The Search for Fuel Economy

Pages from a Designers' Notebook

Jan.1, 2009

The goal: 100 mpg at 70 mph, into a 20 mph headwind, with four bags of groceries.
Chap. 24: Finally! Streamlining reduces fuel consumption by 20% @ 70 mph

Happy New Year

Preparing for a 288 mile freeway trip from Carmel to San Luis Obispo and back
Dec 30, 2008: Highway 101 between Salinas and San Luis Obispo, California, a freeway where the posted speed varies between 65 and 70 mph. We compared the performance and mileage of a stock Helix with a stock but streamlined Helix.

The conditions:

46-70˚ F. Windy. Highway 101 is always very windy.

The first 70 miles is at 65 mph: very strong 30+ mph headwind*.

The next 70 miles is mostly at 70 mph with 2 o'clock side winds.

The next 100 miles changed dramatically: Mostly posted at 65 mph with 3 o'clock side winds for 30 miles, 70 mph posted in no wind for 30 miles. The remainder of trip: 65 mph posted with increasing headwinds from 11 o'clock, becoming very strong at the end of the trip. This is typical for California Highway 101.

*I have ordered a wind anemometer so I won't have to guess about the wind speed.

September streamlining vs. December streamlining

The last time I was on this road, I got 48 mpg into fierce headwinds. Chapter 16. The streamlining was rougher then. Compare the differences around the headlight, handlebars and windshield, above.

The Helix is not still not completely streamlined. The left side is still open, the front wheel is still a scoop. The bottom has no belly pan. But, it is smoother than it was in September.

I was anxious to find out if these changes made any difference in mileage. My son, Zak, rode my stock, red Helix while I rode the streamlined Helix. Other than streamlining, both bikes are equal and un-modified. Altho Zak is bigger and outweighs me, my Streamlined Helix has 15 pounds of lead bolted in the nose. So we are probably very similar in weight.

Zak on the stock Helix: 52.3 mpg
Craig on the streamlined Helix: 64 mpg
Zak was to follow me, riding the same speed. At the beginning, when the wind was strongest, Zak could not go the posted 65 mph. Crouching made no difference. He simply did not have the power.

Later, Zak reported that he ran wide open all the time. Many Helix riders say they do this and the Helix doesn't seem to care. In a "no wind" section, Zak maxed out at 75 mph... the same as mine. In accelleration contests, the two bikes are exactly equal until around 50 mph when the streamlined Helix easily motors ahead.

Zak observed that the winds seemed to blow me around more than him. It is true... I guess I am getting used to it.

I did my best to ride the posted speeds - between 65 and 70 mph. I never had to be wide open, running only partial throttle. Once, to test things in the worst head winds, when Zak could not maintain 65 mph wide open, disappearing in the rear view mirror, I cranked it up to as fast as it would go, which was 75 mph. Easy. In the "no wind" section, I cranked it up again and it topped out at 75 mph.

Do these things have rev-limiters?

Before I left, I noticed that my front wheel was wearing on the sides with the recommended 26 PSI. It must be the 15 pounds of lead bolted up front. 32 PSI definitely made the steering feel more precise.

Bottom Line: 20% decrease in fuel consumption @70 mph

I find this encouraging but there is still much more to be done.

Apparently, 75 mph is all a stock Helix can do, whether it is streamlined or not.

Basic facts:

17.33 horsepower @ 7500 rpm or  12.92 kilowatt

15.16 foot lbs torque @ 5,000 rpm or 20.56 Newton Meters

Below are the corrected readings from my digital tach:

Helix Speed and RPM: My Helix accellerates at 5,980 RPM until about 50 mph. Then it goes:

50 mph: 6040 RPM

55 mph: 6640 RPM

60 mph: 7200 RPM

65 mph: 7800 RPM

70 mph: 8400 RPM

75 mph: 9200 RPM

Actual 64 mpg notes
Son, Morgan: 54.1 mpg - KLR 650
Carol: 59.7 mpg - BMW 650
By the way, I thought you might like the results of the two other motorcycles. We have mileage for only their last 100 miles. Morgan, on the KLR 650, got 54.1 mpg while Carol, on her BMW 650, got 59.7 mpg. These are all very respectable figures. I am still not sure if I would be better applying my streamlining to one of these engines.
Since San Luis Obispo is one of my factory locations from the 70s, many ex-Vetter fairing-wrights still live there. We had an unexpected happy reunion with some dear friends:
Of course, those are our two sons, Zak and Morgan. Charly Perethan, at the far right, won the Craig Vetter Fuel Economy Contests three times! He and his sons, also very accomplished motorcyclists, own and operate Parabellum where they design and make fairings of his design. Duane Anderson, a college buddy from the 1960s, is now a designer for Rifle Fairings. In 1969, Duane did all the metalwork on the original model which became the Triumph Hurricane of 1973.

Henry Tate is one of the smartest people I have ever met. He could - and would - build anything that was needed at Vetter. I am very proud of these people.

My son, Zak, is 26 years old... the age I was when I began designing the Triumph Hurricane so long ago in the summer 1969.

Charly knows motorcycle fuel economy. Of course, that is what we talked about. Charly recalled that after one of the Laguna Seca fuel economy contests, he rode his pretty little Rifle Streamliner home to San Luis Obispo (the same route that we had just taken) sitting up all the way and got 200 mpg!

Well, the US speed limit was only 50 mph... but I'd say we have a lot more we can do. Later, in reflection on our visit, I recalled something Charly had said to me so I dug it out:

Talking with Charly Perethian - mileage pioneer
Charly's bare Yamaha - 200 mpg
Charly's bike streamlined -300 mpg

This is a 100 mpg increase in mileage with the Rifle body streamlining. 100/200 represents a 50% decrease in fuel use with streamlining at the same speed and conditions.

My current Helix actuals are 12/52 or 23% decrease in fuel consumption with streamlining.

I'd say that Charly's 50% increase in mileage may represent the upper end of what is possible on a street legal machine. If a bike begins with 60 mpg, like the Helix, 90 mpg may yet be possible.... but probably no more.

I had hoped for more.

Interesting comments as a result of this post:

It turns out that there is another way to calculate the mileage as Stephen writes:

"The difference is trivial for your initial example but will become significant as you

achieve greater savings - 100% improvement = 50% savings. The math:

52.3 mpg to 64 mpg = 22% improvement

300 miles @ 52.3 = 5.7 gal

300 miles @ 64 = 4.6 gal

1.1 / 5.7 = 19% savings

200 mpg to 300 mpg = 50% improvement

300 miles @ 200 mpg = 1.5 gal

300 miles @ 300 mpg = 1.0 gal

.5 / 1.5 = 33% savings"

Craig says, "It is a good thing they don't pay me to do math"

Jesus said "The Truth shall set us free." The Truth can also be frustrating.

"Craig: Your Helix max. speed was 75 mine top is 79, i guess is because there are diferent engine powers. In Portugal (Europe), Honda say that the early models like mine (1990) got 21 Hp and the latest got 17 Hp at 7000 rpm and top speed 80 mph. Paulo"

Craig replies: My streamliner is a 1994 Helix. It gradually runs up to 9200 rpm. The point is, I want to slow the engine down because I believe it will burn less fuel if it runs slower. To run it slower, I must gear it up. I am pretty sure the headwinds exceeded 20 mph. I don't need to go 75 mph ... only 70. Running the engine slower should help burn less fuel because it will be producing less horsepower.

"Craig: You asked about the rev limiter on the Helix. While it’s not an ignition rev limiter, it does have a “speed” limiter by design, mostly due to the 10” rear tire. I am surprised you got 75 out of it as the highest I ever got was 74.

Craig agrees: There is no room for a larger rear tire.

After many years of following the Helix and seeking out performance modifications, here’s what most of “us” helix nuts have come up with. The ignition, engine design, carb, exhaust, tires, gearing, and everything else is designed for 70MPH. If you push over that, it may gain a few MPH due to down hill, tail wind, or in your case, enhanced AERO, but you won’t get much gain on the top end.

Craig replies: Approaching 75mph is like hitting a wall on both Helixs. It must be the CVT pully sizes. I am certain that if it was geared up, my streamlined Helix would run into the 80s... maybe 90s. But, I don't want to faster. I want to 70mph into a 20 mph headwind. That is all I need to ride anywhere in the US. I don't want or need more power. I have enough power. I don't need trick air cleaners, exhaust systems, or ignition. I want to burn less fuel which means producing less power. Less power is produced at a lower RPM. This is why I want to gear it up.

Is there something wrong with my thinking?

The HP of a Helix was documented here:

“For the next two hours, the Helix was the center of attention. Everyone hanging around that morning found an excuse to take a look at this strange sight. After all, when was the last time you saw a scooter screaming its little heart out on a dyno? After what seemed like countless full-throttle runs, Steve finally shut the Helix down and handed me a computer printout with the results. The Helix produced a whopping 13.2 hp. It would run up to 9000 rpm before power shut off. According to the dyno, in theory, the Helix could run 84 mph.”

Craig replies: The size of the pulleys must be the limiter. Sure would like to see a HP- torque chart for this thing

One comment…84MPH ain’t gonna happen except on a dyno <GRIN>. Eric"

Craig says: I agree, unless it is somehow geared up. I am certain my streamlined Helix would just keep on accelerating into the high 90s. You would be very impressed at - around 50 mph - how fast it motors by Zak riding the unstreamlined Helix.

Eric added later today: "I am sorry, I think I may have misspoke and created a misunderstanding. My error, please correct my statement as needed. What I mean is that everything on the bike is designed to MAKE the bike go 70MPH and no faster, not that you should go hacking stuff up to make it go faster. In fact, the Helix’s sweet spot is about 65 in my experience. The airbox, exhaust, engine speed, etc; everything seems to just settle down and purr at 65. As one approaches 70, the machine quickly gets out of it’s all inclusive designed “sweet” spot. It’s like the engineers said “Ok, the 10 inch tire is going to be safe up to approx 70MPH, so let’s ensure the machine will go that speed but no more.”

Craig says: You bring up a good point. I must admit that 75 mph on the freeway just "Seems" too much for everything on the Helix. 10" rear wheel at 75? That is smaller than the ones on my trailer and I would not pull it at 75 mph! I had a chance last summer to spend time with Dick Mann, who is very knowledgable about motorcycle dynamics. Dick thinks 16" wheels offer the best compromise. While bigger tires should provide a more secure ride, they require a bigger "Smiley Face" cut out at front for turning, making streamlining even harder. Compromises, compromises. This is what design is all about. I love it.
Talking design with Dick Mann

The 150cc Aprilia that I have, however, has a top speed that varies greatly with weight and wind. The Helix though, as previously stated, is approx 70MPH, regardless of wind, weight, etc. No more. That’s what I meant. And yes, I think you are on the right track, trying to make its 65-70MPH speed more efficient with superior aerodynamics"

Thanks again Eric

"Craig: Congratulations! Your point is well taken wrt/ saying it may be better to work on one of the other bikes for max MPG. As you know, the Helix and other scooters are inherently disadvantaged due to their CVT's which are relatively inefficient transmissions (slippage & heat build-up) compared to 6-speeds on the bikes you noted.

You may want to do a quick remount of your fairings on one of the bikes and see what she'll do. I bet you'll be pleasantly surprised. Gary"

Craig says: After I have solved all the streamlining problems I can solve, I will begin again with an EV bike, powered by my solar roof top panels. In addition, I would like to install one of Fred Hayes' Diesels in a machine. They begin at 80 mpg! Really.

"Thanks for the update, Craig! It really is something how your wife gets 60mpg on her bare Beemer and you get 64 on the deluxe rig. Then you had that old tallgeared bike that got 100mpg bare and 200mpg faired... I wonder about testing mpg at lower speeds and quieter conditions... Possibly these solutions are best for surface roads... ? onward, JP"

Craig replies... Lower speeds and quieter conditions can be served by mo peds and small scooters where they can easily eke out 100 mpg. I want this to be for the kind of riding I (we) really do: 70 mph, into headwinds with stuff packed inside. Therefore, this is what I am designing for.

"Awesome Craig,: Glad to know you took the time to test it, and even more glad to know your results were positive! Congratulations! Hopefully you can figure out a way to gear up the Helix... If you haven't already, I'd suggest buying the Helix Service manual and checking out the driveline diagrams loooking for any part that could be re-machined for optimal gearing...there's gotta be some part in the drive line that can be re-made to gear it down.... it may cost a pretty penny though.

Craig says: I have the manual... two gears are needed (gotta gear it up, not down)... very expensive

Here is my main question: Have you been pulled over by the police yet? I only ask because I am assuming you have that vehicle still registered as a stock Helix...Joshua"

Craig replies: Nope! The police, CHP, seem to love me! One CHP patrolman talked to me from his car mounted loud speaker on Highway 101 giving me encouragement! Actually he asked me where my wings were. Every California law official has smiled and waved. It could be because I try to stay within the speed limits...

"Hey Craig, You are having way too much fun! AND happy & healthy 2009 to you and your family. How do you intend to use the anemometer? Just have someone collect roadside data as you are passing by or on board the bike?

Craig thinks: Well, I am just going to use it to measure the headwinds. At the moment, I am just guessing what they are.

Need a weathervane at the same time to get true outside wind direction. Than have to do some math involving trig. If Zak or Morgan don't do that kind of stuff, I can help. Fairly simple spreadsheet. Just like any airfoil of a wing you will be increasing the overall drag coefficient as the relative angle of attack increases. Can't just use the wind component that adds or subtracts from the vehicles forward velocity vector. For a symmetrical airfoil (your streamlined bike) the lowest drag coefficient is always at zero angle of attack.

Arghhh says I! This is what I get from real engineers like Doug ( a dear friend, I should say) . Side winds on the freeways are no more problems than on any faired roadbike. We motorcyclists learned years ago how to anticipate road winds, winds being interrupted by cuts in the hillside, passing trucks, ect. The winds I would like to understand better are those we find in Hurricane Point in Big Sur: Where a sudden gust comes from one side (one never knows which side) and then comes from the other. Then they are gone in a second. No kidding! These conditions move any motorcycle. Recently, a big BMW blew into the side of an RV, scraping off his mirrors. Such winds tend to move my fully streamlined Helix as much or maybe more. Other than some weight in the nose, my best solution is to go thru Hurricane Point slowly. This gives me time to correct for any side movement. Of course, this solution works only if we already know those winds are there. Since I am a local, I know. The poor BMW rider did not.

It would be nice to have little windsocks always in place in such places because they blow RVs around, too.

As that cool video of the mini car dropping down into the angled tube demonstrated (for two points anyway) - drag will continuously vary all the way from zero to 180 degrees angle of attack. BTW - Who was that guy with the handlebar mustache?

Craig answers: That is my brother, Bruce. He did not believe that the round end produced least drag. He convinced himself and anyone else who sees the video.

Should also collect temperature and altitude data for the run. Can then correct all your data back to a standard day sea level condition. Oh boy - just realized that hot day vs cold day even more strongly affects engine power output. Not worth the complication to correct for temp affect on drag performance unless you have engine curves for various altitudes and temps like they did when I was in aircraft flight test. Fugitaboudit...

Craig says: Doug you make me crazy! Of course I apreciate all you have suggested and I thank you for paying attention. When will I find the time to do all these neat tests?

"Craig: I love the door idea!, much better than the outrigger or the device that allows handicapped folks to ride via pneumatically deployed stabilizer wheels I wrote you about, and it's simple.

Craig says yep... the flexible door is everything I hoped it would be. It is just a matter of time before I apply it to the left side and finish up the streamlining.

I've come up with a simple solution/idea that'd help you in determining cross wind direction while riding, what would you think about placing a short steamer along the front top portion of the nose, older aircraft utilized that low tech method for crosswind detection by placing one on the cowl not far from the windscreen, why wouldn't it work for you on the Helix? I had given thought as to your ability to see the streamers reaction to crosswind being the nose slopes down and away from you, the solution would be attach the streamer to a short rod to bring the into view, simple, low tech.

Craig says: I think such streamers would have to be mounted on a 20 foot pole out front to give me fair warning. Really, I can feel these little gusts... probably as quickly as I might see them. We need to design something that softens the effect of the sudden quirky sidewinds as described in the letter above. 15 pounds of lead in the nose does pretty good. What else?

I hope this helps in some way, and Happy New Year to you and Carol.


Thank you Robert and stay tuned. We will figure this out. there is a lot of talent here

"Have you done tuft studies to see how far back the airflow remains attached? Kevin"

Craig answers: Tuft tests are next. I already bought yellow tape and black yarn. Should make a good video. Stay tuned.

"Thanks Craig, enjoy reading about your progress, really looking forward to what you get once you finalize the project. Btw, my 2003 Suzuki 400 Burgman averages about 62 mpg, with plenty of zip to boot (over 90 mph), so that Helix engine/drivetrain might not be your most efficient choice. I also had a BMW 650 (the older one with the rotax engine), that one averaged 60+ mpg as well, and close to 70 if I babied the throttle, and also had loads of power to spare. So, it might be worth thinking about other streamlining targets once you are through with the Helix.

Course, none of these could hold a candle mpg-wise to my Honda CT90 I had as a teenager – pretty sure I was getting almost 100mpg with that one, although top speed was only about 50mph on a level surface. Now, if that bike were streamlined…..Mike

Craig points out: It is easy to get 100 mpg on a CT90. But my experience has been that it just does not have enough power to go 70 into a 20 mph headwind. I have a CT110 and it will barely go 50 if I make myself small. I don't want to make myself small. I want to sit up normal and be comfortable. Right?

I do appreciate your comments and there are a whole bunch more. But, if you are like me, this is too much reading with not enough pictures. Lets continue this on another page.

Happy New Year

Master Index to the Last Vetter Fairing Story

This page posted Jan 1, 2009

Updated Sep 9, 2012