Ch00paKabrA wrote:for one of my builds, I actually used a 21" M/C rim. Depending on which tire you put on it, it will be around 26" in diameter or maybe 27" but not really close to 29". You would need a tire that is 3" tall at the sidewall to get to 27" so unless you can find one that is 4 inches tall, I don't think that you are going to get close to 29 inches. I have a feeling that there would not be many rear triangles that would be wide enough for that.
The real issue is the weight and rotating mass. I don't think that there are many 21" tires that are under 9 lbs, however, there are a few tires in the 19" range that are in the 5 lb range. I have decided to use the IRC rear GP1 dual sport 2.75 x 19 for my latest build (I am using them front and back). These only weigh around 5.5 pounds which will keep my total wheel weight in the 10 lbs range.
Yes there are 21" MC tires in the 26" OD range, I know from personal experience that the MT43s and SR 241s are really close to 28"OD. In fact the SR241 3.00x21 was right at 28" according to my tape. I ran both those tires on my Beta Evo 250 before I sold it. And like you said, any 80/100x21 knobby tire can be found in the 26" range all day long. I looked at the GP1 several times but haven't tested this tire in this size yet. When you get your ride up and going, are you going to be doing offroad riding? If so I would be very interested in hearing how you like the GP1, specifically in dry loose conditions. On my Bomber, I have experimented with 15 tires to date now on the rear. 6 MTB tires in the 24x 2.50-3.00 size range, 4 MC tires various types in the 19x2.75 and 70/100x19, and 5 MC tires, 4 in 17x2.75-3.00 range and 1 in 70/10x17 size. With all of these tires, I keep going back to the SR241. On the 19MC rim, the 2.75x19 SR241 ha sor far shown to be the king of the crop for my intended purposes. On the 17MC rim, the 3.00x17SR241 is king for the same reason. There are better road tires, there are better dirt tires, but so far, I haven't found a better road and dirt tire than the SR241. The two tires that I haven't tested yet that I am very curious about are the SR244 and the GP1 you are going to be going with. So whats the ETA for completion on your latest build? Any pics?