Mountain Bike Reviews Forum banner
1 - 20 of 33 Posts

·
Registered
Joined
·
266 Posts
Discussion Starter · #1 ·
Do you think is possible to machine down a 30.9 mm adjustable seatpost down to 30 mm?

I just saw a 30.9 mm Joplin adjustable seatpost in ebay, and wondered if it would be possible to machine it down to 30 mm, so they will fit mi old Kona Coiler.

How thick the seatpost walls are? I think removing 0.45mm all around wouldn't compromise any strenght...

Give me a reason not to try it :)
 

·
Swedetarded
Joined
·
527 Posts
Removal of OD material and reducing wall thicnkess = less strength.

Less material to absorb stresses means the materials yield strength will be achieved will less stress. It's a law of physics you cannot avoid.

Good luck.
 

·
Registered
Joined
·
291 Posts
How thick the seatpost walls are? I think removing 0.45mm all around wouldn't compromise any strenght...

Give me a reason not to try it :)
Uh this sounds like a painful plan. Bike parts are already pushing the edge of being just strong enough in efforts to save weight taking away half the material will effectively give you a useless pile of aluminum sitting trailside
 

·
Registered
Joined
·
266 Posts
Discussion Starter · #10 ·
I live in Mexico, here a machinist would charge about 20 bucks to do it, The thing is, I don't know how thick the seatpost walls are, but I think the internals in the 30.9 and 31.6 seatposts are the same. The point is, maybe only the outer tube of the 30.9 has an already reduced diameter to fit this measure, It wouldn't hurt to remove a little more (0.45 m is like nothing) and see what happens, haha.

Or, I can buy the seatpost, measure the outer tube OD and ID, and see if it's feasible. If not, I can sell it
 

·
Deadhead
Joined
·
367 Posts
The Joplins are definitely a shaky product, but if you have to have it; you should consider taking the material out of the seat tube on the bike instead. The wall thickness on the Kona seat tubes is pretty thick and I know of a few people that have done this mod with no ill effects.
 

·
Administrator
Joined
·
11,881 Posts
I live in Mexico, here a machinist would charge about 20 bucks to do it, The thing is, I don't know how thick the seatpost walls are, but I think the internals in the 30.9 and 31.6 seatposts are the same. The point is, maybe only the outer tube of the 30.9 has an already reduced diameter to fit this measure, It wouldn't hurt to remove a little more (0.45 m is like nothing) and see what happens, haha.

Or, I can buy the seatpost, measure the outer tube OD and ID, and see if it's feasible. If not, I can sell it
I say go with the seattube on your frame. Take it to a bike shop and they might be able to ream it out with a minimal amount of fuss, or possibly your machinist can do it as well. Be wary of overheating the frame and consider on many bikes there is only a couple of inches at the top of the seat tube that is the size for the seatpost and below it is larger ID.
 

·
Read Only
Joined
·
950 Posts
The walls on any adjustable seatpost are thin enough that you can bind up the post by tightening the seat collar even a little too much. Not only would that issue be amplified by machining the post down, I bet it would be susceptible to getting crushed and deformed when you attempt to tighten down the seat collar. I think your chance of success is less than 1%.
 

·
Registered
Joined
·
266 Posts
Discussion Starter · #16 ·
The walls on any adjustable seatpost are thin enough that you can bind up the post by tightening the seat collar even a little too much. Not only would that issue be amplified by machining the post down, I bet it would be susceptible to getting crushed and deformed when you attempt to tighten down the seat collar. I think your chance of success is less than 1%.
That's what I wanted to read. I thought the outer tube walls were thick enough, given all the hydraulic stuff packed inside.

After all, it was a bad idea, thanks everyone for your input.

I hate Kona for using such an odd seatpost tube haha.
 

·
Registered
Joined
·
124 Posts
The Joplins are definitely a shaky product, but if you have to have it; you should consider taking the material out of the seat tube on the bike instead. The wall thickness on the Kona seat tubes is pretty thick and I know of a few people that have done this mod with no ill effects.
Ask the machinist what it would cost to make you a custom seat post.

6061 is the aluminum you want

heat treating it will be good too.
 

·
Machinist
Joined
·
171 Posts
Ask the machinist what it would cost to make you a custom seat post.

6061 is the aluminum you want

heat treating it will be good too.
Anytime I am asked to make one custom part I let people know that if there is a viable production part/product out there it would be cheaper for them to purchase it than to pay me for my time to produce just one.
 

·
Registered
Joined
·
29 Posts
maybe

I will play devils advocate to everyone else.
Turning it down .9mm is only taking off .45 mm per side, how thick is the tubing? I dont think its going to be half the thickness of the tube. if it is that would make it 2mm thick...
I think you could scratch a tube more than .45 mm scraping on a rock and make that the weak spot in the tube. Would you toss the seat post in that situation?

Maybe you should ask what you weigh, how hard you ride and how sensitive the seat post structure would be in that situation. What are common seat post tubing thicknesses anyway?
 

·
Custom User Title
Joined
·
7,430 Posts
I will play devils advocate to everyone else.
Turning it down .9mm is only taking off .45 mm per side, how thick is the tubing? I dont think its going to be half the thickness of the tube. if it is that would make it 2mm thick...
I think you could scratch a tube more than .45 mm scraping on a rock and make that the weak spot in the tube. Would you toss the seat post in that situation?
.45mm is a lot. It could very well be half the thickness.

Yes, I would toss a seatpost with a half mm gash on it - though I have never got anything even close to that.
 
1 - 20 of 33 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top