turning a forktube into a seatpost

[BrigmasterA]

Anyone got any bright ideas on how to make a steer tube on a fork heavy/sturdy enough to use as a seatpost or seatpost tube?
This is the situation I'm working with...

 

[SkidMark]

Think about what a fork puts up with, it may be strong enough. Steerer tubes are substantially thicker at the bottom (internally butted). I would hate to find out the hard way. Maybe you could sleeve over it with a 1-1/8" tube.

 

[BrigmasterA]

That's kinda how I felt about the abuse a fork takes. But they also have bearings to support them on the top and the bottom. I thought about the 1 1/8 pipe, I also thought about a 7/8 seatpost inside(fits perfectly) and cut holes in the bottom of the forktube in about 4 different spots at the bottom so you could see the seatpost and weld it to the fork and the steertube... Maybe that's hard to understand how I'm explaining it... Wish I knew how to photoshop stuff... Any way I do it, I'm lookin' at the post being no more than two or three inches long...

 

[rick74304]

If you're going to chop it to a total height of 2-3 inches, that will take away most of the torque that leverage would apply. If you haven't already, remove the bearing race from the neck and put a reinforcing weld where it used to be, or just weld it on permanently for even more strength.
Rick

 

[thatismytruck]

I'm with rick. leave the race, weld it. That would be the weakest area of the fork tube. It will act as a nice fillet transition and give it plenty of strength. Those tubes take a bunch of abuse I bet it's strong enough on it's own, but would prefer a bit of insurance.

ok. geek time.

assume rider weight = 250
angle of seat to ground = 70 degrees
assume length of tube = 6" (0.5ft)

constructing a force triangle

tan 20 = X/250
250tan 20= X
X = 90lbs

cos30 = Y/90
90cos 30 = Y
Y=78lbs

torque = 78(0.5ft) = 39 FT-LB

not too bad. But for insurance I would still do what I said above.

 

[ifitsfreeitsforme]

Skip....

 

[thatismytruck]

hey!

my Dad isn't a dork.

 

[BrigmasterA]

Thanks guys. I cut the tube down to about 2.5 inches. I was gonna do it about 3, but it looked like there was a bulge from someone over-tightening a stem in there. Turns out it was just some brazing... But when I cut it off, I discovered it has packed full of rust and dirt right to the point I cut it off at. :x and I can't seem to get it out. :x :x Plus I already cut the race off, so I'll just put a substantial weld around the tube where it meets the fork and I'll plan on using the fork tube as the seatpost. I may get some drilbits and try drilling the crap out if I decide I need a little bit taller stance, but we'll see. I'm gonna approach this like I've approached the rest of this project: Just do it and hope it works. If it doesn't, try something else. :mrgreen: Thanks again guys.

 

[Unchained]

I love that bike, and can't wait to see it finished. I think this is what I would do for the seatpost:

I'd leave the amount of fork tube you have left. I wouldn't cut any more, but I would make one small vertical cut on the fork tube so that it can be clamped with a standard seat post frame tube clamp. Then I would make a series of drill holes as necessary to make it so that I could slide a seat post right through the fork tube and through the sturdiest part of the fork- the crown. (In other words, I would make it so that the seat post would slide right through where the front fender is normally screwed to the forks, underneath the crown). Then you could put a standard seat post clamp over the fork tube (with your seat post in place), get it tight, and you should have no problems, I would think... So long as the seat was staying low to reduce the leverage. This would be simple and would look good? Best of luck with the build, that is a sweet looking bike.