Thorp Air Command - T18.net

Since I dont see this topic I will throw myself to the wolfs. In the process of rebuilding 7077J I cant live with my roll bar. Bought one from some one here awhile ago that was supposed to be a "stock" t-18 buy its not. My bad had it powder coated first and went to use it now. Its 2" to wide! Hmm wonder what that fits And its 4130. So I decided I am going to build one. So here are all my questions and dilemmas. I know there are an abundant amount of ideas/opinions here.
1st convinced I want to go aluminum. plans call out for 4130 1 1/4 at 120 tube. So obviously? I want to stay with 1 1/4" tube. But how thick and what grade? 6061-0 at .035? T-6 I can go anywhere from .035 - .120 I doubt I can bend the thickest though. 2024@ .045? I dont know where to start.
2nd problem. All the tube benders you buy at 1 1/4 are actually bigger (cup diameter) than 1 1/4. Is that normal? Are they adjusted to give some? I was going to pack with sand to hopefully stop it from flattening or am I being ridiculous?
??? Let the comments begin!
Skeet

A steel roll bar makes for a good compass reading, go aluminum.
I built my Thorp from the ground up making all the parts with the exception on the gear leg and roll bar. Reason was I could not buy the materials and make it for what Ken Brock wanted for the finished product. Check with Cubes
I recently straightened an aluminum bar and it took a 32 ton press and a better part of the day to straighten, not an easy job.
I was in Tampa a while back and watched a guy roll bend pipe with a huge hydraulic bender and he was good at it. No he did not stay at the Holiday Inn the night before

Good to hear from you, Skeet. Sounds like you may have stumbled onto an S-18 roll bar. Try a call to Richard Eckland at Eckland Engineering and see what he might have to say. The roll bar that Bill refers to was out of an airplane that get flipped over in the wind. It took a hard hit and got slightly bent. It took a team of horses to take out the slight bend it suffered. That aluminum bar is stout! I know of a steel bar installation back in the day that the builder had degaussed prior to use. It still drove the compass wild.

_________________
Bob Highley
N711SH
SN 835
KLAL

I have built many of these Aluminum roll bars, they are very thick walled, couldn't get my pinky finger into the ID of the tube. It is 6061-0, it is dead soft for forming in a fixture, and even dead soft it is a buger to bend. After it it formed it is sent to heat treat and brought to the T-6 configuration, this is why it is very rigid, tough & hard to straighten if it gets pushed out of shape, which would take a pretty good lick to do. If I were going to use an aluminum unit I would check with Cubes and see what he has and can get you fixed up with as the alum unit does require some process, not that one couldn't get through that process on ones own but it might be easier just to pick one up from Cubes. The 4130 unit lends itself to modification very well, the aluminum unit not so much because of its process to make it rigid, but it does present a very nice area to drill and tap for the Windscreen retention screws!! Dan

Thanks for the quick replies. I have a disease, its called I want to do it myselfitis. No but really all the other holes are drilled and the previous builder put a light bend in the tank deck main stringer which in turned put a slight bend in the firewall stringers....ect As a result instead of buying a roll bar and rebuilding all the rest, I will build the roll bar to "fit" my plane and make necessary adjustments. So does anybody know about that loose fit on a supposed correct size tube bender?

Hi Skeet,

I'm not sure how loose you're talkin'. Stupid question, but are you sure you've got the right size bender? Sometimes benders are made and marked for nominal pipe sizes (ID, plumbing stuff). For instance, a 1" copper pipe in plumbing language is what is called 1 1/8" (which is the actual OD) by pipe fitters & refrigeration guys, and so it goes. Otherwise, the cupped wheel on a lever bender is usually a pretty good fit, but not tight. Same goes for the shape of the shoe. Don't know if that helps or not.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Yeah it's loose, not snug. And I checked it on 2 different benders. I have a hydronic gardener bender with a 1 1/4 marking on it. Then I bought a harbor frieght pipe bender. The die that says 1 1/4 on it fits exactly the same. Not snug. Hmmmm

Ok mystery solved but still have a problem. Bought a 10' piece of 1 1/4 EMT today. Was going to practice on it with my Gardner Bender 1 1/4 inch die. So i went to put it in the die and it fits perfectly! BUT it's OD is 1.5 Bummer! So I guess its like buying a 6" wide piece of wood only to get it home and find out its actually 5.5" I hate SAE. The only thing I wish we would have got out of Jimmy Carter is the metric system. This is retarded. SO what die's where used to bend a REAL 1.25 inch tube?????

... and then there's electrical conduit. So maybe you have a bender for 1 1/4" conduit? Conduit is identified by the approximate ID. For instance, 3/4" EMT has an OD of approximately 15/16". And the ID isn't even exactly 3/4"! If you want to bend 1 1/4 " OD pipe, you need a bender made for that, not an electrician's emt bender labeled as 1 1/4". I don't think that there's a plumber's or an electrician's bender made in that size. As I said, 1" copper pipe to a plumber is 1 1/8" OD. I've purchased plenty of benders and they've all been designed for a specific OD pipe size, and labeled as such (from refrigeration parts suppliers). 1 1/4" OD is not a standard size for copper pipe or emt. 1 1/4" OD tube benders are certainly made. You probably have to get it from an industrial supplier or?

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Skeet,

I had the same issue - an aluminum roll bar 2" wider than I needed. This one was about 3/8" thick. I managed to finesse it just a little at a time over a wood mandrel I made, including clamped-on sides to keep the tube from flattening. It took a lot of strength but I also slipped a pipe over the leg for some leverage. I think the trick is to be patient and do a lot of small adjustments, even if it takes an hour or so to work the material without wrinkles. An inch on each side isn't so far to go but you need to have the 'feel' as it gives each time before kinking.

Thanks guys for the other comments too. I've often wondered about the strength of aluminum roll bars and forgot about the compass. Nice to know there are examples of crash-worthiness too. With the same regard, I'd suggest good scrutiny on the weld at each end of the center post. That would be important to keep the roll bar from tearing away and just folding over in an accident.

Les Krumel,
Albuquerque