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.

Bicycles and Aluminum Corrosion Again

SpaceShuttleYes, this relates to today’s Esoteric Observation on Bicycles and Cycling – Image from

I never meant for this to become Killa’s Esoteric Observations on Aluminum Corrosion blog, but here we are. In a recent post I wrote about a corroded stem face plate on a friend’s bike. Another episode of aluminum corrosion was brought to my attention recently in a pretty spectacular manner.

The rider in front of me (Let’s call him RocketMan) suddenly lost control and fell on straight level ground. He might have hit a bump or something, or not. Read on. I and another rider swerved around him but the third rider back (We’ll call him Mario) T-boned RocketMan, broke a few of RocketMan’s ribs, destroyed his (Mario’s) classic steel Masi frame, launched over the bars, and fractured a vertebra upon touchdown (but that’s another story.)

Pardon a brief diversion to the topic of bicycle wheel engineering. Mario reported to me that despite absorbing enough force to buckle his frame tubes, his front wheel remains absolutely true!


Back to aluminum corrosion. This is not an article about the crash, but rather about the cause. The primary cause of this crash was, in my opinion, a corroded handlebar.

The bicycle of RocketMan ended up in my garage because, well, I’m the guy’s mechanic. Both drops (Drops are the lower curved ends of road bicycle handlebars) of his aluminum handlebars were broken. The right drop was completely broken off, the left was still attached by about a quarter of the circumference of the aluminum tube.

Aluminum bars usually bend. They don’t usually break. And RocketMan fell on his left side, so how did he break the right drop? I only had to un-wrap his handlebar tape to see that corrosion had eaten through most of the metal just above the brake levers – exactly where sweat drips off your hands and collects under the tape.

What I think happened to cause this crash is that RocketMan hit a small bump while riding on the drops, or corrosion simply reached a critical level. I think his right drop broke off first, causing him to lose control and fall. The left drop, I think, broke on impact with the ground.

The evidence suggests crevice corrosion just like the stem face plate I wrote about previously. Here the interface between tape and handlebar creates the crevice, and sweat is the electrolyte. That white stuff is not salt, It’s aluminum hydroxide. And there was a lot of it under the tape!

I’ve seen plenty of corroded handlebars over the years. I’ve seen scary-corroded bars that I thought would crumple like paper that I couldn’t bend or break. These were the worst I’ve ever seen. A metallurgical analysis of the remaining metal would be very interesting, for a certain definition of interesting.

Some riders will go without changing tape for five years. (Oh, you’re supposed to change it?) RocketMan’s bars had been taped up and ridden for about 15 months, not unusually long, but most of it in very sweat-inducing Texas heat and humidity. How much do you sweat? How often do you look under the tape on your bars?

Let’s take a few lessons from this failure:

  • Inspection, inspection, inspection. Look under your tape every six months or so, especially if you ride in hot humid climates. It’s relatively easy to un-tape back to the levers and re-wrap.
  • Or just go ahead and change tape more frequently. It’s pretty cheap, and everyone knows that new tape makes you faster. This is a result of the well-known “New Stuff Principle”.
  • Change your handlebars if they’re corroded.
  • I’ve never been a fan of carbon fiber handlebars (because they tend to break on impact rather than bend) but at least they don’t suffer from this insidious failure mode. It’s something to consider if aluminum bars don’t last for you.

By the way, RocketMan is a retired space shuttle astronaut. In one of those cosmic coincidences of life the aluminum oxidation process that caused his crash is the same chemical reaction that powered his Space Shuttle solid rocket boosters.