Taking a few minutes here to talk about framebag flap options.
Quick Intro: Why our flap design? Because it eliminates the issue of zipper problems, creates easy one-handed access into the bag, allows you to look down into the bag while riding, and allows for more functional volume within the bag. Our flaps wrap over the top tube to close the bag against the frame, and buckle onto the side opposite opening. The challenge then is planning space for the top tube bag.
Summary: For Hard Tail bikes I want to help clarify the 3 main options we can offer: Rear Flap, Full Flap, Front Flap.
For full suspension bikes this is still worth reading, but there is a little more to it and we usually have more flexibility explained below.
Option 1) Rear Flap (standard)
This design works really well on the Carbon and Aluminum frames because the inside of the frame rarely extends all the way to the head tube... Thus some space already exists on the top tube before we design the flap. This is so common, that this is our standard build method.
Pros: Room for the front top tube bag
Cons: Reduces access into the front of the framebag... most pronounced on steel/ti frames, especially ones with shorter top tubes.
Option 2) Full Length Flap
Full length flaps are great... they offer the maximum access into the bag and are easy to access one handed while riding. The full length flap also allows for more space for over-stuffing and generally results in the largest usable volume. These are the most aesthetically clean bags for urban use.
Pros: Best volume, easiest access
Cons: Not compatible with existing top tube bags
Option 3) Forward Flap
Forward Flap is an effective application for bikes with curved or reinforced top tubes at the seatpost (think Surly and full suspensions). It also shifts the opening forward which makes the frame bag very accessible while riding (yes, one handed). In terms of center of gravity - this design also works well because the opening is at the highest point on the frame, and the top tube bag is at the lowest point of the top tube.
I like this design because I usually feel like I already have a lot going on in the cockpit, and don't really want one more bag there. Secondly, the rear top tube bag is not in the way.
Pros: My favorite design right now - easy one hand access into bag. Ergonomically good with gravity.
Cons: Not compatible with front (gas tank) top tube bag.
Full Suspension Considerations:
On full suspension bikes it is generally feasible to accommodate both a front and rear top tube bag. Why?...
A) Because they are typically carbon or Al, which means there will probably already be some extra accessible space along the front of the top tube before the framebag (and flap) begin.
B) Because the framebags on full suspension bikes tend to be smaller (because of the suspension components, and frame tube sizes).
C) Because Full suspension bikes frequently have aggressive top tube curvature and suspension cylinder locations which more clearly define the opening path.
(I will write another post comparing how our design is applied to different full suspension frame configurations)
Opening size relative to size of bag is a
We build our Daly top tube bag 6" long... thus that is our standard length to adjust the flap. Other gas tank bags extend as long as 9". On a small steel frame with as as little as 18" of top tube length - to set the flap back 9" reduces the opening by half and is not ideal (yet people still ride them and love them!).
In contrast a typical modern Carbon/Aluminum frame we typically only have to set the flap back 2-3" because the downtube and top tube are joined for the last 3-4".
Too much to absorb? that's OK... Go ride your bike and forget you ever read any of it. These are pretty minor details in the bigger scheme of you're next awesome trip. We will refuse to build a bag that is a bad idea.