Weld strength & welded vehicle seams

[keith19]

Hi Dave, you are giving me a Tatra example which already has a chassis. Perhaps you can find me an example with a monocoque?

All really heavy-duty vehicles still have a chassis, with one or two exceptions like the tank, which has a shell structure. The bigger the vehicle, the more likely the chassis is to flex, so as you cannot build a huge long vehicle with an absolutely rigid structure, particularly a load-carrying vehicle, a certain amount of flex has to be built in and allowed for, and big vehicles are engineered to reflect this. We learnt all this in tech classes over 30 years ago.

So far as the 4x4s are concerned, the complaints about cracking didn't come from Chelsea mothers, but from US citizens who were giving them what they were supposed to be designed for... The older pickups, less rigid, with chassis, hadn't had these problems, and people wondered why is the new one so bad?

 

[PhillipM]

I don't have any trouble with my off-road chassis being too rigid

 

[zardoz]

However, mistakes are still made, even in the computer age.
Here's Ove Arup and Partners covering their corporate @sses after their cockup with the design of the Millennium Bridge...
http://www.youtube.com/watch?v=gQK21572oSU

One of the pictures of briges they show at the end of that video, is the Queens Park suspension bridge in Chester. That bridge has exhibted the same side to side movement when people 'lock in' with the swaying. It was common knowlege to anyone who walked over it on a summers day when the riverside groves were packed with tourists and locals enjoying a day by the river.

As school kids in the 970's we sometimes waited up at the school side till we had a good mob of peope then marched across,to 'get it swinging' once it was moving from side to side, you could no longer keep in march step and had to adjust your step to the side to side swing. Otherwise you had a job standing upright never mind moving forward.

That bridge was built in the 1920 or 30s. If those mllenium bridge builders didnt know of this effect then I'm a moon man.

 

[keith19]

Zardos, I'd agree. The tradition of soldiers 'breaking step' before going over bridges goes back hundreds of years, even when the bridges were stone bridges and metal was in its industrial infancy.
Most of the rules of thumb for building all kinds of structures have been found to go back to ancient times. The rules for getting proportions right are another example, and go back at least to the ancient Greeks.
But a great deal of modern design is done by computer, so if the proper factors are not entered into the equations in the first place, the result is found to be wanting. This is human error, perhaps due to laziness, lack of training, or even commercial economics.

 

[pedrobedro]

4x4 chassis flex is something they are trying to eradicate not build in.

Watch this and explain what advantage the flex gives

 

[Robotstar5]

4x4 chassis flex is something they are trying to eradicate not build in.

Watch this and explain what advantage the flex gives

I liked the test with load in trashing the Ford!

Saw something similar with a Jeep wagoneer?, the owner had previously had Land Rovers and parked it with one front wheel on a bank, drivers door opened but wouldn't shut and passenger couldn't get his door open due to bodyshell twisting!, next time we saw him he was back in a Land Rover

 

[keith19]

The Chevy test, conducted as an ad for Chevy, only shows a couple of specialised tests on two new vehicles, in particular test applications that would be difficult to simulate in natural driving, even on a farm. We are unlikely to want to carry 3,000 lb up and down such extreme gradients.
The problem with such tests is they highlight only a specific way of showing a supposed advantage for advertising purposes. They do not reflect real life working conditions.

Maybe if they showed two used trucks five, maybe ten or fifteen years down the line, when they are well rusted out, we might get a better indication of reality. The monocoque structured vehicle won't be in quite the same torsional strength field, and any bad distortion will tend to remain permanent. Whereas the more flexy chassis vehicle will be much the same as it is now, although the body on top may be well rusted. That's if stress areas on the Chevy monocoque haven't already broken and brought it to an early demise.
Monocoque is a good rigid design when its new and unworked, but being made of thinner metal than a chassis it rusts out and/or stresses out earlier, and is often more complicated to repair. A chassis is heavy metal, and may well flex more than the monocoque, but will keep going a lot longer and maybe outlast several cabs, and will be much easier to repair.
Just speaking from personal experience in the trade; your experience may be different.
A friend has a 1950's GMC stepside pickup, with a pretty flexible chassis, and great fun to drive. It's had a few bodywork repairs but hardly anything on the chassis. There aren't many monocoque-bodied vehicles half that age still around, all rotted out long since. So I figure flex is best; something good-looking with a bit of flex has a lot going for it...

 

[PhillipM]

The thing with seperate chassis vehicles, is most of them are like triggers old broom, there's nothing left of the original steel after 30 years
Especially where the stress concentrations are from the flexing.

Whereas people don't care so much about repairing a monocoque as they brush it off as bodywork....

 

[Hopefuldave]

Hi Dave, you are giving me a Tatra example which already has a chassis. Perhaps you can find me an example with a monocoque?

All really heavy-duty vehicles still have a chassis, with one or two exceptions like the tank, which has a shell structure. The bigger the vehicle, the more likely the chassis is to flex, so as you cannot build a huge long vehicle with an absolutely rigid structure, particularly a load-carrying vehicle, a certain amount of flex has to be built in and allowed for, and big vehicles are engineered to reflect this. We learnt all this in tech classes over 30 years ago.

So far as the 4x4s are concerned, the complaints about cracking didn't come from Chelsea mothers, but from US citizens who were giving them what they were supposed to be designed for... The older pickups, less rigid, with chassis, hadn't had these problems, and people wondered why is the new one so bad?

Well, if you want heavy duty, monocoque 4x4s with hugely stiff shells, take a look at Paris-Dakar race vehicles, not much that's stiffer than carbon/Kevlar laminate... If they manage to damage one, they can airdrop replacement sections, not necessarily with a chute, and glue 'em on...

They *don't* run live axles, though, they have proper compliant suspension!

The Tatra is the same, hugely rigid and torsionally stiff, it's designed not to, and doesn't, twist (if it did it would be a major problem, as the chassis is also the transmission casing for all 8 driven wheels) - it's a considerably more modern, better thought out than almost all off-road vehicles. The designers started with a blank sheet, no preconceptions, and produced something that thoroughly trounced the competition. If you want to see offroad capability, YouTube has some amusing truck trials videos. The Tatra's weapon of choice in the heavy truck class.

I guess you're a fan of bendy trucks, myself I can't bear 'em - they rattle and crash about, have appalling road manners, and the flex tends to shed bits everywhere. I learned about them 30 years ago too, but I've tried to keep learning since

Dave H. (the other one)

 

[KemppiFrog]

Lots of horses for courses mixup here. Race cars need stiff chassis to alow the suspensions to do their designed job, their drivers expect sharp limits . Monocoques are cheap to build and fairly stiff and.. are expected to deform to protect their occupants, then be thrown away. 4X4 & pick-ups used to be durable, handling was an after thought, so chassis and live axles are/were the norm. Commercial vehicles are about carrying load at least cost, so handling and flex are not an issue. Note the ally wheels on many artics. Each kg. saved on the chassis traslates into better cargo capacity. X a million miles. They just soft mount the cab to reduce driver fatigue.

I remember Practical Mechanics showing how the new era of 'safer' cars was dawning in the 60s. (The mag was in the libary at school) They showed massive chassis as the way forward. Dumb, as developments have shown. And the accident stats show that chassis vehicles (mostly 4X4s) have a higher rate of head injuries as they stop too quickly compared to a monocoque that absorbes the impact and reduces the G force.
KF

 

[keith19]

Hi Dave, I must say your Tatra example is pretty extreme, and is a good example of a very heavy military vehicle. They must have been very expensive to make, even the state-funded Czechs stopped making them way back in the early 1980's. A chassis like that would last a very long time. But the drive and suspension design is pretty exceptional, and made for extreme conditions. Consumer motor and truck manufacturers cannot afford to do more than look at stuff like that.

So far as Kevlar laminates go, I believe compressive failures can occur at relatively low stress levels, which means they are again a rather exotic hybrid, perhaps only affordable when used in competition cars. Such laminates are primarily used for lightness as well as rigidity, and not made to last the lifetime of a standard vehicle.

Yes, I do like old trucks. They have a character and simplicity often lacking in more modern vehicles. They were often over-engineered, because there were no computers then to calculate exactly what a manufacturer could get away with while the vehicle was new. So when strength was calculated, they simply added another 33% on, just to be sure. Perhaps that's why some are still around today, I guess. Mind you, petrol was dead cheap when those things were designed...

I like to keep learning too, but I take everything I'm fed with a pinch of salt, because a lot of today's auto design and manufacturing is gradually taking away from the average owner the ability to do anything on his/her car other than an oil change. The con is to get the manufacturer's agent to do everything, and charge well. Time was when you could do pretty well any job on your car with a set of spanners and sockets, and a timing light...

 

[Badger1bear]

mines fairly heavy but still flexible LOL

https://fbcdn-sphotos-c-a.akamaihd.net/hphotos-ak-ash4/290762_10150266204025793_5363613_o.jpg

 

[keith19]

That's pretty impressive, BB. You been studyin' them Tatra's?

 

[Badger1bear]

That's pretty impressive, BB. You been studyin' them Tatra's?

More like Nick Leeson's financial investment program

 

[Badger1bear]

at last ive got ta grips with not linking to photobucket

 

[t'rusty duck]

Keith,
are you suggesting that the current Chevy pickup is of monocoque construction?
Doesn't look like it here, more like what they'd probably call 'body on frame' ( 3" x 6" x 1/4" box section?) in the USA, comparing the chassis depth to its 17"wheels.

Have to agree that plenty of flex in a chassis does keep you alert, what with having to fight the thing round corners on bumpy roads...

Ken

The Chevy test, conducted as an ad for Chevy, only shows a couple of specialised tests on two new vehicles, in particular test applications that would be difficult to simulate in natural driving, even on a farm. We are unlikely to want to carry 3,000 lb up and down such extreme gradients.
The problem with such tests is they highlight only a specific way of showing a supposed advantage for advertising purposes. They do not reflect real life working conditions.

Maybe if they showed two used trucks five, maybe ten or fifteen years down the line, when they are well rusted out, we might get a better indication of reality. The monocoque structured vehicle won't be in quite the same torsional strength field, and any bad distortion will tend to remain permanent. Whereas the more flexy chassis vehicle will be much the same as it is now, although the body on top may be well rusted. That's if stress areas on the Chevy monocoque haven't already broken and brought it to an early demise.
Monocoque is a good rigid design when its new and unworked, but being made of thinner metal than a chassis it rusts out and/or stresses out earlier, and is often more complicated to repair. A chassis is heavy metal, and may well flex more than the monocoque, but will keep going a lot longer and maybe outlast several cabs, and will be much easier to repair.
Just speaking from personal experience in the trade; your experience may be different.
A friend has a 1950's GMC stepside pickup, with a pretty flexible chassis, and great fun to drive. It's had a few bodywork repairs but hardly anything on the chassis. There aren't many monocoque-bodied vehicles half that age still around, all rotted out long since. So I figure flex is best; something good-looking with a bit of flex has a lot going for it...

 

[keith19]

Ken, that does look like a chassis. If it totally bolts on and off, it's a chassis.
If it's welded on, that makes it a monocoque, or more correctly a unitary body.
Looks to me from your picture that it sits on rubbers, so must be bolt on.

But the same thing applies; the video clip is an ad for Chevy, rather than a straight-forward back-to-back test of two vehicles, whether they have a chassis or not. Also, weight differences are not specified in the clip. Price and vehicle weight are two base comparisons, as is payload.

A bolt-on chassis would generally be expected to have more flex than a frame welded to body construction, weight for weight. This is known and allowed for in the design specifications. It is interesting to note that in the US, government specifications cover the type and strength of chassis to be used in particular types of vehicle, like school buses.

To get back on topic, years back we were taught (for better or worse) to repair chassis of vehicles with either replacement sections, or with specific shapes of patches, which were to be stitch welded. Replacement sections were seam-welded in, because the original would have been. The patches were usually stitch-welded to allow for the flexing of the chassis. Seam welding on patches was thought to introduce brittle areas which would eventually break under the normal flexing of the chassis. Personally I would prefer to replace sections if possible, but sometimes a patch was all that was necessary to pass the MOT, so a patch it was. I am well aware that in later years the MOT tester requires the patch, if any, to be seam-welded rather than stitched, so this is what he gets. On the average chassis vehicle I don't think it's critical enough to make any difference. Over the years, stitch or seam, we never had one come back broken.

 

[t'rusty duck]

Keith,
when all's said and done, that's the statistic which matters.
Hope my record is as good, almost 20 years since the first one went on the road and I'm still waiting...

. Over the years, stitch or seam, we never had one come back broken.