Engineering Advice (Frame)

I am nearing the completion of the first prototye for my trials frame. I have completed most of the crown (all further machining on the crown will simply be done to cut weight). The legs are largely done, except for the hole which need to be drilled and tapped for the bearing holders.

Note: Summary below.

So, I have an issue with the interface between the legs and the crown. The crown and legs are both plenty strong, so the weak link is the interface between the two. I have a 3/8" deep, 3 sided slot cut on each side of the crown to accept the legs. There is a +/- 0.002" slop in one side, and a nearly perfect fit on the other side. I have the legs drilled to make the interface use 4 screws to attach the legs, like a Steve Howard frame. My issue is that I am very concerned that the 4 screws won’t be adaquate to handle the forces of strenuous trials riding. After looking at the frame for a while, I’ve noticed there are 4 convenient spots on the crown to put welds. The shop I’m at has a nice TIG welder and the guy teaching me has told me he is willing to do these welds for me (25 years of TIGing al. and stainless). I am strongly considering taking him up on his offer, but I can’t get the thing re-heat treated if I do so.

Summary: My question is, how much will the loss of the temper of the aluminum hurt the strength, and is it something to worry about? Should I just stick with the screws?


Can’t you try out the screws and if they break, weld it? If so i would go that path.


Seems like you need more practice making frames. Trial and error are lifes best teachers. In an effort to help you out I am willing to take one of these trial and error frames. Just PM me for the address and my UPS shipper number.:smiley:

Welding thick aluminum requires lots of heat. It will probably be dead soft when you’re done welding. Regions of both the crown and the fork legs, right where they join, will be impacted.

SH frames take vertical impact on a lip cut into the crown. The screws just hold the fork legs onto the crown and don’t offer any vertical support in shear during a drop. The crown sits on top of the fork legs.

To me it sounds like you’re doing roughly the same thing. That is I think you’re saying the crown sits on top of the fork legs. If that is the case, and the slots are as you desribe them, the screws might go in from the top of the crown or through the side of the crown, I don’t know which you plan without a sketch. Like the SH frame, if the screws go in from the side they are in shear only as you pull up on the saddle, not when you do drops. If they go in from the top of the crown the screw threads and tapped holes have to support the axial stress of lifting. In either case, I think you’re OK.

I agree with Harper in that the heat of welding will soften the aluminum considerably. However ( and I’m not familiar with your design here), if there is enough mass at the crown/fork leg interface I think it might be very difficult to bend anything here so it might not matter much. I think I’d still consider the welding as opposed to screws. You might start with screw though and see how it works out. Just check them often for loosening.

Also, be careful when doing any additional machining for lightening. You don’t want to compromise strength.

Good luck.

Maybe consider 2 spot welds on each side of the crown, in addition to the screws. That way you get additional strength without ruining the aluminum. And if you ever need to disassemble, just mill off the welds and re-weld later.
OTOH, don’t do this if you’re into aesthetics…



As Greg said, without a sketch it is hard to say. I am also imagining it to be similar to the SH frame.
If so, and if your joint is not machined so that the fit eliminates any movement between the crown and leg, consider adding dowel pins to it. The tight fitting dowels can carry all of the shear forces and the screws just hold the parts together. To get a good fit on the dowels you will need to drill a few thousandths undersize, then use a chucking reamer of the appropriate size for the dowel.


Thanks for the advice yall. I can do one better than a sketch (this part of the crown isn’t very well described in my sketches. I’ll post pics of the crown, with and without the legs.

I’ve been toying with the idea of a 3/8" dowl pin in the center of the fork leg. I also like Harper’s idea of having vertical or lateral screws installed to help deal with the stresses. I am not really concerned with any of the front-back, up down forces, though. My biggest issue is the rotary force conentrated on the crown and lateral forces.

Bugman: Sadly, i already have some people who have volunteered to test the thing. But your advice is quite true and worthy of repeating to others. Nothing ever comes out perfect the first time. On this frame I had the issue of accidentally making the fork legs around 1/4-3/8" too short. I’ve had to compensate in the crown.

I’ll have a couple of pics up soon. I showed the folks on the muni ride it today, and they thought it would be strong enough, but i am still doubtful.

Evan: Rivets wouldn’t work at all. Any rivet that can do make it through 3/4" of metal would be too large to fit on the limited interface area (1.375x1.5")

Here’s a few pics. If I knew how to get them all into one post, I would. I tried to keep the file size down. None of the pics are bigger then 110kb, and the resolution on all of them is only around 640 by X pixels or X pixels by 640.

Here’s the basic pic of the front of the crown.

A side view of the crown, showing the slot that the fork leg fits into. The bottom surface is very smoothe, but has that strange textured finish because I had to do the finish pass with a 1/4" endmill. Sadly the important part of the pic isn’t very well focused.

Here’s an oblique bottom view of the crown, showing what there is to see. I did the basic roughing of the metal on a great 1916 vintage English toolroom lathe. You can see that on the outside I went a bit too far into the support parts of the legs. I used a 1" drill bit and a boring bar to do the center of the crown, where the seattube will go. The line in there is superficial, caused by stopping the lathe and then retracting the boring bar. I used a 3/4" diameter ball end mill to cut the curves on the left, right, and center of the crown. I then used a 3/4" end mill to flatten out the spaces in between. The blue stuff is the remains of my layout on the piece, although in this part I did it basically freehand.

I forgot to mention that cutting the first slot in the leg for the crown took me about 3 hours. The second one took 90 minutes. It took about 30 minutes on each of them to just setup and index the mill to the right areas. Taking readings was also difficult with a calipers because I didn’t want to move the table, knee, and especially the quill unless I absolutely had to. God I want a cnc.

crown, oblique bottom.jpg

Here’s the final pic of the crown and forklegs (still pinned together for convenience and accuracy). One slot on the crown allows 1/4" of lateral movement of the legs, when measured at the end of the legs, about 10-11" away. The other side is 1/16". I rounded the corners of the legs using a circular sander. That part took all of 3 minutes.

Neat bit of machining there. That’s a learning experience.

You might want to round off some of the 90 degree edges where your legs will rub the crown.

I definitely plan to. Most of those edges need to be wrapped in cloth for transportation, since they can easily rip through my camelbak, not to mention my skin.

Good job, Bevan. That is a great learning project.

As for the dowel pins, I would not use only one per side because the leg will be able to rotate around the pin. You would get much more benefit from 2 pins, 3/16" or 1/4" dia., spaced as wide as is convenient. You could even have a dowel in place of each of the current screw locations, and one large socket head screw recessed in the center.

Or for an easy alternative, you could just put a small bead of epoxy like JB Weld in the corner of the crown piece, then assemble the leg with the current screw setup. The epoxy will squeeze up and fill the gap between the leg and crown, removing all the “wiggle room”.

As for rounding the corners, many people don’t realize you can do some pretty cool aluminum machining with a hand held wood router. In this case, you could use a 1/8" radius carbide roundover bit with a bearing. Practice on a scrap first and BE CAREFUL. Let me know if you want more details on the tools or the process.


Thanks for the encouragement. I’m really liking your ideas for the attachment. I’m going to the shop in a few minutes to finish up the machining on it, and then I plan to take a good look at your ideas. I’m seriously leaning towards the single large socket and the 3/16" pins for the interface now. Would it hurt to use 4 pins? I think I will use JB weld, too. I was planning on using something to fill in the space, but I didn’t yet know what.

I am a far better carpenter than I am a machinist. I’ve routed a teeny bit of aluminum before. It wasn’t a pleasent proccess. I considered doing this, but have vetoe’d it for a few reasons. The chips off of a router are easily airborne and lodged in one’s lungs, and I don’t have the correct respirator to deal with it. Also, when routing aluminujm you really should use a slower speed, around 11,000 rpm, and mine only does 22,000. I also don’t have a good 1/8" roundover bit with carbide faces. I think I’ll just use a file.

Thanks for all the help Scott. I’ll post my progress when I get home. Also, if all goes well this frame should be at motorama (Joey will be riding it if I can ship it soon enough). I’m almost sure it’ll be at Moab, so I can hear some better advice from in-person observation of it.

Thanks again for all the help.

This shows how dumb i am: what tools did you use to make the crown? looks like you could have done the top part on a lathe, but i dont know about tose notches, were those done on a mill?

You aren’t stupid, you just didn’t bother to read any of the text that went with the pictures. Don’t worry, most of the people who’ve seen the frame have had no idea what tools were used on it.

Yes, the top of the crown was cut on a lathe. for the finish pass on the radius, I used a hand file on it while it was running, like a wood lathe. Very dangerous.

The bottom was roughed on a lathe and then finished on a mill. The notches were done with a mill.

I just got back from the shop. Thanks to a half day at school today and tomorrow and Friday, I hope to have the frame done by Friday afternoon. All that’s left it to do a bit of a taper on the top of the fork legs and put in all the screws. The legs and the crown fit the wheel, with about 1/8" clearance on every side of a worn monty whiteline on a tryall 47mm wide rim.