Motorcycle aerodynamics, a slippery and windy, slope?

[sendler]

Originally Posted by Grant-53

An ideal set up has equal spacing and no duplicate ratios.

Not exactly. It is better to have a couple gears overlap on either side of the front chain rings. Front chain rings are more difficult to shift and you would not want to constantly HAVE to shift the front just to get the next higher ratio if there was a good chance that you would be soon going right back down.

[fidalgoman]

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.

[Frank Lee]

It is amazing how some weighty cargo in a Microbus transforms it from skittish to rock-stable.

[sendler]

Originally Posted by fidalgoman

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

I had a vw van for a service vehicle once. It was the most dangerous vehicle on the road due to way too much sail area in front of the front wheels. In any snowy conditions, the wind could push the front end to blow you off of the road.

[Grant-53]

I have been riding triple crank bicycles for thirty years and spend an inordinate amount of time plotting gearing patterns. Shifting patterns are always a consideration. The modern index shifters are amazing when adjusted properly. My current bike is a 24 speed Jamis Aragon. Normally I use only the middle front chain ring on level ground. The idea is to have the ranges overlap with out duplicating ratios.

[sendler]

Originally Posted by Grant-53

The idea is to have the ranges overlap with out duplicating ratios.

I'm not trying to be awnry but no bikes come this way. Not even close. If they did you would have to shift the rear 7-8 times (on a 9 speed rear) to get from one end of the cog to the other, every time you shifted the front. This is still obviously not the case even with the new 2X10 mountain bike drive trains. It is more normal to expect to shift the rear maybe 2 cogs lower when changing the front 1 ring higher in order to get the next higher ratio. Lots of overlap is normal.

[Grant-53]

I am not sure we are speaking on the same wavelength. We are getting off topic for this thread. My old Ross Gran Tour was set up with a half step plus granny: 28-48-52T crank gears and 14-28T 5 spd rear cluster. The other end of the spectrum is my 2000 Jamis with 24-36-46T and 11-28T 8spd indexed shifter. Both the Kreidler 50cc race bikes (15 spd) and heavy truck transmissions (12 spd) are worth examining. Plot ratios on semi log scales.
I am looking forward to seeing your streamlined CBR 250. Wish I was well funded enough to try streamlining an electric scooter. I must settle for a 23cc 2 stroke friction drive motor on a full fairing bicycle. Trouble is that it would not be street legal in NY or Ohio. A bicycle fairing would be 7/8 the size of a motorcycle fairing.

[Grant-53]

Let us have a moment of silence for the poor folks blown off the Tampa Sunshine Bridge in their VW microbus in the 1960's. Not so much the area forward of the front wheels
(12 in.) but the area ahead of the CG with the rear engine and no down force.

We are looking to manage airflow and the resulting forces in various scenarios. The cross section radius and pressure forces relative to the vertical and horizontal CG are key to the stability versus response set up. The desired lean angle will affect any spoilers or canards. Down force increases control at the cost of drag. I am looking at the design of vintage glider designs with elliptical hulls (e=0.7) and a modest (4 degrees) negative angle of attack. The ground clearance will be higher on the bicycle, likely 12 in. versus 4-6 inches on a motorcycle.

[Hugh Jim Bissel]

Originally Posted by PeterS

12 or 14 speeds? That's expecting a very narrow power band but the easiest way might be 2 x4 speed bike boxes in series ...messy though .

I've envisioned doing that sort of thing: taking a bike and replacing the engine with the engine and trans from a Ninja or CBR 250 to have a small displacement engine with 2 transmissions in series. I always assumed it would be shifted one box at a time though: ie. both boxes start at 1, shift through the first box and when it's in top gear start shifting through the 2nd box. Adjusting the final drive ratio and/or ratio between the boxes correctly should give you super high gears for cruising at highway speed with low rpm and high load/throttle opening, while still allowing a low 1st gear for starting on hills.

And in regard to fully inclosed fairings stopping on angled pavement, my thought was to use 2 front wheels close together that lean, similar to the piaggio 3 wheeled scooter, with some sort of friction lock between the two front suspension arms to lock the wheels relative to each other so the bike is held vertical at a stop, even if the road is angled.

They both work good in my head, at least!

[renault_megane_dci]

Do we need this much gear reduction ?
The 250 crowd is switching from 1/3 final to close to 1/2 already without particular issues in 1st gear.
Unless you were to change to a diesel engine (and then the motorbike gearbox sounds like a good idea) I can not figure out the need for two gearbox in series on a motorbike engine ...