Stiffer chassis for Miata - please

Believe me, I have carefully studied all the existing methods of getting Miata’s chassis stiffer. The result is that there are as many solutions as there are car builders. Frame Rails, Frog Arms, butterfly brace and front/rear sub frame braces are all marketed to make Your Miata more solid. Well, they most certainly do so, but would they help me to deal with my “chassis issues”?

To find out where my Miata’s chassis flexes I decided to make a series of measurements. I used a “jack it up” method. This was basically the only way to use enough force to make the chassis bend and twist. The conditions are static and they do not simulate actual driving situation, but the measurements give an overall picture how the chassis reacts.

Here is what I found out. Rear is well attached to middle section (the floor and center tunnel). Front instead isn’t. For some reason the front bends down when front wheels are jacked up. Pivot point is more or less where the floor and door sills end and fire wall begins. Chassis also twists. Deflection was measured between six different points. “O” and “P” didn’t change at all and that is where I reasoned that the “rear end was stiff” (Correction: "O" and "P" deflection was about 2 mm once the rear was jacked up. Some error and uncertainty could have come from loose roll bar.) Changes in the other measures are shown in the graphs. Bending is quite heavy. 7 mm deflection in the height of the find screen frame means the front is bending down about 4-5 degrees.

The following pictures show deflection when only on side is jacked up. This time I used a dial indicator for improved accuracy and to verify my rougher tape measurements. It was surprising to find out what kind of an effect the lifting point has. This was another proof that the problem was the weary connection between the middle section and the front.

Conclusion:

Because my Miata’s chassis behaves like this I believe that neither Frog Arms, Frame Rails nor bolt on roll cage will work. Why? Because they don’t attach the front to the rest of the chassis. (See first picture in this post.)

My plan is to first weld a couple of supports to the chassis that connect floor, door sill, fire wall and front chassis. Here is the first template fitted in. After that I will add door bars that connect front and rear. This should help!

Last I must emphasize that I am not building a race car. With this modification I am hoping to get some extra stiffness so that future modifications to suspension can have their full effect.