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Old 07-07-2019, 04:11 PM
BruceConner BruceConner is offline
Join Date: Mar 2017
Location: Tewksbury, MA
Posts: 15
Default Best approach to this shrinking job?

I'm making a piece of the tailcone for a cyclekart based on a 1930s GP car, and there are two ways to go about the job and I thought I would take advantage of the collective wisdom here to see which way might be the best, or at least listen to the half dozen differing opinions!

Due to the engine being in the back of a cyclekart, and some of the proportional changes needed to make the thing the right scale, it's not possible to make the tailcone as a single piece. In fact, it has to be made in 3 pieces - a center longitudinal strip, a lower half cone (to be screwed on semi-permanently to the above strip) and an upper, removable half cone, hooked with Dzus fasteners and a locating device at the rear.

The first piece to make is the center strip (see photo below - the purple hashed section), since everything keys off that. The part is simple enough - Seen from the top, the typical pointed "U" shape (or maybe a rounded "V"?) you'd expect on a single seat racer of the 1039's era. However, it will also have a 3/4 inch wide 90 degree inward facing flange all along the top and bottom edges. This flange will be for fastening the lower half cone, and for the upper half cone to rest on (the upper/lower half cones will also have a matching flange)

In other words, in cross section the longitudinal center strip will look like [.
Obviously, to get the rounded "V" top view profile, the flanges will need shrinking to pull it to shape. That's where my question about workflow comes in.

Two approaches spring to mind:

Given - I have a profile made of the curve I want, so I can work to that.

1. Bend the top flange only, shrink to match the profile, then tip the bottom flange and shrink until everything look straight. Advantage would be good control when shrinking the top flange so that the profile comes out accurately. The shape would "be there" in the top flange as I dialed in the lower flange.

2. Bend both flanges and ping-pong back and forth on both to gradually shrink and bring the piece to the profile. Advantage would be tipping both flanges on a flat piece is pretty straight forward. Disadvantage is (perhaps) chasing the profile shape all over the place due to imbalance of stretch in the upper and lower flanges.

Hope my description makes sense. Anyway, opinions are greatly appreciated.


-Bruce Conner

Last edited by BruceConner; 07-07-2019 at 04:56 PM. Reason: Added photo for clarity
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Old 07-07-2019, 05:54 PM
Kerry Pinkerton's Avatar
Kerry Pinkerton Kerry Pinkerton is offline
Join Date: May 2009
Location: Near Huntsville, Alabama. Just south of the Tennessee line off I65
Posts: 7,723

I'd do #2 for sure. The strength of each flange is not going to allow that curve to change as the other flange changes when you shrink it.
Kerry Pinkerton
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Old 07-09-2019, 02:37 PM
crystallographic crystallographic is offline
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Hi Bruce,
I think you should put a little crown in the central area of your panel, staying back from edges about 6in all around.
Then do your #2, shrinking one edge and then the other, back and forth until you get the panel to lay pretty closely. This is a standard approach for a panel like you have, and also for the upper triangular panel also. Putting in some crown first helps the panel have some geometry for support, otherwise you face the irritating flip-flop of an unshaped panel when you work the edges into their curves.
Might want to practice your edge working method on a strip of scrap/spare material first, though.

"All it takes is a little practical experience to blow the he!! out of a perfectly good theory." --- Lloyd Rosenquist, charter member AWS, 1919.
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Old 07-17-2019, 05:25 PM
BruceConner BruceConner is offline
Join Date: Mar 2017
Location: Tewksbury, MA
Posts: 15

Thanks for the advice, it was right on the money. Result was all I could have asked for, and things stayed under control throughout.

First picture - mostly complete, needs a bit of cleanup and ends trimmed to fit the joggle on the back of the seat.

Finished, and in place!
-Bruce Conner

Last edited by BruceConner; 07-17-2019 at 06:35 PM. Reason: added descriptions to the photos
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