How to Fit a Rimlock

Last updated: Sun, 27 Apr 2008 20:45:00 GMT

When I was advised by People Who Know to fit rimlocks to my DR200 before attempting the Capital Coast Adventure ride, I could hardly refuse.

What the fudge is a rimlock, you ask. It's a device that's fitted inside a wheel and used to clamp the bead of the tyre to the rim of the wheel. When the going gets slippery and traction is at a premium, it's common to drop tyre pressures well below normal running. Numbers like 12 and 14psi are bandied about. At these pressures, there's a risk that your tyres might "creep" around the rim, rotating until they tear the valve stem from the inner tube, resulting in a flat. I looked for a decent definition and this was the best I could find. However, if you're not really sure what a rimlock is, you probably shouldn't be thinking about fitting one right now.

I also looked around for a how-to on fitting rimlocks and found none. Hence this write-up. I'm no expert, but I did get a bit of advice before starting from some guys wiser than I, particularly where online expertise was contradictory. It's probably best that you don't attempt this if you're not already comfortable changing your own tyres and performing your own static balance afterwards.

First off, where to fit the locks? Common sense suggested two options: opposite the valve stem; opposite the heavy spot on the rim. Possibly both the same place. The intarnets also suggested that it didn't matter where they were fitted, or that I should fit them as close to the valve stem as possible. Regular human expertise agreed with my common sense, with the caveat that it was best to fit the lock between groups of spokes, which are normally arranged as nine groups of four spokes, thus it's not possible to fit the lock directly opposite the valve stem.

Remove the wheel. Remove the tyre and tube from the rim, and the old balance weights. Remove the rim tape. As per a static balance, find the heavy spot on the rim to help you decide where to mount the lock. On both my front and rear rims the heavy spot was slightly to one side of the valve stem hole. Accordingly, I decided to drill the rim as close to opposite the heavy spot as possible, between spoke groups.

I marked up by eye, with a scribe and a flexible straight-edge. A bisected line between adjacent spokes looks just about perfect. A tap with a centre-punch makes for easier drilling. My rims were surprisingly soft.

Begin drilling. I started with a 2mm pilot, then 4mm, then 8.5mm. Not a lot of pressure required. Unsurprisingly, this generates a lot of swarf, so it's best done away from the tyres and tubes. With my history, I made sure I was wearing goggles and gloves. Once you're finished drilling and you've offered up the stem of the lock to check, dress up any sharp edges around the hole and clean the swarf out of the rim thoroughly.

Fit the rimlock, then string the tape around the rim and over the "foot" of the lock. This will hold the rimlock in place while you fit the tyre and tube. As before, find the heavy point on the rim. No surprises, it's probably near the rim lock. Both of mine were within an inch of the lock stem. I put a small mark on the rim with a centre-punch so that I know where the new heavy spot is.

Thread the washer and nut onto the stem of the rimlock, don't tighten it, just put it a few turns on. Fit the tyre and tube as normal, place the light spot of the tyre next to the heavy spot on the rim. Received wisdom is to start fitting the tyre at the rimlock and work your way around. You can push the lock in by the stem to get clearance, make sure that the foot of the lock, the bead and the tube aren't fouling on each other.

You can tighten up the nuts on the rimlocks now. Proceed with a static balance as normal. My front rim needed 21g to get it balanced just so. Oddly, my rear tyre needed 56g, which looks like a lot of weight. But both rims are vibe-free right up to 100kph. Job done.

As it turned out, I did benefit from running those rimlocks. There were a couple of slippery clay challenge sections that warranted the 12/14psi front/rear pressures that were recommended, and I don't doubt that I'd have been on my butt more than once otherwise.