Time to Re-tire?
Text: Robert Smith
Photography: Robert Smith
Issue: July/August 2006

Life’s too short to only own one motorcycle, right? But most of us can only ride one at a time. If you own a bike that only does a couple of thousand miles a year, should you wait until the tires are worn out before you replace them? If not, when? And how do you know how old your tires are?

Tire rubber is a complex compound blended with additives and modifiers to give the tire a desired mix of properties like grip, flexibility, compliance, comfort, stability and so on. New tires may take up to six weeks to “cure” properly for their desired properties to fully develop. Over time, some of these additives will evaporate from the tire surface, and the cross-linking in the compound will oxidize and break down. The tire’s properties are compromised, and its structure may even fail completely. In such circumstances, keeping “the rubber side down” may become impossible!

So how can you tell when your tires are due for replacement? All chemical reactions proceed faster at higher temperatures; and sunlight, especially the ultraviolet component, also promotes deterioration. The good news then, for northern climes, is that the process of deterioration may typically take longer than it does in Arizona.

Notwithstanding, tires more than ten years old should be replaced. (And, no, I’m not being paid by any tire makers to say that.) So how can you determine the age of a tire? Unless you keep meticulous records, you may not remember exactly when a tire was replaced; and, of course, it may not have been new even then, especially if it’s one of the specialist tires used on classic and vintage bikes. Made infrequently in small batches, these tires may have been hanging around in the supply chain for years.

Fortunately, tire manufacturers give the game away by stamping a date code in the tire sidewall. This is in the form of three numbers, for example “475” on the sidewall of a Dunlop K70 on my 1970 Bonneville. These characters are usually preceded by “DOT” and a bunch of other letters. More on that later.

Before 2000 the date is indicated by two numbers for the week and one for the year in the decade. In the case of my 475 Bonnie hoop, the tire was made in week 47 of 1995. How do I know it was ‘95, not ‘85 or even ‘75? For the 1990s only, a small triangle was used as a suffix to indicate that decade. Before 1990, you’re on your own. And you probably should have tossed them by now anyway.

From 2000 on, the code is four digits long (though some manufacturers didn’t start stamping four numbers until late in that year); and so, for another example, my Sprint ST’s back tire is a “4704,” created in week 47 of 2004. Simple stuff – after 1990, that is. I have a pile of older rubber awaiting disposal in the back of the Big Shed with year codes that could indicate manufacture in either the Disco or Glam Rock era, but I can’t tell.

Now to the longer DOT designation. The Sprint ST’s front skin reads “DOT ENYO VLK 1704.” We’re interested in the first two letters after DOT and the date code. The remaining characters identify the size and type of tire using codes specific to the manufacturer. The first two letters designate the factory where the tire was made. Using a comprehensive list of manufacturer codes (found at www.harriger.com/tiremakers.htm), I learned “EN” means the tire was made at Bridgestone-Firestone’s Kurosio-Shi factory in Tochigi-Ken, Japan, during week 17 of 2004. Similarly, the rear Michelin Macadam on my Laverda Mirage is stamped “DOT HBBN 188T 4002.” So it was made in week 40 of 2002 by Michelin’s S.A.F.E.N.M. subsidiary in Lasarte, near Bilbao in Spain..

So now you’ve really got no excuse. You can confirm the age of your tire using the date code, and you can even tell where it was made. But if in doubt, replace!

Under pressure
Another factor that will shorten the life of tires is under- or over-inflation. Human nature being what it is, under-inflation is more common than over-inflation as tires lose pressure over time, and laziness is more common that zealousness! However, I’ve also met a motorcycle rental fleet owner who asked me if I thought he could run his bikes’ tires at a lower than recommended pressure on the assumption this would reduce wear (and hence his costs). He assumed that a lower pressure would increase the size of the contact patch and therefore counter the tendency of the tires to “square off.”

He was right about the first part: under inflating will increase contact patch on the basis that, within limits, the contact patch area in square inches is the load on the tire in pounds divided by the tire pressure in pounds per square inch. But under-inflation will also cause the tire to run hotter as it absorbs more energy from increased flexing in the carcass. Hotter rubber wears faster, so the net result is probably a wash. And running tires under-inflated, especially motorcycle tires, is extremely dangerous. I suggested that he best stick to the makers’ recommendation.

By the way, the correct air pressure for your tire is NOT the PSI number printed on the tire sidewall. That’s the maximum safe inflation pressure. The correct running tire pressure will be printed either on the swing arm or the frame, and you can also find it in your bike’s handbook. Plus or minus ten percent is acceptable; so if the recommended pressure is 42psi, a range of 38 to 46 is OK, though I find my bikes handle better when I’m right on the money. Check tire pressures cold with a good tire gauge: Don’t rely on those wonky air-line readouts at the gas station.

Tires unplugged
Few things are more frustrating than collecting a nail in a brand-new tire. It’s happened to me twice: once in a front tire, once in the back. The manufacturer, of course, will recommend replacement. In the case of the front tire, I did just that. The rear hoop I had professionally plugged and vulcanized, and it gave me no trouble until it wore out.

This was my decision, and your mileage may vary: but I was prepared to chance a possible rear tire blowout, and definitely not risk a similar situation at the front. That said, the plug in the rear tire was in the back of my mind all the time I was riding. In a similar situation now, I’d fit a new tire, just for the peace of mind. Money can always be replaced!

New tires slippery?
Yes, they are. And though most of the major manufacturers have now stopped using waxy mold release agents, the surface of a new tire will still take a few tens of miles to achieve maximum grip. So take it easy out there!