Why no wheels with rope instead of spokes?

Question

Spokes in a wheel are (shall) be loaded always in tension. Seems like a simple rope of steel strings would do.

Furthermore, uneven spoke tension can lead to lots of problems (among which loose spokes, broken spokes, cracked rim holes). A single strand of thread, passing through all spoke holes in a star-like fashion would eliminate uneven tension.

And even if such a wheel isn't technically superior to current designs, it would be interesting. People throw thousands at purchasing 3-spoke wheels, grouped spoke wheels and whatnot.

Answer 1

Mavic has made some wheels like you describe, more or less. They're called "R-Sys" and they have aramid (Kevlar) spokes surrounded by carbon fiber sheaths. Here's an exploded view:

For more details about that, see the page the image is from: http://www.pardo.net/bike/pic/fail-001/FAIL-105.html

More seriously, yes, you could make wheels with "rope" spokes only. Why do the Mavic ones have the carbon sheaths then? I would speculate that the carbon acts as armor for the aramid fibers, which must never be nicked or they will fail. The carbon also stabilizes the whole arrangement, reducing lateral flex.

As for your notion that spoke tension would always be even if continuous fibers were used, I'm not so sure. The problem is that to achieve self-evening tension you would need to allow the fibers to run freely through some turning points. And if they run freely they will abrade rather quickly if they actually move at all. So you're at an impasse: either the fibers move and wear out prematurely (and perhaps lead to collapse of the wheel even sooner because of unstable geometry), or the fibers in fact do not move and you have the same tensioning concerns as with conventional wheels.

Finally, consider that spokes are thin and behave not like rods or struts so much as wire. So in a way we are already riding your wheels of the future today.

Answer 2

Getting the tension equal on all of the spokes made from a single length of a flexible string would be very difficult due to friction. And because no rim is perfect, spokes have to be adjusted to moderately different tensions to bring a wheel into true. Threaded spoke nipples work amazingly well on single-strand steel spokes. A spoke material or construction which cannot be threaded requires fittings on one end which can, or unusual and complicated construction as with the helicopter spool in a photo in another comment -- also can't be re-trued like a wheel with threaded spokes.

Answer 3

A single cable as you suggest would mean bad things happening for a single failure. Although lightweight bikes will always push the spoke count as low as possible they can still handle a broken spoke without automatically crashing. You also don't have enough degrees of freedom to dish a rear wheel even if you assume the wheel is self truing.

If instead you consider replacing each spoke of a conventional setup with a cable you add the weight and part count of the fasteners (offsetting any saving in the spokes themselves). You still need the same tensile strength and there's no need for them to be flexible so a single thick strand is no worse than a cable of thinner strands so you're back to the same weight assuming steel. Plus fasteners. You might shave a little weight off in total using something like spectra rope but you would have to ensure no risk of fraying at the fixing points. To overcome all this optimally you'd need to redesign the hub and rim. That's expensive and reduces your market.

There are always titanium spokes for those worried about weight. These would retrofit easily on a standard wheel.

It would be nice to see an old set of wheels rebuilt with cable for demonstration purposes -- you wouldn't want to go fast but you could probably ride on strong fishing line with one piece for each side of the wheel.

Answer 4

You could argue that some carbon fibre disk wheels meet that description...

Compare this, the drive spool for a human powered helicopter with Kevlar "spokes":

Answer 5

Spinergy has something like that - though not with a single rope.

http://www.spinergy.com/catalog/product_info.php?cPath=20&products_id=84

Actually it's not extra light.