I've been busy on a lot of different things since my last post, and could finally get something together worth reporting. Just to let you know where I am going here is a cardboard box 'studio' photo of the result.
These are adapters I found I needed to get all the front wheel related parts I wanted to use mounted up to work together.
Here is a list of the parts:
fork Honda CRF250R
wheel BMW R100GS
axle Honda CRF250R
brake disk Ducati snowflake (320mm)
brake caliper BMW R1200RT (4 piston)
I really like the convenience of the stock GS tubeless wheel. It seems really strong, and I can repair a puncture without removing the wheel from the bike. Plus, I just need to carry a small tire plug set and an MTB bicycle pump to do repairs.
The GS axle is bigger than the CRF's. Here is what I measured:
R100GS CRF250R difference
axle diameter 25.0 mm 20.0 mm -5.0 mm
I didn't want to bore out the CRF fork ends to accept the GS axle since that would need to be done again if the fork ever needed to be replaced. I looked into fitting a bearing that would have the OD of the GS wheel and the ID of the CRF axle. The GS uses a number 6005 bearing with dimensions 25x47x12. The closest standard bearing with the proper ID and OD is a 6204, with dimensions 20x47x14. The GS wheel bearing bore is really just deep enough for the 12mm bearing, and I didn't think it the good to use the thicker 14mm 6204.
I did find that some Suzuki motorcycles use a 20x47x12 bearing in the transmission, but those wouldn't be sealed, so wouldn't be appropriate for wheel bearings.
I would need to make some custom axle spacers to fit the GS wheel between the CRF fork, so in the end, the solution I came up with to fit the bearings was to make a set of stepped spacers that extend into the wheel bearing to adapt the GS wheel bearing to the CRF axle.
I found some kind of stainless round stock at the scrap yard that I though I could use. I made the OD of the new spacers the same as that of the stock GS spacers so I could use the GS bearing protectors on the new spacers. This shows the stock GS axle and spacers on the right, and the CRF axle and custom spacers with the GS bearing protectors switched over on the left.
To get the correct width for each of the new spacers I mounted the GS wheel in the fork with the CRF axle nut screwed on the axle until the outside of the nut was flush with the end of the axle, then positioned the wheel so that the rim was centered between the fork legs. Careful measurement gave me 15.5mm on the right, and 27.0mm on the left for the spacers.
This shows the CRF axle with my new spacers and GS wheel bearings.
I had done a 320mm disk conversion on my PD earlier, so had a good idea of what I needed to get the disk mounted. You can read my write-up of the PD conversion here:
The Ducati snowflake disk has an offset of 9.67mm, but an additional spacer is needed to get the disk far enough over so that the spokes won't hit the inside of the caliper. Also, it seems to me that if the disk is closer to the fork leg there will be less stress in the caliper mount.
I took the springs out of the fork so I could compress it down the check how thick a spacer I needed to get 2mm clearance between the widest part of the fork upper and the outer surface of the brake disk buttons. I measured that about 11mm would work.
I was lucky and found a 1/2"x4"x4" aluminum cutoff at the scrap yard. I didn't have any proper lathe mandrels so I drilled a hole in the stock and pressed in a piece of scrap steel that I turned down and center drilled. That big square spinning around was a little intimidating at first, but I showed it who as in charge, and quickly got it turned down to its 100mm OD.
These photos show how the axle spacers and disk spacer work together to position the wheel centered in the fork, and the disk as far to the left as possible.
To get the brake caliper setup I just positioned the caliper on the disk to where it looked good. It seems a little high in this photo, I thought it would put too much stress on the upper mount.
The geometry of the adapter needed is not simple. I took some measurements, but mainly made the finished piece from a template.
At first I was trying to make templates from thick card stock, but that turned out to be difficult because of the irregularly shaped parts. I hit on this idea to use a thin piece of clear PETE plastic and a sharpie marker.
Once I got the template and knew the size of the adapter I scrounged around at the scrap yard and found a block of aluminum I could use.
The adapter needed two reliefs on the outside for the fork mounts, and one big one on the inside for the caliper. I just sketched the reliefs onto the template to give about 2mm clearance between the adapter and other parts, then just transfered the sketchings to the piece when doing the layout.
This shows the caliper and inside relief, along with some of my fabrication notes.
I plan at some time to make a proper mechanical drawing of the adapter for use by anyone interested.
The three big holes on the side are to reduce weight and give a better appearance. I didn't plan on those at first, but once I got it machined and mounted up it looked like a big bulky hunk of metal sitting there so I added the holes. I think it looks a lot better, and is significantly lighter. The holes are not quite lined up because I positioned them in the center of the thickest parts to give maximum strength.
This adapter ties together a Honda fork, a Ducati disk, and a BMW caliper...
Here is a top view. The 2mm relief gap between the adapter and caliper is in the shadows of this photo, but looks narrow and gives a really good effect.
It was a lot of work to design and machine the parts, but I am very happy with the result. The axle spacers are stainless, which won't rust, and the caliper adapter has a really cool 'one-off' look to it.