Spring rates for na8

[leggem]

Hi all
Have an Na 8 fitted with king springs 8 x15 buddy clubs, 205 x50 tyres , prothane bushes, rebuild bilsteins to om specs , 24/17 sway bars.
My problem is the car is hopeless on anything other than motorway surfaces. I live on Central coast of NSW and would like to have a more compliant setup to tour with.
My question is does anyone have reset standard springs in their car. I've thought about having standard springs reset about an inch lower /25 mil to keep same ride height and have a more user friendly car. I've never had standard springs in the car so am unsure if this will not create other problems

[Magpie]

You are looking at OEM spring rates of 1.7 (R) and 2.7(F), hence why you have a 1:0.63 ratio. Therefore 9kg (f) and 6kg (r).

However you have substantially increased the unspung weight of the car. The one thing a MX5 does not like is increased unsprung mass. This will have adverse impacts on handling, as the suspension is trying to deal with more vibration than it was designed for. Just increasing the spring rates will not be the silver bullet.

My suggestion would be since this is a daily car, go back to the OEM setup (including wheels and sway bars) and see what the car is like as designed. You will probably find it a completely different car.

If you want to go down the path you are headed, then do some calcs on the frequency and this will give you an idea of what spring rates you could use.

[Lokiel]

You seem to have a Frankenstein set-up there based on no logic whatsoever.

As Magpie suggested, adding 15x8 rims on a N/A car is a problem because you're just adding unsprung weight, a big negative, for no benefit other than aesthetics.
Going back to a lighter 6.6" or 7" rim will also increase your acceleration (NA8's don't have a lot of power to start with and heavier rims/wheels are harder to rotate) and require less wear on your brakes.

The MSM/SE had the widest rims of any NA/NB, 7", because the factory deemed that wider tyres were necessary for the additional power.
Your tyres are another inch wider on a N/A car which doesn't need the extra grip.

The MSM/SE had the largest sway bars of any NA/NB, 23mm front and 14mm rear, because the factory deemed it necessary for the additional power and stiffer suspension.
The 14mm rear MSM bar is still used by even highly boosted MX5s so your 17 rear inch sway bar is WAY OVERKILL and possibly why the handling is so bad.
Why so big?

When I first got my car, I remember talking to Richard Larsen of MX5 Plus about handling modz - he warned me that it was a very slippery and complex slope because Mazda designed a very light car that handled exactly how they wanted it to handle. Changing any single component would require changing virtually every other component to correct issues introduced by the new mod - and he was right

Suspension and handling is very much a "black art" to most of us and research is your best tool here - do some searches on what combinations other people have and are happy with (you'll need to filter out those who rave about how good their new cheap shocks are after replacing their 25 year-old OEM shocks since ANYTHING will feel better in that scenario). See MT.net for some good suspension tips/combinations - if anyone has suggested a crap combination it will have been shot down mercilessly and if you're lucky, they may say why they're so bad. Contrarily, good suggestions are usually backed up by others. Whatever you do, DON'T simply create a post there unless you include the details of your question/build and include some research you've already done - they don't tolerate newbie's who can't even be bothered to do some research on questions already asked and answered a million times before (the longer I've been on that site and the older I get, the more I agree with that attittude).

If you can afford them, consider MCA Blues shocks and go back to a stock rear sway bar.
MCA can build your shocks to your preferred specifications - you wont just be hoping that your rebuilt bilsteins and king springs are compatible for your needs (and each other).

[manga_blue]

It will drive like a truck with those sway bars. Put the standard ones back on as a starting point.

[ManiacLachy]

Those sway bars and the prothane bushes are the root cause IMO. If you want a comfortable street ride you'll want OEM rubber bushes, or I.L. Motorsports bushes (stiffer than OEM) at most.

When I first got my car, I remember talking to Richard Larsen of MX5 Plus about handling modz - he warned me that it was a very slippery and complex slope because Mazda designed a very light car that handled exactly how they wanted it to handle. Changing any single component would require changing virtually every other component to correct issues introduced by the new mod - and he was right

Man, do I know this! I've changed just about everything in my suspension (control arms, ball joints, tie rods, shocks, bushes, swaybars), many parts more than once. I think the only original piece is my OEM SE rear sway bar. My car is firm, but not harsh. You won't confuse it for a GT or cruiser, but it's not a teeth rattling race car either. However, I have spent way too much to get here.

IMO, for a near stock power fun weekend car the recipe is: 15x7 wheels, OEM/I.L bushes, OEM sway bars (maybe a slightly bigger front bar like the SE's) and refreshed shocks or high quality aftermarkets with a relatively soft spring rate and lot's of shock travel.

[93_Clubman]

Have an Na8 fitted with king springs, rebuilt bilsteins to oem specs, 24/17 sway bars.

Given above, some of following might be of interest if not already seen:

Benny, a former Pres of NSW MX5 Club, spoke with the Bilstein importers about the mismatch between the Clubman shock & spring rates, & was advised Mazda reverted to the stock NA8 spring at the last moment without telling Bilstein. Result was a very firm ride & bounciness over uneven surfaces as the Bilstiens didn't have enough rebound damping.

In the past most forumites have used Eibach or King springs on the Clubman Bilsteins. From old posts Eibach springs seemed to be preferred on Clubman Bilsteins, but some also preferred King springs. Espelir springs (from MX5Plus) have also been used with Bilsteins to excellent effect by tbro: http://mx5cartalk.com/forum/viewtopic.php?f=30&t=30330

Early NA6 164/97 lb/in with 19/12 mm sways
Later NA6 154/94 lb/in with 19/12 mm sways
Std NA8 154/94 lb/in with 19/11 mm sways
NA8 R package 154/94 lb/in with 20/11 mm sways, but that might have been the US version only
All NB 162/118 lb/in with 22/11 mm sways
NB Mazdaspeed 215/158 with 23/14 mm sways (presumably the same on the AU delivered SE?)

Just going on those figures alone the first model NA6s had the sportiest handling (lowest FRC of 58.9%) then they got progressively worse from there until you got a Toyota Corolla-like 66.9% with the stock NB.

PS divide lb/in by 55.88 to convert to Kg/mm

MX5 sway bar sizes with some slight variation:

NA6: 19mm Front & 12mm Rear (except on the 1993 NA6 red LE which were 19 & 11mm - note USDM 1993 NA6 red LE had Bilsteins which ADM didn't so variation may be possible)
NA8: 19mm F & 11mm R (USDM standard Packages: 19mm F & 11mm R)
NA8 Clubman w TorSen: 20mm F & 11mm R (USDM R Package w TorSen: 20mm F & 12mm R)
NB8A: 22mm F & 11mm R
NB8A 10AE w TorSen: 21mm F & 12mm R
NB8B: 22mm F & 12mm R
SP: 22mm F & 12mm R
SE w Torsen: 23mm F & 14mm R
NC1: 21mm F & 12mm R
NC2: 23.5mm F & 11.5mm R