e30 Cabrio Chasis Stiffening

[agent]

BMW CCA will not let you run track days they sanction in a convertible - period. There are other organizations who will though.

Just something to point out for current and future readers of this topic.

[jeffnhiscars]

I feel like we're talking torsion, tension, and compression. To me it's like a 2"x4"x12" cardboard box versus a 4"x4"x12" cardboard box. One is certainly easier to twist and bend.

But in the end, it's still just a cardboard box. Stack em 10 high and they are fine. Lean on it and it will fold under its own weight.

[validius]

Aren't we talking torsion not tension ?

The chassis does 'twist' but everywhere else besides the center of the twisting is going to experience linear displacement as well. One of the rear mounting points will be moving away from the windshield mounting point on the opposite corner. This would put the hard top in tension along that line. There would be a torsional element, yes but the top would not be nearly as strong in this way.

[jeffnhiscars]

The chassis does 'twist' but everywhere else besides the center of the twisting is going to experience linear displacement as well. One of the rear mounting points will be moving away from the windshield mounting point on the opposite corner. This would put the hard top in tension along that line. There would be a torsional element, yes but the top would not be nearly as strong in this way.

Does that mean, in your opinion the hardtop does little to keep the chassis from flopping like a flounder ?

[validius]

Does that mean, in your opinion the hardtop does little to keep the chassis from flopping like a flounder ?

Not at all. It means that if the hard top (and mounts for the hard top) has meaningful stiffness in the a direction that the chassis flexes significantly it will make a meaningful difference.

Saying that A*B*C=Z doesn't tell me much if i don't know enough to say with confidence that one of the terms isn't 0 ;-)

I will say that i'm quite confident it doesn't hurt stiffness though.

[jeffnhiscars]

I agree 100% that it doesn't hurt. Now if only someone had any evidence that it actually helped :)

[Ether-D]

What if someone were to jack up their e30, one corner at a time, and measure the heights of all of the jack points both with and without the hard top in place? Seems like this could prove or disprove the hardtops' stiffening capabilities.
Not me though. I wanna be right, but not that bad...

[validius]

What if someone were to jack up their e30, one corner at a time, and measure the heights of all of the jack points both with and without the hard top in place? Seems like this could prove or disprove the hardtops' stiffening capabilities.
Not me though. I wanna be right, but not that bad...

Next time we have the robot-mounted laser CMM at work i might try and bring my car in and try this (without a hardtop). If i do, i'll stick a load cell between the jack and the car.

[jeffnhiscars]

What if someone were to jack up their e30, one corner at a time, and measure the heights of all of the jack points both with and without the hard top in place? Seems like this could prove or disprove the hardtops' stiffening capabilities.
Not me though. I wanna be right, but not that bad...

I don't think that will confirm anything if you can't measure the difference in force on the jack both with and without the hardtop on.

Beyond that, the chassis works like a progressive spring in that the further it twists, the harder it is to keep twisting it. This tells me that to REALLY know the answer to this great riddle, you would have to measure the change in force (i.e. The added resistance from the hardtop) in real time. This is now dampers are measured...lbs/in/sec and this would be before we factor in the variation from car to car since we all have different suspension and chassis upgrades. Again, the stiffer your chassis the less potential impact the hardtop has.

I suspect there is a tipping point in real time. Initially, twisting the chassis would take far more force that twisting the hardtop so the chassis would dominate, at a certain point when the chassis is running out of flex and the hardtop is getting stiffer (from being already flexed) the hardtop would add resistance. Do any of our cars ever twist that far ? Who knows but I sure hope not.

I think in the end the result would be statistically insignificant. Call me doubter.....but NOT stubborn lol

[Julien]

I hope that s directed at me. I grew up in NY, dad was an attorney, mom modeled, bro went Ivy League..me..I went to Woodstock in my very first swap...a Porsche powered VW Van so never had the luxury of thinking of myself as a red neck....lol

Could get used to it

So that's where you get your charming good looks from? ;)