I'm having coilovers installed this Friday, and I've been reading about the various AM suspension options chosen by different users here for quite a while. Based on this, a couple of observations and related questions come to mind:

1) Looking at the spring rates quoted in Cannga's thread, it is clear that AM coilovers move in the direction of increasing the front spring rate by a greater percentage than the rear. For example, the stock ratio of rear to front rates is (457/206) 2.22--the rear is 122% stiffer than the front--while the Bilstein Damptronic rear is (565/340=1.66) 66% stiffer than the front, and the Ohlins R&T rear is (685/400=1.71) 71% stiffer than the front. (The KW V3s seem odd by comparison, so I'm treating them as an outlier.)

2) The JRZ/Moton's ratio of rear to front using minimum rate specs(700/500=1.4, rear is 40% stiffer) suggests an even further narrowing of the gap between the two (the front stiffening by a greater percentage than the rear), and Ohlins' R&T specs for their GT2 product show a similar pattern, with the ratio of (800/514) 1.56, realizing this would likely be even smaller--higher front rate--with the added weight of AWD.

3) A general characterization of this information would suggest that, as one moves from a "street comfort" setup to a "track/race" setup, the front spring rates on the 997 TT stiffen more than the rears, percentage-wise.

Why is this? Is it a function of the 911's rear engine placement, vis-à-vis other cars that are more balanced front to rear? That is, should I expect to see the same kind of pattern as one "stiffens" a 50/50 BMW, for example?

Intuitively, the 50/50 split will be starting at a more equal place regarding the difference between front and rear spring rates. But, the other major difference between the front and rear--which is not related to weight bias--is steering versus drive-only wheels. Is this aspect part of the story?

Sorry so long. TIA, Louis

 

L Perm, I think it might be helpful to keep in mind that the spring rates of each coil-over manufacturer exist not as singular components, but rather as integral parts of a compound, the other critical bit being the size of their fluid apertures, whether the fluid is hydraulic oil or nitrogen gas. Some makers may use higher spring rates with larger fluid apertures, others lower spring rates and smaller, i.e. more restrictive, apertures. Still others will use aperture "a", fluid path "b", and spring rate "c".
The result is a particular "recipe" for each manufacturer. Ergo, it is impossible to judge one coil-over against another by comparing only spring rates.


I have personally experienced just AWFUL kidney-bruising rides in P-cars with very low (240-250lb) fronts, and, conversely, amazingly smooth, compliant rides in 600~lb fronts. The difference was not just the brand of coil-over, but ( and this is critical) the set-up done by the installer.


In my case, trying to sort out all of the above leads to headaches and whiskey. The only solution I've found is the kindness of owners letting me try their set-up, and their suspension ace. All else is a waste of time and whiskey.


Enjoy the ride.

 

I agree. I am definitely assuming a "well-matched" damper to spring combination in my discussion. There is surely some range of possibilities that could be described as "well matched," but I wouldn't guess that range to be too extreme. You want the damper to control spring movements, but you don't want the damper taking over the primary spring function.

On one extreme you have the light spring and the restrictive damper where the damper ends up providing primary spring duties, instead of just controlling the movements of the spring. I would liken the 997.1 TT "Sport" PASM mode to this situation. The 997.1 TT "Normal" PASM mode is closer to the other extreme of an uncontrolled spring bouncing indefinitely.

I'm not trying to compare the ride qualities of the coilovers I mention. Instead, I noticed a pattern and was wondering what explains it. A simple explanation, for example, might be that the rear weight bias of the 911 creates the need for a fairly heavy spring in the rear for basic street use. Any "stiffening" that is required for track or race duty will tend to focus on the front more than the rear because the rear is already relatively heavy to begin with. I'm not convinced this is all there is to it.

LP

 

l, i'm sure you're headed in the right direction. Usually, when people describe an arcane discipline as a "black art", what they're saying is they simply have given up on a difficult learning curve. HOWEVER, in this present context, I do believe that suspension tuning, while not a black art, does live perhaps next door to the Prince of Darkness (no offense to John Lucas, the true Prince of Darkness and inventor of the intermittent light bulb).
In the last 2-3 years, I've run into more c2s, tt,gt2, gt3, and 987 guys who are quite happy with one of the brands you cited in your 1st post than the others combined. This impressed me because many different shops did the work, several different models were involved, and very few problems (i.e. quality control issues and/or "mysteries") had occurred. Why is this happening ? I'm inclined to believe a company or a distributor has worked hard and smart, and found a combination that works for a particular family of cars. This seems to happen to makers, by turns, over a period of years. I'm old enough to remember Koni absolutely owning the sports car after market. Slowly, along came Bilstein, and on, and on. Maybe key engineers leave, or management changes focus, or, well who knows. Right now, JRZ seems to be driving the bus, followed by Ohlins new R&T series, with dark horse MCS starting to make an impression.


Seems to force one to abandon the scientific method and just play the current hot hand. May offer the best shot at a good result, and fewer migraines.


Speech over.


Best of luck.

 

Quality shocks ARE the key. I run 700F/800R with JRZ shocks and the performance and ride quality is much much better than the much "softer" PSS9s and H&R coilovers I've run before. I'm actually changing to 900F/1000R next month and even that should be fairly street-able. Having run larger spring splits before, I will also say that a smaller rate split front to rear performs much better on these cars especially if you push the car or track it. For street with occasional track 600/700 or 700/800 is a great set up. In would not go less than 500/700 in any case especially on a stock weight Turbo which is fairly heavy to start.

 

I understand exactly why you asked, and I don't have an answer and not claiming to be authoritative, just want to offer this observation:
From the many posts I've read and questions I've asked about spring rates, 911's spring setup is NOT based on a percentage formula, but a simple rear/front numerical difference, with rear spring being around 150-250 higher than the front. In other words, in street cars you would see 400/600, track cars 600/800, and finally, the serious race cars/cup cars, around 1000/1200.

Intuitively, if you assume the reason for the higher rear spring rate in 911 is the fact that the weight of the rear is higher than the front, say 600 lbs in a 3000 lbs 40/60 distribution car, then it does make sense that spring rates should be a fixed difference and not a percentage. The 600 lbs difference stays at 600 lbs, it doesn't go up in a race car. This is just my musing; definitely did not read any authoritative book about this.

Regarding stiffness variation with same spring rate, among many reasons, IMHO the main one is because of the damping forces of all shock absorbers are not set up the same (JRZ is higher than Bilstein, etc.). Some example: Increasing (high or low shaft velocity) compression damping will increase stiffness as during compression the damping force is additive to spring force in resisting motion. On the other hand, during rebound, increasing rebound force will also increase stiffness because it doesn't allow spring to rebound fast enough and wheel becomes airborne.

I agreed with the posts by others above: although you *do* have a good idea based on spring rate alone, in the end, because both spring rate and damper are factors, you will just have to drive any particular combination to know exactly what it feels like, and then adjust accordingly. BTW, for others reading this, because adjustment of spring and damping force affects roll stiffness distribution and understeer/oversteer behavior, it is not a simple matter and I would only recommend this very fun and educational activity for
a. the most seasoned and advanced of drivers or,
b. most seasoned and advanced of setup techs or,
c. nutty beginner with spare time, effort, and $ to experiment.
My case of course being b & c . Hope this helps.

 

...what he said