One of the enduring myths of cycle racing concerns the relative merits of tubular tyres relative to clinchers. Convention has it that nothing can compare to the performance of a quality tubular. However, lot of anecdotal evidence is bandied around supporting high quality clinchers as superior to tubulars. I was recently sent a link to test data regarding the characteristics of tyres versus tubulars - this gives clear data to support this contention.
Clinchers and tubulars
A couple of images to show the differences between a tubular and a clincher (links to external website images)
Clinchers are held to the rim by the "wires" held by the air pressure against hooked rim walls. Tubulars are held in place by a combination of glue and air pressure - the glue (or rim cement) is a kind of contact adhesive that remains tacky. Applying the glue is time consuming and often messy. Rim tape, a specialised double sided sticky tape, can also be used. Glue/tape characteristics probably affect the rolling resistance of tubulars.
Typically there is less and less difference between clinchers and tubulars in the materials used in their construction, a good example being the Veloflex Record clinchers and tubulars which appear to use identical casing materials.
What is rolling resistance?
Pneumatic tyres offer a significant advantage over solid tyres as they deform to accommodate imperfections in the road or track surface. Rolling resistance describes the frictional losses due to such deformation of the tyre1, 2 - this is influenced strongly by the pressure to which the tyre is inflated, the dimensions of the tyre, and the structure and composition of the tyre and its inner tube, as well as the rotational speed.
Generally speaking, on tarmac roads higher inflation pressure gives lower rolling resistance - though very high pressures eliminate the benefits of pneumatic tyres as they have very little give to them, and traction is lost.
Rolling resistance is expressed as the coefficient of rolling resistance, abbreviated to Crr though sometimes, as in the table, it may be abbreviated to Crr. Here's the maths bit:
F = crr * Nf
In this formula, F is the resistant force, crr is the coefficient of rolling resistance, and Nf is the normal force3 (i.e. the weight supported by each wheel). For some discussion of rolling resistance, see this Wikipedia article)
How the tests were done
From the data table, it would seem that the tests used a bike mounted on Tacx rollers (79mm diameter). The power required for 25mph was measured using SRM power cranks, and the coefficient of rolling resistance calculated using a spreadsheet (which I haven't yet found a copy of). I therefore have no idea how valid this calculation is - I assume it is fine, as the crr values cited in the table are not unusually small or large.
How valid are the data?
There would appear to be technical problems with many methods of determining rolling resistance. Often (as is the case here) the experimental set-up involves running the inflated tyre/wheel against a drum or roller. In this case, 79mm diameter rollers were used. The narrow diameter roller does not effectively mimic the tyre rolling on a flat surface, and will give inaccurate results. That being said, this set of tests is the only one I've seen where the same method was used to investigate such a wide variety of tyres and tubulars, and I have no doubt that it provides a good basis with which to compare tyres within the test.
What do the data mean in terms of race performance?
The major obstacle to riding a bicycle fast is air resistance. The forces resulting from air resistance increase with the square of the frontal area of the cyclist (hence we race crouched own on aero bars). A (probably simplistic - it doesn't take into account rider position) air resistance calculator indicates the typical resistive force at a riding speed of 25mph to be around 16.4N on a still day - with a 10mph headwind, this rises strikingly to 32.2N. How does this resistive force compare to the rolling resistance? Using the formula above, a 70kg rider on a 10kg bike shod with Veloflex Record clinchers would experience a resistance of 1.9N compared with 2.5N using Continental Supersonics - a saving of 0.6N. In relation to air resistance at speeds around 25mph this is small, but significant (about a twentieth of the air resistance).
Nevertheless, the fact remains that the biggest equipment-derived improvement in time trial performance comes about by reducing air resistance, principally by better positioning the rider's upper body.
Is there a place for tubulars?
Wheels with sprint rims are generally lighter than corresponding clincher wheels, so riders seeking ultra-lightweight kit will no doubt see a place for tubs. And my experience (in the only 12 hour time trial I ever rode) is that a properly glued tub is quicker to remove, replace and re-inflate in an unsupported time trial situation than a clincher. That being said, I think most riders would be inclined to pack following a puncture, certainly at events shorter than 100 miles. Furthermore, I stress properly glued. If the tub is very strongly glued on, or if you've used certain brands of tub tape, getting the tub off can be hell - not to mention time-consuming and thumb-skinning.
What tyres will I ride in 2008?
I used to use Veloflex Record clinchers, before switching to Veloflex Record tubulars (caused by a replacement wheel, in turn due to Parcel Force losing a Hed Deep front wheel en route to Hed). In recent years, however, I've suffered so many punctures to lightweight tubulars that just finishing events sometimes felt very much like a lottery with odds stacked against me. One of the greatest advantages of clinchers is the ease of repair following punctures. It's largely for this reason I decided to switch back to clinchers (and in fact used them in the last two or three events of 2007).
At the moment my H3 trispokes have Continetal Supersonics fitted. I'll continue to ride these for a bit. I might try Veloflex Records again, but I'm concerned that there are too many flints on the roads round here for them (I'd rather go for a slightly more robust tyre with higher cr and finish than a low cr tyre and puncture!).
See also:
1. rec.bicycles.tech posting on rolling resistance
2. Sheldon Brown's excellent website has an article on rolling resistance.
3. The normal force actually equals the mass supported by the wheels, divided by the number of wheels, plus the mass of the wheel, all multiplied by g, the gravitational force (9.81 ms-2 on Earth.
Update 6/3/08: See also this page of clincher data, and this page of tubular data, which are in agreement with the above conclusions, albeit with different absolute values of Crr.