Folding Bikes - aluminum vs. chromoly vs. "common steel"

Assuming I have 3 folding bikes with 20" wheels of the same frame design, same accessories and components but one has a frame made of aluminum, one a frame made of chromoly and one a frame made of "common steel". What is going to be the weight of each one of these? The reason why I ask is to try to decide if the extra cost for the lighter frame is going to be worth it, i.e. if the aluminum frame is only 3 pounds lighter than a chromoly frame and is $200 more I may not feel the extra $$ is justifiable.

PS I know that aluminum bikes and steel bikes have different frame designs because of the characteristics of the metals - the question is phrased hypothetically to get an understanding of the relative weights.

3 pounds lighter feels like the difference between a yugo and a porsche .. well lets say VW and BMW
... darn I dont know enough about cars anymore to make a good comparison ....

anyhow 3 lbs for 200 dlr is usually worth it, as the 3 lbs also mean that all the components are nicer and of better quality

If "common steel" means hi ten steel, I can give you my observations. I have a folder made of high-tensile steel (Dahon Yeah). Hi ten is usually used by mass market bike manufacturers and those bikes tend to be cheaper. This steel is heavier that cromoly and much heavier that aluminum.

For the casual rider, hi ten gives you a great value. I may have a romantic attachment to hi ten steel since I grew up on xmart bikes and they were great in my youth. IMHO hi ten bikes seem to "give" a little when going over bumps and undulations. Cromoly, too, since I also own a non-folder made of that. I've owned and ridden aluminum bikes and they are jarring to the hands esp. over long rides (again IMHO). I think bike manufacturers understand this and end up putting heavy suspension forks on alum bikes which negates their weight advantage somewhat. People go a little nuts over weight and end up paying too much for a light bike when a heavy bike can help them lose pounds, at least when they get back into biking.

In terms of bike weight, my hi ten bike weighs 32lbs. A cromoly one would weight 27 lbs (estimate) and an aluminum one would weight 23lbs (estimate). Hope this helps.

Take a look at the specs of the Dahon Speed D7 (chromoly) and Vitesse D7 (aluminum). Unless I'm mistaken, the components are pretty much identical, and the frame is the same design. The Speed is listed at 26.6 lbs and the Vitesse at 24.9. There are hi-tensile folders with frames that look very much like the Speed and the Vitesse. However, their components are usually not as good, and heavier.

http://www.dahon.com/us/speedd7.htm
http://www.dahon.com/us/vitessed7.htm

Here's one example of a hi-tensile clone. Specs read 34lbs "complete" (I don't know what "complete" means relative to the Dahon specs).

http://www.bazookasports.com/viewproduct.php?cat=1&product=2003&details=specs

In terms of performance, a reduction in weight is almost totally worthless and isn't worth the costs.

In terms of carrying, that's up to you. Personally I don't think it's worth it there either.

Get the bike that a) fits you and b) folds or packs to the size you need.

In terms of performance, a reduction in weight is almost totally worthless and isn't worth the costs.

In terms of carrying, that's up to you. Personally I don't think it's worth it there either.

Get the bike that a) fits you and b) folds or packs to the size you need.
Good advice!:):D

Assuming I have 3 folding bikes with 20" wheels of the same frame design, same accessories and components but one has a frame made of aluminum, one a frame made of chromoly and one a frame made of "common steel". What is going to be the weight of each one of these? The reason why I ask is to try to decide if the extra cost for the lighter frame is going to be worth it, i.e. if the aluminum frame is only 3 pounds lighter than a chromoly frame and is $200 more I may not feel the extra $$ is justifiable.

PS I know that aluminum bikes and steel bikes have different frame designs because of the characteristics of the metals - the question is phrased hypothetically to get an understanding of the relative weights.
If the frames were exactly the same (design, tube diameter, wall thickness,...) but made from different materials:
Density of aluminum used for bicycle frames ~2.8g/ccm
Density of most steels used for bicycle frames ~7.9g/ccm

But then either the aluminum frame would be terribly "under-designed"/weak or the steel frames extremely "over-designed"/heavy.

Good advice!:):D

Do you carry a folder around?

IMO, folded weight is way more important than folded size. Most folders are so heavy that they're almost unusable and square nonrolling designs only exacerbate the problem.

Of course, if you're just passing it over to the baggage handlers then I guess it's not really your problem, but that's not what I use my folders for.

If the frames were exactly the same (design, tube diameter, wall thickness,...) but made from different materials:
Density of aluminum used for bicycle frames ~2.8g/ccm
Density of most steels used for bicycle frames ~7.9g/ccm

But then either the aluminum frame would be terribly "under-designed"/weak or the steel frames extremely "over-designed"/heavy.

You are on the right track.

The modulus of aluminum is almost a third that of steel. Basically, the modulus is the spring rate of the material. How far it deflects under a given load.

Modulus:
4130 Steel: 30*10^6 psi
6061-T6 aluminum: 10*10^6 psi

Strength:
4130 Steel: 170 kpsi
6061-T6 aluminum: 40 kpsi

I used 4130 because it's similar to Reynolds 531.

SO...you have to use more aluminum to keep the deflection down! Basically, you make the tubing larger. But this also has the effect of decreasing the bending stress.

That's why Cannondale tubes are so huge...and I do love riding my Cannondale when folding is not required.

Thanks! Good feedback and good thoughts.

Just workout a little bit more, cut on the pizza, mexican food; get rid of those 3 pounds which is much cheaper than $200.00.

I'll carry 3 lb for $200 all day long.

Would you carry 3lb four times a day for 250 days a year for ten years for $200?

TCS

I agree with those who say that the importance of a few pounds is exaggerated by many bike riders. Steel has huge advantages, it's more pliable, far more tolerant of flexing stress and if it's going to break, it gives warning - it doesn't just snap like a carrot and throw you onto the road and under a truck. Oh - and you can repair it too.

I have three aluminium framed bikes, but I wash and minutely examine them every hundred miles looking for cracks and paint damage near the joints. I've never had a problem, but they certainly happen.

Now aluminum rims, on the other hand, are a significant upgrade from steel rims!

...and Aluminum is a perfectly good material for bike frames. I think someone learned that Aluminum doesn't have a fatigue limit, and took that to mean that since it will eventually fail, cyclists are at risk.

Just to give you an idea, to fail an aluminum top tube, you would have to load it with about 900# for a million cycles.

I'll spare you the calculations, as I already posted it on another thread.

There's a lot of aluminum in airplanes, and we don't see too many of them breaking and falling out of the sky, do we?

The safety nannies would never let Cannondale produuce another bike if too many of them started failing (Yes, I own a Cannondale).

Now aluminum rims, on the other hand, are a significant upgrade from steel rims!

...and Aluminum is a perfectly good material for bike frames. I think someone learned that Aluminum doesn't have a fatigue limit, and took that to mean that since it will eventually fail, cyclists are at risk.

Just to give you an idea, to fail an aluminum top tube, you would have to load it with about 900# for a million cycles.

I'll spare you the calculations, as I already posted it on another thread.

There's a lot of aluminum in airplanes, and we don't see too many of them breaking and falling out of the sky, do we?

The safety nannies would never let Cannondale produuce another bike if too many of them started failing (Yes, I own a Cannondale).

Ok - so what about all the dahon failed handleposts and the broken frames we read about on here all the time in the Mechanics section? They do fail, so do alluminium handlebars. Brompton had to change their design and to issue strengtheners when a small number of riders were thrown onto the road as the bar snapped in their hand all of a sudden and without any warning.

All but one of my current bikes has an alluminium frame, bars, wheels and other parts, but I certainly check them over for any sign of failure pretty often.

Ok - so what about all the dahon failed handleposts and the broken frames we read about on here all the time in the Mechanics section? They do fail, so do alluminium handlebars. Brompton had to change their design and to issue strengtheners when a small number of riders were thrown onto the road as the bar snapped in their hand all of a sudden and without any warning.Everything can fail! To make sure thing don't fail, they have to be properly designed, manufactured, used and maintained. This applies to everything, no matter what it is made out of.

BTW, steel parts with thin wall thickness can snap just as sudden with little "warning" as thin walled aluminum parts.

A bicycle can be designed so it won't fail, or at least will be extremely unlikely to fail. It is relatively straightforward to calculate the stresses on the various parts, and then design a part such that the stresses are far below failure mode. Problem is, your design won't be very popular. Or light.

This is all interesting. Who would have thought we would have so many engineers in a folding bike forum? ;-)

What about magnesium?

Finally, check this carbon frame out: http://www.news.com/Is-this-bike-made-from-spider-webs/2100-1008_3-6219404.html?tag=ne.fd.mnbc Is it theoretically possibly to use sequestered carbon from scrubbers to make carbon fiber? Apologies if these questions seem as stupid. I'm no materials scientist.

[QUOTE=CrimsonEclipse;5672612]oh, the argument is the failure mode.

Steel does not suffer work hardening as bad as aluminum.
Aluminum is stronger. This is generally true. Sure there are examples of steel
that are super strong but those are prohibitively expensive. I will limit my
argument to readily available materials.


Actually ChromMoly is a very common material used in bikes . . . and even in its normalized condition it is stronger than ANY aluminum alloy. Even a common 'high carbon' steel is stronger than your common aluminum frame material. Here we are talkin' the 'material itself' . . . not how any particular frame is designed and the materials used.

DON

Given the choice, i'd usually rather ride steel, simply because steel is generally a more comfortable ride. There are other benefits as well, such as being easily repairable, more predictable, and quite strong.

Well, in all fairness, aluminum is usually a bit lighter, stiffer and doesn't rust. I prefer the movement of steel over the harshness of alu though. And with a little care and frame saver, steel isn't going to fall apart.

It comes down to a trade off though, I suppose. 3 extra lbs of lifting, or a rougher ride? And is that $200 a result of different componentry, or simply a much lighter frame?

Yes - I thought that was a mistake about alluminium. Could be that ally is stronger weight for weight - I don't know.Steel frames seem more springy to me. I like that in an unsuspended bike. The other thing to say is that not all steel frames are the same. I remember the difference between an old cromoly road bike and one made out of Reynolds 531. One felt like a bridge, the other like a gazelle. Actually, they were both good strong and effective frames, they just felt different and the 531 was a lot lighter. It had double butted tubes.

I've just recently returned to biking [after a 20-year absence] and my two bikes are steel-framed. I've never ridden an aluminum-framed bike.

I see comments over and over about how aluminum-framed bikes are stiffer. Perhaps so . . . but it is certainly not because aluminum is 'stiffer'. If you compare the modulus of elasticity [Young's modulus] of aluminum and steel you will notice that steel is 2.5 to 3 times stiffer than aluminum, depending on the aluminum alloy.

So if your aluminum bike is 'stiffer' . . . then it is because of its design, not the material. Larger diameter tubing [whether steel or aluminum] is the easiest way to make a tube stiffer. My observations are that most aluminum bikes use larger diameter tubing.

Increasing the wall thickness of a tube [within practical limits] does increase the 'strength' but does little to increase its 'stiffness'.

DON

A bicycle can be designed so it won't fail, or at least will be extremely unlikely to fail. It is relatively straightforward to calculate the stresses on the various parts, and then design a part such that the stresses are far below failure mode. Problem is, your design won't be very popular. Or light.

....or cheap.

(*rolls up newspaper*)
Bad...BAD!! Bad poster.

Bikes are NOT airplanes.

(*rubs nose in post*)
BAD!

Bikes are usually tubular construction. The steel tubes take the stress.
How many aluminum tubular construction airplanes are there? I don't know of any.

Most airplanes are monocoque or semi-monocoque construction meaning a stressed skin
where the skin takes a majority of the loads.

And don't get me started on quality control, design, and preventative maintenance that
airplanes routinely receive.

In short, please don't make the tired argument of airplanes -vs- bicycles.
It's a silly argument and it has no merit.

CE


Hmm.

Well, CrimsonEclipse, I thought the purile insults were the domain of A&S, but apparently they have crept into Foldling Bike forum too.

I've designed aluminum parts for marine applications, armored vehicles, and space systems, too, and none of them have failed to date. I just thought more people would identify with airplanes.

1. Where did I write that airplanes were tubular (and by that, I mean, and I think you meant, tubular frame) construction? You made that up to start your straw agument.

Stress is stress. The aluminimum doesn't care if it goes into a bike, airplane, or an aluminum can. Although the mechanical properties may differ a bit depending on the size, form, or manufacturing technique, it's still aluminum.

Furthermore, the aluminum doesn't care if it's in a tube or block. Again; stress is stress. It's the form of the area under stress that's important. The form of the material may enter in to how well the part performs, but in the end, aluminum is aluminum.

2. Don't airplanes have other flight-critical structural members made of aluminum besides the body?

3. Correct. Lack of maintenance has a tremendous impact on the design of a part. If a part is going to be replaced after a certain number of flight hours, pressure cycles, whatever, it can be designed closer to its limits. Weight is extremely important to aircraft, so they make up for the weight saving with increased inspection, maintenace and/or part replacement.

Part of my job requires that I not only design critical parts, but also write the maintenace and repair procedures for the parts I design.

Since many bikes go unmaintained, it only stands to reason that the parts must be designed to operate in spite of lack or maintenance or inspection. If this is not taken into account, you may get a part that fails.

People apparently jump their folding bikes off curbs (I've never actually SEEN this, but people have written about it in this forum, so it must be true :rolleyes:). If the designer didn't take this into account, you might be getting some stem failures.

Bad design? Maybe.
Not using the right tool for the right job? Maybe.
Bad welding technique and post weld heat treatment? Maybe
Bad material: No!

If a part is designed improperly, you may get a part that fails. By improper design, I mean selection of wrong materials, manufacturing processes, heat treatments, underestimating loads, inclusion of stress risers; I could go on and on.

You wrote in another post about all the handlebar stems that have failed, that you read about in the Mechanics forums. Again, if it's written in a forum, it must be true :rolleyes:! I have actually witnessed ONE single stem failure in my life. It was a steel stem. SO...my observations differ from your research.

In closing, aluminum under stress will behave the same way if it is in an airplane or a bike. It's not a tired argument, and it isn't quantum mechanics. It's a simple matter of proper design, strength of materials and stress analysis.

If you feel my argument was silly, fine. I'll probably post some more items that people think are silly. At lease I've paid for the privilege.

Now, I was able to make my points without once insulting you. Much nicer, eh?

:D

What about monocoque aluminum bike frames?