Engineering Advice (Frame)

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

E.

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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.😄

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Welding thick aluminun requires lots of heat. It will probably be dead
soft when you're done. Both the crown and the fork legs.

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.

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.

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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.

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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...

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"rivets"

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Bevan,

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.

Scott

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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.

S_Wallis said:

*Bevan,

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.

Scott *

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"😉

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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.

+----------------------------------------------------------------+
| Attachment filename: crown, front.jpg |
|Download attachment: http://www.unicyclist.com/attachment/273098|
+----------------------------------------------------------------+

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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.

+----------------------------------------------------------------+
| Attachment filename: crown, side.jpg |
|Download attachment: http://www.unicyclist.com/attachment/273099|
+----------------------------------------------------------------+

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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.

+----------------------------------------------------------------+
| Attachment filename: crown, oblique bottom.jpg |
|Download attachment: http://www.unicyclist.com/attachment/273103|
+----------------------------------------------------------------+

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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.

+----------------------------------------------------------------+
| Attachment filename: crown and forkleg.jpg |
|Download attachment: http://www.unicyclist.com/attachment/273105|
+----------------------------------------------------------------+

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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.

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john_childs said:

*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.

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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.

Scott

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S_Wallis said:

*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.

Scott *

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.

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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?

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total uni said:

*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.

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