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Technical Q&A with Lennard Zinn – Rolling resistance redux

  • By VeloNews.com
  • Published Aug. 7, 2007
  • Updated Oct. 11, 2010 at 2:36 PM EDT

By Lennard Zinn

Dear readers,
Most of today’s column is devoted to a selection of the many, many interesting letters I got in response to my June 26 column, addressing the many factors that add to – or reduce – rolling resistance.
Lennard

Roundness
Lennard,
One additional advantage of any high quality clincher over tubulars is that clinchers are much rounder. An out-of-round tubular is constantly lifting the bike as it rolls down the road.

Certainly, the choice of rim cement vs. rim-tape can create extra hysteresis /distortion. Keep in mind, that better “roundness” can be achieved by a mechanic carefully mounting a tubular while glue is still a bit wet, then rolling on the tire with full body weight under very high pressure and “re-centering” as accurately as possible. The result can be very significant.
Wayne

Pro riders
Dear Lennard,
Why does almost every ProTour team roll on tubulars if the research indicates clinchers are faster? Is it because that is all the teams know (we have always done it this way) or is it comfort?
James

Dear James,
Actually, there are a number of ProTour teams using clinchers these days, although all do use tubulars sometimes. Teams use tubulars due to tradition, weight considerations, safety in the event of a puncture, and, most recently, due to the widespread use of carbon rims. Most time trial wheels (discs and deep section) and superlight carbon climbing wheels in the ProTour are tubular type.
Lennard

Pressure
Dear Lennard,
I have a two-part question regarding your column on rolling resistance. Firstly, for clarification, hysteresis in this case is used to mean the lag in deformation of the tire to match the road? The conclusion being that softer glue slows this process?

Second, I barely saw tire pressure mentioned except when you noted it as a pro for tubular tires. How did you determine what pressure to run during your tests in each tire? Wouldn’t this be an important factor when considering the required power and rolling resistance of the different tires?
Diran

Dear Diran,
You interpreted hysteresis correctly, and a soft, thick glue layer absorbs energy, especially if the gluing surfaces are not well mated and the glue sticks and comes unstuck during each revolution.

During any rolling resistance test, you obviously have to keep tire pressure constant so that you can compare apples to apples. Beyond that, tire pressure should come down when the road is wet or rough, and not just for shock absorption. If the road is rough, the tire has less rolling resistance when softer since the small bumps and gravel chunks are absorbed into the tire, rather than throwing the entire bike and rider up and back as happens with high pressure, costing energy and requiring re-acceleration of the bike. It’s the same reason for why the rolling resistance of a mountain bike is reduced with suspension and low-pressure, tubeless tires – the “sprung weight” is reduced.

As for exactly what pressure is ideal, that depends on the road surface and on the tire. I can at least tell you that the bomber 140-160psi pressures so popular with many triathletes and some bike racers is misled. The bike feels fast and lively because it is bouncing all around, but that is actually slower, since you will be doing more work to maintain the same speed. You are much better off in most cases between 90 and 120psi.
Lennard

Not enough?
Dear Lennard,
When I looked again at the study done at the Continental facility I realized they made a glaring error; the tubulars were under-inflated. They used a maximum pressure in the tubular test of 8.5 bar.

This is a pressure that is at the low end of suggested range for most race tubulars, and under the suggested minimum pressure for the Tufo tires. The study even notes that at higher air pressure tubulars dramatically reduce their rolling resistance. Well, of course, one the advantages to running tubulars on the road is to achieve air pressure of 12-14 bar and therefore significantly reduce rolling resistance. This is impossible on a clincher. I would like to see a study where tires are compared at inflation pressures of 90 to 95 percent of their maximum suggested level. That would be a more useful test.
Carl

And what about that mountain bike?
Dear Lennard,
How much pressure should I put in my tubeless MTB tires? I run regular non-UST tires with Stan’s no tubes. I’m 150 pounds and I normally run 40 to 45 psi. A mechanic told me that the tire will actually have less rolling resistance with 28 to 32 psi because the tire contacts the ground better. Is this true? My goal race this year is the Leadville trail 100, which has a lot of paved roads and/or flat sections.

The increased grip would help me on the down hills, but I don’t want to get slowed down in the critical paved and flat sections.
Warren

Dear Warren,
On varying road surfaces, there will always be a trade-off on tire pressure. On rough surfaces, I agree with your mechanic. On smooth surfaces, you will be faster with a harder tire. I’m not familiar enough with the course to make a suggestion, but I suggest you take into account how much total time you will spend on rough sections and smooth sections, and optimize your tire pressure for where you’ll spend the most time.
Lennard

Beyond rolling resistance
Dear Lennard,
It would be nice if the real advantages of using tubulars were mentioned in the discussions of tires. Keep in mind that tubular offer things likeFewer flats (all things being equal, no pinch flats)Safety, since tubular don’t roll off the rim when flat.Comfort. You can ride at lower pressure, since there is no need for lots of air to prevent pinch flats.Better cornering at speed, tires deforms much like a auto radial tire, rather than the oval clincher aired to 120psi+Rolling resistance has never been an advantage of tubulars but for long distance riding and racing, when the training or race stage is 5-7 hours, tubulars make all the difference.

Furthermore, once you develop the skill set, gluing tires is pretty simple and fast.
Peter

Leiphemer’s time trial position
Lennard,
Very nice articleon the Tour TT, everything you said was spot on as far as I am concerned.I thought I would email you a point of interest about Levi’s TT position,it’s application to the general riding public and for future referencein your articles if you see fit.We have been finding that some riders (Levi included) shoulders keepgetting narrower as their elbows come together. Narrowing the shouldersmakes a big impact on the drag numbers as you can imagine.The arms up tuck position allows for more comfort when squeezing theelbows together and if your arms are short enough, you can tuck your faceinto your hands to make it even faster. In any case, there are many riderswhose shoulders do not get narrower as elbows come together and in factthere are some whose shoulders get wider as their elbows come togetherwhich is why the Levi position only works for only a few people. In additionto this there are some people whose chest gets squeezed too much to allowfor the Levi position as breathing capability is diminished.
Christopher “Dino” Edin
Hed Cycling Products


VeloNews technical writer Lennard Zinn is a frame builder (www.zinncycles.com),a former U.S. national team rider and author of numerous books on bikesand bike maintenance including the pair of successful maintenance guides”Zinnand the Art of Mountain Bike Maintenance” and “Zinnand the Art of Road Bike Maintenance” as well as “Zinn’sCycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s VeloNews.com column is devoted to addressing readers’ technicalquestions about bikes, their care and feeding and how we as riders canuse them as comfortably and efficiently as possible. Readers can send brieftechnical questions directly to Zinn (veloqna@comcast.net)Zinn’s column appears each Tuesday here on VeloNews.com.

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