Been doing some searching, Keo pointed me to the Mike K quote, which led me to find some good posts.
From Steve that explains the old method of thinking:
Post 132
Originally Posted by
98sr20ve
I think that the reason you see the 300/200 kits for the b14 is because no one had the nerve back in 1995 to suggest higher rear rates on our cars (not including cars that have leverage type suspensions that need higher rear rates due to leverage). Plus, the rear toe issue was not known at that time to most people. Once they tried the 300/200 for a while people started complaining and the B15 came along. Then the even rates front to back started to get some attention. Keep in mind that these people are not developing kits for autocrosses only. They are looking at road racing and general use as well. On my car with the even rates it is imperative on the high-speed corners to hit them with the gas down. It makes the car fast but most people don't drive like that. On the street it is a non-issue because you don’t drive 100% at 100mph+ on the street. Auto crossing requires a very unique setup depending on the course. A road racing setup will simple not be a great setup for auto crossing. It can be good but not great. I went from full stiff on my rear bar while auto crossing to full soft and I still would have preferred a stiffer rear setup on the autocross. Fortunately it is relatively easy to swap rear springs on B14/15 and I think that is the way to go on a dual-purpose car.
Originally Posted by
DanglingRegarding higher rates in the rear vs front, I asked the same question to smartbomb back in July:
Higher spring rates in the front of our b15s seems to be the consensus when looking at coilovers or other high end pieces. You seem to recommend 700f/600r for a track only car. This mirrors the coilovers that allow daily driving, like the Tein SS default at 336f/280r, and those who recently bought JICs went with 392f/336r.
So why do all b15 stock springs, lowering springs and the NISMO suspension kit have higher rates in the rear? Does this have to do with progressive vs linear springs? Are higher front rates preferred simply because we have a 62f/38r weight distribution?
Here's his response:
Strangely with the stock trailing arm bushings, I run stiffer rear springs as well. The stock bushings are so gushy, they don't allow the rear bar, including the stock one inside the twist beam to couple well. Once you get rid of the squishy rubber, man does the understeer go away. To me, its like a couple hundred in/lbs difference in spring.
So I would run the Tein/JIC rates most likely as a starting point in a car with urethane trailing arm bushings.
The other reason is that when you start getting stiffer and lower, the transition to oversteer gets more dramatic so a rear biased spring rate gets too snappy for all but the most skilled to handle. Not that the RTR World Chalange Spec V has 1200 in/lb rear and about 900 in/lb front springs. Peter Cunningham likes a lot of oversteer, his car is set up so you hardly have to turn the wheel, you just control the car in the turns with throttle.
A softer stock or street car likes more rear to help the car rotate.
Another great post about bushing softness,
#135
Originally Posted by
Dangling
Here's some more info I dug up searching:
I have been building and racing B14's for quite a
while and have never quite been able to get all of the
understeer out of the chassis and to make the car
respond crisply to attempts to get it to rotate. I
have gotten them to where they rotate slowly with
bending the beam and with ridiculose spring and rear
bar rates but they will still have terminal understeer
if driven in a ham fisted manor.
The B14 is also sort of squirmy under trail braking
and turn in and also has a vauge feeling in the rear
while taking a set.
Well I was looking at the trailing arm bushings and
noted that they were made of really gushy rubber and
had large windows cut out in them for additional flex.
I figured that these bushings were allowing the axle
to walk around giving the unsecure feeling in the back
and also decoupling the antisway bar and the torsion
bar of the rear axle.I had Energy make me a set of
urethane solid bushings to replace the rubber.
The results are oh my god! The car is now so
responsive I spun it on the track on my first lap,
just warming up the tires. The car was way too loose,
a first for a B14. After some shock and tire pressure
adjustment, it was a lot better but still quite loose
and I managed to spin in the race when I had to take a
turn off line to avoid spinning cars.
I feel that this will make a big breakthrough in
B14-B15 handling, i just have to resort my car to take
advantage of the better response but reducing
oversteer a little. I am going to soften the rear bar
and perhaps the rear springs.
These bushings are like going up 300 in/lbs in rear
spring rate! They make a bigger difference in chassis
balance than anything I have done so far. They also
eliminate the squirm and weird feeling out back.
Energy should have them for sale soon. As a warning,
although they should be nice on a street car, if you
have an already on the edge tuned race car, its going
to knock you into wild oversteer.
Bushings aside here are a few good quotes which starts making more sense on why coilovers have stiffer fronts.
Source thread
Originally Posted by
RobertspecV
Incidentally, when you lower the car you need more front roll stiffness, either through springs or ARBs. It's long and complicated to explain, but basically the front roll center drops further than the rear rc for the same amount of lowering. This = more roll in the front than rear for a given drop since roll is a function of weight being transferred and the distance between the roll center and center of gravity. This will tend to loose front traction and give you a push all else being equal. This is part of the reason that all lowering kit springs have a higher rear bias in spring rate than stock. Originally Posted by
Dangling
I gotcha, more rear and less front traction means a lot of understeer. This is a good explanation of why stock spring rates and aftermarket lowering springs have higher rear rates.
Interestingly enough, once you start using much higher spring rates all around, add damping adjustability and go from progressive to linear, you begin to notice that the recommended settings of products like high end coilover systems have higher rates up front vs rear, or even identical in one case:
336F/336R Tein Basic
336F/280R Tein SS
392F/280R JIC FLT-A1
392F/280R JIC FLT-A2
There are other factors. What happens when you install the rear beam pieces of the Nismo bushing kit? That increased durometer rubber acts as a higher spring rate, and Kojima himself contends in a older post that the rear on his b14 would come out at will with harder rubber bushings installed in the rear beam. There are many things to consider in suspension tuning that it's hard to keep track of sometimes.
After all of that reading squirlz was on the right track, but not quite there. Lowering springs are meant for people that leave the rest of the car stock, while Coilovers are meant for people that are serious about handling, which means they will have other mods that stiffen the rear up already.
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