Hi Bernice, I am running one size taller rear tires (596 mm diameter) to get the same final ratio as the standard 5 speed. The ratios of the Stilo C510 gearbox are shorter. The front tires are 577 mm in diameter.
When I bought the car in the 80s it had cut front springs and never handled particularly well. When I revived it two years ago I also got proper coilovers (softest D2s), which made a world of difference.
I think I set them close to stock height, but due the diameter of the rear tires the rear of the car is 10 mm higher than standard and the sill is now close to horizontal with the rear perhaps a mm or two lower.
Would you think the change in rake due to the tire size difference alone would offset the suspension dynamics?
As a start, larger tire/wheel diameter will raise the roll center of rear which reduces grip and alters the grip/traction balance of the chassis/suspension. Depending on wheel offset, that will alter the scrub radius (Braking stability, steering feel), loading on the wheel bearings. Too many install incorrect wheel offsets to gain a wheel that is visually "fashionable" trading off proper dynamics.
As for "coil overs" the as delivered, as designed strut suspension has "coil overs". That term coil over simply means damper coaxial with the spring. What is important is spring rate/damper rates in compression/rebound and how all this interacts with the rest of the entire suspension/chassis as they work as a system.
It is possible to get the "stock" ride height set by adjusting the damper/spring units threaded collar, this will alter the static position of the
lower A-arm which impacts the suspension geometry due to the larger diameter wheel/tire. Keep in mind, the rear suspension has an optimal static height that is designed in. Static height impacts the roll center position, static location rear suspension's camber curve, rear toe curve position (the rear suspension is designed with toe in on bump)..
Take the time to read this recent discussion on suspension/chassis dynamics to gain some idea of how this stuff works.
In another post, @Steve Hoelscher and @Rupunzell were both commenting on not using the Miata suspension as there are much better choices. So, I thought it might be interesting to discuss what other options might exist out there for a custom setup. Note, this is not intended to be a bolt on...
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Change one item on the suspension, from settings/alignment to wheels/tires/tire pressure to spring rates/dampers and such can drastically alter the vehicle dynamics.
It is not a good idea to compensate for gearing by wheel/tire diameter. If the gear ratios need to be altered, alter the gears not the wheel/tires.
Question becomes, what is the point of reference for "handling" ala chassis/suspension dynamics?
Bernice
How do they fit width wise?
