Disc brake question for the brianiacs

[oldpotatoe]

Originally Posted by Mark McM

Well, there's no reason you can't do the calculation.

As mentioned, just the larger diameter contact area of thru-axle hubs is enough to increase "cocking" stiffness, but since you asked, let's look at the axial stiffnesses of an aluminum thru-axle vs. a steel QR skewer. I measured some of my QRs and thru-axles, and also measured drop-out widths ona a thru-axle fork and a QR suspension fork, and used these values below.

The axial stiffness of a column or shaft is: K = E x A / L

where: K is the stiffness, E is the elastic modulus of the material, A is the cross sectional area, and L is the length.

Chrome-Moly steel has an elastic modulus of about 200 GPa, and 7075 aluminum has an eastic modulus of about 70 GPA.

The cross sectional area of a tube is: A = pi x [ D^2 - d^2 ] / 4
where A is the area, D is the outer diameter, and d is the inner diameter (for a solid shaft, d = 0).

My steel QR skewer has D = 5mm and d = 0mm, so A = pi x( 5mm)^2 / 4 = 19.6 mm^2. My aluminum thru-axle has a D 12mm and d - 6mm, so A = pi [ (12mm)^2 - (6mm)^2 ) / 4 = 84.8 mm^2.

Both forks use 100mm spacing between dropouts. My QR dropouts are 7mm thick, so the free length of the QR skewer is 100mm + 2 x 7mm = 114mm. On the thru-axle for, the thickness of the dropout where the head of the thru-axle sits is 6mm, and the thru-axle threads directly into the other dropout, so the free length of the thru-axle is 100mm + 6mm = 106mm.

With these numbers, we can calculate the axial stiffnesses of each shaft:

QR: K = (200 GPa) * (19.6 mm^2) / 114mm = 34,400 N/mm
Thru-axle: K = (70 GPa) * ( 84.8 mm^2) / 106mm = 56,000 N/mm

Of course many suspension forks use 15mm thru-axles, which will be quite a bit stiffer than 12mm thru-axles.

I was write the same thing!!

Nope..