I added a rack to my Redline Zander MTB to make it more usable for me as a "trip" bike (being able to carry food, water, clothes, daughter's inhaler, etc). I have set my Roam up as more of a road/path bike with fenders and Schwalbe commuter tires, and this Zander is for taking out in the woods on trails and creeks, etc. But I still want to be able to carry all my kit with me comfortably. So I needed a rack.
The frame has no rack mounts on the seat stays, so I used a seat post collar designed for mounting a rack. I also found some rack struts in a parts bin and cut them to the appropriate length. It works great and, because I have a Topeak MTX rack on both my Roam and my Zander, the bag and light go with me regardless of the bike I'm on that trip.
Seat post collar:
Giant Seat Collar with Rack Mount
Rack:
Topeak MTX Explorer Disc
Bag:
Topeak MTX Trunkbag DXP
Light:
Planet Bike Blinky 3
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jnjadcock, on Flickr
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jnjadcock, on Flickr
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jnjadcock, on Flickr
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jnjadcock, on Flickr