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New vs. Old…tires
I just replaced my GSX-R’s rear tire last week, but left the front because it still looks like it’s got plenty of miles left. I replaced the rear with the exact same tire I had before (Bridgestone BT-016), but now my bike is suddenly handling weird. When I go into a turn, it feels like the bike doesn’t want to lean over any more, then suddenly it feels like it almost falls into the turn. It’s become spooky to ride in the turns and I don’t know what the deal is. My bike was handling fine before, and I haven’t changed anything with the suspension. What gives?
What you describe is a very common consequence of replacing just one tire. When a bike is difficult to turn up to a certain lean angle, and then seems to fall too easily into the corner beyond that lean angle, that is a sure sign that a tire has worn flat in the middle from a lot of highway miles — imagine your bike with a wide car tire on it as an extreme example. There are a couple of factors at work in your case: Because tires wear over a relatively long period of time, we naturally adapt to, and don’t notice, the changing steering characteristics as tires wear. And because both front and rear tires wear in a similar manner, those changes are usually subtle. Most people notice at least some change in handling when changing to new tires, even when changing them as a set. However, with replacing the rear tire on your GSX-R, the new tire’s nicely rounded profile highlights the squared-off profile of the front tire so it’s much more noticeable. And because it’s an abrupt change, you were more susceptible to noticing that difference. This is why tire manufacturers advise that you always use a matched set of tires — it doesn’t take much of a deviation from the designed profiles to have a detrimental effect on your bike’s handling.
The clutch: Do you really need it?
I was hoping you could help put to rest a disagreement between a few friends and myself on the topic of shifting modern sportbikes without using the clutch (aka clutchless or slip shifting). One half of the argument is that it causes premature wear in the gearbox and clutch, and that the use of the clutch lever for shifting actually keeps the clutch lubed with oil. The other side of the argument is that, if done properly, shifting without the clutch is safe and does not cause any damage or wear.
The technique used to slip shift is described as follows: When accelerating, simply preload the shift lever with a small amount of pressure. Roll off the throttle until the shift lever moves, placing the transmission into the next gear. Allow the shift lever to go back to its center position, then roll the throttle back on. A similar procedure is done to downshift when decelerating, except that with the throttle closed, you need to preload the shift lever, then blip the throttle open until the shift lever allows it to go into the lower gear.
The person in my circle of friends who slip shifts claims to have been doing it for years on several different bikes, without any damage or problems. He says he’s done it for so long now that he doesn’t even need to preload the shifter any more — his timing is so good that he can just back off the throttle and move the shift lever at the same time, and still has never had a problem. He has said though, that he slip shifts mostly for upshifting and doesn’t do it often for downshifting.
We have covered upshifting and downshifting many times in Riding Skills Series, most recently “Clutchless Upshifting” (April ‘07) and “Smooth Downshifting” (Mar. ‘10). When upshifting, briefly closing the throttle without using the clutch is the quickest method, just as you describe it. When your timing is correct and the shift is smooth, there is little chance of damage to the clutch or transmission. Quickshifters work by momentarily cutting the ignition or fuel at precisely the right moment for the shift, and provide smooth, fast shifts, and when a quickshifter is set up properly it poses little threat to the engine’s internals.
Downshiting, however, should always be done with the clutch. While it’s possible to downshift without, as you suggest, the chance of damage to the engine is much greater. When upshifting, there is no need for the rider to match engine speed to road speed. Simply closing the throttle reverses the load on the transmission and allows the shift to take place, and it also slows the engine down to about the right rpm for the next gear. If you downshift without the clutch, however, you must blip the throttle at exactly the right moment and exactly the right amount to properly match engine speed with road speed — any difference will make the shift very abrupt, and is not only potentially damaging to the transmission, but also very unsettling if you are trying to enter a corner smoothly.