aeromoding shaft drive motorcycles

[redyaris]

I have been making slow progress on the Honda VT500. I have mounted a new unertray and am working on the ducting to the radiator. There is a lot of work to do and I will be clearing lots of stuff out of the garage for the winter to keep the progress going.
I have also put the Suzuki GS500 back together and went for a 313km ride averaging around 60mph and got 78.4mpg [196miles/2.5gal] I did some engine off coasting down hills using the engine kill switch on the handle bars. With the clutch pulled in coasting down hill the first thing i notice is that motorcycles don't coast that well when compaired to cars. rear wheel sprocket still drives the transmission and the aero drag is still comparitevely high.

[Varn]

My Virago 700 can go about 80 mph on 400mm^2 of throttle opening, that is a 20 mm carb. My carb has a 34mm throat. It is a Vm34 mikuni mechanical slide carb that I adapted using a manifold. I have been using it for about 3 years not the pair of vacuum carbs that were factory. It gets 50 mpg two up at 65 mph.

Mikuni slide carbs are nice because you can get all the jetting easily.

If I were looking for maximum economy I would consider using a single or twin 20mm carbs on fairly long intake manifolds. Having a longer intake tract might have a better torque at cruising rpm rather than at near maximum.

[dcb]

Originally Posted by redyaris

...rear wheel sprocket still drives the transmission and the aero drag is still comparitevely high.

I've noticed they coast pretty well at slow speeds, and hold their speed better coasting around a corner. But aside from horrible CD (but ok A), they also are comparatively light and so are arrested by drag more quickly when coasting.

[redyaris]

Varn The Honda VT500 has a 52 degree V that makes any thought about changing carbs complex due to space and shape. What you have done with your 700 Verago sound like something I would like to do... I'm thinking...?
dcb I agree; the lower mass of a motocycle generate less momentum [ M = mass x velocity] and has a larger Cd, which together yeald shorter coasting distances. I do recall that as the speed droped the coasting got better, specialy below 50km/hr [30mph], when coasting to a stop on the paved sholder. The problem was that there was just enough traffic to make that slow a speed ill advised, and my plan for the trip was to average 60mph.

[Varn]

I used pvc plumbing pipe that I cut and butt glued to the shape that I wanted then wrapped the contraption with carbon fiber and epoxy. I coated the inside with tank sealer. I braced the carb to the frame to prevent stress cracks. I am sure it can be done if you want it to happen.


Having an efficient running engine is a big part of economy. Make sure your stock carbs are rebuilt. perhaps you can increase the length of the intake runners.

I am not much for driving a motorbike slow. It is a good way to get run off the road.

Stock there were two carbs that were nominally listed as 40 mm. My 34mm slide carb has more performance. I can open the throat fully rather than having to wait for the vacuum to raise the slides on the stock carbs. Strange but true. Each cylinder doesnt draw at the same time so there only really needs to be one carb per two cylinders.

[redyaris]

Varn thanks for the information on how you did yours, I will no dought try somthing similar in the future. The task I am facing now is building the aero body work for the Honda VT500, and have it all ready for the Vetter Challenge next year in May. I am sure that it will take until March of 2012 before the project is ready to roll and then a month of debugging before the trip to Carmel to see which of the shaft drives gets the best mpg. The question we will answer is; can an aero-modded shaft drive motorcycle get into the 80 - 90 mpg Zone under "Vetter Conditions"?

[redyaris]

Low&Slow
I see your 90 day mpg going up slowly, so how is the project going?
I am working on the air inlet for the radiator on the VT500. What needs to be done is quit simple; duct the right amount of air through the radiator for effective cooling and minimum drag, it's the actual doing it that's the problem. I'm directing air that goes between the forks above the front wheel, down past the radiator front face into a chamber that should have a Higher pressure which will drive the air though the radiator. This little detail is taking days to get done due to the proximity of the front wheel and the rotating forks... I have also built a new under tray mount that is much more rigid than the older design. Next on the agenda is the rear license plate mount extension, which will be part of the support structure for the rear aero-bodywork.

[low&slow]

Hi RedYaris, I'm making slow progress with the Vision. I rejetted the carbs to see if it would run Ok with leaner jetting but the jury is out until I see if there is an FE improvement. It definitely doesn't accelerate as well as it did before the change. I trimmed back the rear of rear fender and it seems to help the sidewind wobbing. Next up would be mounting smaller mirrors and turnsignals. I also checked my valves and changed the sparkplugs. I seem to be getting 57-58 mpg pretty regularly and need a bit more to break through into the 60 mpg range on a regular basis.
all the best, L&S

[redyaris]

low&slow
I just looked back at my records for the VT500 and I was getting 56 mpg in 2008 with very little aeromodding. So It looks like I will need to make some major aero improvements if 80mpg+ under Vetter conditions is to be achieved.

[PeterS]

Originally Posted by redyaris

Game on!
2012 Vetter Challenge will be the day of reckoning, when we see whos' mpg is bigger. It would be quit impresive if we could get into the 80mpg range.
What I am thinking is that cars are being aeromoded with very good result without access to changing the final drive gear ratios. Most cars start with much lower Cd's around 0.4 - 0.3. the average motorcycle has a Cd = 0.7 - 0.8. BMW got one of there production bikes down to Cd = 0.4 - 0.5. I would not be overly surprised if we could get down to the Cd = 0.4 - 0.3, at which point we could be in the 80mpg range without a change in final drive gear ratios. It will take time with lots of trial and error, but it is posible.

Hi Redyaris ,
A new bloke here ,could you tell me how you derived the above drag numbers ? I had always assumed a normal motorcycle with sit up and beg riding position had a cd of around 1.2 .Do you have a link that would help me ?