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Why are people with full suspension bikes with several inches of travel be concerned about 'Stiffness' when discussing the advantages of thru axles. I would think this is more of a concern for rigid bikes which don't have suspension. Isn't the advantage that you're just less likely to snap the axle when riding down a gnarly trail not really how 'stiff' the axle is or does this have some other purpose?

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I'm not sure exactly what you're referring to, but it does make sense that on the front of a mountain bike, the axel (or QR) and the hub are the only things keeping the forks parallel and at the same height. Often, one fork leg is for rebound and the other compression, thus making the "stiffness" of the front axel rather important. –  Jack M. Jun 26 '13 at 15:43
    
They're also handy for people using single side mounts, so trike riders are very happy to see them become popular. –  Mσᶎ Jun 27 '13 at 0:16

3 Answers 3

up vote 8 down vote accepted

Thru axles not only keep the wheel in place, they also strongly join the right and left dropouts together. This structurally reinforces the component either being it a fork or a rigid/suspended rear triangle.

This simply makes the bike able to handle greater forces before failure, because it distributes forces between both sides of the structure. It also avoids twisting of the parts. For example, if one arm of a fork twists, it means it may loose alignment relative to the other, thus meaning the wheel is not pointing in the direction the handlebar tells the rider, this leads to a bad feedback and imprecise ride feel. A thru axle keeps both sides of the fork aligned, eliminating (or reducing) this issue. It also keeps the lower ends (bottles) better aligned with the upper parts (stanchions), allowing for a smoother operation and longer life of seals and bushings.

For suspended rear triangles it also makes the structure stiffer, this allows better distribution of forces through the stays, so those forces reach the shock absorber with better alignment, also allowing for smoother operation and longer component life. It also keeps the pivots aligned, thus giving them a longer life for both bearing type or bushing type pivots.

The bottom line is that a stiffer structure is more efficient and gives better feedback to the rider, allowing for a more confident and precise ride. It also allows a lighter bike to handle greater forces.

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Now what is the difference between this and regular nut axles? Why thru axle connects the dropouts better? –  J-unior Jun 27 '13 at 7:36
    
@J-unior the comparison is through axles with QR axles. And you can see on forks through axles of 20mm diameter, vs 9mm on standard QR –  trailmax Jun 27 '13 at 13:15

Stiffness in this context is the ability to resist twisting and bending forces caused by turning and heavy braking (usually on difficult ground). A stiff shock moves up and down freely but allows no twisting or bending back and forward/side to side.

MTB riding benefits from stiffer forks. The main advantage is the precision of tracking by the front wheel. As an over simplification, think what happens when the forks twist - the front wheel no longer points in the same direction as the handlebars. On difficult ground this can change in a split second, making riding more difficult. The stiffer the fork, the more precisely the front wheel can be placed. In the extreme, a flexible fork can throw a rider offline, as best, slowing him, at worst, causing a prang.

For decades the 9mm QR was the standard for virtually all wheels. As MTB forks became longer travel and lighter, and riding more extreme, it was decided that a stiffer connection between the hub and fork was required. The 20mm through axle was developed. This was rejected by many as too heavy, so the 15mm Through was developed as a lighter weight alternate providing nearly as much stiffness as a 20mm, and virtually no more weight than a 9mmQR.

There are few benefits for a through axle for all but the most extreme or elite riders. The additional strength is a benefit to the most extreme downhill riders. The additional stiffness is the only other real benefit - therefore, any discussion on through axles is incomplete without mentioning it.

There is some controversy around the need for the smaller through axles on all but the very best bikes. Many believe the offer little advantage and many disadvantages over QR for a majority of riders, and are more a marketing gimmick than a real advance in bicycle performance (for average riders).

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What are the "many disadvantages" of thru axle over over QR? –  cherouvim Jun 27 '13 at 3:46
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Convinence of wheel removal, requiring tools to achive it (Although modern forks are better most still need tools). Bicycle carriers designed around removing front wheel do not work (many roof rack designs), weight, expensive Incompatability with existing spare wheels, lack of knowledge on how to install wheel reliably..... Do not get me wrong, there is definitly a place for through axle, but is not for everyone, and probably not for most poeple. The median price of bicycles sold is around $500 dollars and at that price point, no one will tell the difference. - OK, maybe not "many" –  mattnz Jun 27 '13 at 4:50
    
Tools for through axles are thing of the past. You can get adapters for most roof racks for 20mm front axle. But yeah, at $500 for you bike you would not get through-axle on your fork. But rather $500 for the fork, it would be expected to get through axle. –  trailmax Jun 27 '13 at 13:19

The stiffness in the front added by the 15mm or 20mm thru axle is very important because it lets the bike go through very rough sections (roots, rock gardens, corrosed terain) without transmitting unecessary feedback on the handlebars. This makes fast riding on rough terrain easier and safer.

In addition, a stiff axle will not allow the wheel to bend laterally too much, even on a smooth terrain, when going fast on a bermed corner etc. This gives more presision and less lost energy.

In any case, snapping an axle, even a 9mm, should be pretty hard. That is unless something has not been set up correctly.

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So, the advantage of this 'stiffness' is that it keeps the fork blades parallel better than a smaller axle, which allows the stanchions to move more freely, enabling the suspension to work more optimally? –  Benzo Jun 26 '13 at 19:01
    
-1: Answer is a pile of confusion and mixed up miss-information. It mixes the concept of fork stiffness, 9mm QR vs stanchion size and shock transmission to handle bars. –  mattnz Jun 26 '13 at 22:34
    
Benzo: yes. Any bending force also slightly distorts the metal and causes extra wear. –  Mσᶎ Jun 27 '13 at 0:17
    
@mattnz: Thanks, I've removed the offending parts of my answer. –  cherouvim Jun 27 '13 at 3:43

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