I won't dispute a single thing you have said here, but will relate these four observations from putting a tunnel brace on my car:While I will always applaud making some effort over no effort, the DDM test is/was not indicative of any real world scenario nor did it follow any standard expectations for a chassis torsion test.
All this is not to say that the brace doesn't improve torsional rigidity, but there's no possible way it improves it as much as the kappa world likes to think it does. KG's 1 or 2% is probably accurate based on my testing of a similar GM chassis, and that test involved going from a 1/4" aluminum piece up to a 3/8" multi-layed carbon/kevlar piece.
I find it hard to believe that a piece of 3/8" thick aluminum adds sufficient strength counteract the rotation on the x axis at one end when the other end is being held in place with as large a cross sectional area as a vehicle has. You have to remember the mechanical advantage due to the axis being in the enter of the car and the forces are being applied one the outside edges of the vehicle.
If you took that backbone brace and attached one end of it to a fixed mount and then on the other end you attached a bar that is 1/2 the width of the vehicle how much force do you think you would need to apply to the end of the bar before the backbone brace started to twist.
I am going to tell you that it's not much. the difference between the stock stamped piece and the solid aluminum one is negligible because of the mechanical advantage. If you had a plate that went from the pinch weld to he pinch weld on the other side and it was bolted to the pinch welds... That would make a difference. because of how narrow the backbone brace is and where the attachment points in the vehicle are it's not a huge help.
Now.... If you boxed the transmission tunnel with 3/8" aluminum and attached either end of the box to the front and rear subframes by use of a cross member you would see a large change in the chassis rigidity.
I am not sure as to why it got named a "backbone" brace when the kappas do not have a backbone style of chassis so there is no backbone to brace in the fist place. The transmission tunnel is not any kind of a backbone. If you boxed the entire transmission tunnel and did the cross members at the subframes you would have then made the car a hybrid chassis where it has both the ladder and backbone styles.
For streetability flexing between the front and rear subframes is OK and is actually needed. The flexing is engineered to take place in very specific spots of the body. Even on a full frame vehicles this is true. Body mounts are the places that have been engineered to flex. Because unibody does not have body mounts key places in the body have been engineered so that they move without causing visible distortion of the painted areas.
As @JohnWR stated with the lifting of a tire off the ground because of a bump. Imagine hitting that bump going around a corner, think it would be better have all 4 tires planted on the ground or only 3 of them? I am pretty sure that 4 is going to be better.
In a race application that is not drag racing excessive twisting between the front and rear subframes is unwanted this is because of the difference in lateral forces between the front and the rear subframes when going around a corner. Ever ride on a roller coaster? Ever sat in the very last seat? You get the snot kicked out of you. the lateral forces in the rear are substantially greater then the front. In a car the same thing applies and if there is too much flex between the front and rear subframes the rear tire on the inside corner will lift of the ground. Back to the previous statement of 4 tires on the ground it better then 3 tires on the ground. It's a delicate balance based on application.
There is a point where stiff can be too stiff. This is when the vehicle is unable to handle variations in the road surface properly. The suspension plays an enormous role in chassis stiffening. Both things need to be approached at the same time. if you stiffen the chassis for a sock suspension and get it right for that suspension and then you put in a race suspension the chassis is going to be far too stiff.
The best thing to concentrate on when stiffening the chassis is what each subframe is doing. You want to keep those rails parallel and square to each other as that is going to effect the handling of the vehicle the most due to changes in in the alignment if the frame rails deviate from each other. Best things to do to help the chassis is to make a box between the shock/strut towers.
for the front of the Kappa something like this for the design would work
View attachment 114955
and for the rear you apply the same principal
triangles = strength that's why they are used in the construction of bridges and tall buildings. Use them to keep portions of the frame square to each other. They take up minimal space and weight for the amount of strength they add.
Convertibles are going to have an inherently large amount of twisting between the front and rear subframes, this is because there is no roof structure that is apart of the chassis. If you stiffen up the chassis between the subframes by using frame connectors and or a roll cage handling performance of the vehicle will diminish when driven on the street. There needs to be some movement so places would need to be engineered into the deign to allow for movement if the vehicle is to be driven on the street.
I cannot even begin to tell you how much of an improvement there is from doing the boxing between the shock towers both front and rear. It's the difference between driving say a 1970's land yacht and a modern day super car.
Now if a tunnel brace was deigned from 4 pieces of 1/2" plate aluminum that formed a box approximatly 1" apart from each other and that was bolted down the full length of the tunnel so that either end could have a cross member attached to it that boletd to each subframe. That would make a large change in reducing the torsional twist. The key is attaching to the front and rear subframes and making a box and having walls inside the box. The "walls" would need to have different angles in different directions.
Don’t blame yourself for missing that one.Thanks for all your help. I found the problem, and it was actually one of the first things I should have checked. 🤦♂️🤦♂️🤦♂️
There were 2 different size tires on the rear!!!!
I had a stock size 245/45R-18 on the left side and a 245/35R-18 on the right. I replaced the tire with the correct size and all is well now.
I think that "hides" may not be appropriate, at least not for all situations........ Adding a mod or reinforcement hides the issue.
He has a 2.4, and from an earlier post it appears to be an early one, which means no traction or stability control, only ABS. So no nannies to get upset.Wouldn't the traction nannies go bananas with the difference in wheel diameters? I know some cars will have 2-3% tolerance. The OP's rear wheel diameters have a 7.2% difference. Maybe our Kappas are more liberal.