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I'm looking at some springs for my car.

They are 6kg front, 4.5kg rear.

Ilm not sure on the factory rates so I can't tell if these are a bit stiffer, or spine breakers.

Any thoughts?

Fatcat motorsports quote 99 model as 162lb fr and 118lb rear so this would equate to 2.9kg front and 2.1 rear..

So your 6.0/4.5 would be a bit over twice as stiff as std..
Whether that is too stiff is highly subjective, and more dependant on what you like..

fwiw, I have 7f/5r in my track car (default was 7/6) and while great on the track I find these very uncomfortable on the road..

I've got 5/4 on mine and find them good enough for an 800km drive around the coast. But as Rascal said, it's all subjective.

The 7/6 rate common on Teins actually has a bad ratio of front to rear bounce rates that makes it more uncomfortable than you'd get from just stiff springing. Your 6/4.5 should be inherently better. It's best to stick around Mazda's original design of 1.37:1 for F/R spring rate ratios.

Hmm, sounds like they might be a bit stuff.
Especially on the factory shocks.

Hmm, I don't mind stiff springs, but I imagine These might be a bit bouncy then?

Got anyone local that have BC's installed?

They come with 8/6 spring rates as standard, as do the D2/G4/K-sport coils.

I think they'll be alright on the standard shocks.

6.0/4.5 you'll be fine. you'll still find them stiffer than the stock springs. but they ain't so bone jarring as higher rates.

if you are using the old stock shocks, you might wanna rethink the spring/shock combo.

Hi hks_kansei,

Comfort depends on suspension frequency, the number of oscillations per minute that the car will bounce at when the suspension is worked. The frequency of the average family sedan is of the order of 80 cycles per minute. For sporty cars this increases to around 100 cpm and for track cars it is 140 cpm plus.

Suspension frequency is proportional to the square root of spring rate, so if you want to double the suspension frequency you need four times the spring rate. If you double the spring rate the frequency will increase by about 40%. I think you will find 6/4.5 kg/mm springs acceptable. I run BCs with 8/6 kgm/mm springs, and as an old Austin Healy Sprite tragic, I don't find them too firm but my mum does. But then her car wallows around like a Manly ferry.

However, as has been already been mentioned, shock rates need to be matched to spring rates. If you want to firm your ride then maybe you just need new shocks. Most cars are under damped, MX5 included, especially if the shocks have already done high kms, so you could probably use firmer shocks with the standard springs. Recall, it was Collin Chapman who first proposed the use of soft springs/firm shocks for better handling. One of the problems with the MX5 is limited rear suspension travel which permits the suspension to hit the bump stops in normal driving situations. This effectively gives a substantially increased spring rate, so increasing the rear spring rate will reduce the tendency of the rear to bottom out so you may find the car handles the bigger road bumps more comfortably than it currently does.