Monday, June 29, 2009

The Truth About Tire Pressure

Part of a series I'm calling Tires 101 with lots of great info to consider when looking for new tires.

When it comes to the pavement; more pressure is better.


Nope. And for several reasons. First, a higher pressure tire is harder to control, is actually a little slower, and rides less comfortably, meaning your feel worse and weaker at the end of a ride. Second, its not as safe. I'll touch on each of these respectively.

Mountain bikers and the cyclo-cross world figured this out a long time ago. Why is it that the road world has taken so long to grasp this concept? Well, I would bet that consumer demand plays a part in this. Somewhere along the line we got it in our heads that more tire pressure capacity makes a better, faster tire and so the tire manufacturers keep pushing the envelope.

So, why is this wrong? Like I said, the off-roaders figured this out early.
A tire at the top of it's pressure range (or above that pressure - "over-inflaters": I'm talking to you...) does not compress and conform to the profile of the terrain it is rolling across. This is easy to understand off road where the dynamics of the terrain are bigger and easier to observe. The rocks, roots, pebbles, and tiny undulations in the trail come fast enough that the tire can't just roll over them all -it has to be able to conform to them in order to roll smoothly, quickly, and without loss of traction. If it doesn't - you bounce over them and the tire loses contact with the trail. What happens when your tire loses contact? You lose traction. What happens when you lose traction? You lose control.

"Well, pavement is a lot smoother than my local trail, Matt." You're correct there, but there are still tiny imperfections in the surface that your tires are forced to deal with. Here in the Northwestern U.S. where we ride on a lot of chip-seal pavement, this is easy to understand. But, wherever you ride - you encounter rough roads from time to time. Pay attention to all that jostling the next time you hit a rough patch. That is your tires (and wheels, honestly) struggling to deal with the rapid onslaught of impacts that comes with such an environment. They're literally bouncing across the bumps. This is why cyclo-crossers run lower pressure. They know that as they navigate the course; if they try to accelerate, brake, or change direction while bouncing around they'll lose control. Again, much more dramatic than on relatively smooth roads; but applicable none-the-less. Slightly softer tires=more control.

Let's deal with speed now. So if my tires are at higher pressure, the contact patch (where the rubber meets the road as it were...) is smaller and therefore has less friction with the surface I'm riding across. True. Especially if you're riding on glass. But, in the countless tens of thousands of miles I've ridden, I don't recall riding across any glass that didn't come from a broken bottle. The road is never perfectly smooth. As we discussed above; what happens to a firmer tire on an imperfect surface? It bounces. How is bouncing different from straight-ahead movement? It involves vertical movement as well. Why is vertical movement bad? It is wasted energy that could have been used to propel you forward instead of up.

Slightly lower pressure allows your tires to better absorb the tiny imperfections in the road that would result in more bouncing at higher pressure. This results in less vertical movement (bouncing) and therefore more efficient forward motion. Couple this greater efficiency with the better traction discussed above and you're now also talking about more confident high-speed cornering. Cool, huh?

Comfort? Not a tough thing to understand. More pressure = less tire deflection with impact. Less deflection means more jostling which means a rougher ride which hurts more - especially on a long stretch of chip-seal. Not fun.

So, let's talk about safety. This, to me is the least understood side effect of high pressure tires. Most of the rims that most of us run around on are actually not designed for higher pressures. Didn't know that? Not your fault - it's not well publicized. But, start digging around on manufacturer's websites and you'll find in the small print that most of them have a recommended maximum inflation pressure - especially when referring to the newly-popular carbon fiber clincher rims. This is because the rim's brake track/side wall is not designed to withstand infinite outward pressure, especially when heated up from aggressive braking. If that were the case, we'd all be riding around on significantly heavier wheels. However, again, consumer demand requires otherwise. Want some examples: Zipp 404 clincher (aluminum brake track): 125 psi max tire pressure; Mavic Open Pro - about the same depending upon tire pressure: wider tire = lower max pressure; Bontrager Race XXX Lite Carbon Clincher: also about 120 lbs depending on tire width.

So, let me ask this: How many of you are running around on Vredestein Fortezza Tri Comps at 150+ psi on the above mentioned rims? How old are those rims and how much wear is there on the brake track? As a brake track wears, it gets thinner and less capable of withstanding all that outward pressure - especially under aggressive braking (i.e. fast, curvy, technical descent). Gonna go let some air out of your tires now? I'll wait here until you return...

O.k., finally to qualify my comments. I'm not an engineer and I don't have a way of testing all of these tires scientifically. All I can go by is my experience riding this equipment. And, I've ridden most of the high-performance/high pressure tires on the market (The afore-mentioned Vredestein: max pressure 175; Vittoria Open Corsa: max 145; Specialized pro-series: max 125; Continental GP 4000s, Attack/Force, GP 3000, GP 4-Season: max 120; Bontrager Race X Lite, Race Lite, Race Lite Hardcase: max 120; Michelin Axial Pro, Pro Race, Pro 2, Krylion Carbon: max 116). I've run these at max pressure, below max pressure, and - admittedly - many of them above max pressure. What is my evaluation? I run my Vredesteins at 135, Vittorias at 125, Continentals at 115 and Michelins at 110. Yup, all below max pressure. Some significantly below.

So what about all that talk about contact patch? Well, the higher-pressure tires (135+ psi) will generally have a softer, more supple casing (the woven fabric body of the tire) which conforms and deflects more at a given pressure. So, sure; you run a Vredestein at 120 it will feel slow because it's contact patch is larger than a Michelin at 110 due to it's more supple casing. You must put more pressure into the more supple tire to bring it's contact patch into line with the less supple tire due to it's softer casing. So, what I'm saying here is this:

Don't exceed your rim's max pressure, but inflate your tires to the point where they feel smooth, corner confidently, and still run fast without risk of pinch-flats from being too soft. And, never exceed the max pressure embossed on the sidewall of your tire.
This will require some trial and error over a few rides, but eventually you'll get it dialed. If you're not able to find max recommended pressure for your rims, error on the safe side. 125 is a pretty common max for aluminum clincher rims.

What about tubulars (sew-ups)? The tire's outward pressure is only exerted on the casing of the tire and is basically only exerting compressive force on the rim in the direction that it is already the most strong. Don't exceed the tire's max pressure - but otherwise, inflate away!

Oh, and buy a floor pump with a good pressure gauge ($40+). It's worth the money.

Thanks for reading! Sorry it's been so long since my last post. We're pleasantly busy at the new store.


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