Giving Skinny a Brake

Fab Kevin
Fab Kevin

Last month we featured a new product from Visionary Cycles in our Horse’s Mouth section, a set of narrowed up OEM style 41mm triple trees. They are ductile iron cast versions of FLH trees that place the fork tubes a mere 6” apart from center to center. When you consider that stock Narrowglide triple trees are a full inch wider, you can see how these trees can make the front of your lane splitter look as skinny as possible. Most customers who use these trees run a spool front wheel and no front brake for an ultra clean look. Since Carmine from Visionary sent a set of trees for the Horse staff to check out, I decided to see how hard it would be to sneak a wheel with a front brake on a set. This isn’t really a home garage type project, but I figured readers might like to see what’s involved in a set up like this.

I placed an ad on Chopper Swapper (that’s @chopperswapper for you Instagram types) for a 41mm front end and got a response from Brandon McKenzie who wanted to trade some trans parts for a nice ’84-’99 FLHT dual disc front end. I didn’t need a dual disc but since I would probably be shaving the fender mounts off anyway, shaving one of the caliper mounts wouldn’t be a big deal. Next I needed a wheel. A Narrowglide wheel was out of the question, since these trees were already an inch narrower than a set of 39mm Narrowglide trees. I knew from experience that shaving a half inch from each side of a 19” HD mag would not leave enough room between the back side of the rotor and the spokes of the mag for a brake caliper, and forget about a laced Narrowglide wheel. I had acquired a 19” front mag wheel from a Harley Softail Rocker, and the narrow cross section of the wheel spokes looked like they might still allow room for a caliper once it was narrowed.

Making parts fit where they aren’t intended to fit usually involves a lot of math, and that’s where I started. Since the wheel spokes are tapered (thicker at the hub, tapering down as they extend toward the rim) I decided to run an 11 ½” rotor instead of trying to run a 10”. The farther the caliper would be away from the hub, the more room I would have to work with. I installed the forks in the trees, slid in the axle and started measuring. My first step was to determine how wide I could make the hub, so I made a mark 5 ¾” up from axle centerline on the inside of the fork leg. The fork legs were tapered slightly, and 5 ¾” was the top edge of the rotor. I measured in between the fork legs at this location and came up with 3.68”. Yikes! I split that number in half, so I could work from the center of the wheel outward. I had 1.84” from wheel centerline to fork leg. I measured a rotor I had at .22”, and I decided on .08” of room from the outside of the rotor to the fork slider. So 1.84” – .08” – .22” = 1.54” for half of the hub width, or 3.08” total hub width. Armed with this number, I started measuring the wheel. I measured the cross section thickness of the spokes at the area where they would pass by the caliper, and came up with .90” total, or .45” out from centerline. Next, I took the hub width of 1.54”. added in HALF the thickness of the rotor which is .11”, then subtracted the spoke width of .45” and came up with 1.2”. Now I could start measuring Brake Calipers to find one that was less than 1.2” from centerline of caliper to the inside edge. My first thought was a Tokico from an SV650 as they are pretty skinny on the back side, but it wouldn’t fit without milling off some material. I hit the jackpot when I measured a used Performance Machine 4 piston caliper to be 1.15” and realized it would fit with .050” to spare! It doesn’t sound like a lot, but the caliper could be shimmed a little if necessary and be off center by another .020” without hurting a thing. It was starting to look like it might work.

I had gathered all of the info I needed to make it work and now I needed to get the hub machined. Besides milling down the hub area, the bearing pockets needed to be milled in deeper. Big Mark to the rescue! Mark of Performance Mold has a fleet of Fadal CNC mills and once I showed him the hub and sketch of what I wanted to do, he was all in. He first clamped the wheel to the table, and located the wheel’s exact centerline using a dial indicator. He then programmed the mill to machine off the hub surface to the desired depth, and cut the bearing pockets to depth as well. The procedure needed to be repeated for both sides of the wheel and when Mark was finished it looked like the wheel was designed to be this way. I bolted on the rotor and slid the caliper on to see if it fit. Luck was on my side, as it fit without hitting the wheel spokes.

This wheel originally came with 1” wheel bearings so I needed to install a set of ¾” sealed wheel bearings and make a new center spacer to fit between the bearings. Then I slid the wheel into the fork assembly. Things were looking great, but with the wheel in place I was able to see how little room there was for the tire in between the cowbells – 3.25” to be exact. I measured a 100/90-19 tire on my FXR at 4”, hence the “100” of the tire’s designation (100mm = roughly 4”). This would mean that even a 90/90-19” would be 3.5”, still too wide. Panic was starting to set in! 

I searched on line to find the skinniest 19” tire I could find, and an Avon Speedmaster in 2.75”X19” was ordered up. While waiting for the tire and tube to be delivered, I designed the caliper bracket to fit the caliper to the fork leg and Big Mark cnc’d it out for me. That only took an hour or two so I paced the shop for another 3 days until I got the tire in. Stuntboy came by and picked up the wheel/tube/tire and had it mounted up and back to the shop within an hour. I frantically measured the overall tire width with my dial calipers, and was relieved to measure it at 3.08” overall. Eureka!

I slipped the wheel back into the forks and centered it perfectly between the fork legs. I use stainless steel wheel spacer stock from Mid-USA to make my spacers. It is 1.12” OD with a .755” ID so you just rough saw the length you need and then trim them to final length on a lathe. I covered the process of measuring and making wheel spacers in issue #108 back in May of 2011, so if you want to dig through your back issues you can re-hash it.

I re-installed the wheel and wheel spacers, along with the caliper bracket. I needed to shim the caliper .25” away from the caliper bracket to get it centered on the rotor, so I added a couple of spacers and bolted that in place. I was surprised and relieved to find that not only did the rotor bolts clear the fork legs as the wheel turned, but the caliper cleared the wheel too. As it turned out, less than 1/16 of an inch either way on any of my math would have caused interference, either caliper to wheel or rotor to fork leg. I am going to design and cut a 5 spoke rotor to match the wheel more closely, but due to the magic of printing deadlines I am not able to have that wrapped up for this issue. The exercise was a success and for a guy like me who really likes front brakes, these skinny Visionary Cycle front trees are an even better option,

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