I love to ride my bike and sometimes need to take some stuff with me - like a powerbank (for the phone attached to the handlebar) or just a knog's chain lock for safe parking ;) What's important - I prefer to hide it in a bag strapped to bike than to ride with backpack. It's just more comfortable for me to be light-and-free, while my bike carries the rest :) And because my bike's frame is not a standard one (Corratec Superbow Trail 2008 - frame looks like this), I always have troubles with choosing the right bag. One I used for a couple of months, needed to be modified to fit, and just teared down - mounting straps just fell off..

So I've decided to build my own bag from the scratch. You know, if you want to have something done well - do it yourself ;)

It isn't rocket science, but can take a while depending on your sewing skills and spare time. Cost will be similar to cheap bike-bags, but quality is far more superior if you'll make it well thought-out.

Stuff you'll need:

some strong textiles - I bought a sheet of backpack-like waterproof and rip-stop material, as well as special material for lining
stiffener - I used some thin (about 0.5mm) PVC sheet, that was lying in my garage in combination with special 3mm foam - it adds stiffeness and also some softeness (foam) to the walls. You can use something like plastic dinner-table pads, cheap and quite stiff, but a thick cardboard will do the job just fine
thick needle for sewing the leather (it's easier with that kind)
strong threads - I bought a 210D/2 high quality polyester sewing thread and additionaly while threading I was folding it twice or more - to make it even stronger
some straps - width is up to you, I used both 50mm and 25mm
velcro - to secure straps around the bike's frame
metal or plastic rings/rectangles (forgive me, don't know how to call it - you'll see it soon) for fastening the straps
rubber tape (if you want to make some fancy mounting aids inside the bag)
some carabiner (optional, if you want to add a place for securing keys etc.)
a zipper
scissors (obviously), some pins, chalk or special tailor-soap for marking on the textiles, some paper, metering tape

Looks like it's a lot of stuff? Well, like I wrote - I wanted to have it done well. You can modify the bag and/or needed materials to suit your expectations. My bag is practical and I hope it will last for quite a long time :)

Ok, after this looong intro, let's get to work!

Step 1: Measure Twice, Cut Once

Measure your bike, I mean the place on the frame where you do plan to hang your bag on.

Make a paper-model, use a sticky-tape to make it 3d and put it on your bike. If something is too big, make it smaller; when it's too small, make it bigger. Perfect your paper mockup and when you are happy with the result, place it on the textile and use chalk/tailor's soap to trace it's shape and dimensions.

As you can see in the photo - remember to add some "bleed" - 10 or 15mm more to the outer outline. This will help you to sew everything properly. Notice, that I'm doing a one-cut-of-material project, meaning that all the bag walls are connected and sewed only where it was impossible to avoid cutting. I see every sewing-connection as a weak link, so if I can avoid this, it means my bag corners and edges will be stronger.

Measure two identical pieces of the stronger material, and measure one piece of the lining (If you choose to use it like I did). Plan where the zipper will go (and on which bike side you want it - I chose my right side) - it will cause the placement of the "ceiling" part.

Measure the stiffing materials (foam, PVC, cardboard - weapons of your choice) one for each triangle walls. The wall which will stick to frame under the seat and the ceiling wall can be stiffened only with foam or cardboard - you have to be able to put the needle through it to sew the straps! If you want, you can also measure some stiffener for the floor.

IMPORTANT: do not cut anything unless you are perfectly sure it's all good! It's better to measure few times than cut and loose all the materials because of a bad measurement.

Now make a "sandwich" with the combination of layers as follows:

strong textile
stiffing foam
stiffing PVC/cardboard/whatever you'll choose
strong textile
lining textile

Of course line-up the stiffeners with corresponding walls.

Use pins to secure all the materials as in the third photo. Now you're ready for the most boring part ;)

Step 2: Sew, Sew, Sew...

Yep, put on the thimble, prepare your coffe/tea/favourite drink and start sewing!

It's relatively simple and even if you are not a professional tailor - you'll get it quick. All you have to do is trace the lines. You don't have to do it on the one day or evening. To be honest - it's probably not possible with doing it by hand - at least that was my case. Even when I was watching some youtube videos in the background, sewing was so boring, that I had to cut the whole process in parts and it took few evenings to make all the stitches.

Remember, that more important is to make it well than make it quick. That kept me going, minute after minute, stitch after stitch... and the result was so good, it automatically made me smile. It just looked much better than my factory-made and bought bag, that fell apart earlier.

Ok, but enough of sweet-talk. When you'll manage to sew-trace all the lines, and your bag looks similar to my third photo, it's time to plan the straps.

Step 3: Plan the Straps And... Sew Again

Make a measure-tool like on the second photo. Now check the third sketch. It's a cross section of the horizontal frame looking from the seat to handlebar. You'll get the idea. Using the measure-tool you've just made, test how long the straps should be (two on the horizontal frame, and two on the vertical, under the seat).

AGAIN: measure twice, cut once! so you won't regret in the future ;) It's not a totally bad idea to cut straps with additional 10-20mm. Remember, that you always can cut down if the strap is too long, but you cant make it longer if it's cut too short (well, ok, I know you can sew it, but it's not the same).

Then again it's time for sewing. First, attach the metal rectangle frames to one end of each strap.

While the whole construction is still flat, and right now it doesn't resemble the triangle bag - sewing of all straps is easier. Just position them on the right walls (two for ceiling and two for the wall that will stick to vertical frame under the bike seat).

Sew it good, so the bag won't fell of the straps. Soon you'll know why there can't be any stiffener other than foam or cardboard in these walls with straps - pushing the needle through the straps and walls is hard already.

Step 4: Fancy Stuff (optional)

What makes a custom bag, if not the additional stuff inside, tailored to your very personal needs :)

While biking I don't like to have too much on me. Of course when riding to work, most of the times I have a backpack, or at least a shoulder bag, but during recreational trips I like to ride through the woods (as I live next to national park), and all the up and downhills, roots sticking above the ground etc. makes me afraid of stuff falling out my pockets, not to mention that backpack/shoulder bag isn't too comfortable then. That's why my bike bag should have all the facilities to fulfill my needs ;)

I've added a carabiner for key transportation. They will hang in the bag in one place, so I'll always have them in my reach.
As I mentioned earlier, sometimes I take my powerbank and connect it to phone which is on the handlebar - so I did a holder at the vertical wall (built with two rubber bands); then, two narrower rubbers will work as cable management stuff, and finally I'll leave a hole in one of the bag's corner (photo no. 4) so the cable could be put out of the bag.
Lastly, the triangle wall that will work as the door - this will be the place for ID/ATM card/wallet holder. A little pocket made from two wide rubber bands.

You can use the inside however you want. Think what do you need and simply do it :)

Step 5: Let's Shape It and Zip It!

Now, as you've finished the inside job and all sewing that was easier to be done with flat construction, it's time to have fun origami-style and make the bag 3-dimentional :)

Fold it and shape it to the triangle, while keeping the bag inside-out like in first photo, then sew two narrow edges (marked on the second diagram) - I kept a hole in one ridge, for cable management (also marked on the diagram).

After all stitches are done, turn the bag from inside-out, to inside-inside position. You know, so the bag looks normal - something like the second picture.

Prepare your zipper, watchout for it's location, so after sewing you'll be able to use it normally, secure it with pins and... back to the sewing. Honestly, that was the worst part of the project. Position it, fold it along with the excess "bleed" part of the material - just like in the 4th picture and sew it. Of course you must attach and sew the zipper to two edges of the triangle "door" of the bag, and then, second part of the zipper to vertical wall and ceiling - and this makes zipper fold with about 90 degrees. Sewing that is a pain, but it can be done - 5th and 6th photo shows the finished zipper.

Yay, bag is ready. Now the mounting straps finish...

Step 6: Sewing the Velcro

Go to your bike. Put the bag on the frame, try it on.

Use pins to secure the straps and mark the place for velcro, so it can be sewed like on the fourth diagram.

Cut the velcro, sew it, cut excess strap fragments. You'll probably need a firelighter to secure edges of the straps. Just make it shrink with heat (carefully, you don't want to burn it all) and voilà!

Next step - official installation.

Step 7: Installation

The easiest part ;)

Just put the bag like in step 6, when you were trying it on and fitting. Secure the straps around the frame with velcros. Put your desired stuff in the bag and go for a ride!

Step 8: Epilogue

Congratulations if you managed to read this all up to here! Kudos, if my probably-too-long instructable didn't scare you off, and you've made, or you're thinking about making your own bag!

Let me know if it was helpful and share a photo with your bag :)

Introduction: Triangle Bike Bag