6 ms·

That sounds like the notorious "cut and shut" thing where criminals create a dangerous vehicle for the second-hand market, except that it's from a different era

by bloak 6y ago

That sounds like the notorious "cut and shut" thing where criminals create a dangerous vehicle for the second-hand market, except that it's from a different era. Seeing as a 1965 Mercedes is built rather differently from a typical modern car, and safety standards have changed since then, too ... any comments on how hard it is to this without compromising safety?

bunje 6y ago
I would guess that it is much more dangerous to modify a modern car because they utilize nontrivial shell geometry to improve the rigidity or to reduce weight. When I was attending driving lessons, my teacher claimed that the specific shape of the windscreen or rear window was a very important safety feature in the scenario that the car rolls over.
- TravelPiglet 6y ago
  Not to mention crumple zones and how the car is intended to protect the people inside
  - jacquesm 6y ago
    Those are typically outside of the safety cell.
- sgt101 6y ago
  The big one is the A pillar on the front. This tends to corrode over time and is often why cars are written off. If it is corroded then the structural integrity of the car is compromised. There is really no repair for it - but a bit of welding can disguise things and get a write off sold as a potential project. The new owner can find out when they take the wings off... or they can find out when the front of the car comes off in the event of a shunt, but in that scenario they only find out very briefly.
  - jacquesm 6y ago
    That write-off is an economic affair, not a technical one. A-pillars too can be re-manufactured, but it is usually better not to because if the A-pillar is compromised you are looking at a body that will have a whole lot of other damage as well. The bad part about damage there is that you need to build it up from the inside out which means 'undressing' the whole box layer by layer so you can rebuild it properly, and most people will not have the time, the skills (or the jigs) to do this properly. But it definitely can be done, but likely not in a way that is economical. I can't imagine an A-pillar that is damaged with the rest of the car being deformed as well, the A-pillar of a modern car is ridiculously strong and meant to be the outer boundary of the safety cage for the passengers. Any deformation there and you'll have a deformed bottom, roof, firewall and probably other parts as well.
Tade0 6y ago
My friend is a car mechanic and his weekend car is a 1985 Nissan Z31, which he welded from two halves - front and back. It's physically possible to do it and maintain reasonable safety but you need proper equipment, skills and time. The frame is generally not designed to be welded like that, so without reinforcement it will be significantly weaker. He did is as a hobby project but generally done properly such operations are not cost-effective. Nevertheless it's still a service some shops provide at the cost of safety. My aunt used to have such a car - she only noticed because she was blowing through sets of tires - the halves were misaligned(rotated) by less than one degree in each axis, but it was enough to make proper tire alignment impossible.
- csunbird 6y ago
  Welding chassis of a car is a practice strongly recommended against, since it makes it significantly weaker at the welding point(s). It is very unsafe, although since it is an oldtimer restoration project, I salute your friend with respect!
  - userbinator 6y ago
    Welding done correctly is as strong if not stronger than the base metal.
    - meowster 6y ago
      The weld will be stronger, but the heat-affected metal will become weaker than the rest of the metal, which is why if it breaks, it will be next to the weld.
      - jacquesm 6y ago
        Only when done improperly. Yes, this is a common failure mode, no, it does not have to be that way. Typically the reason is not that the heat affected metal will be come weaker, typically the reason is that the weld is thicker than the material right next to it creating a stress point in the structure. The hardening can be taken care of by annealing. The best way to fix cars is to increase the length of the weld if you can, the original weld points are there for a balance between economy and strength. If you start welding in 'original new and interesting places' you need to plan how you want to achieve the required strength since economy is likely the least of your worries. Every stretch limo is essentially a demonstration piece to show that this does not need to be a problem, and that you actually can re-inforce a body quite nicely. Keep in mind that the wheelbase of a stretch limo pretty much guarantees much higher forces on all of the welding in the body including the existing seams.
throwaway0a5e 6y ago
Combining vehicles like this is one of those things that isn't bad but gets the Reddit experts screeching because their only experience is that their mom's sister's boyfriend's uncle's roommate got swindled by someone doing a crap job on a Civic that they thought they were getting a good deal on. Vehicles build on frames are cut and extended/shortened all the time as a part of routine modifications for commercial use and it's far less labor intensive to "do right" than a unibody vehicle. Putting two good halfs together is no big deal and is a relatively standard operation, all the known techniques and whatnot transfers right over. (I assume a 60s Mercedes falls into this category.) Unibody vehicles are also routinely repaired in this manner in eastern Europe where it's cheaper to buy wrecked stuff from the west and fix than to buy new. In the west the limo and mobility van industry does very similar things (only they're inserting stuff in the middle rather than repairing the car) People do cut and weld jobs all the time for their own personal vehicles and classic cars. When you're doing it for your own car or a car you don't intend to flip you can afford to spend the time to do it right. Quick hackjobs are the issue, not the technique.
- deleted 6y ago
  [deleted]
- hwillis 6y ago
  There is/was lots of speculative armchairing as well, when HSLAs started coming into the picture. Structural adhesives also get pointed to as a reason not to do big repairs like this. In practice it isn't really borne out. HSLA (high strength low alloy) steels are stronger (and therefore lighter) than mild steels. Many alloy steels (non-HSLA) are sensitive to heat treatment and improper welds make them weaker, so tons of people were being very vocal about HSLAs ruining cars, making repairs impossible, etc etc. Things like saying you'd need proprietary filler to match the composition, that welds would be poison, etc. In reality it doesn't really make a difference. Adhesives are also sometimes/often used to secure panels together instead of welds. The theory was that subtle differences in shape would introduce tensile stressed that would cause adhesives to pop in a crash. Not real. Alternatively stress during modifications could introduce cracks and weaken the adhesive- also bullshit, those adhesives go through stress every time they drive. They're fine. They aren't invalid concerns, but unless you're putting a torch to the adhesive or welding seams down the middle of alloy beams, you aren't doing anything that will cause problems.
blagie 6y ago
I can't imagine it'd be all too tough with a 1965 car and good welding technique. At the time, cars were still pretty over-built and over-engineered. Good welds are pretty darned tough. Most things, you can cut-and-dice without problems. With a 2020 car? I wouldn't trust it unless the person performing the work was a pretty good engineer. As engineering is more optimized, you can't just do stuff like this without being very careful: 1) Welds do leave a heat-affected zone around the weld which is weaker. If you're overengineered (1956) and everything is thicker than necessary, this doesn't matter. If you're near-optimal (2020), and using the exact amount of material needed to avoid failure under expected driving conditions, you might weaken things enough to see failures. 2) If reinforced, you'll affect the overall systems design. Engineered objects flex. Making one part more rigid than expected will put additional strain on other parts. Counterintuitive as it might sound, optimal engineering makes designs safer. You analyze failure modes, put in appropriate safety margins, and do things right. Engineering classes drill that over-engineering is bad engineering. The flip side, though, is that if you can't tweak optimized designs like you can over-engineered ones without being careful and thoughtful about it. Same thing with bikes. You can fix up a $100 Huffy with any old welder. A $2000 steel frame with thin-wall double-butted tubes? You want a really skilled welder and close inspection to make sure it's not compromised.
- throwaway0a5e 6y ago
  Please stop throwing generalized internet engineering tropes at the wall. Just because it all looks like rocket science to someone who doesn't have industry experience doesn't mean it is. The repairs you are proclaiming to be impossible are perfectly possible to do with good results. These repairs are often performed often, just not in the high cost of labor parts of the world where HN lives (the low cost areas buy our wrecked cars to fix and sell to the point where sourcing parts for major repairs is often easier there than in the vehicle's country of origin). There's autobody industry specific tools and literature that help professionals in the industry perform the repairs properly. It's perfectly possible to take a car apart and reassemble (with good parts) the way an OEM would have if you're willing to invest in the tooling (mostly for welding in hard to reach areas). Car passenger cabins aren't supposed to flex (but in practice they do so within acceptable limits) and OEMs provide service literature telling you what you can and can't modify, much repair info can be gleaned by reading between the lines of such service literature. The welding issues you are describing are simply not an issue for skilled welders. Your opinion of engineering education is hard to reconcile with the observed performance of new engineers in any/every field.
southerntofu 6y ago
> any comments on how hard it is to this without compromising safety? I would say it greatly depends on your welding skills. The same welded materials can be very solid of very weak depending on how they were welded. I would not dare to do that, personally.
tyingq 6y ago
Older vehicles are frame+body, such that the body integrity is less important than current unibody cars that don't have a frame. This would be a much more significant change on a unibody car. Though any sort of big modification like this probably changes the characteristics of how it crushes in a crash, flexes under load, etc.
jacquesm 6y ago
The devil is in the details. I know people that could pull this off. I could pull this off. But I also know people that would try to pull it off, it would look good and maybe even pass inspection but I'd never want to be seen in or near that vehicle. I've welded up cars that were so rusted through that you had to re-inforce the body before you could start working on them in order for them to stay true (tubeframe inside the main body to fixate the remains). It's all doable, mostly a matter of time and skill.

by bloak 6y ago

bunje 6y ago

I would guess that it is much more dangerous to modify a modern car because they utilize nontrivial shell geometry to improve the rigidity or to reduce weight. When I was attending driving lessons, my teacher claimed that the specific shape of the windscreen or rear window was a very important safety feature in the scenario that the car rolls over.

TravelPiglet 6y ago
Not to mention crumple zones and how the car is intended to protect the people inside
- jacquesm 6y ago
  Those are typically outside of the safety cell.
sgt101 6y ago
The big one is the A pillar on the front. This tends to corrode over time and is often why cars are written off. If it is corroded then the structural integrity of the car is compromised. There is really no repair for it - but a bit of welding can disguise things and get a write off sold as a potential project. The new owner can find out when they take the wings off... or they can find out when the front of the car comes off in the event of a shunt, but in that scenario they only find out very briefly.
- jacquesm 6y ago
  That write-off is an economic affair, not a technical one. A-pillars too can be re-manufactured, but it is usually better not to because if the A-pillar is compromised you are looking at a body that will have a whole lot of other damage as well. The bad part about damage there is that you need to build it up from the inside out which means 'undressing' the whole box layer by layer so you can rebuild it properly, and most people will not have the time, the skills (or the jigs) to do this properly. But it definitely can be done, but likely not in a way that is economical. I can't imagine an A-pillar that is damaged with the rest of the car being deformed as well, the A-pillar of a modern car is ridiculously strong and meant to be the outer boundary of the safety cage for the passengers. Any deformation there and you'll have a deformed bottom, roof, firewall and probably other parts as well.

Tade0 6y ago

My friend is a car mechanic and his weekend car is a 1985 Nissan Z31, which he welded from two halves - front and back. It's physically possible to do it and maintain reasonable safety but you need proper equipment, skills and time. The frame is generally not designed to be welded like that, so without reinforcement it will be significantly weaker. He did is as a hobby project but generally done properly such operations are not cost-effective. Nevertheless it's still a service some shops provide at the cost of safety. My aunt used to have such a car - she only noticed because she was blowing through sets of tires - the halves were misaligned(rotated) by less than one degree in each axis, but it was enough to make proper tire alignment impossible.

csunbird 6y ago
Welding chassis of a car is a practice strongly recommended against, since it makes it significantly weaker at the welding point(s). It is very unsafe, although since it is an oldtimer restoration project, I salute your friend with respect!
- userbinator 6y ago
  Welding done correctly is as strong if not stronger than the base metal.
  - meowster 6y ago
    The weld will be stronger, but the heat-affected metal will become weaker than the rest of the metal, which is why if it breaks, it will be next to the weld.
    - jacquesm 6y ago
      Only when done improperly. Yes, this is a common failure mode, no, it does not have to be that way. Typically the reason is not that the heat affected metal will be come weaker, typically the reason is that the weld is thicker than the material right next to it creating a stress point in the structure. The hardening can be taken care of by annealing. The best way to fix cars is to increase the length of the weld if you can, the original weld points are there for a balance between economy and strength. If you start welding in 'original new and interesting places' you need to plan how you want to achieve the required strength since economy is likely the least of your worries. Every stretch limo is essentially a demonstration piece to show that this does not need to be a problem, and that you actually can re-inforce a body quite nicely. Keep in mind that the wheelbase of a stretch limo pretty much guarantees much higher forces on all of the welding in the body including the existing seams.

throwaway0a5e 6y ago

Combining vehicles like this is one of those things that isn't bad but gets the Reddit experts screeching because their only experience is that their mom's sister's boyfriend's uncle's roommate got swindled by someone doing a crap job on a Civic that they thought they were getting a good deal on. Vehicles build on frames are cut and extended/shortened all the time as a part of routine modifications for commercial use and it's far less labor intensive to "do right" than a unibody vehicle. Putting two good halfs together is no big deal and is a relatively standard operation, all the known techniques and whatnot transfers right over. (I assume a 60s Mercedes falls into this category.) Unibody vehicles are also routinely repaired in this manner in eastern Europe where it's cheaper to buy wrecked stuff from the west and fix than to buy new. In the west the limo and mobility van industry does very similar things (only they're inserting stuff in the middle rather than repairing the car) People do cut and weld jobs all the time for their own personal vehicles and classic cars. When you're doing it for your own car or a car you don't intend to flip you can afford to spend the time to do it right. Quick hackjobs are the issue, not the technique.

deleted 6y ago
[deleted]
hwillis 6y ago
There is/was lots of speculative armchairing as well, when HSLAs started coming into the picture. Structural adhesives also get pointed to as a reason not to do big repairs like this. In practice it isn't really borne out. HSLA (high strength low alloy) steels are stronger (and therefore lighter) than mild steels. Many alloy steels (non-HSLA) are sensitive to heat treatment and improper welds make them weaker, so tons of people were being very vocal about HSLAs ruining cars, making repairs impossible, etc etc. Things like saying you'd need proprietary filler to match the composition, that welds would be poison, etc. In reality it doesn't really make a difference. Adhesives are also sometimes/often used to secure panels together instead of welds. The theory was that subtle differences in shape would introduce tensile stressed that would cause adhesives to pop in a crash. Not real. Alternatively stress during modifications could introduce cracks and weaken the adhesive- also bullshit, those adhesives go through stress every time they drive. They're fine. They aren't invalid concerns, but unless you're putting a torch to the adhesive or welding seams down the middle of alloy beams, you aren't doing anything that will cause problems.

blagie 6y ago

I can't imagine it'd be all too tough with a 1965 car and good welding technique. At the time, cars were still pretty over-built and over-engineered. Good welds are pretty darned tough. Most things, you can cut-and-dice without problems. With a 2020 car? I wouldn't trust it unless the person performing the work was a pretty good engineer. As engineering is more optimized, you can't just do stuff like this without being very careful: 1) Welds do leave a heat-affected zone around the weld which is weaker. If you're overengineered (1956) and everything is thicker than necessary, this doesn't matter. If you're near-optimal (2020), and using the exact amount of material needed to avoid failure under expected driving conditions, you might weaken things enough to see failures. 2) If reinforced, you'll affect the overall systems design. Engineered objects flex. Making one part more rigid than expected will put additional strain on other parts. Counterintuitive as it might sound, optimal engineering makes designs safer. You analyze failure modes, put in appropriate safety margins, and do things right. Engineering classes drill that over-engineering is bad engineering. The flip side, though, is that if you can't tweak optimized designs like you can over-engineered ones without being careful and thoughtful about it. Same thing with bikes. You can fix up a $100 Huffy with any old welder. A $2000 steel frame with thin-wall double-butted tubes? You want a really skilled welder and close inspection to make sure it's not compromised.

throwaway0a5e 6y ago
Please stop throwing generalized internet engineering tropes at the wall. Just because it all looks like rocket science to someone who doesn't have industry experience doesn't mean it is. The repairs you are proclaiming to be impossible are perfectly possible to do with good results. These repairs are often performed often, just not in the high cost of labor parts of the world where HN lives (the low cost areas buy our wrecked cars to fix and sell to the point where sourcing parts for major repairs is often easier there than in the vehicle's country of origin). There's autobody industry specific tools and literature that help professionals in the industry perform the repairs properly. It's perfectly possible to take a car apart and reassemble (with good parts) the way an OEM would have if you're willing to invest in the tooling (mostly for welding in hard to reach areas). Car passenger cabins aren't supposed to flex (but in practice they do so within acceptable limits) and OEMs provide service literature telling you what you can and can't modify, much repair info can be gleaned by reading between the lines of such service literature. The welding issues you are describing are simply not an issue for skilled welders. Your opinion of engineering education is hard to reconcile with the observed performance of new engineers in any/every field.

southerntofu 6y ago

> any comments on how hard it is to this without compromising safety? I would say it greatly depends on your welding skills. The same welded materials can be very solid of very weak depending on how they were welded. I would not dare to do that, personally.

tyingq 6y ago

Older vehicles are frame+body, such that the body integrity is less important than current unibody cars that don't have a frame. This would be a much more significant change on a unibody car. Though any sort of big modification like this probably changes the characteristics of how it crushes in a crash, flexes under load, etc.

jacquesm 6y ago

The devil is in the details. I know people that could pull this off. I could pull this off. But I also know people that would try to pull it off, it would look good and maybe even pass inspection but I'd never want to be seen in or near that vehicle. I've welded up cars that were so rusted through that you had to re-inforce the body before you could start working on them in order for them to stay true (tubeframe inside the main body to fixate the remains). It's all doable, mostly a matter of time and skill.