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I see triathlon bikes with various different spoke styles, from normal wire to aerodynamically shaped blades to completely solid wheels, but I can't understand why there should be so many styles.

I can understand needing less spokes on an indoor track, as you won't get bumps, so I'm okay with that, but can anyone shed light on the decisions behind the other styles?

I have also seen this question on flat spokes but that wouldn't explain the wing like spokes or the solid wheels, would it?

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Are you asking about the actual spokes, the pattern the spokes are woven in, or both? –  freiheit Sep 13 '11 at 23:28
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I think the spokes themselves - I hadn't even thought about the pattern. If that makes a difference then I guess it is appropriate as well:-) –  Rory Alsop Sep 13 '11 at 23:37
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3 Answers 3

up vote 9 down vote accepted

Yep, there are various types of spokes, as already explained. There are also various ways of "lacing" the spokes, and various spoke counts.

When picking a spoke lacing scheme, there are six basic considerations:

  1. Strength
  2. Flexibility
  3. Torque resistance
  4. Air resistance
  5. Weight
  6. Appearance/sex/stupidly spending money

Strength is determined by the number of spokes, how they are arranged, and how they interface with the hub and rim. If you look at a normal "three cross" wheel, the spoke leaves the hub at an angle nearly tangential to the hub circumference. This way the stress on the hub is minimized. A "four cross" lacing (commonly seen on heavy touring bikes an the like) goes that one better and causes the spoke to be truly tangential to the hub circumference (plus the increased spoke length gives the wheel more "spring").

You rarely see "two cross" and probably never see "one cross" lacings, but "radial" lacing (where the spokes leave the hub at right angles to the circumference) is becoming fairly common, especially on front hubs of "mountain" or "cross" bikes. Radial lacing places considerable stress on a conventional hub, and with radially laced wheels you'll often see the spokes reversed, with the nipples on the hub end, because the hub must be redesigned to stand the stress. Additionally, both because a radially laced wheel is so stiff and because usually radial wheels have low spoke counts, the stress on the rim is higher, such that the nipples would be apt to pull through. So you'll sometimes see the bent end of the (reversed) spoke engaged in a slot in the rim or some such.

Note that while radial laced wheels are stiffer against radial forces on the wheel, they don't resist torque very well at all -- if a rear wheel were to be radially spoked the torque of the drive train would cause the wheel to twist into a sort of spiral, hindering the transmission of power to the wheel circumference. Similarly, if disk brakes were used on a radial front wheel the wheel would twist severely when the disk brakes were applied.

[However, it should be noted that some people like to lace their rear wheel with a cross pattern on one side and radial spokes on the other. This provides the needed torsional rigidity while permitting half the wheel to be radial, but it's not clear that there's any real advantage to the scheme.]

Air resistance obviously increases with spoke count, and is affected by the profile of the spoke as well. Although air resistance is important to pro racers because the top of the wheel is moving forward at twice the speed of the bicycle, it's probably safe to say that the air resistance due to spokes on a standard 32-spoke three-cross wheel would not be noticed by most average bikers, even at fairly high speeds.

The ultimate low-wind-resistance wheel is the disk wheel, of course, and there are other exotic designs with 1, 2, or 3 broad, flat "spokes" made of high-strength composites bonded to rim and hub. But generally you'll only see the disk wheels on rear wheels because the disk becomes a kite if it gets cross-ways to the wind as would happen on the front wheel.

Weight is reduced by using more exotic materials (especially for the rim), and by reducing spoke count. The more exotic the rim, the more intimately the design of the rim ties into the spoking scheme. Some slight weight reduction is also possible by reducing the "cross" of the spoking (and hence reducing spoke length).

And, of course, sex. Many of the above schemes (especially things like the low-spoke-count radially spoked front wheel with reversed spokes) have little practical benefit for the average cyclist, add cost, and reduce reliability. (Ever heard one of those fancy high-tension spokes go? It sounds like a gunshot. And if one breaks you're likely dead in the water until you can get to a bike shop, whereas with a standard three-cross 32-spoke wheel one can usually "limp home" if a roadside repair is not possible.)

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From Sheldon Brown:

Double-butted spokes are thicker at the ends than in the middle. The most popular diameters are 2.0/1.8/2.0 mm (also known as 14/15 gauge) and 1.8/1.6/1.8 (15/16 gauge).

Double-butted spokes do more than save weight. The thick ends make them as strong in the highly-stressed areas as straight-gauge spokes of the same thickness, but the thinner middle sections make the spokes effectively more elastic, allowing them to stretch (temporarily) more than thicker spokes.

As a result, when the wheel is subjected to sharp localized stresses, the most heavily-stressed spokes can elongate enough to shift some of the stress to adjoining spokes. This is particularly desirable when the limiting factor is how much stress the rim can withstand without cracking around the spoke holes.

Triple-butted spokes, such as the DT Alpine III, are the best choice when durability and reliability is the primary aim, as with tandems and bicycles for loaded touring. They share the advantages of single-butted and double-butted spokes. The DT Alpine III, for instance, is 2.34 mm (13 gauge) at the head, 1.8 mm (15 gauge) in the middle, and 2.0 mm (14 gauge) at the threaded end.

Single- and triple-butted spokes solve one of the great problems of wheel design: Since spokes use rolled, not cut threads, the outside diameter of the threads is larger than the base diameter of the spoke wire. Since the holes in the hub flanges must be large enough for the threads to fit through, the holes, in turn, are larger than the wire requires. This is undesirable, because a tight match between the spoke diameter at the elbow and the diameter of the flange hole is crucial to resisting fatigue-related breakage.

Since single- and triple-butted spokes are thicker at the head end than at the thread end, they may be used with hubs that have holes just large enough to pass the thick wire at the head end.

Aero (elliptical) spokes are a variety of double-butted spoke in which the thin part is swaged into an elliptical cross section, making these spokes a bit more ærodynamic than round-section spokes. The most widely available spokes of this type are the Wheelsmith Æro. These are 2.2 x 1.8 mm at the ends, and the middles are equivalent to 16 gauge, but in the form of a 1.8 x 1.2 mm ellipse. The Wheelsmith Æro is my favorite spoke for high-performance applications, not just because of whatever ærodynamic advantage it may offer, but because the flat center section provides an excellent visual indicator to help the wheelbuilder eliminate any residual twist in the spoke. This helps build a wheel that will stay true.

Aero (bladed) spokes have a more pronounced aero shape, flat, rather than elliptical. Although they are the most aerodynamic of spokes, they won't normally fit through the holes in a standard hub because they are too wide. To use "blades", the hub must be slotted with a file. Slotting the holes can weaken the flange, and will usually void the warranty of the hub. It is also a lot of trouble.

In theory, "aero" spokes should be more aerodynamic, however if you think about stressing the spoke, it would be more stiff in the direction the wheel is rotating because it is thicker, and have more flex perpendicular to that, since the spoke is thin or flat on the side.

I have double-butted spokes on my cross-country mountain bike, and raced on them for years without untruing the wheels, and it saves a small amount of weight per spoke but its at a point where the spoke isn't heavily stressed, so it retains most of its strength. Especially good for lighter weight riders on full suspension bikes where the wheels won't be subjected to as much of a beating in the first place.

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Spokes are loaded in tension. They don't resist lateral flex and aren't loaded laterally, so it doesn't matter whether the spoke is round or bladed in that respect. –  Adam Rice Sep 14 '11 at 0:43
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This comes down to aerodynamics, reliability, and cost.

A wheel with many lightweight spokes will be reliable and remain relatively rideable even if one spoke breaks. But all those spokes are like an eggbeater in the wind: not aerodynamic. So wheels like this are mostly used by heavier riders or in touring, where durability trumps speed.

A wheel with fewer spokes puts more load on each of those spokes, and can't tolerate any of them breaking. Also, both to make up for the lower spoke count and because the whole point of reducing spoke count is to improve aerodynamics, these wheels usually have deep-section rims with a teardrop cross-section. These increase the wheel's strength (at the cost of weight), and improve its aerodynamics by replacing what would be the fastest-moving parts of the spokes (at the tips) with solid aerodynamically-shaped rim.

A trispoke or disc wheel is the next logical step. These are much more expensive to fabricate, and discs especially are problematic in crosswinds. They also tend to be heavy, making them inappropriate for some kinds of racing, but not so much triathlons, which are about steady effort.

Bladed spokes are marginally more aerodynamic than round spokes. At the very high end, there are some wheels that use fettucini-like strips of carbon fiber in place of steel spokes—these may cost as much as a very nice bike themselves.

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