5-Ball Racing Chapter 5

Rick Krost also sent me the following golf ball article, which basically explained how dimpling golfballs reduced drag and allowed them to fly faster and more efficiently. But it also pointed out how the shape of a wing far surpassed the round golf ball. Dimpling is not a factor, but on the other hand waxing a sheet metal surface to a slick baby’s ass smooth finish doesn’t matter at all. Here’s what David Freiburger said in his Hot Rod article: “Contrary to what you may read on your favorite message board, well-waxed, smooth paint is no more aero-dynamic than the worst spray-can, flat-black primer job you can imagine.”

Says Rick, “Here are portions of the dimple article:This was brought forth from my favorite aerospace website. It was authored by Jeff Scott. This helps to explain the aerodynamic effect of dimpling on spheres, or as we know it, 'the golf ball theory.' It's pretty brainy, so the pictures tell the story…. for those of us who don't buy Playboy for the reading content. I believe we should employ a combination of these theories on the Assault Weapon.

You will have to bribe me with El Torito before I spill any more beans…….”

–Rick

While few among us can deny that golf is one of the least exciting of all spectator sports, we aerospace engineers are fascinated by its aerodynamics! Even the non-golfers of the world are familiar with the shape of a golf ball, like that pictured below, and have probably wondered why its surface is covered with small indentations called dimples.

The dimples of a typical golf ballBefore explaining the purpose of dimples, we first need to understand the aerodynamic properties of a sphere. Let us start by looking at a smooth sphere without any dimples, like a ping-pong ball. If we lived in an ideal world without any friction, the air flowing around a smooth sphere would behave like that shown in the following diagram. In this figure, the angle q represents position along the surface of the sphere. The leading edge of the sphere that first encounters the incoming airflow is at q=0° while the trailing edge is at q=180°. A position of q=90° is the top of the sphere, q=270° is the bottom, and q=360° brings us back around to the leading edge. Note that in this ideal situation, the air flowing around the sphere forms a perfectly symmetrical pattern. The streamline pattern around the front face, from 270° up to 90°, is the same as that around the back face, from 90° down to 270°.

(a) Ideal frictionless flowfield around a sphere and (b) the resulting pressure distributionThe lower half of this figure also displays the pressure distribution around the surface of the sphere, as represented by the non-dimensional pressure coefficient Cp. Positive (+) values of Cp indicate high pressure while negative (-) values indicate low pressure. It is the differences between high-pressure regions and low-pressure regions that create aerodynamic forces on a body, like lift and drag.

However, this ideal flow pattern tells us something very interesting. Notice that the pressure at the front of the sphere, or q=0°, is very high. This high pressure indicates that the incoming air impacting against the front face creates a drag force. Nonetheless, the pressure at the back of the sphere, or q=180°, is also high and identical to that at the front. This high pressure actually creates a thrust, or negative drag, that cancels out the drag on the front of the sphere. In other words, this theoretical situation tells us that there is no drag on a sphere!

Early aerodynamics researchers were quite puzzled by this theoretical result because it contradicted experimental measurements indicating that a sphere does generate drag. The conflict between theory and experiment was one of the great mysteries of the late 19th century that became known as d'Alembert's Paradox, named for famous French mathematician and physicist Jean le Rond d'Alembert (1717-1783) who first discovered the discrepancy.

The reason d'Alembert's ideal theory failed to explain the true aerodynamic behavior of a sphere is that he ignored the influence of friction in his calculations. The actual flowfield around a sphere looks much different than his theory predicts because friction causes a phenomenon known as flow separation. We can better understand this effect by studying the following diagram of the actual flow around a smooth sphere. Here we see that the flowfield around the sphere is no longer symmetrical. Whereas the flow around the ideal sphere continued to follow the surface along the entire rear face, the actual flow no longer does so. When the airflow follows along the surface, we say that the flow is attached. The point at which the flow breaks away from the surface is called the separation point, and the flow downstream of this point is referred to as separated. The region of separated flow is dominated by unsteady, recirculating vortices that create a wake…

In the case of a golf ball, increasing the speed is not an option since a golfer can only swing the club so fast, and this velocity is insufficient to exceed the transition Reynolds number. That leaves tripping the boundary layer as the only realistic alternative to reducing the drag on a golf ball. The purpose of the dimples is to do just that–to create a rough surface that promotes an early transition to a turbulent boundary layer. This turbulence helps the flow remain attached to the surface of the ball and reduces the size of the separated wake so as to reduce the drag it generates in flight. When the drag is reduced, the ball flies farther. Some golf ball manufacturers have even started including dimples with sharp corners rather than circular dimples since research indicates that these polygonal shapes reduce drag even more.

Comparison of flow separation and drag on blunt and streamlined shapesThe reason we do not see dimples on other shapes, like wings, is that these particular forms of boundary layer trips only work well on a blunt body like a sphere or a cylinder. The most dominant form of drag on these kinds of shapes is caused by pressure, as we have seen throughout this discussion. More streamlined shapes like the airfoils used on wings are dominated by a different kind of drag called skin friction drag. These streamlined bodies, like that pictured above, have a teardrop shape that creates a much more gradual adverse pressure gradient. This less severe gradient promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.

However, there do exist other types of devices commonly used on wings that create a similar effect to the dimples used on golf balls. Though these wing devices also create turbulence in order to delay flow separation, the purpose is not to decrease drag but to increase lift. One of the most popular of these devices is the vortex generator.

So let’s get to work. I don’t have tools for sheet metal fabrication or the training, so I reached out to Custom Chrome for support and the initial fenders and tank to work with. They stepped up and are now a sponsor.

I started by mounting the CCI tank that fit like a dream and contoured to the frame. The tunnel was deep so it set right down on the top tube. I asked our front end maker if he could tell me how far back the tank needed to be mounted to clear his RMD-manufactured girder front end. He started asking questions. He needed the following:

”Let me give you a little more to go on,” Leo said. “Front end fitment for custom bikes is a subject that you should do a article on. To make a front end work from a performance standpoint or from a safe yet perfect stance application is a matter of the front end coming into the picture last. If you ever notice how the bikes built by David Perewitz or Matt Hotch (although completely different ) have that perfect stance yet drive just fine its because they have the front end built based on real dimensions after some parts have been accumulated and mocked up.

“Here is a rough overview of steps for good measurements to help determine front end length and trail. We will deal with this based on a rigid because that's what we are working on. Mount the rear tire assembly in the bike or set the axle height in the frame at the correct height based on the tire diameter (the tire diameter will change based on rim width). Then set the chassis to the desired ride height. At this point,” Leo continued, “you might want to check and see if the axle adjusters are running parallel to the ground.

If not, you should figure approximately where the axle would be, based on chain length and power plant placement. Now that you have these things in place, rake and neck height perpendicular to the ground are truly measurable. As I stated, this is a simple overview but it help with any front-end project.”

A. ground clearance from the bottom of the frame to the ground______________

That's 3 inches.

B. From the bottom of the frame to the center of the neck _______________

Directly down from the neck, it's 24 inches.

C. rake_______

34 Degrees as Leo requested.

D. front tire diameter_________

Right now it's 24.5 inches, but if we shave the tires, it will be less, by maybe one-half inch.

E. no brakes on the front____________

Yep, no brake. This bike ain't about stopping.

F. Approximate neck diameter at at the ends____________

That was approximately 2 3/16 inches.

G. picture of the fork stop______________

Here ya go:

H. distance over bearings in fork cups ______

No fork cups

If any of this doesn't make sense let me know

Some, but we'll figure it out.–Bandit

With all the data in hand Leo sent me a sketch and estimated that if the tank was 2 inches behind the neck I was good to go. Unfortunately, I still didn't have a front end. The Custom Chrome 5-gallon unit came with all the mounting tabs, rubber grommets and fasteners. I worked on spacing, centering and it was as if they built the mounting system and tank shape for this bike. She fit right into place. Some serious mods to the tank are forthcoming, but we needed the basics in place, quick.

To set the rear fender in place the wheel needed to be centered with a chain and sprocket linked to the Baker transmission. We needed the 1-inch Doherty wheel spacer kit which is a lifesaver, but I ran into a problem. I dug out all the transmission plates I had for Softails and none fit. I called Jason at Paughco.

“What are you nuts?” Jason said. “You can’t run a Softail plate.” I ordered the right one and we are working with Paughco on a short tech on what plates fit what. I still don’t get it.

With the proper plate in place, our BDL closed-belt primary arrived and we could bolt up our inner primary and guess about the transmission location. We didn't have an engine, so Rick Krost of U.S. Choppers loaned me a gutted ’48 Pan motor for positioning purposes. I could finally jump in and get to work, although I was squeamish about sheet metal fabrication, but I had no time to lose.

I went to work on positioning the rear fender. With the help of Paughco brackets I built the Nitrous bottle mount and a plate to lock the rear fender in place. Rick came over one afternoon and we burnt through an evening making a plate to house the rear fender and Teflon buffers for the long chain.

Then I plasma-cut the 1/8-inch chunk of steel to support the rear fender and the back of the seat for Valerie. From time to time we called Nyla out of her office to test the positioning, since she’s a similar size to Valerie. She jumps on the lift between martial arts classes and gets in position.

Jeremiah, who has his Shovel at the Bikernet shop for various fabbed parts, has put in a lot of hours working the rear right plate with me, and last night we poured over the arched chunk of 16-gauge steel for the seat backing plate section. This area will also house electric components. We made a tool for bending that plate and still screwed up. It took us a couple of hours to get it close. Wish we had an English wheel. Need more sponsors, but we’re moving forward.

Lot’s of work ahead. Here’s my list:

Neck Bearings installed and measured for Leo—done
Belly Pan—coming up
Kick stand—we still need to figure something out
Rear fender—done

Nitrous mount—done
Fuel pump mount
Nitrous equipment mounts
Pipes
Top motormount
Seat pan
Bars
Tank gauge mount, move gas cap

I gotta knock it off. I can’t handle the pressure. We’ve also been going back and forth with the mysterious Rodan regarding the regs. I’m confused half the time, and natch we’re trying to push the limits in the open bike class. Joel, another Bonneville fan has also been quizzing Rodan as to the rules and his recommendations.

Here are the answers to your questions from Rodan, a SCTA official:

” Rear fender, wheel showing: The minimum rear fender is a flat cover that extends from the seat base of your frame to a point just past the centerline of the rear axle. Like a chopper flat fender. Just has to protect your butt from the tire tread. The max you can have in the open class is a, 'seat or tail section,' kinda like a tt bike seat. It can't extend past the back edge of the rear tire, and it can’t cover the sides of the rear tire or rim to the rear of the centerline of the rear axle. You gotta be able to see the rear half of the rear rim behind a vertical line through the rear axle. Same deal with the number plates, they can't cover up the rear half of the rear wheel.

Nitrous bottles up front? Can’t cover 'em up with a slippery cover.

“You can build the instruments into the gas tank as long as they don’t become a wind breaker/fairing for the rider.

“No belly pan, but you might get by with a flat plate that is the bottom of the frame under the engine. No added aerodynamic stuff at all in open class, and no items that direct air around the rider or the motorcycle.

“Think open, hahah. Yes, you can run a headlight, up to 7 inch. [Good idea.]”