Ortlieb-style adapter for bike trunk bags

2024-02-05

I've been developing an improved solution which I will publish soon to address the shortcomings of this one:

• the bag sits down on the mechanism, thus accelerating wear
• the mechanism requires multiple parts
  • more assembly time
  • more things to break
• the release mechanism requires access to the side of the bag, which could be occluded if panniers are mounted & stuffed with contents
  • case in point:
    • 

2023-09-15

I'm not sure how much more development I'm gonna put into this. Consider this abandoned unless I publish more updates.

How it works

I studied Ortlieb's video: ORTLIEB | E-Trunk - The Trunk Bag For Your E-Bike

& their instruction manual.

Bag locks simply by dropping the trunk bag onto a rack. The mechanism is very clever & auto-adjusts to any tube diameter, & because it's spring-loaded, there is no gap which would generate rattle.

Removing the bag is a simple operation: disengage one side only & peel off the bag.

Ortlieb's mechanism is integrated into the bag's construction however.

I wanted to be able to bring that to any trunk bag that uses traditional straps. For example, many trunk bags features 4 velcro straps, 1 in each corner.

My design

• I didn't care about adjustability to fit different racks since I mostly use this with a single rack
• I wanted to be able to use this mechanism with a traditional trunk bag that has 4 straps: 1 in each corner
• the construction consists of the following non-exhaustive list:

Rev 6.3

Contact

I played around with many different curves for making the contact: ellipses, eccentric circles, etc., but settled on an Archimedean spiral. It was minorly painful to make this in Solidworks.

The inspiration for this curve was watching this video: Was it Worth Building These? | ft. Matthias Wandel. This timestamp mentions the use of a spiral in an analogous application in woodworking.

Spring action

The mechanism requires the use of springs. Ortlieb's prior art uses torsion springs:

I considered several different torsion spring alternatives, but ultimately used torsion springs as well. For experiment's sake, I also added a hook to install a rubber band or o-ring:

Processing & assembly

• torsion spring assembly could be made better, oh well
• used 3.30 mm reamer for PIVOT BLOCKs
  • .137" = 3.48 mm reamer for CONTACTs
  • this combo yielded a nice friction fit with the coiled spring pins, assemblable by hand

Performance

• the SPINE was far too flexible to work effectively, but I suspected as much
• the CONTACT worked decently well
• I didn't trim the ends of the springs because I thought I might need them for different future designs – they interfered with the rack rails & disallowed the CONTACTs from fully seating & locking
• otherwise, I was quite pleasantly surprised at how well it worked

Future improvements

• pelvis
  • need to stiffen: flip print orientation, add large stiffening rib underneath
  • change to m3 CSK instead of m4
  • fillets didn't mirror over
• general comments
  • mark all parts with REV
  • need stronger spring, or preload more
  • capture the spring better to ensure it's captive
  • spring leg is too long & getting caught on tube of rack
• contact
  • deeper rubber band channel
• spine
  • add hook & adjustable stop to prevent the bag from going forward & back
• PIVOT BLOCK
  • add CSK or CBORE to ends where pin inserts