Is it a known thing that fitting an extra disc causes an issue with the spokes?

It depends on the configuration. If the wheel was designed for one brake disc and caliper only, it's more than likely that adding a second disc and caliper (which exerts the same braking force as the first one) will cause an issue when braking hard. A braking force acts tangent to brake disc at the center of the braking pad, which creates a braking torque (Mt), which in turn is transmitted to the rim and tire.

Consider this simplified equation: Assume all spokes have the same angle (alpha) in the central plane towards the rim tangent line. Disregard the out of plane angle.

Let T be a component of the spoke tensile force acting tangent to the rim bead of radius R. Then,

Mt = n * T * R

Substituting the spoke tensile force (S), we derive at

Mt = n * S * R * cos(alpha)

Now, if Mt is doubled, tensile force in the spokes will also double. A manufacturer would have to size for the worst case, meaning thicker spokes are needed. Assuming the same spoke material is used, and safety factors remaining constant, the new thickness d2 = d1 * sqrt(2) = 1.4 * d1 (rounded).

The formula also shows that decreasing the angle (alpha) by (delta), by selecting a 3 crossing vs. 2 crossing pattern, increases the tensile force in the spokes for a given torque. For example, (alpha) = 55 degrees and (delta) = 10 degrees (resulting in a 45 degree slope towards the tangent line) would increase S by 1/( cos(delta)-tan(alpha)*sin(delta) ) = 1.357 meaning spoke stress increases almost 36%. Thus, slanted spokes means thicker spokes also, unfortunately. For this reason, hubs have increased in diameter in recent years, allowing spokes to be kept short.

Adding a second disc in a responsible way inevitably makes the wheel heavier! Namely, by 40% of the initial spoke weight (assuming constant slope or angle). In addition nipples will be a little heavier. We neglect the weight loss at the hub and rim by drilling larges bores for spokes and nipples. The additional brake disc adds to this.

- Knut