'Theoria'

Talking about pressure: getting the most out of your bike’s tires

People ask me all the time about tire pressure, particularly when they are moving into new territory – high volume tires – on their bikes. In Ottawa-Gatineau, tonnes of riders have cyclocross bikes for everything from commuting to racing to dirt road riding. As such, many have been trying out high quality, supple, high volume tires on their CX bikes over the last few years for events like our local Almonte Roubaix, and our club’s Ride of the Damned. Both are run on paved and unpaved roads that demand careful tire choice. As riders branch away from commuter and touring tires like Panaracer Paselas and Continental Gatorskins (read: stiff), they find themselves on a new learning curve. Sure, one runs under 30psi in a good cyclocross tire on grass, but what about a supple Compass tire in 33mm on pavement? More, obviously, but how much more?

I’ve leaned on the Berto tire drop chart since Bicycle Quarterly published it years ago as a starting point for figuring out what pressure to run supple tires at. What surprised me when I moved from Grand Bois’s standard casing tires to their then-new Extra Leger casing was that I had to run MORE pressure than before. I went from about 60psi rear to about 70 in their 32mm tire. This was to prevent folding on pavement. So when I got Compass’s 38mm EL tire last season, I knew I’d have to run them higher than Berto’s chart indicated too. Wait, more than Berto’s chart….where did all this tire nerdery come from?

As I remember, Rodd Heino told me about Grand Bois tires way back when we started riding the dirt roads north of Ottawa. We were on tires like Panaracer Paselas, which were good, but not fast. Rodd learned of tires being sold out of Seattle – Bicycle Quarterly – that came in 700x26mm, 28mm, and 30mm, not to mention 650b x 42mm. We started using them, first the 28s for me, all that would fit my Specialized Roubaix, then the 30s, around which my custom Steelwool all-road bike was designed.

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My custom Steelwool Sector 18 al-road bike, designed to comfortably fit 33mm tires. These are Compass’s 32mm Extra Leger tires, which measure 33mm wide on Woven’s 19mm internal width rims. This setup is FAST!

 

There was a learning curve to get through. We were usually using Continental’s touring tubes inside the 30mm tires to help prevent pinch flats. That was great, except I eventually figured out that since they held air pretty well, I ended up heading out for lots of rides with nothing more than a squeeze test. I wound up flatting plenty, not to mention denting rims. I figured it was just my downhill mountain bike style doing the damage, and that was part of it, but later realized it was more to do with simply running too little air. Once I got onto latex tubes, I had to check air pressure before every ride, and since latex are harder to pinch than butyl tubes, I was doubly ahead! I flatted less.

Vermont Overland GP Dirt Road Race 3
Here’s my cyclocross bike on the left with Compass’s 38mm Extra Leger tires and latex tubes. Iain Radford’s Specialized is rolling on 650b Compass Hetres in 42mm. These are what we rode in the first edition of the Vermont Overland, where they handled the range of paved and dirt road and trails really well.

 

Cyclocross racing on supple tubulars was revelatory, and the keen awareness of tire pressure and its effects on performance wound up carrying over into other riding disciplines. Add fat bike tires to the mix, and you’ve got a situation where tire pressure is top of mind a lot more than it used to be.

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For snow-covered and icy roads, we use Clement’s LAS file tread cyclocross tires in 33mm (36mm on Woven rims) and Continental’s Cyclocross Speed file treads (35mm on Wovens). Both are very supple tires, so they roll fast and conform well to the imperfections of the roads, providing good grip. Latex tubes.

 

Fat bike tires actually end up being the most demonstrative format when it comes to understanding tire pressure’s relationship with rolling resistance. That is, since they are ridden on snow at very low pressures (far under 10psi, usually less than 6) to flatten them out for the sake of flotation, they are always compressing a lot as they roll. A stiff-casing fat bike tire wants to remain fully round, due to its structure, even at low pressure. So when you roll it over and ask it to flatten, that structure resists being flattened. Herein lies ‘rolling resistance.’ The take-home is that a low TPI (threads per inch used in the casing, the fewer the stiffer) tires roll slower than high TPI tires at low pressures. The difference one can feel on the bike is astonishing. If you pump a low or high TPI fat bike tire up to 10 or more PSI, you’ll feel little difference in rolling resistance.

With 23mm tires, the volume is so low you can’t run them at low pressures because you’ll pinch flat. So most riders can use about 100psi and forget about it. But what happens when you up the volume to 25mm? Many riders use the same pressure, and that’ll mean the 25mm tire will roll about the same as the 23mm tire, but will offer a little more grip, because it’s got more rubber on the road. But if the tire is supple, you can drop the pressure to say, 90psi, and you’ll get a smoother ride, more grip, and the tires won’t roll slower, but faster than the 23. If the tire is stiff, however, it will roll slower. This is the reason Jan Heine has just come out with  a blog post on tire pressure, to help riders understand how to get the most out of their tires.

From my perspective, Heine’s most important point about the difference in speed between a supple tire at a higher versus a lower pressure (when both roll equally fast) is that perception is misled by the high frequency buzz of high pressure tires. That is, as speed increases, buzz increases, so riders learn to interpret lots of road buzz with lots of speed. However, one could ride the same speed on a softer tire and not perceive their speed as being as high. My favourite analogy for this is a rigid mountain bike versus a full suspension downhill bike. On the former, 25kph on a trail might feel ‘on the edge,’ with the bike bouncing and deflecting off every rock and root. It’d feel ‘scary fast.’ In contrast, a downhill bike, ridden at the same speed would feel slower, and certainly less exciting and ‘extreme.’ One has to wonder whether it’s actually better to be able to ride faster for a given perception of speed on a mountain bike, versus sticking with a simple platform and simply going slower. I digress….

If you’d like the Cole’s Notes version, here’s the summary on rolling resistance Heine provides. If you’d like to understand more on the ‘why’ side, please jump over to his post. In case you’re wondering, every statement Heine makes aligns 100% with my years of experience as a tire nerd riding all manner of bikes.

Jan Heine’s tire pressure summary:

  • Stiff casings always will be slow. They are even slower at lower pressures.
  • Supple casings are fast, and pressure doesn’t matter.
  • On smooth roads, tire pressure is a matter of personal preference (at least with supple tires). High and low pressures offer the same performance.
  • On rough roads, lower pressures are faster. So if you want to optimize your speed on all roads, including rough ones, go with a relatively low, but safe, pressure.
  • Your tire pressure needs to be high enough to avoid pinch flats. If you get pinch flats, increase your tire pressure, or better, choose wider tires. Pinch flats are rare with wide tires.
  • On pavement, your pressure needs to be high enough that the tire does not collapse during hard cornering.
  • The minimum safe pressure is higher for more supple casings. Stiff casings hold up the bike more, and thus require less air pressure.
  • On gravel, you can run lower pressures than on pavement. On loose surfaces, the tires don’t collapse as easily, because the cornering forces are much lower.
  • Don’t run your tires so low that the casing cords start to break. That happens only at very low pressures, but if you start seeing multiple lines across the casing where cords have broken, inflate the tires a bit more.
  • Berto’s chart still is a good starting point. Inflate your tires to the pressures it recommends, then experiment by adding or letting out some air.
  • See what feels best to you. That is the optimum tire pressure for you. Don’t worry about tire pressure any further! At least on paved roads, you won’t go faster or slower if you change your tire pressure.

To cap things off, at a certain point, riders attending to tire pressure will tend to want to know whether the pressure guage they are using is accurate, or at least consistent. Both are very difficult to know. When using a friend’s floor pump, the question every discerning rider asks is: “Is the guage accurate?” The problem is, nobody knows! I don’t know whether my pump’s 100psi is 100psi, and my friends don’t know whether their pumps’ 100psi is the same as my 100psi! It’s a gong show! Usually a rider will say something like, “Mmmm, I think it reads a bit high.” Or, “I think it reads a bit low….” When it comes to pressures for dirt road riding, I like to use the ‘square edge test’ to determine whether I can bottom the tire. Essentially, I use something like a stair step, and place the front wheel onto it while standing over the bike, front brake clamped. Then I push down with all my body weight and try to bottom the tire. If I can’t, that means I won’t likely bottom the tire on a rock or whatever. I usually want to come close, but not all the way. For cyclocross, you’d easily get all the way there.

I’m at a point where I’d really appreciate a gauge that is accurate, for a loooong time. After reading Josh Poertner’s blog post last year about how he came to purchase the Silca brand and develop the ‘ULTIMATE‘ floor pump, I knew which pump that would be. The amount of time and engineering that has gone into this pump’s gauge is incredible. Aside from people balking at the price (this is ‘investment level), I’ve yet to hear a single negative comment about the actual pump. Hopefully this year I’ll be able to acquire one, and it will be the pump everyone in the parking lot will want to use.

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To learn more about how we all got onto this whole ‘carbon fiber’ ‘wide rim’ ‘high volume tires’ thing in cycling, I encourage you to read Poertner’s series of blog posts, starting here. In case you noticed, yes, we are partnered with Silca, and that is the direct result of having read Poertner’s posts I am referring to, and my reaching out to do something, anything, to support his work and the brand.

Related Posts

Rasputitsa Pre-ride: We’re all in this together

How the Race was Won: Rasputitsa

How the Race was Won: Vermont Overland 2015

Dirty 40 Race: Flying too Close to the Sun

The Ride of the Damned: Recap

 

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