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Technical Q&A with Lennard Zinn: Tire pressure and rolling resistance

  • By VeloNews.com
  • Published Jun. 24, 2003
  • Updated Feb. 8, 2011 at 8:59 PM EDT

By Lennard Zinn

Zipp 404s

Photo:

Dear Lennard Zinn;
Is it true that the Zipp clincher 404/303s will break up when tire pressure exceeds 140? –JV

Dear JV;
Below is a long answer to your question, but it is worth reading, as it addresses, in addition to the specific Zipp question, the general question many of us have about how much pressure a clincher rim can handle.–Lennard

Answer from Zipp
Dear Lennard and JV;
In short, no, a ZIPP 303 or 404 clincher will not break up when tire pressure exceeds 140psi. The source of this concern may stem from one or both of two sources: either from the Maximum psi sticker ZIPP now applies to rims, or from an issue ZIPP experienced last year after our supplier of alloy rim extrusions moved their production facility.

I’ll address the supplier issue first:
After the move of a new facility, the supplier had a problem with consistent heat treating of the alloy segments used in our clincher rims. This problem led to a small percentage of the alloy rims used for 404 and 303 clinchers to flare outward as heat developed during braking. The flaring would occasionally in turn lead to non-structural, sometimes severe cosmetic issues with the carbon sections of the rim, but there were no instances actual rim failure or collapse. ZIPP designs all of its rims with multiple layers of safeguards, which, short of a massive, overwhelming impact, ensure the integrity of the rim.

While as a percentage of total production the heat treatment issue was quite small, due to the nature of the problem it took time to appear initially and longer to address. A massive effort was required to investigate, track and address this issue. As a result, however, we are happy to say that since instituting a number of engineering and procedural changes at our supplier, there have been absolutely no re-occurrences of this problem. While the underlying cause of the issue was initially beyond our control, all of us at ZIPP would like to apologize to those individuals affected by this issue and thank them for their patients and continued support. I can honestly say that today’s ZIPP clincher rims and wheelsets are the finest we have ever produced.

Zipp 303s

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ZIPP now installs a sticker on every clincher rim recommending a maximum tire pressure of 125psi. While this has nothing to do with the alloy rim extrusion supplier issue previously discussed, it occurs to me that the two could easily be seen as related even though they are not. ZIPP recommends a 125psi maximum tire pressure for several reasons, and primary among them is both longevity and safety.

In the last few seasons, there has been something of a disconnect between tire manufacturers, customer expectations and the physical limitations of all (not just ZIPP) alloy rims. Tire manufacturing technology can now produce tires that easily exceed the allowable safe inflation rate achievable by rim manufacturers. The limitation in rims is due to the physical properties of the material and the hook and bead connection of rim and tire. For an alloy rim to include a reasonable margin of safety and longevity, the maximum inflation for a 23C tire should be between 125 and 140psi. Narrower 19mm tires, paradoxically, can be run at significantly higher pressures due to the shape and the lower volume.

The lighter the rim, the lower the maximum pressure allowable, as there is less material to both resist the mechanical forces and to absorb and transfer heat. The alloy rim on a ZIPP clincher weighs approximately 175 grams, less than half the weight of conventional alloy extrusion. As such, and since we here at ZIPP tend to be quite conservative in matters relating to rider safety, we recommend a conservative 125psi maximum to maintain a large margin of safety and ensure a long lasting, great performing and durable wheel set.

I believe that this will continue to be an issue for all manufacturers of clincher rims. To explain in greater detail, with a 23C tire each 10-degree increase in rim surface temperature above an ambient temperature of 70 degrees Fahrenheit will result in a corresponding increase in tire pressure of 1psi. On longer descents with accompanying heavy or near-continuous braking, the energy transferred by braking into the rim will routinely raise the surface temperature to well over 300 degrees Fahrenheit and can under extreme conditions reach over 400 degrees. Again assuming an ambient temperature of 70 degrees, a conservative 300-degree rim surface temperature means an air pressure increase of 23psi.

Now if you recall, I mentioned a bit before that the high end of the safety margin for any manufacturer for a 23C tire is around 140psi. (By way of example, our competitor Mavic recommends a maximum tire pressure of 138psi for any road rim with a 23C tire, a bit higher than the ZIPP rim due to the greater amount of material, but as mentioned before we tend to be quite conservative as well. In real world applications, there should be little or no effective difference.) This means that if a rider begins at 120psi at rest, brakes aggressively or feathers the brakes continuously lot down the hill, then his 120psi tire inflation becomes 143psi or more and is beginning to nudge ever so slightly into that safety margin. With some manufacturers making tires that have a maximum psi in the 140 or even 150 to 160psi range, the safety margin becomes even smaller and the likelihood of tires blowing off any rim becomes more likely. Repeated cycles of heating and cooling also have the effect of potentially shortening the expected life of the rim. The response to this scenario is for rim manufacturers to make progressively heavier and heavier clincher rims to maintain the original margin of safety.

Fueling this race to higher and higher-pressure tires is the generally held notion that higher pressures translate into lower rolling resistance. However, this is not the case. Lowering rolling resistance through extremely high pressure assumes a perfectly smooth surface, which is not something I find on any of the roads I ride these days. Under real-world conditions with the road surfaces a rider is likely to encounter, clincher tires, even those rated at higher maximum pressures, will have a lower rolling energy expenditure at 110 to 130psi, will handle better, puncture less and last longer.

In short, after this long explanation, no, ZIPP 303 and 404 clincher rims will not break apart at 140+ psi tire inflation.
–Bill Vance
National Sales Manager and ZIPP Factory Guy
ZIPP Speed Weaponry


Technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”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.

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