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I have a 2006 Kawasaki ZX-6R. The factory tire sizes are 120/65ZR-17 front and 180/55ZR-17 rear. Right now I’m riding on Michelin Pilot Power tires that correspond in size. I got about 5000 miles out of them, commuting and riding the back roads “enthusiastically” but no track days…yet. I was looking to extend tire life by trying the Michelin Pilot Road 3 tires. Problem — they come in the appropriate size for the rear but the front only comes in 120/60 or 120/70, not 120/65. Of course, Michelin only recommends the Pilot Power in the factory size for my bike. In ‘07 Kawasaki changed to a 120/70 front tire for the ZX-6R. Michelin says there’s about a half inch difference in height between the 120/65 and the 120/70 when the tire is at operational temperature, most likely causing a clearance issue with the fender. They said there’d be no real change in the handling characteristics though as far as the higher sidewall was concerned. But they couldn’t tell/describe to me what those characteristics were. What are those characteristics? Other than clearance, are there any other concerns?
Dunlop recommends a 120/60 front for my bike with the Sportmax Q2 for a sport tire or the Sportmax Roadsmart II, which is supposed to be comparable to the Michelin Pilot Road 3, and don’t even carry a 120/65 at all. What characteristics will change when going smaller? I know I could just as easily go with the Pilot Powers again and be happy with the mileage and performance for the price, but at this point it’s more about understanding what’s going to happen if I do change those tire size measurements. I’ve called Michelin, Dunlop, and even Kawasaki about this. While the information they gave was informative, they never really answered my questions fully. And as I’m sure you know, the internet is full of info but deciphering opinion and educated opinions/fact isn’t always easy. Any help and recommendations you can give would be most appreciated.
The ‘06 ZX-6R came equipped with an OEM-specific variant of Bridgestone’s BT-014, that company’s then-current sport tire, in the slightly odd 120/65 size. Some versions of Yamaha YZF-R6 also came equipped with that size of front tire. The benefits at the time were said to be quicker steering and lighter weight; we found, in general, that the shorter sidewall of the 65-series front tire sacrificed some compliance compared to a taller, 70-series tire. In our track testing of those Yamaha and Kawasaki models, we always swapped the stock tires for DOT race tires with the taller profile, as that was all that was available. In our huge tire test in 2006 (“Jeopardy,” Dec. ‘06), we used an ‘06 ZX-6R with sport-touring, sport, high-performance sport and DOT race tires, all in 70-series sizes, all with favorable results.
Changing to a front tire with a taller profile adds rake and trail, which can slow steering and definitely change handling characteristics. To account for the changed geometry, we raised the fork tubes in the triple clamps (lowering the front end of the motorcycle) to offset the difference in overall tire height. Let’s look at some numbers: Bridgestone states the overall diameter of the ZX-6R’s BT-014 is 589mm. The Dunlop Q2 in a 120/70 size, as an example, measures 605mm in diameter. Theoretically, to keep the same geometry with the taller front tire, you would have to raise the fork tubes in the triple clamps 8mm — half the change in diameter. In practice, we usually raised the fork tubes by 7-10mm, depending on the tire and track. You can find the diameter for most tires on each manufacturer’s website, or you can measure the circumference when installed and calculate diameter. There may be clearance issues, as you say, between the tire and fender, and there may also be an issue between the tire and the radiator or front of the fairing at full compression. You can slot the fender mounts to make clearance there, and you can check the radiator/fairing clearance by (carefully — definitely have a couple of friends help with this) removing the fork caps and letting the front of the bike settle on the suspension. Again, in practice, we didn’t have any issues with the ZX-6R but it’s worth checking yourself.
Using a 60-series front tire would reduce rake and trail and quicken steering, maybe to the point of instability. Ideally you would lower the fork tubes in the triple clamp to account for the difference, but on your bike there may not be enough fork tube protruding and the caps would end up below the top of the triple clamp. In any event, for street riding I think you would benefit most from the additional compliance of the 70-series front tire rather than the quicker steering of the 60-series tire, especially if you adjust your geometry accordingly. One other detail to note: If you do plan on attending a track day with your ZX-6R, consider sticking to a sport tire rather than switching to a sport-touring tire. In our test, we found the additional weight of the sport-touring tires really affected handling, and performance degraded after just a few laps. SR