1. Why so many sizes?
Because there are so many different sizes of tubes out there to attach to.
It's important the clamp be exactly the right size to ensure the amount of grip
needed. We even machine them with the paint thickness taken into account.
2. How do I know which one to get?
First you need to measure the size of the tube you'd like to clamp to. If you
don't have a good measuring tool, like a set of calipers, you can download and
print our handy dandy Tubing Measurement Tool and find out.
Most of the mounts have 3 holes for idlers, to give you up and down options.
Some of the mounts have fewer, because the mount itself is smaller. Our largest
one, 60mm, has two holes, for arcane technical reasons.
3. Where can I put them?
These clamps are designed to go on any tube of the proper size, as long as
the tube is straight or nearly straight where the clamp is. In order to get the
clamping strength required, the fit needs to be good. If the tube being clamped
to is curving, it's not really possible to make a clamp that will fit accurately
enough. It would require some even fancier machining, and it would have to be
custom made for each curve shape. So, you need to stick to straight, round
tubes. Fortunately, that includes most bikes.
The clamps are 1/2 inch (12.7mm) wide, so it's easy to work around obstacles
like seat rails and cable stops.
4. Where are the mounting holes relative to the tube?
For the straightest side to side chainline in the low gears, it's necessary
on most bikes and trikes to allow the power side idler to go as close inboard as
possible. In order to provide enough thread engagement with the idler bolt, the
mounting holes are off the tube centerline. The hole closest to the centerline
varies with the size, but it's usually approximately just inside the tube
The mounts can be flipped up or down, so the idler centerline can be above or
below the tube centerline. For two of the most popular tube sizes, we do make an
adjustable mount where the idler bolt can go on centerline as well as quite a
ways above or below the tube. In order to do this, the closest the idler can
come to the tube is pushed out about 3/8 inch (10mm). This is usually more than
is desirable on a deraileur chainline, but it can work. This mount is primarily
designed for geared hub and tandem timing chain tensioning.