Worth a punt?

[SkipdiverJohn]

I was under the impression that most aluminium alloys fracture or shear rather than distort; guess I was wrong?

There's various types of aluminium alloys with varying degrees of hardness. pure aluminium is very soft, relatively weak, and not much use for structures. It only becomes useful once you start to alloy it with other things to alter it's characteristics.
For example, my Land Rover body is made of an Aluminium-Magnesium alloy known as Birmabright, which is considerably harder and stronger than pure ali. You can still dent or crease it fairly easily though compared to steel.
What you get is plastic deformation as you load the alloy beyond it's yield point. if then left alone, the material will stay together, but in a weakened state. If you try to bend it more though, or bend it back in the opposite direction, you start to compound the structural damage then it will often shear off, not as a result of the original deformation, but the subsequent attempts to put it right.
The bottom line is if you dent an alloy bike frame the best thing to do is leave it alone and live with it, attempts to reshape it are likely to make matters worse.

 

[RichardB]

There's various types of aluminium alloys with varying degrees of hardness. pure aluminium is very soft, relatively weak, and not much use for structures. It only becomes useful once you start to alloy it with other things to alter it's characteristics.

I was reading something the other day about steel vs alloy for bike frames. Quite technical, but in summary: steel distorts elastically under stress, but returns to its original form. Aluminium alloy distorts, but is slightly damaged each time on a molecular level, so that impacts to an ali frame are cumulative over time, and the frame will eventually fail. In other words, steel frames last more or less for ever, but ali frames have a definite lifespan. Hence ali frames tend to be guaranteed by the maker for 1-5 years, whereas steel frames are often guaranteed for a lifetime.

I'm no metallurgist, so I can't really comment, but I haven't heard of ali frames being routinely junked after 10 years due to fatigue, so I rather doubt this information. But as a fan of steel I kind-of want it to be true.

 

[SkipdiverJohn]

Aluminium will still deform elastically below it's yield point, just like steel. Even materials like glass have some degree of elastic deformation. Its not black and white to say that one material will always bend and spring back, whereas another will stay bent or shatter. It's more nuanced than that.
The big advantage of steel is the yield point is closer to the UTS, and in the case of "exotic" heat-treated high strength alloy steels, the yield point can be very close indeed to the UTS. (such as Reynolds 753) This means unless the frame is really heavily loaded/abused, repeated load cycles do not subtract from the material's fatigue life. The downside is the smaller the difference between Yield Strength and UTS, if you do actually put a permanent deformation in the material, the closer it will be to it's failure point.
If you want bodge repairability in the field, it's actually better to employ a lower grade steel. That might sound counter-intuitive deliberately using a poorer material, but they are more tolerant of being reworked.

 

[RichardB]

Aluminium will still deform elastically below it's yield point, just like steel. Even materials like glass have some degree of elastic deformation. Its not black and white to say that one material will always bend and spring back, whereas another will stay bent or shatter. It's more nuanced than that.
The big advantage of steel is the yield point is closer to the UTS, and in the case of "exotic" heat-treated high strength alloy steels, the yield point can be very close indeed to the UTS. (such as Reynolds 753) This means unless the frame is really heavily loaded/abused, repeated load cycles do not subtract from the material's fatigue life. The downside is the smaller the difference between Yield Strength and UTS, if you do actually put a permanent deformation in the material, the closer it will be to it's failure point.
If you want bodge repairability in the field, it's actually better to employ a lower grade steel. That might sound counter-intuitive deliberately using a poorer material, but they are more tolerant of being reworked.

Interesting, thanks.

 

[Gravity Aided]

Aluminum alloy may not be the best for bending back to shape in the method I outlined above. Linguistic disconnect, there. In the States, we refer to both steel and aluminum alloys, like Cr-MO, as alloy.

 

[RichardB]

Aluminum alloy may not be the best for bending back to shape in the method I outlined above. Linguistic disconnect, there. In the States, we refer to both steel and aluminum alloys, like Cr-MO, as alloy.

And you are correct, of course. Steel and all the others are alloys, technically speaking. In the UK we generally refer to steel as ‘steel’, even though it is an alloy of iron, and aluminium-based frames as ‘alloy’. Same with car wheels - steel wheels and alloy wheels (aluminium or magnesium) are the choices. We’re wrong, but the distinction is a useful one.

 

[Gravity Aided]

And you are correct, of course. Steel and all the others are alloys, technically speaking. In the UK we generally refer to steel as ‘steel’, even though it is an alloy of iron, and aluminium-based frames as ‘alloy’. Same with car wheels - steel wheels and alloy wheels (aluminium or magnesium) are the choices. We’re wrong, but the distinction is a useful one.

Not wrong, just different. Often refreshingly so.

 

[paxterg]

The bike is mine for 150. Think they sold for 1100, but if the bike is not safe it’s no good to me! Some people are saying it’s scrap, some it’s ok to ride. Tbh I’m no further forward!

 

[vickster]

The bike is mine for 150. Think they sold for 1100, but if the bike is not safe it’s no good to me! Some people are saying it’s scrap, some it’s ok to ride. Tbh I’m no further forward!

Just get a nice new steel frame and move the parts across