Seatpost-Seatbag

[Smo]

A friend of mine requested a custom seat bag that would run along the seat post and then out behind the seat.  He didn't like how far out most seat bags go and wanted to try something more stable.  I just finished it and I'm pretty happy with it so far.  Haven't taken it on a trail yet, but he plans to do the BCT next weekend and will report back to me.

The first step was to trace his bike on a whiteboard and draw out and plan the bag (see picture of the drawing).  I was super happy to have a whiteboard for this.  With all of the tapering and weird angles, the side panels would either have to be oddly shaped or be made up of several different pieces.  I chose to try the oddly shaped panels for the promise of more difficult math, but easier stitching (I happen to be a math teacher!).  Several rounds of the pythagorean theorem led me to the red distances noted on the whiteboard (which are the actual lengths of the seams or the distances along the fabric).

The picture of the pieces drawn on cordura shows you how I drew them - I started at the left and worked right, using the distances on the whiteboard to triangulate my next points.  The end result was a series of triangles whose distances matched what I calculated.  See the photo of the pieces cut out.

After that it was a matter of sewing everything together and figuring out the velcro - nothing too fancy or difficult, just a bit tedious.  The panels came together really nicely; all of those triangles blended together into smooth curves on the side panels.

The material is 1000d cordura seconds from rockywoods - my friend and I have the same perspective on bikepacking - if it's gonna rain, we'll just go for a hike instead (we live in AZ, it's not too hard to avoid bringing rain gear out here).  So I wasn't concerned about waterproofing, more durability.  And the cordura is CHEAP - $6/yd for the seconds.  The roll top helps to compress the bag, and there are three girth-wise compression straps.  I sewed a strip of webbing onto the bottom (sewed across the webbing every 1.5 inches or so) to keep the compression straps in place.

*look up "thrifty white panelboard" for info on how to get a 4x8 foot whiteboard for $14 at home depot.  They'll even cut into smaller ones for free.

[donnieboy]

Cool, I may have to learn some pythagorean OR make measurements along the seatpost on a parallel to level ground plane every 2" or so to arrive at an acceptable bag capacity as determined by a stencil made from such measurements (a la troubleshoot/grind).  You did a nice job with your math skills, though I'm not sure I'm down for that kinda mathyness...Who knew that I could design awesome adventure gear that could actually improve my life with pythagorean  Math makes me short.  We'll see how mathy I feel once I've ordered my fabric yardage and the new sewing machine arrives.  I have ordered fabric samples from rockywoods - a few camo, different shades of green and black/grays that are waterproof, some being double WP coated denier, others xpac, silnylons, goretex and event materials (we're adventure nuts who refuse to be held back by all but lightning storms, dehydration and threat of hypothermia.  We cycle all winter in Ontario, including during freezing rain and some snow.  It teaches you just how important ventilation, waterproof materials and clothing layers, can be, for adapting to a changing climate, in conjunction with body heat levels.  I'll keep you posted on my bag plan/build progress, as I do have a camera and desire to share.

[Racingguy04]

That's a really interesting bag. How's it with leg rub?

[Smo]

Surprisingly good!  I was a bit worried about it.  When I put it on my friend's bike and pedaled around it rubbed my legs a bit, but when I put it on my bike it didn't.  And when my friend rode my bike it didn't rub him either.  So if you're on a bike that doesn't fit you, not so good apparently.  I should have a more thorough report on how well it works in a week or two.

[Racingguy04]

I'll look forward to reading it, it's definitely an innovative and cool design.

[Smo]

So my friend took it on the Black Canyon Trail and said it worked great!  The only thing he would recommend is to extend the gear loops the full length of the bag for versatility in tying it down.  Also, I'm selling my gear now - PM me if you're interested in one of these.  Cordura or Xpac available.

Here's a little more explanation of the math involved, in case anyone wants to make a bag like this (or any bag with a complex shape like this).  Feel free to ask me questions.  It's really not complicated, it's just very hard to explain in words, let alone over the internet.  I'm used to teaching with a whiteboard and not a keyboard.  Maybe I'll make a quick video if there's demand for it.  I'm tempted to delete what I wrote below, because it's so confusing to explain this without a video, but I'll leave it in case anyone finds it useful.

Anyway, if you look at the whiteboard drawing in my first post, the black circled numbers are how thick/wide we wanted the bag to be at those points.  So it's 1.5 inches wide along the seat post, 6 inches in the back, and 4 at the bottom.  The basic idea is to measure all of the distances we need on the whiteboard, with an actual ruler, and then calculate how long the fabric will actually be along those lines (this is where the pythagorean theorem comes in).

I divided the side panel into triangles by drawing dotted lines.  I measured the lengths from one point to another on the whiteboard for all of those sides.  I only wrote a couple of those down; they are in green.  The red numbers are the final numbers that we will be calculating, which is how long the fabric will actually be along those lines.

In the upper left of the whiteboard you can see a drawing of the next step.  This is a cross-section of the bag, along the dotted line that reads 7 and 7/8th inches (looks like 3/8ths in the picture).  You can see it's 1.5 inches wide at the top, and 6 at the bottom.  It's 7.5 inches tall measured on the whiteboard, that's the solid line in the middle.  Now we're going to make a triangle (on the left side, using the dotted line which is also 7.5 inches).  The bottom of the triangle is (6-1.5)/2 = 2.25 inches.  Why?  Because it's half the difference between the top and the bottom.

Now we know two sides of a right triangle, so we can use the pythagorean theorem: a^2 + b^2 = c^2.  a and b are the short sides and c is the long side, which is what we want to know.  So c^2 = 2.25^2 + 7.5^2, which is 61.3.  So c is the square roof of 61.3, which is 7.83.  Multiply .83 by 8 and you get 6.64, round up to 7 and it's 7/8ths.

Once you have all of those distances figured out, you can lay it out on a piece of fabric, drawing one triangle at a time.  I started in the back of the bag and moved forward.  Basically you're just guessing and checking with a ruler at where the point of the triangle should be, then drawing the two lines towards that point.  You can see what it ends up looking like in the picture I have of my fabric pieces.

[whikidi]

@Smo ... Will you ship to Australia??

[Smo]

I could certainly ship to OZ - send me a PM and we'll work out the details.