# Alloy or aluminium?



## Alan Farroll (24 Aug 2014)

Going to buy my first mountain bike. I notice some are aluminium and others alloy. Is aluminium heavier? what about an alloy frame rusting? What's it an alloy of?

Thanks in advance

Alan


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## Tim Hall (24 Aug 2014)

Generally it's an aluminium alloy. For the pedants/irredeemably accurate, the steel used in bikes is also an alloy. But the adverts you see will be for aluminium alloy.


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## Deleted member 23692 (24 Aug 2014)

'Aluminium' bike are in fact aluminium alloy. Pure aluminium would be too soft to make a bike frame out of, so it's alloyed with various other stuff to make it fit for purpose. Zinc, magnesium and silicone are the usual suspects but many other elements are used. It won't rust (like ferrous metals), but the bare alu-alloy will oxidise - which forms aprotective skin, but it wont corrode.


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## Cycleops (24 Aug 2014)

An aluminium alloy frame cannot rust, but it may oxidize if it doesn't have a protective coating. "Butting" where the tubes are varied internally in thickness throughout its length will affect it's weight.


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## raleighnut (24 Aug 2014)

Some 'alloy' frames are prefixed CrMo Alloy this is a steel frame and superior to a basic steel frame and will normally be lighter than an aluminium frame at the same price level. Quality Aluminium frames tend to cost a lot.


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## Alan Farroll (24 Aug 2014)

Okay. Thanks this is becoming clearer. So if a manufacturer says the frame is for instance "ALUXX-Grade Aluminium" or they simply state in the specifications of a £600 mountain bike that it is Aluminium, then it is not really pure aluminium but an aluminium alloy!!?? And also, if the specifications state that the frame is an "Alloy" then it also is an aluminium alloy??!! Is this correct?


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## Cubist (25 Aug 2014)

Alan Farroll said:


> Okay. Thanks this is becoming clearer. So if a manufacturer says the frame is for instance "ALUXX-Grade Aluminium" or they simply state in the specifications of a £600 mountain bike that it is Aluminium, then it is not really pure aluminium but an aluminium alloy!!?? And also, if the specifications state that the frame is an "Alloy" then it also is an aluminium alloy??!! Is this correct?


Yes. You'll see a couple of different aluminium alloys in today's market, 7005 and 6061. 6061 is an older type of alloy, and needs a particular hardening process to finish it. It contains magnesium and silicon. 7005 is newer, and can be air cooled. It's denser than 6061 which means stronger for a given tube wall thickness. 7005 tends to be found on more expensive bike frames and can be butted (material removed from the middle where it isn't needed, and left in at the ends where it's welded to other tubes. ) Butted tubes tend to be lighter than non-butted. 7005 can be hardened in air, and tubes tend to have sculpted or swoopy shapes allowing the tubes to flex more than straight tubes whilst still retaining strength where it's needed. Straight tubes can be very stiff, fine for transferring power through the frame, but aluminium alloys are stiff to the point of harshness, so shaped tubes give a more comfortable, compliant ride.

Steel varies massively in quality and weight, depending on alloy and metallurgy. As @raleighnut says, chromoly steel is used for more expensive bike frames. Cheaper ones are made of high tensile steel, very, very heavy. Chromoly has several variants, all mostly given a Reynolds number, and this denotes the metallurgy used, and the tubing properties all differ depending on that. Mountain bike frames in chromoly are very popular, with some niche and boutique brands leading the market. However, the type of alloy used varies (as does the price) Steel MTB frames can be strong and bombproof, with neater, slimmer tubes than aluminium, but are rarely lighter (they need different geometries and strengths in different areas to road bikes) The On One 456 is a prime example, as is the Ragley Blue Pig. Stiff, strong and relatively inexpensive, (given the market) with chromoly frames. More exotic and expensive steel alloys, the prime example being Reynolds 853, are quoted as being the "perfect material" for a MTB frame. 853 tubes can be made to the same strength as chromoly for a given tube diameter, but the tube walls can be extruded thinner, and therefore lighter for the same strength. Cotic makes two identical frames, the BFe and the Soul. Their geometry is identical, their intended use is identical, as the framebuilder is at pains to point out, but because the 853 used in the Soul can be made thinner for the same strength, it is lighter, and gives a more compliant, springy ride. What's more, unlike other chromolys, 853 can be air hardened and becomes stronger as it is welded, so doesn't need reinforcing gussets in MTB applications. The heavier, less compliant chromoly BFe is significantly cheaper. To date, steel alloy development stops with the 853. More recent Reynolds variants have proved to be too light. Still as strong for the same tube size, but too flexible, meaning that wider diameter tubes would be needed to prevent the tubes becoming too flexible (or noodley), and this would mean more material would have to be used, thus defeating the weight advantage.


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## Cycleops (25 Aug 2014)

Well, if @Alan Farroll wasn't confused before he sure must be now!


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## raleighnut (25 Aug 2014)

Alan Farroll said:


> Okay. Thanks this is becoming clearer. So if a manufacturer says the frame is for instance "ALUXX-Grade Aluminium" or they simply state in the specifications of a £600 mountain bike that it is Aluminium, then it is not really pure aluminium but an aluminium alloy!!?? And also, if the specifications state that the frame is an "Alloy" then it also is an aluminium alloy??!! Is this correct?


No, a steel alloy is a different material to an aluminium alloy. All metals can be Alloyed even Gold to give 9ct. 18ct and 22ct versions.


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## Cubist (25 Aug 2014)

raleighnut said:


> No, a steel alloy is a different material to an aluminium alloy. All metals can be Alloyed even Gold to give 9ct. 18ct and 22ct versions.


However, in bike marketing terms, a steel alloy frame would be billed as steel or chromoly. Alloy is generally used to denote an aluminium alloy. So if @Alan Farroll is in the market for a new bike, he can be reassured that an "alloy frame " is going to be an alu alloy. Very few entry level MTBs are made of steel alloy, and if they are, the marketing will,make sure you know it.


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## Deleted member 23692 (25 Aug 2014)

Just to compound the confusion... steel is an alloy of iron, so cro-mo is just a different blend of 'steel'


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## Alan Farroll (26 Aug 2014)

Thank you all for your feedback. This helps a lot


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## jack smith (26 Aug 2014)

Basically without going too technical they are all pretty similar so just buy which bike you like!


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## Drago (26 Aug 2014)

What about the Woodelo?


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## 02GF74 (30 Aug 2014)

> 7005 is newer, and can be air cooled. It's denser than 6061 which means stronger for a given tube wall thickness.


 
denser metal alloy does not mean stronger - just think how strng a bike frame mdae from lead would be!!!!
there'd beo ther reasons why the 7005 is stronger and I doubt it is due purely to density.


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## Drago (30 Aug 2014)

In actual practice there's very little difference between 6061 and 7005. Tube manipulation and butting will be the deciders


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## Cubist (31 Aug 2014)

02GF74 said:


> denser metal alloy does not mean stronger - just think how strng a bike frame mdae from lead would be!!!!
> there'd beo ther reasons why the 7005 is stronger and I doubt it is due purely to density.



No, of course, but a denser grained metal can be drawn thinner for the same strength tubing, all other factors being equal, which is why tubes made of 853 Reynolds are lighter than 661 Reynolds tubing for the same diameter, but just as strong.


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## albion (31 Aug 2014)

Cheap bikes use 6061 whilst strong hybrids stick to 7005.

7005 frames nearly vanished but its making a bit of a comeback.


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## Gravity Aided (1 Sep 2014)

6061 T-6 has magnesium and silicon as its alloying agents, and it's ability to withstand shocks and such can be positively or negatively enhanced by loading, gradient, and surface finish, which means that design workmanship, and quality of finish should be quite important in your considerations, as should weld quality. Look for gaps and poor weld finish, as these may have some effect on the overall longevity of the bicycle. I would also consider dropout design very important, as these are integral to the frame and subject to failure in extreme situations. I have seen a few pictures from the more mountainous and hilly areas of Southern Illinois of a fellow who has had some failures on aluminum frames, but he is heavier rider, and some of the trails there can be rather extreme.6061 has a density of 2.70 g/cm³ (0.0975 lb/in³). 7005 has a density of 2.78 g/cm. Its ability to air temper may lead to stronger material, but it is also favored due to its greater ease of welding. Either of these aluminum choices have the proviso that finishing, craftsmanship, and care are very important in the success of the end product. Steel alloys are easier to manufacture and repair. 4130, 4140, and 4145 are alloys often used by bicycle makers. These materials can be case hardened by the presence of carbon monoxides or the use of charcoal in a case, or hardening pack, which infuses these elements into the surface of the softer steel. You could harden all the way through, but this may make the steel brittle, so hardening the exterior is best, giving the best of both worlds for what the steel alloy has to offer.Craftsmanship makes a difference in weight and repair-ability. Last week, I traded in a complete Cannondale H600, weighing 24 pounds built, and 6061 aluminum. I received in exchange a Fuji Touring Series IV frame and fork, weight built 24 pounds. I would consider both bicycles to be equal in speed, but the aluminum ride is harsher. Things can be done to mitigate this, like an alloy fork, but it is still a better ride on steel. For a mountain bicycle, with an emphasis on durability, I would prefer steel, due to its more forgiving nature and repair-ability, given the stresses involved. Fracture lines along the dropout edges and stress points may also appear with age. Steel bends, aluminum can crack instead.


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## Drago (1 Sep 2014)

albion said:


> Cheap bikes use 6061 whilst strong hybrids stick to 7005.



Poppycock. The only thing 'higher' about 7005 is the number. They're different approaches to the same problem and, as Mr Aided points out, manipulation, build and the heat treatment process used are the real deciding factors. 

You will find plenty of excellent frames using both materials, and plenty of dreadful ones.


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## albion (1 Sep 2014)

They switched to 6061 for cost.

http://www.makeitfrom.com/compare-materials/?A=6061-T6-Aluminum&B=7075-T6-Aluminum

As you can see they are chalk and cheese.


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## Drago (1 Sep 2014)

And the raw material is cheaper. So what?

And that data is for just that, the raw material, not the heat treated finished product. Depending on the heat treatment technique/cycle applied there's not a lot in it, at least not enough to justify pub smugness a la 853 v 501.


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## albion (2 Sep 2014)

There a heck of a lot in it., and it will be for finished tube. It seems 7075 got invented by the Japanese for WW2 airplanes, and even gets used in guns

I bet all the small workshops in China churn out 6061 because they can easily get it right. 
But even then all those welds will likely need all the heat treating redone.


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## rojobe (3 Sep 2014)

Has anyone mentioned ride quality? If you're looking for a pure road bike then Alu alloy or Carbon are 99% of the choice. If it's a tourer or more of a do-it-all then steel comes into the frame. Aluminium has no discernable flex so the ride can be harsh and on 23mm tires you'll feel a lot from the road surface. Steel tends to flex and in a good, well designed frame this characteristic can be used to create a ride that is a little more forgiving and comfortable. This becomes more of an advantage over longer distances. So many touring or audax frames are still made from steel.


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## raleighnut (10 Sep 2014)

Cubist said:


> However, in bike marketing terms, a steel alloy frame would be billed as steel or chromoly. Alloy is generally used to denote an aluminium alloy. So if @Alan Farroll is in the market for a new bike, he can be reassured that an "alloy frame " is going to be an alu alloy. Very few entry level MTBs are made of steel alloy, and if they are, the marketing will,make sure you know it.


I've been going out a lot more on the trike lately (my leg is getting better) and have been into a few shops to buy bits and pieces, looked at bikes while in there and have seen a lot with stickers reading CrMo Alloy so I would say any frame described as alloy (as opposed to XXXX Aluminium) is in fact a CrMo frame


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## Gravity Aided (11 Sep 2014)

Some steel bikes may be made of Hi Ten, or High Tensile (Strength) steel, which is a bit more malleable than Cro-Moly. Cro-Moly is better in all aspects, IMO. I think any good bike frame would be made out of 41xx series Cro-Moly, but I also point out that the Trek 800 in its original incarnation was Cro-Moly only as far as the seat tube, and the rest was Hi-Ten. Yet it is a very nice bike, for a bottom of the line model.


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## Psycolist (4 Jun 2015)

Well, that has turned my perception of Aluminium versus Steel on its head. I always believed the ride of an aluminium frame to be more flexible and pliable than steel or cro-mo. The information given in this thread has been given very clearly and has answered many of the questions that were going through my bonce when reading the first couple of posts. However, when all said and done, IMO, nothing beats the looks of a bare, unpainted aluminium finish on a frame. BUT, after reading this thread, I will certainly not dismiss the possibilities of a steel / cro-moly frame build in the future.


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## Gravity Aided (4 Jun 2015)

Chromed steel,also pleasing to the eye,but reputedly more brittle. A painted frame may bend,a chromed one may snap,under the same conditions


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## raleighnut (5 Jun 2015)

Gravity Aided said:


> Chromed steel,also pleasing to the eye,but reputedly more brittle. A painted frame may bend,a chromed one may snap,under the same conditions


That is due to Hydrogen Embrittlement which can be solved by heat treatment. I think it involves holding the plated component at 170 centigrade for 2 hours.
I'll google it.
EDIT -I'm wrong. it may take 22 hours at 190-220 centigrade to remove molecular hydrogen after plating (and that is in a non-hydrogen atmosphere so not a gas-fired oven.)


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## Drago (5 Jun 2015)

Nickel plate instead? Does that affect the steel?


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## raleighnut (5 Jun 2015)

Drago said:


> Nickel plate instead? Does that affect the steel?


I think it can happen with any poor quality plating, and the higher quality the Steel the worse it can be.


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## raleighnut (5 Jun 2015)

Linky
http://www.google.co.uk/url?sa=t&rc...vWNiD-EgVDd_TH48j7Ua4hg&bvm=bv.95039771,d.bGg


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