USD Forks

I actually have not been able to get the calculator you linked me to working yet. I’m not sure why, but I still need to try it on my desktop computer. 

Try this one

www.rbracing-rsr.com/rakeandtrail.html  

That was helpful, thanks again Wookie. I'm still not sure what I'm doing wrong in my drawings to get the incorrect numbers I'm getting, but I'm not gonna worry about it. 

According to that calculator, the current trail of my bike is 4.32in, or 109.7mm. What's strange is that my FSM says that the trail is 104mm, which is what I've been going off of this whole time. So I'd be very curious to find out how Mama Kaw is measuring trail compared to everyone else.

Calculating the trail of the new forks I'm getting 4.07in, or 103.4mm. This includes the 10mm drop I've already drawn into the top triple. A difference of 6.3mm.

Now, what got me going down this rabbit hole and seeking this understanding was because I was curious if increasing or decreasing the triple tree offset could correct the difference in trail. I calculated a 35mm offset instead of the stock 40mm, and the difference in trail dropped down to only 0.8mm. I had hoped this might be a viable method, but checking with my 3D model, I can see that it would cause the fork caps to touch the handlebars. 3mm is about the maximum amount I can decrease the offset. The difference in trail would then only be 2.794mm. Might still be worth doing though, what do you guys think?

You can increase the offset on the lower clamp only to get a bigger difference. That would necessitate cutting the openings for the fork legs at a slight angle from the stem.

Stereordinary wrote:

According to that calculator, the current trail of my bike is 4.32in, or 109.7mm. What's strange is that my FSM says that the trail is 104mm, which is what I've been going off of this whole time. So I'd be very curious to find out how Mama Kaw is measuring trail compared to everyone else.

Calculating the trail of the new forks I'm getting 4.07in, or 103.4mm. This includes the 10mm drop I've already drawn into the top triple. A difference of 6.3mm.

Now, what got me going down this rabbit hole and seeking this understanding was because I was curious if increasing or decreasing the triple tree offset could correct the difference in trail. I calculated a 35mm offset instead of the stock 40mm, and the difference in trail dropped down to only 0.8mm Reducing offset increases trail). I had hoped this might be a viable method, but checking with my 3D model, I can see that it would cause the fork caps to touch the handlebars. 3mm is about the maximum amount I can decrease the offset. The difference in trail would then only be 2.794mm. Might still be worth doing though, what do you guys think?

You need to factor in that shorter forks will reduce the "rake" angle which will shorten trail, the best advice I can give is to extend the forks to "stock" length (somebody way cleverer than me could probably calculate the rake change with shorter forks but would need a lot of data)  and once the tree's are cut there is no turning back. As I said previously sticking with "stock" "fork length" and "offset" will take the guesswork out of it and you can simply replicate the stock tree's (with the holes further apart) to get the correct spacing between the forks

But why settle for simple when complicated is an option? 

Hahaha, indeed! I think for my skill set, making changes in the CAD drawing is simpler than extending the forks, which is another reason I brought it up. But you guys have convinced me to stick with the stock offset and go ahead with the fork extension. 

Kind of an exciting update on this project. I got the first 3D printed prototype part made and mocked up. Overall I’m pretty pleased, there’s some changes that need to be made, but they’re relatively minor and should be easy to do. 

The prototype only has as much as necessary, which meant cutting off one of the fork clamps. Turns out the part is too big to 3D print any affordable way, but chopping it down like this made it cheap. 

 

Now for the bad news. It looks like I’m gonna have a challenge with brake calipers/rotors and getting everything to fit and line up. Again, the forks are ‘03 ZX6R, and all my research says that ‘07-‘08 ZX6R Nissin calipers are a direct swap and a better caliper, so that’s what i got. However the offset between the brake caliper bolts and the rotor center is way off in this case. 



I thought maybe the ‘03 rotors have more offset built into the rotors, but looking at photos they appear to be even less offset than my Zephyr rotors. So I think the ‘03 front wheel hub itself must be wider. 

At this point I have no idea what to do about this issue but I’m open to ideas. 

Stereordinary wrote:

I thought maybe the ‘03 rotors have more offset built into the rotors, but looking at photos they appear to be even less offset than my Zephyr rotors. So I think the ‘03 front wheel hub itself must be wider. 

At this point I have no idea what to do about this issue but I’m open to ideas. 

The offset on all these dished Kawasaki rotors is the same at around 15mm whether they are 310 or 320mm dia.
The discs on most radial equipped bikes are plain flat though..
The usual solution is to fit disc spacers and longer bolts.
 

Agree with Zed on the disc spacers (they must however still locate on the centre hub so you might have to get creative with the spacers) since you haven't cut the trees yet is there an option to reduce the spacing and bring the forks closer to the wheel? Possibly even a combination of spacers and bringing the forks closer? Looks like you need to find approx 8mm each side

I think spacing the forks closer together might be a possibility, I’ll have to look into it. I know it will be complicated around the speedo drive, but as I’ve said before I might just have to lose that altogether anyway. 

Thanks for the suggestions on making spacers for the rotors, seems like that would be an easy part to design. 

Alright, so I'm working on like nine different elements of this (including Wookie's very astute suggestion that perhaps taking some of the extra width off at the fork spacing, and also using axle spacers is the way to go), but I've got some questions I would love opinions on. 

1. Without going too deep into the "why" I came up with this idea, one thing I'm considering doing is making the upper triple flat on the top, with the 10mm drop still there, and a ledge where the top of the fork hits like a stop. The clamp bolt still sits in the same place, and the cutout on top leaves enough room for a socket to break loose the fork cap. Here's a render to hopefully make what I'm explaining clear.

 

My question regarding this is does anyone see any reason not to do it? I do suppose it would be possible for rain water to get stuck in there, which could cause some corrosion over time. But we are talking about alloy parts, and there is still the slit in the clamp for water to drain out of. I think a positive aspect would be consistent fork height, rather than relying on your eyes to make the top of the forks flush with the top of the clamp, there's a built-in stop instead. 

2. The next question is about the speedo drive. I can make new axle spacers to bring the forks in closer, but the speedo drive overlaps with the left fork lower by about 5mm. Now I could omit the speedo drive, but the other thing I could do is cut a notch in order to actually give the speedo drive a place to index so it doesn't rotate. I didn't actually have a solution for that issue before, so this is kind of convenient. Perhaps I split the difference here too. Is that safe enough though? I don't love the idea of ever-so-slightly cutting away at the fork, but we aren't talking about much material here. Most of the "indexing" will be cut into the axle spacer.