*84vvt wrote:Happy to help out
I might have to catch a ride in the Said Red Beast if the track day rules allow .
Absolutely!
Now getting back to suspension for a minute...
I fitted the BC Gold coil-overs recently and have found that I've had to raise the ride height a fair bit to prevent the rear tyres from hitting the guards if I hit a bad bump (pothole etc) during hard cornering, especially if putting the hammer down (which makes the back end squat down a fair bit).
Contrast with the KO coil-overs which were hard as hell, could ride a lot lower, corner flatter and faster, and not have the tyres hit the guards. And not squat when under WOT. Heck if I sat on a front guard, there was literally NO noticeable drop...!
So, spring rates.
Kei Office (KO) were 8k Front, 14k Rear.
BC Gold standard are 5k Front, 8k Rear.
Let's compare the two types of coil-over, mathematically.
Firstly it's important to remember that stiffer rear suspension = more oversteer, softer rear suspension = more understeer. Try fitting an SW rear sway bar to an AW11 and you'll see what I mean.
People who design suspension to be "safe" lean towards an understeer setup whereas people who design suspension for things like serious sports cars or for racing (responsive handling) lean towards oversteer.
Kei Office coil-overs were designed for track use. The ratio of spring stiffness front to rear shows this is the case. An 8:14 ratio = 1:1.75
BC Gold coil-overs are designed as a mass market swap-in for road cars. They can help you on a track but their target market isn't the same as Kei Office. The ratio of spring stiffness is 5:8 = 1:1.6 so you have less responsive steering and less chance of oversteer.
Hence the SW has felt wallowy and fat to me since fitting the BC's. Hasn't helped my confidence while driving it on twisty back roads. It used to feel utterly planted and very precise. Albeit rather harsh.
Now let's look at the overall stiffness of the springs and what that means.
Comparing the front springs we have 8k for the KO versus 5k for the BC.
5 / 8 = 0.625 which means the front springs on the BC are 62.5% the stiffness of the KO front springs. A lot softer eh.
The rear springs are even more interesting. 14k for the KO versus 8k for the BC.
8 / 14 = 0.571 which means the BC rears are only 57.1% the stiffness of the KO. Waaaaaaaay softer...!
Thanks to the far softer rear springs the back end squats down a lot now under wide open throttle (WOT) whereas it didn't squat at all with the KO. This means the rear ride height has had to be raised significantly. And that makes the car feel fat, not planted when cornering.
It also means there's way more "body roll" so no wonder the rear tyres can tap the guards if I hit a bump while cornering hard...
I can firm up the shocks to reduce the chance of tyres hitting guards but at the end of the day what the car really needs is a set of stiffer springs. Firming up the shocks doesn't prevent the car leaning over on a corner, compressing the springs on one side. Firming up the shocks doesn't prevent the rear end squatting right down when you plant your foot and lay down 500 rwhp. And firming up the shocks doesn't help the wheels get back to the ground if the bumps have thrown them up off the deck. Firmer shocks return to Earth slower so they don't help put down the power on a bumpy road.
What might be ideal then? One option would be to leave the soft BC setup as is, with its overly-high ride height at the rear, and fit heavier sway bars. Then I could soften the shocks a little, achieve a nice comfortable ride, put the power down well on a bumpy road, and corner pretty fast too.
However because the thing has 18" rims I've had to raise the back end so much that it now rides about an inch (2.5cm) higher than a standard SW...! That's measured from the ground to the rear cross-member. Still looks ok thanks to the big rims but actually it feels wrong in the corners. Simply doesn't feel planted like it used to. Too high. Doesn't fill me with confidence. So I need to drop it and that means stiffer springs.
Did the maths on various combinations.
8:14 = 1:1.75 Front:Rear ratio for the KO coil-overs.
5:8 = 1:1.6 currently. Standard BC Gold springs for an SW.
5:10 = 1:2 = oversteer to the max and stuff-all gain in terms of firmness on the rear, so ride height would still be way too much.
6:10 = 1:1.66 = reasonable turn-in but still too soft for this car, rear ride height would still be too much.
6:12 = 1:2 = oversteer to the max but the rear ride height would be much better.
7:12 = 1:1.71 = almost the same steering balance as with the KO, which felt excellent in that regard.
In fact 1.71 / 1.75 = 0.977 so the turn-in and handling precision would be about 98% as good as the KO setup was. That's assuming I get other things like camber and toe-in set up as well as can be too of course. Yep, looks like a 7k front and 12k rear spring rate might just be the go.
When I compare the stiffness of a custom 7k BC front spring to the KO 8k one, 7 / 8 = 0.875 so it's 87.5% as stiff as the KO.
Comparing a custom 12k BC rear spring with a 14k KO one, 12 / 14 = 0.857 so that's 85.7% as stiff as the KO.
I reckon if I fit 7k front and 12k rear springs to the BC Golds I will be able to drop the ride again, almost as low as the KO coil-overs allowed, making for a much more planted car. And because the front-rear spring rate ratio is basically the same as the KO, it should mean the car responds very much like it used to. What I'll end up with is a teeny bit softer ride, and shocks that are far more adjustable than the KO units (32 steps of adjustment for the BC versus 4 steps with the KO).
For less than $300 that sounds like a no-brainer to me.
So I've ordered a set of springs and because the 7k front ones are not the usual thing here in NZ I'll be waiting a couple of months for them to turn up. But that's fine. It should still allow me a couple of months to swap them in, mess around with ride height, camber and toe-in again and get the thing working well for the long bumpy squiggly trip down to Fielding and the manic blitzfest around Manfield race track when we get there.
Very much looking forward to it...!
Gives me time to sort out that engine lid heat-shield...
And rebuild the AW's rear suspension perhaps. And make it new engine mount inserts. And replace its naffed muffler. Maybe.
Cars. What they can't do to us eh?
Unless you drive a Micra or Prius, or are in charge of the road rules and looking to drop the speed limits instead of preventing the sale of crap tyres, in which case you likely don't give a rat's...
Meanwhile the SW seems perfectly drivable now even if it does feel fat and wallowy to me.
Probably feels planted as hell to anyone else, LOL.
Guess I've been driving lowered AW's far too long eh?
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I am very glad you still own the thing anyway mate. Some things can never be replaced eh. Especially when you've invested so much time and ingenuity.