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I built an 'Urban Mountain Bike' and I sometimes like to take it off drops. Accordingly, I cut the 'nub' that sticks up above the top tube so that the seat can be removed without a scary open pipe waiting for my sensitive parts.

Seat post clamp underneath top tube

The problem I'm having is in relocating the seat post clamp beneath the top tube. I have cut a vertical I shape under the top tube in the steel frame so that it can collapse into the post. If I remember correctly, it's a 28.6mm OD seatpost with a 31.6mm or so OD (a bit more now with a coat of Herculiner).

Right now I'm using 2x Problem Solvers cable clamps, but I cannot get them tight enough to stop the seatpost from dropping a bit, and if I go any tighter I'm in danger of stripping them out. I'd also prefer a quick release to be able to take it out without an allen wrench.

Does anyone make Quick Release clamps that split to get around my top tube?

The best I can find so far is this, but I have to lose any quick release features:

2 piece split clamp

Does something like this exist already, and I'm reinventing the wheel? Or is there some way I could use two quick release levers on the split clamp?

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The bottom picture is of a shaft collar - it is mounted on a machined shaft to hold components on/limit the stroke of a shaft. It is not designed to compress a seat tube around the post, only to be secured to a shaft. It is worth a try, though. (My go to source for machine bits - mcmaster.com/#shaft-collars/=kh9m2w) –  WTHarper Dec 6 '12 at 22:31
    
Your problem with tightening is likely more related to needing to compress the seat tube and the top tube, which is welded in a position to brace the seat tube, and thus is preventing much compression, I think. –  zenbike Jun 21 '13 at 10:21

3 Answers 3

up vote 5 down vote accepted

DMR make hinged clamps for mounting chain devices and bottle cages. 3 different sizes and there is a 31.8mm which should be perfect. Easy job to retro-fit an existing QR lever and bolt I reckon?

Part Number: DMR-CLP-286

Part Number: DMR-CLP-286

Alternatively here is something from a different application that would probably work. You might be able to contact the company for a 'sample' :)

URL: Staff-Lok Collar

Staff-Lok Collar
(it's steel)

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1  
I have 2 of the equivalent of the DMR style pieces on there, and I just don't think they're strong enough. The Staff-Lok Collar would be, though - I think I'll try one of those if there isn't a bicycle-purposed one. Thanks! –  Ehryk Dec 7 '12 at 0:39
    
Yeah, that first unit isn't really designed as a clamp, just a mounting bracket. Given the setup, you need A LOT of force to be effective. That second unit MIGHT work, but it looks like you might have to order a thousand of them. –  Daniel R Hicks Dec 7 '12 at 0:54

Since you already modified the frame a lot, I would go further and modify it again by welding two "ears" where you could attach a real quick release, something like the picture shown below:

enter image description here

Another possibility would be to use a (cheap?) steel clamp which you could bend open, then wrap around the frame and put the quick release through. Steel will probably not be damaged from the bending, like aluminum would.

enter image description here

Hope this helps!

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Good idea! As it only needs to be bent once the old style steel clamp would work fine I reckon! –  Mere Development Dec 7 '12 at 14:51
    
The problem might be to find a suitable size clamp, since these ones are usually smaller diameter. But industry uses these, too, then a good hardware store could have some. –  heltonbiker Dec 7 '12 at 15:35
    
Are there any quick release levers that are set up to use flat flanges? Usually the clamp is radiused to fit the lever. Is there a similar aluminum quick release clamp I could buy just for the quick release? –  Ehryk Dec 7 '12 at 16:43
    
Great suggestion! Since they are made of steel (a rether soft one) they easily accomodate a wide range of tube diameters, also they can be modifíed to further adapt to even other diameters whith a file and a little effort. –  Jahaziel 17 hours ago

Sounds like a perfect use for a "wedge" seat post. Animal bikes has one, other companies might make longer ones. You'd need to get a Pivotal seat to go with it.

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Would I then need to remove the seat to remove the seat post, and need allen wrenches with me? My goal here is to make it quickly removable on the fly without tools. –  Ehryk Dec 7 '12 at 17:06
    
yea, you'd need an allen wrench. I'm not sure how you could do a quick release in the middle of the seat tube without comprising the integrity of the frame... –  dotjoe Dec 7 '12 at 17:08

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