My seat post on my new bike bent recently and in the process of finding a replacement I thought it might be a good idea to reinforce the new tube with a smaller diameter tube (preferably steel but al would be fine).
The original seat tube has an outside diameter of 27.2 mm and should be about 350 mm from the top of the frame to the seat rails and has to hold my 260 lbs plus a backback. I'm thinking about getting a Thudbuster since they're rated up to 250 lbs, has one that's long enough and then reinforce the inside with another tube. Or get the replacement tube (sent out for a warranty replacement) and reinforce the inside of that. Then either make a small weld at the bottom or if it's snug enough it shouldn't come out.
Since the smaller tube should fit snuggly inside the original one ideally I'd like to go someplace that has a selection (or that has the capability of measuring precisely and ordering a tube) to get the best possible fit. Weight is irrelevant to me - I just want to make sure this doesn't happen again!!
I started another thread looking at Clyde friendly seat posts here (http://www.bikeforums.net/showthread.php?t=238801) - but unless I can find a reinforced seat post I'd really like to find a post that would fit inside or another solution.
If you have any other ideas along this line please let me know too, thanks!
Peterpan1
10-27-06, 12:24 AM
What you are saying is the seat post bent, and you want to reinforce it? I would just get a cork from home depot or your home. force it up the post to a height of about one inch above your clamp. The pour in 2 inches of epoxu. Epoxy is heavy so you can add glass bubbles (microbaloons), and/or add a wooden dowel up the middle. you could also reduce the cork leaving a full size gasket, so that when you jam it up there you get epoixy and them cork, that will worl as long as it's place right. You need hard cork though, not this junk from wine bottles, probably still work.
Another technique that works well is to cut a couple of bulkheads out of plywood, 1/4" is fine, and just epoxy glue them up in the general region, these make it much harder for the tube to deform. If you look inside larger bird flight feathers, like the ones they make pens out of. the tube is like your fingernail, inside the tube is surprisingly minimal series of connected bubble shapes, that pull right out. These stop the wingspars from collapsing. it doesn't take much.
You can go to a boatbuilding store and get structural grade foam shape it and insert it, some of this stuff is VERY hard to compress and weighs very little, sands easily, they make aircraft out of it. Aircraft spruce even sells pourable versions, though I'm not sure they come in small volume cans, maybe some samples. you would need the 6 pound/cubic foot stuff or heavier. Your piece will weigh about 1/1000 of 6 pounds.
Air
10-27-06, 08:03 AM
Those are great ideas! The first one bent (was clamped about 3 inches above the min mark and it bent about 4 inches above that - so I think the entire length from the clamp to the seat should be reinforced), I sent it away for warranty replacement since I've barely had the bike a month and it should be covered. So when the new post comes then I'd like to reinforce it to make sure that won't happen again.
Which of these ideas do you think might be the strongest?
I love the idea of the structural grade foam - will that really stop the tube from bending? They have two types, Last-A-Foam (http://www.aircraftspruce.com/catalog/cmpages/lastafoam.php) and Divinycell Foam PVC (http://www.aircraftspruce.com/catalog/cmpages/divinycellfoam.php) that have ratings above 6 pounds/cubic foot. I like the fact that the Last-A-Foam says it's impervious to water and solvents, sounds like that may be the better bet. They also have 1/4" with 18 cu lb/ft. Seems like if I cut that into strips (like the plywood) and put it in that would be strong and light.
What exactly does the 18 rating mean? Is that just the weight of 1 cubic foot of material or is the the amount of force required to deflect or reshape?
Here's the link for the glass bubbles (http://www.aircraftspruce.com/catalog/cmpages/bubbles.php). Do you know how much volume a 1 pound bottle will take up by any chance? By my calculations I'd need about 17.7 cubic inches of filler, right (less inside diameter)? (Pi*r^2*h = Pi*(1.07/2)^2 * 19.7).
I'm also going to rename the thread [hmmm, didn't work. Oh well!]
Thanks!!
Air
10-27-06, 08:11 AM
By the way, I found these guys (http://www.davidsonpipe.com/gs_index.asp) in Brooklyn who specialize in many variety of pipes. When I get the replacement seat post I may take a run over there and report back.
Air
11-14-06, 10:56 PM
*update*
I got my replacement seatpost back. I went to a pipe place in Brooklyn (41st on the water - really cool over there with the cobblestones and tracks in the street!) but they didn't have anything that would fit. Great people though!
A bike shop sold me a steel older seat post that fit inside the al post and recommended filling the inside with epoxy to lock it in place. He said that people would fix frames that way.
Thoughts?
Air
11-17-06, 12:30 AM
In case someone finds this I posted my solution over in mechanics (http://bikeforums.net/showthread.php?t=245382). Thanks!
hotbike
01-08-07, 01:47 PM
Hi,
I weigh in at 260 lbs, and I could not find a laid-back seatpost that would support my weight without bending.
I resorted to fiberglass, to build my own seat and seatpost from scratch:
http://i134.photobucket.com/albums/q119/hotbike/Type9withkickstand.jpg
http://i134.photobucket.com/albums/q119/hotbike/Type9inthepark.jpg
The bike pictured weighs 55 pounds and has 24x2.125 tires. I sold it (at a $900 profit) and built a BMX version which weighs 40 pounds, and has a more "banana" style seat.
The fiberglass is 1/4" thick on the old , ten layers of 'glass,
3/16" thick on the new, or 7 layers of 'glass
If you want to try the DIY method like I did, make sure you put 'glass reinforcing on the inside between the frame tubes and the foam core. Thus the 'glass is twice as thick as I stated within two inches of the existing frame tubes.