Why I kinda hate NAHBS

First of all let me say I love NAHBS, the bikes that are showcased there aren’t as much bikes as they are pieces of expression. Each one as unique as the builder that made it. They’re pieces of art made in the medium of the bicycle. I personally take the bikes as that.

I love these kind of bikes, my favorite bike is steel, but the fact of the matter is that no matter what you do, there are limitations to the metal bikes. There is good reason why top cyclists all race on carbon, why all new aircraft are almost 100% composite, etc, it’s just better for the application. If Eddy Merckx were racing today (for some reason we seem to romanticize that era in an Amish sort of way), he would be on a carbon bike from Asia. These craft bike thrive in areas where absolute performance of the frame isn’t as important, Cross, city bikes, Fat Bikes, etc.

NAHBS is absolutely part of the larger “craft” movement. And it’s interesting where “craft” industries really thrive. There are some cases where you pay more and legitimately get a better product, like food and beer. These industries make more practical sense because large producers use cheaper inputs (ingredients, processes, etc) to make a uniform and inexpensive product. There are other cases where you’re not necessarily getting a better (functionally speaking) product, but something more unique and artistic, like the stuff at NAHBS. You’re paying just as much for your bike to be produced with cheaper materials and older production techniques, not because you’re interested in functionality, but because you want the bike an expression of yourself.

Once carbon production has become easier and cheaper, you’ll start to see more ‘craft’ carbon shops coming into the NAHBS sphere as well. This is because steel or aluminum isn’t the heart of the NAHBS, it’s just the medium. It is relatively easy for an individual to master the art of building a metal bike, which means that bike is the individual’s creation and expression, thus most bikes at NAHBS are metal. Carbon bikes are more complex.

It takes a team to build a carbon race bike, not an individual. This means that an individual’s vision must match up the other people working on the bike, there is not yet room for much expression. This does not mean that the Engineer creating material layups is not a master of their craft, or the factory worker who must meticulously set up the layups is not passionate about their work, or that the designer who creates the paint scheme does not have vision, but these people must come together to create a single vision for a product that is beautiful AND functional. Just because you don’t know the names of the people who built the bike doesn’t mean they are not craftsman.

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Estimating Bike Rider Drag Coefficient

In response to my last post there was some facebook discussion (always goes well) about wheels and what not, I chimed in with a statement and someone proceeded to correct me on the difference between Frontal Area, CdA and just plain Cd…………….

Anyway.

So this is already implemented in the Wheel Selector so this is a bit of back tracking but I’d like to add explanation, even for my own personal record keeping. I originally thought that estimating the bike/rider system drag coefficients would be easy. I’d just grab a few values form some tables then BOOM done. Nope. Turns out when you’re trying to estimate Drag Coefficient Area you have two moving targets to track: Drag Coefficient and Frontal Area.

F_d\, =\, \tfrac12\, \rho\, v^2\, c_d\, A

So it’s really easy to measure what CdA is. You just plop a guy in a wind tunnel and factor it out. The problem is this method is great for one-of scenarios and is just on the slightly expensive side of things. I’d like to have it very easy to calculate given some pretty well known metrics: height and weight. This means I have to separate out my Cd and Frontal Area. While CdA is relatively easy to measure in a wind tunnel A must be done with 3d scanning, some photo software, or voodoo magic. You can’t actually measure Cd anywhere (i think?) you have to calculate it back out using CdA/Frontal Area.

So I’m going to say we have 4 positions to ride in: Tops, Hoods, Drops, and TT bars. The problem is that both your Frontal Area AND Drag Coefficient change when you change positions (never mind when you stick your fat knees out into the wind while pedaling).

There are a couple studies out there that try to estimate this. The problem is they’re sometimes quite complex. For instance the most accurate way to measure frontal area is to do a 3D scan and import it into a CAD software program. You can also take a front-on image and get your frontal area from the picture as done here. This adds some uncertainty, like having a directly front on image (can be remedied by scaling image), and having a point of scale (although a bike wheel’s always going to be the same size, so there’s that). The problem with both these methods is they’re both a huge pain in the ass.

The next step down in fidelity is taking body angle measurements and plug this into an equation (also huge pain in the ass).

All these methods are detailed in this paper which is a great summary of all the different attempts to calculate frontal area’s and Cd’s.

Highlights from that paper are as follows:

  • A Body Surface Area estimation is only really valid for cyclists 60kg to 80kg.
  • There’s a weak correlation between trunk angle and frontal area
  • Frontal Area’s the primary driver in a cyclists drag
  • There are a TON of formula out there to calculate Frontal Area
  • There’s no one “correct” formula due to the wide variation in body type and position

 

I’m going to go ahead and use the simplest method they detail, using Body Surface Area (BSA) since it only requires your height and weight. I’m sure this will not be super accurate, but then again I’m not looking for minute differences, just a general magnitude to compare to wheel aerodynamics.

BSA (m2) = 0.007184 x Height(cm)0.725 x Weight(kg)0.425

Olds_FA_Estimate

Where Ab is Body area Ap is total including bicycle (sorry about potato quality screen grab)

 

This will give us an estimate for Area in the TT position. Now the problem is your frontal area’s changing with your position. I.E. a position on your tops is going to have a larger frontal area than the frontal area you’re at in your drops (Just going to go ahead and ignore standing for now).

So this is a generalized starting point, remember I’m not SUPER concerned with accuracy, just general magnitude.

As far as getting Cd, a separate case all together, there’s not a lot of consensus. The paper I mentioned earlier has a pretty exhaustive list of data that’s collected from the accurate methods (CAD, 3d scan) to the totally weird methods (towing a rider behind a car with a cable that has a strain gauge). I’m going through those next to see if there’s any sort of pattern.

Also the cyclingpowerlab.com (which seems to be down at the moment) has a list from the Cycle Science Book (wilson 2004) that has another pretty complete table of Cd values, but it condenses down to this:

Tops: 1.15
Hoods: 1
Drops 0.88
TT Position: 0.77

So I now have Cd and A

Thus

Cd A!!!!!!!!!!!

(roughly estimated)

To Be continued

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The Issue with “Cross Winds” in Wind Tunnels

A lot of wheel and bike companies like to put a lot of emphasis on their wheel performance in the cross winds. They show you a pretty graph that looks something like this:

What you’re generally seeing is the dip due to decreased drag then a steep increase when the flow finally and fully separates from the wheel.

The wind tunnel is a perfect environment while the real world is a very imperfect environment (duh right?). What wind tunnels don’t capture from the real world is something called boundary layer, due to the way that they operate.

Think about this, when you’re riding at 20 mph, how fast is the ground moving? Obviously it’s stationary, but relative to the rider and wheels it’s traveling at 20 mph backward (the same speed as the wind). Why does this matter? If you get down to the smallest levels air speed at the ground is going to be the same speed as the ground (zero). This effect is called the boundary layer, where on the surface of everything that’s moving through a fluid there is a very short transition area where stream velocity goes from zero right at the surface of an object to free stream. To remedy this problem and make the wheel “see” wind speed right at the ground level special platforms are used in wind tunnels:

Boundary Layer Table

As you can see on the velocity profile on left, there is a gradient next to the floor, the platform sucks in this low speed air and reestablishes what is the ground (there is inevitably a small BL but it is negligible)

This works great for head on velocity, however it doesn’t capture effects of the BL of the cross wind flow (which IS moving at a velocity different than the ground thus has a boundary layer).

Not only is there a simple ground boundary layer present, there is a larger velocity gradient that is a part of the atmospheric boundary layer (ABL). When the weather stations measure wind velocity it’s typically 10 meters (32 ft).

This is a good illustration of ABL. While most of the gradient is quite high, the steepest gradients are right at the bottom next to the ground (where we’re hopefully riding bikes). This means your 10 meter weather station is reading a significantly higher wind speed than your bike is seeing. This also shows the difference in gradients between different types of terrain

And anyone can tell you this, if you’re riding in the city rarely is it very consistently windy, while in Indiana it’s always windy. However the wheels are the closest component to the ground (again, hopefully).

Alright so now circle back to the neat drag image we first looked at. Zipp in particular likes to emphasize their performance at 15 degrees AoA. This is probably the point where the wheel drag data varies the most, we have also observed this divergence in wind tunnel and CFD results. However let’s think about this for a minute, if you’re traveling at 25 mph (pretty quick, but a good race speed) a 15 degree AoA means that you have almost a 7 mph of direct cross wind. A 7 mph wind seems pretty reasonable and average right?

Remember the wind table essentially eliminates the BL in both components of wind velocity, forward speed (which we want to eliminate) and cross winds (which we’d like to not eliminate but is not physically possible).

So when Zipp says the wheel is saying a 7 mph wind, what does the REAL wind speed have to be to get a 15 degree AoA?

I used this calculator: http://es.ucsc.edu/~jnoble/wind/extrap/index.html

The second set of calculations are applicable to heights under 100 meters, given that the height of a wheel is at the lower limit of this equations usefulness so take this with a grain of salt. To get the magical 7 mph cross wind, the weather station is going to have to measure a 20 mph wind (assuming Agricultural fields as your surface roughness, obviously a town, forest or city will be much higher).

So 20 mph is a VERY windy day without doubt. All I’m suggesting here is that bike companies have determined that 15 degrees AoA is a  mythical regime that we’re all riding in all the time, but it’s not QUITE the case.

So a real world measurement of this is what Mavic actually did with respect to the Kona Ironman course:

http://www.engineerstalk.mavic.com/yaw-angle-measurement-in-real-conditions-on-kona-ironman-course/

Kona is a notoriously windy place to ride and more importantly it’s this desolate lava field with nothing to protect you from the wind. They go into this exhaustive breakdown of each portion of the course (OMG triathletes YAWN) A more realistic distribution of wind speeds is observed when they take their setup to France:

A much more real world riding area in France…almost no cross wind!!!!!

Given this is a low wind day and they don’t actually tell you what the real wind speed is so I guess it’s not a perfect example. I also am VERY aware that the times at which you’re pinned in the gutter are usually the hardest parts of races.

However, bottom line is manufacturers like to play up their drag at high angles of attack, I think, because pretty much everything below 5 degrees is very closely packed together and it’s hard to really differentiate your brand. Is <2 grams of drag worth an extra $1000 of proprietary technology?

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Rough Draft

So I’ve finished my rough draft of wheel selector, it’s live here:

http://chrisuberti.com/wheelselector/

Mainly the biggest hurdle I’m having is coding the damn thing into a website.

So right now what it is essentially a power estimator given there’s zero cross wind, no rolling resistance, and you’re riding by yourself (aka Ironman). It’s estimating pretty high on the power/work numbers based on my own rides.

What it needs to be a viable wheel selector:

  • Wind included (with inclusion of atmospheric boundary layer effects)
  • Wheel drag across range of wind angles (currently only running for 0 degrees, which means all wheels are pretty much the same)
  • Add rolling resistance
  • Time spent Drafting?
  • TT position
  • Refined bicycle rider CdA (this is pretty rough currently, the topic of it’s own blog post)
  • Add more explanation to what the hell is going on

Other things I plan on doing

  • Some sort of submit to database, involving entering your ride metrics (from power meter). This could help refine CdA measurments, which are particularly difficult to estimate
  • Better interface (ajax implementation, oh the confusing computer languages, why cant everything be like matlab…easy)
  • Step 2:  ?????????
  • Step 3: Profit!

Ultimately I’d like to incorporate something like this into what this guy did here.

But that’s a LONG way off for a side project.

I don’t want a silly plain power estimator, that doesn’t really give you much (plus that’s kinda the reverse solving process from what you need to benefit you as a racing cyclist). With this tool, you could essentially calculate your power needed for say a mountain cilmb, pick which wheelset is best, etc.

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A Hairy Vengeance

I like to go through and watch what other bike companies are doing in terms of Aero-ness. It’s hit and miss, largely miss. However the funniest one I’ve come across is this following Specialized #AeroisEverything video:

If you want the bit I’m talking about FF to 2:25. They propose that shaving your hairy trunks will save 70 seconds on average over a 40km TT. “To put that in perspective that’s like going from a traditional round tubed bike to something Aero like the Venge.”Capture

Think about that, you can spend $3500 on a bike frame….or just spend $3 on a razor to shave your legs.

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