Seat post keeps sliding down while riding
 

I replaced the seat post on my 84 Schwinn Voyageur SP recently, and now it frequently slides down while riding. I've made sure the bolt is tight, but it doesn't seem to ever hold completely. The seat post is 26.6, which should be correct for the year and model. I'm wondering if the bolt is a replacement, and isn't able to tighten properly due to being too long, or if the Voyageur's cable hanger could be the culprit. Has anyone else with a Voyageur (or similar cable hanger) had this problem before?

I've attached some cell phone shots of the seat post bolt — boosted the shadows a bit to try showing it more clearly.

https://cimg8.ibsrv.net/gimg/bikefor...cb70e04c4e.jpg
https://cimg0.ibsrv.net/gimg/bikefor...2fbe00ff74.jpg
https://cimg1.ibsrv.net/gimg/bikefor...cd93339afc.jpg

Simple experiment is to remove the brake hanger/stop and then ride the bike and see what happens. Or sub a different bolt that isn't an Allen wrench one. In the as imaged condition can you pivot the cable hanger about (with the cable tension released)? If not then the hanger's thickness is possibly the reason the lug won't clamp the post fully. Andy

Looks like the cable hanger. If you cannot loose the hanger, then get a 26.7/26.8 seat post. Adapter sleeves may work.
Also make sure that everything is squeaky clean, use alcohol to make sure there are no oils and such

It's late enough I'm not going to ride again tonight, but tomorrow I'll try tightening with the hanger removed to see what happens. The hanger can't be moved so that may be the problem, but it's original to the bike so I feel like it shouldn't be the cause, right? I'll try a different bolt as well.

I hadn't thought of an adapter sleeve, thanks! I guess it's worth noting, this didn't happen until the seat post changed. Maybe this new one isn't as truly 26.6 as the last (if the last even was — no markings of any kind). I'll also be sure to give everything a good cleaning before doing anything else.

EDIT: Measured the seat post with a caliper and got anywhere from 26.492 to 26.516. Are seat posts typically smaller than marked? Fully tightened (hanger in-place), the seat post opening measured 26.67 with the calipers.

The old seat post probably is marked, but with 30+ year old seat posts, I often have to use both a bright LED task light and a magnifier to find it.

Are the bolt/nut bottoming out?

Is the cable hanger original?

Also, if you have it, there's nothing wrong with trying some carbon assembly paste in there.

There you go, the opening is larger than the post, so it will not clamp tightly. I would suggest moving the cable hanger to one side as suggested and see if you get different results.

Measuring a seat tube with calipers will give you inconsistent measurements. The insert type of gauge is best. I'd say that from your measurements, the 26.6 spec is probably accurate. It's clear that the cable hanger is interfering with the clamping, and this is easy to check. To fix, you can move it to the side as others have recommended, mount a different hanger (like the one in the picture below).

I would recommend first to file the hanger a bit thinner at the clamp end. Half a mm make make all the difference.


https://cimg8.ibsrv.net/gimg/bikefor...89420ec994.png

Yes, posts are often slightly undersized. I'll also add that measuring the seat tube ID is not going to get you a specific dimension. There's too much distortion from heat unevenly applied during construction and then the binder slot further changes both the "roundness" and the spread of that upper portion of the ID.

Here's a link to a seat post size thread from a few years ago. I decided to actually measure the various posts I had on hand with a nice micrometer. Scroll down to post 8 https://www.bikeforums.net/bicycle-m...post-size.html Andy

Then check oy post 12. This last one is the key IMO. Andy

This solution will address the root cause of the problem (and provide a handy quick-release).

Aluminum soda can. Cut one up and use it as a shim.

And carbon assembly paste

Carbon Fiber grip FTW...

I'm on the side of the >rear brake hanger is interfering with the seat post clamp squeezing the post sufficiently..< team.

& it looks like the post is chromed steel too..

I'm learning quite a bit from these responses, thank you! I have a background in woodworking so I'm familiar with listed dimensions not being a true representation. I wasn't sure if that also applied here — now I've got my answer. I tried moving the hanger to the side but still had slippage. It actually seemed less secure. I'm gonna see if the bike shop has some carbon grip since so many people have suggested it.

What's the significance of it being chromed steel? More likely to slide???

...I don't know about the newest ones, but over the years they vary as much as .1 mm from the diameter marked on the post. It's best practice when replacing a post to measure the old one with calipers, and try to replace it with a new one you measure with calipers to make certain they are close. The best measure of a post to seat tube fit is sliding it in and feeling it make contact with the seat tube as it slides down, which is often not possible when you are buying a new one off the internet. You want to feel just the slightest bit of resistance as it slides down into the greased tube, ideally.

So I’ll give you some bad advice. Take a thin file and remove a little thickness off the ears and see if it helps. If we are really talking about .2mm (.008”) I doubt the bike will fall apart. Mine didn’t... at least not yet.

John

It's a harder surface.

But I put one in a Schwinn Merada and it did not slip

So with the brake hanger out of the way, you were still unable to get the ears to come together enough to grip the post tightly enough? Did the ears come all the way together with the brake hanger no longer between them? If not, then you need to check (still no answer) if the bolt and nut are bottoming out.

I definitely should have checked the old seat post more carefully. It was pretty trashed when I bought the bike and wouldn't maintain the seat angle. I'd start riding with a nice level saddle, and 20 minutes later it'd be pointed towards the sky, applying lots of unwanted pressure. There were no markings to be found. The 26.6 was obscured enough that I checked the old Schwinn catalogs to make sure it was 26.6 and not 26.8. I tossed it as soon as the new one arrived, trusting the numbers a little too much it appears. Live and learn!

I ran out (in some stupid hot heat) and grabbed a tube of Park Tool Supergrip Carbon and Alloy Assembly Compound yada-yada — what a name! Applied it to the seat post, installed it (noticed more resistance even at this stage), and gave it a weight test in the apartment. The seat post seems to be unmoved. It's still stupidly hot, so I'll wait until this evening to give a test ride, but I'm feeling cautiously optimistic right now.

At this point, I don't believe the bolt is bottoming out. The ears definitely came together more tightly but the post still slid pretty easily with pressure applied. The carbon grip seems to have made a difference. I'll find out later tonight.

Posted to Rooney just now in the C&V forum that my 1984 has a 27.2mm SR Foursir seatpost, so the catalog and PDF are incorrect for the SP at least.

https://cimg4.ibsrv.net/gimg/bikefor...64468bb98e.jpg

^^^^^ Wow, HUGE difference. That means that with a 26.2 post, the seat lug ears are being squeezed too close together. OP should definitely go for a larger post. Fortunately, 27.2 is a very common size and easily found.

Double-checked this morning and it is 27.2. I don't know much about different tubesets, but perhaps 26.6 is correct for the Tenax tubeset used on the Voyageur, but incorrect for the SP/SLX tubset on the Voyageur SP. Both the 1984 and 1985 catalogs show 26.6 for both variants..

https://cimg6.ibsrv.net/gimg/bikefor...2f1ee215b3.jpg

I went for a ride today without any problems. The carbon grip seems to have solved the issue for now. I'll continue keeping an eye on things. I am still interested to hear what either of the other 84/85 SP owners in the Voyageur thread have to say about their seatpost diameter.

UPDATE:

I posted this in the Voyageur thread, but I'll post it here as well in case anyone comes across this later. This thread has three more 84/85 VSP owners, in addition to Schreck, saying their seatposts are marked 27.2. So, it would seem I do have the wrong size on my bike currently. The carbon grip has been holding, but I think I'll be looking for a 27.2 seatpost now.