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SOLOII CSP Miata sway bar combinations

by Randy Stocker

NOTE:The information below is for 1990-1997 Mk1 Miata's. I know little about the 1999+ Mk2's.

Installing different (anti)sway bars is one of the best and most economical ways to improve handling. The stock sizes are a 19mm front and 12mm for the rear. The exceptions are a 20mm front for 94+ 'R' models and 11mm rear for the 93 LE and 94 models with an open diff. The I have tried every sway bar comination for my CSP autocross Miata that you can imagine. Below are some of the results of the combinations I have tried. Use this table as a starting point for your sway bar selection (or collection as I call it). Keep in mind that a SOLOII Street Prepared car will generally not be able to get by with just one set of bars, you will need to have many assorted sizes on hand to install as courses and surfaces change to be competitive. The below table is only a generalization as driving style, handling preferences (oversteer/understeer), tire choice, spring rates, shock valving and the lot surface type (sealcoat to brushed concrete) will all affect the characteristics of your car. Every car is different and every driver is different.

MY CAR. My autocross Miata is a 1992 Base with no air. I have installed a late model 1.8 motor in it. The front springs are currently 375lb Carerra coil-over's and the rears are 185 lb Racing Beat fronts. (I use reletively soft springs because of the very rough lots in my region). I use Koni Sport adjustable shocks, typically set at full stiff in the front and full soft in the rear. The OEM bumpstops have been removed and replaced with shorter generic ones. The wheels are Kiezer 13x7" in the front with 5" backspace and 13x8" in the rear with 5" backspace. It weighs a portly 2116 2103 lbs (45lb rollbar and 13lb trailer hitch!). As of 3/1998 I now run 550# front and 200# rear springs.

SOLOII CSP SWAY BAR COMBO'S:
Handeling Characteristics
Front:Rear:slight understeerneutralslight oversteer
20mm 11mm12mm
13/16" (20.6mm) 11mm12mm1/2" (12.7)mm
7/8 (22.2mm) 12mm1/2" (12.7mm)9/16" (14.3mm)
15/16" (23.8mm) 1/2" (12.7mm)9/16" (14.3mm)15mm
24mm 1/2" (12.7mm)9/16" (14.3mm)15mm
1" (25.4mm) 9/16" (14.3mm)15mm5/8" (15.8mm)

TUNING: Most of these bars I tested were 2 or 3 position adjustable. Making stiffer or softer adjustments affected handling but not to the point of the next size bar. A bar's stiffness is mostly a function of it's diameter (to the 4th power actually. For your first bar purchase I would suggest buying a combo that fell into the neutral handeling range and the adjust from there.

BAR SIZE: The stiffer the bars the flatter the car will corner, however, the downside is that the bigger the bar the quicker the load is tranfered making the car less forgiving to harsh steering inputs. ('Mr. Spin'). A nice byproduct of increasingly larger bars (and the corresponding poly bushings) is better transitional response (slaloms, garages, etc). I recommend starting with small bars so they will be forgiving and when your driving style gets smoother as you get used to the new setup you should switch to something larger. Many sway bar 'kits' come with a rear bar that is too large for autocrossing and will typically cause oversteer. You probably will have to buy singles to get the combo you like. Torsen LSD equipped cars can get away with larger rear bars than the earlier Viscous LSD cars. The key to a fast viscous or open diff car is to keep the rear planted and not spin the inside rear tire in a power-on corner exit. Select a combo that slightly understeers for these cars.

TIRES: Different tires will most likely require different size bars too. I used the 7/8" front with the OEM 12mm with the BFG's 230 R1's because the stiffer tire construction caused too much snap-oversteer in the rear. (tires have spring rates too). I am now using the Hoosier Radials and they like a larger bar combo. The Hoosier's seem to like a 15/16" front with the 9/16" rear, as a consequence my steering response is much better now.

VENDORS: Companies that I am aware of that sell sway bars for a 1.6 chassis.

Front:
20mm - Mazda
13/16" - Addco
22mm - Mazdaspeed
7/8" - Addco, BSP, Flyin' Miata (1.6), Jackson Racing
24mm - Eibach
15/16" - Racing Beat, Motorsports, PBC
1" - Pettit, Helwig, Ground Control, Suspension Techniques, Rod Millen, Flyin' Miata (1.8)

Note: to install the 20mm 1994+ front 'R' bar in a early Miata just drill a new hole about 3/4" in closer toward the bar bend.

Rear:
11mm - Mazda
12mm - Mazda
1/2" - Addco, Ground Control
14mm - Mazdaspeed
9/16" - Addco
15mm - Eibach
5/8" Addco, BSP, Motorsports, Racing Beat, Flyin' Miata, PBC, Jackson Racing, Rod Millen.

There are other vendors than these but they typically private label the same bars. Some vendors switch suppliers (Addco, Ranchero, Helwig, Quickor, etc) frequently so be sure to just ask for the diameters and whether they are adjustable when you are ready to buy. There are some companies marketing 3/4" rear bars. My suggestion is to stay away from these, they are just too big. The only time I have ever driven a Miata that handled well with a 3/4" rear was in a 1996 'M' (heavy) that was Torsen equipped. It had a huge 27mm front bar and very heavy 13x7 steel wheels.

Update 6/23/98.I get asked frequently about sway bar sizes for a strickly street driven Miata. All I can say is that it is a matter of strong personal taste and the environment you drive in. Street sway bar kits are usually designed with low speed street driving in mind (i.e., under 50 mph), they are tuned to provide good low speed steering response and low speed chassis balance to give the car the sensation it is handling well. A street driven car will never see the g-forces, lateral loads or transitional load tranfers that an autox or roadrace car sees so you can probably go with a combination that oversteers more than the above table recommends as 'neutral'.

Keep in mind though that the faster you go the more front bias the setup needs to be to maintain the same level of balance. In other words, if you typically drive 90% of your time around town then your Miata will feel very racy with a relativly larger rear bias that a street kit provides. However, as you do go faster, such as an interstate off ramp, your Miata will likely get nervous and will be unforgiving to corrections. If you are not comfortable with your high speed or emergency car control skills then I would personally keep that in consideration when determining sway bar sizes for a street setup. FWIW, I have set up my wife's daily driver Miata with a 1" front and 5/8" rear. YMMV

I also get asked about sway bar sizes for ITA Miata's. I have never driven in an IT race so I have no experience with it. I did however participate in the Midwest Council autox series which is really a SoloI. Note that each track is very different and what works at one track will probably not work for the next. I found that at Blackhawk and Road America that the car was much faster with a setup that was even more front heavy than the autox sizes. I tried and liked both a 15/16" front w/12mm rear and a 7/8" w/no rear. Good Luck.

Update 1/30/99.The shape of the 94-97 1.8 front swaybars are identical to the front 1.6 bars with the exception of the length of the bar after the bend for the endlinks, the 94-97 is about 3/4" longer (the rears are the same). -> |______|

If you have a 1.6 aftermarket bar that is 3 position adjustable you might be able to use it in a 1.8 chassis if you get lucky and the outer most mounting hole (soft position) will match up with length of the 1.8. A 1.8 bar in a 1.6 chassis would be the opposite using the inner most (stiffest position). Since you will be using the last adjustment position you'll loose the adjustability feature though. Otherwise, to install a front 1.8 bar on a 1.6 car you can redrill a new hole approx 3/4" closer to the bend (always compare and measure.) Most of the aftermarket bars have enough of a flat mounting surface to support the new hole, otherwise you might have to put the bar on a bench grinder to make the flat mounting area larger. This works fine on a solid bar but a hollow bar may not have enough meat in that area for that to work. (again, always compare and measure).

Question: I want a coil-over suspension but the kits are just too expensive.

Answer: I made DIY home-made coil-overs on Koni's back in 1995 from a $42 Carrera lower 1 5/8" threaded perch and $55 Carrera "Barrel" spring (2.5" lower and 3.5" upper). Carerra spring part number 8MBxxx where xxx is the rate. The bonus of the barrel spring is that it uses the factory upper perch as-is and there is no upper adapter perch which saves a little extra travel.

coilover.jpg (54 KB)

Note the rubber spacers. I made them from the hard rubber spring spacers you can buy at any parts store (such as Pep Boys) and then cut and shaped them to fit. Commonly called "NASCAR donuts" the effectively raise the spring rate by taking one active coil out of service. The springs in the photo are 375# and I a calculated I needed more like 400#. These did the trick for an investment of $4.

Question: I have aftermarket springs and I have decided I want to raise my ride height slightly for better shock travel and speed bump clearance.

Answer: Try a spacer on the lower perch. I made two sets out of 3.5" aluminum solid bar stock, both on a lathe. Any machine shop could duplicate it for about $30-50 or you could DIY it using many small drill holes on a press to hollow the center and then smooth with a file.

The first set I made in 1993. I was running Illuminas at the time and they have a very tall lower perch centering ring (easily double the factory shocks). This allowed the use of the 1/4" spacer and still allowed enough height for the spring to be centered. I would not run the spacers with the stock shocks (as in the photo) since there would be nothing to keep the spring from moving around.

The second set I made in 1994 is smaller in diameter than the first set and also beveled to fit inside the lower perch cup of the Koni's. I got the idea from the 'R' model drivers because back in the days before Koni made the correct 'R' perch groove on the shock body the most common remedy to get back to the factory perch height was to use the factory Bilstein lower perch set inside the Koni lower perch.

springperchspacers.jpg (38 KB)

Question: How much shock travel is there if I lower my car?

Answer: Expertly answered by Jyri J. Virkki on his bump stop web page .

Question: I've lowered my car and now the steering rods angle upward substantially. I'm told this produces "bump steer". What can I do?

Answer: The Miata has great suspension geometry at stock ride height but lower it more than 12.5" from wheel center to fender lip and it goes to pot. As you note one of the worst effects is bump steer from the steering rods being so out of alignment with the control arms.

What I did was do two things. First I used the 1993 LE tie rod ends which have approx 3/16" more stud height which lowers the rod (in the USA the 93 LE was the lowest of all M1 Miata's from the factory and the factory supplied them with these special tie rod ends - part number 8AN2-32-280 and only available through Mazda Comp). I then machined 1/4" thick spacers for the steering rack to raise it back up (if you attempt to do this be very careful and know what you are doing as the width and hole size must be precise in order for the rack not to move or separate under load!). The idea is to get the steering rods as close to parallel with the control arms as possible.

bumpsteerspacers.jpg (13 KB) spacers bumpsteerspacers.jpg (13 KB) new angles

Question: I've lowered my Miata and now it handles even worse. It feels racy but the fact is that I could take corners faster when it was stock.

Answer: Lowered Miata have poor suspension geometry. There, I said it. If your Miata is lowered more than an inch you have undone the wonderful suspension geometry the Mazda engineers designed. (and have less suspension travel which means you are riding on the bumpstops all the time - but that is for another day). For the chassis engineers in the audience you have lowered your roll center but raised the "roll couple" - which is far worse, and you also removed the negative chamber gain as the suspension compresses. Congratulations, you just made a 323! One way to get it back is to relocate the control arms 1 inch upwards to put the suspensions relative location back the way it was.

That's what I did by having new rod-end control arms fabricated. By using the rod-ends and a turnbuckle arrangement for alignment adjustment I could use a single attachment hole instead of the slotted subframe hole from the factory. This also gave me a consistant alignment that will never come out (the eccentrics will slip in autocross or track situations undoing the $60 alignment you just got) and the "dynamic" steer has been eliminated from the removing the rubber bushing compliance.

rod-end-rear-control-arms.jpg (23 KB)

rod-end-rear-control-arms2.jpg (23 KB) The rears are converted factory arms.

rod-end-front-tubular-control-arms.jpg (23 KB) The fronts are full replacement tubular chrome molly with a built-in ball joint. These are the real deal and are real racecar parts. Half the weight and twice as strong as the factory items

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