Aluminum vs. Steel: the truth about stiffness


Apr 9, 2001


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The Bottom Line Flex is good

You have all heard the hype about aluminum, how it is ligher faster and better than steel. Accept this for what it is hype. Aluminum bikes are only marginally lighter than steel bikes. The lightest production aluminum frame I know of is the Cu92 Giant XTC frame at 3lbs and only a third of a pound lighter than a 3.3lbs Ritchey P-21. Therefore weight should not be a big concern.

That leaves stiffness. Aluminum frames are supposed to be faster because they are stiff, because they do not flex, so they are therefore more efficient. This rides on an assumption that flex is inherantly inefficient which is not totally correct. Think back to high school physics, you probably did an experiment with a spring and a weight, you put a small amount of energy into the system and it would go for minutes. It was a really flexy and really efficient system. The only reason it would come to a stop is due to material damping, that is throught the energy moving through the material some of it went out as heat. Ineffecieny in flex is due to the materials damping characteristics.

So how does this translate to bike frames. Materials like steel and titanium have very low material damping characteristics, meaning that relatively littel energy is lost to heat. That makes sense becuse we make almost all of our springs out of steel or titanium. When you are building up a frame out of these materials you are actually building a spring. On the other hand when one builds a frame out of aluminum they are making the stiffest structure possible, imagine an I Beam. So when you think of flex you shoudn't think of effiency but rather of the ride of a spring vs. an I beam.

Imagine a trapoline next to a wood table. Put one person on the trapoline and ask them to jump, do the same with the person on the table. The person on the table will spend the same huge amount of energy everytime trying to jump higher whereas the person on the trapoline would jump much much higher whith the same or less energy expenditure. That is because the person on the trapoline has a temporaily store energy and then use it later when it is more needed. The person on the table can't do this.

Now forget about the trapoline and get on your bike on some nice twisty singletrack or a sweet road decent. You lean your bike into a corner, carving a nice right hand curve. Now up ahead you see a tight left hand corner coming up. On the aluminum bike you would need to physically lift the front ent up and over to get it to go the other way, however on the steel bike one simply needs to release the spring to have that energy released, boinging the rider into the left hand carve. A faster and less tiring act than the rider on the aluminum frame. Ride a couple of bikes side by side you'll see that it is immensely more difficult to hande the really stiff frame.

This concept reiminds me of a saying in the Italian motorcyle industry "If something is too perfect you tire of it quickly" That is often true of aluminum frames, they are too perfect. In order to make the bike go as fast as it is supposed to go the rider must apply energy at the perfect times in the perfect direction. pull on the handlebars a certain way while cranking with a certain amount of power, and having your butt on a certain perfect point on the seat. This is really difficult to do.

The Moral of the Story FLEX IS GOOD. You wouldn't buy a ski or snowboard or golf club or hockey stick that didn't flex why a bike frame? I still accept that some people really like the ride of aluminum but most of us are not given the choice. The most popular offroad pricepoints between $850 to $2000 have no steel or ti bike. CALL THE MANUFACTURER AND TELL THEM YOU WANT A CHOICE, THAT YOU WANT STEEL.

******RESPONSE TO GIOVAGNS "FRAME MATERIALS"*******

Giovagn expressed a rather elloquent and well written rebute, however I will maintain my original stance that the overall "real" speed in almost all riding conditions of steel and titainium is near equal to that of aluminum and that the advantage of aluminum is primarialy psychological in that it has a greater percieved speed.

There is one fundemental flaw in Giovagns argument and that is the idea that flex is primarily a function of the vertical plane, sort of like a full suspension bike. While this flex does occur it is really marginal, second off if you were to lose a race because of too much flex on the vertical plane you would have lost the same race on the basis of bad technique. If the net force of your two legs is primarily in the downward direction you are only using one leg at a time, and we all know this is a bad idea.

To clarify things one must understand that the flex that I talk about is the twisting that occurs between the handlebars and the bottom bracket, not vertical flex. When one views flex in this way the rebound of the frame is concurrent with the efforts of the rider. The onlw real difference then is the "pop" of the frame, that energy transmitted in that "split second" in a sprint. I will say that the amount of difference in "pop" between steel and aluminum is not a major determining factor in the outcome of a sprint, in which the major factors are raw power over time, timing and position.

What the difference between the materials would be psychological, do you like the flavor of steel or aluminum, pretty simple.

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