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My "to see" bicycle light shifts out of position. While I need to fix it (fork bridge mounted dynamo), how do I determine when it is correctly aligned?

In particular how are "to see" lights correctly aligned so they don't blind on-coming traffic?

This question is primarily about how to adjust beam position for courtesy and safety, rather than the mechanics of mounting any particular light.

related question: How to correctly align "be seen" lights

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3  
Bah, why bother, no-one else does. Blind everyone! –  Mσᶎ Jun 23 at 3:57
    
whatever happened to front and rear? But note that "to see" is only one purpose of these lights, the other is "be seen". To an extent, you want the beam to face toward the traffic and not just the road. –  PeteH Jun 23 at 7:54

3 Answers 3

Alignment is anything from trivial to impossible. Good lights have a cut-off beam pattern that is more rectangular than circular. That way you can point the light at the road in front of you and not have the central peak that's pointing up into the eyes of people coming towards you. Even expensive lights often don't do this, so you might be out of luck.

The hassle with dyno lights is that you have to be moving to see the beam. I find that getting it roughly right in the workshop than nudging it while I'm riding works best. Having a nylock nut on the vertical adjustment bolt means I can set it to be "just tight enough" and it won't shift while I play with it later. Then I move the light a little while I'm riding - just reach down and push/pull the light a little until it's pointed the right way. If you don't have a locking nut you'll have to stop the bike at that point and tighten the nut fully to lock the light in place.

Actually aligning the beam is a matter of deciding how close to you you want the beam to hit the road and aiming it accordingly. Too close and you have a small, very bright spot right in front of you that actally makes it harder to see than not having the light at all. Too far and you're putting most of your light into other people's eyes. With a circular beam you're going to have light going up, there's no way around it.

My preference is to aim the light such that the top of the beam is just below horizontal. Or with my B&M Eyc that has a nice rectangular beam and sits just over a 406 wheel, actually horizontal. What I do is wait until I'm riding either towards a reflective sign or white wall, or next to a wall. Then I work out where the top of the beam is and push it down/pull it up until it's right. That way I'm getting the light on the road out in front of me without blinding anyone.


That's specific to the type of light and how it's mounted. The only general advice is "make sure it has a proper mount and is installed correctly". Of course, for may lights the answer is "it did not come with a proper mount".

For your fork bridge mounted dyno light the mounts are often cheap steel and if you're lucky there will be a fold or two to strengthen it in places. Those usually end up not working very well.

If the light slips around on the mounting bolt that's usually easy to fix. Get a locking nut and if necessary a longer bolt so you can fit it. The nut you want is deeper than a normal nut and has a plastic insert to make it less likely to come undone. They're cheap, so most bike shops will buy a box of 100-200 for 5c-20c each (more for stainless steel ones) and charge you 20c-50c for one. Your call as to whether $10 for a box is worth it. If you are right on the end of the bolt don't fart round with removing washers, just buy a longer bolt. Most cities have an industrial fastener shop, find it or use an online one (but $5 shipping on $1 of bolts... ow).

fork crown mounts

Note that the two leftmost brackets in this picture use the curve of the head tube bulge to stabilise the bracket. The other two rely entirely on the bolt being really tight. If you're buying a bracket, try for something like the first two.

If your bracket is bending you need another one. Your LBS or local bike fix crew might have a box of light mounting bits you can go through, and that's generally the easy way. This is where having access to a vice and power drill is helpful, or it's going to be cheaper to get the bike shop to do the work for you. $20 in labour cost is easier than buying a cheap vice and not having any further use for it afterwards.

But, if you have a few tools: Find something that your light will attach to easily and is more or less right to attach to your bike. Or go to a metal sales merchant and see if you can buy offcuts (most sell them by weight), and buy a 10cm piece of angle iron (aluminium "iron", ideally) that you can cut to shape.

Notch the angle iron so you can bend it forward, but don't completely cut off the angled part - you want that there so the light can't be bent too far forward. This is where the cheap pressed brackets fail and you don't want that.

Once it fits the bike and light bend the bottom of the flat so the corners are angled in to fit onto that bulge. It's a little tweak but it makes a surprising amount of difference.

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This answer could be improved by dealing with light alignment, rather than fixing the light to the bike. Its also a great answer to another question (correct and efficient fork bridge mounting of lights.) –  Samuel Russell Jun 23 at 4:21
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@SamuelRussell you're right. Added :) –  Mσᶎ Jun 23 at 4:30

I don't know whether you intend to use the light you own or are considering to buy a new one. As Mσᶎ pointed out, symmetric beams are practically impossible to adjust sensibly because in order to not blind oncoming traffic you need to point them too near your bike.

Car lights have highly non-symmetric beams to address this problem and some countries like Germany also require cyclists to equip lights with well-shaped beams. If you search the Internet for StVZO-compliant (Straßenverkehrs-Zulassungs-Ordnung) bicycle lights you will find lots of models to choose from.

StVZO-compliant lights cast a beam that has a flat top, so they are very easy to adjust: I get immediately behind a parked car, as I would be when stopped at a traffic light, and tilt the light so that the top of the beam falls just below the car's wing mirrors. If the beam doesn't hit the mirror, it won't hit any other mirror that is farther away and, of course, it won't hit the eyes of any driver, since they are positioned higher than the mirrors.

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This answer could be improved by indicating how to correctly align a non-symmetric beam. –  Samuel Russell Jun 23 at 22:20
    
It's a nice method, but you might want to check them against wing mirrors as well -- if you're passing a lien of parked cars you don't particularly want to dazzle one who's thinking about changing lanes. –  Chris H Jun 24 at 10:34
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Those were the ones I was referring to (in my mind; i'm not a native speaker), hence my comment that the drivers' eyes are higher than the mirrors. Thanks for pointing out the correct name ;) –  Asuranceturix Jun 24 at 10:38

First you need a good strong light, with a reasonable beam width/height. Then it should be aligned so that the beam is pointed immediately forward and a bit down, with the center of the beam striking the ground maybe 15 feet in front of the bike. With proper spread this will illuminate an area of the road from 25-30 feet in front to about 5 feet in front, and about 5 feet wide. Adjust from there based on local conditions/personal preferences.

When adjusted this way there should be no significant problem with "blinding" oncoming traffic.

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