cango

See the graphic - is there a reason that they are different? Or is it just because?

In the grip part road is 23.8 and mountain is 22.2.

Makes it difficult to move parts around from mountain to road and vice versa.

BNSF

You're probably heard of "Planned Obsolescence"? This is "Planned Incompatibility". They don't want you to swap parts around and from brand to brand.

AlgarveCycling

Road bars receive tape whereas MTB, BMX etc receive thicker grips. The resulting thickness is generally similar.

Different folks like different levels of tape thickness too on road bars but the base diameter needs to be as it is to accommodate 1.8mm tape and still be a decent diameter for grip, hence greater diameter. Some will use thicker tape for more comfort if they want.

Ironfish653

“they, They, THEY!!!!” Omg, guys, loosen up the tinfoil. No big conspiracy.

25.4 (not 26.0, unless you’re French) was the clamp size when bikes were based around the 1” threaded headsets.

Once the 31.8mm threadless became standard, bar clamps moved to 31.8 along with it. That was like 20 years ago, so it’s not like it was something that ‘just happened.’

Grip diameter is a whole other thing. MTB straight bars are typically used with a thick rubber grip, so the bar tube is thinner, to keep from making the grip too fat.

Road bars are usually taped, which is a thinner layer than MTB grips, ergo, the bar is a bigger diameter.

Besides; the form and function of most of the controls ( brakes and shifter) don’t really work if you go from flat bar to drop bar, so WTBD?

Leinster

What the guys above said. Drop bar brake levers from the 1920s will fit on road Handlebars from the 2020s. The brake and shift levers that were used on the original mountain bikes in the 1970s will work on the same handlebars installed in the latest Santa Cruz Hightower. The bit that has changed is the clamp diameter.

cango

There is only 1.6mm difference between the road and mountain bar grip area diameter. This is dumb except to make sure you can't move parts back and forth.

HD3andMe

You're going to be REALLY upset when you learn that MTB bars have 35mm clamp areas too.

Phil_gretz

Try dealing with 50 years of quill stems and handlebars. Modern stuff (threadless stems with faceplates) are easy by comparison.

I'm working on an 1987 Schwinn Circuit. It comes with Cinelli stem and bars. So the quill diameter is standard 22.2 (not French 22.0), but I'd prefer ~1cm shorter reach. The current stem: 115cm quill type with 26.4mm clamp. Yup, Cinelli's 26.4 because, why?

So I'll spend nearly $60 for a particular Cinelli quill stem.... and so it goes. And I'll have to remove the bars and unwrap one side, because quill...

ljsense

It's a good question.

Maybe someone will come along with better history on this than I have, but I think you should look at the early days of mountain biking and those clunkers of Marin County and so on.

Remember their big gnarly brake levers? Huge alloy jobs that held all your fingers with a curlique knob to spare? I'm guessing those were borrowed from motorcycles, which have a common 7/8" bar size. Of course, 7/8" is 22.2 mm.

So if you were Gary Fisher or whoever, and you wanted to start making mountain bikes, having a handlebar that fit in the quill stems available that could take motorcyle style brake levers, you ended up with a 25.4 mm to 22.2 mm handlebar. Tooling up, or contracting out, to do that would be a lot easier than launching a new component, like a new brake lever line.

I'd be happy to read a better history, as this is just a semi-informed guess.

Leinster

You know what else stops you moving parts back and forth? The shape of brake and shift levers. They’re not designed to go on both. But by all means go ahead and install a pair of 105 levers on your flat bars, and do let us all know how great they would be if not for the bar diameter.

Also, this is why lights, computer mounts, accessories etc all come with extra spacers; so you can install them on mountain or road bikes.

JohnDThompson 

You know what they say about "standards:" the best thing about them is that you have so many to choose from!

cyccommute 

What parts do you think can be moved between road and mountain bikes? Mountain brake levers and shift levers won’t work on the curved bar of a road bike. You can use road levers on flat bars if you wan’t but they aren’t optimum. Outside of not fitting well on different bars, modern mountain and road shift and brake levers aren’t compatible with the drivetrain and brake components.

desconhecido 

Right.

Quill stem diameters that I've seen: 26.4 mm (old Cinelli), 26.0 mm, 25.8 mm (old 3TTT), 25.4 mm, 25.0 mm (old French) 22.0 mm and 24.0 on old Raleighs with steel bars, e.g. Sports.

HD3andMe

Modern mountain and road shift and brake levers are compatible with the drivetrain and brake components.

https://blog.3t.bike/2019/09/12028/g...s-drivetrains/

You can also mix and match some di2 components.

https://www.cxmagazine.com/mechanica...train-shifters

Gresp15C

The best rule is: Know the dimensions of the parts on your bike, and take a caliper with you when shopping for parts. And don't forget pull ratios for brake and shift systems.

The different diameter standards predate the mountain bike.

Trakhak

Thank goodness all 26" tires are interchangeable.

cyccommute 

You have a point on SRAM but SRAM has a fairly small market share. Shimano parts aren’t compatible across disciplines and their market share is far larger.

As for Di2, how many people are going to have those?

And, finally, there is also the issue of mountain shift and brake levers won’t work on curved bars.

Gresp15C

... unless you've got an old Schwinn cruiser.

dedhed

I recently found that out with 24" too

Miele Man

When they started making ALLOY dropbars they increased the clamping area size to maintain strength. I have steel dropbars that do have the same diameter clamp area as do a MTB bar.

Cheers

Paul Barnard

You know what else annoys me? They made it so that I can't move my mountain bike wheels and tires over to my road bike. There is no reason extra room can't be built into road frames.

HD3andMe

Thank you for admitting that you were wrong when you claimed "modern mountain and road shift and brake levers aren’t compatible with the drivetrain and brake components."

Please provide some data that supports your contention that SRAM's market share is "fairly small." Please be specific by market - road, gravel/cross, and MTB.

Finally, if someone wanted to run AXS mtb shifters on "curved bars" they certainly could run them on the flats. That would be pretty goofy since you can already run Blips to shift.

p.s. to answer your question, many people run Di2.

Darth Lefty 

No one tell OP about motorcycle bars

cyccommute 

I only admit to being wrong about SRAM. Shimano is not compatible.

Based on observation. I see thousands of bicycles per year at my local co-op. SRAM mountain is more prevalent than road. I can not recall the last time I saw a SRAM road system. We have boxes and boxes of Shimano shifters and derailers of both road and mountain. We have very few SRAM mountain drivetrain components in our bins (I watch for them and usually pick them up for my use). I may have seen a single SRAM road component in our parts bin 10 years of working at the co-op.

I also don’t see SRAM road components on bikes that we work on. I can tell you that most anything that comes into the shop I’m asked about and SRAM road just doesn’t come up.

That’s my point. You might be able to put mountain bike shifters on road levers but it’s not going to work all that well.

Again, based on my co-op experience, I’ve seen very few of them in a bicycle crazy town.

HD3andMe

No, you're wrong again. I even provided a link in my previous reply. Shimano road and MTB is compatible - Di2. Read, or watch the linked video here once again:

https://www.cxmagazine.com/mechanica...train-shifters

Thank you for admitting that your claim is only based on your limited experience working at a co-op. The reality is that the market is much different than you describe it.

Yeah, you're missing the point. You don't need mountain shifters to actuate mountain derailleurs with Di2 or AXS. They can work with road shifters...