Yesterday I finally comnpleted my first ever aluminum trials frame. Actually, it’s the first frame I’ve ever built, and my first ever machining project, ignoring the bc plates, which I started after it anyways. It has about 50-70 hours of shop time put into it. No more than 20 of those hours actually involved the cutter touching the metal, but setup on a manual mill when you’re a beginner takes about 70% of the time. I think that if I made this exact same thing over again, I could do it in 30-40 hours.
I ended up using 2 major tools on this, the lathe and the mill. Bandsaws were also used, but just because they are fast, efficient, and easier to setup.
The material is 6061-T6 aluminum. I cut the parts out of 3 original forms. A 2"x5"x22" piece of bar stock, a piece of 1/2" plate, and a piece of 1.250"x0.180" wall thickness tubing. The bar was what the crown came out of, the legs came out of the plate, and the seattube was cut out of the tubing.
The frame is 4.75" wide at it’s widest point, but in the end it is effectively 4.5" wide at the crown, where it matters. There is an 8" seattube, which is plenty for me. Good thing, too, since it took 3 tries to cut the right size for the seatpost in it, and by the 3rd try 8" of tubing was all that was left. The seattube is made to fit a 27.2mm seatpost, and there’s about 6-7" of space for the post to fit in. Past that the boring bar chattered too much to be usable on the tube, so I gave up. THe bvearing holders are machined into the frame, and are of a main cap style. They are for 40mm bearings, and have a small lip on the outside to help to retain the bearings. The bottoms are just stamped steel UDC bearing holders. I used 10-32" screws to attach the holders to the frame, which isn’t ideal, but it works.
A quick overview is that I haven’t ridden the frame much, for 2 reasons. The first is I haven’t had time to make a seatclamp for the thing, and the second is that my trials wheel needs a new hub, so I can’t really do any good rides on it yet. The frame felt extremely stiff at the the shop, and there shouldn’t be any of the issues with it that were found on Justin’s SH frame.
The basic design for the frame was taken off of a SH frame design, pictures of which can be found here. You can see a resemblence in the fork legs and crown/forkleg interface, but other than that, everything’s changed. Rather than being 1.5" wide, the crown is 2" wide. There are 2 cross-membery thingies instead of 1, and they are of a different shape. That frame is for a muni, this one’s for a trials. Mine has pins in the crown and seattube, while that one doesn’t. The bearing holders are also drastically changed. My seattube is also thicker walled near the bottom and longer. The crtown-forkeleg is so different there’s no reason to bother trying to explain it. There are other differences, but they aren’t as major.
I plan to put pins in the crown for foot on crown tricks, but my initial attempts at 1fww and standup ww have been already gone well with plenty of grip. Sadly it’s raining right now, so I’m not going to try much more for a while.
The crown-forkleg interface uses 2 1/4" spring steel roll pins and 2 stainless 10-32 screws to attach it, with the heads of the screws reccessed. They are placed at opposite corners to eachother.
The seattube-crown interface went amazingly well. It has a 0.002" interference fit with the crown, plus 2 3/16" stainless roll pins on each side. There is also semi-permanent locktite that I slathered on there before inserting the tube. I put the tube in a freezer for 90 minutes, and then when I tested it on the crown, it had a nearly perfect fit. Now it’s as solid or more so than if it were welded.
If anyone wants details or pictures of fabrication of the frame I’ll provide them, but otherwise I won’t bother. I don’t have pics of the cuts in progress (holdign a camera and running a mill at the same time sucks), but I can get pics of the tools used.
I think I will anodize this frame blue to fit my rim (blue try-all), but I’m open to other ideas. Anything but red, since red, silver, and blue is just disgustingly patrotic (no offense, but I refuse to ride the Americycle). I found a site describing how to anodize at home, and I checked with the shop, and we have all the equipment neccessary for the task. I may anodize it Monday or Friday.
I would like to hear any input you can t hink of on ways to lighten or strengthen the frame, or other possible improvements or changes, since this is just a prototype.
Thanks for reading.