Tuesday, April 12, 2011

More on tires (and spokes)...

I already feel, to some extent, that I've beaten a dead horse on the topic of tire size and air pressure. Besides a properly clean and lubed drivetrain, I think tire selection and air pressure is the biggest thing you can do to make your bike ride as comfortably and fast as you can make it go. A recent ride and a new video posted on YouTube got me thinking more about tires and pressure.

Starting with the video, which is making the rounds on Facebook, it portrays a very telling story of what a bike's tire/wheel goes through over the cobbles of the Paris-Roubaix race. The same thing actually happens to your tires when you ride on the road, albeit on a much smaller scale. The film's slow motion really does a great job of capturing what happens to a tire and wheel on a road surface.



Check out what happens to the rear tire as it rolls off of a piece of pavé and momentarily gets airborne at 1:36. The tire, even with lower air pressure, momentarily slips and loses traction. This is the same thing that happens, at a much smaller scale, on a rougher road with high tire pressure. The tire bounces, loses traction. Any time traction is lost, the rider must somehow make up for that loss by increasing output. This video really shows how tough those guy are who race Paris-Roubaix.

The other cool shot happens at about 2:05. As the front tire rolls over a rock, the tire absorbs some of the shock, but not all. The rim also absorbs some shock and ever so slightly deforms causing the spoke that is perpendicular to the impact to lose some tension momentarily. You can see the spoke wiggle. Every time that happens, the spoke nipple could potentially loosen. The moment this happens in the video lasts all of a fraction of a fraction of a second considering these guys are probably moving across the pavé at 25-30 mph. Imagine what happens to a wheel with 28 sections of pavé during the race's 162 miles. It's no wonder teams spend so much time and thought on tire and wheel selection for the race.

(What's playing: Tom Waits Who Are You)

6 comments:

John anning said...

Do you think it's possible they would be even faster with somewhat wider tires? Are 25-28c tires just tradition, or are they the fastest option?

blackmountaincycles said...

First, most of those frames wouldn't be able to physically accept a larger tire without compromising clearances. Second, for the level that they are riding, I think there is diminishing returns the bigger they go with tires.

james said...

What air pressure do you run your 28's? How low can you go before you sacrifice reliability and performance due to lower rolling resistance?

blackmountaincycles said...

I run 80 in the rear and about 78 in front. I'm about 170-175 lbs. I've not tried any lower, but I probably wouldn't run lower than about 5psi lower simply because of the rough roads.

I'm not sure if this correlation is valid, but I ran 100psi in my 23's. The difference between a 28 and a 23 is about 20%. The difference between 100psi and 80psi is 20%.

alex wetmore said...

I don't think your 20% correlation is really valid. You shouldn't compare the widths of the tires, but the volume of air in them. Those two things don't track linearly (a 20% increase in width from 40mm to 50mm wide adds a higher percentage in volume (56%) than going from 23mm to 27mm (37%)). I'm not going to figure out what the right factor is though.

The 15% drop rule seems to work pretty well in my experience: http://www.bikequarterly.com/images/TireDrop.pdf

rigtenzin said...

The chains are hopping around too. It's easy to see why chains sometimes fall off of well adjusted drivetrains.