The Melvin bolts to the frame where the derailleur would normally be and allows the use of a one speed rear wheel on bikes with vertical dropouts.
A vertical dropout has the slot for the hub axle going up and down. It keeps the wheel in a fixed spot, and 99 times out of 100 it does not put the wheel in the right spot for perfect chain tension. (With a horizontal dropout the wheel can be moved fore or aft to remove slack from the chain.) The Melvin takes care of all that.
Spacers on the mount can be rearranged to place the pulley wheels under the cog. The top pulley has some side to side float to fine tune the chain line also.
There are other chain tensioners out there but only the Melvin uses two pulleys which provides the most chain wrap. One of the coolest things about the Melvin is that it allows two front chainrings to be used (of course this is no longer a single speed, but can be very handy for loaded commuting) Because we use two pulleys, two chain rings with a maximum difference of twenty teeth can be used up front.
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5.0 GEARS 1 review
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I've put these on two mountain bikes with tripple chainrings. Supposedly the maximum difference is 20 teeth, but one of the bikes has a 22-32-44 and it works great. Setting up another one soon. One speed for climbs, one for level and one for downhill. Simple.