Motorcycle Aerobody Research

[sendler]

Originally Posted by visionary

Its from Kraig Schultz’s Deltabike site and it draws on some opinions of highly influential designers.
Delta Bike USA - Forge Design Competition A - Cross Wind Stability

The Delta 11 has an inherent design disadvantage in cross winds of an ultra low CG from the batteries. But he still seems to manage. The important thing to keep in mind is that the lift (side) of a fast moving vehicle in side winds is nowhere near where you think it is by looking at the static side CoP. And having it centered on the steering head may be better than having it further back, like an arrow, until you get up to salt flat speeds where the tires are barely contributing to the direction control.
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Is a big streamliner easier to blow off the stand? Yes. Is it worse in side winds on the road? Not necessarily.

[sendler]

I would have also thought the Rifle bikes would have a lower cd. Too short I guess. The other big surprise is the race bike at still way over .6cd. This shows that the front of a vehicle is way less important than the rear.
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[godscountry]

Do you remember BMW's concept vehicle the Simple ?I maybe off a little, but I thought it had a cd of 0.18.What's interesting is the body consist's of mostly flat panels.With some photographs and scaling it may be a good choice for your vehicle.It looks good and it would keep the cost's down.You could in fact build it out of aluminum honey comb panels and some Kevlar.I would suggest a couple of cross bars[chromemolly] with disconnects to protect your sides or if you use a door,incorporate it into the door or doors.

[aerohead]

Originally Posted by visionary

What’s it all about?

Motorcycles with extensive or “full” bodywork are rare beasts indeed. Informed opinion, let alone genuine academic research into their aerodynamic performance is hard to come by. I’ve started this thread in order to open the debate and deepen the pool of knowledge, as well as supporting my own project, which could benefit from better sources of information.

If you have knowledge of good sources please post links. Personally I have found very little real scientific knowledge to be available. I do however come across quite a lot of misinformation and misapplication of aero principles from other disciplines.

My “pet” hate is the oft cited “centre of pressure must be behind the centre of gravity” which applies to bodies in free flight and is not directly applicable to road vehicles. Yet this phrase is routinely applied by “experts” in motorcycle dynamics. Even if true, it must be seen through the complex relationship between front and rear tyre slip angles, which I have never seen mentioned.

There seems to be a lot of conflicting theories about sidewind stability and how a low drag motorcycle would be affected. Is it more, or less prone to disturbance than a normal rider seated on a naked bike, and if so – why? My personal opinion is that the critical factor is body height, since a lower body also has a smaller moment around the contact patches.

I have been looking at some youtube videos of small-scale wind tunnels, with a view to building a simple version solely to investigate these issues. Making provision for a balance instrument that measures overturning forces at various angles of lean and yaw. I have never seen a motorcycle in a wind tunnel at anything other than upright and “zero yaw”.

Since the accuracy of numbers is not critical, we only need to understand the relationships, would a small-scale tunnel purpose-made for this task bring any benefit? For those with windtunnel knowledge, what would you imagine a suitable design to be – just to get a feel for the principles.

*There is a very good SAE Paper that was published for the HONDA Hawk MC streamliner project which has heavy emphasis on directional stability.
*The Easy Riders' dual-engine Harley-Davidson remains the fastest bike I believe.It's body was tested at Texas Tech in 1990 and returned Cd 0.11.
*The E-Z HOOK MC streamliner is the fastest single-engine bike and its body was developed in the Cal Tech wind tunnel and was reported as the lowest drag of any wheeled vehicle ever tested there,at Cd 0.103 If memory serves me.
*Early 1950s (NSU,Moto-Guzzi,etc.) streamliners from Germany,Russia,and Italy in most cases had provisions for rider body re-positioning/weight shifting as an aid in crosswind conditions by way of body openings.
*The IHPV competitions and USFRA racing at Bonneville have limits for crosswind conditions on the course.I've seen cyclists in the weeds at Battle Mountain.They have no provision for weight-shifting inside the body shells and are at the mercy of gusts.

[sendler]

Unfortunately, these land speed vehicles are so far removed from daily drivers that very few of their design characteristics apply.

[DonBarletta]

Here's the AeroBike, made by some guy in the UK over the last couple of years.

AeroBike Enclosed Motorcycle

[Grant-53]

The lean of a body might cause a bit more lift from side wind than at vertical. Angle of attack affects lift and in turn traction. I would like to try using stretch material such as Lycra to allow a rider to put a knee down. It might be that the change in shape would reduce side lift and add a bit drag to steer into the turn.
Bicycles using front disc wheels have not worked in crosswinds. Trek has an article on its aero shaped tubing which is a 5:1 airfoil with a Kamm length to meet rule requirement.
Sport Rider magazine did an article on wind testing motorcycles in 2006. The racing rules limit the height of the body work behind the rider.

[Grant-53]

http://www.vintagesailplaner.com/Sheet1bis-9.pdf
This is the drawing of a vintage glider fuselage that I believe is a useful starting point for a motorcycle fairing. The number 6 bulkhead is about optimum for a Kamm tail. Zoom out to 25% size and rotate right (clockwise)

[3-Wheeler]

Originally Posted by visionary

....The figures for the rifle faired Vetter Challenge bike were a shock –Cd 0.3627.
This is way higher than I would have imagined for a streamliner.....

A Cd of 0.36 seems high to me as well, but several comments are in order:

1) The Insight has a advertised Cd of 0.25
2) At 35 mph, it coasts about 1/4 mile and ends up at about 20. This is an estimate so don't quote me on it.
3) My Honda VF500 only coasts about 1/8 mile and is not going 20 mph at the end. It has a custom full size fairing, and I would guess that it's Cd has to be closer to 0.8.
4) The car gets 40 more mpg than the bike in the summer (120 vs. 80)
5) The egg-shell fairings at the end of Craig Vetter's High Mileage Contest were good enough to achieve 480 mpg, so a motorcycle *can* be very efficient with fuel.

Jim.

[Jyden]

In 1928 a man named Ernst Henne bought a BMW R37, he then took it apart and thew most of the bits away. Once he had the bike back to its bare essentials he set about building one of the most successful record breaking motorcycles ever built.

All in all, Ernst Henne’s BMW took 76 world records with the bike staying in a state of active development for 6 years between 1929 and 1935. In ’29 he set the fastest ever recorded speed for a motorcycle at a blistering pace of 216.75 km/h, by ’35 Henne had developed the car to the point he set an all new motorcycle world speed record of 256 km\h. That’s fast by today’s standards and he did it all in a backyard shed before the start of WW2.

The BMW R37 had a twin cylinder engine in a boxer configuration, the total capacity was 749cc and the bore/stroke was 83mm/68 mm. It’s estimated that with the supercharger installed, the engine was producing 100+hp, a staggeringly high number in 1929 and significantly higher than almost any road car of the era.

Ernst Henne's Supercharged BMW R37 - (SILODROME)