Chasing a little better handling for next month's race. 2410# NB w/driver, 250rwhp/240rwt. Portland International raceway with chicane.

Working on upgrading the car little by little. Working on a racers budget, so it's presently not possible to make large/expensive changes. Still, I would like to improve the handling by using the basic parts that I already have, which are: Spec miata sway bars and spring rates, non-adjustable Bilstein shocks, Hoosier R7's on 9" wheels.

My baseline set-up so far is: ride height 4 1/4" front pinch weld, 4 1/2" rear. -2.5 front camber, -2 1/4 rear. 0 toe front, 1/16 toe-in rear. 700#/325# spring rate. FCM bump stops. I sent the shocks to Blistein West for re-valve, but did not get a dyno sheet.

What I wish to change:

1) not enough neg camber. I have not used a pyrometer as I'm currently sans the race crew; but I'm picking up rubber on the inside 1/3 of the tire. How do you do this without bending something, cut/weld/extend lower ball joints or using the offset bushings (that have been dissed by other MT members)?

2) I'm either bottoming or getting bump steer at the end of a long sweeper curve (4a,b and entry to 5) which is unsettling the car in the braking zone for t-5. increase ride height vs. live with it? Would "R"-package tie rod ends help here?

3) Just a little more neutral throttle over-steer would help me get around t-12. I'm currently at middle of three settings at rear bar. Full stiff was tail happy. Does putting one side at full stiff, and leaving the other side at middle setting split the difference? i.e does position 1 + position 2 = 1 1/2?

So far, I am thinking of leaving the ride height alone and increasing my spring rates to 1000/500; installing the "R" package tie rod ends and then using the offset bushings for the front upper control arms (with wheel spacers to keep the tires from from rubbing the shock).

Does this seem to be a reasonable plan?

Does anyone have an opinion on these drop spindles: http://www.miataroadster.com/keisler...s/g-74087.aspx

Here's a pic of the car from the rear entering the chicane (slowest corner on the track). It looks a little higher than the same pics of similar SM cars.



on the throttle pic:

a little faster corner:


Thx for the suggestions.

 

re 1) camber: are you picking up rubber on the inside of both front & rear, or moreso the fronts? In my experience it does not necessarily indicate the need for more camber. I have always done this (autox, not track) largely from dragging the inside edge of the *inside* wheel on corners. This less-loaded wheel is not the one you are optimizing the camber for, and with the stock arms you are almost certainly not going to have anything close to optimal camber on sides of the car, especially in the front, while turning. I would get someone to apply a pyrometer, or at least look at long-term wear (not cruft pickup) before adding more camber. Or if you can get front-on and rear-on pics of your car in mid-corner you can maybe judge... that said, if you are all the way adjusted via the cams then you need to do something to change geometry if you do want more neg camber. Options are replacement adjustable arms or offset bushings or monkeying with stuff.

re 2) tie rod ends: you should not be either bottoming out or getting bump steer at the end of a corner (well, unless there is a significant bump in the surface there). If anything the front should be lightly loaded at corner exit. Can you explain what is happening a bit more? That said, tie rod ends will not affect bottoming out but would potentially reduce bump steer on a lowered car.

re 3) rear bar: what bar are you running - eibach? It is a small enough bar that I would not try to interpolate settings; the differences are pretty small. If you are really that close, adjust tire pressure or something. By my math the difference in wheel rate between full stiff and medium settings on the eibach bar is equivalent to about a 30# difference in rear spring rate (and the difference between medium and soft is even smaller). And no, I don't think that in the real world the different arm lengths would work out as a "1.5" setting.

The drop spindles look beautiful but I have not run them.

 

1) I am picking up rubber on both front and back. No test days in immediate future, so no pyrometer data.

2)Portland International Raceway // Track Map

I am having trouble picking up the pace on T-4. This is a right hand sweeper that feeds into a tighter right hander. You enter it fast, with just a little trailbraking. You point a bit as you drift into the braking zone for T-5. There is a small rise/fall just in this transition that makes the car settle side to side and you have to input a steering correction. This side to side motion makes the car feel loose when under heavy braking... it feels kind of like bump steer, but maybe there's just a lot going on here. I used to be fast there, now in the mazda... not so fast.

3) Yes Eibach.

I have just now ordered the upper control arm offset bushings; derlin was all that mazdamotorsports had in stock. I was assured by local racer that the 9" 6ul's with 225x45 hoosiers wouldn't rub if you used 5mm spacers. I'm hoping that he is right as I remember Emilio discussing his worries about possible rubbing issues when using wide wheels in conjunction with the upper strut offset bushings.

 

OK, now I understand (sort of) the turn situation. Thanks for the context. I would suspect bottoming out in the left front rather than bump steer if the instability happens as you are braking and turning (harder) to the right. If you don't want to change springs & bars you may want to just raise the car 1/2" everywhere for one session and see if the problem goes away (obviously this affects many other things and may not be what you want, but it would at least diagnose the problem). I don't think the bilstein SM shock packages are adjustable but if they are you may be able to work around the issue with a bit more low speed compression (to keep the car from whacking into the bump stop during this quick transition) or less low speed rebound in the front if it may be "jacking down" during the transitions (but it shouldn't be unless you have WAY too much rebound).

Are you intending to use those bump stops, or to stay off them except during exceptional events (actual bumps, like curbing)? You can always do the old zip-tie-on-the-shock trick to see if you are getting onto them. They can definitely produce sudden, destabilizing dynamic changes like you are reporting when one end (or really one wheel) suddenly has a different spring rate.... Some people like to use bump stops as part of the intended tuning package, but I don't. Emergency only, outside the intended travel range.

You might also be able to diagnose transitional behavior issues by doing a bit of a slalom in a parking lot somewhere .

The upper strut offset bushings do pull the wheels in, so having spacers on hand is smart. You can get wheel to a-arm contact (at or near steering lock) as well as tire-to-whatever contact.

 

Thank you for your suggestions.

I think that the SM racers use the FatCatMS progressive bumpstops as part of their overall spring package. So, yes I think it is OK to be into the stop a little. I will try raising the suspension a little.

Since I will be disassembling the suspension to put in the offset bushings anyway, and also getting into the rears to install new coilover threaded bodies (as mine have issues). It would be cheap and easy at that point to install heavier springs. I know that there is a lot of experienced knowledge on this forum to probably get me pretty close in rates. The goal would be to get me close enough to neutral so that it was in the range of a quick rear bar setting change. (as the Oregon SCCA race format has done away with practice; instead doing a qual, a sprint race and then a main race... so no test/tune time)

Q) I'm not sure how much increase in rate that my current bilstein shock could handle. I'm presently 700f/400r. I've seen spring package suggestions in the 1000f/500r. Do you have an opinion on spring rate choice for my situation?

BTW: What's nice about these new handling problems showing up is that, with the advise of the Forum and the great parts from the Forum vendors, I am getting faster!

 

Most of my experience is autocross, which is more transition-centric than full size tracks and I don't know the SM bilsteins, so I don't think I can offer more specific advice.

Two general things I will offer:

1) the less spring, the more compliant the car. More spring does allow you to get lower but there is always a tradeoff. I've run front springs up to 800# (and that was on a miata that was almost 400# lighter than yours), and 900-1000/500 is not unheard of. Just consider that more spring will tend to make the car notice bumps/uneven surfaces such as the one you describe between turn 4 & 5 *more*, not *less* (assuming that both suspensions are set up to avoid bottoming out the suspension, of course).

2) this may be more of an issue in lower speed corners, but a lot of us autocrossers have had MUCH better luck putting down power coming out of corners with minimal or no rear bar. If you do change spring rates I might aim for something that tips the car away from that stiffest setting on the eibach bar and toward either the lowest setting or disconnecting the bar entirely. My last setup was 650/440 with the huge ISC racing front sway bar & no rear bar; before that I ran 800/685 with a more normal front bar (1.25" hollow, I think) & no rear bar, but that setup had tire stagger so the equations were a bit different. I also ran for years with 400/335 & the 1.125" or 1.25" hollow front bar and no rear bar. But the common denominator of these setups is a bit more rear spring than some other people run, with less bar. So I said I would not offer specific suggestions, but I guess I might think something like 800/500, for example, if you do want to stiffen the car, rather than 1000/500, and start with the eibach rear bar @ the softest setting.

 

Delrin is fine for the upper mounts, it's the bottom ones where they'll bind when they go non-co-linear after you adjust the alignment.

I ran 15x9 6ULs with 225/45R15 A6s for a few seasons using the ISC upper offset bushings, they did not rub on the shock. They *did* rub on the sway bar and control arm, but only under full lock, so it wasn't an issue as long as I remembered to be careful when putting it on the trailer.

--Ian

 

Thanks Ian. It's one less thing to worry about!

 

The parts arrived and I installed them yesterday.

The offset bushings were a real pain to get in correctly. It took several tries to get the alignment of the offset symmetric so that the sleeves wouldn't bind.

The "R" package tie rod ends (pn# N021-32-280A) were a cheap and easy swap, but I couldn't see any real differences between these and the regular NB ends. I believe that they merely shim the tie-rod alignment down .060" or so to correct for toe-out on bump.

I was able to use the FatCatMotorsports suspension calculator to size my spring rate. By his calculator, moving from a 700f/325r to an 850f/400r spring rate should give a 15% increase in roll stiffness with little change in the front roll couple. Hopefully, it will be "neutral" right out of the box. (?)!