Aerodynamic Front Wheels and Steering Stability

posted Mar 26, 2010, 5:07 AM by Donald Vescio   [ updated Sep 12, 2011, 8:11 AM ]

Frequently, I’ll hear a rider say that he/she can’t ride a deep front wheel during windy conditions because it is too hard to handle.  While I certainly can identify with the concern of being pushed across the road because of a sudden gust of wind, it is during these windy conditions that the advantages of deep aero wheels really become apparent.  There are two considerations that need to be factored when selecting front wheel choice during windy days:  the concept of “center of pressure”; your relative handling skills. 

Center of pressure really is a simple concept.  Think of a cross wind that is exactly perpendicular to you  and your bike as you ride down the road; now imagine the location/point on your bike that receives the greatest side pressure (eg, that catches the most crosswind) from the wind.  The center of pressure is directly related to the surface areas of your wheels.  If you have more surface area in your front wheel than your rear, the center of pressure moves forward; if you’re running a disc wheel in back and a relatively narrow profile rim in front, the center of pressure moves backward on the bike. 

Now, where the center of pressure is located has a big impact on the handling of your bike.   The front wheel of your bike pivots on its axis, while the rear wheel only can rotate in plane with the frame.  What this means is that lots of wind can hit your rear wheel without having any noticeable impact on the handling of your bike—in fact, one can use a rear disc even in near-gale conditions without worry.  Because the front wheel pivots, a rider has to continuously adjust to side pressures from the wind, constantly adjusting steering to match the force of crosswinds. 


Franceso Moser on an hour record bid.


The easiest way to shift the center of pressure to the rear of your bike is to run a deeper profile rear wheel than front, with running a rear disc being most optimal.  As long as the rear wheel profile is deeper than the front, the impact of crosswinds on steering is can be lessened; the greater the difference in profile between the rear and front wheels, the more stable steering (i.e., the further back the center of pressure) will be.  Factor in that deeper profile wheels tend to be faster than shallower profile wheels in crosswinds, it makes sense to run as deep of a front wheel as possible to maximize aerodynamics.  If one wants to run a deep front wheel, then use a disc to take advantage of not only its aerodynamic qualities, but also its positive impact on shifting the center of pressure away from the front wheel.   

The whole center of pressure concept is great when one is dealing with a constant velocity crosswind; very quickly, the rider can adjust the side pressure of the wind and ride as normal.   Problems arise, however, when the wind is gusty, or if the rider happens to be hit by a crosswind when passing by roadside buildings, fences, a passing vehicle, etc.    The pressures associated with wind gusts are considered transient—in other words, that appear and disappear suddenly.  Because transient phenomena are difficult to anticipate, there is a natural tendency for a rider to over-correct when the front wheel is pushed by a sudden gust of wind.  It is these over-corrections that are the fundamental causes of front wheel instability and handling problems. 

To put this another way, deep profile front wheels are difficult to handle in gusty crosswinds because riders do not anticipate changes in pressure on the front of their bikes and because riders don’t take advantage of a rear disc’s stability properties.  There are two ways to remedy this problem:  you can run a shallower front wheel that is less impacted by cross winds; you can work on your technique.   Assuming that you are already running a rear wheel that is deeper than your front, the former strategy will provide greater steering stability at the loss of aerodynamics, while the later will enable you to realize a significant aerodynamic advantage over your competitors.  All that it takes is practice—ride relaxed, float with the changes in the wind, and try not to dramatically correct your steering when a gust of wind hits you.



Fabian Cancellera running a deep front wheel.


For an interesting discussion on center of pressure and bike handling, see John Cobb’s article at: 

http://www.blackwellresearch.com/pdf/tech_articles/Steering%20Stability%20Explained.pdf

 

 

 Other articles by John Cobb are below.

 

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Donald Vescio,
Sep 12, 2011, 8:11 AM
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Donald Vescio,
Sep 12, 2011, 8:11 AM
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Donald Vescio,
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Donald Vescio,
Sep 12, 2011, 8:11 AM
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Donald Vescio,
Sep 12, 2011, 8:11 AM
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Donald Vescio,
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Donald Vescio,
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Donald Vescio,
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