What gives a car body its rigidity.

[Roverageous]

First things first - I know nothing about vehicle construction & where stressed are placed on the car chassis / body / whatever.

 

I gather that modern cars do not have a 'chassis' any more, but the body of the vehicle provides the actual strength. Which parts of the body are stresses placed during use?

 

Do rotten sills risk the car 'folding up' - providing that both sills are rotten in the same place on either side?

 

Do sill repairs affect the strength of the vehicle?

 

What about the floor? The footwells on most cars I've had seem to be very thin metal - will a rotten floorpan cause the rest of the body to flex / twist?

 

How rotten would a car have to be to introduce extra flex during driving?

 

Sorry for all the questions!

[Snipes]

Anywhere you can see box section is a good indication of where a vehicles strength lies but, basically, the whole is greater than the sum of it's parts. So everything contributes.

[Tamworthbay]

Search monocoque on the Internet as a good starting point. It depends massively car to car, some have unstressed panels bolted on e.g the original DS. The basic structure is designed to give enough strength (and presumably a bit spare) in original form. Decent repairs should maintain this strength and I have been amazed by some of the bodges I have seen over the years. The problem comes when they are in a smash and the strength isn’t where it’s supposed to be.

[Partridge]

Underseal. Well, it did in my old W124, I don't think much else was giving it rigidity.

[chaseracer]

It's the rust elves holding hands, isn't it?

[BavarianRetro]

Try cutting the roof off and jacking one up before opening a door.

[pshome]

A shoe box explains it very well. It's very rigid, until you remove the cover.

Same "box" principle gives cars their rigidity.

[MikeR]

Try cutting the roof off and jacking one up before opening a door.

some cars dont even need the roof off to do that ,

 

I had a Fiat 127 that would go banana shaped when it was jacked up and the doors would not close  !!

[Asimo]

Make youself some models with paper, (panels) and straws ( sills etc). Explains a lot.

[PhilA]

With regards to repair sections, so long as it retains the same shape and the new metal is securely fixed in place then it's going to have the same strength as the original.

 

Modern metal is very thin overall compared to, say, a Morris Minor; instead computer designed geometry has taken the place of thicker steel.

The cabin is built more like a roll cage these days, with the roof edges, screen, doors, door posts and sills forming the "safety cell", and crumple zones outside the safety cell that are designed to deform in a wreck, absorbing the energy of the movement of the vehicle(s) before deforming the safety cell space.

 

As with any geometric shapes, their strength relies upon the strength of the whole. Begin to remove elements of that strength and you do lose the rigidity of the shape in general.

 

As for folding up, yes. There is a risk of it but if the sills are gone then likely the A posts are also, the strong points around the forward firewall and the shell is less strong in many places. However, if you're going to be in a wreck that crushes the car that badly then you're going to be in a pretty poor shape.

 

In short, yes. Modern cars do rely more on the shape of their panel work (hell, torsional stiffness now in a lot of cars is provided by the windshield glass!) for strength but can be repaired to be just as strong as they were when they left the factory.

 

Phil

[richardmorris]

some cars dont even need the roof off to do that ,

 

I had a Fiat 127 that would go banana shaped when it was jacked up and the doors would not close  !!

 

My last Citroen Cx was a bit like that- you couldn’t open the doors when it was on the four post lift. And could visibly see the panel gaps move!

Happily the fiat and merc are better.

[PhilA]

My Renault can be jacked up on the front frame rail ahead of the A post and the doors open and close like it's sat flat on the floor on its wheels. Apparently that was part of the design, to have superior torsional rigidity to keep the suspension in alignment under hard maneuvering.

 

This being where the old "park one wheel on a kerb and try open the doors" comes from. The door might open- closing again can be another story as the shell twists out of shape...

 

Phil

[Pillock]

If you jack an Allegro up, the steering wheel goes square.

[Dick Cheeseburger]

Rigidity wasn't invented until the 1980's.

[Simonsays]

In the case of my Streetka?

 

It was mainly my hopes and prayers that kept it rigid.

 

You could feel the body flexing if you drove it hard round corners.

[chaseracer]

Rigidity wasn't invented until the 1980's.

 

Issigonis clearly didn't get that memo when he was designing the ADO17 1800.

 

[spike60]

I had a 5 year old Manta hatch that you couldn't open or close the doors on when jacked, my Alfa Sprint needed the door hinges bending back up with a trolley jack about every 3 weeks. . It wasn't rotten.

[mat_the_cat]

In the case of my Streetka?

 

It was mainly my hopes and prayers that kept it rigid.

 

You could feel the body flexing if you drove it hard round corners.

 

Before I took the Stellar off the road in the early 2000s, you could actually get the interior light to flash on during enthusiastic cornering! Sills have now been replaced, with extra internal reinforcement, and I've also strengthened the rear axle mounting points. Noticeably stiffer now

[bezzabsa]

buy a Land rover with a propa chassis!!!!!!!

 

 

 

 

 

until it rusts!

[forddeliveryboy]

My last Citroen Cx was a bit like that- you couldn’t open the doors when it was on the four post lift. And could visibly see the panel gaps move!

Happily the fiat and merc are better.

The Series 2 models were made from significantly less good steel, but CXs always surprised in odd ways. Yes, jacking one up (even from the correct place, which plenty didn't) could suggest all sorts of severe structural weaknesses yet this never appeared in the way the cars drove or crashed.

 

Rather than judging the integrity of the structure when jacked in a way which doesn't reflect reality, I find a far more telling simple test is to press your fingers into the door-body gap when pressing on over demanding roads - when a car's structure is working hard. CXs never showed much movement in this respect, if anything less than the V70 I use day-to-day.

 

'Rigidity' is an odd concept, the strongest trees are those which are most easily able to move when necessary, within limits, without doing any damage. And as chaseracer points out, the Landcrab was demonstrating inner strengths decades before it became fashionable, just as Saab, Citroën and others did with secondary (and primary) safety.

[chodweaver]

Regarding weakness in the sills - yes, a most dramatic example would be where there was serious weakness in both sills at the same point along their length - for a car with either a non-structural or no roof (I.e. a convertible), opening both doors and jumping up and down in the middle could get it to sag and then fold up. Somewhere here there are pics of a Cortina estate that has done the opposite - repeated rotting out at the rear axle or further back having been repaired with welded patches without straightening the body first have left this poor car bent like a banana - the boot floor and the front bumper both being lower than the middle of the sills.

 

In practice, weakness in both sills will rarely be in exactly the same place, so the car will tend to twist rather than fold, and do so unevenly according to whether turning left or right - leading to some very unpleasant and likely properly dangerous handling characteristics. The shoe box example given above is worth extending - try twisting one, then cut a slot out of each of the long bottom edges, in different positions, simulating rusty sill sections and see how it flexes.

 

Much of the gentler shaping in larger non-exposed panels or floor will be to stop those panels from flexing, resonating and emitting booming noises on the move. Older car designs had far less of this built in, and sound deadening was achieved through lots of wadding and tar-like materials which worked but added weight, and was limited to the better built and more costly models.

[New POD]

Rigidity wasn't invented until the 1980's.

And can now be obtained from a Pharmacist without prescription.

[Paul Dupart]

As long as the tin worms keep holding hands you're ok....

[HMC]

Interesting thread, and not an easy question to answer- for us nor engineers when lots of our cars where designed; especially when perhaps a different design team works on a new variant of some sort.

 

My most flexible car was a classic shape 900 convertible; my recent beetle convertible was impressively rigid. I suspect the beetle team knew a Cabriolet was going to be planned. Especially impressive as the MK IV golf underpinnings never sired a golf convertible directly. Sometimes the orignal design is modified and becomes less stiff. My 405 estate had an impressively open load space but was more flexible, inevitably, than the saloon that had a rear bulkhead and a high rear boot sill. The skoda favorit facelift, at least on the later ones, had a front strut brace fitted from the factory.

[steveo3002]

why are you asking ...kinda sounds like you want to drive around in a rotten car

 

will the car bend in half with rusty sills?   probably not if its raised carefully , care needs to be taken when replacing them that the body is braced and panel gaps checked before welding 

 

do repairs effect the strength ...well if done to a high standard good as new , you do see alot of hobby welding that wont be as strong as it should  

[DSdriver]

You can tell when a DS needs the sills doing because the door pops open when you hit a bump.

[stephen01]

jack up a Triumph Herald! 

[Junkman]

some cars dont even need the roof off to do that ,

 

I had a Fiat 127 that would go banana shaped when it was jacked up and the doors would not close  !!

 

Same thing with the Granada MKII and actually pretty much any car.

Because cars are deliberately anything but rigid.

 

This might be a surprise for many, but body repairs, especially amateurish ones,

actually add unwanted rigidity to car, which can seriously affect its strength.

[captain_70s]

In my case it's mostly underseal and tape.

 

I understand some cars use metal but I see no reason to add that sort of weight to my already under powered cars.

[davehedgehog31]

If it's British and more than 25 years old, filler mainly.