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What is a stressed chassis!
Zagato - 30/3/13 at 08:10 AM

What do they mean by stressed chassis? Think I also read the body/chassis on a Caterham for instance was stressed. Are the ali panels actually part of the strength?

JoelP - 30/3/13 at 08:34 AM

You'd have to say that any chassis is stressed (if it makes it put off the garage at least!). Having your panels bonded does add to the strength of the chassis though.

smart51 - 30/3/13 at 08:42 AM

Stressed body panels are those that are rigidly fastened to the chassis such that they add to the stiffness. or strength of the chassis. Cymtrick's chassis mods do this in places by replacing diagonal tubes with welded in panels. Done right, this can make the chassis both stiffer and lighter.

GRP bonded on with rubber doesn't add strength.

britishtrident - 30/3/13 at 08:47 AM

All chassis are stress bearing, that is what they are for, I think you are really asking about alloy body panels.

The stiffness contribution of thin aluminium alloy body panels pop rivet to a steel spaceframe chassis to the torsional stiffness of the chassis is very low.

Where thicker panels of suitable grade light alloy sheet are properly riveted and or bonded to the frame they can contribute to chassis stiffness.

Peteff - 30/3/13 at 09:09 AM

Why the ? It's common practise to use the panels to stiffen the chassis.

Sam_68 - 30/3/13 at 09:14 AM

quote:
Originally posted by Zagato
What do they mean by stressed chassis? Think I also read the body/chassis on a Caterham for instance was stressed. Are the ali panels actually part of the strength?


I think we need to know the context.

As has been said, all chassis are stressed - that's their entire purpose.

Caterham (and many other spaceframes) can be correctly stated to have stressed skins/panels, and yes that means the ali panels contribute substantially to the chassis overall strength and stiffness.

In come contexts, to say that a chassis has been 'stressed' may simply mean that the designer has undertaken the relevant stress calculations and/or testing to ensure its fitness for purpose (perhaps surprisingly, this is not standard practice - many spaceframes are merely designed by best-guess experience and few are tested for stiffness once complete).

Zagato - 30/3/13 at 07:26 PM

Thanks for the replies. I think I must have read somewhere about the body panels being part of the stressed body by what you are saying. I was a bit shocked by the panels actually adding stiffness on a Caterham as they are just riveted on, and I would of thought the thin ali wouldn't give much added strength but I am obviously wrong. I think Caterham use some king of gunk to go in between the chassis and ali panels to reduce electrolysis but don't know if it's a cement of any type.

The bodies I looked at today in the show room old & new looked well built. Still can't get over how small everything is compared to Land Rovers that I'm used to - a lot easier to work on I hope

Sam_68 - 30/3/13 at 09:55 PM

quote:
Originally posted by ZagatoI was a bit shocked by the panels actually adding stiffness on a Caterham as they are just riveted on, and I would of thought the thin ali wouldn't give much added strength but I am obviously wrong.


It's possibly worth mentioning that:

1) The side panels on the Caterham cockpit are made from aluminium honeycomb sandwich (which is very stiff indeed, for its weight) not just plain aluminium sheet.

2) Prior to the introduction of aluminium honeycomb materials (and later composites), Formula 1 car chassis like the Lotus 25 and 72 were made pretty much entirely from single skin aluminium panels, riveted together - and were substantially stiffer than the spaceframes that went before them.

Certainly, bonding and riveting is better than just riveting (rivets 'fret' and allow the structure to go baggy over time; so does bonding and riveting, to be fair, but to a much lesser degree), and the grade of aluminium and types of rivets used can make a big difference, but every little helps.

In the early days, the entire transmission tunnel of the Lotus/Caterham Seven was stressed sheet aluminium, with no tubes in it.

britishtrident - 30/3/13 at 10:06 PM

err actually

Lotus Sevens up to and early S3 Caterhams didn't really have a proper transmission tunnel never mind a structural one , the chassis was unbelievable sparse.
see http://gglotus.org/ggtech/7-hamai-restore/projseve.htm S2 chassis but S3 was almost identical

The S3 chassis gained a couple of extra tubes (most notably the diagonal in the engine bay) after S3 production go underway but after that remained virtually unchanged for 3 decades.

Basically you can have a light alloy (or steel or composite) monocoque chassis or a steel tube chassis but if mixing the two techniques it is difficult to get light alloy panels to carry a share of the loads.



[Edited on 30/3/13 by britishtrident]

Sam_68 - 30/3/13 at 10:41 PM

quote:
Originally posted by britishtrident
Lotus Sevens up to and early S3 Caterhams didn't really have a proper transmission tunnel never mind a structural one...


The transmission tunnel was in 18 swg. aluminium, riveted in place, and was designed (along with the floorpan) to contribute to the structural stiffness.

And I would dispute your assertion that it's difficult to mix spaceframe and alloy stressed skin construction: it's well established that riveted and bonded alloy panels (or even just riveted, until they go baggy) contribute substantial additional stiffness to a spaceframe.

Slimy38 - 31/3/13 at 08:09 AM

I wonder if I could ask a similar question? I seem to remember one manufacturer uses welded steel panels, to me that has an obvious benefit in structural strength but the trade off that you have steel in a very vulnerable place, and that I guess it's weight is far more than it's equivalent in aluminium. Is aluminium still the first choice or should I consider steel? Is there a case for aluminium for some panels and steel for others?

Sam_68 - 31/3/13 at 10:13 AM

quote:
Originally posted by Slimy38
I wonder if I could ask a similar question? I seem to remember one manufacturer uses welded steel panels, to me that has an obvious benefit in structural strength but the trade off that you have steel in a very vulnerable place, and that I guess it's weight is far more than it's equivalent in aluminium.


Horses for courses, as you've already worked out for yourself. As a general rule, I'd try to avoid sheet steel panels if at all possible, simply on weight grounds... you can get equal or better stiffness and impact protection using aluminium honeycomb, but at a cost. If your budget doesn't stretch to it, there is sometimes justification for small amounts of steel panelling in key areas, but it usually smacks of cost cutting or bad design.

britishtrident - 31/3/13 at 11:17 AM

quote:
Originally posted by Slimy38
I wonder if I could ask a similar question? I seem to remember one manufacturer uses welded steel panels, to me that has an obvious benefit in structural strength but the trade off that you have steel in a very vulnerable place, and that I guess it's weight is far more than it's equivalent in aluminium. Is aluminium still the first choice or should I consider steel? Is there a case for aluminium for some panels and steel for others?



The modulus of elasticity ( Youngs Modulus " E" of steel is very roughly 2.4 to 2.9 times that of aluminium. In simple terms in pure tension (ie stretching ) when a bar of light aluminium alloy is subjected to a pull load it will increase in length (stretch) 2.5 times more than a steel bar of the same dimensions. This disparity in the

In pure bending the same applies to beam in bending but if you compare weight for weight with steel you can use a much thicker section of aluminium and weight for weight the aluminium beam will be stiffer than the steel beam, this is one of the main reasons why Aluminium and Magnesium alloys are the material of choice for road wheels.

Thin gauge steel sheet works well for stiffening a steel spaceframe but on larger panels it is prone to buckling -- which is the cause the infamous "twang" from the floor when getting out some Locost style cars with steel floors. This why on production cars the floor has swage channels pressed into the sheet material to act as stiffeners.

Light alloy floor panels have to be thicker and because of the thicker section are less prone to buckling but because of the large difference in Youngs Modulus it is a challenge to get the light alloy to make a worthwhile contribution to the structures stiffness.

Back in the early 1960s Lotus launched the the Lotus 25 F1 car it was a pure monocoque made of aluminium alloy, it was much stiffer that the spaceframe Lotus 24 however strange as it may seem it was also heavier. The ever practical Ron Tauranac at Brabham stuck with a spaceframe chassis but made the light alloy body work to stiffen the chassis. But because he had to use thicker section material he ended up with a car that was a bit stiffer than the spaceframe cars but heavier than both the spaceframe and monocoque.


Take a look at the design of the Lotus Seven S4 chassis there is a web page somewhere with a structural analysis of it and FEM evaluation of modifications to it. http://cnx.org/content/m37138/1.4/

Also look at Cymtricks mods



You will find lots of info here http://locost7.info/mirror/aussiemods.php


[Edited on 31/3/13 by britishtrident]

[Edited on 31/3/13 by britishtrident]