Streamlining Question

[donpearsall]

On a motorcycle, since you have to move the frontal area mass of air (the flat plate equivalent) that comprises the bike, engine, radiator, rider, bodywork, etc anyway, does it make sense to also streamline the front tire with fender, narrow tire, etc?
 
 It seems to me that since the front tire is within the frontal flat plate area anyway, streamlining is redundant.
 Since I don't have a wind tunnel handy, what are some opinions and real life experiments?

[hawkwind]

Don, frontal area is only half of the equation ,the other part is Cd and this is effected by the shape of the object ,so when you add them together you get the important CdA ,the best shape for subsonic objects is the classic tear drop with the (pointy end at the rear ),a "dustbin" fairing coupled together with a "boattail" rear will give you the best Cd ,also be aware the best gains for a M/C will come from a boattail rear ,the dustbin is the icing on top    
 Gary

[donpearsall]

Thank for the reply, Gary. I do not know what a "dustbin" fairing is, though.
 
 And for a boattail rear, you mean like a canoe with the pointed rear, or a boat, with the truncated transom? I am assuming you mean the canoe shape.
 Don

[StraightSix]

The classic dustbin fairing as on the legendary Guzzi V8:
 
    [img width= height= alt= - ]http://www.mgcycle.com/Mg-v8.jpg[/img]

[k.h.]

72 hp, 178 mph
 
 Moto Guzzi V8, Cycle magazine, 1972:
 
 "For 1956 an increase in the carb sizes of one millimeter, and an increase in the exhaust pipe diameter of two millimeters, and raising the compression ratio to the highest ever at just over 10:1, put the power to 72 bhp at 12000 rpm. This certainly wasn?t the maximum power point; the crankshaft and big end life dictated a 12000 limit."
 
 Campbell and the eight took control at the Belgium Grand Prix at Spa. Campbell simply smoked off the Gilera and MV teams along the long Masta Straight; Campbell stacked one lap record on top of another until the mark stood at 118.5 mph; on the Masta Straight, the eight made believers out of those manning the speed trap; the bike whistled past at 178 mph. Then it stopped. A battery wire had broken.
 
 The end came swiftly. In October 1957, Moto Guzzi, together with Gilera and Mondial, pulled out of racing. Lomas knew that 1958 would have been the "payoff year." The V8 would have gone into the next season armed with more than 80 horsepower. "A new carburetor system had been designed; each carb had its own integral float chamber. This added six horsepower due to better fuel distribution. Coupled with other small mods, the power went up to 82 bhp at 12000 rpm." But Guzzi?s withdrawal cancelled further development. 1958 was the year that never was. Lomas, so intimately tied with Moto Guzzi?s racing efforts, retired. "And everything was packed away."
 
 Too bad.  Mix modern materials and an enclosed rear section, who knows?
 
  <small>[ May 25, 2005, 05:27 PM: Message edited by: k.h. ]</small>

[peter stevens]

Don, there is one problem with the 'dustbin' type fairing and that is side wind stability. That's why such fairings are not allowed in present day circuit racing. What this means is that if the motorcycle encounters a side wind the machine can be pushed off course. This is because the center of pressure on the side of the bike tends to be forward of the center of gravity, an unstable situation. The cure is to fit a long gently tappering tail fairing, this moves the center of pressure to behind the center of gravity, a stable situation. Good luck with the project.
 Peter

[panic]

The John Player Norton team discovered that if the leading surface of the conventional fairly was shaped properly it would create a "plug" of still air, which enlosed the front wheel at speed. Whether this added area for the fairing lost more than the reduced Cd I don't know.

[whitworthsocket]

Don, frontal area is only half of the equation ,the other part is Cd and this is effected by the shape of the object ,so when you add them together you get the important CdA ,the best shape for subsonic objects is the classic tear drop with the (pointy end at the rear ),a "dustbin" fairing coupled together with a "boattail" rear will give you the best Cd ,also be aware the best gains for a M/C will come from a boattail rear ,the dustbin is the icing on top    
 Gary

Don I would like to add to Gary's teardrop suggestion. If memory serves me right. I think the ratio of the teardrop is length vs width. Is a ratio of 8:1
Regards Whitworthsocket

[panic]

The transom effect in aero is if the length of the tail at a good taper angle is so long (or the skin friction so high, etc.) to be impractical.
An effective alternative design is Kamm, where the small taper (14° included) extends back until the cross-sectional area is reduced by 50%, then ends with a flush (vertical, or normal to the tail axis) panel, with no radius, blend, etc. - must be a sharp edge. Shorter tail or more taper = less effective = more drag.

[bak189]

Gentlman and ladies, a couple of small corrections and observations...........................
The main reason that Dustbin fairings were "outlawed" for circuit racing......was not really due to cross winds (although it did factor in) it was due to the fact the fairing retained the exhaust fumes within the fairing.....putting the rider to "sleep"..........................................................
Also....a boat-tail fairing look great in the wind-tunnel......but on the 1973 Can-_Am record bike
it was not rideable past 80mph............................

[hawkwind]

Bob ,any Idea why the can-am tail failed ,didnt they only try it one time and had a speed wobble ? and due to time restraints did not try it again ,then again look at that wonderfull machine Wilhelm Herz used in 56 an NSU 110 HP 500cc 210 mph dust bin up front ,boat tail out back ,no stinkin PC insurance /safety concerns back then 
cheers

[panic]

No ifo but my first suspicion is lift - the flow top vs. bottom wasn't what they anticipated.

[Blue]

Don I would like to add to Gary's teardrop suggestion. If memory serves me right. I think the ratio of the teardrop is length vs width. Is a ratio of 8:1

The lowest drag for a subsonic "body of revolution" is achieved at a fineness ratio of ~2.5:1.  The lower the fineness ratio for a given frontal area, the lower the wetted area.  This reduces drag until we reach a point where some part of the flow separates.  In cars and aircraft ground interference and other artifacts (wings, tires, parachute fairings, etc.) interfere with having such a steep pressure recovery as exists at anything close to a 2.5:1 ratio. 

The best we've seen in aircraft is about 3:1 for the forward section of a sailplane fuselage and 4:1 for a practical general aviation airframe (Questair Venture).  The lowest drag for a faired strut is achieved at a section fineness ratio of 3 or 4:1.  Wing sections are thinner at 6 to 20:1 because the need to generate lift compromises the short pressure recovery.

From my own theoretical view of a dustbin fairing, it appears to be simply too wide and too big too far forward.  The best low-drag two wheeled vehicles are the fully enclosed endurance bicycle competitors.  Because these bikes are recumbants, their thickest point is well aft of the front wheel.  This allows for a long laminar run along the fairing of more than 50%.  Closure angle is not constant, and in fact the key is a reflexed pressure recovery:  a steep closure followed by a more gradual angle.  Trailing edge "bluntness" should be limited to less than 1/2% of the overall length. 

Despite running at much lower Reynolds numbers where drag should be higher, these bikes have CdA's of <.03 compared to the best LSR motorcycle streamliners at .05 to .08 (Wheeler/BUB-7/Ack).  The same principles scaled up to LSR motorcycles would yield CdA's of .02 to .03.  Since all 3 of these bikes have gone ~350, imagine them going 495 on the same HP and tires...

In absolutely no case should there be a blunt parachute or push bar at the rear.  The base drag this causes is greater than the skin friction for most of the current LSR designs.

[gazza414]

Some rear ends to look over



and the ACK

[Blue]

It seems to me that since the front tire is within the frontal flat plate area anyway, streamlining is redundant.

One other point, drag is created by an object exchanging momentum with the air: "dragging" air along with it.  Since the front wheel assembly has features and area that the air has to flow around and against, it makes drag whether it is followed by a fairing or not.

People in LSR hate me for this, but stop thinking of drag as related to "frontal area".  It is convenient to think of vehicles "punching a hole" in the air as a concept, but it's wrong. Subsonic drag is separation, stagnation, pressure drag, and skin friction.  With good design we can eliminate the first two and minimize the third.  CdA is useful to represent separation, stagnation and pressure drag, but is crude and misleading when used to quantify skin friction. 

In the next decade, high-separation designs common in LSR and the production automotive world will give way to low separation designs whose drag is limited to skin friction.  CdA, even as a concept, will have to give way to the more specific skin area and section Cd's used for most of the last century in aeronautics.