Originally Posted by
dookie
as sheldon sort of said, spokes have way more stiffness along their axis than they do perpendicular to it. i mean, pull on a spoke. does it deform (stretch)? now push it sideways. sure, they're stiffer under tension...so push a spoke on a built wheel sideways. it still moves a hell of a lot more than it does axially.
that's why radial spoking in the rear is a bad idea.
up front, the primary stresses on the wheel are in line with the spokes. your weight bearing down on the hub, pushing on the spoke(s) at the bottom of the wheel and pulling on the one(s) at the top. even cornering or rocking the bike in a sprint the lateral loads on the rim are still in line with the spokes as they are stretched or compressed. no problem for a radial build.
the same is true in the rear, but there is an additional force...you pedaling and pulling on the chain, trying to spin the hub independently from the rim/tire, which is stuck to the ground. it's a job for the spokes to transfer this force to the rim. a radial lace has the hub pushing on them sideways, just resulting in flex. whereas a crossed lacing places the spokes more or less tangent to the hub flange, aligning the hub's rotational torque with the axis of the spoke. no flex.
mostly true, i just want to clarify a couple things:
spokes function only in tension. a wheelbuild has a certain amount of static pre-tension that keeps it together; riding it applies a range of dynamic tension that depends on rider weight, rim/spoke type, pattern, etc. with a normal build the dynamic tension is absorbed by the elasticity of the system. however, if you break a spoke on a 16-spoke wheel, it's possible for the remaining forces to exceed the capacity of the rest of the system, and then you get overall failure.
a radial laced drive wheel doesn't push spokes sideways, per se; rather it provides leverage for the hub to apply a large dynamic load on the spoke, radically changing its tension with only a small torque. this can fatigue all parts of the system prematurely. conversely, in a cross lacing, there are always a series of spokes that are tangent to those rotational forces and can absorb them without significantly altering their tension. this is sometimes claimed as the reason for high-flange track hubs in the rear— the moment arm between the axle and the spoke drilling is longer, so each spoke has more effective leverage to resist the changing torque of a fixed drivetrain.
but as long as you remember not to radially lace a drive wheel, you'll be ok.