Service

SERVICE

PAUL DEAN

Striking a cord

Q I was wondering what the actual life expectancy of a tire might be. I just finished a Border 2 Border Insanity run on my 2007 Triumph Speed Triple with a 180/55-17 Bridgestone BT-023 on the rear. On my way back to Phoenix, I noticed I had worn through the tread and was nowhere near a tire shop, so I plodded along until I got home. Happily, I didn’t make it through to the cords (not that riding on that tire was very safe in the first place). I am curious as to how far you can actually push a tire if you really had to do so. Will it last long enough to get you to a shop for a replacement? Or was I just damned lucky? Mark Houlden

Phoenix, Arizona

Ain pure clinical terms, you’ve asked the unanswerable. I’ve seen tires that had been worn down so ridiculously far that loose cords were whipping around and flailing against the rear fender, yet the tires were still holding air. I’ve also seen others that began losing nressure the moment the first cord peeked through the rubber.

Once, I watched a rider pull into a restaurant parking lot on a Sportster with a rear tire that had a wide stripe of exposed cord running all around the middle. At that point, the tire looked fully inflated. But when I walked out of the restaurant about a half-hour later,

I heard a loud hissing sound and saw the rear end of the Sportster slowly dropping as the air rushed out of that tire. Even though the bike hadn’t moved since being parked, air finally found a way to get through the carcass. That’s how unpredictable such matters can be.

It’s also important to remember that not all tires are alike. Some have more and better materials (heavier plies, thicker belts, stronger belt construction, etc.) under the tread area than others, which helps prolong their possible survival when completely worn out. Plus, the farther the tread wears, the easier it is for a nail or other sharp object to penetrate to the interior of the tire. When that happens on a tire that is worn down to its cords, the result might not be a tvnical slow air loss that gives a bit of a warning; it instead is more likely to be an abrupt, sudden deflation—a “blow-out” like is common with tubetype tires.

So, were you lucky? You bet!

The dirt on four-strokes

QWhy do high-end four-stroke dirtbike engines have such a short shelf life (just 20 hours on a piston and rings, for example). Any modern sportbike engine is arguably very high-tech and is as much, if not more, a racebike as any four-stroke dirtbike. With the exception of oil changes and very long valve-adjustment intervals, sportbikes last many thousands (if not tens of thousands) of miles with little if any performance decrease.

I ask because I’m considering purchasing a new 2009 KTM 250 XCF-W from a dealer who installed a KTM dual-sport kit, making it my ultimate lightweight 90/10 (dirt-to-street) dream bike. The most miles will be street riding, but the most time will be off-road doing intermediate-level riding. Could vou nlease confirm that mv intended use will actually require these 20/40/50hour engine overhauls and explain why, when other equally or greater highperformance engines do not?

Cary Bertera Pittsburgh, Pennsylvania

A Many people tend to think of modern four-stroke dirtbike engines as little more than four-cylinder sportbike motors with three cylinders lopped off. But although those dirt engines do often borrow certain technology and design elements from sportbikes, they are different animals operating under entirely different conditions.

Weight is a perfect example. As fourstrokes began emerging as the direction in which off-road sport was heading, there was widespread concern about the greater weight of these new “Thumpers” compared to two-strokes. Riders liked a four-stroke’s power characteristics but felt the engines made the bikes too heavy and more difficult to handle, especially in rough going. So, manufacturers began building engines as light as possible, but at the price of some durability. The result has been thinner cases, smaller oil and coolant capacities, lighter crankshafts and pistons, etc.

In addition, while a Four has the firing of three other cylinders to help it overcome the considerable inertia of the piston stopping completely and then restarting at both TDC and BDC, a Single does not, and so the constant and considerable changes in engine speed and the resultant vibration also take their toll. Then figure in the even greater impact of the extremely abrupt changes in engine speed caused by normal off-road abuse such as landings from jumps and bounding across whoops and other rough terrain at full throttle.

Now consider the effect of heat and speed in this equation. At any given rpm in any given gear, a dirtbike is moving at a slower speed than a sportbike would under those same parameters, yet is probably generating at least as much engine heat; with less of an airflow passing through its radiators, which are part of a more marginal cooling system, the dirtbike engine tends to run hotter, making it more likely to tax the oil’s lubricating capabilities and accelerate the wear of critical components. Throw in the fact that off-road bikes operate in a much, urn, “dirtier” environment than a streetbike, thus increasing the possibility of contamination reaching the engine’s interior, and the picture should start to become clear. The stresses imposed on a single-cylinder four-stroke dirtbike are different and proportionately greater than they are on a multi-cylinder sportbike, but the ability of the Single to cope with those stresses over time has been deliberately compromised, largely in the pursuit of less weight.

Recall Roster NHTSA Recall No. 10V488000 Manufacturer: BMW of North America Models: 2004-’08 K1200GT. K1200R. K1200S; ’09-’10 K1300GT, K1300S Problem: On certain of these motorcycles, the bearings in the rear suspension linkage could be subject to corrosion, which could lead to reduced movement of the link lever. This could affect the handling of the vehicle and possibly cause the rider to lose control, resulting in a crash. Remedy: Dealers will replace the linkage bearings free of charge. Owners not receiving this remedy can contact BMW at 800/525-7417.

So, should you follow the factory’s engine-maintenance recommendations? With your proposed street-to-dirt ratio of riding miles and time, you probably could cheat a little. Having designed the 250 XCF-W as an off-road-only competition bike, KTM assumes it will be ridden quite hard and so created a rebuild timetable to suit. Usage that is less punishing offers some relief from that schedule, but I suggest you not push it too far. After all, unless you just stick to cruising around the neighborhood, riding a 250 Single at road speeds isn’t that much easier on the engine than riding it in the dirt.

Is anti-lock anti-stop?

A debate has been raging among

the members of my motorcycle club concerning whether ABS brakes are a help or a hindrance to stopping on gravel, very bumpy roads, rumble strips or a series of speed bumps. Bikes intended for both offand on-road riding have provisions for turning the ABS off, which suggests that ABS is not good for stopping off-road. The proposed rule change in Europe, however, will prohibit that feature once ABS becomes mandatory. FEMA and some manufacturers have decried this, and many riders have commented that trying to stop with ABS on gravel or over bumps is like having no brakes, arguing that a locked wheel is

better than a wheel without any braking. This raises concerns for machines on which you cannot deactivate the ABS and you find yourself on a gravel road or need to stop quickly on rumble strips. Has ABS been tested on these surfaces? Any assistance in ending the debate will be appreciated. Barry Krivisky

Dix Hills, New York

A Anti-lock braking tends to be a sensitive subject for most manufacturers, making them reluctant to talk about R&D on that subject, so I know little about their testing of ABS on lessthan-grippy surfaces. Our experience here, though, has shown that when traction is at a premium—such as on loose dirt, sand, gravel and any surface that has a very low coefficient of friction—ABS does indeed cause stopping distances to be considerably longer than usual, often many times more so than on a dry road at the same speed. We’ve not conducted any scientific ABS testing on speed bumps and rumble strips, so I can’t offer much insight there. But we have observed that

M' W.I WWflW '*f| ' PW WIP—f »3 1

stopping distances sometimes are a bit longer with ABS when braking hard on rippled surfaces, such as what often happens to asphalt roads leading up to stop signs and redlights. Problem is, we can’t say for certain that the stopping distances in those instances would have been any shorter with a non-ABS system.

None of this should be a surprise to anyone giving it much thought. If you have a braking system that prevents wheel lockup, and you apply the brakes on a surface that offers only, say, half the grip provided by clean, dry pavement, it only makes sense that the stopping distance will probably be somewhere around twice as long, give or take. ABS does not increase the traction between tire and ground; it only prevents the brakes from halting wheel rotation until the vehicle is no longer moving. And whether or not a locked wheel is better as a stopping device than a rolling wheel with very little braking is really a matter of surface conditions. On a road that is merely wet, modem tires are capable of maintaining surprisingly good grip, so stopping distances should not only be shorter with ABS than without it, the bike also is less likely to fall over as it would be if the wheels were locked and the tires sliding. But on dirt, gravel and many other surfaces that provide very poor grip, a bike with locked wheels ridden by a competent rider probably could stop more quickly—if for no other reason than the rider’s ability to modulate the braking force between rolling wheels with light braking and wheels just barely locked up.

I doubt that anything I’ve said here will stop your club’s debate. All we can do is hope that before ABS is made mandatory, some legislator with a little common sense and a lot of political horsepower can be convinced to infuse some degree of logic into the final decision.

Track-day re-tirement

QOver the past few years, I’ve made several modifications to enhance my 2003 Honda RC51 ’s performance. This included a set of 10-spoke Marchesini wheels that alone made a huge difference in performance. I now would like to change the rear tire size from the stock 190/50ZR17 to a 190/55. Everybody I talk to seems to agree that the increased size will provide a larger contact patch for grip, which is important to me because I participate in track days. But I’ve read that when switching to a larger rear tire size, you have to adjust the suspension to compensate for the change in ride height, but I’ve also read that no suspension changes are required after a swap such as this.

I imagine that the 190/55 tire will help my RC’s performance, but I would really appreciate your thoughts and tips before I get myself into a “suspension setup” nightmare. Sam Nienaber

St. Louis, Missouri

Alt’s quite possible, Sam, that you’re sweating bullets over something that, in the end, might not be a problem in the least. You didn’t specify which brand of tire you currently use or are considering as a replacement, so I can’t give you exact numbers regarding dimensional differences in tire sizes. I also know nothing about your riding style, your skill level and the present suspension setup on your RC51. So, I’m kind of in the dark when it comes to giving you detailed advice one way or the other.

But I can offer this suggestion: Try the 190/55 tire and see first-hand how it affect the bike’s handling. For all you know right now, the taller tire might improve the RC51 ’s handling in some way you hadn’t imagined. Besides, the differences in size are not enormous. A 190/55ZR17 Dunlop Sportmax Q2, for example, is just Vs-inch taller than the 190/50, so it would only raise the rear axle by 3/i6 of an inch. That’s not a lot of ride-height change on a stock bike ridden by (I assume) a non-professional rider on track days.

But if you’re still worried, take this approach: With the 190/50 tire still in place and properly inflated, have a friend carefully determine the bike’s rear ride height with you in the saddle, measuring from the ground to a sturdy point at the back of the chassis with the bike perfectly vertical. Then change to the 190/55 tire, ride the bike at a track day and decide if you like the resultant handling. If you do, game over. If you don’t, simply adjust the rear spring preload while you’re at the track to achieve the same ride height as before, then go try it again. That way, you’ll know for sure if the taller tire is helping or hindering your bike’s handling. □