Technical FAQ: Thru-axles and frame stiffness

This week, Lennard Zinn addresses questions pertaining to thru-axles and carbon fiber frames.

Why thru-axles?

Dear Lennard,
I’ve been riding for years and have raced road, track, tandem, and mountain bike.

I’ve never had a problem with my forks or wheels. Why is the industry shifting to thru-axles? It seems like marketing, but there must be an engineering reason behind this.
— Bill

Dear Bill,
There are two arguments for thru-axles; one has to do with safety and the other has to do with performance.

I’m sure you understand why we have the universal presence of wheel-retention tabs on fork ends for bikes with quick-release skewers and rim brakes; they are there to prevent the front wheel from falling out. They are often called “lawyer tabs,” in reference to the huge lawsuits that have been won by plaintiffs whose front wheel had fallen out, even if they had not tightened the quick release properly.

With disc brakes, those safety issues with front-wheel retention are compounded, and thru-axles are a solution to them. When the front disc brake is applied, the fulcrum of an imaginary lever connecting the contact point with the ground and one on the top of the tire moves from being at the hub axle — the center of the wheel — to the point on the rotor where the brake pads are clamping it. This means that there is now a torque about that point that is trying to force the front axle almost straight down, out of the ends of the fork ends, if their slots are vertical.

On mountain bikes, there have been incidents where, despite lawyer tabs on the forks, the wheel was forced out of the fork under hard braking. This is because a suspension fork’s magnesium outer legs are soft, and hard braking with the QR even slightly loose files the fork ends down between the clamping faces on the ends of the axle and the clamping faces of the quick-release skewer. It should be somewhat apparent that this filing of the faces of the fork ends will not be uniform; rather, it will tend to file the lower part of the fork end thinner than the upper end. This means that, even if the skewer is tight enough when the axle is fully seated in the fork ends, it will be too loose as soon as the axle moves down in the fork ends at all. This up-and-down axle movement also grinds down the soft lawyer tabs, and they can be knocked off completely when the axle comes forcefully down the slot, thus releasing the wheel.

Some disc brake-fork manufacturers initially addressed this concern by angling the slots in the fork ends forward, rather than down. The torque produced upon hard braking about the point on the rotor where the brake pads were clamping it would now be forcing the hub axle down against the lower side of the slot in the fork end, rather than straight down the (vertical) slot in the fork end.

A much more foolproof solution to this issue, however, is a thru-axle. Once you screw in the axle and clamp it down, it is irrelevant what the disc brake does; it will not dislodge the wheel.

The performance improvement comes from added rigidity to the fork. The thru-axle ties both fork legs together. With a suspension fork, you are no longer relying on the stiffness of the fork brake to prevent independent movement of the two legs and hence fork binding. A thru-axle is also the only way an upside-down fork, like a Manitou Dorado or a Maverick (or a motorcycle fork), will work. And with a rigid fork, the thru-axle, crown, and legs together form a rigid box, resulting in more precise steering control, thanks to a reduction in lateral fork flex.

Now that all QR forks have lawyer tabs anyway, there is no longer a significant speed of wheel change issue between that and a quick-lever type of thru-axle.
― Lennard

Carbon fiber frame properties

Dear Lennard,
Two related questions:

1. How much vertical compliance does a carbon fiber frame actually offer? I realize this is dependent on stay design, rider weight, tire psi, road condition, etc., but generally speaking, is it possible for the average amateur cyclist to feel appreciable vertical compliance with carbon (versus, say, aluminum)?

My belief is that it might be felt by a rider weighing 90 kilograms (198 pounds) riding a road bike running 700×23 tires at 100psi, but that there is no way any vertical compliance from the frame can be felt when a rider at 65 kilos (143 pounds) is running 29″ x 2.25″ tires at 25psi on a hardtail mountain bike — the tires will provide the initial compliance and then would have to max out at their expansion limit before the frame even begins to comply (before which, presumably, the tire would have blown out anyway.)

2. More or less orthogonal to the answer of the first question, does carbon lose its ability to dampen vibrations (and, perhaps, offer vertical compliance) over time? I’m not referring to catastrophic failure here, but rather I’m wondering if material fatigue will eventually result in a carbon frame with apparent structural integrity losing its damping capabilities. Or can we just ride these things with full damping capability until they become obsolete?
— Jeff

Dear Jeff,
1. A frame’s vertical compliance is of course dependent on the frame design, the carbon layup, the tube diameters, and the quality of construction (how much resin beyond what is required to bond all of the layers together has been squeezed out). Beyond that, you are absolutely right that your bike’s first line of suspension is the tires. If you have big, soft tires, the frame’s vertical compliance will be so tiny relative to that of the tires that you will not be able to tell the difference between the vertical compliance of frames of similar quality. With skinny tires pumped up hard, you can probably notice a compliance difference between frames with a significant difference in vertical stiffness.

2. Carbon fiber composites do not tend to fatigue. So as long as you don’t damage the layup with impacts bigger than it can absorb without delamination or cracking, your frame’s stiffness and damping characteristics will not tend to change over time.
― Lennard