My Super Commuter 8S Rear Bag Solution


New Member
On my Super Commuter 8S, I needed a rear rack-bag capable of carrying two extra Bosch batteries, tools, accessories, snacks, etc. The minuscule stock SC8 rack essentially limits rear-bag options to panniers.

My solution was to buy a Topeak QR Beam seat post rack (Link Removed - No Longer Exists). As delivered, the problem with the Topeak QR Beam is its cantilever design. This limits the useful weight limit and makes it susceptible to lateral movement while riding. To fix this issue, I fabricated two stabilizing struts out of thin-wall 4130 steel tubing and connected the racks together with 5/16 inch adel clamps. I have ridden with this setup on approximately a dozen long rides with no major issues. It works very well. Even with the weight of two extra batteries in the bag, it is all rock solid. The bag is a Topeak MTX Trunk Bag DX (Link Removed - No Longer Exists). The bag can be attached and removed from the bike in seconds with the built-in quick release. The bag's top is expandable and it also has two side pockets for wallets, keys, tools, etc. It's a well thought out design and very convenient. Only modification so far is that I will be adding a thin steel cable with a quick connect clip to prevent the bag from inadvertently sliding off the rack from the rear (this happened once after going over an unexpected large speed-bump at a too high speed).

Photos are below. Later this month I will be powder coating to flat-black all of the bare Topeak aluminum and the bare steel struts.

Thanks to Luv2ride on this forum for inspiring this!

Wow. That is absolutely 100% perfection! I was contemplating something along these very lines but got stumped at the point where you have the 4130 tubing and adel clamps. I could not figure out what to use, how to do it. After studying the photos, I'm a big fan. Getting the new Allant+, and will have the very same need (stupid Trek, even after all the desires of existing users, they still make a bike with those stupid pannier only rails), So, with that said, can you possibly share some sort of explanation, blueprint, exact process to replicate those stabilizing struts? Alternately, you could start a side gig and make them? I'd pay you.

Glad you like the design. It should work equally well on your new Allant+.

It is really easy to fabricate the struts yourself. Should take less than an hour to make both.

Tools needed are:
  1. Hack saw or any powered metal saw to cut the steel tubes.
  2. Bench vise to pinch the ends of the steel tube.
  3. File or grinder round off ends of the steel tube.
  4. Electric drill to drill holes on each end.
I bought the thin-wall 4130 tubing from Aircraft Spruce. You can probably find similar tubing on Amazon. 4130 thin-wall steel tubing is high strength and very malleable so it doesn't split when you pinch it in the vice. I used aircraft quality screws and fiber lock-nuts.

For aesthetic reasons only, I will be disassembling the Topeak rack and powder coating all the bare aluminum and 4130 steel struts to an all flat-black color. All flat-black would also look great on your new Allant+.

Additional photos follow:

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I have the same bag and rack. I had previously asked a local metal shop about doing something very similar. After this post, pretty sure I can do it myself.