Menu

Technical Q&A with Lennard Zinn – That stubborn bead

  • By VeloNews.com
  • Published Aug. 12, 2008
  • Updated Oct. 11, 2010 at 2:20 PM EST

By Lennard Zinn

The Zipp 530

Photo:

Dear Lennard,
Why would changing a tire on a Zipp 530 be so difficult? I beat my fingers and snap tire levers trying to get a tire off.
Steve

Dear Steve,
It’s a good question, but one I thought might be best answered by the folks at Zipp.
Lennard

Answer from Zipp

Lennard,
Because the intended use of Zipp wheels is racing/high performance, the design is one that holds the tire very securely even under the most severe conditions. Tires can blow off of clincher rims for three reasons, either the rim is too small, the tire is too large, or the air pressure/force is too high for the tire, rim, or the interface of the two.

Since our co-molding process requires the full tooling of the outer aluminum rim as well as the structural inner carbon hoop, the outer aluminum ring must be controlled to a diameter more than two times tighter than the ISO/ETRTO standard. To ensure secure tire fit we fit our tolerance into the upper 50 percent of the allowable size, so the rims can range from the nominal specified diameter to about 90 percent of the MAX specified diameter under the international standard, essentially the rims always tend to run larger rather than smaller. Likewise, many tire manufacturers try to run their tires in the smaller range of the tire size tolerance band due to the stretching of the tire and the bead with use, time, and heat. Clearly this combination can lead to a tight fit, but it is agreed by all that this is a very safe and secure fit.

On the pressure/force side of the equation, there are multiple critical aspects, but in particular the pressure you pump your tires to before the ride represents the minimum pressure the tire/rim will see during the ride. Tire pressure increases with brake heat in the rim, plus the tire becomes more elastic and will stretch with heat, so for example if you pump your tire to the Max 125psi and then descend the Alpe d’Huez, your rim may achieve 300 degrees F, the air pressure will rise due to the heat to roughly 150psi, and the tire will show increased elasticity…add in a high speed corner and the combined forces on the rim/tire interface can be significant. The Zipp rim has been designed to hold the tire when the system is hot and loaded, likewise the maximum pressure rating on the Zipp rim is set based on extensive lab testing of both burst pressure and fitment of dozens of tires, but also the hot bursting conditions of these tires. We feel that our fitment is one of the most secure on the market currently and offers additional security even when hot, as a result, on our rims most all tires will physically fail through the sidewall before blowing off of the rim, even when hot.

For tire installation, we recommend using a thin rim tape like Zipp, Rox, Ritchey or Michelin plastic. The added thickness of a tape like Velox makes the tire fit too tightly at the bead seat of the channel, making it very hard to stretch the tire over the bead on the opposite end. Also using the narrowest allowable inner tube helps by eliminating bulk under the tire during the install and drastically reduces the likelihood of pinching the tube during the installation.
Josh Poertner
Category Manager / Technical Director
Zipp Speed Weaponry


Dura-Ace electric
Dear Lennard,
I have some questions regarding the upcoming Dura Ace electronic group.

The 2009 mechanical group will support an 11-28 rear cogset paired with a compact 50/34 chainring. I’ve looked at what Shimano has on its website and cannot find anything about what chainrings will work with the electronic group, nor any mention of why the electronic rear derailleur is limited to 27 teeth maximum.

As an old guy who could go faster on the level with an 11-tooth small cog but who’d like a bit more help up the steeps, I’m wondering why no 11-28 compatibility. I’m hoping that even if I cannot get the 28-tooth grunt gear, I’ll still be able to consider the high-tech group when it comes time to replace the stuff on my Specialized S-works Roubaix. I’m in love with this bike’s compact cranks and haven’t had to get off yet on any Sonoma County, California, climbs, but I’m considering a pilgrimage to Ventoux and L’Alpe d’Huez next spring. I know that one extra cog won’t transform me from Lunchalot into Lance, but I’ll take any help I can get!

By the way, how much of the decision to make it “fly-by-wire” rather than wireless was because of concerns about activating one’s neighbor in the peloton’s derailleur with a tap on the shift lever?
Jim

Answer from Shimano

Dear Lennard,
The Di2 (electronic Dura Ace) components are compatible with any Dura Ace 7900 series products. That means that it would be compatible with the new compact Dura Ace 7900 crank and provide a solid mountain climbing option.

BTW, the Di2 does not use a wireless signal for actuation of the derailleurs because doing so would require each component (both shifters, front derailleur and rear derailleur) to have its own power source. By using a wiring harness with a central battery, we can reduce the weight of the entire system. Also, going wireless would require a transmitter and receiver built into each part which would make each component larger, heavier, and cause it to consume more power – requiring a larger, heavier battery. Sending the shift signal through the same wiring harness as the power became a no-brainer at that point.
Devin Walton
Shimano Media Relations Officer


Tire talk continues

Dear Readers,

I know I have already said that the rolling resistance debate has gone on long enough. I would really like to give it a rest after today. That said, of the numerous letters I got this week on the subject, there are a couple of them that really need to be posted to give the other side of claims and theories put forward in previous columns.

I also want to keep rolling resistance in context by once again reminding you all that, in the grand scheme of things, rolling resistance is only a very minor contributor to the total resistance that you oppose when riding a road bike includes. We’re splitting hairs, here.

However, on a mountain bike, rolling resistance becomes more important because of the variations in texture of the trail and due to the lower speeds, which de-emphasize the importance of aerodynamic efficiency. This is an interesting test of mountain bike tires that factors pressure vs. tire width on various kinds of courses.

Keith sent this link and notes that you can save up to 50 watts. And more important than fasting rolling is staying upright. You will go faster if you don’t fall down, and it is interesting to notice the differences in wet grip of various tires.
Lennard


Dear Lennard,
I read the latest Tire Talk installment and I’m compelled to comment on one thing (I could comment on a lot more than that, but I’ll be charitable). The second and third letters published (from “Jonathan” and “Alan”) both perpetuate a common misconception about tire flexibility and resultant energy losses.

The problem is essentially one of basic physics: flexibility and hysteresis are entirely independent physical parameters. One can have a mechanical structure which is highly flexible yet has low energy losses; conversely, there are mechanical structures which are (fairly) rigid yet have high hysteresis.

A good example of the former is a light weight, easily-stretched metal spring; stretch it, then relax it. So long as the spring returns to its original length, the losses are negligible. Bicycle spokes in normal operation follow this model: if they didn’t, I assure you people would’ve figured out how to avoid that energy loss long ago! An example of the latter is a high-performance (HP) “speed-rated” car tire. HP tires are actually a lot stiffer than their lower-performance (LP) brethren, primarily due to the carcass design and construction. What allows an HP tire to maintain very high speeds (as well as improved handling on good roads) is that the stiffer carcass doesn’t flex, which generates heat due to hysteresis. Running a LP tire at continuous high speed is downright dangerous due to the potential for tire failure from overheating caused by this sidewall flexure.

Chances are that Jonathan’s HP tires had a higher rolling resistance due to greater suspension losses resulting from the stiff tire carcass, not because of the (perceived) soft rubber tread.

In the immediate instance of bicycle tires, it’s not carcass flexibility per se that causes loss, but rather, how much hysteresis is associated with carcass (and tread rubber) flexure. Highly flexible, low-loss carcasses are exactly what you want; either thin-wall, high-thread-count tubulars or clinchers will suffice. Tread flexure also results in some hysteretic loss is a function of rubber compounding, thickness, tread design (with smooth treads being lowest loss), and tire size, shape, inflation pressure, and even road surface smoothness, since that also influences how much flexure there is.

Not mentioned in any of the rolling resistance tests you reference is the one conducted by Jan Heine and published in Bicycle Quarterly, Vol. 5, No. 1, Autumn 2006. The two best tires in this real-world roll-down test were a Dede Tre Giro d’Italia 700X23 (actual width 24.5 mm) clincher and a Clement Campionato del Mondo 700X28 (actual width 28 mm) tubular. Notable is that rolling resistance is only a weak function of tire width (some of the other top contenders were even wider than the del Mondo), but what these two tires do have in common is very supple carcasses (hand-made, high-thread-count cotton or silk, respectively) and thin, smooth (or nearly so) treads.

Along with many other interesting observations and analysis (including a wonderfully concise article, “The Physics of Tire Rolling Resistance”) in this issue is the conclusion that in real-world riding, high-quality tubulars can rival the rolling resistance of the best clinchers. Good thing, since all my road bikes have tubulars!
Dave


Dear Lennard,
A VeloNews reader named Jonathan used the example of his cars 2 mpg increase after changing from a “stiffer” hi-performance tire to a standard tire to try to disprove Vittoria’s claims about tubular v. clincher rolling resistance. I’m afraid he is a bit mixed up. While it is true that a hi-performance car tire does have a softer rubber compound then a standard passenger car tire, the hi-performance tire has a substantially stiffer sidewall in order to prevent the tire from deforming under a heavy cornering load. The more flexible sidewall of a standard car tire would not keep the tires tread as flat on the pavement as a hi-performance tire with a stiffer, thereby reducing the contact patch (and traction) under cornering. His increase in mileage is due to both the more pliable nature of his new tires “softer” carcass and the reduced adhesion of the harder rubber compound.
Phil


Dear Lennard,
“At the same pressure, both tires, tubular and clincher, must have the same total contact area with the ground, since that is the force (pressure times area) that is resisting gravity. The shape of the contact area may be different, as the 29er converts have been arguing vociferously about 26-inch vs. 29-inch tires for years, but the total area at the same tire pressure should be the same.”

This fellow is way off target.

While he correctly states 1) that force (F) is the product of pressure and area ( F = P x A ) and 2) that the force acting at the contact patch is numerically equivalently to the distributed force of gravity acting on the bike and rider, his logic suggests that the contact patch (A) must be the same in all cases where the tire pressure is identical. The huge mistake here is that he’s assumed that the pressure (P) applied at the contact patch is the tire’s inflation pressure, which it certainly is not!

To dispense with the physics and mathematics, let me offer an extreme case in point: Imagine that you had a conventional, say, 36-psi tire, on the one hand, and a rigid, non-pneumatic tire on the other. Further imagine that the rigid tire was constructed in such a way that it presented an identical contact patch to the road surface as the pneumatic tire. For a given bike and rider, would the pressure at the contact patches be the same for both tires? Indeed they would be, although the inflation pressures would differ by 36 psi. In reality, there, in fact, is no correlation between inflation pressure and contact patch pressure, except to the extent that changes in inflation pressure cause the tire to deform and consequently present different contact patches.
Jack


Dear Lennard,
Regarding all the tubular versus clincher stuff. I personally would rather be on a tubular in a race cornering at high speeds and staying upright in the case of a flat or a blowout. Just an old Cat. 2 talking.
Nick


Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

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.

FILED UNDER: Bikes and Tech TAGS: / / / /

Stay updated on all things VeloNews

Subscribe to the FREE VeloNews newsletter