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Absolutely agree DoctoRot, I paid £400 for a hardly used 1 year old "Seig" lathe with 400mm centres. It has it's limitations but for what I needed it has paid for itself many times over on this project alone both in time and money

 

More great stuff to think about guys, thank you once again.

I would love to have a lathe... and a mill, and a sandblasting cabinet, and a CNC machine... space is the issue. My shop is 10x14' and I use it for more than just my bikes. What I would love is some place I could go to use machines that are already set up, like some places have maker spaces and that sort of thing. There is one across the river from me, so maybe I need to think about becoming a member.

Regarding DocotRot's comment about the intended use of the bike, you are spot-on. The bike is not designed or intended to be used on the track, and mostly around town, highways as needed, and weekend fun rides. But I do fancy the occasional longer trip. 

I guess then, I can figure that the 15mm axle has done well so far, it should continue to be sufficient, right? But you do raise the point of increasing the span across which the axle is mounted. I'm wondering then if you think perhaps it might be wise to go back to the stock 200mm Zephyr fork spacing? This would put the fork "centerlines" directly back in line with where they were originally relative to the axle, and thus also the wheel bearings. I could still explore bigger, stiffer axle options down the road, but for now this might be the quickest way to get to a stable setup. Thoughts? 

Personally I think the 205mm spacing with 4.5mm rotor spacers and the 15mm axle is the way to go. Even the stock ZX6R spacing is only 5mm wider so any additional stress on the axle will be minimal and as you have said you aren't looking to "set a lap record of the Nürnberg Ring" When the factory settled on the front axle size they would have factored in the performance of the engine, how the bike was likely to be used, the combined wheight of a rider and passenger and then added a "healthy" safety margin to the calculation. I admire your dedication to "getting it right" but at some point as long as you are using parts that weren't made to go together there will have to be compromises (never with safety but I don't think that's a factor with this issue) this is of course only my opinion.

The main reason i would consider a larger diameter axle is not so much the fork span increase, which is marginal, but rather you are loosing the fork brace that the stock fender had and now relying solely on a 15mm axle to control the fork lowers of a much more rigid fork. In reality its probably adequate for this application since it doesnt seem like you will be riding this hard but if the front end feels vague thats where I would start looking.

 

This is all making a lot of sense. I so wish I could teleport you guys to my neighborhood, and take you out for tacos and beer. 

I did some research last night and found that an axle from a 94-98 ZX600 has a 20mm main shaft, and unlike other 20mm axle bikes I've looked at, this one also has a speedometer worm gear drive. I managed to find one with the drive for only $10 on Ebay, so I pulled the trigger. Whether the drive actually fits up in my wheel or not is tbd, but we'll see. 

Part of me thinks that going with a 200mm fork spacing would be nice just for a sort of sense of cohesion, like because that's what it was originally, that's what it should be. I feel like it's no coincidence that my stock spacing is 200mm and front tire is 110, and stock ZX6R is 210mm with a 120 from tire. That's just a perfect 10mm difference in fork spacing based on a wider tire. I can make that work (even if there's some other difficulties), but what I can't do is make the disc rotor spacers only 2mm thick. There's just no way to design it such that it retains the hub center flange. Looking at it closely though, the disc rotor actually still sits on the real hub center. At only 1mm of contact, I'm not sure that's enough. But maybe?

I know I'm going back and forth over and over on a lot of this stuff, and you guys are probably pulling your hair out thinking "just do it already!" So I want to say thank you for your patience, and taking the time to give me feedback. As we talked about this bike isn't meant to be ridden super hard, but I am always kind of a do it once, and do it right kind of person. I don't mind taking the time, doing the research, and spending the money to do things correctly. 

It's understandable, in the case of Zed and to a lesser degree me if something doesn't work out we have only lost a few bucks in materials. When you are outsourcing and paying hundreds of dollars and also waiting weeks to get things done you don't want something not to work. If it's made to your design and dimensions the buck stops with you and the MKII will be hundreds of dollars and weeks more.