Written by Josh at Zipp – Supplement to LZ Column

Written by Josh at Zipp:Here is an excerpt from a PM I sent to somebody a few weeks ago on this identical topic. I hope this helps clarify things somewhat…

The outer aluminum hoop in our clincher rims weighs only 200 grams, and therefore will get much hotter than a standard aluminum rim. Since the temperature of something is relative to the energy input/mass, a 200 gram rim will get twice as hot as a 400 gram rim for the same energy input. What happens is that if your descending a mountain at 70 degrees and you brake for 2 minutes and your standard aluminum rims heat to 220 degrees (an increase of 150) this same condition with a 200 gram clincher rim will result in a rim temp of roughly 370. Since the carbon is such a good insulator, the rims just have nowhere else to put the heat, so they get hot. This in itself is not the end of the world, except that tubes can melt at these temps, but considering that your tire pressure will rise at a rate of roughly 1psi per 10 degrees, you will see a corresponding tire pressure increase of 30psi.

Assuming your tire pressures are at 125, everything is fine almost regardless of the tire you are using, but if you were at say 160 and then had this same situation, your pressure could elevate to 190 which could be a terminal situation causing the tire to blow off of the rim.

We have seen temperatures intermittently hit 350-400 degrees in mountain testing in the Alps, so the case described here can actually happen. Since there is a pretty wide band of tolerance for tire fit, we can only guarantee that your tires will remain properly seated at their recommended pressures, and must recommend a pressure of our own. We make our rims at the large end of the diameter tolerance for ISO classification, meaning it is harder to get tires onto the rims and therefore harder for them to blow off, but airpressure is such a powerful and explosive force, that when combined with heat we must be very, very safe, not to mention that as tires age and stretch, their pressure capability is reduced, so there are numerous factors which must be accounted for here. Every rim manufacturer has a recommended pressure in this range in their literature, some just advertise it morethan others, Mavic for example has a nifty sliding scale relative totire width allowing for 135psi MAX for a 19mm down 115 psi MAX for a25mm tire, and we have advised similarly, and for all the same reasons.

The other issue to consider is that most every tire on the market runsat optimal rolling resistance between 105 and 125psi depending on loadand road surface condition. We have seen data from numerousmanufacturers and had enlightening talks with others to learn thatnylon cased tires like Michelin or Continental tend to run optimallyaround 105-115 and cotton or bias cased tires like Vittoria orVredestein tend to run optimally at 115-125 maybe as high as 130 forVredestein, but all of them actually will increase in rr at higherpressures due to the tread rubber beginning to fail in shear as itlocally deforms to meet the contour of the road imperfections when thecasing is too rigid. Think of it in terms of heat input, as theoverinflated tire struggles to conform to all the tiny surfaceimperfections to make the necessary contact patch, a lot of heat isgenerated. Not only is there higher rr, but faster tire wear as well athigher pressures, not to mention the tires decreasing ability to staymounted on the rim as pressure increases. In an ideal world tiremanufacturers would list a recommended pressure and not just a MAXpressure (the max pressure is simply a predetermined percentage of thebursting pressure of a given tire as set out by industry standards andhas nothing to do with the pressure you should actually run) but theyare between the rock and hard place as consumers continually push forhigher and higher pressures feeling that ‘if some is good, more isbetter’. Of course none of this even mentions comfort, which we believeto be of increasing importance as more and more data has shown fatigueto be caused by vibration. It may be that by increasing tire pressureby 20-25 psi, you feel faster as your tires are transfering more of thehigh frequency vibration to your body (you’re feeling a higherfrequency ‘faster’ vibrations’ so it really does ‘feel’ faster) but areactually expending more energy to do it, while simultaneously wearingout the tires faster and inducing fatigue.

Looking to pro road teams, most of them are running 100-110 psi intubulars and 105-120 psi in clinchers, and this has been a bit of aknock against the clinchers from the pros, that they prefer the lowerpressures for improved handling, grip and feel, but need additional airto prevent pinch flats. Especially of issue are rainy races, where theymay even lower pressure to 95 psi or so for better grip and control inthe corners. The only real exception here would be track racing,especially on wood, where the surface is so smooth that very high tirepressures can yield excellent RR results, but still generally reducegrip slightly, but even this is specific as a board track may runn wellat 220psi, but a concrete track favors 150-160psi, and some tracksurfaces are no better than most roads… For an analogous example,look to Inline Skate racing, where they carefully select the durometer(firmness) of their wheels to the surface they are racing on, this isnearly as critical as ski wax in ski racing as it can win or lose racesfor you. It is so critical as too hard a wheel will get you droppedlike a hot rock on most road surfaces, whereas that same wheel is theonly way to be competitive indoors on wood.
Josh