Flyin' Miata : Measuring chassis stiffness

We spend a lot of time under Miatas. The factory frame rails take a beating on these cars and obviously they're good things to have. We designed some new braces to replace them and had to determine how effective they were.

The test car was a 1993 Miata with 83,600 miles. It has a Hard Dog Sport rollbar and an FM shock tower brace along with some unindentified triangular brace that is attached to the differential in the back. This brace is not designed to counteract twist.

To test the twisting of the chassis, we supported the car on three jackstands. Two were at the front of the factory frame rails and one was on one of the rear control arm mounting points. The doors on the car were opened to give a worst-case scenario. The distance from the floor to the stock rear jacking point was measured. A jack was then put beside the measured point and the car was lifted until the front jack stand started to unload. The measurement was repeated. We performed this several times to ensure accurate measurements.

The car was then lifted off the jackstands and all measurement and support points were marked. Since we were using the lift, the car did not move forward or back at all when lifted. The new braces were installed, then the car was lowered back down on to perform the test again - and like the unmodified version, we performed it several times.

Pre-modification, measurements showed 5.5-6 mm of twist. Post-modification, we found 3.5-4 mm. A 2 mm change in twist over a 6 mm range is a 33% improvement, we rounded that down to 30%.

The braces are now available for all Miatas.

Testing the twist.
The braces on the car.