Suspension & Steering
Coil and Shock Selection Info
I have received a few privite mesaages regarding suspension components for the 928...as well as the one posted regarding progressive springs vs linear springs. Hopefully, this will help answer some questions...
Progressive vs. linear springs:
Let's begin with a short description of how a car rolls around its roll center during cornering and its effect on springs. The roll center is the axis made by connecting the front roll and rear roll center of a vehicle. You can sort of look at this axis as a line running from the front to the rear of the vehicle at a centerline and somewhat parallel to the ground. During cornering, the vehicle will "roll" about this axis.
Now let's look at the vehicle at rest and what the springs are doing...the spings will be compressed to "x" distance. During a corner, the springs on one side (outside) will compress at a given rate and the springs on the other side (inside) will uncompress/extend at a given rate.
A linear spring has a given rate of resistance that is constant, usually given in lbs/in compression. If a spring has a rate of 400 lbs per inch and when the 928 is sitting still or at equilibrium, that spring is compressed 2.25" (assume the 928 weighs 3600 lbs).
Now you enter a corner...one side compresses and one side extends.... A linear spring will allow the vehicle to roll "evenly" since one spring is compressing, say one inch, and the other spring is extending one inch...one spring loads with 200 lbs and one spring unloads with 200 lbs. The roll of the vehicle is balanced and controlled.
Now let's say you encounter a bump during the corner, and the deflection is an additional two inches...again, the suspension loads and unloads evenly, and again, roll is controlled, the vehicle maintains a constant attitude.
This is how Porsche designed the springs on the 928. This is the reason why the 928s will rise and fall during a bumby corner, but its "attitude" does not change! In other words, the 928 does not rotate about its roll axis during a corner. This is one of the features that makes this vehicle one of the best high speed handling vehicles of all time!
Now lets look at a progressive spring. WARNING: This is a very simplified view.....
A progressive spring has two or more rates on one spring...let's assume that for the first 3 inches or three coils, the rates is 300 lbs per inch and that for the next two inches, the rate is 400 lbs per inch. Therefore, when at rest or equilibrium, the 928 will be fully compressed on the 300 lbs spring section, and the next inch of movement will be in the 400 lb per inch range. And the 928 will now rest at approximately 3" ... remember, this is a simplification!
Now let's use the same scenario......a corner is entered and the outside spring is compressed one inch and the inside spring is extended .75 inch...rememeber the rates!
Now you encounter a bump in the corner...and since you are currently using the outside spring at a rate of 400 lbs per inch and the inside spring rate of 300 lb per inch... and the suspension is compressed the additional two inches....what happens to the spring rates? Since they differ, the chassis rotates around the roll axis... and the vehicle attitude changes/rotates differently than when in steady state cornering!
Its motion becomes slightly skewed since to offset the bump deflection each spring must work through a different spring rate! If you were to then hit another bump, the attitude would change again...and so forth.
Progressive springs can be made to work in very limited conditions where the parameteres are well known, like a specific road course where telemetry is constantly giving feedback to help select the right rates.....and the designers are experts on vehicle dynamics.
Moving the front springs to the rear:
When we first performed our research to develop our coilover kits, we needed a baseline, so we used the then popular progressive front spring, those around long enough know the spring I am refering too, and placed the old fronts on the rear of the "white car" and learned alot about springs, rates and front to rear ratios. We used the recommended Bilsteins. We experienced rear "hop" that was so uncontrolled during hard bumpy cornering that we almost lost the car! We later learned about leverage ratios and front to rear spring balance. This really helped us choose spring rates to match shock dampening.
What about the "Club Sport" spring?
When Porsche built the "Club Sport", they used a front spring that was about 8% stiffer than the standard spring. That part number is noted on the spring and there is also a color marking identifing it as such. If you have those springs, just tell me the color and part number and will let you know if they are the original club sport springs. These springs are not progressive rate springs.
Are DEVEK coilovers for racing?
NO, our standard internal and externally adjustable coilovers are used to improve street vehicle handling and provide a better and more controlled ride. These are great for everyday driving and occaisional track usage. Our other two levels are for those who are serious about extracting the maximum out of their 928 in aggressive street and track situations. The levels are as follows:
Level 1 (standard) for normal street/mildly aggressive driving/track schools..available in either internally or externally adjustable configs
Level 2 a track level/aggressive street setup with custom rebound and compression rates along with higher rate springs....a perfect balance!..available in either internally or externally adjustable configs
Level 3 a very aggressive track coilover setup with maximum control and fully user adjustable (external) in compression/rebound....
Why not use Bilsteins?
Bilsteins are an excellent shock. They are expensive new and are non-rebound adjustable/tunable. Rebound adjustment allows the user to configure the shock to changing road conditions and keep the rubber on the road by easy adjustment. We like Konis due to the ease of adjustment...not all drivers like the same setup!
Also, we can have your old Bilsteins rebuilt for about $100 per shock and get them valved any way you request!
I hope this helps those who have inquired and others on the list ...if there is enough interest, I will post some information on ride heights and what our research revealed in the future or as time permits. Let me know....
Dear fellow sharksters:
This "spring" discussion does prove one thing, "There are at least two ways
to skin a cat." There are well-versed people on both sides of the spring
argument. Kim Crumb is on the side of "Progressive is better for street
and track use."
Let's see, he has been the PCA 928 model expert FOREVER, and he has the
winningest competition record of any 928 driver, PERIOD. (Look it up if
you doubt what I just wrote.) He spent about two years developing the
progressive springs that are now sold by Weltmeister (from which he gets
NOTHING), yet, in spite of the fact that he is not compensated in any way
for their development, he still uses them.
He clearly does not say or think that linear springs are no-good. It seems
that others have not been as open-minded about the progressive springs.
On this issue there is no right or wrong answer. You might as well ask,
"What's the best Baskin Robbins' flavor?" or "the best Porsche color?" As
Sly Stone said, "Different strokes for different folks." YMMV.
Merry motoring, Ed.
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