I. What they are and why they are needed:
Last year I started a thread when I started playing around with tender springs and experimenting with my first dual spring coilover system. In that thread I wrote the following:
For all of you out there with upgraded Koni Yellow shocks and stiffer coil over springs. Have you ever thought,
“Man I like how the car handles the twisties with the upgraded springs, but man this thing just feels way too primitive on the streets, I mean it handles the potholes, bumps, and ruts like an old wild west chuck wagon. I mean, it’s a Porsche for crying out loud, shouldn’t it be a little more civilized?”
Well, maybe that’s a little exaggerated, but not by much. Anyway, that’s how I felt after driving around these old Napa Valley back roads with upgraded #350 front springs for over a year now. The ride was not horrible, but in comparison to my Mercedes Benz commute car, I was simply not satisfied with how primitive the 951 suspension felt.
Since I had already spent money on Koni Yellows, I decided to make to most of what I already had and improve the suspension system. Although I am using Koni Yellows as a starting point, any adjustable strut should do just fine.
This year's design has been more than satisfying…in fact, for a mere $272
it is down right mind blowing awesome!
II. What can it do? – Street - Rain - Race track
Well, I am so impressed with this set-up that think I’m going to call it the “Wolf in sheep’s clothing”
suspension set-up. It has surpasses all my expectations!!!
In street mode. The car ride is very comfortable, the suspension is extremely compliant over potholes, street ruts, and road bumps. It basically feels as comfortable as your stock 951 suspension. Except that, on simulated back road twisties, the car has practically no simulated body roll. No matter how hard it is tossed into the corners, the suspension provides a very stable platform, yet compliant to all road irregularities, the best feeling in the world.
For this set-up to work properly it is necessary to use a stiff enough tender spring that is NOT fully collapsed under the car’s normal weight. I am using a 4” free length 350# tender which incidently still has 5/8” of travel available under its installed height of 2.0 inches. This is just enough to act as a “shock absorber” and soften the impact to the main 550# spring whenever road irregularities are encountered. However, after that initial slack is taken up, it’s all up to the 550# main spring to resist the effects of weight transfer and hold body roll at bay.
Now the beauty of a dual spring set-up is that under the softer initial “combo spring rate”, the damper settings can be set as soft as desired
. This is one of the major benefits that helps do the trick. Damper rates can be set as soft as ZERO rebound adjustment turns and the Koni’s damping forces are still good enough to provide critical damping
for this spring package.
A stiff single spring system cannot be tamed like this. I tried setting ZERO rebound when I had my suspension mocked-up using only the 550# main spring, and yes the ride quality was good on potholes, but after a number of potholes the car was bouncing up and down the road like a pogo stick. At soft rebound settings the Koni’s damping cannot control the stiff 550# spring. Also, going over bumps, even with zero bump setting, the oomph!!! transmitted to the driver and passenger, is simply unacceptable. Increase the damper stiffness to control the 550# springs, and now we are back to a primitive suspension that is not comfortable any longer.
Rain, rain, rain…. We have had lots of rain here in the valley, especially in month of February. This gave me a great opportunity to take the car out and see how the suspension dealt with slippery roads. I have only two words to say about it - Confidence Inspiring
. Being able to set the damper settings at very soft settings greatly improves the cars wet traction performance (having new Fuzion ZRi’s doesn’t hurt either).
“Race Mode”. A second major design parameter for this system was to design a suspension that did not bottom out during heavy cornering at race track speeds. Ever since I learned here on Rennlist that a number Rennlisters had discovered that their 951’s were riding on the bump stops while heavy cornering (especially lowered cars with sticky tires) at race track events, I decided to experiment with my car and see for myself if my car was bottoming out also. It was very simple to do. I simply placed a zip tie on the strut shaft went for a few laps around Infineon race track avoiding rough pavement and avoiding the apex and trackout berms. When I came back, I removed the wheel and looked for the zip tie. It had been pushed up way into the bump stop. Well I was very disappointed to say the least.
However, this gave me the motivation to undertake this winter design project. I started out with a clean sheet of paper and decided to go back to basic physics and figure out what forces where at play here, and what would be the minimum spring stiffness necessary to keep the suspension working freely - unobstructed. All spring math calculations, measurements, and estimates will follow shortly in a follow up post.
Slapped on some Toyo RA-1’s, Hawk HT-10 racing pads, set camber close to – 3 deg., set 1/16” toe out, Koni’s to 1 ½ turns firm, and headed to Infineon Raceway just 20 minutes away.
I went to the NASA DE at Infineon Raceway on March 21, 2009. The car was so easy to drive, it put a big grin on my face the entire day. Turn-in response has been greatly improved. The car turns in so obediently and does not roll much even under heavy trail braking scenarios. What was most apparent was how well it behaved at this track’s signature section, the Infineon esses, an absolute blast, again this dual spring suspension set-up has exceeded all my expectations.
One would think that with 550# front main springs and mere 28 mm rear torsion bars, the car would push like a pig, but NOT so!!!! The esses section is a great place to evaluate the car’s turn-in response as one modulates the throttle, and the hairpin turn at the end of the back straight a great place to evaluate the degree of inherent understeer.
Braking Zone. Go up to 120+ mph on the back straight on the approach to Turn #11 (the hairpin) and stomp on the brakes. The brakes are much easier to modulate over the undulating payment. Last year with the 350# springs there were some spots where the car used to bottom out under this heavy braking condition, the screech of tires, and flat spots soon followed as the tires searched for grip over the bumps.
This is a favorite passing zone for the really fast guys. This braking zone has to be respected the most, however, because of the proximity of outer a wall and the lack of run off area. However, lots of braking battles occur here and the only winners seem to be the flatbed tow trucks, they seem to conduct a lot of business at Turn 11.
I was also pleasantly surprised how well I could navigate through the downhill Turn #4. This turn has in the past given me problems, because with a downhill braking zone, and a greater than 90 degree off-camber right turn, the front outside spring would tend bottom out at turn-in and the car would tend to “push” no matter how much trail braking I dared to dial in - a tad too much trail braking and get ready for the spin!!! NOW, with the 550# main springs, what a difference, I like Turn 4. Easy as pie.
In summary, this suspension set up is perfect for a heavy street car, i.e. one that sees dual purpose Street/DE car.
The only thing I didn’t get to do on 3/21/09, was get a good clean lap in order to compare to last year’s 2:01 times. This was because the track was so crowded. NASA combined the Time Trialers (TT’s) with the HPDE-4 group into one session, with so many cars on track at the same time, there was just too much traffic, but on the upside, it gave me a good opportunity to practice passing. YMMV
Baseline: Koni Yellow struts, #350 x 10” front springs, and adjustable ride height kit. Not a good street/track compromise.
So the revised suspension.
Upgrade #2: dual spring package (2 ½” ID springs):
Hypercoil 8” x 550# main spring
Hypercoil Stacked Spring Divider,
Hypercoil 4” x 350# (tender spring)
2 550# Main Springs = HYPCO8-550 = $108
2 350# Tender Springs = HYPCO4-350= $108
2 Spring Dividers = HY-18DS100 = $56
Sub Total = $272
Upgrade #3 (Although not necessary, but for the more ambitious in the crowd, one can also convert to Koni Dual Adjustable Race Struts)
Koni Race Yellows dual adjustable #8611-1259
2 Koni Race Struts = KON-86111259RACE = $620
Sub Total = $892.00
2.5” ID Dual Spring Combo (4” 350#Tender + 8” 550#Main)
The combined spring rate constant is calculated as follows: 1/Cf = 1/C1 + 1/C2
Initial combo rate = 214 lb/in.
In the picture below, you can see the adjustable height aluminum sleeve after being cut XX” (Do as I say, not as I do. Measure twice, cut only once.)
It is necessary to cut the threaded sleeve so that the spring separator (slider) can slide up and down above the spring perch on the smooth portion of the strut tube.
I highly recommend that suspension be mocked-up, wrap tape around the threaded collar to protect the threads (since one cannot use the spring slider at this point), assemble the dual spring combo and drive around for a week or two to let the suspension settle. Then take final measurements and cut the necessary threads off the aluminum threaded collar.
Depicted in the middle. Hypercoil spring Divider/slider HY-18DS100.
All the parts needed:
The assembled package - at full droop. The scratches seen is where the plastic slider is rubbing off the black painted tube.
Coming Soon, in a few days….
Section III. How to Design your own Dual Spring Rate Coilover suspension
(all measurements, specs, formulas, and estimates) - See post #24.
Section IV, "Lowering the car to improve cornering potential”
- See post #35
Section V. Converting Single Adjustable Koni Yellows to Dual Adjustable (Bump and Rebound) Race Koni’s
- see post #37
This has been a really fun project.