Originally Posted by
sweeks
I have the exact same problem on my Airborne, which has a Co-Motion front fork. I currently have my dyno-hub-powered light mounted on the left-side brake post.
I considered drilling a hole through the front of the fork, but the metal there is not really thick enough to tap threads in. I suppose I could use a bolt long enough to engage the threads at the back to hold the light on, and then use a locknut to hold the fender on. For the present I've decided to leave things as they are.
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As noted above, I put my light on the right side, not left on my rando bike. I did that because I did not want shadow from the tire to block the light on the right side of the road where you could have curbing or edge of pavement, etc. Since my light is above the fender, opposing traffic can still see my light just as well as if I had it on the left.
If you have the tools to drill the steerer tube, that is what I would do. Then cover up the metal where you drilled it with finger nail polish to prevent rust.
Different bike in photo below, if a long bolt is impractical, use threaded rod. In the photo below, I used a threaded rod for extra length, as I also had a fender mount to deal with. And my headtube was so big that I needed to move the light mount further forward from the fork. This is a touring bike and I do not carry lots of open end wrenches, so one of the nuts was one of those allen wrench type nuts that are used on rim brake blocks so I can use an allen wrench on it if I needed to remove the fender. This fork hole was not threaded, so the nuts on the threaded rod are torqued tight enough that it should not move.
The light mount is similar to a standard B&M, but that is not what I used, I bent a different mount to fit my needs. So, you will not see an identical one for sale anywhere, but there are similar ones.
If you drill it, if you have a drill press, that would be best. You want to make sure that the hole is directly ahead and not off to one side. My first dyno powered headlight went on a fork where the hole was not exactly straight ahead, the light beam was clearly in the wrong place. My light mount was plastic so I could not bend it for aim. Eventually I realized that I could use some of those weird spherical shaped washers like on rim brake blocks to allow me to aim it. But it is best if you do not need to resort to that, so getting the hole straight is better.
This placement of course assumes you have enough clearance for a light under your handlebar bag.