As you know, it is quite obvious that bicycle spokes attach the hub in the center to the rim. What else do they do? If you compare the wheels today with the ones from ancient times, there are more spokes now on motor bikes and bicycles than of a wheel of a chariot. Why is that? What effect does it have on the vehicle if there is a higher number of spokes on the wheels?
1$\begingroup$ Just to let anybody interested know, I've added this question to my list of interesting examples of cross-site questions on meta. $\endgroup$– hippietrailJan 18, 2014 at 6:51
$\begingroup$ See also this $\endgroup$– FlorisJun 7, 2017 at 22:57
In comparing wheels of today to those in history, there are traditionally more spokes now. However, that's because wheels in the past (even large wagon wheels in not-so-ancient times) used relatively thick wooden spokes that behaved like a column and dealt with the load of the wheel with compression.
However, modern spokes are very thin. Far too thin to actually support any compressive load without buckling. Modern metal spokes are very easy to bend. However, when wheels are built, the spokes are threaded into nipples and the nipples are tightened so that the spokes are in tension at all times. A rod under tension does not buckle, so the instability is gone from the spoke.
How much tension is very important of course but this isn't an answer about wheel building (although if you want information on that, hit me up in chat or ask over on Bicycles.SE, I've built many, many bicycle wheels).
The real benefit to using thin spokes is two-fold. First, they are considerably lighter weight than the giant column-type spokes used before. Second, they are also considerably more comfortable because they do flex some under loading, how much can be tuned by the number, material, lacing pattern, and tension of the spokes. So there's a much greater control over the characteristics of the wheel with modern spokes than the giant column-type spokes of yesteryear.
In addition to carrying the load, bicycle and motorcycle wheels have to handle the transfer of power. On a wagon or chariot, the wheels just respond and have to roll. On a bicycle or motorcycle, the power from the rider or engine is transmitted to the hub, forcing the hub to rotate. The spokes then need to transfer that power to the rim to make the wheel spin. This shearing rotation is why rear wheels rarely have a radial spoke pattern and instead have spokes that are at various angles to the hub.
10$\begingroup$ A spoke doesn't even need to be rigid, see: fiberfixspoke.com $\endgroup$ Jan 16, 2014 at 22:40
1$\begingroup$ There are also aerodynamic effects between the spoke and the air which don't matter to a casual rider. But in extreme cases such as world-class velodrome racing you might see advantages from a cowled or rigid wheel due to the elimination of drag from the spokes themselves. $\endgroup$– RBerteigJan 17, 2014 at 2:20
3$\begingroup$ @RBerteig Also true. Specialized (who later sold it to Hed) make a tri-spoke wheel that is back to the column-type wheel rather than spoked wheels and disc wheels are commonly used that have no spokes but are solid instead. But covering that in the answer is tangential and really off-topic here, no matter how many of them I may own and race on the velodome :) $\endgroup$– tpg2114Jan 17, 2014 at 2:25
$\begingroup$ @tpg2114 I'd be interested to know if the advent of disc brakes has influenced the design of front wheels - as I imagine spokes can be arranged in a radial pattern when the braking is done at the rim - but would, like the rear wheel, need to be able to resist a "shearing rotation" when the braking is done at the hub. $\endgroup$ Jan 18, 2014 at 18:06
1$\begingroup$ @dav1dsm1th they just used the existing torque transfer lacing patterns $\endgroup$ Jan 21, 2014 at 20:48
If there is weight on the axle the rim gets pushed down into the ground and tries to deform by flattening on the bottom and bulging right besides the ground. Properly tensioned spokes will counteract this bulging and lessen the deformation allowing for an easier and smoother ride.
This means that the rim does not have to be super resistant to deformation which allows it to be much lighter compared to a column-type rim.
$\begingroup$ I would actually think that the rim gets pushed and pulled down by the weight on the axle. $\endgroup$ Jan 17, 2014 at 11:51
2$\begingroup$ @HenkLangeveld semantics aren't important just that the hub mostly hangs of the top half of the rim which in turn pushes the entire rim down $\endgroup$ Jan 17, 2014 at 11:53
All of the answers given are very good, and delivered well. Therefore, I have little to add to the question, but would like to mention that metallurgy has a great effect on the number of spokes needed for a given application in our modern era. The greater the tensile strength of the spoke material used; the fewer spokes that are required to transmit a given load value. Ferrari used spoked wheels on many of their early race vehicles, for the purpose of weight reduction. To insure maximum strength, while minimizing wheel weight, their engineers chose to use very thin spokes made from titanium alloy. Today, if you happen to sock a curb while making a high-speed grocery run with your vintage Ferrari, be prepared to pay in excess of $10,000 for a replacement wheel, if you can find one. Otherwise, the cost will be higher, to have one custom fabricated. Ah, the challenges presented to the unimaginably wealthy.
Note that the actual mechanics of the spoke structure is somewhat non-intuitive. I hope to clarify it here.
Spokes do NOT support the weight by "bearing" the weight as it goes down from hub to bottom of the wheel. They support the weight by keeping the hub "pulled up" towards the top of the wheel. That's why they can be flexible, because they are essentially working as ropes.
For clarity.. the old style wheels with wooden peg like columns were the kind that were bearing the load (i.e. getting compressed). The bicycle and similar configurations the spokes are getting "stretched" when doing their job.
By connecting to the rim of wheel to the axle through the spokes the mass of the week gets concentrated at its rim . This increases its moment of inertia . This ensures its uniform speed .
$\begingroup$ As a bicycle racer, I can assure you that you don't want to increase your wheel's moment of inertia. $\endgroup$– pelaFeb 22, 2017 at 14:18