I’ve played with the ARB’s on my Mk3 a bit & found a difference, ment to put this summary up ages ago, but only just got round to logging on to site.
Here is a summary of what I have found trawling
various forums, as everywhere I looked only had snippets of info on a couple of
bars. Some dimensions are approximate.<o:p></o:p>
Stiffness of bar is dictated by its polar moment of
inertia Ip = pi * (D^4 – d^4) / 32
I don’t know if the arm lengths are the same or not,
this can make a big difference too, I assume they are.
MX5 Standard 6-speed 2.0 sport set up (blue dot) roll
bars
Diameter front bar - 22.1 mm, wall ~2.55 mm - Ip=15,220
Diameter rear bar - 12 mm, solid bar - Ip=2,030
MX5 Eibach / Mazda Speed (MS) bars
Diameter front
bar – 26mm, wall 4mm - Ip=34,555
Front
arm adjustable at 172 or 192 mm on one side only (stnd ~182) effective Ip ~
35,522, or 33,614<o:p></o:p>
Diameter rear
bar - 16mm, solid bar - Ip=6,430<o:p></o:p>
RX8 Red dot is original S1 & S2 standard
suspension<o:p></o:p>
Diameter front bar – 25.5 mm, wall ~3.5(guess) mm - Ip=30,010<o:p></o:p>
Diameter rear bar - 16 mm, wall ~1.9 mm - Ip=4,260<o:p></o:p>
RX8 Yellow dot is stiffer S1 & S2 sports option set up<o:p></o:p>
Diameter front bar – 26.5 mm, wall ~3.5 mm - Ip=34,220<o:p></o:p>
Diameter rear bar - 16 mm, wall ~2.3 mm - Ip=4,775<o:p></o:p>
RX8 Orange dot is stiffer still From 09-11 R3 models<o:p></o:p>
Diameter front bar – 26.5 mm, wall ~4 mm - Ip=36,910
Diameter rear bar - 16 mm, solid bar -Ip=6,430
On my car with MS/Eibach 30mm lowering springs, I fitted
Yellow dot roll bars front & rear. Reduced body roll further, sharper
initial response. Felt to make a big improvement on track, response seemed
better entering corner, less roll & more grip mid corner, but did feel to
have a bias toward power on understeer at all speeds. <o:p></o:p>
Orange rear roll bar then fitted with Yellow front
roll bar. When pushing hard on road, back end feels a little looser. On track,
balance was much better (less understeer) & more fun.<o:p></o:p>
Comments on above set up from a friend who drives a
Mk3 sport with stock suspension: Surprised how little ride is compromised (the
Mrs noticed though). Body roll reduced, but not fully eliminated. Low speed
understeer on road, seems to need to carry some speed into corners to get some
weight transfer for it to bite. More tail happy at higher speed cornering.
On various forums, people have suggested Red front
& Yellow rear roll bars give good balance. These can both be picked up on
Ebay quite cheap from scrapped RX8s. That set up should be similar in terms of
balance (just a bit softer overall) to the Yellow front & Orange rear that
I like. If you don’t mind buying new bits, the MS/Eibach adjustable roll set
would seem a good option, as that is similar to above (yellow front, orange
rear), but with some easy adjustability.
Fitting: RX8 bars fit MX5 mk3 with no mods & don’t
foul anything. Drop links are a sod to get undone if rusty, mine were ground
off & 4 new ones fitted! Rear roll bar is a doddle to change. Front
required the radiator to be unfixed & lifted a few inches (coolant hoses
not disconnected though), to allow radiator to be raised enough to get the roll
bar out & in.
