Wheel Design How-To Part 1

I think it’d be interesting to write a little bit about what all went into a wheel design for Boyd cycling this past year. The rims which are currently in production I’m super proud of because as a Pro-level racer they’re a set of wheels that I’d choose over almost any other wheels, most of this design work took place in 2014. This will probably be broken up into several manageable parts, enjoy.

Design Goals

Arguably the most important part of any design is setting out the initial design requirements. This sets the ‘ship of design’ on course and becomes very difficult to change once you have sunk costs in a given design philosophy. Working with Boyd (who has extensive feedback of what cyclist want out of their wheels) we set out a number of goals to strive for: 

  • Aerodynamics
  • Wide Rim/Tire Compatibility
  • Clincher rim brake track heat dissipation
  • Tubeless compatibility

We weighted these components in a design matrix to help evaluate different rim shapes.

The primary improvement we hoped to achieve was to design an Aerodynamic wheelset. A secondary but closely related goal was to design a wheelset to accommodate a wider tire. Traditionally road cyclists use narrow road tires measuring 21mm or 23mm, however the current trend is towards wider tires 25mm and larger. The larger tire has numerous advantages over a narrow tire: better grip, improved rolling resistance and is less prone to puncture. The two main disadvantages are weight and aerodynamics. The goal of an Aerodynamic wheelset and a wide tire are closely coupled, especially for the increasingly prevalent clincher wheelset. This means that some of the negative aerodynamic effects of a wide tire can be negated with proper design of the rim.

Composite clinchers have, in the past, had a dubious reputation. The thermal conductivity of Carbon Fiber is two orders of magnitude less than Aluminum. Low thermal conductivity coupled with the low melting point of the resin used in the some Carbon Fibers meant that under heavy braking the large heat built up through friction was not dissipated as it is with Aluminum (where heat build-up is not a concern). A two fold effect then takes place: the heat in the rim is transferred to the air in the tire, increasing the tire’s pressure and load on the rim hooks, and the material around the brake track softens allowing the rim to open up. Since braking is always heavier in the front, this leads to a catastrophic blow-out. This is a relatively rare failure, but due to its catastrophic nature (front wheel, downhill, usually at high speed), has been a fairly singular deterrent for people purchasing a carbon clincher wheelset.

The final goal is achieving improved tubeless compatibility with the rims. A tubeless rim replaces a traditional tire/tube setup with a heavier duty tire with stronger bead and latex based sealant. Nearly every Mountain Bike rim sold today is setup without tubes due to their puncture resistance, lower tire pressure that can be used, and better grip. There has been much slower adoption among road users. This can be attributed to a number of factors including but not limited to: limited tire availability, limited tubeless ready rim availability, the unproven technology factor, and difficulty of setup. There are essentially no disadvantages to producing a rim that is compatible with a tubeless and normal tube setup versus a tube setup alone.



Bike Wheels and Ocean Creatures

On to part 2!!

Ground Boundary Layers

If I did not make it clear in the past post, I really enjoy new innovations in bicycling. However, there is a difference between some new kick-starter trinket that solves a problem in an innovative way, and a $4,000 wheel-set without any published data yet besides “It looks like humpback whales”.

Anyway, back to birds and whales.

The other important lesson to take from the Albatross doesn’t have to do with it’s wings, but HOW it actually fly’s 1000’s of miles without flapping:

Once over the open ocean, the albatross will perform a circular swooping flight to cheat the wind. Flying 30 or so feat above the ocean the albatross will dive down with the wind to the ocean surface gaining velocity from gravity and the wind. Once at the surface it’ll turn around right at the surface and head into the wind. Once angled towards the wind the bird then pulls up, using it’s high speed and incoming wind to climb back to it’s initial flight level. It’ll then repeat this process thousands of times to cross an ocean.  (or just watch the video below)

Albatross flight

The albatross takes advantage of the surface boundary layer to generate “free speed”. Wind speed is always lower the closer you are to the ground and this gradient is a VERY well known phenomenon. This is important because a large wind gradient close to the ground undermines the whole idea of minimizing crosswind drag. If you want a more exhaustive discussion of Atmospheric Boundary Layer check out This Post. In short , and in my humble opinion, the magical 15 degrees of cross wind that wheel and bike industry has adopted as the targeted drag angle to minimize drag for is WRONG!!!!!!!!!!!!!!!!!!!WindAngleIsTooDamnHigh

This high crosswind angle is Zipp’s ENTIRE marketing shtick. It’s fine to try to reduce drag at high angles of attack but every design choice you make when making a wheel, bike, plane, whatever represents a trade-off. Usually, high angle of attack drag reduction results in higher low angle drag, especially if you introduce drag creating turbulators, which is exactly what Zipp has done.

Now I will add, that between the testing that we did when designing our wheels we noticed that for near zero angles of attack, drag results were more insensitive to actual rim shape than at the magical 15-degree: 60mmcfd

These are results from a variety of the “best-case” 60mm rim shapes from our CFD runs along with some industry rim shapes (cough, zipp, cough). If you look closely the rims that perform best at high angles of cross wind tend to perform worse at near zero angles of attack (but results are all closer at the Zero point). I believe that in order to get a true 15-degree wind angle on your wheel you have to have near hurricane force winds and that most people would rather have a wheel that performs slightly better during the remaining 98% non-hurricane-force wind conditions. The cynic in me even would go so far as to say most bike industry has focused on 15-degrees simply because it’s the low point on the drag curve, the magical point where 90mm wheels get to the ‘negative drag’ point.

Scaling the Flow

When I posted the last post a friend brought up the important point of flow similarity between objects at different sizes and speeds. In Aerospace design its usually pretty difficult to fit a full sized aircraft or rocket into your wind tunnel. Fortunately flow is somewhat scale-able. Generally speaking if as long as Reynolds Number and Mach number of a flow is the same between say a full sized aircraft and a model in a wind tunnel.

So lets take a look at the various Re and Mach numbers for the 787, Albatross, Humpack Whale, and a Zipp 454:reynoldsnumberwheels
 What this tells us is that none of these are REALLY like any other. However it does tell us that most of the flows above are generally in the turbulent region (what this is is even highly dependent on the individual situation). If anything the bike wheelset is the one closest to the laminar region of flow.

Zipp modeled these fins off the ‘tubercles’ of a humpback whales and there’s some decent research on the topic. However, the question of the humpback whale brings up a few interesting questions and points. The paper does indeed show that flow separation on the flipper is delayed due to the tubercles. However, the tubercles are only found on the high Aspect Ratio (long) flippers of the Humpback. Faster whales like the Orca (ok litterally just learned Orca’s actually a dolphin) and dolphins have smooth contoured fins, additionally no birds that I’m familiar with have tubercles (although they do have more intricate flow control). So why do humpbacks have these features while other animals do not?figure-3-humpback-whale-megaptera-novaeangliae-flipper-leading-edge-tubercles-left

The other important question to ask about the tubercles is how it’s applied to the Zipp wheels. On the whales the tubercles are on the leading edge of the fin. With a wheelset, only a quarter of the rim is showing leading edge tubercles to the wind at any given cross-wind moment. The rest of the time, the flow that’s being turbulated by the Zipp fins is flowing off into the free-stream and not working to help flow remain attached. Also…um importantly…the humpback fins don’t rotate.


So the tubercles have to provide a large benefit in that small area to offset the detriment they cause around the rest of wheel.

Most good engineering innovations have sort of an ‘ah-ha’ sense to them where the design just sort of makes sense before you even look at any data. For instance, once the Wright brothers first solved the problem of powered flight there were dozens of  other non-wright airplanes in the sky within a few years because they quickly recognized that control/balance was one of the primary problems that the wrights solved and had a bit of an ‘ah-ha’ moment.

I personally don’t get that “Oh that makes a lot of sense” feeling with the Zipp tubercles. I don’t think there’s some special magical design that’s going to cheat wind. Of course I might be very wrong since I don’t know the actual numbers, and I’m just some stupid blogger on the internet…..but I do know you don’t see airplanes that look like this:Stupid Looking Airplane


Why Zipp Should Study the Albatross (or Oh Good Lord this….Thing)

Seriously WTF Wheelset

I’m going to break this up into a two part-er of why I think these aren’t all they’re cracked up to be. So here goes:

First off: I am a HUGE fan of biologically inspired design. Nature generally does everything better than we can. Case in point: wings. High efficiency gliders and recently developed composite passenger aircraft.

The albatross is (arguably) the most fascinating bird out there, it can fly literally 1000’s of miles without flapping it’s wings once. Half of the the equation explaining how it can do this are its wings:

Albadross WingsNow, aside from looking totally bad-ass, you’ll notice that the wings gentle curvature and tapered ends, which look remarkably similar to the new Boeing 787 Dreamliner:


It’s almost like a 747 and a sailplane had a baby: the 787. The reason why the 787 has such a funky wing shape is because the name of the aircraft design game is: efficiency. Now take a look at this wing you’d find on mondern-ish to old aircraft:6a4eabc2-1678-4a25-afc8-3feda38b05c0

You should notice two things: first, are the small nubs halfway down the wing, these are called ‘vortex generators’ and second, the wing sticking straight up at the end of the wing (insert Yo Dawg joke). These two devices try to accomplish the same thing: reduction of drag caused by lift (lift causes drag, which is a whole Aero chapter onto itself).

However these are not elegant solutions. The vortex generators are literally creating turbulent drag in order prevent stalls in the wing during take-off and landing, but generate a not insignificant and unwanted amount of drag during 99.999% of the rest of the flight.

The vertical wing-tip also reduces drag caused by blocking the swirling wing tip vorticies, HOWEVER the price you pay is you have to drag two of these vertical wings that don’t contribute to lift of the aircraft. Both of these devices reduce overall drag by creating more drag in a particular way, but the benefits outweigh the drag penalties of these devices.

Now comes Boeing, and more importantly the new carbon manufacturing techniques of the 21st century that allows ANY shape to be crafted with the structural integrity required for a wing. With exhaustive design and testing they came to the shape seen above which, lo and behold, looks an awfully like one of the most efficient wing designs crafted by eon’s of evolution. The wing essentially accomplishes the same goal as the wing tip and vortex generators, but without the massive drag penalty they incur.

Now, I would not say off hand that all the dimples and bumps put on a Zipp wheel make it less aerodynamic since I do not have any data to back that claim up. However, dimples and and bumps represent a trade-off: their mechanism has to benefit the design more than hurt it. Generally the trade off works like this: increased skin friction drag for lower pressure drag.

The golf ball is a great example, the dimples increase the surface roughness of the ball (increasing the skin drag), but reduces pressure drag. The reason why this works so well on a golf ball is because….it’s a ball….it has to be round. An aerodynamic rim on the other hand, has a lot more play room with regards to it’s shape, if you were to design the optimal golf ball, it’d look a lot more like a rain drop (it just wouldn’t be very hit-able).

This is why I think that things like these dimples or hump bumps (I’ve so named since the design seems to come from humpback whales), are somewhat gimmicky. Like Boeing showed, solutions can be found for complex flow problems without using these flow devices to ‘force’ the flow to do what you want it to do. I’m sure the micro-vorticies do help in the way that Zipp claims, but I’m skeptical if the improvements they generate are offset by their disturbance.

Great design in nature is elegant, smooth, and most importantly simple (because physics). We should all try to mimic it.

Stay tuned for part 2!


Last word on Aero Riding Positions

Ok one last post on the Super-Tuck. There’s some mis-information out there. I think everyone agree’s that it’s empirically faster, however great caution must be exercised when using it. Trying it out mid-race will endanger you and people around you. As with most things cycling related: practice first.

OK, warnings aside, onto some practical information.

Thanks to Hincapie Rider Mac Brennan for the following information from the A2 wind tunnel in Charlotte. 31.02a Left Side 33.0 Left Side35.0 Left Side 37.0 Left Side

As you can see there’s 4 positions: Drops, Hoods, Elbow’s on Bars, and finally ‘Super-Tuck’. One other note is that all positions were done while pedaling except the Super-Tuck….which is usually pretty appropriate.

Recalling the Drag equation,  CdA is a measure of how ‘aero’ something is and is directly proportional to the amount of drag generated by an object.

Power to overcome drag (30mph) CdA (m^2)
Drops 415w 0.28
Aero on Hoods 409w 0.276
Aero Postion tops 383w 0.259
Super tuck 279w 0.188

Recall, that CFD report that came out mid-TdF busting the Aero benefits of the ‘super-tuck’ claimed that the super-tuck was 0.6% LESS aero than a standard drops position. However Mac’s wind tunnel results show that Mac’s position in the Super-Tuck is 32% MORE aerodynamic than his position in the drops. I’m sure this delta is exacerbated by the fact that he’s pedaling in the drops…but still. Even if it’s only 25% without pedaling in the drops that difference is HUGE.

The answer to what to do with regards to the super-tuck is…….it’s complicated.

For instance We just did a Time Trial this weekend as part of the River Gorge Omnium. If you’re not familiar with the courses there are several relatively long downhills during the course of the 4 mile TT. We don’t have specific time trial bikes since we don’t really do many during the course of the year. Riding a standard road bike changes the type of ride you’ll have to do for a TT. On the main downhill that occurs mid-way through the race:


I mean yeah I’m 6 seconds slower than the eventual winner from this year’s TT: Aaron Beebe, but I’m also fairly certain I did less pedaling and probably recovered a bit thanks to coasting for a whole minute.

More specifically you can compare it like this: given the CdA values supplied by Mac above you get the following picture. SuperTuck Drag Profile at -5 gradient

This kinda paints a picture of what kinda gains you’re getting in the Super-Tuck. At 25 mph, for instance, the difference is pretty small and you probably should be pedaling. However the faster you go the more benefit you get. Hypothetically on a smooth, straight road going down a 5% gradient mac could do ~45 mph in the super tuck without pedaling, while the Aero Tops position (which is BASICALLY a TT position) he’d have to pedal at 340 watts, and in the Drops 445.

Bang for your buck wise, when I’m racing, I’m not pedaling over 35 mph. If you’re on a TT bike and strong enough to win a TT it’s a different story, but most crit racers are rarely in that situation.

However, if you’re mid-pack and you’re super tucked…..YOU ARE A MORON

**cough** looking at several riders at River Gorge Omnium, especially the kid with his jersey unzipped while he was super-tucking **cough**


The problems with the ‘Super-Tuck’

In the last post I did I outlined the benefits of the ‘Super-Tuck’. Now onto it’s problems. Like I said earlier looking at the benefits only takes into account one side of the dynamic bicycle system: Aerodynamics/Power. The second type of system you have to look at, which is the reason why you’re a freaking idiot if you use this position a lot, is control systems.

Every engineer is familiar with the following (unless you’re an Industrial Engineer *cough* *black sheep* *cough*)

This is a mass/spring/damper system. It’s a simplified version of A LOT of real world applications. Enginineers use this simplified model to figure out how to make cruise control, autopilots, heaters, generally anything where you’re trying to help control the behavior of a physical/electrical system. It’s essentially a mass connected by a spring and damper to some either stationary or moving anchor point.

If you apply this system to a bike/rider and your chubby ass is the block (mass – m), then your legs and arms act as both the spring and damper. When you go through a turn/rough patch of road, your arms and legs act to smooth out any bumps you ride over and the diagram becomes something more closely resembling this:

An Inverted Mass/Spring/Damper system with variable displacement

An Inverted Mass/Spring/Damper system with variable displacement

You can obviously get infinitely more complicated than this but the math and physics of the simple mass/spring/damper system remains. This type of modeling is very familiar to mechanical engineers due to its obvious applications to motor vehicles.

Now the problem with the top tube surfing position is that since you’re resting your ass on the top tube you completely you not only increase your spring stiffness by eliminating any bump absorption of your arms and legs, but you also GREATLY under-damp the system.

The example below shows a block with gravity acting on it with a starting position, the blocks are dropped and eventually achieve equilibrium.



The thing to notice about the under-damped system is how dramatic the OVERSHOOT is (that is the amount the blue block overshoots it’s equilibrium point). Anyone who’s tuned MTB suspension will begin to see the correlation now. When you setup a MTB fork and don’t have any damping, the fork will absorb the bump…..but then translate that displacement further up to your hands. This is great if you’re doing dirt jumps or something….not so great in the ‘Super-Tuck’.

In short, if you hit an unexpected bump in the road (cuz your foolishly super-tucking in the pack) not just your wheel/tire but everything will now go flying in the air.

This same effect will be seen in your steering system, albeit in a much more complected manner. However the end result will be the same: no ability to quickly turn to avoid hazard and any outside input to your steering will send you wildly off course due to your inability to correct the course.

Just watch the dudes at Rio tackle the technical descent into the finish:


No super-tuck-ing there.