[stretch]

Well, I plotted a stock 2005 STI shock dyno (courtesy of Whiteline) to my Suspension Calculator.

The stock shocks are roughly 65% critically damped to the stock spring rates, shown in the graph below. The fronts are darn near perfectly 65% critically damped, and this is considered to be ideal by most engineers. So there you have it, proof that the stock shocks are not overdamped. In fact, the rear is a little underdamped but close enough.

Ergo, riding on your bump stops and/or moving to firmer springs makes you underdamped. JDM Pinks rates are such a minor increase in spring rate that the dampers are still adequate up front. The rear starts to look a bit underdamped. However, that assumes you're not riding on the bump stops, which you would be up front with any lowering springs. Once on the bump stops, the stock shocks are roughly 45% critically damped. I feel this is yet another compelling reason not to lower your car on the stock shocks, ever, not even a small amount.

There's a neat little application here that will help you visualize what 45% critically damped means:
http://www.engin.brown.edu/courses/e...mpedvibes.html

Scroll half way down the page. Set the natural frequency as high as it'll go, then move the damping coefficient to 0.45. Click "start". Notice the secondary oscillation- that's exactly what I reported feeling months ago on my car on the stock springs, and it went away when I trimmed my bump stops. That secondary oscillation is a likely contributer to "bobblehead".

[ConradSTI]

Interesting. I've always felt that numbers don't lie...assuming, all things being equal and we've got an accurate starting base to run our equations. Now I'll quickly bow out as to let the engineering crew attack this subject.

[Rootus]

I've always thought that people were missing the target a bit when accusing the stock setup of bouncing. In my experience the suspension was okay, it was the seat springs that were underdamped .

[twiSTies]

Yeah but they're overdamped the moment you hit the bumpstop, with the 1.5" of strut travel they provide, correct?

[Rootus]

Originally Posted by twiSTies

Yeah but they're overdamped the moment you hit the bumpstop, with the 1.5" of strut travel they provide, correct?

Hitting the bumpstop causes the effective spring rate to go way up, not down, so they would become underdamped.

[Boxer]

Originally Posted by Rootus

I've always thought that people were missing the target a bit when accusing the stock setup of bouncing. In my experience the suspension was okay, it was the seat springs that were underdamped .

Agreed.

Everyone should do a simple test. Drive down a road you know to be smooth as a baby's bottom and then put your hand under your rear end (palm down) and feel what the seat is doing. You'll be amazed.

[stretch]

Yes, hitting the bump stops makes the system quite underdamped.

This data seems to further validates that Prodrive is the only company to have actually done real engineering when designing springs rather than relying on a butt dyno. Their very mild spring rate increase, for which they seemed to receive some criticism (even from me), now seems to be a smart decision- and they tackled the much larger issue via a softer bump stop and conservative ride height. If my data is wrong, please show me where- it very well could be.

However, while I think Prodrive did the engineering, I don't think they provided an ideal solution to the problem. Any lowering whatsoever via springs is a bad idea IMO on the stock shocks, which means "lowering springs" need the assistance of other parts to make the package work. I'm a broken records saying this, but I think the best setup currently available would be the stock springs with Prodrive bump stops and RCE lowering camber plates, as that'd have even more suspension travel. Edit: no, Ground Control plates with Ground Control springs- see later in post.

Speaking of, I want to thank 4banger for doing the legwork on figuring out the damper calculations. Wes, all your work appeared to be correct when I checked it, and I've obviously added parts to my suspension setup spreadsheet. Thanks a ton, man.

Personally, I now think the stock shocks have some potential. Their issue is limited stroke, but a vendor could make better use of it if so inclined. It'd take four parts:

1) A camber/caster plate that adds an inch or more of bump travel
2) A redesigned spring tophat to accommodate the camber/caster plate (stock tophats would rub if raised an inch).
3) ~275lb/in linear front pretensioned ~100lbs, ~240lb/in rear springs (with dead coils).
4) Prodrive bump stops front, trimmed stock bump stops rear

Without special lowering plates, such springs would raise the car half in inch- all the lowering must be done via the plates. But with the plates, such a system could trade about 1/2" of droop for bump travel and lower the car the other half inch as most people would want. The system would be only 62% critically damped, but the higher rates are a compromise of having insufficient total suspension travel. Lower spring rates require more travel which the stock struts don't have, so a balance must be struck between being underdamped and not having enough stroke. Having sufficient travel is king here and gets priority.

Actually, ya know what? It just occurred to me that this is pretty much what Ground Control offers, I think- camber plates that add about an inch of travel and springs that could be adjusted to keep that bump travel. I probably would have tried that if I had this data before ordering coilovers! Such a system would cost $800 from GC and offer a lot of adjustability, both more than what I think some would want.

TiC or others, is it time for you to make your own camber plates? How about some 1" lowering plates? Heck, just give me a permanent 1.5 degrees of extra camber and leave the adjustability to the bolt (allowing a range of ~0.6 to ~2.4 degrees), that'd keep it simple and cheap. Bundle them with smaller diameter spring tophats and a custom smaller diameter spring so it all fits, and I think you'll have a spring package that would give an STI excellent ride quality and handling. I dunno, maybe there's no market for such a comprehensive (and inevitably expensive) kit, but it sure as hell would be cheaper than Ohlins + camber plates to accomplish much the same thing.

Racecomp was on the right track with their lowering camber plates but really jumped ship by making lowering springs designed for their non-lowering plates.

[Rootus]

Originally Posted by stretch

Racecomp was on the right track with their lowering camber plates but really jumped ship by making lowering springs designed for their non-lowering plates.

They were probably on a different track altogether. I suspect their lowering camber plates were more of a response to the design of the JDM STI Pinks than anything else. But only RCE could really say for sure.

I like all the work you've done on this topic, stretch, it's much appreciated. Now the only issue is that even if you can come up with a good plan to utilize the stock struts, none of this solves the single biggest problem (IMHO ) -- clunk, clunk, clunk!

[05_GD7]

What about RCE's upcoming "normal" springs? A slightly increased spring rate (with emphasis on higher rate increase on the rear) and a near stock ride height?

[stretch]

Originally Posted by Rootus

They were probably on a different track altogether. I suspect their lowering camber plates were more of a response to the design of the JDM STI Pinks than anything else. But only RCE could really say for sure.

I like all the work you've done on this topic, stretch, it's much appreciated. Now the only issue is that even if you can come up with a good plan to utilize the stock struts, none of this solves the single biggest problem (IMHO ) -- clunk, clunk, clunk!

That's been solved ages ago- lube the inside of the struts! I don't recall anyone having a problem within a year of doing so. Personally, all clunks I've had in the rear weren't related to the struts- what I thought was strut clunk was my rear window... but there are other threads for that.

What about RCE's upcoming "normal" springs? A slightly increased spring rate (with emphasis on higher rate increase on the rear) and a near stock ride height?

The changes RCE made seem to be very minimal, but they're surely going to outperform their existing springs, as less is more with regards to drop. However, I feel- and I know RCE will disagree with me on this, which is fine- that they're missing the much larger opportunity to sell springs that would maintain the stock ride height or even raise it 1/4 inch, then lower it back down with their own lowering plates. That's more expensive but how lowering should be done. Furthermore, RCE is keeping the stock bump stops, which are overly stiff and not progressive enough. You will be underdamped, riding on 300lb/in bump stops, and have insufficient bump travel just like almost all other lowering springs.

So no, I don't like any of RCE's springs. I haven't tried them, but I did try similar rates and ride heights while running Ground Control springs and sleeves. Either RCE is choosing (like all the big spring companies) to completely ignore the very important suspension travel problem or they've got some long-travel fixed perch dampers in the works (which would also be a great solution).

[Turninconcepts.com]

Originally Posted by stretch

TiC or others, is it time for you to make your own camber plates? How about some 1" lowering plates? Heck, just give me a permanent 1.5 degrees of extra camber and leave the adjustability to the bolt (allowing a range of ~0.6 to ~2.4 degrees), that'd keep it simple and cheap. Bundle them with smaller diameter spring tophats and a custom smaller diameter spring so it all fits, and I think you'll have a spring package that would give an STI excellent ride quality and handling. I dunno, maybe there's no market for such a comprehensive (and inevitably expensive) kit, but it sure as hell would be cheaper than Ohlins + camber plates to accomplish much the same thing.

Actually, we've been working with another shop to make plates. I'll talk to them about doing some kind of lowering setup. I think there's a way you could keep the stock diameter springs if you did an upper spring perch that didn't have such a large conical section.

[SWortham]

If you make such a drastic camber plate you'll also have to be careful with tire clearance. After all, the more travel you give the car, the closer those tires can come to rubbing the fenders.

For example, with my RCE lowering camber plates, Ohlins struts, and stock springs I have more travel than ever before and as a result in extreme situations my front 245/45-17 tires will rub under heavy compression. Granted, those oversized tires have a lot to do with that but still it's something to think about.

[SWortham]

Oh, I wanted to pose a question. What about compression damping? I just wonder if a lot of the harshness of the stock struts comes from the compression side of things in addition to the limited travel.

It sure would be helpful to see a damping graph of the Ohlins struts next to the stock struts. It could be that the Ohlins are far more digressive on the compression side of things. And along with the fact that you get more travel with the Ohlins, that could explain the vast improvement in ride quality.

[stretch]

The compression damping is pretty similar to what many other manufacturers use and looks good to me at a glance. Heck, Koni uses more compression damping and they're still comfortable.

In regards to rubbing- yeah, that can become a problem especially if you start running larger diameter tires. You've discovered the reason why all cars must have fender gap to be comfortable. Your car must be awesomely comfortable if you actually have that much suspension travel, though- to hit the fender, your car (at its ride height) would have to use more than 2" of bump stroke, I think. Nice. Ideally, you want to use as much stroke as possible, then hit a bump stop just prior to the fender to prevent damage. On my last car, I had enough stroke to allow the tire to hit the fender, but it was extremely rare in practice so I lived with it. The ride quality and performance was worth it.

[SWortham]

Originally Posted by stretch

Ideally, you want to use as much stroke as possible, then hit a bump stop just prior to the fender to prevent damage. On my last car, I had enough stroke to allow the tire to hit the fender, but it was extremely rare in practice so I lived with it. The ride quality and performance was worth it.

It has only happened once so far when I was doing my Petter Solberg impression and hit a bump at 70 mph. I imagine the car was on the bump stops at that point but it was progressive enough that I didn't get the distinct feeling of the suspension bottoming out. The car handled the bump fine aside from that little bit of rubbing.