I have a Lezyne Strip Drive 150 but my saddle bag is in the way so I can't use it on my seatpost. If I increase the height of my seatpost then that would be uncomfortable, so what can I do?
You could attach the light to something else, like another part of the frame, or a tab on the saddle bag, or your backpack.
You could remove the saddle bag, attaching onto e.g. the handlebars or putting it into a rucksack.
You could buy a different light which does attach to the bag or another part of the bike.
Time to get logical
You obviously have very little seatpost exposed. Most cyclists on compact (i.e. sloping top tube) bikes should not be in this situation, but some of us are on level top tube bikes and some of us have relatively short legs. NB: all products mentioned below are not necessarily endorsements; these are merely ones that I remember existing, and I have no commercial interest.
Many saddlebags have a loop on the back that's often got a reflective strip. Here's the Lezyne Caddy M:
However, the Lezyne Strip Drive doesn't have a corresponding clip that would mount to such a loop. You would have to get a light that clips to a strip like this. Moreover, not all saddle bags have these loops! I think that some panniers, if you use them, may have mounting loops or other places you could clip a light.
One possible alternative is if there are lights that clip to seat stays. However, these sit lower and may be less visible to drivers. I'm not sure which lights, if any, are designed to be mounted here. Another possible alternative is something mounted on your helmet, perhaps something like this Topeak light.
Less commonly, some saddles have dedicated but proprietary mounting points accessories. Specialized's mount for one particular taillight is here. Fizik makes two lights that can be mounted to its ICS saddle mount. If you use the Garmin Varia, which is admittedly more expensive, some companies on Shapeways offer dedicated mounts. Here is one for Specialized saddles. I think there may be more general saddle mounts, but these might need to clamp to the back of the saddle rails, and some saddles may be incompatible (e.g. Selle SMP saddles are incompatible with Bup Labs' 3d printed general mounts because their rails are tilted at too sharp an angle). These would apply to other lights using a Garmin mount, but I'm not sure any others exist yet.
I use the hard case luggage rack with the extra Hide My Bell adapter that is likely made not exactly for this purpose but somehow matches the desired shape and serves the goal very well. I bought it "from the view", was last in the shop under heavy discount. You only need to drill single hole to attach it:
It got a little bit angled for me but same way as the seat post is so the light sits even better.
Attaching such a thing to the bag from the flexible material like your seat post would be more challenging but it might work if to put some large enough plate with the hole on the inner side. You may also find some other place to screw it on.
The comparable part should not be very difficult to make even in a simple workshop, by attaching a L-shape elbow to something like thin plastic bottle of the suitable diameter.
Sounds like you want to keep the saddle bag in place. You can get lights which attach to the seat rails, see below, so you would be able to hang this behind the saddle bag no problem.
Not only will it bring you fleeting amusement, but on group rides you'll have the bonus advantage that nobody will ask you to take a turn on the front, you can sit on the back of the group the whole way.
I'm not sure I'd use it as the only light on my setup, but I often attach a bright flashing additional light to my seatstay, on the side more visible to the traffic. Check the viewing angles - it may be an option for you. Or you could double up and fit a cheaper light on the other side. I've done that too
One more option: Attach the light to the back of your helmet.
A riding buddy of mine has a lightweight rear light there. I am not actually sure how he has it attached. I believe it is clipped to a strap or something/somehow, so this may not be applicable to every helmet. He also is pretty safety conscious so he was careful to not negate the safety features of his helmet by doing so. He only uses it when his seatpost-mounted camera/taillight battery is drained, so it only gets occasional use. The only real con is that it is more eye-level so riding behind someone with this setup is not as pleasant as with one mounted lower.
There are other helmet-mounted attachments for lights out there. I have one for a forward light, so maybe there is another out there for attaching a taillight to the top of the helmet.
Since you want 2 things to exist in the same place, you'll have to fashion some kind of thing for your light to attach to. I would find a flat piece of metal from the hardware store and bend it in shape to attach your light to. A simpler solution is to get a rear rack and attach your light at the end, or other places as others have suggested. Why the need for the saddle bag though?
I commute at night in dark areas, so rear lights are important to me. I have:
- one flasher on the rear of my helmet
- one throbber hanging on the outside of my backpack
- one solid inside my backpack (a red-LED flashlight, and this lights up the white material of the backpack nicely
- one blinky on the seatpost
- one solid between the seat stays.
I also have two more blinkies on the seat post as cold spares in case of batteries going flat or loss. Higher mounting is helpful because it can be seen over obstacles like cars, where lower mounted lights can be obscured.
You could move the light to the frame's seat post strapped just above or below the top tube. Don't go too low between the seat stays because then it can be hidden. The wheel itself can hide the light if its on a stay.
Consider hooking the light to your riding clothes - a jersey pocket or a trouser belt. And also contemplate adding additional lights as backups too.