Are freeride tires faster than XC race tires?

Disclaimer: I wrote this story three years ago for VeloNews, and it was so intricate and demanding to read that even they wouldn’t run it. (Heck, I didn’t consider running it until I posted this story.)

The VeloNews editor also didn’t like the idea that, on a long trail ride, freeride tires might be faster than XC race tires. Trying to convince him that skill, cornering speed and confidence are more important than low weight and rolling resistance was a lost cause.

You’ve been warned. Now check out this action:

Mountain biking has its own brand of religious dogma. Light tires climb way faster than heavy ones, low rolling resistance is the key to fast trail riding, light wheels and tires make your bike much faster, and so on. These pronouncements are certainly popular, but are they true?

Several online calculators make it possible to calculate how various bike setups, terrain and conditions affect your power output and speed. As an example, let’s look at the effect of switching from light cross country tires to burly freeride tires. Mark Twain said, “If you torture a statistic long enough, it will confess to anything.” Well, let’s torture us some statistics!

Click for big.

Assumptions
– Rider weight: 180 pounds

– Rider position: Upright

– Bike weight: 26 pounds with light XC wheels and 2.0 cross country tires. 27.2 pounds with burly 2.4 freeride tires.

– Added weight: Changing from 2.0 cross country tires to 2.4 freeride tires adds 1.2 pounds to the wheels.

– Added rolling resistance: Hard to measure, but my neighborhood tests put it at +10 percent.

– Added cornering traction: Hard to measure, but photos and race times make +50 percent seem conservative.

– Power outputs: 100-watt cruise, 300-watt hammer, 800-watt sprint

– All other variables: Technique, wind, sand, rocks, berms and moon phase are all ignored. Remember the movie Tron? Let’s hop on our light cycles and see what happens.

Climbs
The skinny tire leaves his fat friend in the dust, but not by much. The steeper the hill and the harder you pedal, the smaller the effect of tire weight and rolling resistance. Fatso might lose a mellow one-mile climb by 3.3 percent timewise, but on a hard steep climb he’ll stay within 1.5 percent. Heck, if the climb is really steep and loose, Fatso might actually win.

Time to hammer up a one-mile climb:

Next time you feel like you’re pulling a tractor tire up a hill, remember your macho tires aren’t slowing you down as much as you think. Keep pedaling and hope they pay for themselves later.

Flats
Air drag increases exponentially with speed, so while your buzzing knobs might drive you crazy, your real impediments are all those displaced air molecules. Keep pushing. The harder you go, the less your big tires are slowing you down.

Time to ride one mile flat:

Descents
Now those big meats earn their keep! Coasting down a steep descent, weight means nothing, and you push so much air that those growling knobs become insignificant. Meaty tires cost you a few seconds on gentle grades, but when the pitch steepens to 10 percent they’re just as fast as the puny ones. When your bike wants to roll at 40 miles per hour, the limiting factor becomes confidence.

Time to coast (no brakes) a one-mile straight downhill:

Acceleration
Without a doubt, lighter tires make a bike feel much quicker. In that first instant of acceleration, before the rider starts moving, the lighter tires bestow a 40 percent reduction in rotational inertia — enough to turn a hippo into a jackrabbit. But once you start pushing your body through the air, that advantage all but disappears.

Distance traveled after two seconds of accelerating from a stop.

Mountain cross racers like Brian Lopes swear by light rims and tires because they deliver faster starts. Light wheels are indeed faster, but by only one or two percent. Lopes only needs an inch to take a world championship. If your powers are sub-Lopesian, ask yourself whether a narrow lead is worth blowing the first turn.

Corners
Cornering forces are a product of radius and speed. For a given turn, increasing your speed exponentially increases the Gs — and the amount of traction you need to stay out of the bushes. Let’s assume the skinny little XC tire will hold 0.3 Gs in a turn and the wider, side-knobbier FR tire will hold 0.45 Gs. (By the way, those forces correspond to 17- and 22-degree lean angles.)

In our imaginary world, the burlier tires save you 1/3 second in every turn — not to mention reducing the need for post-turn acceleration. On really curvy trails you never reach full speed, so you end up riding at your cornering speed, which is about 20 percent higher with the knobbier tires. Ride a mile of pure turns, and freeride tires will save you almost a minute and a half.

Riding a 90-degree turn with a 15-foot radius

A virtual race
How do these tires compare over a fantasy mix of climbs, descents, flats, straights and curves? Over this 32-mile ride, the XC tires make time on the climbs and the straight flats, but they quickly lose ground on steep descents and curvy sections. Increased confidence and cornering speed make the freeride tires more than three minutes faster overall.

Click for big.

Go forth and calculate
You can tweak these numbers however you like, but the fact is lighter, faster-rolling tires might not always be faster on the trail. At the very least, think about how you dress your rims. If you’re serious about optimizing your bike, run some experiments. Ride your favorite trails with a timer and a heart rate monitor or wattage meter, and find out which tires ride fastest.

If you’re feeling nerdy, you can find the spreadsheets I used for these calculations at www.leelikesbikes.com/calculators.html. These let you calculate the effects of everything you can imagine. Plug in your numbers and torture some statistics of your own.

Whew! If you made it this far, you rock. What do you think?

— Lee