Dear bike industry: really, a 'new' 20x110mm axle standard?!?!

Why Boost 20mm drives me mad

Here's some news that just about everyone wants to hear! There's a new front axle standard for mountain bikes and it's the same as the old standard, just not quite! 

Yes, it's an old familiar face, a 20 x 110mm thru-axle for the front hub just like downhill forks have had since the 20th century. Surely you remember 20mm thru-axles: stiff, secure, even lightweight. 

The old 20mm thru-axle standard should have become the standard, but somehow someone decided that saving 2.3 grams was important enough to create 15 x 100mm thru-axles. Was it really the only way XC riders and racers would embrace thru-axles? I kind of doubt it.

But 15 x 100mm soon wasn't wide enough, so Boost had to come along with an additional 10mm of spacing. Which, if you're paying attention, you'll recall is the same width as the original 20mm thru-axle. Damn it.

It's the same, but different

Back to the old, new 20 x 110mm standard. Now it's Boost so it won't work with forks that use the previous 20 x 110mm standard. Yah, progress!

Will there be an adapter for an old 20mm fork to run new Boost 20mm front hubs?

What's the difference? This new Boost hub features the same 110mm axle spacing as the 20mm thru-axle spacing from back in the day, but bumps out the disc rotor mount 5mm laterally to increase the hub flange width. 

What does that 5mm do? It supposedly allows front wheels to be built stronger, with the spokes having more triangulation (a wider base of the triangle) because 29-inch wheeled downhill bikes are now a reality. 

But what it really does is force fork manufacturers to have to create an entire new lower left leg casting because the disc caliper mount now needs to be re-positioned 5mm wider because the rotor is 5mm farther to the outside. Arghhhh!

Backwards compatibility? Sort of

Alas, there's good news in all this nonsense. If you still have your old (or are they new again?) 20 x 110mm downhill hubs you can bump the rotor over 5mm with spacers and run your old hubs on your new 20mm Boost-compatible fork. 

Will there be an adapter for an old 20mm fork to run new Boost 20mm front hubs? Maybe? For forks with 200mm disc direct mounts that's unlikely, but for forks that are able to take 160mm or 180mm rotors it's entirely likely some company will make a 5mm offset adapter for the brake caliper. 

I get that without progress the sport will become stagnant, and in racing it's no-holds-barred for every millisecond, but these tiny incremental gains are challenging. Especially when we had a very good standard to begin with, the 20 x 110mm thru axle.

Russell Eich

Tech Writer, US
Russell fell head over heels in love with bikes in the '90s, and has been involved in the bike industry ever since. Between wrenching in bike shops, guiding professionally, and writing about bikes, Russell has honed an appreciation for what works, gained knowledge of what doesn't, and can barely contain his enthusiasm for what comes next. His two-wheeled passion continues in the Rocky Mountains high above Boulder, Colorado.
  • Discipline: Mountain, road, cyclocross
  • Preferred Terrain: High altitudes, forgotten singletracks, bike parks, roads without cars
  • Current Bikes: Custom Meriwether steel hardtail, Specialized S-Works Enduro 29, Kona Jake the Snake, Trek 69er, and a bunch more
  • Dream Bike: Yeti SB5c, Intense Tracer 275C, Black Cat custom road
  • Beer of Choice: Gin + Tonic
  • Location: Rollinsville, CO, USA

Related Articles

Back to top