It took a while, but I finally got this done in late January; not a dull moment from start to finish. I knew I'd have to mod the swingarm, I did
not realize I'd have to do it twice.
If reduced RWT (rear wheel travel) and/or possibly changing the shock's damping characteristics aren't a big deal - as they are for me - it's much easier to just send the shock out to be shortened .40"/10mm, which also solves the swingarm travel-limitation problem (more on that later).
I wanted to
1) lower the bike's seat height, which is needlessly high for a 9"/228mm-travel machine
2) knock down the strong anti-squat effect from the huge rear sprocket, to make the front end feel lighter
3) add more fork rake, for greater stability over rough ground and a more neutral-feeling steering response.
I've only done two rides since the mods, but I noticed the difference immediately once in the dirt. The small amount of residual headshake was completely gone (finally!), and the bike tracked like on rails - I could actually take my hands off the bars in rough sections, which is about as good as it gets in a dirt bike. The added rake gave the front a better angle of attack on bumps, so it felt smoother with less bouncing; the rear end also felt more 'planted' and stable (the shock worked about the same). The front end didn't feel any lighter, though.
Stock swingarm on left, modded on right:
The first step was to drill a pair of M10 lower-shock-mount holes in the swingarm mounting ears, 10mm directly beneath the existing ones (the LH swingarm above has the location marked; the edges of the old & new holes just about touch). Since the new holes are slightly below the arm top edges, the .3906" (25/64") 8"/200mm-long drill bit had to be stepped @ 2mm in diameter from 1"/25mm to 4.75"/120mm (starting from chuck end) to clear the arms. A drill guide was created from a cube of mild steel, using an M6 bolt & washer thru the old holes for placement clamping and the modified drill to make a same-size guide hole. The guide was positioned on the outer side of one mount to drill the 1st hole, then moved to the inner side of the other mount to guide the drill for the 2nd one while maintaining the original alignment. Resistance was futile.
The new holes were too close to the swingarm crosstube for the lower shock eye to fit, so the top of the crosstube between the mounts was removed with a hole saw, Dremel tool, & files. A 6061 aluminum channel was machined to fit between & below the mounts and restore the integrity and strength of the crosstube, then welded in place. The old holes were then filled in, and the mounts shortened.
Channel diagram (the 1.270" dimension may vary arm to arm):
Channel in place, pre-welding:
Stock arm on top, modified below:
Notice the wider mounting-ear spacing on the top arm; my street FX's ears are slightly closer together than the bottom one (maybe they're all slightly different?). As often happens when aluminum is welded, the position and orientation of the mounting ears changed slightly. During the weld bead grinding/clean-up process most of this was corrected, but the mounts were still slightly farther apart than before and - now being shorter - they no longer had enough flex to take up this additional space just by tightening. A new set of stainless collars were made for the Fox shock, with about .002"/.05mm axial clearance to the mounts.
A little paint, and done? Nope. The Zero swingarm has very restricted travel compared to every ICE swingarm I've worked on, all of which moved way up and way down with the shock off. On the '14 FX, the swingarm hits the frame side rails at 6.3"/160mm RWT up from level, and at 4.0"/100mm down it hits the kickstand mount. This was a real buzzkill for a long(ish) travel lowering operation.
With over 75mm of fender-tire clearance at bottoming pre-mods, I figured I could easily lower the bike 30mm without issues. Turns out the Zero design allows the wheel to move only 8mm past shock bottoming before the swingarm pivot extensions hit the frame; clearance at these points is barely 2mm at full travel. As shock bottoming involves a compressible rubber bumper and hard swingarm-frame contact will violently wedge the frame rails apart, 2mm clearance seems minimal to me.
Anyway... I spent quite a while grinding angled reliefs into the tops of the swingarm extensions to clear the frame, 1mm at a time (not visible in pics, this was done later). 7.5mm was needed in total, resulting in 9.25"/235mm of upward swingarm travel from the new full-extension point. I used the Dremel for the bulk of the material removal, then a straight round file to level out the cut & maximize clearance.
If it weren't for the travel limitation - which may not be an issue on other Zero models - the swingarm could be easily modified for lowering during pre-welding fabrication; just rotate the crosstube 180° around the motorcycle centerline (w/bulge down) & weld as before, then weld the shock ears onto the straight(er) tube top. It might've made a cool optional part for all the Zero owners looking to lower; probably about the same $$ as shock mods if dealer labor's involved, and IMO better than throwing travel away.
A side note for Dremel owners... using a Variac at about 95VAC and an aggressive double-cutting bit works amazing for quickly & easily removing large amounts of softer materials like aluminum. Instead of the usual high-power behavior using that type of bit (tool digging in and trying to yank itself out of your hands while throwing chips everywhere), it's a much more controllable shaving process, with the cuttings forming a compact pile right around the work. Way cool.
Ray
Author
Topic: 30mm rear lowering, the hard way (no loss of travel) (Read 1259 times)