I am not the only one saying it is a bad idea....if you go back and check you'll see that. As for whether I have tried it, no I have not laced a 21" front rim to a cb750 hub. I did have a 21" front rim bolted to the back of a cb750 roller so I could push it around the shop. Just by my eyeballing it I could see there were going to be considerable clearance issues with a stock frame, espically with suspension compression (remember just by sitting on the bike the ride height is 1/2" lower than parked) and the top of the rim/tire combo hitting the cross braces in the frame. I had no interest then to do it because it is an unsafe idea overall (oh and it looks stupid).
But let's tackle the obvious here - most 21" streetbike (aka harley) rims are at most 2.15 inches wide which is nowhere near the 2.75 you need for a 120 tire. At most you are maybe looking at a 80mm-90mm wide tire. Most dirtbike front widths are 1.6- 1.85 inches (and most dirtbike rims are 36 hole to boot). So now you have a really heavy bike on a really narrow contact patch that frictionally will not be able to support the lateral load of taking any kind of turn at speed.
Although Spin Werkes makes a 21" rim that is 3 and 3.50 inches wide - it is also $312 (there goes the $400 budget). So assuming you get a windfall and everything else is free you still need a 21" tire that will fit the rim. With a 3 inch rim you can use a 120-130, with a 3.50 in rim you can go as big as 150mm. Someone out there must make that tire because the rim is being made but I can gaurantee it will not be cheap. We have seen 200mm 21" tires in this post and those were a couple hundred $$$$. so his budget is blown.
I am not even going to comment on the 15" shocks and change in rake angle that is already going on on the bike, but to make the whole thing taller will bring any handling problems (speed wobbles maybe) right to the front. The taller you make a rimand tire the more leverage it has on the chassis and the contact patch. Vintage race bikes usually use an 18" front and rear, starting about the 1980s superbikes used a 16" and 17" rims respectivley. I could bore you all day with camber thrust and how you have now radically not just how the chassis responds to loads but what loads are actually being placed where but frankly you all have the internet and can search for yourselves and I really don't feel like investing much more into this thread. Start with tony Foale's site just so you know how a motorcyce tire handles:
http://www.tonyfoale.com/Articles/Tyres/TYRES.htmMotorcycle rims are not like car rims. Without crunching the numbers as to what you are doing to the suspension geometry, center of gravity, weight distribution the bike could very well fall over the first time you lean it into a corner or nothing could happen - nobody here seems to know and the one guy who wants to do it is "eyeballing" it all the way.
We have wasted 7 pages on this nonsense already, the dare has been laid out - you want to do it so hard then do it already - at least babyfood's first post was a pic of his actual project in progress.
