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I read this question posted by someone else (details below) but without any response:

One thing I've noticed about newer bikes vs bikes like this [referring to a Specialized Hardrock aluminum frame MTB] from around 20 years ago is that the rear triangle of the older bikes are typically a lot beefier. on some of the newer bikes the rear looks paper thin, particularly the seat stays. What is your opinion on that trend, are the beefier rear triangles from yesteryear better or the newer, more forgiving modern frames that look like they have a lot more flex to them?

(From a comment to video

by 'madcapper6' -- though the comment was tangential to the main subject).

Firstly, is the core observation about rear triangles getting smaller accurate? Assuming its true, what's the reason for it?

This is somewhat of an opposite observation to another question (Why are bike frames getting so much bigger) which was about the embiggening of frame & some components.

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There is some validity to the observation.

In aluminum, hydroforming on mainstream bikes only started in the early to mid aughts and got more prevalent and sophisticated from there, eventually to the point where even fairly low end bikes often have heavily manipulated tube shapes that would have been much more expensive to produce, if feasible at all, in the past. Hydroforming is often used now to create thin profiles on chain and seat stays that in the past on a comparable price point bike would have been a chunkier, continuous diameter tube.

In carbon, many companies have veered towards dainty, compliant rear end sections. There was less of this in the earlier days of carbon.

In steel however, there's been some of the opposite effect. Steel stays on many contemporary bikes are much larger diameter than comparable older bikes. In some cases this is because fancier modern alloys beget larger tubes with thinner walls. More commonly though, it's a mix of consumers being visually accustomed to bigger tubes and also the structural requirements of disc brakes.

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First, let me stipulate that the real measure of "beefiness" would be tube-wall thickness, and that's something we can't determine just by looking at the outside. I am not aware that tube walls have gotten thinner.

Second, in terms of tube diameter, I think we may see more variation than we did 20 or 30 years ago. The individual tubes that make up bikes (assuming they are not monocoque carbon fiber or something more exotic) are shaped in ways that were very uncommon before. Some manufacturers are using slender seatstay tubes to make the ride a little more compliant. But you could find examples of bikes from the 80s with wispy seatstays. I don't this applies to chainstays.

Third, a lot of road bikes these days have dropped seatstays. Part of this is for aerodynamics, part is for increased compliance. This is a relatively new trend over the past ~5 years.

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  • Could you clarify the comment about tube-wall thickness being the measure of “beefiness”? Stiffness of the tube should be linear in tube-wall thickness, but quadratic in diameter. – RLH Jul 8 at 3:29
  • @RLH I'm assuming "beefiness" would be the layman's definition of strength, which is simply how solid and substantial a part is. – MaplePanda Jul 8 at 5:36
  • By "dropped" do you mean that they are lower to the ground? Not that they are eliminated entirely? – UuDdLrLrSs Jul 8 at 11:39
  • By "dropped" I mean the seatstay joins the seat tube at its midpoint, rather than at the top-tube joint. – Adam Rice Jul 8 at 13:17
  • @MaplePanda Yes, but which strength or substantialness? For bending, diameter contributes more to strength than wall thickness. For visual appearance, diameter is the only thing that contributes. For tension, diameter and wall thickness contribute equally. Wall thickness helps protect against crushing, but without further clarification, that wouldn’t seem to be the quality most in question. – RLH Jul 8 at 14:18

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