[Doug Fattic]

Fixture design is a little like religion, there are lots of ways to make them and everyone thinks theirs is right. What kind one gets depends on what kind of method is used to put a frame together. I suggest you ask your question on one of the several framebuilding forums where builders hang out. Velocipede Salon, MTBR and BikeForums. In addition Google Groups harbors another group of builders that communicate by email.

I’ve been thinking of how to design fixtures ever since I apprenticed in England in the 70’s. I’m not a fan of the Arcos fixture although that doesn’t mean it can’t work for someone else. One of the challenges of 80/20 aluminum extrusion is having enough accuracy in the material to do the rear triangle based off of the fixture itself. They always have a bit of twist and bow. Accuracy is the problem with all fixtures and why an Anvil or Sputnik costs around $5000. The cost of building them with enough precision within themselves to center a rear wheel to be off by less than 1mm is pretty high. This is why many pro builders buy a pro fixture instead of making something out of wonky 80/20.

There are 2 things that are important to me in fixture design. 1st I want to be able to set the fixture with a minimum of outside tools like rulers and a protractor. And 2nd if I need to adjust one thing after it is set I don’t want its other pieces to have to be reset again because of the change.

If I were going to use 80/20 I would support the pieces with a “picture frame” that the pieces holding the tubes can attach to. That way the markings on this outside frame would make it easy to set the seat and head tube angles. I would also make the cones on slots so they can be moved to be in alignment with each other (instead of relying on the 80/20 to put them there). That also means the bottom bracket shell adjustment needs to be able to move too. This outside frame can allow the fixture to be in horizontal or vertical or if held by some kind of stand spun around.

I have a fixture laser cut and etched out of stainless steel that lays on an alignment table so its accuracy does not have to be based in itself but rather off the table. Almost everything can be set by reading fixture markings instead of having to use rulers or protractors (although sometimes I do need to use those tools). Because it is representitive of a full-scale drawing and I can design the frame right on the fixture that matches the users bicycle position to orient the tubes. Fixture design is a complex subject that can’t even be introduced well in a single post. Good luck.