Topic: Input Requested -- Coilover Spring Rates

[Ben L]

Title says it all.  Car pushes like a beeeatch. 

Its also riding a tad high on the Star Specs. 

The mind races to one possible solution to both problems -- coilovers!!!  Looking at BC Racing's SAs for the Kappa as an economical alternative to the nicey but pricey KW ones.

The BCs come with 7k spring rates, front and rear.  Usually, in competition apps., the rate on the fronts is higher than the rears.

All you race suspension fountains of wisdom, am I crazy to try dialing out understeer by going with softer springs up front, and leaving the stock GXP front bar alone?

Because I want pillowball upper mounts, its a special order anyway.  The Circuit Motorsports guy said if I wrote and posted a review, he spring them how I wanted for the same price.  Which comes to about $250 per corner.

Like maybe 8k fronts and 10k rears?  Snap On?  Anyone?

[Ben L]

Nobody??     Bueller?   

[DeepBlueGXP]

DDM has a coilover that they are selling, I'll send Dave a PM to respond...

[Yamatr3]

+1 on what Deep said.  I would talk to Dave at DDM, he is very good at setting up suspension.

[Chemist]

 

+1 on what Deep said.  I would talk to Dave at DDM, he is very good at setting up suspension.

+1   

Dave's alignment on my GMPP suspension is primo! 

[snaponbob]

Title says it all.  Car pushes like a beeeatch. 

Its also riding a tad high on the Star Specs. 

The mind races to one possible solution to both problems -- coilovers!!!  Looking at BC Racing's SAs for the Kappa as an economical alternative to the nicey but pricey KW ones.

The BCs come with 7k spring rates, front and rear.  Usually, in competition apps., the rate on the fronts is higher than the rears.

All you race suspension fountains of wisdom, am I crazy to try dialing out understeer by going with softer springs up front, and leaving the stock GXP front bar alone?

Because I want pillowball upper mounts, its a special order anyway.  The Circuit Motorsports guy said if I wrote and posted a review, he spring them how I wanted for the same price.  Which comes to about $250 per corner.

Like maybe 8k fronts and 10k rears?  Snap On?  Anyone?

Confusing about waht 7K, etc., is all about. We already have "coil overs". It si important to understand that GM has a weird set up on the Kappas from the afctory. The GXP/RL has 175 fronts and 220 rears. BACKWARDS from most front engine rear drive cars. Send me a PM, we'll talk. 

[MGar]

I think part of the reason for the confusing setup is due to the difference in travel.  The rear shock/spring combo has significantly less travel in it than the front, so it makes sense that the rear would have an increased rate, as to compensate so it does not hit the bump stops in the rear.

[Ben L]

I would certainly get DDM's KWs if I could afford the nearly $2k it would cost.  But I do not feel right picking Dave's brain when considering someone else's product

I definitely prefer to buy from our vendors here whenever possible.

But the BC BRs I am looking at are at a price point almost half of the KWs offered by DDM.  The BCs are quite popular and well reviewed on other platforms, and a couple of BB folks have them installed on Kappas.  They seem like an option at least worth considering.

I was confused by the "7k" rate I was given, too.  I think it may be metric -- 7kg per mm of travel??

I understand our stock suspensions come with coil springs over Bilstein shocks, but what I meant by coilovers was the adjustable perch, spherical bearing top, type.   Interesting that the Kappa has lower rate fronts.  As Snapon confirms, usually we see stiffer fronts on a RWD sports car.   I may be on to something after all . . .  Snapon, I will PM you.  I understand you brewed up some coilovers by revalving Koni Sports, and selecting appropriate springs.

Alignment is another story.  Re-alignment, and also corner weighting, I consider absolutely necessary if I switch the shocks and springs and drop the ride height the inch or so it needs now.  Most places quote a flat rate for a four wheel alignment (about $100+/- locally) and 2-3 shop hours for corner weighting.

That adds a couple hundred more $$$ to the switch, which also has to be factored in to the budget impact.

[Dave@DDMworks]

Feel free to pick our brains anytime That is why we are here 

As for the spring rates, that is really a tough answer to give you without seeing the shock dyno's, knowing what alignment specs you are running, what kind of driving you are going to be doing, etc. There are a lot of factors that can affect how the car handles other than the spring rates. Other front engine rear wheel drive cars we usually setup do like a higher rate spring in the front with a softer spring in the back. The Kappa's seem to favor a little more even setup front to back, so 7kg all around does not sound crazy. I am not sure what Bob ended up with on his car at this point, he was trying a bunch of different rates on his car, and I believe he ended up with a little higher rate in the back than the front, but do not remember the exact specs he ended up with.

Hope that helps a little and we will see what Bob has to say also.

Thanks,
Dave

[Ben L]

You're a good man, Dave.

I think Bob ended up with something like 175 lb front and 225 lb rear.  He is running Z0K GXP springs over revalved Koni Sports. 

Dunno what the metric conversion would be, but this validates my sense that we need to bias spring rates toward stiffer rears, the opposite of what usually is the case with RWD front engined cars.

Seriously considering going the Koni Sports/Z0K FE3 spring route as an alternative to the BC BR coilovers.  If I go that route, I will get the Konis from you guys!!

Completing the FFR coupe build is sucking up all my $$$ at the moment, so it may be a couple months.

[snaponbob]

You're a good man, Dave.

I think Bob ended up with something like 175 lb front and 225 lb rear.  He is running Z0K GXP springs over revalved Koni Sports. 

Dunno what the metric conversion would be, but this validates my sense that we need to bias spring rates toward stiffer rears, the opposite of what usually is the case with RWD front engined cars.

Seriously considering going the Koni Sports/Z0K FE3 spring route as an alternative to the BC BR coilovers.  If I go that route, I will get the Konis from you guys!!

Completing the FFR coupe build is sucking up all my $$$ at the moment, so it may be a couple months.

I installed Koni SA's as soon as they came out in the US and am pretty sure I had the first retail SA's sold. So that would be late August 2007. In September 2008 I had the compression valving redone by the Koni guys at the Solo Nationals. When I went to adjustable spring perches in February they were simply sleeves heat shrink installed on the Koni bodies. At that time the STOCK FE3 springs (175 front and 220 rear) came off and the experiments started from there. Given how I wanted the car to feel I roughly doubled the rates. Polyurethane bushings went in all the control arms (ooo, THAT was fun) and I have a DDM ProBeam. Believe it or not, after having the entire suspension out and back in the car I only had to adjust the toe at the right rear. Everything else was spot on where it was when I took it apart. No alignment needed!! As for corner weighting, all I did was adjust the spring perches to set the car where I wanted it (for ride height), took it to a local shop that has the local SCCA Regions scales and it was with 4-6 pounds of perfect at the four corners. NO adjustments needed !!! The front/rear was 51.8%/48.2% and the side to side delta was 4 pounds rear and 6 pounds front. Too close to play with.

I will caution that assumptions and conventional wisdom was WRONG when I started throwing springs at the car. The front/rear difference in rates is LESS than stock and that was a huge surprise. With a 32mm solid front bar and FE3 rear bar the car is nearly dead level in turns, sinfully neutral in most modes (still a bit pushy in REAL tight turns, and that can be resolved with technique) but is loose under power. FANTASTIC. NOBODY could believe what this can can turn in to (except, maybe, Alex Jones -AKA Dr Jones with his B/SP GXP). Slaloms like a snake at crazy speeds. This car is NOTHING like what it started as.

HTH.