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how to attach bike trailer without coupler
Partly
Baler twine
Someone in the boot holding onto it
Zipties, the modern baler twine now bales are more often wrapped than tied.
To attach a bike trailer without a coupler, you can use a universal bike trailer hitch that attaches to the rear axle of your bike, or make your own coupler using a piece of metal tubing and bolts. However, it's important to note that doing so may void warranties and pose safety risks. It's always best to use the manufacturer-recommended coupler or hitch for your specific trailer and bike combination, and consult with a professional if unsure.