The bearings inner race sit directly on an inner metal tube which is the hub axle,
inside which is the thru-axle, with spacer between them and end caps on both ends*

*unless it doesn't.

there are a fair bit of proprietary systems out there but mostly work the same**

**unless it doesn't.

 

You're over-thinking it. A thru-axle is just a more reliable and secure quick release lever.

 

I understand that may be the case practically, but not technically. They work fundamentally differently.

In particular, in a QR axle, the locknuts that hit the frame dropout are usually locked tight to the same axle that the bearings run on, and the bearings themselves run against a shoulder (sealed bearings) or cone (loose bearings) that's machined right onto the axle. That means the bearings have a very direct tie between themselves, the bike frame, and the hub shell, and the axle and the axle lock-nuts. The QR primarily influences just the interface between the frame and the lock-nuts.

On a through-axle bike, the axle has to slide through the "lock nuts/spacers", and through the bearings. In any precision application, this would only work if the bearings and spacers were a light press fit or a "freeze fit" to the axle, but in order for the through-axle to work, there has to be clearance. And for it to work in the real world, it must be quite a lot of clearance at that. So it's a much less solid connection, and the bearings can fret or work against either the axle or the "lock nuts/spacers", or both. I understand that bike wheels aren't high-speed electric motors, but it does look like a big step backward. Frankly it seems a bit sketchy to me. With QR, the tightness of the QR only impacted the interface between the lock nuts and the frame, but it practically didn't impact the interface between the bearings, axle, and locknuts/spacers.

But then again maybe I'm having a GCF (gross conceptual failure) about how through-axles actually work. Nobody seems to talk about this aspect of through-axles, which is why I started the thread.

 

This is a pic of my axle and a diagram of my Pro 4 rear hub as an example:-

A thru axle is basically a fat quick release skewer that slides thru the above axle. No inner bearing races touch the actual thru axle.

Two shoulders on the inside of the axle press against the inner race of the hub bearings. There are few spacer and another inbetween the two freehub bearings included within the assembly.

When the end caps are on the end of the axle and the thru axle is screwed in it pulls the dropouts together compressing all assembly and the inner bearing races together and clamps them so they don't rotate.

The most common Thru axles measure 12,15,20mm outer diameter, the inner diameter will depend how thick a thru axle is.

 

This is a pic of my axle and a diagram of my Pro 4 rear hub as an example:-

...

A thru axle is basically a fat quick release skewer that slides thru the above axle. No inner bearing races touch the actual thru axle....

Ok, this answers my question. In this case, the through-axle isn't really the axle at all, there is still a "real" axle in the hub, and the through-axle is just like a giant QR. This is a LOT different situation than previous answers that said the bearings run directly on the through-axle. I understand not everyone appreciates the difference but the engineering implications of the difference are huge. I guess if this is the normal setup they really shouldn't be called through-axles at all, but rather through-skewers or something...

 

As I recall, thru axles were mostly introduced to provide a more secure method to keep the front wheel from coming out of the dropouts from the increased braking force of disc brakes. QR's and the dropout angle could sometimes release under extreme braking. Not uncommon with downhill mt. bikes.

 

Another major one was the fact that very small variations in qr lever pressure could mean that your disc brake caliper wouldn't line up exactly the same on the rotor every time you swapped wheels. Thru axles give you consistency of locating the front wheel and getting the position of the caliper/rotor aligned the same every time.

But as to OP's desire to get overly technical, there's no 100% consistency. You've got to look at exploded diagrams of every hub to compare, see how they're similar, and how they differ. Frame dropouts vary. Some are replaceable. Some aren't. Same with forks. Some have replaceable inserts. Some don't. @JackOfDiamonds can look all that **** up himself for all I care.

 

I was of the belief that the overarching adaptation to through axles was primarily for "stiffness" independently of the wheel axle. It provides much more robust triangulation to a rear triangle or fork.

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I was of the belief that the overarching adaptation to through axles was primarily for "stiffness" independently of the wheel axle. It provides much more robust triangulation to a rear triangle or fork.

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I can definitely believe the final result with through axles is stiffer and has more precise wheel placement than the traditional QR, especially with suspension. My questions were more about the bearing questions above.

For my QR bikes with disk brakes I have my personal incantations that I do that enable me to be able to remove and replace a wheel without bodging the disc alignment. It can be done, but I'm sure through axle requires less incantations to maintain disc alignment.

 

Its kinda like a Pineapple upside down cake. Its not REALLY upside down, when all is said and done. You just gotta kind of go with it.

 

As noted earlier you are overthinking this. Do you really think the bike industry didn't design thru axles correctly?

I'll never go back to Quick Release ever again. I've had the front wheel loosen up on me on a quick release system

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Agreed. I couldn't really get what the OP was asking either. I sorta got the impression he was questioning whether a TA was any better than a QR skewer. Like somehow everyone got it wrong.

I'll do you one better than "never going back to QR".... I've gotten rid of all my 12x148 'lever' QR axles. So sick of the levers getting hit on stuff and getting scraped to sh!t, or getting knocked loose.
I'm on straight-up Fox Kabolts and Robert Axle Project bolts with a firm snug-up.

I once had the opportunity to ride a pair QR wheels and a pair of TA wheels back to back, same day. Same tires, same bike. I had a wheelset with QR Hope hubs and WTB Scraper rims, and a pair of 12x142 TA hope hubs with Arc i40 rims.
All I had to do was pop in new PMW slider inserts into my frame and go. Lets say the difference was quite apparent and 100% sold me on TA.

 

I’m confused as to why some are crucifying the guy for wanting to understand how thru-axles work. It’s a legit question when you don’t have one to look at yourself.


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Maybe the ulterior motive of the thread wasn't clear, but I also build frames for myself Mainly bmx and single speed road, but I'm looking at maybe building a fatbike or something. For MTB, it looks like I should jump on the through axle train so I will need to do a little more research on the frame threading and spacing and whatnot.

 

you definitely should. most rear hubs for sale these days are 148x12 ta.

 

I don't have a bike with through-axles, and the articles online are all BS articles that talk about "what through axles mean for you", or complain about standards proliferation, and they don't actually talk about the nerdy technology or the specs.

Through-axles are hollow aluminum, right? Anyone know how big the hole is through them?

Do the inner races of the hub bearings run directly on the through-axle? If so, it must require pretty close tolerances for the axle to avoid fretting or sloppiness--does anyone know what the tolerances actually are? Is there any problem having the steel bearing races running on the aluminum through-axle? Are you supposed to grease the axle?

Do they get tight from screwing into the opposite dropout, or is there a cam mechanism like a QR to make them tight? In the pictures, it looks like the axle first screws into the bike until it bottoms, then it gets tightened with a cam. Is this right?

Are the threads replaceable? Like what happens if your frame strips out?

THE BIG QUESTION: Normally, in order to "squeeze" the wheel into the bike, something has to oppose the squeezing force, or else the bearings will bind. On a QR hub, whether it's sealed or cone bearings, the bearings are locked to the axle by the machined axle. But then the axle won't slide through the bearings. To have the axle slide through, you would need a spacer inside the hub between the two bearings, so that when you tighten the axle, the bearings don't bind. This is like the spacer that goes between the BB bearings on BMX cranks. Is there a spacer like this inside the hub to take up the space between the bearings?

Thru axles are a silly way to fix wheels between the drop outs of the fork or rear drops outs. clamping force isn't nearly as high as a good QR.

The bearing do not interface with the thru axle at all its just a long alloy bolt with limited clamping force

 

clamping force isn't the issue.