V shape vs U shape profiles
The Effect of Shape
As you can see from this illustration if you have a V shape you will have a larger contact patch during a lean. This allows you to lean further, and go faster. If you run a U shape your bike becomes unstable during a lean reducing your ability to lean.
Improper tire size
If you go to a shop and they sell you the wrong profile/size tire for your bike you are likely to end up with a U shape. This happened to me, and because at the time I was clueless, I went with what the shop expert said. This was my mistake, but as I readily admit I didn’t know any better. Thus the reason for me writing this up is to save other people from enduring this learning experience.
Today a friend with a similar bike asked me about what tires to get. I went online to look them up, and the tires I would have suggested didn’t come in the correct size. I thought, well they must have discontinued the proper size since I couldn’t find them anywhere. But that wasn’t good enough for me. I went out to the bike and looked at it. Sure enough it didn't have the proper tire size!
Upon further investigation I noticed that the bike with the improper tire had much wider chicken strips (the area on the tire from the edge towards the center that is never ridden on because you are too much of a chicken). Let’s take a look.
Large area of unused tire
On the older GPR 16" wheels the front should play host to a 90/90-16 tire. I was sold an incorrect tire of 100/80-16. Now if we do the math we find a diameter of 568.40, and 566.40 respectively. Not much difference at all, right? Maybe so, but the profile is enough difference that the evidence will shock you.
To compare I will use a 90/80-17 tire, since I currently only have one 16" wheeled bike. On this bike the tire is the correct size. I did the research, and chose this one on my own without the help of an expert at the store.
The 2000 GPR 16" front wheel running an Avon Viper 100/80-16. Notice the huge chicken strip. The bike clearly becomes unstable even at a moderate lean angle.
The 2003 GPR 17" wheel running a Sava MC18 90/90-17. Notice there is no chicken strip! This bike can be ridden right to the edge of the tire with full confidence, and control at full lean angle. The difference in the handling of the two bike (both the same model) was astounding.
Proper tire size
Now no doubt people could come up with any number of arguments like I didn’t know how to ride as well with the 16" wheels, or the bikes are different, or something. Well these bikes are of the same year with nearly identical parts. They have both been ridden by multiple riders (many of which were the same), and thus most if not all of the variables can be eliminated.
Additionally, both sets of wheels with the same tires have at some point in time been on each of the bikes for a significant number of miles. The only real difference at all points in time are one set has the correct sized tires, and one set does not.
I should also point out that the rear wheel on both bikes had the correct sized tire installed.
Entire tire is employed
What are the correct tire sizes? Below is a table with the proper tire sizes for the various years of Derbi GPRs.
Note: later RS50 and RS4 bikes from 2006 on are like the 2005 Derbi, and the earlier RS50s with 17" wheels are like the 2003 Derbi.