Real Talk (for road racers) with Jerry

Posted by Jerry Chabot on

I like wind tunnel data. As an engineer I loves me some data. But, I also question methods and error percentage and above all, the context and scale of the results being presented. Numbers with no context are meaningless, or at best very misleading. 20! That is a big number that sounds meaningful! A difference of 20 in the context of an absolute measurement of 1,000, however, is only 2%. Similarly, comparative data should be taken in the same tunnel and set up, as the difference in drag results from tunnel to tunnel is going to be meaningful in proportion to the differences in rim to rim results, at least in my engineering opinion. I could go into a discussion between the mindset of an engineer verses that of a scientist, but suffice to say, engineers care about “meaningful” variances whereas a scientist is looking for anything “measurable”.

What is the context of the drag differential in various rims in a bike/rider system? Total drag for a rider in a crouched, racing position is something like 3,600 grams. Of that total, the wheels are around 750 grams and of that 750 grams the frontal area of the wheel is in the range of 60-70 percent. The difference in drag in wheels from one aero wheel to another of the same depth is single digit percentages of 750 grams. My point is that the rider and position are by far the biggest component of drag and easily outpace the difference in one wheel to another, though marginal gains are available. Full disclosure, this is taken from readily available data taken the many published test results available, and is not data that I have collected.

It is generally accepted, again from many well vetted tests and calculations, that saving 100 grams of drag saves 1 second per kilometer at 30mph, the oft quoted “40 seconds over a 40km TT”. That sounds like a lot if you do many 40km TTs. I don’t, gosh that sounds terrible. Triathletes, though, do this regularly.

It is also generally accepted that a “deep” aero wheel is about 100gr less drag than a traditional (aka old fashioned) box section wheel. Everyone’s marketing literature quotes the wheel drag savings off of this baseline, which is sort of crap right out of the gates because no one rides a box section rim when they are trying to go fast, and it is unlikely to be the wheel you are considering upgrading from.

Wind tunnel testing shows that even a 30mm deep “semi aero” rim achieves most of the 100gr savings. From the various published drag data that I have found, going from a 30mm to a 60mm to a 90mm and so on saves about 10 seconds over 40km per step in depth - in general in “significant digits” not just “digits” (think like an engineer here not a scientist). A full, maximum depth commercially available, 90mm rim therefore saves about 90 seconds over 40km vs the box section when riding at 30mph. The tiny little 30mm semi aero saves about 70 seconds. Only a 20 second difference! So, for scale (engineering) a 30mm deep rim, which barely qualifies as aero, gets you the bulk of the savings in drag!

The smallest commercially available carbon rim is 25mm deep and would be considered a fragile, special use only, uphill TT wheel. A 30mm rim is only 5mmdeeper than a special use only uphill TT rim. Take a second to go look at a ruler, please, as I want you to fully grasp the scale we are talking about here.

Going from a "climbing" wheel at 30mm deep to a huge, heavy, deep 90mm flywheel saves you only about 20 seconds over 40km at 30mph. 30 miles per hour, braining yourself for 50 DAMN MINUTES! 30mph is FAST! And you save just 20 seconds, and that assumes you are out front riding in clean air, like a time trial or triathlon, in TT mode. Get into a pack, like a road race or a crit, and the difference in drag is meaningless. There, I said it. In the context of road racing, the difference in drag between the climbing wheels and the ridiculous 90mm deep wheels is meaningless while you are in the pack. Note to Reader: Opinions found here, I’ve got a few.

Let’s bring this down to the differences in drag between NEXT wheels and all the other wheels out there at various price points and with various dimples, dips, waves, ridges, bulges, razorbacks (??) and so on. We are all within the “noise” (that is a measurement term) of that 10-20 second window of clean air savings potential between the 30mm and 90mm rims. A 55mm deep NEXT wheel is going to perform similarly, aerodynamically, in a road racing environment, to a 55mm deep wheel from anyone of the other guys. There are percentage point differences at various yaw angles, and every manufacturer’s tests cherry picks the best yaw angle for their rims, which is valid, and I have no problem with that. But realistically, unless you do triathlon or are a TT specialist, buying carbon wheels based upon the aerodynamic performance between brands is a red herring. Sorry, not sorry. (Upcoming blog on our TT and triathlon value propositon). 

Here is some comparative data I should find the testing source for (it was in my research notes) showing about a 9 gram spread between several major brands:

Grams of Total Drag:
Enve SES 6.7:                         766 grams
Hed Jet 6/9:                           766 grams
Bontrager Aeolus D3 7:         757 grams
Zipp 404/808:                       757 grams

If you want a really good “sciencey” deep dive, verses my “engineeringy” point of view, I would suggest you check out the great blog of Tom Anhalt. You should start with this awesome wheel comparison he did at arguably best available wind tunnel, the new Specialized tunnel in California. 

Be note that he is measuring drag in terms of Drag Area (CdA, or Cd*Area) not grams of drag. To give CdA some scale, the Cd of a rider on the hoods is around 1.0, in a racing crouched position 0.88, and on aero bars is 0.7 (from Bicycling Science, Wilson 2004). Their area is maybe around 0.36m^2, and therefore a CdA of 0.88x0.36=0.32m^2 in a racing position. Tom’s great test results show a range of CdA from about 0.0175 for the best tested wheel/tire combo to 0.0195 for a Zipp 101 (a 30mm deep alloy rim), both with the same tire, at 0 degrees yaw (full headwind). On a chart that looks like a very measurable difference, and in fact it is! But what percentage difference is that in terms of total bike/rider system (recall that to be about 3,600 grams total)? It is (0.0195-0.0175)/0.32x100 = 0.064%. Note that is zero point zero zero six four percent. Converting that into grams of drag: 3,600gr x 0.00064 = 2.3 grams of drag difference between the two, riding straight ahead in clean air.

At 10 degrees of yaw, which is becoming an accepted baseline yaw angle comparison point for a rider traveling fast (the faster you go, the lower your average yaw angle experienced), the spread between the 30mm rim (which is a much better baseline that a 14mm box section!) and the best performer is 0.019 vs. 0.0105, or 101 grams of drag, or about 40 seconds over 40km at 30mph, between an alloy 30mm all arounder (with really pretty poor performance at increasing yaw) wheel and the very best aero stuff out there. Sound familiar? But wait! Look at the spread between similar wheels at that same yaw! They range from about 0.012 to 0.0105 CdA, or 17 grams of drag difference. Recall that 100 grams of drag reduction can save you about 40 seconds over 40km at 30mph riding solo. So 17 grams is about 7 seconds difference in a 40km TT. And some of that is certainly due to the different tires being tested. The point I am trying to make here is that in real world road racing (let's table triathlon for now) “does this make a meaningful difference to me athletically” terms, any good quality carbon wheel is going to perform pretty much the same. Measurably the same? No. Meaningfully the same? Yes! So in real world terms, I am going to argue that a very well built wheel with great after sales support but with very average aero performance is going to be a better wheel (probably at a lower price) for the typical road rider/racer than a production built wheel with a lot of sunk costs in aero optimization and testing. My 2 cents.

TLDR? Maybe, but if you did read this far I am proud of you which should make you feel all warm and fuzzy, like a hug but not a hug because I don't hug. But honestly why did I make you go through that? I wanted set the stage for my answers to the frequently asked questions “Why should I buy carbon wheels?” and “Which model wheels should I buy?”.

Why carbon? Because carbon wheels are lighter than the minimum comparable alloy wheel that gets you meaningful aero savings. If you go to a 30 deep alloy, you are talking a pretty heavy rim and it goes up from there. Another great reason is that a deep wheel is way STIFFER. That is why they feel fast and responsive. That is why sprinters like really deep wheels. Also, sprinters are going FAST, 40mph plus in clean air if they are doing it right at the end, all while at their maximum watts. 10 more watts matters in that racing moment. They also look cool, tubeless up really nicely, look cool, are actually tougher than an alloy rim, and they look really cool.

Which model? If you want to go fast but worry about side winds pushing you around, get the Quick. If you do hilly road racing or live in a hilly area, don’t ride the Truth for that. If you are a smaller guy or a bit more weight focused or love the wrapped logo style and want an all-around wheel, go with the Quick over the Fast. If you prefer more stiffness and  want an all-around wicked fast wheel, get the Fast over the Quick. If you like hilly road racing more than crits, or do a lot of hilly riding, the Quick should get you 80% of the aero benefits of the deeper wheels and feel livelier on climbs. If you want max performance for TT or if you are a dedicated crit sprinter, take a look at the Truth. If you are a time trialist or triathlete, I think the aero gains of the deeper Truth are obvious and from a pure performance standpoint, which is what those events are all about. If you can afford it, get the Truth bundle with the rear disc wheel. The difference between the BEST 80mm rim and a bad solid disc is very measurable, and our disc is one of the nicest, certainly among the lightest, disc wheels available. Not to mention, we offer the whole Truth Triple Threat package for around the price of a big name brand disc wheel alone.

Thanks for hanging in there, now go check out the wheel store and buy yourself something nice, you've earned it.