Chassis supports and door bars – project ready

Chassis supports are now ready and welded in. Door bars still need to be painted. Templates were accurate enough to make the final fitment easy. The biggest effort was cutting the 3 mm steel plate with angle grinder. Top looks wasn’t the priority this time.

I was lucky to get help from a fellow race car builder who suggested a couple of smart changes: to add triangle plates, use sturdier tube for door bars and make them removable. It’s a street car after all. Door bars are made of 38x3 mm steel tube and bolted on to the support in the front and to the roll bar in the rear.

The idea was to weld the door bar front mounts as high as possible, and still keep the car entry easy.

And here are the results: before and after comparison. Bending reduced roughly to the half. On the road chassis feels more solid and car responds to steering inputs crispier than before. In general it felt like new set of stiffer shocks was installed. Shakiness and 60 mph shim are also gone. Am I satisfied? Yes I am. It wasn’t realistic to expect bending go down to zero. Current situation is a clear improvement and stiffer bushes are next on the list.

Chassis support preparations

In previous post I wrote that no deflection was noted when measuring “O” and “P”. That was incorrect. Once the rear was jacked up deflection in “C” and “D” increased about 2 mm. In my notes I had written that “it was difficult to do measure “O” and “P” accurately (alone with measure tape…)”. So in the future let’s just concentrate on “C” to “F”. Also slightly loose roll bar bolts could play a role here. Oh well, I must tighten the bolts for next round.

Here is a photo from miataturbo.net (http://www.miataturbo.net/race-prep-75/some-seam-weld-photos-59541/page2/#post761185) forum. There is a great thread about race prepping the chassis. I draw my supports in black. Number one connects floor to the sill and side frame, and number 2 connects fire wall. I am not going to take the dashboard out to make things easier…and harder.

So far I have made models from 1 mm thick aluminium sheet that I found from scrap bin. That is why there are odd holes. The idea is to make the final support (support 1) from 3 mm thick steel and weld it in. I’ll also add a traditional triangular steel sheet (support 2) to the corner.

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.