mx5cartalk.com

Do you guys know what the 2F guys run on their cars?

On my track pig I am running Tein SS with 6/5kg springs, as these were kicking around from my street car. I also have SE swaybars and an SE tower brace. My alignment is as per http://949racing.com/miata-race-alignment-info.aspx.

On the last track day we played with the shock stiffness and found with all 4 set to the 'middle of the road' setting the car would get crazy oversteer on rebound. After dialling up the rear shocks a bit (very minor adjustment) the car seemed to have worse turn in but it was more stable and faster.

Is this kind of compromise fairly typical?

The yanks seem to think I need a stiffer front swaybar and/or heavier springs.

Cheers

We must both be noobs as you were a second quicker than me (number 82):
http://www.natsoft.com.au/cgi-bin/resul ... 011.QR1.P1

Something you have to be careful with when getting info from 949, and information from the states in general, is what tyres they are using! The yanks/Canadians get access, and use, much stickier (slicks/ proper semi-slicks) or wider tyres (such as 225/45R15). Generally, you need increase spring stiffness (rate) as the tyre gets stickier! This is the main reason why they recommend such high spring rates.

For 2f, i have found a lot of people run fairly mild/soft spring rates:
<5kg/mm front
<4kg/mm rear
I believe this is due to the combination of (generally) 205/50R15 tyres and only street legal semi-slicks being used. Plus our tracks aren’t exactly the smoothest. Having also spoken to DMS on the matter of shocks, I found they run 4kg in the front and less in the rear. Being such a light car, it is hard to justify the 8/6 kg/mm spring rates on street tyres.

.....

But, just keep at it and tweaking your wheel alignment and tyre pressures (what tyres were you using?), maybe look in too a front roll bar. Experience will make you faster!

In terms of roll bars, some quick research yesterday showed that the Racing beat ones are the lightest for relative stiffness. I was possibly going to order one from 949 in a few weeks.

Experience has a great deal to do with it, I agree. Zoomzoom was a couple of seconds quicker in my car with no other changes.

It's quite interesting to hear they are running much softer springs!

We are just using up the street tyres that came with the 6UL wheels I recently picked up. They are Kumho KU36 and the tyre pressures were 35psi front & 34psi rear hot.

Yeah, spring rates have been doing my head in. If I get some time I want to punch some numbers on it to give an idea of the difference wheel frequencies. Hopefully that might shed some light.

Maybe play around with the tyre pressures a bit more next time. Or leave the settings the same and just try and get the best time for what you have.

6kg front and 5 kg rear will have alot of oversteer. unless you change to a more sensible pair matched front/rear, you'll be understeering for life. even just switching the fronts to 7kgs will have massive improvements.

or you could soften the rear, weaker swaybay and/or softer springs.

did you actually try full stiff on the front and full soft on the rear while at the track?

Davex3 wrote:

project.r.racing wrote:6kg front and 5 kg rear will have alot of oversteer. unless you change to a more sensible pair matched front/rear, you'll be understeering for life. even just switching the fronts to 7kgs will have massive improvements.

or you could soften the rear, weaker swaybay and/or softer springs.

did you actually try full stiff on the front and full soft on the rear while at the track?

+1 on that. From what i can tell the mx5 likes to have atleast 2kg more spring in the front. I'm currently on 5.5f / 4r kg spring (moving to 8/6 shortly) and had to loosen my rear sway bar right up to stop oversteer.

Curiously, what shocks are you runing?

Hmmm, Do you know what the standard spring rates are? You could be finding that the shocks aren't damped correctly for the springs. Are you getting coilovers or just changing to heavier spring rates? Sorry, just really interested in this stuff at the moment.

My turbo race car runs either 220 width slicks or 205 semi slicks. It has Tein coilovers and poly bushes throughout and a Whiteline anti-roll bar on the front.

I dont remember the spring rates off the top of my head but the shocks are full soft on the rear and NO anti-roll bar at all on the rear. Front shocks are set to about 6 or 7 clicks from full soft. Front anti-roll bar is on its softest setting. No idea how having a full, welded in roll cage will affect this.

A track MX5, especially if you run decent power CAN be to stiff!

My old NA8 turbo road car was an absolute handful on track and not that quick as it was so stiff....sure did teach me a lot about catching oversteer though

Are those SE Bilstiens monos? Probably a very nice shock.

saboteur wrote::
On my track pig I am running Tein SS with 6/5kg springs, as these were kicking around from my street car.
:
Cheers

I did some investigation on spring rates a while ago and this is a summary of what I found:

The larger the ratio between front and rear spring rates, the better the car will handle when pushed hard.
Tein SS have close F/R spring rates to make the ride more comfortable.
The most popular track ratio is 9:6 (as recommended by Keith Tanner).

My BC Racing BR Series coilovers are 8Kg Front, 6kg Rear - a common compromise .

Lokiel wrote:I did some investigation on spring rates a while ago and this is a summary of what I found:

The larger the ratio between front and rear spring rates, the better the car will handle when pushed hard

Hmm, wouldn't be my definition of better.
The larger the ratio between front and rear the more understeer the car will have.

Lokiel wrote: Tein SS have close F/R spring rates to make the ride more comfortable.

The closer they are the more oversteer it will have.
Not sure you'd describe that as comfortable....

keeping a ratio of 3:2 F-R, regardless of spring rate is a better idea, hence the 9:6, 6:4 combinations out there..

It's all about balancing front/rear spring rates with sway bars. You can't set rigid rules about springs or sways in isolation. There are probably hundreds of combinations that can work and each one is a compromise.

Have a look at FatCatMotorsports calculators and start plugging in different spring and sway values. Personal driving style does come into it but for a track car you might want to start looking around:
1. FRC between 52% and 55% (best to ignore Shaihk's and Andy's specialised motorkhana settings with high FRC values), and
2. Front bounce frequency >= rear bounce frequency, and
3. Front bounce frequency >= 2Hz, and
4. Roll stiffness between 2,000 and 2,400lb-ft/deg

http://www.fatcatmotorsports.com/FRC_1_ ... _1_6NA.htm
http://www.fatcatmotorsports.com/FRC_1_ ... _1_8NA.htm
http://www.fatcatmotorsports.com/FRC_NB ... _1_8NB.htm
http://www.fatcatmotorsports.com/FRC_NC/FCM_MSDS_NC.htm

It was my feeling driving this car, that the push on corner entry was caused as the front would dive quite heavily under braking, then as I would lift off the brakes at the same time turning in, the shock would rebound and cause the weight to lift off the front wheels.

I reckon that stiffer springs in the front may help to reduce dive, and with more track time to play with it, I think it would be wise to increase the front shock rebound to dampen the rebound from the dive.

Even as it was I was blown away by the lap times for an otherwise stock car on street tyres, it just shows what a great package these cars are.

Cheers,

Tim

zoomzoom wrote:It was my feeling driving this car, that the push on corner entry was caused as the front would dive quite heavily under braking, then as I would lift off the brakes at the same time turning in, the shock would rebound and cause the weight to lift off the front wheels.

Try carrying the brake further into the corner and being smoother coming off the brakes.
This will a) stop the car susp rebounding so quickly, and b) allow you to corner faster as there is more weight on the front tyres, therefore more grip.