Weld strength & welded vehicle seams

[grim_d]

Hi all,

Just having a bit of a discussion about my welding on my project with a friend and one section in particular got me to thinking.

On my offside sill I have replaced a large portion of the inner sill, at least the bottom 1/3 all the way along seam welding the repair sections to the floor panel as it was the original spot welded seam that had rusted out and required replacing. I chose not to reinstate the seam for 2 reasons,1. it would be a rust trap and 2. as per mot requirements all repair sections should be fully seam welded unless and entire new panel is being used.

Now we all know that fully seam welding a race car is bad as it makes it far too stiff and can cause ripping or weld fatigue cracking and that it should be stitch welded to allow a bit of flex.

I am curious as to whether or not something similar may occur here as it is quite a large structural point that is now fully seam welded, I have no doubt however as to the strength of the weld itself as I am confident in my welding.

My friend is of the opinion that all my welds, not just the ones in question, will fatigue and crack and that my car will collapse in a ball of flames as soon as I hit a pothole.

Thoughts?

 

[langy]

Now we all know that fully seam welding a race car is bad as it makes it far too stiff and can cause ripping or weld fatigue cracking and that it should be stitch welded to allow a bit of flex.

Its quite common to seam weld a race car, They want stiffness

 

[pedrobedro]

Did it collapse and kill you before you welded it, why should it be different after you welded it ? I don't understand his logic or you listening to him. Just carry on and prove him wrong. Race cars are often seam welded so the suspension is doing the job it should and the stress cracks are caused when the body flexes not avoided by allowing it to flex "a bit"

 

[grim_d]

Now we all know that fully seam welding a race car is bad as it makes it far too stiff and can cause ripping or weld fatigue cracking and that it should be stitch welded to allow a bit of flex.

Its quite common to seam weld a race car, They want stiffness

It's common to stitch weld, fully seam welding makes it too stiff and actually causes damage to the shell!

Did it collapse and kill you before you welded it, why should it be different after you welded it ? I don't understand his logic or you listening to him. Just carry on and prove him wrong. Race cars are often seam welded so the suspension is doing the job it should and the stress cracks are caused when the body flexes not avoided by allowing it to flex "a bit"

I'm not worried about it collapsing at all, I trust my work 100%. As above they are often stitch welded, where as here there is a solid seam, I'm not concerned about it just curious for others opinions if this section will now be too stiff, the other side has far less repair so I suppose there is a bit of imbalance.

 

[piman]

Hello Grim ,

I'm with Langy, not only racing cars but rally cars also; if that weakend the shell they wouldn't do as there is far more stress to a rally car shell.

Alec

 

[BillJ]

2. as per mot requirements all repair sections should be fully seam welded unless and entire new panel is being used.

I don't believe that is correct. If you are welding a seam where there was none before (e.g. welding a patch to an existing panel, then it must be a continuous seam (to replicate the continuous metal that was there originally). However, if replacing an original seam then it is acceptable to weld it in a manner similar to the original (e.g. plug-weld it if the original seam was spot-welded).

I've had a good few plug-welded seams pass MOT without a second look.

Regarding racing cars, it is indeed common practice to stitch-weld production car bodies to stiffen them but I know some works teams backed off from fully seam-welding them because it made them too stiff, with consequences that I'm not too sure of.

 

[t'rusty duck]

Grim-D & langy,
I'm with pedrobedro on this one, in fact I'd use the quote that 'there's no such thing as a chassis which is too stiff', with the exception being those offroaders where (primitive?) leaf spring suspension may need the chassis to make up for any shortcomings in the suspension travel department.

Here's some light reading on the subject of a vehicle's torsional stiffness:

MGB Heritage Competition Bodyshells
http://www.britishv8.org/Articles/BritishMotorHeritage-Competition.htm
Note the continuous seam welding where necessary, most probably on a joint which has been found to be prone to 'unzipping' if only spot or stitch welding was used.

A rundown of the torsional stiffness of a selection of bodyshells, in which it's no surprise to see the Koenigsegg Agera R and the Bugatti Veyron topping the leader board.
http://www.germancarforum.com/community/threads/the-list-torsional-rigidity.12334/

For the record, I've sold over 2,500 chassis for Citroen's (in)famous upside down pram during the last 20 years and make very sure that those I put together to take a BMW 1150 bike engine (109 bhp compared with the original 34 bhp) are much stiffer than the standard version.

Ken

 

[the snooper]

I don't believe that is correct.

you are right, if you cut a section of sill out that was to include the lower spot welded seam, you can then plug weld the lower seam of the repair but the rest must be fully welded as bill says.

simply put it has to be like for like

 

[grim_d]

I stand corrected then in regards to the mot requirements, not that it would have changed anything I don't think as welding it directly to the floor was quicker.

As I say I'm happy with everything just interested in others opinions on the subject.

Duck, thanks for links, interesting reading.

 

[langy]

It's common to stitch weld, fully seam welding makes it too stiff and actually causes damage to the shell!



I'm not worried about it collapsing at all, I trust my work 100%. As above they are often stitch welded, where as here there is a solid seam, I'm not concerned about it just curious for others opinions if this section will now be too stiff, the other side has far less repair so I suppose there is a bit of imbalance.

Oh well i guess I was taught wrong all those years ago

 

[normspanners]

There is such a thing as TOO stiff a chassis, in a truck part of the suspension is chassis flex, if carrying large pipes, bridge sections etc the truck feels a LOT stiffer and effects the handling, also they can break springs when carrying that type of load an that is why the chassis are bolted/rivetted not welded.
Every chassis except maybe a tank or excavator are designed to flex
Rally cars are stitch welded to save fractures,and therefore shear points, it is the rollcage that gives the whole shell the stiffness to improve the handling but still enables the car to flex to relieve stress. On some chassis sections filling with expanding foam strengthens up that section increadably.
Again in motorcycle frames joins where/are brazed to stop welding shear points

 

[t'rusty duck]

Since the original discussion was about passenger cars, not wagons, I guess it depends on whether you consider safe, predictable handling to be a priority.
Chassis flex is undamped suspension movement, after all.

They say that a picture is worth a thousand words, so here are some moving pictures...

http://www.youtube.com/watch?v=TKMQjm7i2Jw

http://www.youtube.com/watch?v=EWjTbiYo3x0


I have no experience with motorcycle frames, however I do know that on high quality bicycle frames, the tubes used to be silver soldered (or brazed?) into the lugs so that the strength of the high tensile tubing was not compromised.

I'm not quite sure what you mean by 'welding shear points', unless you're referring to the heat affected zone around a weld.

Ken

There is such a thing as TOO stiff a chassis, in a truck part of the suspension is chassis flex, if carrying large pipes, bridge sections etc the truck feels a LOT stiffer and effects the handling, also they can break springs when carrying that type of load an that is why the chassis are bolted/rivetted not welded.
Every chassis except maybe a tank or excavator are designed to flex
Rally cars are stitch welded to save fractures,and therefore shear points, it is the rollcage that gives the whole shell the stiffness to improve the handling but still enables the car to flex to relieve stress. On some chassis sections filling with expanding foam strengthens up that section increadably.
Again in motorcycle frames joins where/are brazed to stop welding shear points

 

[normspanners]

A chassis is the frame of a vehicle in this context and in some parts of the world it is actually illegal to weld the structural chassis /frame of a vehicle for the reason of the weld altering the designed flex in the structure, most vehicles are designed to flex to save cracks or shear points.
Yes a welding shear point or shear line is the area either side of the weld affected by the weld and if that part is likely to break and that weld is continuous its the parent steel that will crack usually one side or the other of the weld due to stresses imposed by the welding and the load applied, but if that weld is stitched there isnt the same continuous stress line and welding shear point therefore the joint is stronger due to the parent metal passing between the welds in places.
Rally cars if made to rigid will crack the original panels usually before a weld will break,
I never try to do a continuous weld on a highly stressed component for that reason.

 

[Piero]

I cant stand plug welds done with a mig welder.

Seen to many done wrong!!!!!!

The plug hole needs to be a little bigger than most people use and the weld should be started out side of the hole till the metals get hot.

I am lucky enough to own a good spot welder.

I kid you not. You would struggle to brake a good spot weld.
The metal gives way first.

As to any welding. Its mostly down to the skills of the welder.

The weld should simply do the job thats needed of it. Simple! !!!!

As to stiffness or additional stiffness. Adding metal for stiffness can be lighter than loads of welds.

 

[PhillipM]

Stitch welding is done on race/rally cars because you get 99% of the stiffness benefit of a seam weld, with much less distortion and only 1/3rd of the weight of wire added to the car...
For rally cars that often crack the odd weld on a rough landing, having gaps between the welds stops a crack propagating all the way down the car too.

 

[grim_d]

A chassis is the frame of a vehicle in this context and in some parts of the world it is actually illegal to weld the structural chassis /frame of a vehicle for the reason of the weld altering the designed flex in the structure, most vehicles are designed to flex to save cracks or shear points.
Yes a welding shear point or shear line is the area either side of the weld affected by the weld and if that part is likely to break and that weld is continuous its the parent steel that will crack usually one side or the other of the weld due to stresses imposed by the welding and the load applied, but if that weld is stitched there isnt the same continuous stress line and welding shear point therefore the joint is stronger due to the parent metal passing between the welds in places.
Rally cars if made to rigid will crack the original panels usually before a weld will break,
I never try to do a continuous weld on a highly stressed component for that reason.

This is kind of what I was getting at, in my situation what would you do?

I did think about cutting gaps in the seam or just splitting the seam then re stitch welding it.

 

[Sean]

This is kind of what I was getting at, in my situation what would you do?

I did think about cutting gaps in the seam or just splitting the seam then re stitch welding it.

yeah back to the original post... your mate is talking ******** your weld will be fine as it is, there is no need for "stress relief" it not like its so stressed its going to fall apart ( i presume it didnt when it was mostly rust)


and id have done exactly what you have done except id have extended the patch and plug welded it to the flange in a similar manner to the original.

 

[grim_d]

yeah back to the original post... your mate is talking ******** your weld will be fine as it is, there is no need for "stress relief" it not like its so stressed its going to fall apart ( i presume it didnt when it was mostly rust)


and id have done exactly what you have done except id have extended the patch and plug welded it to the flange in a similar manner to the original.

I would have normally plug welded it to the flange but the flange was cut out along with the bottom of the sill as that's what was rusty and since I'm being a bit gash and not too bothered about how the hidden repairs look cosmetically because I'm desperate to get it back in on the road I just welded it to the floor rather than recreating the flange to weld to.

 

[keith19]

Your repairs will be fine; I've seen a lot done very much worse in the trade over the years, then covered over...

There is normally little or no 'stress' of the sort your mate is talking about in a repair section like that. Unless you intend weaving on a race track at well over the ton, on a regular basis.

So far as to stitch or not to stitch is concerned, it's only relatively recently that this continuous seam business came in for the MOT, along with thinner and newer types of steel. Back in the 70's and 80's we were taught at college to stitch weld trucks and cars, particularly when patching chassis work, for reasons very similar to those given by Normspanners.

The steel used for various applications was generally a bit thicker then, of course. We even used to stitch-weld floor pans in with arc sometimes, when the mig plant wasn't available.

 

[t'rusty duck]

Normspanners,
I've no idea where you got the idea that bodyshells of passenger vehicles are designed to flex, when most manufacturers nowadays are using everything in their power, including computer aided design and high tensile steel, to ensure the exact opposite.

As an example, for the 2006 Ford Explorer:
The 2006 Explorer features an all-new frame that is better in all significant measures: It's more rigid, safer and more capable than ever before.*
Extensive computer-aided engineering (CAE) modeling enabled engineers to add strength where it was needed. For example, the new frame rails feature taller and wider sections than before. In crucial areas, the sections are also thicker for additional strength where it's needed, without added extra weight where it's not.*

Possibly the most significant change was moving from traditional flat joints — with the cross beams attached to the top of the frame rails — to box joints. The Explorer adopts the F-150's tube-through-tube frame design, where the cross beams pass through the frame-rails, creating an inherently stronger joint. These joints are then completely welded around the perimeter.*
As a result, the Explorer frame boasts a 55 percent increase in torsional stiffness, and a 63 percent increase in bending resistance, improving ride and handling. The stiffer frame also helps eliminate squeaks and rattles in the cabin, improving long-term customer satisfaction.

New suspension attachment points up to 593 percent stiffer, body mount attachment points up to 169 percent stiffer for improved NVH
These brackets also benefited from extensive CAE modeling, adding strength without adding unnecessary weight.

http://media.ford.com/press_kits_detail.cfm?presskit_id=1303&item_id=4067&press_section_id=2871

If you dig around on the internet, you'll find much the same from any other manufacturer, more refined designs and/or higher grade materials to increase torsional stiffness and bending resistance without increasing the weight of their vehicles.

BMW are very forthcoming with technical detail, even disclosing information about the average gauge of the sheet metal they use and its physical properties, with the final words being; "Due to new Design of Carrier Structure Torsional Stiffness raised from 23500 Nm/°to 27000 Nm/°"

http://www.autosteel.org/~/media/Fi... Mixed Material Concept of the New BMW X5.pdf

I've got no formal training in motor vehicle design, the closest I got was designing bridges, but the same rule probably applies in both disciplines, "do as much as you can with the minimum amount of material to ensure the thing doesn't bend, twist or fall apart".

Although that doesn't apply to Bailey Bridges, as the b@ggers are pin-jointed and frighten the life out of you when you load them up with 600 tons of wrought iron...

Ken



Whitley Bridge, 1977 & 1997 by slcchassis, on Flickr

A chassis is the frame of a vehicle in this context and in some parts of the world it is actually illegal to weld the structural chassis /frame of a vehicle for the reason of the weld altering the designed flex in the structure, most vehicles are designed to flex to save cracks or shear points.
Yes a welding shear point or shear line is the area either side of the weld affected by the weld and if that part is likely to break and that weld is continuous its the parent steel that will crack usually one side or the other of the weld due to stresses imposed by the welding and the load applied, but if that weld is stitched there isnt the same continuous stress line and welding shear point therefore the joint is stronger due to the parent metal passing between the welds in places.
Rally cars if made to rigid will crack the original panels usually before a weld will break,
I never try to do a continuous weld on a highly stressed component for that reason.