In my chasing of the
terminal understeer issues with my car, it has become quite apparent that Miata's run really high rates relative to other platforms. Just for reference, the front MR is 0.69:1 and the rear is 0.75:1 on a miata. An s2k is 0.589 Front and 0.578 Rear yet command lower spring rates. I am aware that there are other factors that play into spring rates like roll center, wheel rate, frequencies, aero, tires, etc. But why is it that our cars just want more than others? Iv'e never really understood that, and when I tell people I run 1100/600 on a car this light, they just say, "Well that's ur problem, ur way too over-sprung".

So yeah, if anyone could shed some light on this topic, that would be cool.

 

To prevent tires from bottoming out in the wheel wells. That seems to be the best answer I can find.

 

I mean, that's what bump stops are for.

 

Heh, and preventing the tires from bottoming out in the wheel wells is something that Xidas DON'T do particularly well.

--Ian

 

Feels like you answered your question in your original post.

 

If you are on XIDA's and chasing terminal understeer at 1100/600 you have a different problem?


Have to ask if your experienced / fast enough to know difference?
Rear sway bar ??
Wing & angle? If no wing you might be a bit oversprung but not much.
Splitter, 1100 matches with a real one.
Are you too low or to high?
You are on square tire setup correct?

 

I mean, I race w2w. Been driving for 5 years, I'd like to think I know what i'm talking about by now.

Sways: 1.25" RB (soft setting) - SPM 14mm rear bar (stiff setting)
Wing: APR GT250, negative AOA.
2 splitter setups: 1 is no protrusion, flush with air dam. As you might expect, this makes the car push. 2 is a 4.5" splitter. Pushing still though not as bad.
4.5-4.75" ride heights
245 AR1 square/
3* front camber, 0 toe, 6.6* caster
2.5* rear camber, 1/16" toe in

This probably isn't a good thread to discuss this though, I might make a new one for that.

 

Had to ask....

You are right in the range of a bunch of us. Have you called the guys at Supermiata?

How are your bushings?

To the original question. Everyone else is too soft.

 

Bushings are new poly/delrin mix with bronze oilite sleeves.

I haven't called them.

 

Any chance you got hold of a wimpy / copy / mismarked rear bar? How are those rear bar bushings? Firm urethane and tight?

Did you verify markings on springs? Easy to mix up 500 w 600.

Doing some light reading today and came across this in the May 2021 issue of Racecar Engineering written by Claude Rouelle w https://optimumg.com

"When an engineer asks the driver what the car balance is, and the driver describes the car as understeering, the engineer has two solutions: softening the front anti-roll bar or stiffening the rear one. remember playing with the front or rear springs is not a good idea as it could destroy your mechanical grip and / or aero balance.
To make the right choice, the engineer needs to ask a second question: is the car too lazy or too nervous? If the answer is the former, the engineer will increase rear anti-roll bar stiffness as, usually, stiffer cars tend to be more responsive. If the answer is too nervous, the engineer will soften the front anti-roll bar."

Some fantastic "light" reading.

https://optimumg.com/springsdampers1/

https://optimumg.com/springs-dampers-part-two/

 

Rear sway was purchased new though SPM. Springs were verified upon install.

The car doesnt feel "nervous" per say. It just lacks that confidence as you "load up" the front end (it will wash out). It would lean more towards the "lazy" side of things on turn in. But again, my rear bar is already full stiff.

 

I still think your steering rack and/or steering system is fucked up. You're having to give about 30-50* more of wheel angle than other miatas running similar times to turn the same amount. So maybe you end up putting too much angle, which can induce understeer?

 

It would be backwards. My rack is slow therefore I give more input for less steering angle.

You are the 2nd person to comment about how my rack looks slow, but I just cant see how it would actually be slow. Its just gearing.

 

You've got all our goodies, we are happy to help with setup via email or phone call. We still here for you bro.

Try a smaller front bar. 54107/54103. See how that goes.

There's also other variables like alignment but I'm thinking you've got too much front bar.

Are the tires hitting the tub hard?

Xidas use *all* available bump travel. On some cars and with big tires, sometimes it's too much. Check it out and install tophat spacers if needed.

 

Combination of things, relative to some other cars:
Relatively high motion ratios (low wheel rates)
Greatly lowered roll centers compared to OEM so CG has a bunch more leverage over RC
Narrow track compared to most others cars with about the same lateral grip.
2x the OEM lateral grip the car was designed around. OEM was design to corner on the bumpstops. Stops are effectively ultra high rate springs. Good race suspension will be design to corner off the stops in steady state and only engage them over kerbs. So that lost ultra high rate stop has to be replaced.

More to it than that but that's the rough sketch. In any case, your FSB is too high a rate for the rest of you set up. Like Ed said, 54107 will probably bring the balance back. Make sure you're not much below 4.5" front pinch weld, assuming you're still on Xidas. If a different shock (different length shock, different durometer and length and stops, different low speed compression damping rates.. that Xida specific set up advice may not work.

 

Thanks for the info.

Whats the difference between the 54107/3 bars? I see on your site it differentiates based on model year. Does that matter? What's the reasoning for a 107 bar on my NB?

Edit: Also, I just found out that my driver front bump stop is gone. I have no idea how (It either esploded or I neglected to put it back.....probably the latter). But its just not there anymore. The passenger side is in. So that certainly wont help my issues. I guess ill be sending you guys an email.

 

103 and 107 are same spring rate. 107 is NA version which clears thicc shocks completely. 103, NB version puts ends real close to the fatness, often causing contact when used with OEM bushings (moar deflection). Ping Ed and he'll get a stop out to you. Don't drive until you get that stop installed, otherwise you could destroy the shock and some other stuff. Check rear stops while you're at it. They're half height on Xida Race.

 

Interesting. I was having an issue with my endlink on my passenger side making contact with my shock. I had to move the Endlink to the stiff position on that side of the bar to move it farther way from the shock, but now the FRC is even stiffer.

I sent an email, but it looks like ill be ordering a 107 bar.

 

--Ian

 

It's me. I'm the gnome.