New coilovers with confusing spring rates

[Matt-23]

Hello all,

Recently purchased a set of custom made coilovers from a company here in Australia, the spring rate they sent me is 9kg/mm (500lb/in) front and 10kg/mm (560lb/in) rear (I did ask to be set up for roll racing / drag racing). I've had to add 50mm pre-load to get the rear wheels to sit out of the wheel well, I can't get the fronts to sit right with 0 preload. The car drives well, it knocked 0.5sec off 0-100km/h however it does have some lift off oversteer which is a little un-nerving. I have run the company and asked and they have said they have set up a few Mustangs with this set up. It's $50 for new springs so I'm thinking of going a 5kg/mm (280lb/in) front and 14kg/mm (784lb/in) rear which is their heaviest spring. Any assistance would be appreciated.

Thanks

Matt

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[Bluemustang]

I assume this is a divorced rear spring? If so, 560 lb. rear is even softer than the stock GT PP rate.

 

[Matt-23]

Hi Bluemustang I'm not too sure what you mean by divorced rear spring but I'll attach a photo to help

 

[SteveW]

That's a really soft rear spring rate and perhaps that along with shocks with too much rebound for that rate is where the lift off oversteer is coming from. Just my guess. I have no idea what a proper drag setup looks like so not going to comment on what you should do for rear springs if the focus is on drag racing.

When you get front rates above ~400 lbs-in on the front struts you will not be able to have preload and a normal or lowered ride height. It is common to add small helper springs and a coupler to keep the main spring under tension at full droop.

There does appear to be one of those helper springs and couplers in place on top of your rear springs in the picture you posted.

 

[BmacIL]

It seems like a lot of the aftermarket coilovers that aren't motorsports-focused have ridiculously soft rear springs, probably for ride at the expense of everything else. Those rears you posted are ~25% softer than stock GT PP rear springs, and ~17% softer than base GT springs. I know the BC/Pedders can work fine when paired with a significantly stiffer rear spring via the Mike Maier adjusters. @strengthrehab has them with 1300 lb rears and the stock 391 lb/in fronts.

With the rears being so soft, the weight transfer will be so much and so quick that you'll lose overall tire grip. The rear springs will have more difficulty than stock with overcoming the sway bar, as well as the huge bushing rate for the rear LCA. You're likely barely loading the inside rear tire.

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