McBTC

Comparing the modulus of elasticity, alloy is springier than steel. With the advent of downward sloping top tubes, we see a lot of road bike frames that significantly depart for the lines of vintage steel road bikes. My guess is, a modern road bike with a lot of exposed aluminum setback seat post probably provides as much road compliance as the traditional steel road bike with a short alloy or even CF seat post.

TimothyH

Aluminum schmaluminum.

The material is only part of it. An aluminum bike can be made as stiff or compliant as the designer wants.

I have a custom True Temper S3 steel frame which is as stiff as any bike. I also have a Bianchi Pista steel bike which is like riding a giant leaf spring. Both are steel and they have vastly different ride qualities.


-Tim-

DrIsotope

No matter how well made the frame, aluminum has an indistinct fatigue limit. Used long enough, it will fail. Inevitable. Steel and titanium on the other hand have an endurance limit-- if they are exposed to cyclic stresses below the fatigue limit, they will last indefinitely.

So while framebuilding methods can make an aluminum frame as stiff or compliant as you'd like, at some point, it's going to fail.

TimothyH

Fail point, scmail point.

The fail point of aluminum is definitely past 16 years or 65,000 miles.

DrIsotope

I snapped a chainstay at ~19,000 miles. The failure point of an aluminum frame is indistinct. Might last 20 years or more. Might die in two.

Campag4life

You are nibbling around the edges a bit and so a pop quiz you may appreciate. If you have a natively more flexible material like Al compared to steel, why isn't an Al bike considered 'generally' more comfortable than a steel bike because it would flex more and more flex is generally more forgiving. Why is the opposite perception true?
I would like to hear your thoughts and then will provide the reason. It isn't directly related to amount of seat post exposed.

Campag4life

Just don't agree. Fatigue limit can be tested and is in the laboratory by major manufacturers. I personally have been riding Al bikes a long time and have never seen a fatigue failure. I am sure they maybe out there but so incredibly rare. Force/deflection X's no. of cycles to fatigue. Any frame can be tested in the laboratory at higher forces and larger frame bending amplitudes than a frame would see in the field to failure. Likely some Al frames stroking in the lab throughout the world as I type this.

DrIsotope

You can argue whatever you want, you'll still be wrong. The failure point of aluminum is indistinct. That's a basic fact. Out in my workshop, I have an aluminum frame with a snapped chainstay. It lasted 19,143.8 miles.

Nowhere in this thread have I stated anything resembling an opinion, nothing at all subjective. The failure point of aluminum is indistinct. My chainstay snapped at the above mentioned mileage. Facts.

StephenH

The modulus of elasticity is the stiffness per unit area. However, an aluminum frame inevitably requires more cross-sectional area in the members, so the stiffness of actual members may be greater than steel. Making members larger diameter makes them stiffer for bending and torsion, which may nor may not be a good thing.

79pmooney

As you say, fatigue limit can be tested for but with aluminum, that limit will not be found. So aluminum frames are overbuilt to ensure that the stresses stay so low that the bike will hopefully outlast the original owner. (Klein started the overbuilt model, closely followed by Cannondale. With careful engineering, it has been possible to slim things down a little. The ever increasing desire for super stiff makes the engineers' jobs a lot easier. None of this changes the fact that all aluminum bikes will fail, often suddenly, if ridden enough. There are a lot of aluminum bikes that haven't failed yet. By contrast, a non-high end steel or a ti bike constructed carefully without flaws will last until a big stresser (crash or the like) happens. My Peter Mooney has seen just short of 50,000 miles, Wouldn't surprise me to see it go that far again except it won't be me riding it. (It has not been babied. It has crashed hard several times. On its second fork. Now a modern, bigger tubed and stiffer/lighter modern steel bike would be pretty tired after the same abuse.)

Ben

Darth Lefty 

Racing Dan

Sure, but for whatever reason, steel frames tend to go soft with use.

Racing Dan

Sure, but they build passenger jets from "aluminium".

79pmooney

Light steel frames where the stresses are above the fatigue limit are subject to minute cracks from those stresses and yes, may start feeling "soft" as the integrity is compromised. My (probably 531) Mooney is not that light and is fully as stiff as new 40 years ago.

Ben

Brofessor

Whichever comes first.

79pmooney

Those planes are on schedules so that aluminum is replaced before it has seen too many cycles. Now, some of the older planes are flown near forever by third world airlines and periodically some of them do fail.

Ben

ColonelSanders

I have broken two aluminium frames due to fatigue(on the top tube, about an inch from where the top tube meets the seat tube), neither bike lasted 12 months.

I am 400lbs though.

Racing Dan

and carbon supposedly have indefinite fatigue life, except just like steel, titanium and alloy, they too fail in real world application. All of them can be overbuild to the point of lasting forever.

mstateglfr 

you post some wonked stiff, but this is a doozer.

aluminum is also weaker. It's overbuilt and oversized to account for it being a weaker metal. This then, in general, makes it stiffer since larger tubes flex less.

but regardless of that, frame geometry, tubing shape and butting, and tire width all play significant parts in how a bike feels. Stiff or flexible. Plush or harsh. Etc etc.
seatpost has something to do with it, sure, but indont think as much as the 3 variables I listed.


regardless of even all that now- so what? If a compact aluminum frame with an absurdly long seatpost is as compliant as a traditional level top tube steel frane with traditional seat post...so what? What's the reason for posting?

mstateglfr 

no. So much no. For the sake of accuracy, read up on frames going soft so you dont feel the urge to post the claim again.

Racing Dan

Im well aware some claim it doesn't happen. Except it does. Im sure you can build a steel bike that lasts forever and Im sure some do, but definitely not all of them. In that sense they are no different than alloy or what ever else you can think of.

DOS

McBTC

True, true... e.g.,

CliffordK

A lot of bicycles are made out of aluminum. Of course some are made of steel.

And then the "boutique" bikes made of Carbon Fiber. They catch people's attention, but the work horses are still aluminum and steel.

Does Cannondale make "modern" bikes?

Then of course, back to the components. Lots of aluminum components including aluminum seatposts mentioned by the OP.

DOS