Fatigue life: carbon vs. aluminum

[JackL]

I was always told that aluminum is vulnerable to metal fatigue, which is one reason alu frames are built stiff. I also thought that aluminum crank arms have a "service life" and can crack & fail after years of hard use. I don't know if this also applies to alu stems and handlebars.

I also undeerstand that carbon fiber has a very long fatigue life, though this may just apply to the fibers and not the plastic matrix. My Easton carbon bars claim "superior fatigue life" in their literature.

So the question: will a conservatively engineered carbon component outlast its aluminum counterpart?

[zap]

Yes, but then we aren't seeing airplanes fall out off the sky either because some aluminum parts on a wing failed.

But new planes are being designed with more carbon components to take advantage of carbons light weight.

I still have 140g carbon bars I purchased in 1989. My oldest AL bar is 5 years old and will probably get tossed this winter.

As with all things, engineering and manufacturing of any material is important. Ten years ago, who would have thought that some AL and carbon bike frames could outlast steel and ti?

[froze]

The airplane thing is not a good example of aluminum durability, the frame consist of aircraft aluminum tubing that are bonded together with lugs not welded, because in aircraft the constant flexing of the airframe would cause welds to fail. Bike manufactures first used bonded lug aluminum frames but it wasn't cost effective due to the extra labor involved so they went to welding. Notice you don't see bridges or skyscrappers made of Aluminum, but in airplanes steel would be to heavy to fly. Also aircraft do have a life expendency and once the aircraft has reached that point it is retired.

Carbon is used in military planes for many reasons, absorbs radar, makes the plane lighter thus faster, if shot the bullet goes straight thru both sides instead of entering the first skin and bouncing around tearing up more stuff. The military also knows that the life expendency of CF is not great, but they are willing to put up with that to gain the other; Uncle Sam has deep pockets.

I believe that CF is a fad that will go away unless they come up with something that can give it better durability. CF frames get easily destroyed from just chain suck! the chain gouges gapping holes into the rear chain stay, nice uh? But CF can take a very hard blow without breaking, but once it breaks it fails fast with little or no warning.

Bike manufactures embraced aluminum because it was light-an excellent selling point, but it saved the biking manufactures tons of money because they no longer had to pay some person to assemble a frame by hand a braze using lugs, all of this was very labor extensive and the bike manufactures back in the early 80's were having financial nightmares. Now they can have some robotic shop in Taiwan or China spit out a frame in 5 seconds at a cost of $20 and then gets ship here where some manufacture charges $500 and up just for the frame; even if it's built in the USA it still is assembled by robots and probably cost $80 to make. Not all frames fall into this pricing structure, there are some that are better than others. But AL frames do not like to flex thus they do not last as long as TI or Steel. Just watch these forums (and others) long enough and you will quickly learn that AL has a higher failure rate.

One most ponder one's own navel when the worlds best custom frame builders all build with steel and uses lugs. Why? Why would they make bikes out of steel and lugs when they could custom hand weld AL frames faster and cheaper? Because these guys know that their product is meant to last a lifetime and they will not build with a inferior product and ruining their reputations.

[vaxn8r]

No, it's because they can't compete performance per dollar with the likes of Cannondale, Specialized etc. Aluminum frames are not typically thought of as "one off" custom. What are they going to show off anyway? A really excellent toothpaste weld? With steel they can show off.

BTW Co-Motion is one of the premier custom builders out there and they do both custom steel and AL....half bikes too ;)

[froze]

Really? Then why were these custom bike builders building bikes when the rest of the bicycle manufactures 15 years and more ago all built with lugs? To show off their "one off custom"? an excellent lugging technique? their skillful brazing? That was a lame brain excuse for custom guys to continue to use steel...good try though. I don't put Co-Motion in the same category as Kirk, Rivendell and Sach...sorry.

But I know this is going to run into a running argument so here are some web sites to help you understand why they use steel:

http://www.henryjames.com/faq.html
http://www.kirkframeworks.com/Philosophy.htm
http://www.exploratorium.edu/cycling/frames1.html
http://rivendellbicycles.com/html/101_pureopinions.html
http://rivendellbicycles.com/html/101_framemanufacture.html
http://rivendellbicycles.com/html/101_framematerials.html
http://rivendellbicycles.com/html/101_lugs.html
http://www.richardsachs.com/articles/rsachslug1.html
http://www.richardsachs.com/articles/rsachswatchtime.html
http://www.richardsachs.com/articles/rsachscigar.html

[vaxn8r]

Sorry, somewhere in that rant I missed your point.

[froze]

Let's see if I can word it differently for you. You said the reason custom frame builders use steel now is to show off their abilities. I said that before the mid 80's all those custom guys AS WELL AS the mass bike producers MOSTLY built bikes with lugs. My question is to you, that if both, the custom and mass producers were making lug steel bikes then who was showing off back then? Could you tell a mass lug construction from custom lug construction? Probably not, since all lugged bikes were hand assembled and brazed anyway! So again your thoughts about custom guys using steel to show off their talents doesn't hold water, because all those guys were making bikes before most of the manufactures switched to aluminum, and there was no one to show off too.

Custom frame builders aren't stupid, if they felt that aluminum bikes were better then steel they would all have switched to aluminum. Making fancy toothpaste welds for them would be easy, and profits would soar. They would be in the same position they were in before the mid 80's when they all made lugged steel bikes, except now they would be more profitable making aluminum frames. There is a reason the custom guys make bikes out of steel, just read those web sites to find out why.

[vaxn8r]

Froze, I'm not sure I agree with your logic but I do appreciate a more reasoned response. BTW, I've read all those articles. I have all their sites bookmarked.

I don't appreciate the condescending attitude and I surely didn't understand the anger from your post. What gives?

Anyway, can you explain exactly why Co-Motion could not be in the same league as the others you mentioned?

[zap]

There are plenty of custom aluminum frame builders out there. There will be more custom carbon frame builders too!

Steel has it's place, but not in this post.

The gentleman asked about fatigue life between carbon and Aluminum.

A number of aluminum alloys need to be heat treated after welding. Easy to fill a heat chamber with a number of bicycle frames and keep the frames in alignment. Can't do that with large complex components such as aircraft wings.

The point I was attempting to make is that there is a lot of flexing going on in aircraft. Aluminum can handle a certain amount of flex repeatedly. Check out http://www.castellanodesigns.com/tech.html (al frame builder with a flexible rear stay). I'm sure that aircraft go through more flying hours than bicycle frames have riding hours.

I'm not sure how military aircraft enter the equation. Plenty of passenger and recreational aircraft have critical components made out of composites. I'm quite certain that the aerospace industry has conducted extensive durability tests on a number of different materials.

Composites are here to stay with plenty of exiting new developments on the way.

[JohnS]

I fatigue well before my bike ever gets tired... :beer:

[froze]

I don't appreciate the condescending attitude and I surely didn't understand the anger from your post. What gives?

Anyway, can you explain exactly why Co-Motion could not be in the same league as the others you mentioned?

Wasn't trying to be condescending, I apologize to you for being a jerk.

The reason Co-Motion is not in the same "league" is because they do not have a "master" frame builder recognition. Example I hope will clear up my weird thoughts: Lets assume that today all the bike builders I mentioned and CoMotion, went out of business; 30 years from now no one will remember Co-Motion and the value of their bikes will be similar to a Waterford...better then Trek! However people who know bikes know that Richard Sachs, Bob Jackson, Peter Mooney and Grant Peterson, were the best frame builders in the USA, and their bikes would more then likely increase in value and be looked at as highly collectable...kind of like the original Eisentraut bikes when they were actually made by him.

Don't get me wrong, I can't afford that level of bike myself, so I have to settle for the likes of a Trek; and the next year I've decided on a Cinelli SuperCorsa...still not in the league of those above-but that's in my price range.

By the way this is a quote from Peter Mooney: "The price of a high quality lug set, bottom bracket and fork crown can be close to the price of the entire tube set. It's no wonder so many frame builders have chosen welding as a replacement. Lugs reinforce high wear areas on a frame. They provide an extra thickness where headset cups and seatpost clamps can distort thin walled tubes or require the use of thicker, heavier tubing. Lugs can also facilitate easier tube replacement and repairs, should they be necessary. And practicality aside, lugs unquestionably add much to the beauty of your bicycle. All in all, I continue using steel for its smooth road handling personality, and lugs for their aesthetic charm. The finished result is one I take great pride in and trust you will too."

[bcm119]

Froze- in regard to the Co-Motion comment, there is also alot of regional bias at work here, and having recently moved to Oregon I can tell you that Co-Motion is respected very, very highly in these parts... but perhaps not as well known in other areas. Reputations are affected by distribution patterns, which are unrelated to quality. I haven't ridden a Co Motion and can't comment on how they compare, but I'm pretty sure that in 30 years Oregonians will not have forgotten about them, at least not the ones I've met.

[zap]

bcm-Co-Motions are very well represented here in the Maryland burbs of Washington, D.C.

Ours is a 12 year old fillet brazed double espresso. (No rust :banana: ) Folks that had Santana's, trek, cannondale's have made the switch to CoMo.

CoMo will be remember 50 years from now.....