My Apollo folding bike has an annoying issue with the seatpost/seat tube interface. The seatpost fits nicely into the top of the seat tube, but when it gets below a certain point, it becomes extremely stiff and has to be forced down. This wouldn't normally be an issue, but the folding mechanism for the bike involves dropping the seatpost, and it's taking a lot longer to fold the bike than it otherwise would. I've tried flipping the bike over and inserting the seatpost from the bottom of the seat tube, and it starts out stiff when I do that, so I think it's the frame not the seatpost that's the issue.

The stiffness seems to begin when the bottom of the seatpost is level with the area where the main tube of the frame is welded to the seat tube, so I suspect it's got something to do with distortion during welding. I know it's a cheap bike, so this isn't a massive shock, but what can I do to fix this? The frame is aluminium, is there a way to ream out the seat tube?

Thanks in advance, and sorry for the huge post. I seem to say that a lot on here....

you may find a machine shop tool , an adjustable reamer, at a machine shop.
and bring the ID of the tube in the welding distorted area out
to what it is in the rest of the seat tube .

The tool cost more than your bike, so just figure the shop charge
to have the service done, a bargain

it sounds like there's some distortion in the frame's seat tube at the joint. There are two possibilities and the signs will be slightly different for both.

If it's only local distortion, which is fairly common at brazed or welded joints, then the post will bind as it comes down to the distorted area, but the binding will be fairly constant as you work the post into the area.

If the tube is slightly bent at the joint, which also happens, then the post will start to bind, and the binding will get steadily worse as you push the post deeper.

The former is easiest to correct, and most bike shops have reamers that can handle this, though the may not have handles, or extensions able to reach deep enough. If the binding is fairly slight you may be able to improve it yourself by using medium grit lapping compound, and working the post to lap out the binding area. Lapping compound comes in pint cans but if you have a good relationship with an auto mechanic or engine rebuilder you might be able to "borrow" a teaspoon, though it might pay to have a beer in hand when you ask.

It definitely doesn't get much worse as the post goes down

How deep can they normally go? The tube is open at the bottom, and the suspect area is about 6 inches in from there. Is that likely to be too far for a bike shop reamer?

Airburst, I wonder if this could help.
https://www.amazon.com/Brake-Cylinder.../dp/B0030F15FQ

Brad

Six inches is within the range regular seatposts fit into frames, and the typical reamer should reach it fine. Look for a smaller bike shop that seems more driven by service work vs. sales and it should be a fairly straightforward job, not costing a lot.

Cylinder hones can work, but they can easily remove more material than needed, from places where it ought not to have been removed.

An adjustable reamer will only remove material from the distorted area of the tube and leave the rest alone.