Seat tube plate for twin TT

[Meriwether]

Can anyone recommend a thickness for steel plate used as a brace/plate/platform for the Retrotec style twin toptube frames? Anyone know what he uses for this purpose?
Do you think 0.032" is thick enough? I'll try asking Curtis if nobody knows.

Photo stolen off the internets to show what I mean:

[j-ro]

That looks more like 16ga to me so,060ish.
.032 might be on the thin side, it would be plenty strong but just wouldn't look right

[Brad Bedell]

Walt did a 'box' for my twin tube on the 36er. I assume he took some square tubing drilled a seat tube hole and mitered to fit the tubes.

[Walt]

I think it started life as a piece of 3/4x1 1/2" rectangular 4130. Probably .065" wall. It was
relatively easy to miter and fit in, and IMO will spread the load out quite a bit better than a piece of plate. Some of the welding is a little bit annoying because of the tight spaces in that area, but it's not bad. I'm sure you could use brass or even silver and just braze it in if you wanted to.

But then again, I have done it a grand total of one time. So my "expertise" isn't all that great here...

-Walt

Walt did a 'box' for my twin tube on the 36er. I assume he took some square tubing drilled a seat tube hole and mitered to fit the tubes.

[Brad Bedell]

Welp, I've not broke or bent it. So I think it's good. I was thinking a plate would be what I would have done, when I saw the box I acknowledged that was a better solution.

Have you built your 'bean' a bike trailer yet?

[Thylacine]

The plate is better off welded to the top of the stays, not at 3 o'clock, and make sure the end of the plate is not loaded and therefore causing a stress-riser. The picture sample sort of does it, but it could be better.

Did I mention twin top tubes are dumb?

[jeff]

The plate is better off welded to the top of the stays, not at 3 o'clock, and make sure the end of the plate is not loaded and therefore causing a stress-riser. The picture sample sort of does it, but it could be better.

Did I mention twin top tubes are dumb?

Yes...

[Meriwether]

The plate is better off welded to the top of the stays, not at 3 o'clock, and make sure the end of the plate is not loaded and therefore causing a stress-riser. The picture sample sort of does it, but it could be better.

Did I mention twin top tubes are dumb?

haha, if i were only doing a twin TT! The bike i'm making is even dumber!

It appears to me in the photo, and others i've seen, that the stays are indeed at 3 or in the middle of the tube, not on top or bottom. Do you have and example you could share of what you mean - where the stays are under the plate?

[curtis inglis]

I use two different wall thickness depending on what type of bike I am building. .040 or .050 4130 plate

thylacine: thanks for the tips on making my bikes better!

[Meriwether]

I use two different wall thickness depending on what type of bike I am building. .040 or .050 4130 plate

Thanks Curtis! Great to hear it from the horses mouth.
Since trying this type of build, I have gained a new level of respect for twin top tube frames, and any frame that has curved tubes. It definitely adds a level of complexity to the mitering and fixturing IMO.

I switched from 0.032" to 0.062"-ish since that's what Aircraft Spruce carries and glad I did. The 0.032" is pretty thin and flexy, but easy to cut with metal clips. It'd be nice to have a bandsaw for the 0.06 but a hacksaw works good enough.