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Old 07-19-08, 12:01 AM   #1
marxspiro
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Downtube Removal

I'm considering getting wild with my aluminum commuter bike and removing the downtube and replacing it with a cable. I've on seen this one the Slingshot Bicycles but they turn it into some sort of funky suspension thing. My thinking is that the downtube is only in tension (unless you're crashing into something) so why not replace it with a cable to save weight (my bike has an oversized downtube). Will this really save me any weight? Could this cause my top tube to torsion as I pedal, and subsequently cause my bottom bracket to "wag"? I would appreciate any advice on this and also possibly some where to find some other examples. I sure someone else has tried this.

here is slingshots website.

http://www.slingshotbikes.com/bikes/1
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Old 07-19-08, 10:28 AM   #2
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Removing downtube

Dear Marxspiro:
This is my opinion.
First, the downtube replaced with a cable will "work" for the upward movement of the front wheel, but what about when the front wheel is pushed downwards? Cable is useless in compression and all the stress will be on the top tube. Which brings me to the second point.
I think it would be better to experiment with this design using a Cromoly frame. Aluminum does not flex well.
Good Luck,
CK
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Old 07-19-08, 12:11 PM   #3
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A standard bicycle fram transmits the torsion (turning force) of pedaling through the downtube (a rider often pushes down with left hand and right foot... or right hand and left foot... in order to balance herself).

Another issue is that each time the rider hits a pothole or sharp bump, the force on the downtube of a normal bike goes from tension to compression for a short time.

Quote:
Will this really save me any weight?
Here's how I think it might actually save weight: you design and build a bike with a compromise between the design you describe (keep the tension cable) and the combined top tube & down tube design like this folding bike

Last edited by cerewa; 07-19-08 at 12:20 PM.
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Old 07-19-08, 04:28 PM   #4
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You are missing the key feature that makes the Slingshot frame work so well, the hinge between the tt and st. It's made to allow the frame to flex, smoothing out the ride and allowing it to fold for packing.
Also: no, you won't save any significant weight- and yes, your frame will snap right at the st/tt joint.
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Old 07-20-08, 02:36 PM   #5
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Well, the bike is a GT so it's seat-stays go past the seat tube and connect to the top tube about 8 inches from the seat tube junction so I'm not incredibally worried about that joint snapping because its overly reinforced, which is why I am considering this mod in the first place. I have also just considered cutting off those reinforcemend to shed some weight, but that would not look nearly as kewl. Other concerns are duly noted though.
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Old 07-20-08, 10:47 PM   #6
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Removing down tube could get you nominated for a Darwin Award.
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Old 07-21-08, 03:45 PM   #7
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Originally Posted by marxspiro View Post
Well, the bike is a GT so it's seat-stays go past the seat tube and connect to the top tube about 8 inches from the seat tube junction so I'm not incredibally worried about that joint snapping because its overly reinforced, which is why I am considering this mod in the first place. I have also just considered cutting off those reinforcemend to shed some weight, but that would not look nearly as kewl. Other concerns are duly noted though.
You just might have found the only practical application for a frame with a Hellenic-style stay attachment......it's still gonna break.
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Old 07-21-08, 05:42 PM   #8
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There is absolutely no reason why this won't work. I strongly recommend you do it. Just make sure you videotape the first ride. So we can see how wonderfully it works, of course...
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Old 07-22-08, 08:57 PM   #9
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well unless someone says "I tried that and it nearly killed me!" I think I'm gonna go for it. If in the case that I have a "sudden and catastrophic" frame collapse and survive, then at least that may give my wife the impotice needed to allow me to purchase a new frame.
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Old 07-23-08, 04:40 PM   #10
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Remember: videotape!
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Old 07-23-08, 06:25 PM   #11
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Quote:
Originally Posted by marxspiro View Post
well unless someone says "I tried that and it nearly killed me!" I think I'm gonna go for it. If in the case that I have a "sudden and catastrophic" frame collapse and survive, then at least that may give my wife the impetus needed to allow me to purchase a new frame.
+1 to the videotape idea. Be sure to videotape over some rough terrain (potholes, curbs, etc) so we can visually evaluate its performance.
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Old 07-23-08, 08:18 PM   #12
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Is "Funniest Home Videos" still accepting submissions? This might be a money maker for the OP.
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Old 07-23-08, 09:58 PM   #13
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Old 07-24-08, 08:44 AM   #14
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videotape or it didn't happen.
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