What springs should I get?

Tumbles · Postby **Tumbles** » Sat Sep 19, 2015 1:25 am

I'm thinking about having a F/R 6/4kg setup with KYB AGX or Koni shocks. I've gotten one pair of shocks cheap right now, which is why I decided against coilovers. (Cost.)

Not sure on spring rates for each spring offered and not sure what springs are considered okay? I saw an old thread which had some spring rates but that was like 4 years old and I'm not sure if it changed. Also something about progressive rears? (I get that it's a piece spiral of metal, but yeah.)

Ideally I want something slightly firmer that's going to give me less body roll under braking and cornering.

I'll be running PS3 tyres with 1.8-2 deg camber in front, 1.5 in the rear, 0 toe front, 0.2 toe in rear and max identical caster I can run on the front. I stop and start and corner a lot.

Magpie · Postby **Magpie** » Sat Sep 19, 2015 6:00 am

What is the main use of the car? Softer springs for bumpy roads, harder springs if smooth roads (in a nut shell).

A decrease in f/r Spring rates decreases roll resistance and does not affect over/under steer.

Tumbles · Postby **Tumbles** » Sat Sep 19, 2015 10:22 pm

Magpie wrote:What is the main use of the car? Softer springs for bumpy roads, harder springs if smooth roads (in a nut shell).

A decrease in f/r Spring rates decreases roll resistance and does not affect over/under steer.

Hard driving on the street?

Smooth roads.

I thought 6kg/mm and 4kg/mm for front and rear respectively was a higher spring rate than stock?

Magpie · Postby **Magpie** » Sun Sep 20, 2015 6:28 am

Old thread http://mx5cartalk.com/forum/viewtopic.php?f=30&t=50506

For the record I run 10/6.

Tumbles · Postby **Tumbles** » Sun Sep 20, 2015 4:33 pm

Magpie wrote:Old thread http://mx5cartalk.com/forum/viewtopic.php?f=30&t=50506

For the record I run 10/6.

manga_blue wrote:Stock 91-93 is 154/94 lb/in. Conversion factor is 55.9 lb/in = 1 kg/mm. So that makes it 2.8/1.7 Kg/mm

So my F6/R4 suggestion is stiffer than stock.

I'm confused by your posts overall, since it seems like my suggestion for spring rates was fine?

manga_blue · Postby **manga_blue** » Sun Sep 20, 2015 5:24 pm

6/4 is a good setting for a touring car. It's great on the highways and mountains while still pretty comfortable in the city. 6/4 will do occasional duty as a track car but you'll find 7/5 a better road/track compromise and 10/6 or 10/7 a better track focussed combo.

The difficulty you'll strike with AGXs is that they're not height adjustable. You'll need to be very precise about calculating free length and crush height before you buy so that you don't end up either with the arse dragging on the ground or a 4WD. I've also got my doubts about the ability of AGXs to handle spring rates as high as 6Kg/mm. Better to get a matched spring/damper combination with height adjustment in a coilover.

edit: Avoid progressives unless it's purely a soft road car. They lurch badly on turn-in when you put them on a track

Magpie · Postby **Magpie** » Sun Sep 20, 2015 5:42 pm

And with the low spring rates the bump stops become part of the suspension setup. If the bump stops are old then this will give a very harsh ride when hitting the bump stops. You can get progressive rate bump stops that can help ease the ride as well. If running high spring rates bump stops are a little more forgiving.

I can tell you my combo is very nasty on bumpy roads wearing a normal seatbelt, you get bounced around a lot, however on the track with a harness it is a nice sweet ride.

manga_blue · Postby **manga_blue** » Sun Sep 20, 2015 5:46 pm

Magpie, I'm running 10/7 on B-grade coast and mountain roads all the time. Bump stops are irrelevant to me. I never hit them, but yeah, different story if you're going to run softer springs.

Magpie · Postby **Magpie** » Sun Sep 20, 2015 6:05 pm

Agree manga_blue, my comment was meant to help the OP that if he is going to run softer springs make sure he considers his bump stops and not just cut them down like some people do

Like you I hardly hit the bump stops except under very hard braking/cornering howevr with the bump stops I have there is not a 'crash' when you hit them rather a progressive motion.

KevGoat · Postby **KevGoat** » Sun Sep 20, 2015 6:48 pm

This page from Fatcat Motorsport may help a little:

http://fatcatmotorsports.com/FCM_app_guide.htm

madjak · Postby **madjak** » Mon Sep 21, 2015 4:58 pm

The AST shocks I just picked up have a spring rate of 550 lbs/in in the rear. Well that's what I think is engraved on the spring. I think that's equivalent to around 10 kg/mm which is really high for the back end.

I'm guessing I'd need to match that to 14 - 16 kg/mm in the front. Hmm

Anyone running that much spring on a track car?

manga_blue · Postby **manga_blue** » Mon Sep 21, 2015 7:03 pm

There are some front running NBs running 14/10 with the help of a fair bit of chassis bracing. I don't think NAs are stiff enough in the frames to take spring rates that high. All you'd be doing is making the suspension rigid and relying on chassis flex instead and I can't see that helping you tune the suspension or set good times.

10/7 seems to about the sensible limit for NAs. There's no reason you can't put those 10Kg/mm springs in the front if they're the right ID and free length.

madjak · Postby **madjak** » Mon Sep 21, 2015 7:11 pm

yeah I had thought of moving them to the front and getting some 7/8k for the rear.

I have a full cage, which at the rear is braced 100mm from the suspension points which should add a lot of chassis strength. In the front I'm not sure what the classes allow but I could add a triangulated bolt in strut brace.

I figure I'll drop them in the car and run it as is and see how it feels. The next event my main competition is doing an engine rebuild for the nationals so I'll only by up against some slower lotuses. The event is a good mixture of a really bumpy hillclimb course and a smooth track with kerbs and a single nasty bump on the main sweeper... ideal for testing suspension. After that I'll make a judgement as to the fronts.

I'm actually going to get to test my 3D printed rear wing for the first time too! exciting!

Thanks for the response... exactly what I was looking for.

Tumbles · Postby **Tumbles** » Tue Sep 22, 2015 2:22 am

That was actually a lot to take in and I'm not 100% that I have it.

My rear shocks deal fell through so this thread is pretty much defunct since I'll just offput suspension changes probably for another 6 months to a year.

http://www.fatcatmotorsports.com/FRC_TU ... TORIAL.htm

Regardless, I plugged in 7kg front and 4kg rear, and it keeps bounce frequency below 1.8, keeps really close to original FRC and increases roll stiffness. Australian roads aren't ultra-smooth but it'll be smooth enough.

Is this what I want? Why does the 3:2 ratio not hold here? (I heard that somewhere that was the go for MX-5s.)

Tumbles · Postby **Tumbles** » Tue Sep 22, 2015 2:24 am

Magpie wrote:Agree manga_blue, my comment was meant to help the OP that if he is going to run softer springs make sure he considers his bump stops and not just cut them down like some people do

Why would people cut down bump stops?

I'll try and remember this, but to be honest at this point in time it seems like coilovers are such an easy option for not thinking.