On the Road

Underinflated tires don't work very well

January 3, 2012

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Regular readers of Heavy Duty Trucking may know me as The Tire Guy. I've written the magazine's tire feature every month for the past four years or so. I got a dose of my own medicine on Christmas Day, trying to take my motorcycle out for a rare winter spin.
Riding a motorcycle on soft tires proves once and for all the negative effects of under inflation.
Riding a motorcycle on soft tires proves once and for all the negative effects of under inflation.


I live near Buffalo, N.Y., not the kind of place you'd expect to do any two-wheeling around Christmastime. But we're having an odd winter this year. Temperatures have been hovering well above freezing since November. The mercury has dipped below freezing only a couple of times so far this season, so, on a rainy Christmas day with temperatures in the mid-40s, your scribe decided it would be a great day to get the bike out.

I mean, how often do we northerners get to ride on Christmas day?

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Because of the wet roads, I decided to switch out my rear tire -- which was due for a change in the spring -- for a tire with more tread on it. I have a few parts bikes in the garage because parts for my 28-year-old Virago are getting hard to find. One of the tires is nearly new with lots of tread for slippery wet roads, and it's still on a wheel, so the change would be simple.

That tire had not been ridden in a couple of years, so I fully expected it to be a little soft. There's a 7-11 up the road from my house with a coin-operated tire pump, so I figured I'd ride up there and fill the tire before I tore off into the wilderness. It's a tubeless tire, and it was firm and still tight on the rim, so I figured it would get me the mile or so up to the corner store.

The bike barely rolled down my driveway. It labored in first gear, and nearly stopped when I shifted into second. Had I tightened the wheel too much? Were the bearings under too much load? Was the brake dragging? No, I had hand-spun the wheel before taking it off the stand. So what was dragging?

I nursed the bike up to the store, fished four quarters out of my pocket for the tire pump, and set to inflating the tire. The manual recommends 28 psi inflation pressure, but I had less than 10 psi in the tire -- about one-third the suggested pressure.

Topped up and saddled up, I wheeled out of the 7-11 parking lot like the wind. The drag was gone and the bike rolled along as smoothly as ever. Could a soft tire have made that much difference?

The sidewalls on a motorcycle tire are fairly stiff, and the tire didn't look visibly mishapened while I sat on the bike, but there was certainly something in that tire that didn't like running soft.

A few years ago, Tim Miller of Goodyear provided me with a quote about underinflated tires that resonated because I had been there. He said driving on soft tires was like walking barefoot on a beach with deep sand. Pushing the shifting sand aside with your foot requires energy, and that's why your legs get tired walking in deep sand. It takes some effort; like rolling a 400-pound motorcycle with a 200-pound rider on a tire with just one-third of its recommended pressure.

I'll grant you right now, my tire was severely underinflated, but it didn't look or feel underinflated. Much like a truck tire with 50 or 60 psi instead of 100 or so. You can't tell much by thumping a tire, but your engine would sure notice a difference, especially if several tires on the truck were under-inflated.

So, I'll call it a lesson learned -- the hard way as usual. A little more experience I can bring to my monthly tire column. And you, dear reader, can believe me or not. If not, you'll have to learn the lesson the hard way yourself.

If that's how you operate, take Tim Miller's advice and go for a walk on the beach rather than mine, trying to roll soft tires down the road.

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Author Bio

Jim Park

Equipment Editor

Truck journalist 13 years, commercial driver 20 years. Joined us in 2007. Specializes in technical/equipment material (including Tire Report), brings real-world perspective to test drives.

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