Technical Q&A with Lennard Zinn: Weight v. wind
Weight or air resistance?Dear Lennard,
This question has been plaguing me and my riding buddies for the past year. I guess there were some studies done, specifically by Ridley or Cervelo, that aero’ road frames are worth more than weight savings when selecting a bike. So, here's my dilemma: I'm a lifetime Cat 3/Masters racer and am in the market for a new bike. My racing weight is 132 lbs and I've always tried to find the lightest equipment out there, namely because I can ride it without fear of breakage.
By Lennard Zinn
Weight or air resistance?Dear Lennard,
This question has been plaguing me and my riding buddies for the past year.
I guess there were some studies done, specifically by Ridley or Cervelo, that aero’ road frames are worth more than weight savings when selecting a bike.
So, here’s my dilemma: I’m a lifetime Cat 3/Masters racer and am in the market for a new bike. My racing weight is 132 lbs and I’ve always tried to find the lightest equipment out there, namely because I can ride it without fear of breakage.
This latest information that an aero’ frame’s benefits outweigh (pun intended) the benefits of a lighter frame has me reeling. Can you please shed some light on this conundrum? I don’t ride time trials and most of my time on the road is spent in the pack in criteriums and road races.
I realize that this is the hot new category, but I think for the people actually buying those aero’ road bikes, rather than pros sponsored by their manufacturers, that the aerodynamic shape of the frame makes almost zero difference and could be the slower option for the average rider when compared to a lighter frame of equal stiffness.
In a straight-on headwind, yes, the aero’ frame is faster, and the little rough air-tripping “R-Surface” strips on the Ridley head tube and seatmast work great at keeping the passing air attached to the frame, running in smooth laminar flow with a minimum of turbulence. I’m sure that Jens Voigt’s Cervelo aero’ road bike was useful to him on his long, solo breakaways when the air was calm or the wind was parallel to his direction of travel.
But nestled in the pack, the aero’ tubes are irrelevant, and low weight is a boon when accelerating out of corners in a criterium. On climbs, the speed is so slow that the aerodynamics of the frame tubes is irrelevant and once again, low weight is king here.
Crosswinds on large aero’ tubes are a liability beyond a very narrow range of relative wind angles (the relative wind velocity, which has both magnitude and direction, is the vector sum of the rider velocity and the wind velocity). To understand this better, look at the graph at the bottom of Hed’s aerodynamics technology page Hed’s aerodynamics technology page. Click on various wheels, and you’ll see that drag drops on any deep-section wheel as yaw angle (relative wind angle) increases at first, but all of them reach a point, generally before 8 degrees of yaw, where the drag suddenly goes up and continues to climb from then on. The same thing happens with aero’ tubes on a frame.
A strong solo rider may be able to go fast enough in a strong sidewind that the net relative wind angle is low enough to make the aero’ tubes useful, but somebody slogging along alone on a training ride or RAAM in a crosswind is not going to see a benefit. And while the pack speeds would often be enough to cut the relative wind angle even in a strong crosswind, the fact that the bike is hidden in a pack make the question of a frame’s aerodynamics moot anyway.
Stick with what you’ve been doing.
High pressure and carbon rimsDear Lennard,
It is my understanding that one of the big challenges in building an all carbon fiber rim for clincher tires is the high pressures the tires put on the outside of the rims. Why haven’t the carbon fiber rim manufactures embraced tubeless clinchers, which run at a much lower pressure? At first glance this appears to be a natural fit.
That’s a terrific question, but one I thought best answered by someone who works with carbon, and its challenges, on a regular basis. So, I went to Paul Lew, founder of Lew Racing, who now works for Reynolds.
Answer from Paul Lew:
Full carbon hook-bead clinchers are susceptible to the high heat generated from brake pad friction in a manner that is not similar to carbon tubular wheels. The primary difference is that the open clincher design requires a cantilevered hook portion, which can flare outward because of heat generated from brake pad friction, and the air pressure of the inner tube. Because advanced carbon composite construction is limited to lower temperature performance than aluminum, based on the “glass-transition temperature” (see the note at the bottom for an explanation of what that means)* of the epoxy resin system, deformation can occur with significant pressure from the tire or tire inner tube when temperatures reach about 250 degrees Fahrenheit. It is typical for the temperature of the brake track to reach 300 degrees during hard braking after high-speed descents.
There are several solutions to remedy the problem of deformation in a typical hook-bead style clincher. The first and best solution is to increase the performance of the epoxy resin system to perform continuously in excess of 300 deg Fahrenheit. The second remedy is to increase the modulus of the fiber in the cantilevered hook-bead portion of the rim. Increasing the modulus of the fiber means that while the epoxy performance may drop, the increased stiffness of the fiber will help to offset the decrease of the epoxy performance. The third remedy it to modify the geometry of the cantilevered portion. Tapering the cantilever section so that the base of the cantilever tapers wider than the hook-end will create stiffness and strength, just as a tree widens at its base. At Reynolds Cycling, we take advantage of all three of these performance-increasing features.
Inflating tires to a low tire pressure, which is common in tubeless designs, will reduce the likelihood of rim deformation due to the increased temperature of brake pad friction. Lowering tire pressure may not be the best solution for performance related wheels. While there are exceptions, higher tire pressure as a rule results in lower rolling resistance. The idea that a wider tire at a lower pressure results in lower rolling resistance has been misunderstood. The fact is, at the same tire pressure, a wider tire as compared to a narrower tire has lower rolling resistance. If the tire pressure is lower, the rolling resistance will increase, regardless of tire width. Therefore, lowering the tire pressure to accommodate the shortcomings of composite design may be an acceptable solution, but not for a performance-based wheel design.
Tire and rim manufacturers recommend a tire pressure range for safe operation. Following those tire pressure recommendations is an important safety guideline. For performance-based wheels and tires, finding wheels and tires with the highest-pressure rating is important. For commuting or touring, where high tire pressure, and low rolling resistance is not as important as comfort, lower tire pressure requirements may result in a wider selection of wheels and rims which include high-performance, “designed for racing” components as well as lower priced options.
* The Glass Transition Temperature (Tg), is the temperature at which an amorphous solid, such as glass or a polymer, becomes brittle on cooling, or soft on heating. Above Tg these clusters become large facilitating the flow of material. In organic polymers, secondary, non-covalent bonds between the polymer chains become weak above Tg. Above Tg glasses and organic polymers become soft and capable of plastic deformation.
Director of Technology and Innovation
Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.