also, the BB is a metal sleeve with very tight tolerances on metal bearnings. the BB doesn't flex, nor does the BB shell, nor do the cranks or the spider. it's the rings themselves. cheap, thin metal flexes easily, and if it isn't compliant, it stays flexed (read bent).

I'm going to disagree with most of what you wrote.

The new 105 is hardly flawless. Older 5500 brakes and cranks/bottom bracket are noticeably stiffer than the newer 5800. That's my direct experience, and why I am going to upgrade the cranks to Ultegra and brakes to Dura Ace.

Also, flex in drivetrain is not limited to just chainrings. Any mechanical engineer will tell you that all metal flexes - some more and some less but it is the nature of metal to flex.


-Tim-

I had mentioned in the original post a set of XT cranks. These are the ones I was talking about.

So, applying the wisdom of the contributed & benevolent posts herein, a modern 105 or ultegra wouldn't offer up any imperical improvement in stiffness over these older XT's I presume. Perhaps a slight weight difference too.

There's a couple of ways a crankset can be stiffer, given the same quantity of the same materials, and the simplest is to have tubular (hollow) arms and bottom bracket. If you take the same cross section area of that old XT crank arm, and make it a tube, it's going to be stiffer. If you take the same BB spindle and make it a tube, it's going to be stiffer. Even a cheap crank now has a C-shaped section instead of a straight one like the old XT to add some torsional stiffness without doing any really clever manufacturing.

With better quality materials you can make the tubes ever-fatter with ever-thinner walls, getting the same performance with less and less mass. The chain rings that look like disks are hollow too.

I was lucky NAHBS was in my town this year, and Shimano (a sponsor) had a nice display of an entire alloy crankset cut in half to show the hollow arms and spindle. I can't find a photo of it, but it was neato. It wasn't obvious how they'd made it.

This article has a nice photo of a modern crank arm cut in half to show its features
https://www.bikerumor.com/2016/03/24/sram-eagle-12-speed-mountain-bike-groups-let-you-soar-far-and-wide-complete-tech-overview/


Here's a Hollowglide chain ring cut away
https://www.spoke.ie/2009/04/new-shim...egra-groupset/

On a tube, the farther the material is from the center, the more it contributes to stiffness. So, it's possible to hollow out quite a bit of mass from the center and save quite a lot of weight, while losing relatively little stiffness. However, you still lose a little stiffness; it's not like the air in the tube is helping hold the aluminum around it in place!

When tubular cranks are stiff, it's because the arms are wide; we're getting to where some of the premium cranksets look absolutely silly to a lot of people.

You see the same thing in frame design. Increasing the thickness of the tube's walls increases the bike's stiffness, it doesn't decrease it. However, it's a fairly small effect compared with increasing the tube's diameter. Hence, steel frames with skinny tubing are often pretty flexy, even when they weigh more than aluminum frames, even though steel is similarly stiff-per-weight as aluminum.

The difference is often hard for most people to notice, other than the weight. Most riders are not going to be able to pedal hard enough cause a lot of flex, and even if they do pedal that hard, they will also notice issues like frame flex and other issues.

Last year I upgraded the old square taper crank on my Birdy to a new Dura Ace unit. The Dura Ace crankset weighed half as much, was far easier to install, and spins much more freely. As for issues like flex, there was no discernible difference. The old unit was heavy and strong, the new one was light and strong.