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Chassis Flex

rtune
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rtune Experienced

rtune

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So i jacked up my r34 by the front right hard point and noticed when it was

on the jack that the drivers door hangs differently. Not by much but enough

to make a clack sound on the latch as it closes or opens.. passenger door opens

closes the same way however.

lower the car back down by that point and it closes normally again.

Is there really that much chassis flex/twist ?

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BHDave Grand Master

BHDave

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I don't have links handy but if you are going to foam fill the do A, B and C pillars as well as the chassis rails and sills. There are improvements in rigidity to be had according to some of the OEM manufacturers research but no quantifiable results in our cars.

Or just get a decent cage with some gussets to the a and b pillars...

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  • 2 months later...
Duncan Grand Master

Duncan

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wow I am late.

Yes the standard chassis flexes like crazy, but Ronin 09 said try it on most other cars!

In our new sports sedan we have a very serious cage but have also foam filled the A and B pillars, and the sills and chasis rails to get the most stiffness we can (and it adds stuff all weight). The problem is I can't tell if it made any difference because the didn't exactly do before and after tests on a 6 post shaker rig. But it is dirt cheap and light so it can't hurt.

The most common ways to spot the chasis flex is to close the door on a coupe while it is on a jack (it will clunk not just close), or jack it up from one corner and see how many wheels come off the ground. Our race car will lift 3 wheels off the ground when jacking it up from one corner.....

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rtune Experienced

rtune

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  • Author
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wow I am late.

Yes the standard chassis flexes like crazy, but Ronin 09 said try it on most other cars!

In our new sports sedan we have a very serious cage but have also foam filled the A and B pillars, and the sills and chasis rails to get the most stiffness we can (and it adds stuff all weight). The problem is I can't tell if it made any difference because the didn't exactly do before and after tests on a 6 post shaker rig. But it is dirt cheap and light so it can't hurt.

The most common ways to spot the chasis flex is to close the door on a coupe while it is on a jack (it will clunk not just close), or jack it up from one corner and see how many wheels come off the ground. Our race car will lift 3 wheels off the ground when jacking it up from one corner.....

I read some info on foam filling and don't really like what I read. Nissan or Nismo recommend against it too. Why? because

although it appears to add rigidity, it will not flex, it will simply snap. So it might be great for a road car and add rigidity to

a point, but hammer it around a circuit and sooner or later (maybe sooner) it is going to snap at the point(s) that are doing

most work, and once snapped it is totally useless and nearly impossible to remove.

Stands to reason, I mean, take the popular foam products and make a tube, let them set. Very tough, like an iron bar?

but would you lean it against a wall at 45 degrees and stand on it? jump on it? you can do that with a roll cage, no problem,

but the foam will snap like a stick of rock candy. You can probably hit 2g vertical on some corners on some circuits.. thats

a 500 kg load hammering against the foam .. which has no flex at all ..

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Duncan Grand Master

Duncan

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yeah fair call and not just for foam filling. there is no doubt that a car that flexes more will last longer (it will take the shock and then flex back into place). the stiffer the car (or even any solid member) the more likely it is to crack instead of break.

In the case that we are using (light/stiff race car) I am happy to take that tradeoff. The less the suspension mounts can move compared to each other the more predictable the handling will be. If the chasis is stuffed in a couple of years of racing its just $5k for a new one.

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Roy Grand Master

Roy

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Posted

I think you will find that the foam in rails is an 60s and 70s thing, mostly from rallying. There was thinking that it helped with stiffness, but the main reason they did it was to keep mud and garbage out of the rails which over time would cause corrosion.

Again it was noted to help the stifness by a bees dick but the other reason i know rally guys do it to the B and C pillars is to improve the acustics in the cabin and help the driver hear the navigator with pace note calls. It tones down the NVH from all the hollow spaces in the cabin

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