Really great comments and I think I can add some detail.
Tom A. you are right, the term ‘increased air volume’ has been used in terribly misleading ways…this is much the same that wide rim formats are now being equated with ‘stability’…and this is further muddled by the difference between a wide rim and a wide bead seat on a clincher, which may be on the same rim, but may also be mutually exclusive of each other. for instance the original 808 was launched at 27mm wide but only 21mm at the brake track. it definitely fits the bill for ‘wide’ and was (for the time period) a rather stable handling wheel, but we just didn’t have the techniques or thought process to design for stability yet, and we were dealing with riders wanting 21mm tires, bike companies shrink wrapping chainstays, and brakes that wouldn’t open past 28mm in most cases…so the hurdles to overcome to acheive Firecrest were considerably and were not limited to issues within any single company.
Ultimately the ‘increased air volume’ benefit is purely limited to the zone of the tire between the road and the rim. Wider tires also have larger diameters, so they keep the rim further from the ground, which is beneficial in pinch flat situations. widening the stance of the tire increases the cross sectional area of the tire and requires more energy for an impacting object to effectively reach the rim.
As for wide rims in general, deep rims need width in order to have curvature…the curvature is how we control and manipulate the airflow… look at 90mm deep rims of today with V and U shapes…they are comparable to 45 and 60mm toroidal and Firecrest shaped rims. The wide tire bead format is a slightly different story which came from touring rims and was more a result of the assumption that you would be riding wider tires. if we were all still running 19mm tires it is unlikely 16-17c clinchers would be in vogue…largely because the ETRTO specifically states that 19-20mm tires cannot be used with 17c rims for safety reasons. so with the assumption that people were choosing 23-25mm tires, the wider bead seat began to make sense. This was very similar to the progression of tubular rims. In 2004 we launched the 27mm wide 808 with a tire well designed for a 21mm tire. This was the direct result of a 2002 wind tunnel test I attended with Lance, Postal, Trek, John Cobb and others where the 3 spoke wheels were all being tested with 19mm tires and the team was talking about Bruyneel’s mandate of a 21mm minimum for safety reasons… I took the lesson back and began redesigning all the Zipp stuff around 21mm tires instead of the 19mm we had used for the previous 10 years. In 2006 we began designing our tubular tire wells from laser scans of actual inflated tires. when we did the original Super-Toroidal 303 as the first carbon Pave wheel in 2008, we optimized the tire well around 25-28mm tires as tires that wide don’t glue well onto small radius tire wells…and found that tire disbond forces were much higher due to the better interface between the rim and tire… since you have to design around some width of tire, you might as well design around the most likely one…though that doesn’t mean the others are inferior.
Having said all of that, there is a lot of baggage and misleading banter about ‘increased air volume rims and tires’. For one, you can measure the change in contact patch, but we have never been able to measure a decrease in rolling resistance. If it’s there, it is in the noise of the various test equipment options, we’ve never seen it, and I don’t know of anybody who has data showing it to be true…and at that, if it were true, it is would technically be the shorter/wider contact patch (and the larger effective casing radius) to credit and not the additional air volume…so Tom is right, all the talk about air volume is confusing at best, though without writing a paragraph every time, I’m not sure how we change the language to make it more clear.
Secondly, wider bead seat really has nothing to do with wheel stability from a bike handling point of view…it changes pneumatic trail, it changes lateral squirm and slip angle for a given pressure, but it isn’t inherently any more ‘stable’…though that term is generally lacking definition. It is amazing how quickly this notion has spread through everybody’s marketing and into the media, and I think there has been some amazing power of suggestion happening as pretty much every new wide rim has to make stability claims, and everybody generally seems to be buying in. I can argue that the shorter wider contact patch is slightly less stable due to the reduced resistance to steering inputs by the contact patch…this is why many riders feel the need to reduce pressures to regain the steering response they are used to.
The stability we talk about with Firecrest is based on aerodynamic stability where we have designed the aerodynamic center of pressure to be very near the steering axis, and to only move a very tiny amount with yaw angle changes…essentially the wind is pushing the wheel around with the tiniest possible lever arm compared to other designs. The thing that the wide rim and wide tire bed help here is that they allow the designed shape of the wheel to work with wider tires and all narrower options, whereas with narrower tire beds, the aero properties of the wheel change dramatically beyond a certain tire size. I’m not throwing this is an a plug for product, but just to say that is our definition of ‘stability’ and it is not a direct result of a wider tire bed, but the wider tire bed does make it more applicable across a wider range of tire choices.
Lastly, we hear frequently about designing for narrow tires, but to that I say try 20’s on your Firecrest rims…it’s smoking fast, and since the rim is so wide it can clean up the dirty air off of the tire at even higher yaw angles, so the stall angle of the wheel is very high. the only place you pay for the rim width is near 0 degrees, where the added frontal area of the system remains similar to when the rim has a 21 or 23mm tire…but past that, it is very, very fast. Lemme go find some data…
Josh
