How Many Standards Do We Need for Bicycle Rear Axle Size?


From as far back as I can remember until the mid-1980’s, road bicycles – commonly called “ten-speeds” – were built to a de facto standard. They all looked pretty much alike. Granted there were cheap department store versions and expensive hand-built models with Italian names like Colnago and Bottecchia. But critical interface dimensions were common among bikes back then, with the exception of a few French and Italian thread selections.

Since the mid-80s cycling has existed in a state of continual innovation. In fact I’ll send half a dozen Killa’s Garage stickers to the first reader to email me the name of the bicycle manufacturer that uses the phrase “Innovate or Die” as their corporate mantra. Granted, innovation can be a really good thing. It’s the business equivalent of evolution through natural selection. But natural selection is painful. Things go extinct. I have a garage full of bicycle fossils to prove it.

I could rant about the rise of multiple bottom bracket and headset sizes in the past ten (twenty?) years, but let’s stay focused on a more recent phenomenon, the explosion of rear wheel axle options.

For eons the distance between rear dropouts (where the rear wheel fits) was 120mm. Freewheels had five cogs. Then one day somebody decided that we needed six cogs on our freewheels. Six cogs wouldn’t fit where five did. So a new standard distance between rear dropouts was introduced – 126mm.

Just like that, your 120mm bike frame and rear hub became extinct.

Somebody decided to cram seven cogs in the same 126mm space and it worked, but when they tried to fit eight, they said, we need a new wider standard – and 130mm dropout spacing was born.

Boom, your 126mm frame and rear hub went the way of the dodo!

Oh, and about that same time they switched from freewheels to cassettes/freehubs. A cassette/freehub combination does the same thing as a freewheel. It’s just a different design, and it’s not interchangeable with a freewheel.

Nine, ten and eleven cog cassettes followed, and thankfully they all fit in the same 130mm spacing, even on the same freehub (sometimes). Twelve cog cassettes are coming, and apparently they fit in the same 130mm spacing. Phwew, evolutionary convergence!

On a separate but related evolutionary path, mountain bike builders set their own standard of 135mm. The extra width was said to allow wider spoke flange spacing for greater lateral stiffness. Or maybe it was just to ensure that you couldn’t use your road hub on a mountain bike. Whatever.

So far this was evolution plodding along at a reasonable pace, 3 to 4 standards over something like 30 years. The Cambrian Explosion of rear axle evolution began about five years ago with the introduction of thru-bolt axles and the evolution shows no sign of abating.

Manufacturers began marketing disc brakes for first mountain bikes, then road bicycles, which led to the use of thru-bolt axles for reasons beyond the scope of this discussion, making obsolete the quick-release system we have used since its invention in 1930 by Tullio Campagnolo, god rest his soul.

A thru-bolt axle requires a thru-bolt compatible frame, so your bicycle frame just went extinct again. Daaah!

Thru-bolt axles were a primordial soup, so manufacturers rushed to evolve their preferred design standard. As a result, at least three different “standard” thru-bolt dropout spacings and three axle diameter choices were developed in a period of less than five years.

Rear Dropout Spacings (mm):

Quick release – 120, 126, 130, 135 (MTB), and 140 thru 160 (Tandem), all being driven to extinction by thru-bolt axle development.

 Thru-Bolt – 142, 142+*, and 148 aka Boost

Thru-Bolt Axle Diameters (mm): 12, 15, and 20

*The difference between 142 and 142+ seems to be a 2mm lateral shift in position of the right spoke flange and the cassette.

And that’s just the rear axle. One bicycle manufacturer recently decided that the front dropout spacing of 100mm that we’ve used just about forever needs to be increased to 110mm. Daaaaah!

Look out, here comes an asteroid!!!!

For a more serious discussion of bicycle rear hubs, complete with pictures that have arrows pointing to cool things, visit In fact, if it has anything to do with a bike, BikeGremlin has probably written about it.

3 thoughts on “How Many Standards Do We Need for Bicycle Rear Axle Size?”

  1. The fat bike (snow bike) I purchased while living in Alaska had a 197-mm rear spacing and 150-mm front spacing. These were to accommodate the 100-mm wide rims wearing 5” wide tires.

  2. “Innovate or die”

    Also, more cassette cogs aren’t the only thing motivating wider rear spacing for MTBs these days. Frames are now designed to accept wider tires, so certain drive train components need to be bumped out to prevent chain/tire interference.

    1. Hi Darin, You’re a little late with the correct answer of “Specialized”, but I’ll send you some stickers anyway. Good point about moving the whole drive train out to clear wider tires. I have wondered about an off center rear end. Does the left dropout have to be as far to the left as the right dropout has to be to the right? OK, maybe yes with disc brakes.

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