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This is just a general interest kind of question, but I've really been wondering about this lately. We've had a lot of advancements in bikes over the years, and people are always trying to make them lighter, yet for some reason it seems that the frames keep on getting bigger. Just comparing my old steel road bike to my new aluminum touring bike, here's a quick list of things that are bigger.

  • Wider diameter handle bars
  • 1 1/8 inch steerer vs. a 1 inch steerer tube.
  • Wider seat and chain stays.
  • All main triangle tubes, but the biggest culprit is the down tube.
  • Fork is a little beefier.
  • Stem is almost twice as thick.

After you add all this up, I have an aluminum bike that isn't any lighter than the old steel bike I have. Why has virtually ever part of the frame been made so much larger?

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Because "bigger is better", to the marketing types at least. Why do humongous pickup trucks continue to sell? Not because every Tom, Dick, and Sam needs to haul stuff all the time, and not because they're fuel efficient. –  Daniel R Hicks Sep 29 '12 at 1:22
And, of course, by changing all the dimensions they make repair parts for older bikes unobtainable (or at least too expensive), forcing you to buy a new bike. –  Daniel R Hicks Sep 29 '12 at 1:23
@DanielRHicks Yeah, but for road bikes, light and sleek seems to be what everyone is after. I could see the justification for mountain bikes from department stores, but for quality bike shop bikes, it seems a little odd. I was just out in the garage and decided to test the weight. the newer bike is a bit lighter, even with the much beefier frame, but the question still stands, I know some parts have to be larger because of the different materials, but even my cromoly fork on my new bike is quite a bit larger than the old one. –  Kibbee Sep 29 '12 at 2:20
You are comparing a touring bike to a road bike and wonder that it's beefier? –  Baarn Sep 29 '12 at 10:24
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2 Answers

up vote 13 down vote accepted

From metallurgy for cyclists:

a tube's diameter increases (D), the stiffness increases to the third power of that number (d is the inside diameter). Comparing a one-inch tube and a two-inch tube of equal wall thickness., the fatty is going to be eight times as stiff as the little weenie tube. And the weight will only double. Now does the ride of those Kleins and Cannondales start to make sense?

So wider diameter tubes actually allow for less material to be used for the same amount of stiffness, making for a lighter bike with a similar ride.

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Great answer. Stiffness is important so that you don't waste energy. Also, it shows that the walls can even be less than equal thickness on the bike with wider tubes, and still be more stiff. Although I guess there's a limit to how thin you can make the walls while still maintaining enough strength. –  Kibbee Sep 29 '12 at 16:39
Also, it's a good way to offset the fact that metals such as aluminium aren't as strong as steel. An oversized ali tube still weighs less than a standard gauge steel tube for same stiffness. –  GordonM Sep 29 '12 at 19:17
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It's because of the differing characteristics of steel and aluminum.

With either material, you can apply stress to them, they will bend, and then when you remove the stress, they will bend back. However, if you keep doing that, they behave very differently.

For steel (and titanium), if you keep the stress below a given level (known as the fatigue limit), you can essentially bend them and they will come back for an infinite number of cycles. Aluminum, on the other hand, does not have this behavior; each time you bend it, you fatigue the metal a bit, and over time, this accumulates.

So, for bike frames, you can build a frame using steel tubing that will flex a little during use, and as long as it doesn't flex too much, it will last a long, long time. Try to do this with an aluminum frame, you will get fatigue, and it will break. To try to get around this, aluminum frames are made with larger tubing, which is more rigid and bends less.

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