I have tried every sway bar comination over that last 5 years 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 21162103 lbs (45lb rollbar and 13lb trailer hitch!)
SWAY BAR COMBO'S:
|Front:||Rear:||slight understeer||neutral||slight oversteer|
|13/16" (20.6mm)||11mm||12mm||1/2" (12.7)mm|
|7/8 (22.2mm)||12mm||1/2" (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)||15mm||5/8" (15.8mm)|
BAR SIZE: The stiffer the bars the better the transitional response can be (slaloms, garages, etc), however, they will prove to be less forgiving to harsh steering inputs. ('Mr. Spin'). 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.
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. (Although I could not tell the difference between the 24mm Eibach and the 15/16" Racing Beat). For your first bar purchase I would suggest buying a combo that fell into the neutral handeling range and the adjust from there.
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 because the stiffer tire construction caused too much snap-oversteer in the rear. (tires have spring rates too). I am now using the Hoosiers 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.
20mm - Mazda
13/16" - Addco
7/8" - Addco, BSP, Dealer Alternative, Jackson Racing
24mm - Eibach
15/16" - Racing Beat, Motorsports, PBC
1" - Pettit, Helwig, Ground Control, Suspension Techniques, Rod Millen
11mm - Mazda
12mm - Mazda
1/2" - Addco, Ground Control
9/16" - Addco
15mm - Eibach
5/8" Addco, BSP, Motorsports, Racing Beat, Dealer Alternative, PBC, Jackson Racing, Rod Millen.
There are other vendors than these but they typically private label the same bars. Some vendors switch suppliers 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 eqipped. 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. My wifes car is set up with a 1" front and 5/8" rear. A street driven car will never see the g-forces, lateral loads or transitional loads that an autox car sees so you can probably go with a combination that oversteers more than the above table recommends as 'neutral'. I personally like to concider emergency maneuvers when determining sway bar sizes for the street too. 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.I think this explains it all, if you have any further question please contact me at SOLOMIATA@AOL.COM