1984 Craig Vetter High Mileage Contest
Laguna Seca GP, Monterrey, CA
Page 2 - More on Streamlining and Lessons Learned
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Streamlining - A Short Primer
A correctly-designed streamlined shape minimizes separated flow. The drag coefficient can be a factor of 8 or 10 lower than a blunt body for the same frontal area. If you can achieve a reduction in CdA (drag coefficient times frontal area) by a factor of 8, this equates to a reduction of power consumption by a factor of 2. Here's a link to the page I snagged this graphic from.
This is tempting! Who wouldn't want to move at exactly 1/2 the power required for the same speed?
But, these are wind-tunnel models, not real vehicles with wheels. Wheels pump air into the vehicle and can add up to roughly 1/3 of the total drag on a full streamliner. This is why people use covered or disc wheels, and design in ways to minimize air pumping in near the wheels. Note that air sitting still near the ground, if you scoop it up, has to be accelerated to the speed of the vehicle very quickly. F=ma. (force equals mass times acceleration). The additional drag force in picking up still air is the mass of the air times it's acceleration. The less air you pick up, the less drag. If you must pick up air, the slower you do it, the better. This is why you would be better off bringing in some air that is already moving along your fairing surface than just scooping some up suddenly-lower "a."
The separated flow in the above example is at a lower pressure than the air at the front of the sphere. This is a classic example of pressure drag. Pressure is force times unit area. If you knew all there is to know about the pressure in the front of the sphere and the back, you could calculate the additional drag just by the area and difference in pressure.
There is a bit of a trade-off to consider, because the added surface area of a streamlined shape has more skin friction. But, if separation is minimized, the additional skin friction is way less than the pressure drag.
The worst combination would be an attempt at a streamlined shape that has separation anyway, so you get pressure drag and more skin friction as well. You slow yourself down for nothing.
Full streamlining works, but it is important to ensure that air does not separate and minimize the air that is pumped in. (Then you have to think about breathing and keeping your motor cool-everything is a trade-off.)
Here are our attempts at full streamlining from college days. I think they worked pretty good overall. On the left, that is Marshall Chandler taping up the HPV. His responsibility was the fairing and mine was the frame and drivetrain. On the right, I'm the skinny kid in the fashionable shorts. I think I got maybe 2 haircuts since. :)
"Full" Streamlining vs Partial Streamlining, and the World's Fastest Mountain Bike:
I was really interested in Ken Miyako's design for the 1984 Vetter rally. He's got a full fairing, but it's not a "full streamliner." When the rider tucks in, the rider completes the shape. I wish I had a better profile pic, but you can tell enough from this pic. He's still going to get separation, but is it a good trade-off? It would seem so. He still got second with 345 mpg. If memory is correct, this was a Honda R&D entry.
I'm looking at this thing thinking,"My next project will be a 10-speed that uses this approach." I was actually so anxious to get this going, I never wrote the Fresno Yamaha Streamliner up for a senior project! (It didn't work out anyway, because it was during the summer break. Fresno Yamaha got the bike back before the fall semester-so bad plan-no senior project.)
When I got home from the Vetter Rally, I started drawing up a 10-speed with a frame-mounted fairing that was bigger than anyone was using on bicycles at the time, and a rear piece that looked like the back of Miyako's Honda. The rider would complete the shape.
First, the idea needed a wind-tunnel test.
Notice the end result! The rear piece is gone. It didn't work at all. Myself and two other students, Marshall Chandler (the guy that designed the black fully-streamlined fairing above) and Pablo Gutierrez talked our prof into letting us use this idea for a semester design project.
We made a wire model of a bicycle, wired on a GI Joe, and started testing shapes in a small windtunnel. The bigger the front fairing, the better it got. The rider's knees are where we ended up cutting the fairing. But, I could not make the rear fairing work. We dropped it.
This one was my favorite aero project. We made one for the ASME college races and even though we didn't have the sprint speed of the streamliners, our guy managed to do a 36 mph run on flat ground, no wind, in a 200 meter flying start sprint. There was also a road course. In practice, it didn't take long to see that we were going to kick everyone's ass, real bad. So, the race director banned it! What an a-hole!
A few years later, one of the IHPVA founders, Chester Kyle, knew about this fairing, and knew about a race across Australia for HPVs, anything goes. One of the riders that wanted to do this race across Australia got my number from Dr Kyle. Could we make one for a mountain bike?
Long story short-yes. Kent Riches from Air Tech got interested as well, and he used my mold and made a primo fairing out of Kevlar. The rider was Henry Kingman of San Francisco. Before Kingman's trip to Australia, he and I went to the IHPVA world championships in Yreka, CA, and he won two gold medals on this thing.
The reduction in drag was about 1/2 the drag of a bike without the fairing. This is roughly the same benefit that cycle racers get when drafting.
As an interesting reality check, the Yreka IHPVA event also invited a championship 4-man road bike team. They had full-on aero stuff-disc wheels, aero helmets, all that. They road a one-hour time trial and took turns drafting. Kingman on the aero mtn bike matched their time almost exactly. He didn't even bother with disc wheels, and just to mess with people, he actually taped cards in his spokes to make motor sounds. :) (And he was on a frickin mtn bike with fat tires!)
Why bring this up in an article about motorcycles?
1. It shows that, as a rough estimate, the benefits of a really good front fairing alone are half the drag of no fairing. This does not mean half the power consumption! That requires 8 times less drag (It's proportional to the cube of the velocity).
2. Don't just assume that a rear fairing works because it looks more like a full streamliner. It may be slowing you down. Good ideas still need to be tested.
Problems with Full Streamlining:
In theory, a full-streamliner works. In practice, there are issues.
Most of the top finishers at the Vetter rallies were technically not full streamliners, but some got pretty close and they were getting amazing results. They were building small bikes, even recumbants, to get frontal area down. This more than made up for opening the fairing up a bit. The Yamaha 185s squeezed into a Rifle fairing could get over 300 mpg, and I never got better than 200 on mine, it was way too big.
Note that Hannebrink's entry, which was both small and a full streamliner, totally kicked everyone's ass in qualifying. He has both a low Cd, and a low A. He got 577 mpg on Saturday.
A pic of Hannebrink's streamliner from 1983 that I snagged off Vetter's site. He's got the low Cd, and low frontal area. The next year, when he got 577 mpg in qualifying, the second best was 339 in a full Rifle fairing with an open top.
Since he is low to the ground and the side profile was much smaller than mine, I don't think he had near the crosswind problems I did. Additionally, I would estimate that keeping the thing sealed helps a lot in terms of not giving gusts of wind a chance to make sudden changes that you have to respond to.
Wrapping Up - Lessons Learned, and Some Tips for 2011:
-Streamlining works, but a bad streamliner is scary! My own streamliner was tall and had a high CdA, but it still got 200 mpg in testing at 55 mph. The fairing flopped around because it was a one-time use experiment that came apart in transport.
-I think the example of Hannebrick's shows that full streamlining can be made to work, but you end up with a tiny vehicle and you are tucked in low. Hey, if you want to win, you do what it takes.
-When you push the envelope in a competition like this, you lose some practicality. We found the same in our fully streamlined HPV by riding it on the street in Fresno. Our rider could literally time the lights and ride it across town and keep up with cars, but it was slow low to the ground, it was very, very dangerous.
-A lot of people have rear fairings as well as front, but due to separation and added skin friction, there is a chance that they are counter productive. Not everyone has a windtunnel, so there is a simple way around this. See "Tips" below.
-You can make the gearing changes you need to make with sprockets. If you need radical changes, go to an industrial supply and get some hardware and make your own.
-I got done too late and neglected finding a way to cover the wheels, oops.
-Making big fairings can be very time consuming. You can get the job done with Mylar and strips, but it's pretty much one-time use and it has some disadvantages too.
-A well-done front fairing for a partially faired vehicle is highest priority. The shape needs to minimize the chances for separation. Just because you have a lot of the bike covered, it doesn't mean that it is "aero" if the air separates. Note the faired mtn bike example above. That one reduced drag by 50% and a rear piece didn't do anything at all.
Tips:
-Finish your vehicle early and test, test, test. This is easy to say, and hard to do. But, it is critical. I totally got my butt kicked by the experienced guys at the Vetter rally. If you look at their mileage over the different years, they were able to improve a lot each year.
-Steal ideas from cycling. Cyclists don't use those long aero helmets and smooth riding shorts and jerseys for nothing. Unless you make a full streamliner, add a fairing to the back of your helmet, and get some smooth clothes.
-For a full streamliner, never cut the tail short thinking that you will reduce skin friction. The added pressure drag is worse! Of course, a tail that is just too long is a waste too. You need to know what the optimum is.
-Use covered wheels, and minimize flow into your vehicle due to the wheels. Consider ways to get the fairing as close to the wheels as possible, and solve airflow problems to the motor and rider another way. It is always better to bring air for your motor or for the rider in a place where air is already moving, not from where air is standing still (unless you like air braking). Take a peek at F1 cars, they have copied air scoops from aircraft and bring air in at the mid point. They don't scoop it up.
-Here is an easy and cheap way to find your improvement (or lack of) in terms of drag (CdA)-
If you want to compare your vehicle with or without fairing, or see if your rear piece is slowing you down, you can do it with a coast down test and compare numbers.
All you need is a hill with a steady grade, a calm day and patience.
If your hill has a steady grade, and you coast down it at a steady rate (terminal velocity), the component of gravity in the direction of travel balances with your total drag.
Rolling resistance is proportional to weight on the wheels and not speed, so for constant weight, this cancels out in a comparison.
If you find your terminal velocity with different shapes, the difference in velocity is due to aerodynamics only - assuming no other variable like dragging brakes.
To calculate the difference in CdA, you take the ratio of the squares of your terminal velocities.
Example-
Our 10 speed coasted down a hill at 29 mph without the fairing. (This was a long, steep hill!) With the fairing, it coasted down at 42 mph. I was the crash test dummy, this was fun!
The difference in CdA was 29^2/42^2, or .48 - basically 1/2.
This is about the same benefit a pro cyclist gets from drafting the bike ahead of him. Note that Henry Kingman pretty much verified this because he matched the speed of a 4-man cycle team that was taking turns drafting and he was on his own on a bike with the fairing.
We use CdA instead of Cd and A separately because there is no use in worrying about the frontal area differences in this case, you don't need it.
I haven't tried this with motorcycles, but it should work just the same. To keep it fair, the total weight of the bike needs to be identical each time, so if you take off 20 lb of fairing, add 20 lb of ballast, or the test is meaningless. If you try it and your numbers are too close to trust, do more tests, or find a steeper hill. The steeper the hill, the more the difference will be easy to see. Remember to make sure the speed levels off too, so gravity and drag are the only two forces working on the bike.
Last Word: Top Ten Reasons I'm Glad I Did the Vetter Rally Back in the Day!
10 - Big League. At least for me, it was an upgrade over the usual ASME stuff for engineering students. This wasn't just a student-level project, this was motorcycles, real stuff.
9 - Interviewing Ammo. When I was job hunting after engineering school, I had to send out the obligatory resume. You always hear that you need a hook to stand out. I would send out resumes and toss in a photo of my Vetter entry. That got attention every time. The HPV stuff would too, but those were team efforts that a dozen other people said the same things about.
8 - It kicked the Human Powered racing up a gear, and that just got better and better. I got back from the rally and went right to work on designing a 10-speed that borrowed ideas I saw at the Vetter rally. I wanted a pedal-version of Ken Miyako's Honda!
7 - Networking. A few months after the rally, there was a conference in Southern California (SAE, I think) and some of the top finishers were there to show slides and talk about their experiences. Vetter and Malewicki were there, and being a rally participant was like being one of the cool guys or something (except I am too humble for that, right!?)
6 - There is no substitute for getting out there-none. There is a saying in supercross/motocross that "results get results," which means "you have to win," but honestly, everyone that puts in a good effort and gets good numbers at an event like this gets results. There is nothing else like coming up with your ideas, doing the design, building it, getting it done, and getting to the event and participating. So what if the people that did it 4 times already kick your ass? Doing the Vetter mileage event was one of the best decisions I ever made. That was 27 years ago and here I sit still writing about it with the same enthusiasm I had when I did it.
5 - Spinoffs. We have all heard about how NASA's moon trip in the 1960s was worth the money and effort many times over due to spinoffs. On a personal level, the Vetter event was like that for me. This is mainly about the bike fairing. Once I drew up the bicycle I wanted to make after the rally, it became a semester design project and I brought in 2 of my buddies because it was not independent study. I got practical windtunnel time on something I was interested in and not just a boring fluids lab. We made our changes, got our bike built and ended up with something unique. The bike with fairing became our ASME club's second entry bike at the races for a couple of years. Best of all-
4 - I kicked Cal Poly's ass! To get credit for our semester design project, I had to do an 80-page paper on what we did, why we did it, what worked and what didn't. Our faculty advisor for this was the same guy that oversaw our ASME racing, and he wasn't about to give us a break because we had a head start on aero stuff and making vehicles already. Quite the opposite. He was a ball-buster and made us work even harder.
Once the paper was done, I signed up to present the paper at an ASME regional student conference. The deal is you get like 15 minutes to show slides and give a talk, just like a professional conference, but this has professional engineers as judges. It's a competition. Our worthy racing rival was Cal Poly, which was famous for its strong participation in hands-on projects. Our ball-busting faculty advisor was just as interested in kicking their butts as we were. Cal Poly beat us at the races, but at this conference, I got second and Cal Poly's guy got third-ha! Did it! I think I won some money too.
3 - More networking. I was on the planning committee for the ASME conference, because it was CSU Fresno's turn to host it. There was no question who we wanted as guest speakers, Dr Chester Kyle, a founder of the IHPVA and lead designer for Team USA's bikes at the 1984 Olympics in LA, and Doug Malewicki. They agreed. We were stylin. These two engineers were our role models.
We had a pinic at the conference, and I rolled out the bike with fairing that was inspired by the Ken Miyako Vetter entry. Kyle and Malewicki took it for a ride, and they honestly loved it. For a student engineer, it can't get any better than that, really. It would be like making a completely new motorcycle and Rossi telling you it's killer.
2- Copied! In fact, Malewicki himself went home and made one too! He made one that was a bit smaller and was going to hook up with John Howard, a famous cyclist. It was going to be named "The Outlaw" in French or something. It made a magazine or two but kind of fizzled after that, but I didn't mind. I thought it was cool. Kyle remembered it too and a few years later, when the HPV race across Australia was announced, Kyle was invited to go along as advisor and observer. One of the Americans that wanted to go was Henry Kingman, and he called Kyle. According to Kingman, Kyle asked him "Do you want to win?" "Of course I do." "Call Steve Bruhn and get his fairing." Kingman found me working in LA at the time, and I agreed to hook him up.
1 - Gold Medals. I still had the mold, but no part. I called Kent Riches at Air Tech, and told him about the race across Australia. He said bring the mold to his shop in Oceanside. Soon we had the best ever example of the fairing, made by the best motorcycle fairing shop-and this one was Kevlar. It was killer. For a warmup, Kingman and I went to the IHPVA championships in Yreka, and that was when he won 2 gold medals. He also made it to Australia, but that is a long story. Dr Kyle was there too, and he got lots of pics of the mtn bike with fairing in the outback that I never got to see. :(
Sorry for the ending that was more bicyling than motorcycle, but that is how things are. You just can never know how things turn out, and how just doing one event can mean so much over time.
I still have a strong interest in bicycle aerodynamics, but cyclists are notoriously stubborn. I took the bike and fairing in one configuration or another to some inventor's and bike conferences, and cyclists just turned their noses at it! "Looks heavy..." "I would never ride that..." I guess Craig Vetter probably heard the same things about his fairings in the 1970s.
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