Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Motorcycle aerodynamics, a slippery and windy, slope?

fidalgoman · 08-31-2013, 11:57 PM

I can tell you from experience that one of the most exciting near death experience's I have ever had was in a 1964 VW van which is basically a shoe box on wheels. A sudden side gust blew us from our lane past the oncoming traffic lane onto the shoulder of the road and the tires actually tracked on the rounded edge of the gravel shoulder at the edge of the river we were following. I put in as large a steering input as I thought prudent to keep from rolling the van the instant the gust hit. Point being even square boxes can have issues with side winds.

An airfoil shape has an aerodynamic center (AC) of reaction aprox. 25 - 30 % back from the leading edge on a true foil shape. That means the center of gravity should be equal or ahead of the AC. If it is not then the shape is aerodynamically unstable. The closer you get to the ideal the more stable it will be even if it's not perfect.

You will notice that Vetter's scooter has a long tail not only to make it slippery but to make it stable as well. The subject can be long winded but an airfoil "will" react with the wind more than a rough shape. It can be controlled to a great extent by balancing out the CG to the AC.

To keep the shape from reacting too much stall strips can be added vertically along the shape to trip the flow when the wind angle exceeds a certain acceptable amount thus limiting the side-wind effect. It will take some adjusting. Perhaps this is a bit too much information.

Actually the FIM's prohibition of true aero shapes has more to do with the high speeds attained from low drag shapes than instability. Early on some competitors used torpedo noses that covered the front wheel blending back in an streamlined form. The speeds were reaching scary limits for road courses even in the early days and the rule fix was you can make it look sleek as long as for the most part it's style only. Then the speeds went back down.

Now just about all racing places limits as technology increases, be it engine size, down force in cars or anything else that allows the efficiency to produce too much speed for the drivers reaction time. In a low powered street vehicle this is not going to be much of an issue. Like at least one poster illustrated you want to keep the sail area in front of the CG as short and small as practical.

08-31-2013, 11:57 PM	#212 (permalink)
fidalgoman EcoModding Lurker Join Date: Jul 2009 Location: NW Montana Posts: 89 Thanks: 6 Thanked 21 Times in 19 Posts	I can tell you from experience that one of the most exciting near death experience's I have ever had was in a 1964 VW van which is basically a shoe box on wheels. A sudden side gust blew us from our lane past the oncoming traffic lane onto the shoulder of the road and the tires actually tracked on the rounded edge of the gravel shoulder at the edge of the river we were following. I put in as large a steering input as I thought prudent to keep from rolling the van the instant the gust hit. Point being even square boxes can have issues with side winds. An airfoil shape has an aerodynamic center (AC) of reaction aprox. 25 - 30 % back from the leading edge on a true foil shape. That means the center of gravity should be equal or ahead of the AC. If it is not then the shape is aerodynamically unstable. The closer you get to the ideal the more stable it will be even if it's not perfect. You will notice that Vetter's scooter has a long tail not only to make it slippery but to make it stable as well. The subject can be long winded but an airfoil "will" react with the wind more than a rough shape. It can be controlled to a great extent by balancing out the CG to the AC. To keep the shape from reacting too much stall strips can be added vertically along the shape to trip the flow when the wind angle exceeds a certain acceptable amount thus limiting the side-wind effect. It will take some adjusting. Perhaps this is a bit too much information. Actually the FIM's prohibition of true aero shapes has more to do with the high speeds attained from low drag shapes than instability. Early on some competitors used torpedo noses that covered the front wheel blending back in an streamlined form. The speeds were reaching scary limits for road courses even in the early days and the rule fix was you can make it look sleek as long as for the most part it's style only. Then the speeds went back down. Now just about all racing places limits as technology increases, be it engine size, down force in cars or anything else that allows the efficiency to produce too much speed for the drivers reaction time. In a low powered street vehicle this is not going to be much of an issue. Like at least one poster illustrated you want to keep the sail area in front of the CG as short and small as practical.