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Motorcycle Forum / General / Sportbikes / September 2005A question about tires
I recently bought a used 1992 Kawasaki ZX-7. The cycle came with Dunlop D220 tires front and rear. The front tire was a 120/60 while the cycle is specced by Kawasaki for a 120/70. As soon as I started riding the bike I was unnerved by a random wandering feel, especially when trying to go in a straight line. The rear tire seemed to be worse than the front. When I changed over to Metzler SportTec tires the trouble went away completely. I checked the tires for odd wear or other problems but they looked fine and were not anywere near worn out. I also looked for looseness in the steering bearings and swing arm but didn't find any. The cycle was banged up by the first owner and the right handlebar was bent back a centimeter or so. If the tires hadn't worked to fix the problem my next stop would have been a frame specialist shop. So my question is: is the difference in front tire dimensions really that important? Was it the brand? I should also add that I'm about 240 pounds if that makes a difference. This is also my first motorcycle with radial tires. Thanx. > So my question is: is the difference in front tire > dimensions really that important? Yes, the difference in diameter between a 120/60-17 front tire and a 120/70-17 front tire is 0.945 inches. That would lower your front axle almost half an inch and you would certainly pick up on the quicker steering, as the steering geometry is disrupted by such a change in tire section without making other changes, such as pushing the fork tubes down through the triple clamps to compensate. Rider say that they can feel the difference in steering is the front end is only 5 mm lower. Half an inch is over 12 mm lower... Some sporty tires even have an intermediate 65% section to go between the 60% section and the 70% section. Racers like the 60% section for quick turn-in. 70% section is better for sport touring. But some tires won't let you use all of the tread, you have really wide "chicken strips" with the taller profile. Case in point, the Bridgestone BT-020. I selected it, instead of the Dunlop D-220, which I thought would be ridiculously tall in the 130/70-17 size. When I switched from a 130X60-17 Metzeler Me-Z2 to a 120X70-17 BT-020 Bridgestone, the handling of my Yamaha went all to Hell. It began to feel like some old 1970's UJM with a 19-inch front tire. That Bridgestone was very vague feeling in long sweepers, followed rain grooves, even did a dance on the grooves at fairly low speeds while the Metzeler didn't. On the plus side, the tire felt very precise at very low speeds. > Was it the brand? Could be. The brand, the tread pattern, the stiffness of the sidewalls, and how "pointy" the tire section is and the section height will all change the feel of the steering. All of the possible differences make original equipment tires, replaced as a set, a very reasonable option. If you watch the AMA superbike races on TV, you can tell which riders are uncomfortable with the quick steering of their pointy tires. They set up for a corner by sticking their knee out while they are still straight up. Then they lean the WRONG WAY while still straight up to stop the bike from turning into the corner too early! If the bike steers that easily, why stick the knee out early? > I should also add > that I'm about 240 pounds if that makes a difference. So you would need to use more pressure in the tire. Maybe halfway between the recommendation for solo riding and carrying a passenger. But your extra weight up high will make a pointy tire feel too pointy, you might want a rounder profile front tire to avoid that "falling into the corner" feel... > If the bike steers that easily, why stick the knee out early? So you don't have to add that to the list of things you need to do at the turn point. ab > > If the bike steers that easily, why stick the knee out early? > > So you don't have to add that to the list of things you need to do at the > turn point. And, if you have long legs and a competitor is drafting you closely, sticking a leg out could discourage him from an inside pass... > > > If the bike steers that easily, why stick the knee out early? > > [quoted text clipped - 3 lines] > And, if you have long legs and a competitor is drafting you closely, > sticking a leg out could discourage him from an inside pass... Or give him a convenient lever with which to punt you off the track. ;-) "Racing incident, it was!"  SignatureTweak > > > > If the bike steers that easily, why stick the knee out early? > > > [quoted text clipped - 6 lines] > Or give him a convenient lever with which to punt you off the track. > ;-) I don't think a following rider should try an inside pass on a rider who's leaning the opposite way that his knee is pointing. If a rider is on a machine that turns in too quick, he's likely to take at least two tries at setting up a line through the corner. He's going to notice that his motorbike is headed toward the apex too soon, he may even think he's going to run off the course to the inside, so he's going to straighten up and take another stab at countersteering the bike down. So, two guys collide and the guy who hit the lead rider from behind says, "Well, that's just racing." IMO, the following rider should have gotten a major clue by the lead rider's weird opposite leaning with his knee out in the other direction he planned to turn toward, and the following rider should either not pass, or know some strategy for passing a rider who turns in early, straightens up and tries again. What do you did when the rider ahead straightens up his line and tries again? I would stay the f.ck away from him and try to pass on a straight. > > > > > If the bike steers that easily, why stick the knee out early? > > > > [quoted text clipped - 9 lines] > I don't think a following rider should try an inside pass on a rider > who's leaning the opposite way that his knee is pointing. Depends on the the area and setup of a track. Sometimes you do stay hung off the side while you go around a curve to the opposite direction. Say, in a L-R-L, where the right is not too sharp, you would stay off the left side of the bike in order to have 2 less body movements. The "esses" at Road Atlanta are also ridden this way by many people. In the old setup, Schwantz hangs off the left at the T2 exit and stays there till T5 exit. > If a rider is on a machine that turns in too quick, he's likely to take > at least two tries at setting up a line through the corner. He's going [quoted text clipped - 16 lines] > again? I would stay the f.ck away from him and try to pass on a > straight. Pass on the outside. Anybody doing that can't be carrying much corner speed, because if you can change your line without crashing...you could be going faster. SignatureTweak closely, > > I don't think a following rider should try an inside pass on a rider > > who's leaning the opposite way that his knee is pointing. [quoted text clipped - 6 lines] > old setup, Schwantz hangs off the left at the T2 exit and stays there > till T5 exit. I was really thinking about riders who are slowing down from high speed and trying to set up for a sweeper. That's when they get their knee out so early. But, does anybody actually do any *passing* in the esses at RA? It looked to me like the fast guys were flicking their motorbikes back and forth on faith. No passing room, except maybe for snakes slithering in formation... I was watching Yates on TV and it looked like he had the esses down to body "zen", his muscles and reflexes seemed to be on autopilot, "in the zone", there was just no time to think, "Well, maybe I should start turning in *here*, no, wait, that's too soon, I'll try turning in here." > > What do you DO when the rider ahead straightens up his line and tries > > again? I would stay the f.ck away from him and try to pass on a [quoted text clipped - 3 lines] > speed, because if you can change your line without crashing...you could > be going faster. That's how another rider hit me at the end of T8 at Willow Springs. T8 is an increasing radius RH sweeper. Fast guys will get to the right side all the way around T8. That gives them somewhere to slide to if they start drifting. Experts say that 135 to 140 mph T8 is actually faster than the front straight. Experts also say you need "big balls" to go that fast around T8. T9 is a decreasing radius RH sweeper going onto the front straight. Fast guys ride T9 out, all the way from the apex, across the front straight, and they look like they're going into the pit exit, they use part of that pavement. They use all the road to get onto the front straight at high speed. I was setting up for T9, keeping to the right, but I straightened up just a tad to move a little to the left so I could take T9 a little wider. That's when the other rider brushed me going around on the outside at about 10 mph faster than me. A speed weave started, I thought I was going dirt riding. Good thing no really fast VIP was trying to pass both of us. I would have gotten center punched. We were both novices, but the club's special VIP buddies were allowed out on the track any time they wanted to practice and they would sometimes buzz us with a 30 mph speed diferential... > closely, > [quoted text clipped - 17 lines] > forth on faith. No passing room, except maybe for snakes slithering in > formation... > I was watching Yates on TV and it looked like he had the esses down to > body "zen", his muscles and reflexes seemed to be on autopilot, "in the > zone", there was just no time to think, "Well, maybe I should start > turning in *here*, no, wait, that's too soon, I'll try turning in > here." Doug Polen passed me through there once...it was "poetry in motion" watching him glide down through there at a SIGNIFICANTLY higher pace than I can carry, all the while looking like it was completely without effort. > > > What do you DO when the rider ahead straightens up his line and tries > > > again? I would stay the f.ck away from him and try to pass on a [quoted text clipped - 23 lines] > wider. That's when the other rider brushed me going around on the > outside at about 10 mph faster than me. As a career backmarker, one thing I make certain to never do is change my line, especially during a race. I've been passed on either side by 3 or 4 bikes in the middle of turns that were close enough that I could reach out and get an autograph when they go by. LWT races might have + 50 bikes on the grid with mixed expert and novice, and some of those guys are so much faster than me it's not even funny. > A speed weave started, I thought I was going dirt riding. Good thing no > really fast VIP was trying to pass both of us. I would have gotten [quoted text clipped - 3 lines] > out on the track any time they wanted to practice and they would > sometimes buzz us with a 30 mph speed diferential... You know, when that happens I find it simultaneously depressing and inspirational. Depressing that I "be so slow", but inspirational in that "if he can go that fast, I can certainly go a little faster myself". SignatureTweak > Experts say that 135 to 140 mph T8 is actually faster than the front > straight. I don't think that's the case these days, as most literbikes will accelerate to beyond 145 on the front straight. T8 is a fast turn either way. ab >> So my question is: is the difference in front tire >> dimensions really that important? [quoted text clipped - 8 lines] > Rider say that they can feel the difference in steering is the front > end is only 5 mm lower. Half an inch is over 12 mm lower... Thanks for the replies, they were helpful. >I recently bought a used 1992 Kawasaki ZX-7. The cycle came with >Dunlop D220 tires front and rear. The front tire was a 120/60 while [quoted text clipped - 14 lines] > >Thanx. Inflation (proper) certainly matters. Aspect ratio might matter, but sometimes it's subtle. Some bikes "like" one kind of tire better than another, for whatever reason, though I can't exactly prove that (just an observation). '----------------------------------------------------- ' Daniel Bannon ' NW WA State, U.S.A. ' 2003 ZX636B Hercusaki, 1999 CBR1100XX '----------------------------------------------------- > I recently bought a used 1992 Kawasaki ZX-7. The cycle came with > Dunlop D220 tires front and rear. Those were OEM tires on the bike in 1992. They might actually be that old. I'd guess that a lot of the wierdness you felt was due to the tires being old and dried out. ab |
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