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iTrader: (7)

I just got some KSport Kontrol Pro coilovers, front 10(kg), rear 7 spings supplied from the vendor. I have some 8kg springs lying around, and was thinking about swapping the 7s for 8s before I install them. They are exactly the same physical dimensions in terms of coil diameter and height.

I read today that ideally front and rear springs should vary by no more than about 15% in stiffness. (These are true coilovers BTW) I saw an article recently in Modified regarding an RX-8 with 7kg rear springs that had to be swapped to 8s due to bottoming out the suspension on some turns. I could imagine the same thing happening to the Z, plus I carry a tire/tool hauling trailer on the highway. The suspension travel will be kind of short regardless with these coilovers. I know that Tanabe and a couple other companies offer a 10/8 setup.

Considering all of this, is there any good reason to stick with the 7kg springs in the rear and only swap them if I'm dissatisfied? It might help the rear grip better exiting turns, which is a perrenial problem.

Finally, can anyone with KSports offer some initial damper settings to get me started? I will be using them for HPDE and occasional autocross.

jnathan68

iTrader: (29)

I have never heard about the 15% difference in stiffness although I am not a suspension expert so maybe it is ideal to have no more than that. Where did you read that and were they talking about street cars or race cars?

Either way, as you probably know, changing spring rates is one way you to change the characteristics of how the car handles. For example, if your car oversteers, you could change the rear or front spring rate to reduce the oversteer as you are thinking.

So, to answer your question about changing to the 8k spring, it is fine to do so but since you don't know if it will change the handling for the better of for the worse, might want to start with the 7's and see how it handles.

Some rules I do know are:

1) You really shouldn't go more than 2k up or down in spring rate without having the struts revalved.

2) If you do change the rate of the spring, you can (and sometimes should) change the length of the spring to compensate.

3) If you are not sure of a change, call the suspension company and talk to their tech people.

terrasmak

iTrader: (8)

7's will be roughly the same as your front 10's. Typically a z with 10's up front and true coilovers will run a 5 in the rear. The 5 mounted in the outer location vs the stock location is equal to an 8 giving you the normal 10f 8r stagger we typically run.

INTIMAZY

iTrader: (15)

Many high end companies (arguably the best) use a 7k rear.

I personally have no issues with my 10k/7k Cusco setup

12AutoX

iTrader: (7)

http://www.racing-car-technology.com...takessetup.htm

I suppose I can try the 7s. My only concern is that the KSport dampers have 2 7/8" of travel in the rear when uncompressed. That works out to 794 lb/in on the rear axle, and about 1550 lbs static weight on that axle. So if the static weight compresses the springs 2", there would be less than an inch of suspension travel. Can that be right? If I run 8s, That's 900 lb/in compressed 1.72", leaving 1.15" travel.

I'm pretty certain my math is off due to the geometry. The mounting point is at least 6" inbound of the tire. The stock spring is further inbound, but not much. I'm trying to think of it by visualizing a nut cracker having more crushing power near the fulcrum. That means you need less pressure to crack the nut as it approaches the fulcrum, and my pickup point is inbound quite a ways from the wheel hub.

If it's good enough for Cusco, I guess somebody sorted it out! I would like to know the formula though, just for the geek factor. I will have to measure the static compression once I get them installed.

Zazz93

iTrader: (2)

What is the spring bind point on the inboard mounting position vs the on shock position. I'm guessing the spring binds sooner on the shock.

Zazz93

iTrader: (2)

I believe the stock Z's motion ratio is in the .6 range (the amount of shock travel for a given amount of wheel travel) so if that is 2.875 inches of spring travel you should, ideally, have about 4.79 inches of wheel travel. However, to translate that into spring rate to static weight movement you would have to calculate the new (as in true coilover) fulcrum effect. Like Terrasmak says the inbound position should yield a softer effective spring rate in comparision to the outbound position. This is partially what jnathan68 was referring to when he suggested a longer spring. This is why I thought there may be too many calculations involved with changing to that style of coilover. However, I only have a basic understanding of these dynamics so I'm interested in seeing what the "Gurus" say.

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iTrader: (7)

I installed the 10's and 7's, and took the car out for a drive around the block. Maybe the rear dampers were set too soft, but it felt like there was a Cavalier welded to the back half of the car. I swapped in the 8's and let everything settle overnight. I stiffened the rear dampers and took it out again, and it felt really pulled together.

This morning I adjusted the ride height, did a string alignment, and went for a test drive. The car feels great- very little dive and squat, quick transitions, relatively sharp response. I need to make some minor adjustments to get a little toe out in the front, but it was a great weekend. Thank God for perscription strength Ibuprofen. I'll have to get the corner balancing done in the near future. I really had doubts about KSport, but for now I'm a believer. The proof is in the pudding- the next track day is 10/15, so we'll see if my times improve.

jnathan68

iTrader: (29)

Good to hear. Let us know how it goes.

terrasmak

iTrader: (8)

I would also like to know. I have a feeling its will be oversteer city with traction problem on corner exit.

12AutoX

iTrader: (7)

Hopefully I will have a vid up here shortly. First off, the 8kg springs in the rear did not induce oversteer. I think my tires are about shot, but I didn't get any more rear step-out than before at the track. Lateral holding power (midcorner) was improved. Turn in improved. I also set my Cusco sway bar from the soft setting to hard, and met no additional understeer. On smooth track, the car felt more stable and planted.

However, I found out that my softer suspension had been masking some undulations on the third-longest straight. It's actually an increasing radius turn, but it is a full throttle affair involving a third to fourth upshift. My head hit the ceiling despite my harness and the car bounced in a way that prevented me from keeping on the throttle or judging my braking point accurately. I think that section slowed me down. I also had difficulty judging the best entry speed into the big carousel leading into a straight. That will just take some getting used to and confidence-building. Overall, I like the KSports a lot, but have to figure out how to set the dampers for a bumpy course. Next time out I will try softening them a little. My lap time was not improved overall, and actually increased by about .5 seconds. I think it will improve on a smoother track though.