4 point lower cross brace

[petrolhead]

Honestly, i think they do flex quite a bit (nothing compared to my 73 charger, before installing weld in frame connectors). But if i jack up my e30 form a corner, or even put it on a 4 post lift, the doors are now more difficult to open/close. This is on a rust free Arizona car with the only accident being the one that i inflicted but was only a fender scuff, the rest of the panels are rust free and original.

I'm not saying there isn't room for improvement , or that there aren't weak points in the chassis (firewall , A-pillars/windscreen frame, etc..) .

But , one example.
When you take apart an average oldtimer and put it in a rotisserie, its advisable not to climb in there because it'll bend the chassis. Not so with a 2d E30. I measured (tried to) how much front and rear subframes / their mounting points flex relative to each other, by loadind up the cabin with ~200kgs of 'stuff' , then by jacking up the whole chassis from the centerpoint of the floor. Measurements didn't go off.

[Fidhle007]

I'm sure the chassis is reasonably stiff but I'd imagine there would be some give in the subframe/unibody junction that a cross brace could help out with. Also, the loads seen under driving conditions (especially on a track) are going to much more then you can achieve by weighting or jacking a car. Like a few thousand Lbs more... I'm not saying every car should have a cross brace (I've stated before that I would never put one on a street car) but I can't think of a reason not to install one on my track toy.

[Jaxx_]

I can't think of a reason not to install one on my track toy.

extra weight? lol

there are other ways to get your chassis more rigid that no one mentions in this thread, like expanding foam.

[petrolhead]

Stretching my English skills for the following so pardon me..

I'm sure the chassis is reasonably stiff but I'd imagine there would be some give in the subframe/unibody junction that a cross brace could help out with. Also, the loads seen under driving conditions (especially on a track) are going to much more then you can achieve by weighting or jacking a car. Like a few thousand Lbs more... I'm not saying every car should have a cross brace (I've stated before that I would never put one on a street car) but I can't think of a reason not to install one on my track toy.

The lateral and longitudinal forces that suspension/arms/subframe/chassis face, are limited by grip , right? for a 1000 kg car (~2200lbs ?) they're no way "a few thousand pounds" per axle. Not when you're pulling max 1 G in cornering. I saw a calculation somewhere , about a strut brace. 2200 lb (1100 lb front end) , 1 G cornering with 100% weight transfer, "leverage ratios" of E30 front suspension , add up to less than 150 kg lateral force on outside wheel strut tower.

Vertical forces in acceleration/braking/cornering are limited by the car's weight. Not "few thousand lb" per corner either. Bigger forces than that, come from tire hitting a bump, kerb etc. The forces are there , no argument there. But , if you say that these forces considerably twist the chassis , but the same twisting can't be detected at all by putting say 200kg load in one direction then another, I disagree.
If those vertical shocks are so very big , how come they don't cause permanent deformation. When you measure an uncrashed e30, strut tower heights are likely +- couple mm right , as is pretty much everything else.

As for not being able to detect chassis deflection by simply jacking up a car etc , I disagree (when the chassis in question isn't that stiff). My buddy wanted me to weld him mounting brackets for a front strut brace (no BMW , big 80's coupe..). I was sceptical and told him he'd get a nice placebo effect from it, while driving and nothing more. Well we did this while the car was of the ground , and cut a piece of pipe to go between the towers , took measurements and made bolt holes. Now, what happened , was that the bolt holes were ~1,5 cm off once we put the car back on the floor. And this was not just the McPhersons' upper ends moving about/flexing as they always do with their soft rubber, but the chassis itself deflecting that much.

extra weight? lol

Well , I CAN think of 100kgs worth of other useless stuff I don't want in my car. But , more seriously, I'm a sceptic by nature. Especially when it comes to things people want to sell me. Budget being (very) limited and weight being a major concern, I'm trying to build my "toy" by getting everything I need , and nothing I really don't need.