Deciding on Spring Rates for Coilovers.

[shaunwil]

My coil-overs are out for refurbish and I'm looking at making them much more road focused.

My current coil-overs are an ohlin damper sold by cusco about 15yrs ago. The have 15 & 14 kg/mm springs . The chappy refurbishing can valve them to suit any spring within reason. They are just struggling getting the bodies plated at the minute.

I'm considering matching the mazdaspeed springs with 7&5 kg/mm, but also considering 8&6 kg/mm.

I'm running racing beat antiroll bars and want the car to be more road friendly.

There is so much info out there its quite hard to choose.

 

[JoeLanger]

My HSDs are 9/10 they're still stiff but not excessive IMO

And if you're swapping the springs, I wouldn't mind the scrap!

 

[shaunwil]

I should get the springs back when I pick them up, they are rock hard. Not sure what grade steel they will be (been watching a lot of Forged in Fire & Alex Steel).

I think at the minute I'm airing towards the 7/5, but depends on the result on the dyno when they are back.

 

[Ceylon]

I originally intended to go 10k front and 8k rear. But after trying out the ohlins at 11k all round I'm really quite happy with this spring rate so didn't bother changing. My car is pretty much solely used on the roads.

 

[jdmr_neil]

My meisters are 10/9, i think 10/8 is standard but it all depends on which model.

If your planning a road car i would 100% say your plan of 8/6 is bang on the money.

Weekend track car 10/8

Track legend doesn't really need advice but north of that. Super lap stars are around 22/18 but that would make a truly awful street car.

 

[shaunwil]

Its mental to think just how many variations of spring rates are out there. There is everything from 5kg/mm to 24kg/mm !

The stock springs are progressive - First front inch is 4.7kg/mm, 2nd inch is 9.4kg/mm, 3rd inch is 14.1kg/mm (from https://www.rx7club.com/suspension-wheels-tires-brakes-20/howard-colemans-fd-chassis-setup-723617/ ) Rears 1st inch are 3.5kg/mm - no info for the rest.

I do remember stock springs to be way too light for my liking, even with the uprated anti-roll bars.

Maybe 8/6 would be a good option.......................

 

[shaunwil]

I think the really high rates are made for slicks and trackdays. They were on the heavy side on mine, too much I would say.

 

[Tangelos]

My HKS coilovers are 8/8 and i absolutely love them for a road car

 

[shaunwil]

My HKS coilovers are 8/8 and i absolutely love them for a road car

How does the car feel balanced ? Read somewhere the Japs liked the fronts harder than the rears to induce a bit of under-steer first.

 

[Tangelos]

The car does feel Pretty balanced, and I was surprised they were 8s because I’ve seen most Japanese coilovers running pretty high spring rates. If I put it all the way on its softest setting (they are 30 way adjustable) then the car feels bouncy but it also feels like you’re riding on a cloud. I just need better tyres

 

[shaunwil]

I think I'm going to go for 8/6 and see how I go, can always swap the rears out to 8kg at a later date.

 

[Nas80]

10/8 is able to easily do bumpy B roads on my car.

 

[marco]

Who is doing the revalving? I might need mine doing as they are set up for some pretty high spring rates

 

[shaunwil]

They are at cornering force in Harrogate at the minute. http://www.corneringforce.com/?page_id=24

 

[BenLewisMAV]

The different spring rates front and rear can also be due to the motion ratio being different from front to rear..

For every 1mm of ride height change, the springs don't compress or extend by exactly 1mm - how much less varies with the design of the suspension geometry and resulting kinematic curves.
(A large movement at the wheel translates into a smaller movement at the spring.. )

For an FD the ratio is something like
0.6 for the Fronts
0.68 for the Rear
(e.g. Rear: 10mm @ wheel = 6.8mm @ spring)

As a result, the bit you really care about is what impact the spring rate is having at the wheel and tyre (called the wheel rate). That, along with the damper and ARB, can adjust how much the wheel moves during impacts or the body rolls during cornering.

In the Front, an 8KG/mm spring rate will roughly have 2.84KG/mm wheel rate

In the Rear, An 8KG/mm spring rate is around 3.64KG/mm wheel rate

Alternately, A 6KG/mm spring rate in the Rear is around 2.73KG/mm wheel rate

So you see that 8KG Front & 6KG Rear, actually requires a similar amount of force to move the wheel closer to the body (~27-28KG for 10mm compression).
An 8KG/mm spring BOTH front and rear means that the rear end will be around 22% stiffer than the front. This is likely to promote oversteer during cornering - although in reality this is a little more complicated due to other factors like the anti roll bar - which adds a ton more spring rate the more the body rolls during cornering anyway.. (roll stiffness)

For reference: Wheel rate = Spring rate * (motionratio^2) * suspension angle [use .985]

As a handy aside, you can also use this (installation) ratio to speed up setting the ride height of your car. Once it has initially settled (this will normally take a few days) you can measure how much you want to raise or lower the car by, then use the ratio to see how much you need to adjust it on the coilover itself.

 

[shaunwil]

Awesome, everyday is a school day

 

[MILLION]

I think I'm going to go for 8/6 and see how I go, can always swap the rears out to 8kg at a later date.

I bought a load of springs to try out with my Tein's. I settled on 8/6. For what it's worth, i'm really happy with it

 

[jandesousa]

My two pence worth ….
I'm running 8/6 as well on BC's and really happy with them. Still stiff enough and improved so much over my old Teins which was rock solid