Turtle Speed

Got some orange mountain bike second hand, and it doesn't seem to have any hardware for securing the seat post in place. What type of type post clamp or hardware do I need to replace? It looks like this:





Can I just pick any type of clamp as long as the bolt is long enough? Would any random-ass bolt and nut from the hardware store work as long as the size is OK?


...


The other thing about this bike is that the seat post is almost stuck, despite not having a clamp. I can just barely twist the seat, but it won't easily slide up and down. I'm pretty sure I could eventually twist it out, but if I want to make the bike nicer and more functional, should I replace the post? I don't know if this corrosion near the bottom of the post is part of the problem:



I have no problem dropping $5-10 on a new post if it means the user can actually slide it around properly. Is it easy to replace a seat's post? Sounds dumb, but I've never seen Park or Sheldon describe how to do it. My digital caliper reads about 25.5m.

Turtle Speed

Another stupid question, when looking at the top of the seat tube where those flats for the bolt/skewer would go, are they supposed to be "pinched in" like that (see 1st picture)? I almost wonder if the bolt fell out, or was taken out, and someone took a pair of pliers and went HRRRGGHH and pinched those together in order to keep the post from moving (easily).

Uh... or is it supposed to look like that?

Andrew R Stewart 

TS- You should first get the seat post out and figure out why the clamp ears are so bent/distorted in toward each other. Is it because the post is undersized, the ST is not thick enough to resist bending when enough clamping pressure is applied or is the weld/braze attaching the clamp ears beginning to fail/crack? Until this is understood and gotten past i would not spend an other dime on the bike. The ears take a simple nut and bolt, likely an 8mm bolt diameter. But you could use a fancy QR bolt. The distorted ears will challenge a bolt and possible cause the bolt to bend. Good luck... Andy.

dabac

since you've got the "ears" on the seat tube, you want a bolt or a skewer as opposed to a collar.

And yes, it is possible to use "any" bolt of the correct diameter/length. But be aware that there are some advantages to the bike-specific stuff. Often there's a single spline below the head of the bolt, and a slot in one of the ears on the seat tube, making adjusting saddle height an one-tool job if you use bike parts. A "real" bike part may take Allen keys instead of wrenches, which reduces the risk of paint chipping from tools on the frame.

But as you suspect, and Andrew R points out - the ears are not supposed to lean like that.
I understand where you get the "pair of pliers" scenario, but it would be stupid well beyond the regular reach of stupidity. A lot of force goes through the post when a sitting rider hits even a smallish bump, so there's no way an uncorroded post would remain in place with simply a pinched tube.
Do a search and you'll see there's a few threads on this site about getting properly mounted posts to stop sliding.
A more probable reason is that someone either have used a too thin post, or simply over torqued the fastener.

Try measuring the outer diameter of the seat tube further down, the wall thickness of the tube and the seat post diameter. Do some simple math to try to determine if you've got the right post. 25.5 mm sounds like it's on the thin side for a MTB post.

That too is a common problem. Posts needs to be lubed(well, actually protected against corrosion, but grease does that fairly well) before insertion.

Not necessarily, unless you get really bad gouging when you pull it out. But you should pull it out, use whatever means available to scour the remaining corrosion from both post and tube off, and lube lightly before reinserting.
If you decide to replace, make sure you get the right post size for your frame, and be aware that posts are available in two versions - w/ saddle clamp and w/o saddle clamp(like your current one)

Really depends on the state of the parts. The theory is easy enough. Undo clamp(easy for you), pull post out(might be more challenging), measure diameter(...assuming it's the right kind of post to start with - which you probably shouldn't), decide on w/or w/o saddle clamp), measure length, buy new post, transfer saddle. Insert post. Adjust saddle tilt, aft forward position and height and you're done.

Unfortunately, about may not be good enough when it comes to posts, you often need to be spot on to find one that'll fit and stay put.

If someone had used a too narrow post, I'd expect the slit between the ears to narrow towards the top -which it doesn't seem to do. So probably simple over torque caused the issue. I'd suggest - apart from researching the proper post size - that you use a Q/R skewer with an external cam skewer. With the nut at the straightest ear, it should do well despite the misalignment.

If your hardware store is really good, you can use some spherical washers to deal with the misalignment instead.

TiBikeGuy

First, try to get the seatpost out. You may need to pry the two ears slightly apart. Once you get the seatpost out, sand of the rusted area and take a proper measurement with your calipers. Most of the cheap China imports use 25.4mm steel seatpost. You may want to replace with a slightly longer aluminium ones. That will give you a wider range of height adjustment.

Use a wire brush to remove the rust from inside the frame. Lightly coat the new seatpost with grease. While the post is out you can straighten the two ears with a spanner or screwdriver. You could use a nut & bolt, allen screw & bolt or a quick-release seat post binder to secure the new seatpost to the frame.

fietsbob

At the basics : a bolt that is long enough to go through those ears, and take a washer on both sides ,
and nut.
and small diameter enough to fit in the hole ..

need 2 wrenches , 1 to hold the bolt, 1 to turn the nut. ..[ QR post binders were used ]

Jed19

Obviously if he knew the answer, or extremely knowledgeable about bicycles, he won't ask the question. There is no excuse for humiliating people bike calling their questions/enquiries stupid. Please................

Tpcorr

Jed, Turtle Speed is the OP.

Turtle Speed

Thank you all so much. You guys are so helpful.

So I got the seat and seat post out! Bent the ears back open a little bit with a screwdriver and dripped some oil along the post, then was able to twist it out. The post isn't scratched or gouged.

I am 99% sure that the post fits the tube, and I strongly suspect it's the stock post. The saddle has the same branding as on the frame, so I'm sure they came together. I can't prove the post wasn't changed at some point, but a visual inspection hints that it's about the same age as the bike. Plus, I know people rarely ever change parts on these low-end bikes. And finally, after the oil kinda got spread around the inside of the seat tube, the post seems to telescope into the tube normally - snug, but not slipping all over the place. This makes me happy, cuz I'd like to reuse the post.

Next I'll clean up the rust on the post and inside the seat tube, regrease, and then see how things fit with a bolt. Will keep an eye on whether the ears want to smoosh inwards again under force. I didn't see any cracks forming at the joint, but I'll try to keep tabs on that.

I am so excited to work on this bike. I like to overhaul all the bearings, true wheels, etc. That will be no problem, it's just that I have these little pockets of ignorance on stuff like this which I've never encountered before. You guys are the best. The rider will be so happy.

Turtle Speed

Random other question on another bike.
(Sorry for piggybacking, but I didn't want to clutter up the Bicycle Mechanics subforum with multiple topics.)

Broken-down, low-end 24" youth mountain bike.

Triple chainring, but it has no front derailer at all, just some rusty scarring on the seat tube from where the old one was removed.

The seat tube measures 31.8mm.
Biggest ring has 42 teeth.
The bike is set up for it to be a top-pull design.

I'm having problems finding a compatible cheapo derailer to buy. The Shimano Tourney TZ-30 doesn't list the right clamp size, and according to the shimano tech docs, the TZ-31 is made for a 48-tooth ring.
Do I need something like a TX-50? Any other cheapo derailers that will fit & work?

Jed19

Oops! Mea culpa.