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I have seen thru axles mentioned in various places but never explained or even described.

I found a post on another site where someone asked the same question and got lots of debate in response, but no explanation.

What is a thru axle? What problem does it solve?

Thanks.

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With a traditional non-through axle, there's a slot at the bottom of the fork (or the dropouts), the axle is hollow, and there is a skewer through axle. You use the quick-release to loosen the grip around that slot to slide the skewer (vertically) in and out of that slot, while it's still going through the wheel.

With a through axle, there is simply a hole on either side, instead of a slot. You remove the axle entirely out the side of the hub in order to remove the wheel.

Typically, through axles are much thicker axles. The thicker axle is stronger and stiffer. Downhill mountain bikers were breaking axles, and it's a lot harder to break a 15mm through axle than to break a 9mm traditional axle. The stiffness also helps the bike handle better ("more responsive").

From what I've heard from a framebuilder friend, the move to through axles is also because frame manufacturing techniques improved to allow them. With a traditional dropout, if the two sides were a millimeter off, the framebuilder could simply file a millimeter out of the slot on one side to make things work right. Once it's all polished and painted you'd never notice. With a through axle the framebuilder absolutely has to get the alignment just right, because an oval hole just won't work.

The only downside of a through axle is that changing a flat might take an extra second or two.

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    I'm not convinced that changing a flat takes longer, IME it's at least as fast. The difference is that you need an allen key. The big thing is that everything slides apart and back together really quickly and easily, there's no trying to jam things into place. But I suspect I've been lucky?
    – Móż
    Feb 7, 2014 at 6:08
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    @Ӎσᶎ Most of the through axles I've seen were quick-release and didn't involve an allen key. I don't know about you, but it takes me a few minutes to change a flat, so an extra second really wouldn't matter.
    – freiheit
    Feb 7, 2014 at 7:02
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    I don't think that it would take much longer if you were changing out the whole tube, as most of the time spent there is actually dismounting and mounting the tire, as well as pumping up the tire. If you're patching the tube, then just waiting for the glue to dry is probably the most time consuming part. If you're in a race and you're swapping out an entire wheel, then the time could be increased. However, in downhill (which I don't do, correct me if I'm wrong) I assume your run is pretty much over if you get a flat. There's not enough time to catch up after losing that much time.
    – Kibbee
    Feb 7, 2014 at 15:31
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    Why can't they make a frame with 15 mm slots, and have a 15 mm non-removable axle through the wheel hub.
    – Kaz
    Aug 8, 2016 at 21:01
  • With a through axle the framebuilder absolutely has to get the alignment just right. This is also the case with vertical rear dropouts and front fork ends. Only rear horizontal dropouts or track ends have the play that allows for some inaccuracy.
    – Kaz
    Aug 8, 2016 at 21:03
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This article does an excellent job of describing through-axles:

http://cyclingtips.com/2015/10/road-bikes-are-headed-towards-through-axels-but-why/

Chiefly:

  1. They're inserted through the forks, not slotted into dropouts like Quick Release axles
  2. More symmetric forces are applied to the forks/suspension.
  3. Provides a solution against unwinding, pop-out QR axles when using disc brakes. I'm not sure how often this happens. Also, the article cites different solutions that don't require Through Axles like angling dropouts or re-positioning brakes.

To me it sounds like the most value is in the context of mountain biking.

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