how to reinforce a 1302 convertible chassis?

[lelef]

Hi,
I've noticed that the chassis isn't torsionally very strong
Just to better understand the problem
I've completely restored the car 4 year ago, so the chassis is good
The front mcpherson is a mix with vw Golf suspension lowered springs stronger sway bar all red polybush ,adjustable domes and dome bar.
the front is 80mm wider
The rear is 100mm wider ,brake disks, all red polybush .
I use a Renalt R5 transalex (5speed) it has a steel case and engine and trans assebly has 3 points rigid mounts.
It could be done something better at the rear because whit the supercharged engine there is some flex but seems to me a nonsense to reinforce the rear when lifting the front from one wheel is noticeable that the chassis isn't really strong torsionally .
In a convertible isn't easy to find a way to reinforce the chassis
An idea is to weld on the central tube the upper half of another central tube raised 10mm on the front and 20 or more ad the rear plus some reinforcement in the rear section that will remain under the rear seat, a series of spot welds between the original central tube and the new tube cover should add more rigidity.
The car handles really well ,the chassis torsion should be normal like it is but I would like to fix it

[Q-Dog]

Roll cage.
_________________
Brian

'69 Dune Buggy
'69 Beetle Convertible
'70 Beetle

[lelef]

Hi,
yes, roll cage is the easy way
I've also a buggy and I know how good should be done a frame if you have no doors and with a roll cage but this is a car to use in summer with the family .
I would like to find something to stiffen the chassis with no visible mods

[slalombuggy]

You have a box with no top and sides that open (doors), you need to make something that replicates the top. and ties it together front to back

A cage is the only way to stiffen the car substatially

Nothing you add down the middle will make it anywhere near as stiff as a cage.

brad

[lelef]

A rool cage on a convertible dosn't seems to me looking good

[Erik G]

I dunno, I sure like this one

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[lelef]

my idea is to make something more like this Lotus backbone chassis

[slalombuggy]

That Elan chassis will still twist. I doesn't matter how big you make the center spline, torsional rigidity comes from making the box as wide and tall as will fit in the package. The bigger the box with the most cross bracing you can fit the stiffer it will be. A cage might not be optimal but it's the only real solution. You need to get diagonal strength and tie the front and back of the car together with more than just the door latches.


brad

[chrisflstf]

You need to choose between appearance or performance. One of them has to give

[Q-Dog]

You could weld the doors shut.
_________________
Brian

'69 Dune Buggy
'69 Beetle Convertible
'70 Beetle

[lelef]

this car is a collection of compromises
I've never liked small engines ,when I built the buggy I immediately excluded the vw engine ending with a 13b
My wife liked the beetle convertible so why not to restore one?
at the end I was surprised how pleasant and easy it is to drive it and unlike other car this doesn't try continuously to kill me
Every convertible chassis is the half resistant of the same model with a roof, so adding a roof is the easiest way, a roll cage is a wireframe roof so it will be no more a convertible !
Yes ,I've done some improvement ,better suspensions but not too hard, better tyres bu not a nightmare if it starts raining,better brakes not racing brakes, some more hp but full of torque from idle to hi-rev, 5 speed but longer rastios.
In the same way there should be something in the middle between appearance and performance to reinforce the chassis, probably reinforcing in other ways adding the same weight of a roll cage will result a fraction of the improvement but for this car will be enough.
Nobody have never tried to stiffen a convertible chassis without doing it like a racing car?

[theKbStockpiler]

What about making custom reinforcement rails that are thicker? Would the connection points be strong enough to make it a possible solution?
_________________
My beetle is not competing with your beetle. I have the yellow beetle in my town. There is a red one, a green one ......
Use all safety devices including a mask.

[Chad1376]

Just throwing spaghetti at the wall to see if anything sticks..please.don't flame me if you think they're garbage ideas.

A) - Sort of a 1/2 cage

b) - Gusset just across the door sills. It would need some reinforcement, beyond just sheet metal, to prevent buckling.

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My never ending 1302 Autocross Project:
http://www.shoptalkforums.com/viewtopic.php?f=51&a...02b8f83f22

[slalombuggy]

[lelef]

Yes theKbStockpiler , It seems that for Miata there is something like that
Chad1376 , I,ve welded some reinforcing like this (yellow) but really smaller when I've restored the car
I could add 2 bolted brace from the bulkhead to the central tube, with the front mcPherson the bulkhead should be structural and also 2 bolted brace at the rear of the rear seats ,in this luggage area there is the idraulic system for the electric roof and the sound system so it isn't used for storage I should weld 2 bars to the central tube or weld another central tube on the existing one this will not help a lot in torsion but it will limit the flexing (red)

I'm unable to find again the topic but someone have welded a reinforcement of the central tube under the chassis but in my car I think there should be a ground clearance limit

[chrisflstf]

Instead of pan stiffeners, maybe use some better seats with side bolsters and a 5 point harness, of a half cage covered by a toneau cover to hide most of it

[Chad1376]

I'm going to throw out some super simplified structural theory: Moment of Inertia.

I=MR^2

I=Moment of Inertia - This directly relates to strength in bending and torsion.

M=Mass - Think of this as the amount of structural material

R=Radius - Think of this as the distance from the center of the tunnel

The key to all of this is the R squared. The result is you get exponential gains by moving as much structural material away from the center as possible. I little bit of material far away will be as strong as lot of heavy structure near the tunnel (ignoring buckling and a whole bunch of detail.)

It's making a bigger box, that Slalombuggy is referring to.
_________________
My never ending 1302 Autocross Project:
http://www.shoptalkforums.com/viewtopic.php?f=51&a...02b8f83f22

[lelef]

I think you wont to say torsional moment(if my translation is good), ok the formula is quite the same but dynamically with suspension ,brakes and the engine torque the center of the moment dosn't correspond with the center of the chassis like in a static test

[Chad1376]

Talking about center of structure...not center of the whole car's mass.

It's why and I-beam is shaped like an I. It's about moving material away from the center.
_________________
My never ending 1302 Autocross Project:
http://www.shoptalkforums.com/viewtopic.php?f=51&a...02b8f83f22

[ach60]

A VW pan really has no structural element other than the central tube.
When they make convertibles they add stiffening rails to the body, and gussets at the doors & cowl.
They added no additional stiffeners to the pan.
Now look at a modern convertible or even a big BMW 7 series, and you see the Door Sills(Rocker panels) have a huge section width.
For the front and rear suspension you need to triangulate the mounting points to stiffen.
You could section the center tube of the pan, and raise it 3", and raise the inner door sills 6".
These changes would not be noticeable, and wouldn't really entry or exit from the vehicle.
The whole deal with a roll cage is that except in race cars where a helmet is always worn,
is that it really only adds more shit for your head to smack around on in the event of an accident.
Yes you’ll be alive, but Mommy will be changing your diaper, and feeding you, until you die from complications from brain injuries.
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Good Luck
Al