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Spoke Crossing(8 posts)

Spoke CrossingVVS
Dec 13, 2002 6:44 AM
Can anyone take out the hype and explain simply why spokes are crossed? Also, with rear wheels, I read that the drive side of the hub is different from the non-drive side, so much so that many wheels have different crossing of the drive and non-drive sides. To me, that says that the hub twists under load so that the drive side is basically doing more work than the non-drive side. I find this hard to believe given the low power outputs of bicycle motors (us mortals) and what at least looks like the relative strength of hubs. I think it means that if the hub twists, then the drive side would be radially ahead of the rim, which would have to pull the rim out of its plane toward the drive side or stretch the spokes. I know that the dish is different on drive and non-drive side and that this must effect wheel design, spoke tension, etc. too.

PS, I am a returning rider with a job and a family and not a lot of available riding time. I bought a nice bike from this site. It rides great, nicest ride I've ever enjoyed - no complaints. It has low spoke count, aero wheels; I like them because they look cool, they are easy to keep clean and it's easy to add air.... not too techie, but honest.
re: Spoke Crossingbrider
Dec 13, 2002 7:58 AM
Well, crossing IS important in the rear wheel to prevent the hub from turning relative to the rim. You essentially have "pulling" spokes, and to balance that, you have spokes that are laced at the other tangent. You're really never going to see the hub flanges work independent of each other, so a radial lacing on the non-drinve side of a rear wheel is only placing more load on the pulling spokes of the drive side. You could potentially go to a 2x on the non-drive side while keeping a 3x on the drive side, but I'd stay entirely away from radial on ANY spokes of a rear wheel. For a front wheel, any crossing is acceptable, as there's no torque between the rim and hub.
Eh?Eager Beagle
Dec 13, 2002 8:09 AM
I only ask cos I find this stuff intresting, but there must be SOME torque between hub and rim up front.. The rear drives the bike, pushes the frame forward, and the front hub (connected to the frame via skewers and bearings) rolls the front rim via the spokes - hence torque, n'est pas?
You should try Campy hubs, no torque in the bearings ;-pSpunout
Dec 13, 2002 9:05 AM
But, there is an unloading from weight, maybe some side-to-side work when cornering. No radial torque though.
re: Spoke CrossingChen2
Dec 13, 2002 11:20 AM
May I recommend the book "The Bicycle Wheel" by Jobst Brandt. He does a great job of explaining wheel technology and physics.
~Al
The primary reason to cross spokes isKerry
Dec 13, 2002 4:06 PM
you want to distribute the forces of "holding up" the wheel to more than one or two spokes. Once you decide to do that (a wise decision), you start having spokes leaving the hub at angles other than 90 degrees. A relatively simple force diagram will show the need for the angled spoke, and crossing comes right after that. Look at any mechanical structure (like a bridge or truss) and you will see diagonal bracing to spread the load. Crossing spokes is the comparable thing in a wheel. Then, crossing the spokes "over/under" is a way share the load between the spokes that cross. It's a way of reducing the load on any given spoke. It's not a huge spreading of load, but it does do some.
Spokes are laced tangentially to transfer torque.the other Tim
Dec 13, 2002 5:30 PM
The only way to have tangential spokes is to have them cross.

Since the spokes remain in tension, they (and the rim's compression) share the work of "holding up" the wheel. Spokes connected to the rim near where the wheel is in contact with the road see a reduced tension. All other spokes have an increased tension (compared to an unloaded wheel), and it's pretty evenly distributed. Think about it - you cannot change the number of spokes with increased or decreased tension by crossing them.
re: Spoke CrossingJofa
Dec 14, 2002 10:14 AM
The practice was originally developed in response to the proliferation of dramatic failures of poorly made, very long, spokes in high-wheelers (penny-farthings). These broken spokes made effective flails. Crossing them enabled adjacent spokes to be tied together at the crossing; so when a spoke failed it was restrained by its neighbour, minimising the damage it might cause to rider or passer-by.

When geared bicycle designs called for much smaller wheels, as they are today, the problem evaporated. Spokes were still crossed, probably out of tradition (the same reason people continued to tie them at their crossings for a while, and a few still ludicrously do), but they were also eventually shown to handle drive forces better than their radial predecessors (which isn't as obvious as one might think). Retaining the pattern in the front wheel allowed spoke lengths to remain the same on both sides of both wheels (before multiple gears and dishing, remember), so a mechanic need only carry around one box of spokes, rather than 2 or 3. This was not a minor concern, as in the early and mid- century spokes were still failing regularly due to the variable quality of their manufacture, and anything to simplify repairs was appreciated. (It is for this same reason that 'high flange' hubs originated: the size was dictated by the smallest flange that would allow a broken drive-side spoke to be removed without requiring removal of the sprocket; this size was then repeated for the other 3 flanges).

We have retained the crossed (tangential) patterns for this same -first- reason: they respond better to drive loads in the rear wheel. The axial dislocation due to load of a radially spoked rear wheel is very small, and would be undetectable to the rider; except in that it is significant enough to cause serious wear in the spoke holes, which will hasten spoke failure. So, now that our spokes are diffent lengths anyway, because of all our gears pushing the rear hub off to one side, why not revert to radial in the front? Well, there is one good reason: most front hubs now are designed for tangential spoking, which subjects them to little radial load, enabling the hub flange to be quite shallow. If a wheel is built non-tangentially, however, with one of these hubs, the spoke flange is liable to fail. This is not an uncommon failure.

Jofa