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zondag 8 april 2012

The flip point: extremely narrow tires on velos

Tires have been one of my favorite subjects to investigate from the day I found out that the rolling resistance at moderate velo speeds is almost as high as the aerodynamic drag. Apart from reading a lot about it on the Internet, I have switched tires countless times (I have a closet full with half used tires) and almost every time I ride home I roll down from a small bridge to see how far the tires of choice will roll. This is of course not very scientific, but over the course of the years I have gathered a lot of insight on tires this way.
Rolling resistance is the energy that is lost when the tire deforms in contact with the road and the rubbing of the rubber molecules amongst each other. There are several ways to make these losses smaller. Narrow racing tires typically use the most advanced techniques to lower the rolling resistance (RR) like more and thinner threads to make a thinner and more supple tire possible and better rubber compounds. For wider touring tires, the tire producers tend to give more attention to puncture proofness and durability. I think I can safely say that the HPV scene is slowly changing this, because there are a lot of geeks among them that like to push the tire producers towards Speed.
Especially velos do not really need the low aerodynamic drag of a narrow tire, because they usually are embedded in wheel arches where they are almost completely out of  the airflow anyway. Since a wide tire deforms less than a narrow tire they can have a lower rolling resistance, but only when the rubber compound and the tire casing shows up similar techniques as the narrow racing tire. However, the narrow tire has a last line of defense! It can be pumped up much harder than a wide tire before bursting. A hard tire deforms less in contact with the road, so less RR.

The following question came to my mind: how practical is it to use narrow racing tires on a velo?
To come up with an answer, I have ridden Schwalbe Durano 28-406 on the front wheels all winter long and for a shorter period the most extreme 20" tire on the market: Schwalbe Ultremo 23-406.

Durano: a racy tire but much more durable than it's predecessor the Stelvio (not in produktion anymore). I managed to wreck the sidewall of a Stelvio in one race, but the Duranos' can handle almost anything I throw at them. Almost, for the one thing that took one of the Duranos' out prematurely was heavy braking to the point that I lock up the wheel. The abrasion was too much and the road simply scrubbed right through the canvas, also destroying the tube inside.

destroyed Durano, the canvas scrubbed through

Well, since then I have been braking in  a more sensible manner and didn't have that problem anymore.
As to the riding experience: the Duranos' are fast tires and they give the Mango the smallest possible turning circle, so agility is also better. The feel in corners is very direct, so I know exactly what is going on. Because of the 9,5 bar of pressure I keep in them,  they do not bulge under the heavy load in fast cornering like wide tires do, especially when the pressure is down to a very comfortable level. Especially in roundabouts the velo may become a "Gallopin' Gertie"  with the loaded front tire hopping up and down uncontrollably until you slow down. I like to go fast in corners, so this all is a big plus for me.
Not related to RR, but the light weight does also contribute to the riding experience, since I do relatively much riding in town. A lighter tire accelerates easier, since there is less  rotational weight to bring into movement. Likewise, braking is a bit quicker but as written before, it is wise to not let the wheel lock up completely.
On a sidenote: I have put on lighter wheels with narrower rims to have even less rotational weight. The narrow rim is also better suited to such a narrow tire.
Comfort is of course less than what I was used to, but since the Mango has good shocks it's not much of an issue. I got used to it soon enough, but I do avoid getting next to the road and ride much slower on cobblestones. High pressure in the tires does make the velo noisier as impacts are simply more intense. Despite the high pressure and ensuing small contact patch, there still is a reasonable amount of grip on the road. I was surprised by that, since I had the idea that the high durability (for a narrow racing tire) must have something to do with a hard, slippery compound. Not so: the grip is not much less than that of the wider Kojak.
To make things more interesting, we had snow for a while this winter and one might think that a narrow tire is not good for that. Sometimes it's completely the other way around then you'd expect: the Durano cuts through fresh snow easily and does not have to displace as much snow as wide tires, so riding takes less effort and you stay in contact with the road. The non-driven front tires do not need to give traction, so that is not an issue either.
 Apart from the above mentioned flat because of the abrasion of the tire, I had no flats all winter. The thin Raceguard protection that Schwalbe puts in seems to do a pretty good job.
Maybe you have noticed that I put in 1 bar more pressure than Schwalbe gives as maximum pressure. I do so, because high pressure cuts the RR from narrow tires enormously and I somehow have a lot of confidence that the Durano is strong enough, unlike the old Stelvio.
So overall the Durano is pretty practical, be it that you won't like to do dirt trails with it as it sinks away too easily there.

Ultremo: Given the rather good impression I got from the Duranos' it only seemed logical to take it even further. The even lighter, narrower Ultremo tire. The ultimate in the line-up from Schwalbe.
Ultremo: best kept on the shelve until racing day?
Putting the tire on the rim was a bit more tedious than with the Durano, but not really very hard to do. It got a whole lot trickier when I wanted to pump them up to their max. 11bar! It's not simply a question of a few pump strokes more. Most pumps do not get up to that pressure at all, but we have an excellent track pump at the shop that managed it. Alas the pressure gauge was bust, so I had to check the pressure with a separate digital gauge from Schwalbe. The problem was to not lose too much pressure while putting the gauge on the valve stem. It was almost funny how many times I had to pump and pump again to get close to the desired pressure, because the moment I pulled the gauge off the valve it could easily lose one bar in pressure! When I tried to pump it up a few days later at home again I found that my own big floor pump only just could manage to get enough pressure in the tire. Certainly my little pump that I always take with me in the Mango, cannot get even close to the desired pressure. Since I usually swap tube and tire in case of a flat it is no issue on a daily basis (simply put on a tire that does not need extreme pressure) but on multiday trips it IS a serious problem. I know, I just keep on rattling about high pressure, but it is truly vital for such a tire to have it topped up at all time. I noticed it on my roll-out course that the variation in distance rolled varied a lot, depending on how much pressure I managed to get inside! Furthermore the tire will no doubt be destroyed easily when the pressure gets too low. It's the high pressure that has to maintain the shape of the tire. Too low a pressure and the sidewalls will bulge the very supple sidewalls until the rim cuts it up. The infamous snakebite flat is therefore a very likely thing to happen when you do not keep a close eye on pressure.
In all other respects the Ultremo is a Durano on steroids and so far it also seems to be strong enough. Only the pressure that is needed to keep this tire going good, is enough for me to completely disqualify it as a practical tire. If you seriously want the max in speed however, this is it.

As you may know, I have an enormous collection of YouTube vids. Here is one featuring the Duranos:
Yep, they also come pimped (white tread)

Don't forget to alter the setting on the bicycle computer:
circumference Durano @9,5bar=146cm
circumference Ultremo @11bar=144cm