Weld strength & welded vehicle seams

[keith19]

Ken, interesting that you have designed bridges. You will be aware that long bridges are designed to flex, otherwise they would break up under wind stress. Longer vehicles are designed to flex to some degree too, as do most other vehicles with a chassis.

Monocoque construction largely put an end to chassis use in cars, and they continue to make cars stiffer, with thinner metal, and lighter weight, since that time. Whether this is intrinsically a good thing is quite another matter. As you rightly state: "do as much as you can with the minimum amount of material to ensure the thing doesn't bend, twist or fall apart"is indeed the order of the day. Minimum cost also figures greatly.

 

[PhillipM]

The idea is that all the flex is carefully controlled in the bushes, mounts and suspension then, rather than undamped, unpredictable flex in the chassis.

There's no doubt at all that a truck/trailer chassis that was considerably stiffened and the suspension designed to take the flex instead would perform much better dynamically, the problem is you're either going to add a lot of cost, or a lot of weight, in order to do it, which doesn't add up the same way as improving handling/suspension and NVH in a passenger vehicle does.
Same thing with bridges.

 

[langy]

Porsche Cup cars are fully seam welded, They obviously have no idea they have got it wrong

 

[PhillipM]

What do they know, they put the engine in the wrong place!

 

[Piero]

Keith 19

can you explain why you belive the car body or chassis would have a need to flex if the suspention is doing its job properly?????

 

[keith19]

I would not say that the behaviour of a given chassis under stress is 'unpredictable'. The movements of a chassis under stress can be ascertained, predicted, and the chassis built accordingly.

The vehicles with a chassis that I have driven in the past would, when pushed in cornering, for example, always give you notice that you were approaching the off-road limit sometime before you actually reached it.
Later monocoque vehicles generally can be pushed much harder in this respect, but also break away more suddenly, resulting in less time for driver reaction.

Most of us, one hopes, do not push either kind that hard on normal roads, but I have seen many incidents after modern cars have gone off 'suddenly', to use the words of the hapless drivers. Perhaps modern cars give too great a feeling of cocooned safety, and people are inclined to push them too hard...

 

[PhillipM]

That's more the way they are setup with regards to feedback and being massively over tyred than anything with the chassis being 'too stiff'

 

[Piero]

It sounds like your saying that if you as a driver can read the flex in a car, you have a better better chance of any sudden handeling issues

BUT NO CAR is gluged to the tar mac, there will always be a brake off point.
with no flex and a well set up suspention, that brake off point is far less

 

[keith19]

It sounds like your saying that if you as a driver can read the flex in a car, you have a better better chance of any sudden handeling issues

BUT NO CAR is gluged to the tar mac, there will always be a brake off point.
with no flex and a well set up suspention, that brake off point is far less

To some extent I would agree, but as I pointed out above: Later monocoque vehicles generally can be pushed much harder in this respect, but also break away more suddenly, resulting in less time for driver reaction.

When you state ' brake off point is far less', what exactly do you mean? Less than what? The brake off point will always exist, I fully agree, but what I am pointing out is that in a modern car it will come more suddenly. Not earlier, just more suddenly when it comes. I find it difficult to believe that average modern drivers are a great deal worse than those in the past, and are unable to ascertain when they are approaching the point of no return. Therefore to some degree the car itself must be masking the approach of the break off point, occasionally with dire results.

I am not here looking at racing or rally drivers, who have much experience of reading their vehicle under adverse conditions, and can usually cope, but of all those sad blokes I would find with their car in the ditch on a corner on a country road, with no other vehicle involved. 'It suddenly spun off' They'd say. Usually they were unharmed, their car was generally a write-off.

 

[Piero]

lets just look at a simple sernario (if you dont know by now, I am dyslexic, so please forgive my spelling)

ok a car is driving alone at x miles an hour, driver needs or desides to change direction.

if you have a VERY flexible car/chassis etc etc, all the forces needed to turn the car will be absorbed in the flex

with a stiff car etc that forces can be directed into turning the car, no ?????

how can that be a bad thing?????

 

[keith19]

Just as you can have a vehicle too stiff, so you can have one too flexible, I would agree.
I have had experience of driving a vehicle with a chassis, somewhat overloaded, downhill at some speed, with heavy traffic coming from the other direction. Due to going downhill, the cargo weight shifts the centre of gravity forward and loads the front suspension and the front wheels. The steering becomes much less responsive, and the vehicle begins to 'snake' a little
This is more likely in a vehicle with a flexible chassis. But of course these were exceptional circumstances, and nobody should ever overload their vehicle.

To answer your earlier question about why a chassis might need to flex, if a vehicle is designed to have a super-rigid body, all the road or off-road stress is being handled by the suspension system. Since no suspension system can be perfect, even moderate wear on a suspension system can result in effects under stress that the manufacturer may not have predicted.
As no metal construction is perfect either, vibration from worn suspension and lack of flex can then give rise to rapid metal fatigue in particular areas; whether these are weld HAZ or component fitting areas, every vehicle has dozens of them.
However if the chassis is able to flex to a limited degree, this absorbs some of the stress, and what stress exists is spread around the system. There is quite an history of smaller 4x4 trucks being given very rigid bodies, and the resultant manufacturer's recalls for metal cracking one or two years down the line. If you are off-road, a degree of flex helps you keep all 4 wheels on the ground, too.

 

[hotrodder]

I think it's one of those things that's too complicated to discuss as genralised 'rules of thumb' personally. The 'over tyred' thing just mentioned for example... common myth that wider tyres = more rubber on the road/bigger contact patch when in fact contact patch size is a function of the weight the tyre's supporting and it's pressure. Fit wider tyres and you change the shape of the contact patch- it gets shorter and wider which has some positive effects, some negative. More grip under cornering at the limit could be said to be +ve, the drawback is when the limit is exceeded they let go more quickly and with less warning than a narrower tyre (still a gross generalisation as tyre grip is horrifically complicated)

Back to chassis stiffness and the thread started with cars then trucks got mentioned and then bridges... all very different especially bridges It's harder to make a long chassis super stiff and if you do then you need posher suspension with more travel if the thing isn't gonna be picking up wheels under certain circumstances compared to a chassis that's designed to flex some
A road car is very different to a competition car, the formers suspension needs be as cheap as it can be while providing enough comfort with acceptable handling.
Old versus modern cars... suspension design has got to play at least as big role a role as chassis stiffness- most things these days are front engine/front drive, wierd and wonderful suspension geometries are a thing of the past and bland/safe gentle understeer with a hint of lift off oversteer has replaced character that let you know it'd bite long before you have a 'watch this' moment. Character that ment it was possible to understand oversteer at 15mph on a wet roundabout.
Seam weld something old/characterful like a Mk1 Escort or an early 911, throw a proper climbing frame of a cage in it (as seam welding alone is just chasing the next weakest link in the chain) and while they'll react faster one will still be an easy to drive, grin inducing, oversteering toy while the other will still punish anyone that prods it without first having an understanding of what lift off oversteer really is

 

[Piero]

Just as you can have a vehicle too stiff, so you can have one too flexible, I would agree.
I have had experience of driving a vehicle with a chassis, somewhat overloaded, downhill at some speed, with heavy traffic coming from the other direction. Due to going downhill, the cargo weight shifts the centre of gravity forward and loads the front suspension and the front wheels. The steering becomes much less responsive, and the vehicle begins to 'snake' a little
This is more likely in a vehicle with a flexible chassis. But of course these were exceptional circumstances, and nobody should ever overload their vehicle.

To answer your earlier question about why a chassis might need to flex, if a vehicle is designed to have a super-rigid body, all the road or off-road stress is being handled by the suspension system. Since no suspension system can be perfect, even moderate wear on a suspension system can result in effects under stress that the manufacturer may not have predicted.
As no metal construction is perfect either, vibration from worn suspension and lack of flex can then give rise to rapid metal fatigue in particular areas; whether these are weld HAZ or component fitting areas, every vehicle has dozens of them.
However if the chassis is able to flex to a limited degree, this absorbs some of the stress, and what stress exists is spread around the system. There is quite an history of smaller 4x4 trucks being given very rigid bodies, and the resultant manufacturer's recalls for metal cracking one or two years down the line. If you are off-road, a degree of flex helps you keep all 4 wheels on the ground, too.

Your referance to worn suspention adding extra stress to the car is wrong.
if any thing its the opersite.

change any worn bush, bulljoint etc etc, adds to tightening up the suspention,

all the flex and movment in a worn suspention is obsorving force
and not allowing correct desngned movment.

its no differant if you was to let your tyers down some, then try and drive fast round a corner.

it takes energy to flex the tyre.

the trye example can be used to show what is happening to chassis/car body flex

 

[PhillipM]

Depends how knackered they are, if you start running out of travel in the rubber bushes or bouncing off the bumpstops the stress in the chassis goes up enormously.

 

[keith19]

"Your referance to worn suspention adding extra stress to the car is wrong.
if any thing its the opersite."
How?
A worn suspension system throws more stress on the tyres, the drive-joints, the other parts of the suspension that are not so worn, and also on the driver's ability to control the vehicle. A worn suspension can be lethal, or it can merely be a noisy inconvenience, depending what components are affected.

Sure you can change suspension parts, if you have some mechanical aptitude and know something is wrong.

But very many people are not aware of this. They just feel their car is not so responsive has it was, or it gets through tyres more quickly, or it maybe wants to leave the road on right-hand bends. I used to work in the trade, and what some people are happy(?) to drive about in is frightening. Worn ball-joints you can pop off with your hand, for example. It's generally only during the MOT that these things come to light for some people, by which time other damage can already have been done.

 

[t'rusty duck]

Keith,
I think you've put the cart before the horse, as one of the aims of bridge design is to minimise the deflection which inevitably occurs under dead, live and wind loading.
Modern suspension or cable-stayed bridges have stiff deck structures and streamlined profile for that reason.

The collapse of 'Galloping Gertie' in 1940 put paid to the use of decks with low rigidity.
http://www.youtube.com/watch?v=3mclp9QmCGs
It also spelt the end of the designer's career and Leon Moisseiff died 3 years later of a heart attack.

"18.3.4 Wind-Resistant Design
General
In the first half of the 19th century, suspension bridges occasionally collapsed under wind loads because girders tended to have insufficient rigidity. In the latter half of the 19th century, such collapses decreased because the importance of making girders sufficiently stiff was recognized.
In the beginning of the 20th century, stiffening girders with less rigidity reappeared as the deflection theory was applied to long-span suspension bridges.
The Tacoma Narrows Bridge collapsed 4 months after its completion in 1940 under a wind velocity of only 19 m/s. The deck of the bridge was stiffened with I-girders formed from built-up plates. The I-girders had low rigidity and aerodynamic stability was very inferior as shown in recent wind-resistant design.
After this accident, wind tunnel tests for stiffening girders became routine in the investigation of aerodynamic stability.
Truss-type stiffening girders, which give sufficient rigidity and combined partially with open deck grating, have dominated the design of modern suspension bridges in the United States.
A new type of stiffening girder, however, a streamlined box girder with sufficient aerodynamic stability was adopted for the Severn Bridge in the United Kingdom in 1966.
In the 1980s, it was confirmed that a box girder, with big fairings (stabilizers) on each side and longitudinal openings on upper and lower decks, had excellent aerodynamic stability.
This concept was adopted for the Tsing Ma Bridge, completed in 1997. The Akashi Kaikyo Bridge has a vertical stabilizer in the centre span located along the centerline of the truss-type stiffening girder just below the deck to improve aerodynamic stability.
In the 1990s, in Italy, a new girder type has been proposed for the Messina Straits Bridge, which would have a center span of 3300 m. The 60-m-wide girder would be made up of three oval box girders which support the highway and railway traffic.
Stiffening girders in recent suspension bridges are shown in Figure 18.15."

Minimum cost is not always the best option either, if this theory on the collapse of the Tay Rail Bridge is correct.
http://taybridgedisaster.co.uk/index/wind-theory

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

To drift back 'on topic', one of the major problem regarding the handling of a vehicle with a chassis of low torsional stiffness is that its movement is undamped, so if a bump is encountered in mid corner, the vehicle will be prone to skip sideways.
NVH ( noise, vibration and harshness) are other downsides, with 'scuttle shake' being the most obvious symptom.

Ken.

Ken, interesting that you have designed bridges. You will be aware that long bridges are designed to flex, otherwise they would break up under wind stress. Longer vehicles are designed to flex to some degree too, as do most other vehicles with a chassis.

Monocoque construction largely put an end to chassis use in cars, and they continue to make cars stiffer, with thinner metal, and lighter weight, since that time. Whether this is intrinsically a good thing is quite another matter. As you rightly state: "do as much as you can with the minimum amount of material to ensure the thing doesn't bend, twist or fall apart"is indeed the order of the day. Minimum cost also figures greatly.

 

[keith19]

Very interesting stuff on the bridges, Ken. But they still have to give to some degree, don't they? Just like very tall buildings move several metres at the top in the wind This is designed into them, of course.
Sorry to get it off topic a bit, one thing leads to another...

"one of the major problem regarding the handling of a vehicle with a chassis of low torsional stiffness is that its movement is undamped, so if a bump is encountered in mid corner, the vehicle will be prone to skip sideways."
Yes, I realise this is so, we happen to have had a Morris Minor van amongst our collection. But I cannot recall it ever happening with the Elan. Horses for courses in chassis design as well as anything else.

 

[t'rusty duck]

Keith,
aye, it's that blooming Young's modulus (stress/strain), which is the problem.
For steel, it's only 29,000,000 lbf/in², but if it was the same as something like a diamond at 175,000,000 lbf/in², a designer's life might be simpler.
Although brittle failure could well be an even bigger problem...

Ken

Very interesting stuff on the bridges, Ken. But they still have to give to some degree, don't they? Just like very tall buildings move several metres at the top in the wind This is designed into them, of course.
Sorry to get it off topic a bit, one thing leads to another...

"one of the major problem regarding the handling of a vehicle with a chassis of low torsional stiffness is that its movement is undamped, so if a bump is encountered in mid corner, the vehicle will be prone to skip sideways."
Yes, I realise this is so, we happen to have had a Morris Minor van amongst our collection. But I cannot recall it ever happening with the Elan. Horses for courses in chassis design as well as anything else.

 

[PhillipM]

Not as big a problem as a man with a van. And a pickaxe.
Or an accountant.

 

[Hopefuldave]

[QUOTE Keith]
However if the chassis is able to flex to a limited degree, this absorbs some of the stress, and what stress exists is spread around the system. There is quite an history of smaller 4x4 trucks being given very rigid bodies, and the resultant manufacturer's recalls for metal cracking one or two years down the line. If you are off-road, a degree of flex helps you keep all 4 wheels on the ground, too.
[/QUOTE]

Nope, what keeps wheels on the ground is properly designed suspension with good wheel articulation - a perfect example would be the Tatra 813 off-road trucks, a massively stiff central tube "spine and outriggers" chassis with relatively compliantly sprung swing-axles pivoted at the centre line - trucks with 15-ton payloads that can out off-road a trials landrover...

Bear in mind that the majority of "consumer" 4x4s are still built on WW2 Jeep principles, with rigid live axles (at least at the rear) because few ever have to cope with worse than pulling a horsebox across a lawn they're not designed with the necessary combination of rigid chassis and compliant suspension because Chelsea mums couldn't drive something.that rolled so much...

On the Tatra the "bodywork" is held rigid by the chassis, and is a bit of an afterthought!

The only vehicles I know of that really need some torsional flex designed in are motorcycles, and then mainly for racing - too stiff a rear swingarm will transfer bumps into the bike "sideways", in the plane of the swingarm, so MotoGP bikes are designed with some controlled torsional flex in their swingarms...

Dave H. (the other one)