and I was bottoming the bump stop %100 in the rear, but not the front... So I band aided it with some #650lb rear springs, which got rid of the REAL bad harshness (guessing the hard bottoming out) and helped the car a fair amount.
Though I also added a mountain of front camber, and didn't change the rear... Soooo it is/was a bit tail happy.

Was the car faster/slower... that I don't know... But it is more enjoyable to drive, and was given some pretty good praise from a pro driver.

However the rear still feels soft compared to the front, MAJORLY when going over bumps.

so I did some math. And based off the cars weight (from the corner balance) I'm getting 292# wheel rate and thus 1.96hz. borderline too stiff. So don't really want to go stiffer.

"How To Make Your Car Handle" by Fred Puhn states that front frequency should be slower, because of heave rate over bumps. (.0568*wheelbase in " / speed in mph) so 55mph give .1s and 100mph gives .055s.
so front should be 1.63hz-1.77hz A far cry from the current 2.13hz!

So this math suggests a front spring of 201# to 240#...

NOW! The another book "Race Car Vehicle Dynamics" by Milliken Milliken also goes on to state that a %10 stiffer rear bias will improve ride comfort over bumps, and general ride comfort. It more or less goes out the windows for race cars. As the one of the next statements is roughly "race cars tend to run much stiffer undriven wheels compared to driven wheels. This keeps the driven wheels more evenly loaded for traction" Also the commonly known stiffer means faster transitions and all that.

So I guess that puts me right back at sticking with these spring rates... You can run a lot more front spring on these cars, because we sit so far back, and thus the ride comfort issues normally produced are less. But the performance gains are still there. Of course personal preference, tire choice and size, and rest of car setup, is going to play a pretty big impact as well.

Sorry this got rambly. I started looking at spring rate info for a rather new to me e36 M3. But as i read more I thought back to the Z, and I started writing...
I highly suggest reading chapters 12 16 and 22 of the "Race Car Vehicle Dynamics" by Milliken Milliken book. Which can be found free online.

Cheers

-Levi

[rf900rkw]

Competition cars running on billiard table smooth tracks can get away taking large amounts of the roll couple on the non-driven axle. I'm still not convinced it's such a good thing on the street, even with our unique layout.

In addition to the Milliken^2 tomb (William and Douglas Milliken, for those wondering), Competition Car Suspension by Allan Staniforth gets a thumbs up. The last few chapters layout out the math in manner that is far less dry than the Milliken^2 book. I don't know about current availability, online or otherwise; both have been a fixture on my desk for two decades (at least) Milliken is published by SAE, Saniforth by Haynes.,

[saber63]

Ah yes, that was the other book i was eyeing. I'll see if I can track down a reasonably priced copy.

Well, it seems partly "race cars" run really stiff front springs to keep the front roll "normalish", when the E36 front gets lowered. I know from experience how the front end goes to hell when to low and not stiff enough. And than the seating position stuff means it's easier to get away with.
We also don't know how the car's "comfort" and "grip" change front and back with regards to spring rate and damping ratios. Milliken^2 has one great graph about this, but it appears the numbers are chasis specific, and very tedious to calculate. Enough so, it's an entirely different SAE paper :|

If we're talking road cars... The %10 split of wheel rates almost perfectly matches the math for having an ideal heave rate at 100mph.... And based on how poor the rear of my Z feels going over railroad tracks and the like with the stiff front. And how all my other cars Don't (one still feels harsh, but doesn't heave)

So I would guess that it's all about compromises in comfort/preference, and dynamics/grip.
For a %90+ daily driver, I'd guess ~%10 stiffer rear wheel rate would be better and between 1-2hz depending on choice.
When lower lap times start to matter, I think that's when the large difference might make more sense, but only if the data logging/lap times agree.

I am honestly very very tempted to pick up a pair of 250lb front springs. And maybe some adjustable sway bars...

[kornfeld]

Bumping this again....I started with 400lb front and rear, and the rear was too soft. Went to 500lb rear. I'm still finding it too soft. I still feel like I'm close to bottoming out on longer whooomps on the highway, and I don't like it.

Thoughts on 600 vs 700 lb rear springs? I'm tempted to go straight to 700lb springs....