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Old May 7th, 2006, 02:20 PM   #1
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Ideas for curved track

Looking for ideas for making curved track from PVC pipe for my skate dolly.




One thing I am considering is using tensioned cable to bend the pipe - think of a long bow ala Robin Hood- but how to tension the cable??

open to ideas and suggestions please
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Old May 7th, 2006, 02:35 PM   #2
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Bending PVC is going to be a challenge because of its mechanical properties. I think at room temperature its really brittle and to bend it it might need to be heated to the right temp if your talking about PCV pipe you would get at a hardwood store.

Materials have what is called a stress strain curve. Their is a linear part that is elastic and if you bend it anywhere along this line it will return to its original shape but their is a critical point that moves to plastic deformation where it will be permanently deformed but it will bend back. You need to apply enough force so that you leave the elastic area and move to the plastic area to retain shape. Metals like Steel and Aluminum have large plastic areas where they can permanently and easily be deformed to whereas plastics like PVC might have a much smaller area. For this reason it might be easier to actually permanently deform aluminum, copper, or thin wall steel tubing compared to PVC which will probably either shatter or return to near original shape if tension is removed.
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Old May 7th, 2006, 05:04 PM   #3
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You might consider something like the Losmandy Flex Track system which uses a roll of flexible track:

http://www.porta-jib.com/flex-trak.htm

I've used this with the Losmandy dolly and it's very good - even on tight turns.

Robin
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Old May 8th, 2006, 06:48 AM   #4
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Keith;

I was thinking of keeping the cable in place under tension like a bow. For example a 3m length when arced to a quarter circle will have an end to end chord of 2.7m, thats a strut compression of only 10%


Robin;

I see they use 40mm 85 hard rubber hose; I wonder if this or equivalent can easily be obtained in the UK?

Is it difficult to level on rough ground?
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Old May 8th, 2006, 08:14 AM   #5
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John:

Out of curiousity, how often do you feel you have a need for curved track when shooting? What sort of shots?
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Old May 8th, 2006, 09:46 AM   #6
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Quote:
Originally Posted by John Jay
Keith;

I was thinking of keeping the cable in place under tension like a bow. For example a 3m length when arced to a quarter circle will have an end to end chord of 2.7m, thats a strut compression of only 10%


Robin;

I see they use 40mm 85 hard rubber hose; I wonder if this or equivalent can easily be obtained in the UK?

Is it difficult to level on rough ground?
John,

Unless the PVC in the UK is different than the US, your idea will work. It actually bends very easily, but once bent it will be nearly impossible to straighten it out again, but hey it's cheap.

The main problem you will have is that you will not be able to bend two pieces into an identical shape. You will need to use a dolly which has movable wheels on one of the sides. One solidly mounted side will follow one of the tubes, and the other half will "float" in order to mate with the other PVC rail.

If you are going to use a tripod on it, some dollies that are made with only three sets of wheels instead of four. Two fixed and one that slides in and out. To make it the best and easiest to use, the two fixed sets of wheels should be able to rotate slightly, because the track will not be straight of course. If your curve is not too much, even straight fixed wheels will work.

One last point. When you tie your cord between the ends, you will probably find that it will bend more toward the center than the ends. It will not be an even curve, and that is just the property of the material. If you wish to make it a smooth curve you will need to find something to bend it around and leave it for a while. For example, if you want say a 12 ft radius in it, you might want to bend it around an object that has say a 8 ft radius and let it stay that way for a few days, especially if it is set in the sun. You do get sun there sometimes don't you? :) Use water tanks, cattle drinking troughs, anything large and round, and clamp at the very ends. If you use this procedure, the two lengths will be fairly close and you will probably be able to secure one section with tape and let the other float and move under the load of the dolly and the dolly itself will cause it to bend slightly and conform to the fixed rail, even with fixed dolly wheels.

Good luck Mike
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Old May 8th, 2006, 02:52 PM   #7
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Mike;

Thanks for those pointers; very helpful

Charles;

I've two shots coming up, one is a slooooow truck about a round-table brainstorming session (lots of 2's n 3's etc) intercut with a static of a project manager scribbling on a whiteboard, t'other is the project manager overseeing consultants at workstations - a snakey weave through the open plan office. I need to hold perspective so that means constant focal length.

I am coming to the realisation that snakey hose pipe track could actually improve productivity, provided your shot list is worked out in advance.
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Old May 10th, 2006, 09:49 PM   #8
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try this company for curved PVC track

www.longvalleyequipment.com/
6' section of curved track for 90.00 and it takes 8 sections to make a complete circle.
sounds like a deal to me
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Old May 10th, 2006, 09:50 PM   #9
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curved track. right address! sorry!

www.longvalleyequip.com for the curved track
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Old May 10th, 2006, 10:00 PM   #10
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One thing I will mention is that with even the most sophisticated, precision machined track, it's not always the easiest thing to get them to join cleanly in a full circle--sometimes the seams open a bit, or the sections will lift up.

John, for the roundy-round you describe it sounds like a full circle of track will be the ticket. Flextrack is cool but you wouldn't be able to join it to make a continuous circle, and it would be really annoying to have to "rewind" each time.

The other shot sounds like a stabilizer might be the ticket (not the worst idea for the first shot either...!)
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Old May 10th, 2006, 11:18 PM   #11
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Are there any single-track dolly systems? It seems like it would be much easier to have a curve with only one track. The dolly would be more like a bipod than a dolly, but this would take the weight off the operater (assuming a steady-cam would be used instead) and keep the pathway identical per shot.
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Old May 23rd, 2006, 10:58 AM   #12
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FTR;

After giving up the search for rubber pipe we have found a good alternative as 32mm MDPE (Medium Density Polyethylene) pipe which is used as mains water pipe in the UK. It comes in ARRI blue, has decent crush resistance for a ride-on and rolls up into a coil quite nicely so it fits in the car boot*(US trunk) and can be bought for around $50 for a 25m coil. We also got an industrial hot air blower (used in paint stripping - light stokes to warm it up to remove the curl ) to soften it up when track laying.
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Old May 30th, 2006, 11:16 PM   #13
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I am yet to try this, but here's what seems like a decent idea for bending PVC:

http://www.pvcworkshop.com/bendpipe.htm
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Old May 31st, 2006, 10:34 AM   #14
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Quote:
Originally Posted by Mike Toledano
I am yet to try this, but here's what seems like a decent idea for bending PVC:

http://www.pvcworkshop.com/bendpipe.htm
Would be great for furniture, but not necessarly for track. Track needs to have large radiuses for use with a dolly. This is more for doing small radiuses. But a good way of doing them!

Mike
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