Aero Wheels? Roads Yes, Trails No

Affiliations: Jason Nugent BSc Hons
Acknowledgments: Arnie Baker (reviewer), Joe Friel (reviewer), Will Hopkins (consultant physicist), Mary Ann Wallace (editing)
Correspondence: ap958=AT=ccn.cs.dal.ca (Jason Nugent)
Reference: Nugent, J. M. (1997). Aero wheels? Roads Yes, Trails No. Sportscience Training & Technology http://www.sportsci.org/traintech/aerowheels/jmn.html
Date: May 1997

Summary. The lower air resistance of aero wheels compared with conventional spoked wheels will reduce your time in most road events by a valuable 2%. At the slower speeds of most off-road events, the difference in air resistance has a negligible effect on performance. Aero wheels also weigh more, cost more, and most can't be trued. My recommendation: use conventional spoked wheels, unless you race competitive downhill or cross country time trials.
Reviewers' comments

Aero wheels are nothing new. In 1891 the British publication Cycling considered disk wheels the "greatest invention to affect the modern bicycle", but they began to appear in competitive road racing only a decade ago. Manufacturers of aero road wheels, including Spinergy, Spin, Zipp, and Hed, have now designed models to be used by the fat tire set. Several influential off-road race teams are sponsored by these companies, increasing the exposure of aero wheels designed specifically for mountain bikes. Do they work on trails? There isn't much published data, but what there is, and some simple physics, makes it reasonably clear that aero wheels are worthwhile in the faster off-road events.

What's the big deal with aero wheels anyway? You probably know the answer already. The wire spokes on a normal wheel experience a resistive force as they slice through the air, just like the wires of an egg whisk in a bowl of water. An aero wheel effectively reduces the number of spokes and the drag of each spoke. The drop in resistance is enough to give you a valuable increase in speed of a few percent in road events. But as we'll see, the gains off-road are generally less because of the slower speed of these events.

Literature
 
Reviewed are contents from the Zipp Components and Spinergy commercial websites, the Mountain Bike Review website, a study reported in the European Journal of Applied Physiology in abstract form only (Capelli et al., 1993), a classic paper on energy expenditure in cycling (di Prampero et al., 1979), and an article in Inside Triathlon (Zinn, 1995). The BikePro website provided data for wheel prices. The published data are for the effects of air resistance on road bikes and road wheels; I've had to resort to physics to adapt these findings to off-road races.

Findings
 
The advantage of aero wheels in road races is undisputed. According to a mathematical model devised by Jim Martin, Director of Sports Science for Team EDS, an elite cyclist can expect to complete a 40-km individual time trial in 55:57 with conventional spoked wheels in calm conditions and excellent body position. With aero wheels (the Specialized Tri Spoke in front and a lenticular rear disk), the time saving is 1:26. The faster time equates to an average speed of 44 km per hour. Under various conditions, the time savings with aero wheels in the model are consistently 1-2 minutes over a distance of 40 km, equivalent to 2% for a slow rider and closer to 3% for elite cyclists (Zipp Components). In a study with real bikes and riders, Capelli et al. (1993) reported savings of 3% over distances ranging from 1 to 20 km when aero wheels replaced conventional spoked wheels. These results fit nicely with the increase in world-record speeds of about 4% that occurred after the adoption of the new aero bikes. Most of this increase is due to the aero wheels; the rest comes from improved aerodynamics of the frame and handlebars.

Aero wheels were originally disks, but they are now also made with aero spokes. Lennard Zinn (1995), technical writer for Inside Triathlon and a well-known frame builder, claims disk wheels perform best under nearly all conditions and even offer negative drag readings at extreme wind angles (negative drag means the disk wheel propels the bike forward). The downside is that disk wheels pose handling problems when you ride in 90 degree crosswinds, so they are unsuitable for group riding and mass-start races. New aero spoke wheels have virtually eliminated this disadvantage while providing almost the same time savings during an event.

The table below, from Zinn's article, shows that for a given relative wind angle, spoked aero wheels perform nearly as well as a disk. Relative wind angle is the direction of wind experienced by the rider (and bike). Relative wind angles of greater than 15° are rare, even in a strong 90° crosswind, because the bike is usually going a lot faster than the wind. Spoked aero wheels fare well, but a disk wheel pulls away from all challengers when the wind is angling in.

Two factors come into play to reduce the possible advantage of aero wheels for off-road racing: the reduced speed in most off-road events, and the greater weight of aero wheels. The difference in speed is the crucial factor.

Off-road events are usually slower, for a number of reasons. In a road time trial, a cyclist assumes a tight aerodynamic position, but in a mountain bike event, most riders spend a large portion of the course using aggressive body English to negotiate the technical sections. Air resistance from the rider is therefore greater in off-road events. The rolling resistance of mountain-bike tires also greatly exceeds that of a smooth road-bike tubular tire (although tires designed with relatively smooth tread have begun to appear in Europe). Roughness in the off-road surface adds even more to the rolling resistance. All these factors equate to slower average speeds in off-road events. Hilly courses are also slower if the rider has to brake on the downhill sections, and the variations in power output in a hilly course result in lower average power output and speed, for physiological reasons I won't go into. But these are all usually minor factors. The main reason for the slower speeds in off-road racing is simply that the courses don't give the rider the opportunity to maintain high speeds.

How does slower speed affect air resistance? Drag force increases in relation to the square of the speed. In other words, when you double your speed the drag force increases fourfold, because "twice as many air molecules collide with you and your bike twice as fast" (Burke, 1995). But the energy expended against a resistive force is also directly related to the speed so the overall effect of air resistance is proportional to the cube of speed. Let's turn this around and think about the effect of going slower. If your gain with aero wheels in a 40-km time trial is 2%, and you do the time trial at 40 km/hr, then your savings in an off-road race at half the speed will be only 2%/8, or about 0.25%. That's assuming the aerodynamic characteristics of off-road and on-road aero wheels are the same. There are no data on this question.

These savings need to be adjusted in various ways. If the speed varies a lot around 20 km/hr, air resistance will contribute relatively more at the faster speeds, and the savings might go up to 0.4%. But this figure was reached by assuming that an off-road race is just like a slow on-road race. It's not. Arnie Baker pointed out in his first reading of this article that riders often "float" technical sections of off-road races, not braking nor pedaling. At such times the rider isn't working against air resistance so the gain might be 0.2-0.3%. And this gain is for 20 km/hr. Most off-road events are a lot slower. Last weekend, the race I was at had the pros turning 8 km lap times every 35 minutes, averaging less than 15 km/hr. At that speed, aero wheels simply won't help.

Speeds for some events are increasing. Recently there has been a proliferation in the number of off-road stage races. These events are patterned after their road counterparts and usually consist of an off-road time trial, a circuit style criterion, and a "normal" off-road race thrown in for good measure. In the 1997 Sea Otter Classic, Hubert Pallhuber (Diamondback) won the 7.7 km time trial stage in 14:23:7, giving him an average speed of 32 km/hr. At that speed, aero wheels could be a small benefit. Downhill events show much more promise for aero wheels. At certain events such as Mammoth Mountain, cyclists have been clocked at speeds exceeding 90 km/hr. At speeds this high, aerodynamics play a very important role.

Now, what about the weight factor? Weight is crucial in cycling. In spite of the availability of full suspension bikes, nearly all professional riders still choose to ride on hardtails, which have only front suspension. The reason? A full suspension bike weighs too much. Aero wheels are heavier than normal wheels, but how much heavier? Although Spinergy does not claim weight for their rear wheel, they do mention that their front wheel weighs 895 grams. A standard spoked wheel consisting of a Shimano XT front hub laced to a 32 hole Mavic 217 SUP rim with DT 14/15 gauge spokes weighs 794 grams, including skewer. This equates to a difference of 101 grams, or almost a quarter of a pound. If a more exotic front wheel is desired, a Hershey Naked hub can be substituted. The weight difference then rises to 239 grams, or over half a pound.

The effect of this extra weight of aero wheels on performance is less than you might think, according to my consultant physicist. If we assume a rider weighs 65 kg and a bike weighs 10 kg, the extra weight represents about 0.3% of the combined weight of the bike and rider. When you go up a steep climb, it will take you 0.3% longer because of the extra weight. Coming down again, you could go this much faster with the extra weight, and you'll recover the lost time if you don't have to brake. For hills that aren't steep, the loss in time is less than 0.3%. On the flat, the extra weight has no effect, provided you don't change speed. But every time you accelerate, it will take you about 0.5% longer to reach a faster speed (not 0.3%: see note). Again, you will recoup the lost time, provided it's the course that slows you down after the burst of speed; if you have to brake to slow down, you won't make it up. It's often the hills that slow you down, not just the next technical bit that you have to brake for, so I would guess that the loss of time due to the weight of aero wheels is maybe 0.4% of the time spent accelerating and 0.2% of the time going up hills. And how much time is spent going up hills or accelerating? Let's say a quarter of the race for each, which makes a total loss of time of 0.15%. Negligible. I find it hard to believe, but there it is. In spite of calculations by my consultant physicist that prove the insignificance of the weight argument, many off-road racers who depend on their ability to climb will not tolerate an increase of any magnitude. Although an increase of just a percent or two may seem small, the mental aspect of thinking that you are adding weight by using aero wheels (even if it is just a little) may cause you to slow down. Never discount the psychological factors!

A final consideration is the durability and cost of aero wheels. The rigors of racing off-road increases the potential for broken components. Aerodynamic wheels are considerably more expensive than their spoked counterparts. A pair of Spinergy wheels costs US$800. In comparison, the pair of Hershey hubs, Mavic 217 rims and DT 14/15 gauge spokes costs $420. So the Spinergy wheels would have to last twice as long to be cost effective. Unfortunately, it is not possible to true most aero wheels, so the lifespan may be shorter, especially if the rider uses them for downhill racing. Spinergy and Spin both offer replacement policies for a tweaked wheel at a significantly lower cost than a new aero wheel--about half the price of a new wheel, including postage, but still far more than having wheels trued at a bike shop.

Ultimately, cyclists will decide if aero wheels are worthwhile. At the Mountain Bike Review website, the section providing comments on Spinergy wheels is split roughly 50/50, with half the comments raving about how great these wheels are and the other half condemning them. Most reviewers do agree that the wheels flex considerably when riding. Some riders feel that this helps provide a cushion effect which lets them ride longer with less fatigue, while others feel that this motion is disconcerting and actually saps energy.

Conclusions
 
It's fairly certain that the aerodynamic benefits of aero wheels designed for mountain biking are limited to fast races. Expect no gains in speed in normal off-road races, but worthwhile benefits of a percent or more as speeds get close to road speeds. And for downhill races, aero wheels seem certain to give you a healthy lead.

Further Research
 
More specific tests designed with mountain bike conditions need to be done to determine how pronounced the aero benefits are for these wheels. According to information presented at the Zipp Components website, the lack of "established standards" poses serious problems when conducting aerodynamic studies. Several manufacturers who are conducting internal testing may be guilty of biasing their own results to ensure that their product appears in a favorable light. Studies that are conducted under the supervision of third party examiners need to be performed. These results then need to be published regardless of which set of wheel performed the best. Roll down tests could be performed, as well as real studies and mathematical modeling that includes hills and the effects of weight and rolling resistance (rotational inertia). The results of this research ideally would be guidelines for cyclists to determine whether they would gain by using aero wheels for a particular race.

References
 
Burke, E. R. (1995). Serious Cycling. Human Kinetics. p.188.

Capelli, C., Rosa, G., Butti, F., Guido, F., Veicsteinas, A., di Prampero, P. E. (1993). Energy cost and efficiency of riding aerodynamic bicycles. European Journal of Applied Physiology, 67, 144-149.

di Prampero, P. E., Cortili, G., Mognoni, P., & Saibene, F. (1979). Equation of motion of a cyclist. Journal of Applied Physiology, 47, 201-206.

Zinn, L. (1995). Why are disc wheels so rare in triathlon? Inside Triathlon, September 1995, 50. 
 
Estimates of the effects of changing speeds and increased weight (including the note below) were provided by Will Hopkins.
 
Note: When you accelerate, an increase in mass of a wheel by 0.3% slows you a bit more than 0.3%, because you give the wheel rotational energy as well as linear energy. It depends on where the extra mass of the aero wheel is distributed. If it's all around the rim, add an extra 0.3%. If it's all at the hub, there is no extra effect. Let's say 0.5% for aero wheels.

I agree with the author's conclusion: aerodynamic wheels can't have meaningful aerodynamic benefits for mountain biking, because air resistance plays a much smaller role in the slower off-road races. The wider mountain-bike rim may also shroud the spokes, which would reduce the effects of air resistance and thereby reduce even further the saving of aero wheels relative to normal wheels.

The author correctly identifies that weight is an issue. As important as the climbing issue, which the author notes, is the fact that mountain biking is a sport of surging--large power fluctuations. Heavy wheels do not accelerate well. Dean Golich, a sports scientist with USA Cycling at the time, studied cross-country riders on the Atlanta course prior to the 1996 Olympics. Using an SRM crank (a device for recording power developed by the cyclist), he found almost continuously repeated power fluctuations over 100 watts. Although road cyclists will use heavier wheels in relatively steady-state time trials, they will not accept this weight penalty in criteriums, where repeated surging out of corners is crucial to success.

The author raises the issue of crosswinds. Crosswinds can be a factor in exposed downhill mountain runs, but are rarely an issue in cross-country mountain biking. Handling difficulties are usually only a problem with an aero front wheel, not an aero rear wheel.

The author states that aero wheels cannot be trued; that may be true of Spinergy; but Zipp, Buzz and Hed aero wheels can be trued. Flex may also be less of an issue with these wheels.

The lack of data points up a need for research to establish the speeds at which air resistance starts to have an effect with mountain-bike aero wheels. The mountain bike market is larger than the road market; so it's logical that manufacturers will try to exploit this market to the fullest.

Jason Nugent has identified the one bike component in which mountain bike technology lags behind that of its older cousin, road cycling. I agree with his conclusions that there are good reasons the off-road wheel remains rather low-tech in an otherwise high-tech sport, at least in the cross-country event. It's obvious to me that at the speed of most off-road races--hovering around 20 km/hr at most for top male riders-- the contribution of work done against air resistance must be tiny compared with the work the rider puts out to negotiate hills, change speed continuously, and overcome the rolling resistance of the tires. In the downhill event, with speeds exceeding 80 kph at times, air resistance is exceptionally large and has a definite impact on finish times. Aero wheels are an advantage here.

As explained by Jason, cost is an important consideration in a sport that places great stress on equipment. Few non-sponsored riders are willing to shell out the additional money to replace damaged aero wheels when standard, spoked wheels can often be trued at a fraction of the cost.

Even one of the manufacturers' executives, Zipp's Bill Vance, concedes that aero mountain wheels aren't going to make you fast at typical mountain bike speeds. Vance does claim that the wheels will help you accelerate more quickly and give you a more comfortable ride and control over the bike. Perhaps instead of aero wheels, we should call these "suspension" wheels and suspend usage until we get some practical test results. Mary Ann Wallace