Technical Q&A with Lennard Zinn – Do split fork blades give an edge?

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By Lennard Zinn

Predictor-Lotto star Cadel Evans came within a scant 23 seconds of winning this year’s Tour de France and narrowly held on to second place, a mere eight seconds ahead of third-place Levi Leipheimer. It’s fair to say that Evans’ podium performance is thanks in part to his two breakthrough time trial rides.

Evans placed second in both long time trials, behind Alexandre Vinokourov in stage 13 and behind Leipheimer in stage 19. With final time differences that slim, it is interesting to look at small time contributions among the top three riders overall, especially in long time trials, where there is no teamwork or drafting to regroup the riders at the end of the stage.

To say that Evans’ TT results were breakthroughs and pivotal in his overall placing is a bit of an understatement. By comparison, the Aussie placed 11th and eighth in the long time trials of the 2006 Tour, ultimately placing fifth overall. While some would argue that Evans can credit his 2007 second place overall to a 10-second time penalty Leipheimer got earlier in the race, he was in the position to benefit and to almost be on the brink of outright winning the Tour on the penultimate stage thanks to the sum of a number of small contributors to his performance in the time trials.

Evans’ TT position has certainly improved, as has his equipment choice. I’ve discussed the former here, and it is a piece of the latter which I’d like to discuss now.

The top riders in the tour have very similar TT equipment, from the skinsuit and aero helmet to the aero bar, aero frame, deep-section front wheel and disc rear wheel. However, Evans had one thing on his bike significantly different from the two Discovery riders he ended up sandwiched between overall, and that was his Oval Concepts Jetstream fork, which instead of one fork blade on each side of the wheel, it has two on each side, separated by a thin, 9mm-wide gap.

The Lugano, Switzerland, company holds a number of design and utility patents on the JetStream fork. In the past five years, four different riders using the fork have chalked up five grand tour victories and Evans came within a hair’s breadth of a sixth this year.

In a wind tunnel in 2006, world-renowned aerodynamics expert Mark Drela, Ph.D., Professor of Aerodynamics and Astronautics and a Terry J. Kohler Professor of Fluid Dynamics at MIT in Cambridge, Massachusetts, performed aerodynamic tests on the Oval JetStream fork. Drela found a greater than 20 percent reduction in front wheel drag at a bike speed of 50 kph (31 mph) for the JetStream’s twin-aero-blade design over the best possible single blade design.

Drela compared optimized aero shapes for both single- and double-blade forks and studied the drag effects of the fork blades at varying distances from the spokes traveling at double the bike speed as they do on the top of the front wheel. This “spoke velocity = double bike velocity” contributes to making the top of the front wheel a primary mechanical drag component on a bike.

Drela’s study found that the best single-blade shape pushes about 40 percent of the air hitting the front of the blade inside toward the spokes (Figure A). This additional air hitting the spokes increases the drag of the wheel by 15-20 percent compared to in the absence of a fork blade (Figure B). While aerodynamics might improve by moving the fork blades farther away from the spokes, the consequent stiffness loss and weight gain would be undesirable.

Drela found that the JetStream fork’s twin blades have quite a different effect. Again the first blade encountering the air pushes about 40 percent of the air inside toward the spokes. However, the positioning and shape of the second blade and shape and width of the gap between the blades create a vacuum effect, pulling air through the gap between the blades and away from the spokes (Figure C).

Thus the spokes encounter less air when passing between the fork blades and therefore create less drag. The air sucked outward through the gap joins the air pushed outside the first blade where it forms a laminar jet stream behind the fork, further reducing wheel drag. Drela discovered that this flow difference results in the JetStream fork not only creating less of an increase in the air drag on the front wheel but actually causing there to be less net drag on the wheel than if there were no fork at all! The Jetstream fork reduced the drag created by the spokes between 8 and 12 percent compared to if there were no fork blades present! (Figure D)

The overall effect of using a JetStream fork vs. the best conventional single blade fork is a 20-30 percent reduction in the drag created by the front wheel. It is mere conjecture whether that kind of a reduction over 109km of time trialing could make the 8-second difference over Leipheimer, but it seems reasonable to conclude that it could do that and then some.

Below is a listing of the JetStream fork’s success in Grand Tours over the past five years:

Other significant victories in 2006 using JetStream forks include Michellie Jones’ 2006 Ford Hawaiian IronMan World Championships and five other IronMan distance events worldwide.

Zinn’s VeloNews.com column is devoted to addressing readers’ technicalquestions about bikes, their care and feeding and how we as riders canuse them as comfortably and efficiently as possible. Readers can send brieftechnical questions directly to Zinn (veloqna@comcast.net)Zinn’s column appears each Tuesday here on VeloNews.com.

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