acotrel said:
If you are changing the yoke offset and the rake, when you first ride the bike take care and be sensitive to how the bike is behaving when entering and leaving corners. If it stands up when braking into corner - that is really bad. If it self-steers and tightens it's line when powering out of corners, that can be good if you ride in a way that can use it to advantage, but might not be good on a road bike - neutral steering might be better. In any case the handling should inspire confidence, not destroy it. The difference between good handling and bad handling is very small when you are making fork yokes.
I noticed in that photo of the rear brake you will be using, that the end of the stay for the calliper is attached to the swing arm - attached to the engine plate at the rear of the gearbox with a rose joints might be better. That way it might tend to lower the rear end of the bike when you use it.
I rarely use the rear brake on the street, Alan, so I think it will probably be fine with the brake stay on the swing arm. I agree that you can change it's effect on the bike under braking by moving the attachment point around on the frame, but the theoretical benefit just doesn't look worth the effort for a street bike. There are a lot of production bikes and race bikes with the caliper mounted to the swing arm with no problems. Besides, I used this same setup on my race Commando for many years, so I know it works fine.
I've already mentioned this in another post, so I'll keep it brief. The critical parameter at the front end for proper handling is trail. Rake and yoke offset are only really significant for their effect on trail, with a secondary impact on wheelbase and front/rear weight ratio. Within the normal range of rake you might try on a Commando, say 24 to 28 degrees, as long as you choose a yoke offset to give a reasonable trail with the tire size you are using, the bike will handle well. A little too much trail will give a bike that is solid in high speed sweepers, but needs a bit more force to start the turn-in, and needs a bit more effort on the bars to hold a line while accelerating out of the corner. But it really isn't that sensitive to a little extra trail. A little less trail makes it really easy to turn in, but not quite as stable feeling in sweepers, and too little can make it quite scary. I raced the same Commando PR for almost two decades, and tried a lot of things that affect trail, including 19, 18, and 17 inch tires, longer rear shocks, raising and lowering the front forks, cutting and welding the steering head for less rake, but most significantly a set of adjustable front yokes (from Spondon). I tried three different offsets on the yokes, and settled on one that gave 45 mm offset. That's a lot less than a stock Commando, but I had jacked up the rear a bit, lowered the front, and was running 18" slicks. The end result was slightly more trail that the stock Commando. Contrary to your concerns, I didn't really find handling to be sensitive to small changes. Larger ones, yes, but still not in terms of life threatening handling quirks. I ended up with a setup that gave me very neutral handling, which I preferred, and that's what I'm planning for this bike.
From what you've said before on the forum, it sounds to me like you've formed your opinions on bike setup from smaller, lighter weight, shorter wheelbase bikes like the Seeley, which are more sensitive to changes than the Commando. I've also raced bikes of that sort, and they are nowhere near as forgiving as the Commando is, and much more sensitive to changes. Getting the rake and yoke offset right for a street Commando is nowhere near as difficult as you seem to think.
Ken