What are the pros and cons of bigger or thicker or smaller diameters in handlebars and matching diameter stems?
Thanks.
Perry

As with everything bicycle, marketing.

Pros: They all work.

Pros of 31.8, it's the current industry standard, fitted to just about all bikes, road, MTB (some 35mm here as well), CX Hybrid etc - easy to get hold of, and at all price points, a lot of specs only available in larger diameter - i.e wide MTB bars, carbon bars.

Cons of 25.4 and especially 26, their effectively dead standards, only found on entry level bikes (and some retro/niche) - harder to find spares/replacements, your buying into obsolescence if your buying a new setup

As AnkleWorks says, they all work.

you can shim a 1"bar, to fit in a 1.25" stem.. and 26>31.8.. [but not the inverse] (1.25"=31.75mm..)

I have Trekking and flat bars that are 1", center, 7/8" MTB controls fit.. [22.2mm]


now if you are leaving out you want aero-carbonfiber drop bars,
and get in a lather over the latest race bike gear, the Pro's ride, that is a different set of needs..

C&V Collectible Italian stuff , uses 26, older Cinelli 26.4mm




...

LOL @ "EVERYTHING NEW IS MARKETING HYPE!!!1!!!!1!!1!"

Pros - 31.8 has removable face plates on the stem making handlebar and stem swaps/adjustments easier (you don't have to unwrap the bar tape and remove the shifters every time you want to change stems.)

Also 31.8 is arguably less flexy/more sturdy although I'm sure someone is going to respond by saying they dropped their quill stem off the empire state building and it survived but a threaded stem broke once and killed their family.

31.8 Pros... none
31.8 Cons... likes like shiit.
25.4 and 26 Pros.... there is enough space to mount headlight or electronic device without the handlebar having a hideous bump in it.
Cons...... none

All other properties being equal, larger diameter tubes are stiffer, which would be a pro.

31.8 mm is the current standard for road bars, or at least it has been for the last two decades or so. The adoption of 31.8 mm handlebars coincides with threadless stems.

25.0 (French), 25.4, 26.0, and 26.4 (Cinelli) are past standards, typically used with quill stems.

Since 31.8 mm bars and threadless came into general use at the same time, and threadless stems typically have removable faceplates, it is generally true that stems for 31.8 mm bars have removable faceplates.

BUT... Removable faceplates aren't unique to 31.8 mm handlebars. There are/were stems for 25.x and 26.x mm bars that also had removable faceplates. Both quill and threadless.

Standards change. The thing I like best about 31.8 is that finally road and mountain/hybrid stem sizes have found a matching standard, until the next one comes along. What this means is that pretty much any stem will work with any bar drop or flat, for now. I could take the stem off my mountain bike and use it with the bars on my road bike if I wanted to. Also, it has been said by manufacturers that 31.8 bars are stronger, yet lighter. In the past, when I was young, soon after the earth's crust had cooled, it was recommended that handlebars with those wimpy 25.4, 26, and those oddball 26.4 diameters be replaced on a regular basis due to fatigue caused by those "tiny" bars. I have actually broken 25.4 bars more than once, so it must be a thing, nothing to do with the fact that I bought the least expensive bars I could find. These days I ride in the full knowledge that my 31.8 bars on my road and mountain bikes offer me complete security

31.8 pros: it's not a dead standard.

Sure, it would be a pro if stiffer always = better. Sometimes stiffer just = stiffer.

Nah. Threadless stems were around for at least ten years before 31.8mm became the "standard." It started with downhill mountain bikes, probably using motocross handlebars, hence the non-metric 1¼" size. No idea why it became the road standard. There's really no need for that much stiffness in a road handlebar. I'd say it coincides with the widespread use of CFRP for handlebars.

Plus, the aesthetics of the bar/stem combination are improved if both are about the same size tubing. Oversize-tube frames and forks with oversize steerers and aluminum and CFRP stems and bars kind of need each other.

Yes, because that's pretty much all there were. A few builders made "threadless" stems that fit onto an extension brazed into the steerer tube, but the headsets were still threaded.

Truth.

neither are the others, kid.. you are just letting marketing affecting your perceptions.


not every one lusts after carbon race bikes.. though disproportionately here..


Lots of the standards for tube in bikes, is fractional , though using metric equivalents..






.....

I have a steel frame and fork and 4cm of spacers under my upright stem. Carbon racing bikes are the last things I care about.

The selection for 25.4-26mm compact drop handlebars tho... Almost none existent. So it's not marketing affecting my perceptions, I just don't want to limit my options since the main thing I care about is being comfortable on my bike and the handlebar plays a big part in that.

I got some 30 years back,1" Asian Randonneur bend , but yea drop bars have gone to 26, in the better brands.

Nitto offers their Mustache bar 25.4 center, but 23.8 tube for road levers.. bar end shifters

Compacts became a follow on from Brifters , they had to be longer to get all that shifter stuff in them.



Bickering over minutiae, its what gets done here.. you should be working now... , right?





..

Sorry but I find vintage road bars simply unbearable. Those tiny brake levers compared with the deep bends are just so unbelievably uncomfortable for me. I also really like a nice fat top. So if I had to build a comfortable drop bar bike using the 25.4-26mm standard I simply couldn't do it.

Other than liking the look of quill stems, I don't see a reason why anyone would go with the old standard. If you're building a modern bike it's a no brainer to go 31.8mm, the selection is so much bigger, because like it or not that has been the new standard for a few years now, and not just on carbon race bikes... Accept it or not, 25.4mm and 26mm is pretty much dead. Only the niche vintage and some low end stuff is what keeps it sort of alive. Even discussing the pros and cons is sort of pointless...

It's a national holiday here so that's a no.

If you find it pointless, don't do it.

There really is a point for some of us though, and that is to try to tease out possible reasons why things change. Pros and cons is what it's all about. It's easy to toss out pat answers like "stiffer=better" and "marketing" and "corporate conspiracy" or what have you. But IMO simplicity and reality have an inversely proportional relationship.

There are lots of reasons for why standards or dimensions come and go, and they all have to do with market forces. Which are not the same as marketing. Sometimes new technology actually does provide an unqualified improvement over old. On bikes, however, this is rarely the case, because of the level of refinement. Genuine improvements are marginally smaller and smaller, and cost marginally more and more. Maybe your new bike is "better" than a bike from ten years ago, but also costs twice as much, for a maybe 1% "improvement," depending on how you want to quantify that.

I prefer to consider it merely a desire for change that drives the market, not improvement. Newer, stiffer, lighter, more speeds, etc. are what people want. This is, of course, marketed as improvement, but it does not come easily without costs and compromises. Paying attention to those costs and compromises is the entire point.

In theory, the 31.8 bars should be slightly stiffer than the 25.4 and 26.0 bars. It probably doesn't make any difference for most riding, and in some cases, flex is deliberately put into the bars for comfort (Specialized Zertz).

I think there are a few issues.

More 1" stems (both quill and threadless) come in 25.4 and 26.0 sizes than in 31.8 sizes. This means that the smaller bar sizes might be better for working on vintage bikes. However, there are ways to get around this limitation.

Almost all 1 1/8" threadless stems come in 31.8, so working on newer bikes, one should stick with 31.8. Also, the few rare 26.0 stems for 1 1/8" threadless often have 2-bolt faceplates, when a 4 bolt faceplate is likely a better choice.

I have come to like the flat-top "aero" bars. Unfortunately there are very few 26.0 flat top bars available. And they still typically require an open face stem which is rare for quill stems.

I'm not a big fan of "ergo/anatomic" curve bars, but I do like a tight top curve to the bars which gives better brake lever placement. Thus, either "ergo" or "compact". There are a couple available in 25.4 or 26.0, but most of the more modern shapes are coming out in 31.8. Also, if buying used parts, the 31.8 are much more common.

Also, almost all new carbon fiber bars are being made in 31.8, with a few vintage TT bars still showing up.

Anyway, the choice really depends on what you're mounting it on, but if I was building a "modern" bike, I'd stick with 1 1/8" stem and 31.8 bars.

Bmx and motorcycle still use 22.2mm. Lol to stiffness/strength.

And generally, they are both made from steel, very few bike handlebars are made from steel now. different applications, different materials

Motorcycle Moto-cross and road sport/superbike handlebars have been aluminum for decades.

Oddly, I was looking at track quills recently and discovered quite a few made in steel.

Apparently a few track bars.

Nitto B123 CrMo Track Handlebar Keirin in Tree Fort Bikes Drop Handlebars

It looks like Soma also has steel bars.

Soma Fabrications Walker Racer Steel Track Bar in Tree Fort Bikes Drop Handlebars

But, you do have a point. The cheap BMX bars are quite different than the high-end road bars.

Nitto makes lots of 1 1/8" threadless stems with removable faceplates for smaller diameter bars.


-Tim-

Road bikes went to 31.8 to allow easier clamping on more delicate carbon bars.

People buy carbon bars sometimes because they think they will damp more vibration.

25.4/26.0 aluminum bars probably damp more vibration that 31.8 carbon bars.

Any diameter stem can have removable face plates. I have a box full of 25.4mm stems and a few 26mm with removable face plates.

I am fine with 31.8 if it means better cross-compatablity between road and mtb bars.

I find 31.8 on drop bars to be of no real benefit for me, and find the extra bulge makes it a little harder to utilized some of the bar around the stem.

However, on MTBs I think they make a lot of sense, in light of bars being up to 800mm wide (mine are 750mm) and the amount I lean into and pull on them.

So while would slightly prefer a narrower clamp size for road bikes and drop bars, I am willing to accept the 31.8 standard if it means that stems are interchangable between mtb and road.