Aerodynamic Stability

Aerodynamic Stability

We can all remember the first time we pushed the pedals of our bike with a newly upgraded high-end wheel set.  It was a pure speed sensation in comparison to the wheel sets we had replaced.  Going from a high-end “non-aero” wheel to a high-end aero wheel is an equally thrilling experience.  The first time you ride an aero wheel in a road or TT bike is a special moment because you can’t believe how fast they feel. The performance improvement over a standard wheel is very significant.

Yet once someone has ridden a super-fast wheel set for five minutes, attention invariably turns to the stability of the bike through varying cross winds.  To date, most aero wheels are sold on the basis of their axial drag performance.

Traditionally, a deeper wheel gives lower drag through increased side force, so there is more load on the side of the wheel, which the rider needs to counteract with steering force.  At a certain wind angle the airflow will separate, and then the steering force has to be reduced making it difficult for the rider to maintain their desired line.  Often the condition becomes unstable and takes several steering inputs to get the bike back on line, which can potentially be very dangerous in a TT or triathlon.  This instability can cost valuable time when you need to come off the extensions to stabilize the bicycle.

As a result, before development began, the Smart ENVE team established an overall objective of developing wheels that optimized aerodynamics and stability.  The team established a process to put a metric to the level of stability of the bike in a crosswind.  Thus, the ENVE Stability Index (ESI) was born.

ENVE Stability Index (ESI)

This is by far the most important variable, and it has taken extensive research to define a wheel shape that is both fast and stable. All three wheel systems have a very similar level of stability, and are optimized for speed and ride-ability.

Traditionally, wheel testing in the wind tunnel is done in steady state; where the wheel is held for a period of time at a constant angle.  In the real world wind gusts, gaps in the hedgerows and buildings will cause instantaneous changes in wind angle.  The science of wheel stability has led us to look at transient aerodynamics or unsteady aerodynamics.

In fact, it’s only in recent years that Formula One has started to look at transient aerodynamics.  Smart’s experience in F1 aerodynamics actually started when he was a trackside engineer and the racecar did not produce the down force that it did in the wind tunnel.  This led to an extensive correlation study to understand the differences between steady state wind tunnel data and transient real world aerodynamics.
 
The team’s research in the wind tunnel showed that the wheels with poor stability had a non-linear steering torque response curve.  So when the wind angle fluctuates by more than a few degrees at a given yaw angle, the variation in steering torque can be quite high.  In certain cases its necessary to reduce the steering force (away from the wind) and this tends to make the rider feel unbalanced and often oscillate and lose the desired path of travel.?

The team’s key conclusions were:

• Within reason riders could handle a build-up in side force on the wheels by shifting their weight around slightly on the seat and turning the wheel into the wind so that it felt stable.
• Having some steering force build up was preferable to a totally neutral steering force in a crosswind.  However, this needs to be limited so that the difference in steering torque is not too high in very gusty situation where the wind angle changes.
• The biggest problem for all the testers was steering stability and the unpredictability that most of the test wheels demonstrated.  To have total confidence, the wheel needed to be very predictable so that required changes in steering input were minimal.

From this, the team set out to establish the following performance metrics and objectives:

All wheels should ideally have a linear response to steering input vs. steering angle.  In the real world this is very difficult to achieve so the metric was defined which related to the linearity of the curve.  The Conventional wheel has an index of 0.27; ENVE/Smart aimed to get our products as close to 1 as possible.

ENVE Benchmark Test at 50 KPH

Steering Torque in Nm at  50kph vs. Yaw Angle (degrees) Apparatus drag removed, data normalised to 50 kph. Front hubs only.

ENVE Steering Torque Gradient (ESG)

To explain the concept of steering input it is necessary to understand that a gradient of zero would represent a wheel with no force change.  However, we found that riders wanted some steering force so that they could turn into the wind and “balance” rather than have a wheel that tended to wander.

Most riders liked the feeling of increased steering torque with wind angle.  But if the gradient was too high, then when the wind switched from 15 degrees to 0 there was a large difference in steering torque.

Some of the difficult to handle wheels had gradients of 0.15 NM per degree.  The team decided to limit the ENVE steering gradient (ESG) on the wheels as follows, so that the stability performance in gusty conditions for each application was met.

Graph 1 (Steering Torque (Nm) vs. Yaw Angle (deg.))

The Conventional wheel exhibits a very non-linear steering torque vs. yaw angle response. This is a wheel that we field-tested and all riders reported stability problems. A perfect stability gradient is 1.0.

Wheel Type	ENVE Steering Gradient	ENVE Stability Index
Popular Road Wheel	-0.027	0.602
Popular Aero Wheel	0.022	0.060
Smart ENVE System 3.4	0.067	0.988
Smart ENVE System 6.7	0.089	0.981
Smart Enve System 8.9	0.108	0.977

The Smart ENVE Systems

Rather than selling a huge range of depths and leaving the consumer to decide which wheel was appropriate for the application and conditions, the team wanted to minimize the wheel choice and make it as simple as possible for the consumer.  The wheel range consists of:

Smart ENVE System 3.4 – Multi Purpose Wheel System (now shipping)

• Light and stiff as any climbing rim.
• Shallow steering gradient and excellent stability so can be ridden in any wind conditions.
• Main category of use: Sportive, Novice multipurpose wheel for road, tri and TT, Professional /Amateur racer in hilly terrain, and for use in extreme weather conditions.

Smart ENVE System 6.7 – Road Racing/Multi-Purpose TT/Triathlon Wheel System (now shipping)

• Fastest wheel set when installed on a bike in the mid depth category.
• Stability Index suitable for amateur and pro riders in all wind conditions, and novice on calmer days.  Sufficiently stable to be used all day long in TT/Triathlon endurance rides.
• Best weight and stiffness in class.

Smart ENVE System 8.9 – TT/Triathlon Wheel System (available in 2012)

• Fastest wheel set when installed in a TT frame.
• Higher steering gradient for TT and triathlon. Usable by TT and experienced triathletes in all but the windiest conditions.
• Class leading weight and stiffness.