One of your assumptions is nonsenseMarch 25 2012 at 9:36 PM
scott (Login stinkwheel)
Response to Which wheel size is best between 16" and 18"?
You CAN have different size wheels. In fact, it is reasonably commonplace.
You just need to understand what it does to the handling of the bike.
Assuming your bike was designed to have both wheels the same size here's (very roughly, there's more to it) what would happen.
18" rear with 16" front:
Effectively drops the front end of the bike by 1". This sharpens the rake angle, more weight is taken on the front tyre, steering input has more effect. You will tend to be closer to the edge of the tyre for a given angle of lean, this is sometimes desireable on dual compound tyres. The handling will sharpen, the bike will become twitchier and more prone to tankslappers under power.
16" rear with 18" front:
Raises the front end by 1". Opens out the rake angle. More weight is on the rear tyre, steering input has less effect. The bike will tend to try to stay upright then flop over into corners.
All of this is fairly irrelevant given how you can raise and lower the front and rear of your bike at a whim to bring the rake angle back to something more appropriate by dropping the forks through the yokes or by fitting longer or shorter struts in place of your shocks.
One further thing you did not consider is that a larger diameter wheel will be less affected by lumps and bumps in the road (for the same reason as they fit 21" and 23" wheels on dirt bikes). With a hardtail, this is more of a consideration.
The final consideration is that given how you are tighter than a ducks backside, you should fit square cross section rear and a groove cut front tyre (best at clearing rainwater) for extra longevity so you need to keep the 18" ones to keep the option of the various Avon tyres open to you. I can see you running round on a safety mileage rear and a speedmaster front.
Do not give in to the propoganda, a square section tyre has a comparatively large footprint on the road and if you run it at the right pressure, the tread stays on the road and the sidewalls flex. I can get the stand down on either side on the enfield with speedmasters fitted.