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Long time ago, bicycles had threaded 1 inch headsets and fork steerer tubes were always made of steel. Then, approximately the same time, 1+1/8 inch headsets, threadless headsets and steerer tubes made of materials other than steel began to be widely used.

The 1 inch headset always was bit of marginal, especially if used with steerer tubes made of other materials than steel, so the move to 1+1/8 inch headsets is understandable. Tubes with larger diameter are stronger.

However, recently there has been a trend to make the steerer tube tapered. So it could be the case that the lower headset bearing is 1.5 inch whereas only the upper headset bearing is the standard 1+1/8 inch size. Then standard stems can be used but at the same time the lower headset bearing is much bigger.

What are the benefits of this strange setup where the lower headset bearing is larger and the steerer tube is tapered?

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There is more load on the lower bearing because it bears all the weight and receives all vertical impacts. Making the lower part of the head tube bigger also allows for bigger down tubes.

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The shift to larger bearings may in part be marketing hype. The manufacturer can say "look how big ours is, that make it so much better". Another aspect may be as a result of warranty claims or law suits. If they see a lot of cracked head or steerer tubes a design change will be required to protect the brand image. The larger tubes are stronger. The larger tube requires a larger bearing. The majority of the load is on the lower bearing as it is supporting the riders weight. The same principal applies to the axles. Larger bearings can support more weight. Hence the proliferation of thru axles. Most riders don't realize there are only a few brands that manufacture there own frames. Most frames come from just a few factories. From the standardization aspect it makes sense that most of the headtubes be the same size. The final reason I can think of is the way and where people ride. Riders are going places and doing things that would have been unheard of just a few years ago. We are at the point that the limiting factor for most of us isn't the bike but the riders ability.

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  • In theory, larger-diameter wheel axles could use a QR type mechanism rather than thru-axle. I believe a major reason for moving to through axles is that it was discovered around 2005ish that front disk brake forces would over time unscrew a quick release endcap, leading to separation of the wheel from the fork. A thru axle wheel can't fall off.
    – Armand
    Feb 8, 2021 at 22:57
  • "Most frames come from just a few factories" I have heard this before but think it is apocryphal. Anyone have any cites for this?
    – Armand
    Feb 8, 2021 at 22:59
  • @Armand You mean like a thru bolt system? QR is slightly inferior to TA because the retaining force is friction between locknut and dropout instead of shear like with a TA, so I prefer we don't revert to that system. The QR disc brake problem has mostly been fixed now with stiffer forks.
    – MaplePanda
    Feb 9, 2021 at 0:11
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    @MaplePanda I was interpreting "few" as meaning "2 or 3". I could believe that there are 5 or so main companies, but would expect the number of factories total to be more like 10-20 or more.
    – Armand
    Feb 9, 2021 at 5:10
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    @MaplePanda Yes, QR is inferior to thru-axle. mikes suggested that thru-axles were prompted by larger diameter wheel axles and my point was that a QR-type system would also work for larger diameter wheel axles, so it was something else rather than diameter increase that prompted the switch to thru-axle from QR.
    – Armand
    Feb 9, 2021 at 5:17
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One noticeable difference with a tapered headset is that the lower bearing can be moved inside the frame, creating a cleaner look. With a standard 1-1/8 headset, road bikes could easily get lower bearing inside the headtube and I would think that mountain bike producers also thought this was a good idea and therefore had to come up with a solution to get larger bearings into the head tube and a tapered headset was really the only solution that made sense.

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    Semi- and fully- integrated headsets had already brought the bearings inside the head tube with 1 1/8 straight steerers.
    – Armand
    Feb 8, 2021 at 22:49

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