Using a rear tire size of 120/80/R17 instead of the original size of 140/60/R17 on your Yamaha FZ-S can have several impacts on your bike:
Handling: The tire size affects the contact patch and the overall grip of the bike. A narrower tire (120mm) may provide slightly different handling characteristics compared to the wider original tire (140mm). The bike may feel slightly lighter and more agile, but it may also impact stability, especially during high-speed cornering.
Ground Clearance: A smaller tire may slightly increase the ground clearance of the bike, which could affect the bike's geometry and overall stability. It's important to ensure that the altered ground clearance doesn't lead to any clearance issues, such as scraping the undercarriage or compromising the bike's suspension travel.
Speedometer Accuracy: Changing the tire size can also affect the accuracy of your bike's speedometer and odometer. Since the diameter of the tire is smaller, the speedometer may indicate a higher speed than you are actually traveling. It's advisable to recalibrate or consult a professional to ensure accurate readings.
Load Capacity: The original tire size is specifically chosen by the manufacturer to handle the load capacity of the bike. With a narrower tire, there may be a slight reduction in load-carrying capacity. Make sure that the new tire can still handle the weight requirements of the bike and any potential passengers.
It's important to note that altering the tire size may also have legal implications, as it could violate local regulations or affect the bike's roadworthiness certification. It's generally recommended to use the tire size specified by the manufacturer for optimal performance and safety.
If you have already installed a rear tire with a different size, it's advisable to monitor the bike's performance and handling closely. If you notice any significant changes or concerns, it's best to consult a Yamaha authorized service center or a qualified motorcycle mechanic for further guidance and to ensure that your bike remains in a safe and roadworthy condition.