Ducati’s Jurassic Tail

Admin · September 15, 2022

Behold Ducati’s new “stegosaurus” tailsection. Is it an aerodynamic evolution (devolution?) or a clever Italian smoke screen from Luigi Dall’Igna’s carbon fiber box of tricks? (MotoGP/)

Lately Ducati’s MotoGP team have tested an assembly of four vanes attached to the top of the rider’s seat back—their purpose unstated. The press has named this the “stegosaurus seat” after the small-headed late Jurassic dinosaur which had a long row of nearly vertical plates projecting above its spine and tail.

Two per side, Ducati’s vanes are angled outward in such a way that in a corner the inner pair are nearly horizontal while the outer pair are close to vertical. Each pair of vanes is arranged somewhat like the relationship between an aircraft’s deployed leading-edge slat and the main wing—to increase the overall ability to turn the direction of a flow during low-speed flight.

So far, commentary has identified at least three possible purposes:

A means of providing some in-corner rear downforce (think of how many riders who have, in the past year, complained of inadequate rear grip during corner entry). Suggesting that this is <i>not</i> the case: While the vanes that are horizontal in a corner produce downforce, those that are vertical will produce an outward force that rear-tire grip must overcome.<br/>
To act as guide vanes to convert the normal motorcycle wake of constantly shed von Kármán vortices into a more orderly central high-speed flow. This could be a step toward closing a motorcycle’s wake, thus reducing drag.
A red herring devised by the wily <a href="https://www.cycleworld.com/tags/gigi-dalligna/">Luigi Dall’Igna</a>, intended to cause the other teams to waste time and R&D resources discovering that the vanes have no purpose.

Related: Motorcycles And Their Boundary Layers

Several riders on both the factory and satellite teams have used the new stegosaurus tail. Shown here is “La Bestia” (Enea Bastianini) on the Gresini Racing bike. (MotoGP/)

Tending to support No. 3 is the fact that motorcycle seat backs, being so often in separated airflow, could just as well be left off altogether or even made as giant pine cones—all with little effect on drag. How do we know (or strongly suspect) airflow behind the rider to be separated?

First, we can still see riders raising their butts onto the seat back on fast straights. It has long been claimed that reducing the downward angle of the rider’s back can cause separated airflow to reattach, the rider adjusting position as he feels his leathers press against his back rather than balloon outward. Why do this? Many riders claim to gain 200–300 extra revs from the practice on the straight.

Related: Honda Working On Rear End Aerodynamics

Jack Miller on the Lenovo Duc shows off the vanes’ layout from above. Will his KTM be sporting them next year? (MotoGP/)

Second, observing a misty first 500 practice at Eastern Creek (Australia), I could see that the flow around several million dollars’ worth of 190 hp GP bikes produced turbulent wakes no different from those of much less pricey box trucks. Any motorcyclist who has closely followed such trucks has felt the right-left-right series of buffets—these result from vortices curling around the back of the truck, first on one side, then on the other. As soon as one such vortex grows large enough to encounter the flow coming past the other side of the box, the first vortex gets peeled off by it, a fresh vortex grows on the opposite side, and the cycle repeats. Each whirling vortex carries away energy that cannot be used to drive the truck or motorcycle.

During one of the many strange eras in Formula 1, it was the done thing to place little guide vanes on sticks (all right, struts if you must), spaced away from the car here and there. Something similar has been researched in the highly competitive trucking industry as a possible means of reducing drag (and therefore fuel bills). The idea: providing a bit of guidance from vanes-on-sticks in certain key places might replace wasteful vortex shedding with a more orderly wake.

Vanes-on-sticks disappeared from F1 as quickly as they had arrived; presumably the wind-tunnel people went off in a new direction. Or it may have been that the vanes worked too well, and the FIA’s tech people paid their inventor the deep compliment of banning them.

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