Thursday, February 9, 2012

Where the Rubber Meets the Road


People talk endlessly about frames.  They talk about aero wheels, bottom bracket rigidity, carbon lay-ups, internal cables, external cables, sealed pseudo-half-internal cables, and how many seconds they all will save you if you can average 50kph over a 40km ITT.  I get asked which bike is the lightest, which gloves grip the tightest, which bearings spin the fastest, and which bar tape is most aerodynamic.  I take the utmost care in answering these questions because they matter.  There is a difference to be made and an advantage to be gained through them.  But when you get down to where the rubber meets the road, most people overlook just that: where the rubber meets the road.

Tires and tire pressure are crucial to dialing the way your bike handles, rides, and responds on the road—or dirt for that matter.  These two factors are inextricably related and it is a mistake to consider one without thinking about how it will affect the other.  Often issues of rubber on a bicycle are shrouded in confusion, misinformation, and outdated theories about what is best. 

I will now commence the unshrouding.

It is my personal belief that all bicycle tires should be like your childhood dreams: they should be big.  They should be as big as your frame will allow—which for most common road bikes is 25-28c size tire.  At minimum, they should be 23c or larger.  If, for some reason, you have a tire on your road bike that is smaller than a 23c size (yes you should measure it, the labels are often dishonest), I beseech you to immediately rip them from your wheels and damn them to a musty closet, or the darkest corner of your basement.  On the other hand, you could also just replace them with a larger size when they are worn or need replacing—whichever suits your druthers. 

My Michelins are labeled as 25c, but you can see they don't quite measure that way.  I LOVE the ride of these tires.

But in a more serious sense, bigger tires can make a big difference on any bike.  The old adage that narrower tires mean less rolling resistance has been debunked.  To be honest, it was probably never bunked to begin with.  This theory falls guilty of considering tire size independently of tire pressure, which, as I said before, is a dangerous thing to do.  In a sense, it is simultaneously right and wrong, if that is possible. 
All rolling resistance is caused by the deformation of the tire as it meets the road.  In a perfect world, where all the roads were paved with perfectly polished concrete, any reduction in that deformation would theoretically decrease your rolling resistance—meaning that a narrow, highly inflated tire would roll the fastest.  In our very imperfect world, however, an over inflated, overly narrow tire does a poor job of absorbing all the imperfections of the road.  It actually rolls slower than a properly inflated wider tire.  This article provides a great diagram of why this is true.  The amount of deflection for a wide tire is less in relationship to its total volume.  This allows the tire to better absorb the imperfections of the road. 

There are other issues, like a tires flexibility (TPI), the type of inner tube used, and weight that can all affect the speed with which a tire rolls.  In an effort to avoid a endless harangue, however, I will limit this post to size and…


Pressure.  It’s what makes bicycles go.  Can you imagine riding a bike with tires of solid rubber?  It’s preposterous.  The advent of pneumatic tires changed everything.  I am not exaggerating when I say that tire pressure is one of the most important factors in cycling.  Seriously.

The key to understanding proper tire pressure is recognizing the fact that pressure is directly linked to, and affected by the volume of your tires.  Larger tires should—nay, must be run at lower pressures.  It’s physics and not debatable.  Sheldon Brown, who forgot more about bikes in his lifetime than I will ever know, does a really eloquent job of illustrating a great example of why this must be.  You can read his writings on the subject here, and I would encourage you to explore the rest of his site as well.  There is a wealth of knowledge and experience there that is unrivaled.  If you’re not in the mood for extracurricular reading, though, I will do my best to give a brief synopsis of his ideas. 

Tire pressure is measured in PSI or pounds per square inch.  Europeans (and some Canadians) use a metric called Bars, but you needn’t worry about that.  The main point here is that pressure is measured as a force distributed over a defined space.  In our case, this is pounds over inches.  In light of this, it is easy to see how closely related the volume of your tire is to the pressure you should run.  For example, (this is lifted directly from Sheldon Brown) if you have a tire that is one inch wide and you inflate it to 100psi, there is 50lbs of load on each bead of the tire.  If you double the volume of the tire to two inches and still inflate the tire to 100psi, there is now 100lbs of load on each bead.  The tire that is twice as large should be run at half the pressure.  Simple, right?

So what does all this mean for you? Well it has the potential to change the way you feel on the bike drastically.  I would venture to say that most cyclists—somewhere in the neighborhood of 85 to 90% or so—overinflate their tires, regardless of the size.  It’s common practice to pump tires up to the maximum pressure listed on the side of the tire.  The funny thing about the pressure that is printed on the side of your tire is that it is probably determined more by people in the marketing and legal department than in the technical department.  The real truth of the matter is that tire pressure is best determined on a case by case basis.  Issues like rider weight, riding conditions, tire volume, and the personal preferences of the rider all need to be accounted for.  Because the front tire bears far less of the rider weight, it should also be run at a lower pressure (about 10% lower in my personal practice). 

If you were to drive (or ride) to Cadence right now, and put some fat 25c tires on your bike and get them inflated  to the correct pressure (consult Jamie or I about what pressure is best for you), I will guarantee you will notice a difference.  When done correctly, running fat tires will reduce pinch flats and puncture flats, exponentially increase the comfort of your bike, and dramatically improve how your bike corners and handles.  All of this without making sacrifices in rolling resistance! 

My rule of thumb is "Fat as you can fit."

I know you’re thinking, “Wow Scott, you’re a magician.  You just made me faster and more comfortable.”  But I’m just a tuba-playin’, blog-writin’, music-lovin’ bike wrench that is trying to do his job.  You’re welcome. 

This really is a topic that deserves much more time and space than I have given it here.  So if you are interested, stop in, see me, and we can discuss tires in all of their wonderful fat glory.    

For additional tire reading:

1 comment:

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