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Home Aerodynamics Forks Do Aero Forks Work?

Do Aero Forks Work?

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Do Aero Forks Work?

“Fifteen seconds”.  The official states emotionlessly as you prepare to push yourself to your limits in the race of truth.  You’ve prepared your body through diligent training.  You’ve worked on your body positioning to punch as small a hole in the wind as possible.  “Ten seconds.”  She drones.  You have equipped your bike with the latest the bike industry has to offer.  You’ve got your aero bars.  “Five.  Four.”  You’ve got fast wheels.  “Three.  Two.”  You’ve got your skinsuit and aero helmet. “One.  GO!”  You hurl yourself down the start ramp and accelerate up to speed.  You’ve convinced yourself that you have looked after every detail when it has come to your body and equipment; but have you really?

True Temper, Oval, and Reynolds are among the many manufacturers who would claim that you have not covered all the aero bases if you haven’t taken a look at the fork under your nose.  Thanks to the hard work of Chester Kyle, Jim Martin and others, consumers have received sporadic information about the complex world of cycling aerodynamics.  These aero gurus have demonstrated that positioning matters, wheels matter, and even handlebars matter in the pursuit of speed.  Clearly, there are factors other than aerodynamics, such as pricing, weight, and durability that will affect the purchase process; however, these topics are beyond the scope of this investigation.  The goal of this testing session that was financed by the author last January was to objectively determine how much, or even if, aero forks make one go faster via superior aerodynamics.

Test Samples

Three aero forks were tested and their properties were compared to a conventionally shaped 1996 Kestrel EMS fork.  The True Temper Aero with its unique Truspeed coating (roughened sandpaper-like surface), the Oval JetStream with its dual element blade legs and the more traditional looking Reynolds Aero were tested.  The Kestrel EMS, Reynolds Aero, and Zipp 404 wheel used for testing were borrowed from acquaintances of the author, while the Oval and True Temper forks were supplied by the respective manufacturers.


Figure 1 .  Fork terminology and description of tabulated geometric data.

The key aerodynamic design variables of blade chord length to width ratio (L/D), frontal area, side area and blade separation distance at the crown are tabulated below.


Table 1 .  Relevant geometrical quantities.

To purchase the updated version of this geometrical table, which includes data for the Easton EC90 Aero fork and the Cervelo Wolf TT fork one can purchase the results in our new online store.

A quick comment on the issue of UCI legality is necessary.  Unless one is a professional that competes in UCI sanctioned events, the 3:1 L/D ratio limit imposed by the UCI is unimportant.  Both True Temper and Oval claim that their forks are UCI legal, while Reynolds does not make this claim about their aero fork.

Test Methods

Nestled in the small university town of College Station , Texas , the Oran W. Nicks Low Speed Wind Tunnel has an undeniable reputation as the place to go for cycling related aerodynamic research.  With a test section that measures seven feet wide by ten feet tall, adequate flow qualities, and dedicated cycling fixtures, this facility is convenient for industry outsiders while still providing quality data.  Tunnel director, Jorge Martinez, has claimed that the wind tunnel balance will provide force data that is accurate to +/- 0.05 lbs.  While not as accurate as is desirable, it is still good enough to identify trends and shed some light on a variety of cycling specific aerodynamic questions.

All testing was done using a wheel attachment fixture developed by the tunnel staff and an external consultant.  Two streamlined struts allow a wheel and fork combination to be attached to the tunnel force measurement system (also known as a balance).  This setup is as realistic as one can get while still minimizing the variables in the experiment.  It could have been possible to mount an entire bicycle, complete with brakes, etc., but these additional components add to the noise and repeatability of the experiment.  For simplicity and to minimize experimental variables, a single rotating Zipp 404 clincher wheel (with a Kevlar 700x20c Continental Ultra 200 tire inflated to 110 psi) was used as the test platform.  This type of setup allows for a comparative evaluation while still respecting the most important of “real world” concerns.


Figure 2 .  Experimental setup at Texas A&M low speed wind tunnel.

A small electric motor spins an inch and a half steel roller that, under a light spring force, ultimately rotates the wheel.  Wheel rotation is matched to the wind tunnel speed using an infinitely adjustable speed controller and a speed sensing system that is very similar to modern day cyclecomputers.


Figure 3 .  Close-up of wheel strut and motor.

In order to insure that all forks were tested identically despite differences in steer tube length and diameter, a cover was placed over all steer tubes.  A round, ten inch long piece of 1 ¼” inner diameter PVC pipe was roughened and installed prior to the forks being tested.


Figure 4 .  Steer tube fairing and measuring the head tube angle.

One of the benefits of testing bike parts in a wind tunnel is the ability to simulate different wind conditions independent of other variables.  A word that may be familiar to some is the term “yaw” which is a carryover from the aerospace industry.  Yaw is the angle the bike velocity vector makes with the relative wind vector.  For example, if there is no wind and the rider is traveling 25 mph, the yaw angle would be zero degrees.  However, if the same rider experienced a pure crosswind of nine mph the yaw angle would be 20°.


Figure 5 .  A top view of the experimental setup and definition of the wind-axis terms (drag, side force) as well as the body-axis term (axial force).


Figure 6 .  Side view of the fork test configuration and head tube angle.

In order to evaluate overall aerodynamic performance, it is essential to measure the sample’s properties over a representative range of yaw angles.  It was assumed that the appropriate range of yaw values for cycling is 0° to 20°.  There are certainly occasions in which the yaw angle exceeds 20 degrees, but it can be argued that these situations are rare.

Another terminology issue that needs to be discussed is that of lift, drag, side force, and axial force.  The force of interest for determining overall cycling performance is the axial force, or the force that opposes the direction of travel.  Drag is the force that acts in a parallel direction to the wind vector, and is only applicable when referring to the wind-axis coordinate system common in the aerospace industry.  Lift is the force that acts in a direction perpendicular to drag (usually “up” when thinking about an airplane wing).

In cycling, lift is not important, since forces in that direction do not affect performance.  Of importance, though, is the side force, or the force that tends to make the bike steer to the left or right.  If anyone has ridden aero wheels on a windy day, they have experienced the adverse affects of the side force component; too much side force and the bike will become difficult to control.

It is the ratio of side force to drag that helps determine the overall aerodynamic efficiency of wheels, forks, or any component on a bicycle.  This ratio is the fundamental principle behind the often referred to “sail effect” - it is theoretically possible to have a propulsive resultant force which allows the bike part to “sail upwind”.  In order for this to occur, though, the side force to drag ratio must be greater than one, and the flow must remain attached for the extreme high yaw angles.  Simply put, an overall higher ratio of side force to drag means that the axial force (the force opposing forward motion) will be lower – and a lower axial force means higher speed for a given power output.  If one wants the gory math, it can be shown that the axial force is derived as follows:

Faxial = Fside*sin(yaw°)+Fdrag*cos(yaw°)
Equation 1 .  Coordinate transformation from wind axis to body axis.

The discussion of axial force and drag may seem like a debate in semantics, but it should be re-emphasized that the variable of interest when it comes to determining overall cycling aerodynamic performance is the axial force.  Drag is only directly applicable when the yaw angle is equal to zero.  For this reason, fork data will be reported using axial force and not the more familiar quantity of drag.

Results & Discussion

A little over six years of fork development has netted the consumer approximately 0.15 lb less axial force at 30 mph, which corresponds to a wattage savings of 9 watts or around 25 seconds for a flat 40 k time trial.


Figure 7 .  Average axial force values over the 0° to 20° yaw angle range (measurement error +/- 0.05 lbs).

It should be clear that all three aero forks offer a measurable improvement over the traditionally shaped fork.  The Reynolds and Oval forks perform nearly identically, with the True Temper fork performing just within the uncertainty of the measurement method.  The above results can also be presented slightly differently in order to better differentiate the aero fork products.

When the data is summarized for crosswind (yaw of 10° to 20°) and calm conditions (yaw of  0° to 10°), the Reynolds and Oval products are still extremely close; however, the difference at larger yaw angles between these forks and the True Temper fork becomes slightly more apparent.


Figure 8 .  Axial force averages for calm and crosswind conditions (measurement error +/- 0.05 lbs).

Yet another way to look at the aero data is to determine the efficiency of the forks in reducing the axial force at yaw.  The higher the side force to drag ratio, the more efficient the fork.  Again, it can be seen that all of the aero forks offer an improvement over the traditional fork.


Figure 9 .  Average side force to drag ratio for 0° to 20° yaw angles (error +/- 0.07).

The same trends in tunnel acquired side force to drag ratios can be seen in the geometric area ratios of the forks.  This observation is consistent with aerodynamic theory since it has been shown that drag is proportional to frontal area and side force is proportional to projected side area.  In the chart below, the ordering of geometrical area ratios is seen to be an accurate predictor of tunnel performance.


Figure 10 . Geometric area ratios of the forks correlate well with wind tunnel results (+/- 0.1).

Of particular interest in comparing these two plots is that the wind tunnel results show a smaller difference between forks than do the geometrical results.  This is almost certainly due to the dominating influence of the Zipp 404 wheel on the overall aero performance of the combination.  However, the fork still does play a role as can be clearly seen in comparing the Kestrel data with the aero fork data.

How can these results be translated into information that is useful to mortals unable to ride consistently at 30 mph?  Below is a table summarizing time savings and wattage savings (relative to the traditional Kestrel EMS fork) at different speeds.  It should be noted that the greater time savings at slower speeds are a result of the aero benefits occurring over a longer duration of time.


Table 2 .  Typical wattage and time savings for several different average speeds on a flat time trial course (note: minor variations are due to rounding).

The data above appears to be consistent with aerodynamic theory.  The aero forks have more streamlined blade shapes than the traditionally shaped fork.  The result is a larger side force to drag ratio and, therefore, a lower axial force.

Summary

If one takes their racing seriously, attention to detail is important.  Cyclists need to train their body, their position, and then they need to start thinking about what equipment to use.  In the absence of wind tunnel data, a good approach to determining the aerodynamic performance of a fork is by comparing area ratios and/or blade length to width ratios.  An aero fork is a bit further down on the list of equipment that one should address, but if a championship is on the line, the fork under your nose matters.  Aero forks work.

Last Updated on Tuesday, 23 February 2010 04:50  

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