One interesting note is that the wheels become stiffer with higher spoke counts, but in a decreasing order of magnitude. You will notice that the 4 additional spokes from 16-20 spokes increase stiffness by 9.2% while the 4 additional spokes from 20 to 24 increase stiffness by only 7.5%. The 4 additional spokes between 28 and 32 yields only a 3.2% increase in stiffness. (This is largely due to the relative ratios of spoke counts and design of the study where spoke count increases were all performed by adding the same number of spokes (4). Proportionally 20 spokes is 125% more than 16, but 32 spokes is 114% more than 28 and hence the decreasing ratio, but also of note is that 25% more spokes yields only 9.2% more stiffness).

What this graph and others will not show, is how this wheel feels to the rider. One goal in optimizing spoke count is to determine what ‘feels good’ to the athletes, and optimize the product to that goal. In this instance, a 404 front wheel in production is built with 18 spokes, because that gives the largest range of acceptability among rider weights. Zipp generally tells customers that standard spoke counts are good for triathletes up to 225 lbs and road riders up to 225. Heavier riders are recommended to move up to the Clydesdale wheels which utilize higher spoke counts for greater stiffness. The standard 404 rim has identical construction and durability to a Clydesdale rim, so the only differentiator is in spoke count, and therefore stiffness. At the other end of the coin, very light riders may special order 16 spoke wheels if they wish to save the 9 grams in spoke weight and are not as concerned with wheel stiffness. For a rider only weighing 125lbs, the loss of 9% stiffness is of little issue, particularly on a wheel which a 200 lb rider would find acceptably stiff.

Rear wheels utilize more spokes as they encounter higher lateral forces due to rider weight distribution, must deal with torsional loading due to tractive forces, and simultaneously have decreased flange spacing due to geometric conditions (drive side flange competes for real-estate with the drivetrain). Generally, Zipp wheels shoot for identical front and rear wheel stiffnesses, but the advent of 10 speed systems has made this increasingly difficult. For 2005, the rear hub geometry has changed substantially and now utilizes a larger drive side flange spaced closer into the cassette in order to build stiffer rear wheels. One effect of this is that the 303 wheels have moved from 28 to 24 spokes with no sacrifice in stiffness. 404 wheels on the other hand remain at 24 spoke rear as 20 spokes resulted in a slight decrease in stiffness over last year, and the straight pull hub geometry makes a 22 spoke rear hub impossible, this means that the 404 rear wheel gains an increase in stiffness of roughly 6.5% with no increase in weight.

Other than reducing weight, the other often touted reason for reducing spoke count is for aerodynamics. Special spoke counts are often requested for special events, and just as we will not argue that decreased spoke count does have some positive aerodynamic benefits, we will argue that these benefits are of ever decreasing proportion, and may have other consequences. For this reason, Zipp refuses to build wheels utilizing fewer than 16 spokes. We feel that this is the best blend of aerodynamics and acceptable stiffness, while also allowing for long spoke life without fear of premature spoke fatigue and failure. One other important feature is that a 16 spokes seem to be about the lowest number possible which will still leave produce a ridable wheel in the event of a spoke failure.

The graph below shows the differences in aerodynamics between the spoke counts ranging from 16-28, and may be surprising in its lack of major differences. In fact, the 4 spoke differences between wheels res are largely within the margin of error of the wind tunnel itself. It should be noted that each line on this graph represents a minimum of 3 runs averaged together in order to minimize margin of error.

At first glance many will look at this and determine that the lower spoke count wheels have a distinct advantage, although the 18 spoke wheel has the lowest total drag of all. One reason for this may be due to its average of 14 separate runs as opposed to only 2-4 runs for the other wheels. Since these particular runs were all taken over a 3 day period, there could be other factors as well, such as temperature and humidity effects. Because of this, we accept the margin of uncertainty to be 0.5 pounds per data point. This means that almost all of these wheels have overlapping uncertainties at almost every data point. Without hundreds of additional runs conducted in extremely controlled test situation, it is difficult to say that there is a distinct advantage to one spoke count or the other. In fact, much of what one may read into this graph is a result of the scaling of the drag on Y axis. Also, we have thrown in an 18 spoke 404 with 19mm Continental track tire to show the differences a tire can make. I this instance, the wheel appears to possibly be slightly slower than the 404 with Corsa CX tire and 18 spokes, but considering margin of error, we would have to say that the two curves are essentially the same, with the 19mm tire curve tending to have higher drag.

To highlight this scaling issue, we will look at the identical graph with the inclusion of a very popular aluminum wheel, and a classic standard wheel from 10 years ago. The popular current wheel is a machined aluminum rim with bladed aluminum spokes, with machined reliefs between spoke nodes. The classic standard wheel is a GL330 tubular rim from 1995 utilizing 32 round 14/15 spokes and a Record hub. Both wheels were tested with the identical Corsa CX tire as the other Zipp wheels. The higher drag skews the scale somewhat and makes the differences in the various 404’s seem even smaller, particularly in light of the massive drag reductions of the deep section wheels compared to the standard aluminum rims.

Error bars have been included on this graph to show how many of the uncertainties in the deep section wheels overlap each other, but the margins between the deep section wheels of any spoke count and the two traditional type wheels are vastly separated.

The differences in the two aluminum wheels show essentially the difference between spoke count and shape for two relatively standard rims of similar geometry. The reduction of 12 spokes, combined with the bladed spoke shape compared to round makes a considerable difference between the two. This highlights the second part of this exercise, a look into spoke shape.

Spoke shape
One of the key drivers of the decreased spoke count movement was that early testing showed large drag decreases with decreased spoke count. At the time, however, what was not being thought of so much was that the decreases were in spokes that were inherently poorly shaped. Round is a very non-ideal shape for anything which must be propelled through the wind. This is one of the reasons that rim shape is so critical to blending and smoothing the air coming off of a tire, the tire is essentially round and the only way to improve its performance significantly is to control the downstream airflow in such a way as to reduce pressure drag. With spokes, however, we do not have the luxury of being able to build structure onto the trailing edge. Spokes are generally designed primarily for structural properties including fatigue and ultimate strength, with aerodynamics being more of an afterthought coming about in the late 1980’s and early 1990’s. The first aerodynamic spokes were bladed, essentially, a round spoke was smashed in the center to form a rectangular cross section. This was an improvement, but hubs now had to be slotted to accept these special spokes, and they tended to be heavy as only straight gauge spokes were generally bladed. The next improvement came in the form of ovalized spokes. These had the advantage of not requiring slotted hubs, and were in some cases ovalized on a butted section allowing for slightly lighter spokes. The beauty of the oval profile, is that it is very aerodynamic over a wide range of wind angles and conditions. Bladed spokes are more aerodynamic than round spokes at most every possible condition, however, they do add some side force to the wheel in a cross wind, whereas oval spokes tended to yield an aerodynamic improvement and reduced side force. Zipp switched over to ovalized spokes in 1998 after testing showed them to be a more beneficial design overall.

In 2002, Zipp made the move to a radically new spoke from a small Belgian company. This spoke had taken the pro peloton by storm, and in a visit to Europe in 2001, Zipp engineers saw mechanics from some 7 teams actually rebuilding their sponsored wheels with these. The spoke was the CX-Ray, a double butted spoke, ovalized through coining, and post heat treated. The test results showed a more than doubling of fatigue life, and a slight improvement in aerodynamics over the existing 14 gauge ovalized spoke. The aerodynamic benefit came with a weight benefit as well since these spokes were ovalized on a 16 gauge butted section, so Sapim had produced an aero spoke with the weight of a superlight butted spoke.

The chart above shows the various spoke shapes and sizes, with their computer predicted pressure wakes at 30mph. The computer simulations predict numbers which do correlate with real world testing of built wheels. Note with the round spoke, that the wake will generally be equal to the diameter of the spoke, such that even the thinnest butted spokes have pressure wakes larger than heavier gauge bladed or ovalized spokes. Some have theorized that bladed or ovalized spokes will ‘stall’ at higher wind angles causing increased pressure drag, but wind tunnel testing has thus far shown this to not necessarily be true. The graph below comes from data taken at Texas A&M in January 2004. Two different 303 wheels were used, both with the 2001 ‘V’ shape which has since been changed, however, the only difference between them is spoke shape. Also shown are two competiting carbon wheel models, one from an American company, another Italian. These rims are nearly identical in shape, with one using round and the other bladed spokes.

At first glance, one notices the improvement from bladed to oval spokes in the older ‘V’ shaped 303 wheels. Also of note is that the wheels of that time period used 24 spokes instead of the current 20. Amazingly, the difference between the 2001 303 with 24 spokes and ‘V’ shaped rim is huge when compared to the latest generations 303 rim shape utilizing Zipp’s patented bulged rim shape and only 20 CX Ray spokes. It should also be noted that there were 3 major revisions to rim shape and spoke type between 2001 and 2004 resulting in this major improvement.

The second thing one might notice is that the wheel with 16 round spokes has slightly lower drag than the one with 18 bladed spokes, which is confusing because the article just stated that blades were more aerodynamic. But now for the rest of the story. The aerodynamic forces on the wheel are only one component of the drag of the wheel itself. The wheel in the wind tunnel is being spun at a constant 30mph during testing, and by measuring the wattage required to spin the wheel during the testing we can determine effectiveness of some of these small changes.

Below is the graph of the identical test shown above, however the data represented is not from drag, but wattage required to spin the wheel during this drag test. What you will notice is that while round or bladed spokes, or spoke count may not drastically affect the aerodynamic loads on the entire wheel, the power necessary to spin that wheel can vary dramatically.