Rack Building Basics — Fork Crown Mount
In this entry I’ll show how I build the fork crown mount for a rack. This is the part that goes from theback of the rackthrough the fork crown.
On a rack that is designed for very heavy loads I would recommend connecting the rackto the fork crown using two stays, one over each fork blade. This single stay setup is not as good at resisting high lateral loads. It does work with almost everyfork though, where thetwo stay method only works with forks that have extra eyelets on thetops of the fork blades.
I don’t have a lathe, so I use a bolt as the threaded stud that runs through the fork crown. This is a M6 bolt that I’m removing the head from:
The bolt will be held inside some larger diameter tubing. First I need to cut the tubing square. Note that to make sure that it is square I need to check the cut in two orientations 90 degrees from each other.
This is an exploded view of what is going on. I have a 6mm (close to 1/4″) threaded stud that will be inserted into some 5/16 x 0.035″ tubing. This is then inserted into some 3/8 x 0.035″ tubing. The 3/8″ tubing is bent and will be brazed into the fork crown. The 5/16″ tubing is pulled out from the 3/8″ tubing in this view, but when brazed together the 3/8″ and 5/16″ tubing will be flush. This creates a nice face to sit against the fork’s crown.
Everything is loaded up with flux and ready for brazing:
A view after the three parts are brazed together.
Here is a shot showing how I hold the piece in place while brazing it to the back of the rack. I’m using vice grips on the rack and a piece of scrap metal (PCI slot cover from a PC) to hold the piece in place. I checked with a square to make sure that the threaded stud is square with the back of the rack.
Here I am checking the angle. The bike that this is going on has a 73 degree head tube angle, so the face of the fork crown is also 73 degrees. I want it to sit flat (or close to it), so I need to angle my mount at 17 degrees (73 + 17 = 90). I’m using a simple engineers protractor to check the angle. My reference is one of the stays on the rack. I’m a little shallow here, but that is okay. It will make the front of the rack slightly higher than the rear, and that is acceptable.
The other option for doing this is to mount the fork crown mount into a fork and then use a fixture to hold the rack in place. The advantage of this method is that you can see how everything will look before it is brazed together. Alistair designed this nice little jig out of a test tube holder for holding the rack in place while brazing. I like the jig, but find that things are a little more secure using my method. Give both a try and see what works best.
The fork crown mount is half brazed onto the rack. I flipped the rack over to finish the brazing on this side.
All done
