Styling Roof rails.

[Rogereld]

Now I have the Panda 100 running I am moving on to looking at some additional enhancements. I have found a pair of roof rails, offered cheap from a breaker. One of the rails is slightly bent which may be the reason for a lower price. I am not an expert in bending metal but I am wondering what my chances are of straightening a bent roof rail?

 

[koalar]

Photo please

Depending on where and how much will alter the best course of action

Also your skill level and equipment might also come into play

 

[Rogereld]

 

[koalar]

Cheers

Gental bend over it's whole length

I'd be tempted to turn it over on some scaffolding board and place some wood on top in the middle about 2 foot long and squeeze the sandwich together with a g clamp.

Measure how much it springs back and add that much under each end and repeat

Should get you close

 

[porta]

It’s aluminium but quite brittle, do as @koalar suggests, don’t try doing it in one go

 

[jrkitching]

It’s aluminium but quite brittle

That.

Even if you manage to straighten it, it will have been weakened, and how can you know it's not going to deform again. Worse, what if it breaks in service when you've got a load attached, and that load falls onto a busy road?

Unless you've already bought this, I'd pass and look for a better one.

 

[AndyRKett]

Measure how much it springs back and add that much under each end and repeat

The elastic deformation is about 1.5 - 2 degrees on mild aluminum

That said the more Aluminum is bent the more it hardens and the more brittle it become, so it could crack trying to bend it back unless you can anneal it (which you can't do with it being plastic covered) so....

Unless you've already bought this, I'd pass and look for a better one.

^^^^ that

 

[jrkitching]

That said the more Aluminum is bent the more it hardens and the more brittle it become

This happens even when it's not bent beyond its elastic limit, which is why aircraft structures have a fatigue life and mandatory periodic inspections.
On aircraft, once it's bent beyond this (as the one in the picture has), the part must be scrapped.

 

[Rogereld]

I have not purchased the rails. They are on a car at a breakers yard and would need dismantling if anyone wants them. Thank you all for various comments. I will look elsewhere.

 

[koalar]

This happens even when it's not bent beyond its elastic limit, which is why aircraft structures have a fatigue life and mandatory periodic inspections.
On aircraft, once it's bent beyond this (as the one in the picture has), the part must be scrapped.

While true

Aircraft don't use much aliminium it's normally an alloy such as duralumin, avional or silumin (might be spelt it wrong)

It quite a cool process. Large sheets come out of an anealing oven then immediately get bent and fastened into shape, It the hardens over the next few days at room temperature, as the cyristine structure alters

There are other ways now such as composite

 

[koalar]

The elastic deformation is about 1.5 - 2 degrees on mild aluminum

Are you sure

I just cashed some aluminum in at the scrap merchants, I bent the tube 180 degrees to fit them into the car

Same with aluminum cans and they are already extruded and work hardened but not super brittle


We don't know what the alloy is but looking at the bend, it looks pretty ductil to me,

More so than steel which gets pulled and jigged back to shape after a crash

You normaly would not be carrying any weight in the centre of the bars

 

[koalar]

I have not purchased the rails. They are on a car at a breakers yard and would need dismantling if anyone wants them. Thank you all for various comments. I will look elsewhere.

Probably wise

Getting them to look right would take skill or luck as they are slightly curved from new

 

[AndyRKett]

Are you sure

I just cashed some aluminum in at the scrap merchants, I bent the tube 180 degrees to fit them into the car

Same with aluminum cans and they are already extruded and work hardened but not super brittle


We don't know what the alloy is but looking at the bend, it looks pretty ductil to me,

More so than steel which gets pulled and jigged back to shape after a crash

You normaly would not be carrying any weight in the centre of the bars

Elastic deformation (1.5-2 degrees) means you need to bend the metal 1.5 - 2 degrees more than the final shape that you are aiming for as this is the "bounce - back" I think you are misunderstanding and thinking that this is the level it will bend to before breaking or becoming brittle?
That said as @jrkitching points out correctly you don't need to bend it for it to become brittle, in aircraft the act of pressurizing and depressurizing is enough to make the metal brittle over time which is why there are massive bone yards full of multi-million dollar aircraft which have reached the end of there serviceable life because the fuselage has reached its max-permitted number of pressurization cycles.

Just to note Aluminum cans are not extruded.

Steel doesn't have the same properties as aluminum and therefore can tolerate being bent back into shape more so than steel but is will still fatigue and fracture, (which is how Uri gella made his money bending spoons till they break) When jigging cars they still quite often cut out a lot of the seriously damaged metal even after pulling the car back into shape. They use the damaged metal to pull against to make sure the overall geometry of the car is right, then cut out the bits that will have bent beyond repair.

Aircraft don't use much aliminium it's normally an alloy such as duralumin, avional or silumin (might be spelt it wrong)

It quite a cool process. Large sheets come out of an anealing oven then immediately get bent and fastened into shape, It the hardens over the next few days at room temperature, as the cyristine structure alters

There are other ways now such as composite

These are pretty old fashioned names for alloys, but they are all still "alloys" and modern aircraft use alloys such as "Aluminum 7075" or "Aluminum 6063", the clue being they are all "alloys" of aluminum and so are mostly comprised of aluminum with a bit of other metals and elements like silicon added to them for different properties. All alloys of aluminum work harden or fatigue. as they are all comprised of usually >90% aluminum. thought Silumin usually has a high silicon concentration making it very cheap. This makes up 3000 and 4000 series alloys which don't tend to be used in aircraft.

Metal fatigue and cracking can be avoided by annealing the metal, heating it up, something you'll find if ever doing metal work on the body of an old Land Rover which is often made of Birmabright, an alloy with magnesium and Manganese. as soon as you start trying to knock out even small dents the metal can tear and split apart.

You can't anneal painted or plastic coated surfaces as you just burn the paint off, so you probably could get those bars back into shape but you're have to get them recoated afterwards, which would probably cost more than a bar that's not bent.


There you go, bit of random metallurgy science.

 

[koalar]

Just to note Aluminum cans are not extruded.

drawing and ironing still work hardens the aluminum the same as impact extrusion

Here's part of an aluminum walking stick, the only aluminium that's not cast I could find

I bent about a 2 cm curve and straightened it again, over a piece of wood

Not perfect as you can now see a bit of rippling down its Length, pretty strong stuff had to use all my weight but it bent and straightened again

 

[AndyRKett]

drawing and ironing still work hardens the aluminum the same as impact extrusion

Here's part of an aluminum walking stick, the only aluminium that's not cast I could find

I bent about a 2 cm curve and straightened it again, over a piece of wood

Not perfect as you can now see a bit of rippling down its Length, pretty strong stuff had to use all my weight but it bent and straightened againView attachment 428352

but as you point out its not straight.

Also that has been bent and straightened once.

Instead of bending a bit of a pole just once do this instead, put a lot of weight on it side ways take it of do that repeatedly. Subject it to vibration, road salt, advere weather of hot and cold for several years.. Then bend it round a pole and see how well it straightens or if it crasks, I am sure you can't tell me if there are cracks appearing on the inside, or at a microscopic level or if there are now any floors in the structure of the metal where you made your bend.

Even if it doesn't break or crack from being bent once, are you going to attach a still obviously bent bit of metal to your car and trust it to hold loads on the roof when doing 60mph into a head wind?

@jrkitching is still right here, its not worth using a damaged roof rail on a car, same as you wouldn't trust a crash helmet or baby seat that's already been in a crash.

 

[koalar]

but as you point out its not straight.

Also that has been bent and straightened once.

Instead of bending a bit of a pole just once do this instead, put a lot of weight on it side ways take it of do that repeatedly. Subject it to vibration, road salt, advere weather of hot and cold for several years.. Then bend it round a pole and see how well it straightens or if it crasks, I am sure you can't tell me if there are cracks appearing on the inside, or at a microscopic level or if there are now any floors in the structure of the metal where you made your bend.

Even if it doesn't break or crack from being bent once, are you going to attach a still obviously bent bit of metal to your car and trust it to hold loads on the roof when doing 60mph into a head wind?

@jrkitching is still right here, its not worth using a damaged roof rail on a car, same as you wouldn't trust a crash helmet or baby seat that's already been in a crash.

I don't know what more I can do

I bent some extruded aluminum

I straighten it back, it's pretty dam good, from 2 meter away you would not be able to tell but it's not up to Rolls Royce body standards. On the curve of the original it would be even harder to tell

No cracking, no fuss, no anealing

Which is exactly as I suspected from one of the most malleable and ductil metals

It's not Birmabright, duralumin, avional, silumin, cast aluminium which have very different properties

Whether I would take the job on myself I don't know, now I have looked at my rails a form would be needed to avoid crushing the top of tube

 

[AndyRKett]

I don't know what more I can do

I bent some extruded aluminum

I straighten it back, it's pretty dam good, from 2 meter away you would not be able to tell but it's not up to Rolls Royce body standards. On the curve of the original it would be even harder to tell

No cracking, no fuss, no anealing

No but also not being used as a roof rail on a car, and even if its not being fitted to carry loads then its being fitted for aesthetic purposes and so a bent and roughly repaired roof rail is not going to look good, as the owner of the car will always see the fault.

I can guarantee you your piece of pipe is not pure aluminum and will be some alloy with other materials in it to make it more durable, no one is making domestic products from pure aluminum, they are all alloys.

Pure aluminium is quite soft and lacking in strength. In most applications various aluminium alloys are used instead because of their higher strength and hardness. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. See more

 

[koalar]

No but also not being used as a roof rail on a car, and even if its not being fitted to carry loads then its being fitted for aesthetic purposes and so a bent and roughly repaired roof rail is not going to look good, as the owner of the car will always see the fault.

What

Already said you would not be able to tell from 2 meters away

Which one was bent and straightened

I can guarantee you your piece of pipe is not pure aluminum and will be some alloy with other materials in it to make it more durable, no one is making domestic products from pure aluminum, they are all alloys.

Correct,

It look to me from the bend it's fairly soft, other have already said in the threads the bend easy if you put weight in the middle

Until someone tries we will never know

There's a lot of negative doom and gloom,. Most of the forces should be focused and evenly spread where the bolts are, the middle should mainly have compression and stretching forces.

 

[AndyRKett]

What

Already said you would not be able to tell from 2 meters away

Which one was bent and straightened

Pictures of a pipe you bent are irrelevant, we're not talking about a uniform pipe and from one picture you can't say what state the metal is in. you will never bend it back perfectly. They are pretty common and the OP might as well (as he has already said he is going to) get a set that are not damaged.

regardless of you bending a bit of pipe, the thread is about roof rails which are already damaged and an unknown commodity.

There's a lot of negative doom and gloom,. Most of the forces should be focused and evenly spread where the bolts are, the middle should mainly have compression and stretching forces.

The forces are through roof bars normally attached to the rails, so the weight of the load is focused on 4 small points on the roof rails. The one side is significantly damaged and is unlikely to ever be as strong as it would be undamaged. To try and repair it is only going to weaken it further. I am not sure how you propose something will be simultaneously in compression and tension at the same time ??

I don't suspect the OP plans on carrying loads of weight around on the roof of his 100hp, I don't suppose their weight carrying capacity is really the main issue anyway. I do suspect he wants them fitted for aesthetic reasons, and as you will never get them 100% right, the OP will always look at the damage rail and see any imperfections as he will always know it's there and what he is looking at

So really regardless of all my waffle up above, you'll never get it perfectly straight, and it could break or fail in an unexpected way should you try and use it to carry weight.

 

[koalar]

Most of the forces should be focused and evenly spread where the bolts are, the middle should mainly have compression and stretching forces.

I am not sure how you propose something will be simultaneously in compression and tension at the same time ??

I never said at the same time

As you accelerate and brake the 4 corners will try and move slight independently but they can't because they are connected by the bar. Like I said most of the forces in the middle of the bar should be push and pulling