550 aftermarket springs and shocks

Terry,
I have not seen HGTC ride height data. I too am assuming they are consistent with the FHP data. It does appear that my car rides "more level" than a HGTC equipped 575 when I look at the photos of the car CNC is selling in another thread.

As an aside, when I tracked my old 308, I went through many iterations of springs and ARBs, alignments and ride height to get the car to "stick", particularly through a high speed corner like the kink at Road America. Too much rake (1 1/2 inches in that case) made the car too responsive at the front (very small movements of the steering wheel causing big changes in path). Running 3/4 inch changed the handling and made it much more manageable.

While I have no interest in tracking the 575, the experience is a reminder of the importance of the variable.

I assume the PO lowered the rear to make the rear stick more or because it improved the car visually. Ditto the front. I am also assuming that the front was set to within the tolerances for the Euro height and the springs have settled by 3-4 mm over the last ten years. As the car shows few signs of wear and the springs stayed stock, I am assuming the later. Will find out more when the alignment gets checked / reset after the spring changes.

Philip

 

Philip- Seems like somewhere in the WSM it says that settling of 5 mm or so is to be expected, so your assumption is probably good.

 

Carl,
I finally wrestled the paper from the SAE. As i mentioned, DRM software does not work well on a Mac.

The paper was an interesting read. As you note, it's pretty academic for the street (the aero testing work on the Corvette described in the paper was done in a wind tunnel running at 120 mph). To those reading without the paper, while the aero changes from rear wing angle, front splitter extension, rear wing end plates and wing chord length and so on were useful, the ride height was the most influential variable in increasing downforce.

As i read their conclusions:
(1) the (front of the) car should be run as low as possible (practical). For example, in the base configuration, the front downforce was about 50lbs. Dropping the front by an inch created about 130lbs of downforce at the front (and took about 50lbs of DF from the rear).
(2) the aero balance can be trimmed with the rear ride height. Dropping the rear of the car reduced front downforce (and rear downforce about 30% of this amount) but reduced drag (by about 1%)

So, if you are running at sustained high speeds (or tuning for sustained high speeds) we know what to do! This, as Carl noted, is pretty irrelevant for the street but the paper does provide some useful input on the impact of changing rear ride height in response to my question. Thank you Carl.

For my car where the rear RH is lower than stock, I have not felt it particularly "floaty" at speed. I may dial in a little more rake to push the front down a bit more, but the primary issue has been the front to rear weight transfer causing porpoising, hence going with stiffer springs.

I will take some pics as I pull it all apart. To anyone that has removed their front shocks, I am trying to see the minimum disassembly required and am wondering if the shock can be removed without disconnecting either the upper or lower control arms. I.e., can it be threaded out through the top of the upper arm or toward the rear between the two arms while leaving them attached. Of course, more components can be undone...

Philip

 

Yes, it can be wiggled out the bottom but the sway bar brackets must be removed so the bar hangs down and you get full droop on the suspension arms.

 

Thanks, appreciate the input. Philip

 

I have driver's side shock removed. Painted marks on spring and collar denote ride height setting. Will measure, count threads tomorrow.

One note, and related question. The lower control arm fasteners (eccentrics) are mounted with the bolt heads facing the front and the nuts the rear. The upper control arm bolts are set with the heads on the outside and the nuts on the inside. I.e., the front bolt is in the same orientation as the lower control arm, where as the rearmost bolt has the head toward the rear. How are everybody else's located. I remember doing this on my 308 and a Ferrari master tech subsequently scolded me for having the orientation incorrect.

Philip

(Now where is the Carroll Smith book on chassis rake changes!)

 

u should
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Have a race car to scratch that itch. Weighs about 1100lbs with 200 bhp. Compare that to the 575...

Anyway, glad you had fun with yours
Philip

 

Carl, I don't pretend to be the oracle on this. So I consulted (a book by) one.

Symptom: floaty ride with excessive vertical chassis movement.
Cause: too little spring overall.

- Carroll Smith

The HGTC spec calls stiffer springs at the font with a larger rear ARB and alignment. The handling of the HGTC car was described to me as much improved by a source I trust.

What would you do differently given my aspiration?

Philip

 

Philip- Afraid all the cars run together. What springs, rear anti-roll bar, and Shock ECU are you running right now?

 

Stock front springs, rears, ARB and the upgraded shock ECU, Terry.

I am putting on 400lb front springs (which Henry or Rick measured were the HGTC values), 25% - 30% more than stock, and the HGTC rear ARB.

At the moment I am holding off on rear springs as the HGTCs were only measured at 10lbs heavier.

I pulled off the front shocks today. Removed the lock rings and measured the bottom of the collar to the center of the shock bolt. 148mm. The lower spring platform is about 6mm.

I'll get the springs removed and the new ones installed this week.

Philip

 

Philip- Remember that the OEM racing springs (FHP and HGTC) were shorter (~275 mm vs 238 mm F and 284 mm vs 259 mm R) than the original springs and this is one reason the ride height was lower. The rear FHP springs were ~12.5% stiffer than the OEM springs at ~.225 mm deflection/N vs ~.257 mm/N. That works out to way more than 10 lbs and Henry said the HGTC springs were slightly stiffer than the FHP springs.

Here are all the numbers for OEM and FHP. Do not have them for HGTC.
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I just typed a long reply with lots of math and lost it so I'll save you from my calcs, Terry!

The bottom line: front
- I calc the stock front spring rate to be 308 lb/in. It foots with my calcs of suspension travel and Ferrari's table for static load and compression at static load.
- Rick's meurement was 320. Lb/in.
So, close.

- I am told the HGTC F springs were +35% versus stock. This equates to 415 lb/in.
- Rick's meurement was 400lb/in.

-The FHP spring I calc as 422 lb/in.
-Rick measured as 375.
Can't explain this.

I went with 400 lb.

I'll look at the rears more closely. Thanks.

Philip

 

Don't have that one Carl. Can you share the SR and RH you adopted along with your objective?

 

Terry
Thanks for the prod on the rears. Repeating the calcs for the rears and comparing o Rick's measurements I get:

Stock:
Calculated spring rate= 225 lb/in
Measured in Rick's post = 235 lb/in
Close

FHP = HGTC (per Rick's measurements)
Calculated spring rate (Terry's table)= 255lb
Measured per Rick's post = 245lb/in
So close again

However, in total, Terry's right. There's a 10% difference in SR and the rear springs should be upgraded to about 250lb springs

Philip

 

Philip- My calculations also showed the FHP springs slightly stiffer than the HGTC springs, but we do not have accurate measurements at static and compressed lengths for the HGTC springs and I think the tester just compressed the springs an inch and took the readings. Will not give the same measurements.

 

Yes, we don't know how they were tested unfortunately. Hopefully the tester would put some preload on the spring to adjust for the initial creep and then measure the force required.

If they were tested similarly, while the absolute values may differ from those measured over a broader range, the relative values should be comparable.

Also, Terry, looking at "your" table above, and comparing dynamic load to static load and the corresponding change in shock height, to the unladen to static load and corresponding change in free length to static load length, the calculated spring rates are the same to 2 decimal places so I am not sure how material the "creep" is. One source (an Indy car racing spring specialist I checked online) opined spring creep wasn't usually found to be material in SR below 500 lb/in.

 

Philip- Affirmative, I had checked those numbers, too.

 

Anyone have an answer on this? I looked at the parts book on the Eurospares website and it looks like the bolts should both face head front, threaded section toward the rear but if someone could check their own car, I'd appreciate it.

Philip

 

Philip- Does this photo help?
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Perfect, thanks Terry. It was how they were installed on the car originally.

Separately, I took the shocks and swapped out the springs tonight, putting the adjustable perches 15mm lower on the shock body (which should keep the same ride height as before if my geometry and spring rate mathematics is correct).

John, we were able to get the protective sleeves (which contain the spring guide) into the 400lb springs with a bit of lubricant to help everything along. Outside of the spring color (blue) it will all look factory.

I hope to get the front of car re-assembled later this week father I've cleaned up everything with the shocks out. I'll then address the rear.

Philip

 

Good to hear Philip; out of curiosity, and for the benefit of the community, where/who did you source your 400# springs from? Sounds like they're not the Lensing springs if they're blue?

 

John, I purchased the springs from Suspension Spring Specialist, on the web as "blue coil springs". (The race shop i use put me on to them.)

The closed end springs we want are "rears" (the fronts are closed / open configuration). As I think I said earlier in the thread, I purchased 10.5" long, 5" OD, 400 lb springs. they are very reasonably priced at about $50 per coil. Their standard supply color is blue, but they will supply unpainted too so you can have them finished in OEM black. (i think the blue looks rather interesting but in reality, it is all hidden in every day use). Incidentally, SSS will wind custom springs to any specification for an additional fee so they will be one source for the rears springs if I upgrade those too.

As I have them set I believe they will leave the car with the same static ride height and will not interfere with any other suspension components but the proof in that pudding will be determined over the next week or so when I put them back on the car. More to come.

Philip