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Old 06-10-09, 07:03 AM   #1
MarkS
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How to keep seat from going down?

Ok, my spouse's bike was pretty cheap. Fuji Crosstown. Had it for 3 years. Get's used 4 times a week. 8 miles each time.

The problem is that the seat keeps going down. It has a quickset style seat clamp, which I tighten down about as hard as my hand can tolerate, but 2 days later it needs to be done again. We've replaced the seat post hoping that that would work, but no magic.

Would a different seat clamp, like one that uses a allen wrench, work better? What can we do to keep the seat in place?

Thanks!
Mark
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Old 06-10-09, 07:13 AM   #2
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Is there an allen bolt on one side and the quick release on the other? If so, I would close the quick release then tighten the allen bolt.
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Old 06-10-09, 07:17 AM   #3
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Hmmm.

If it was my bike the first thing that I would do would be to look at the slot in the top of the seat tube. If the sides of the slot taper noticeably inward at the top I'd try to find a cheap seat post that's 0.2mm larger diameter. You'll probably have to work a bit to round out the top of the seat tube again so that the larger seatpost will fit.
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Old 06-10-09, 07:19 AM   #4
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Grease it first. Then look at other option for your clamping mechanism.
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Old 06-10-09, 08:39 AM   #5
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Used hairspray on my bike to keep the seatpost from slipping.
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Old 06-10-09, 09:01 AM   #6
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If we're talking metal frame and metal post, I'd have an LBS (if you don't have calipers) verify that your diameters match (correct size seatpost for the tube).

Personally, I've only had carbon-fiber seatposts slip. Rule out functional causes first ... including wrong size, and inadequate torque.

After that, you can use hairspray, or "carbon fiber assembly paste" to help limit the slippage ... but ... I'd make sure nothing simpler is going on, first.
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Old 06-10-09, 09:09 AM   #7
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You don't need special 'carbon assembly' anything grease. Moly lube from your local auto parts store or Park Polylube work great and are dirt cheap.

The grease increases the holding power of the seat tube/clamp. Never had it not work. I'm assuming this is the stock seatpost so I'm sure the diameter is right. If you aren't able to get enough force on the quick release, look at a Constrictor clamp. If your current clamp has a hex head on one side, get long handled allen keys.

It'll hold, promise.
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Old 06-10-09, 09:10 AM   #8
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Either tighten by the allen bolt at the other side of the quick release lever (and forget about using the quick release), or do as I did and replace the lever with just an allen bolt as you described.

If it won't hand tighten properly, then there might be a small bump inside the seat tube that prevents an even clamping force.
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Old 06-10-09, 09:18 AM   #9
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btw:

"How to keep seat from going down?"

In my experience ... marry her.

[sorry]
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Old 06-10-09, 09:54 AM   #10
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Quote:
Originally Posted by SaladShooter View Post
You don't need special 'carbon assembly' anything grease. Moly lube from your local auto parts store or Park Polylube work great and are dirt cheap.

The grease increases the holding power of the seat tube/clamp. Never had it not work. I'm assuming this is the stock seatpost so I'm sure the diameter is right. If you aren't able to get enough force on the quick release, look at a Constrictor clamp. If your current clamp has a hex head on one side, get long handled allen keys.

It'll hold, promise.
Do you mean on the seat post itself? Or just on the restraining bolt assembly? Putting grease on the post sounds like it should make the post slip down faster ... but maybe I'm not getting the physics.

Thanks!
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Old 06-10-09, 10:03 AM   #11
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This post helped me

Vitesse seat post slides down

BTW, the Ritchey Liquid Torque is also sold under this name and can be used also for metal on metal

http://www.cswestbikes.com/servlet/t...ssembly/Detail

Edit: somebody mentioned in a different post that a white chalk can be used instead of the Ritchey Liquid Torque to increase the friction. Never tried it though.

Kam
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Old 06-10-09, 10:10 AM   #12
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Yup, on the post itself. You just take a tiny bit of whatever grease you choose (the Park stuff is at least bike specific if that matters to you, almost any synthetic grease will do) and smear it very thinly over the area below the seat tube.

Grease IS slippery, yes, but it increases the resistance for seatpost to slide against the seat tube, and most importantly prevents the two from bonding to one another. The grease allows the clamp to hold tighter (and I'm going off memory from advice years ago, it's worked every time so I haven't questioned it in 20 years) because as it tightens down the seat tube can get a tighter grip without binding as it squeezes from metal to metal friction.

Example... I had issues with my road post sliding down a few months ago. Turns out I had forgotten to grease it during assembly. I greased it with moly grease. Holds like a champ now.
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Old 06-10-09, 10:54 AM   #13
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When I got my lightly used Fuji Touring, the previous owner had never been able to get the seatpost to hold tight with the stock Fuji clamp. Why? Fuji shipped the bike with the wrong diameter seatpost clamp!

$5 later my new, proper diameter clamp has solved all seatpost-related issues instantly.

Just like replacing the headset-mounted canti cable housing stop with a fork crown housing stop solved the braking problems the previous owner never could solve... And filing off the alu dustcaps of the front hub solved the rubbing problem they had...

Ah well, what I'm trying to say, is don't think just because Fuji made this bike the way it is, that they took the time to make sure they were using the proper components to build it up.
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Old 06-10-09, 11:35 AM   #14
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Quote:
Originally Posted by SaladShooter View Post
You don't need special 'carbon assembly' anything grease. Moly lube from your local auto parts store or Park Polylube work great and are dirt cheap.
The resins used to assemble carbon-fiber parts are generally very sensitive to solvents and detergents. Standard grease may cause long-term damage.
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Old 06-10-09, 11:48 AM   #15
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I had the same problem on a Fuji, I used a little sand paper on the stem, solved the problem.
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Old 06-10-09, 11:51 AM   #16
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The resins used to assemble carbon-fiber parts are generally very sensitive to solvents and detergents. Standard grease may cause long-term damage.
This is a metal to metal connection, not a carbon post.
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Old 06-10-09, 12:23 PM   #17
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This is a metal to metal connection, not a carbon post.
Yes, but if you DO have a carbon post, you need the carbon paste.
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Old 06-10-09, 01:44 PM   #18
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Yes, but if you DO have a carbon post, you need the carbon paste.
Yes, and if you have two cats and are making anti-gravity device, you need super glue.
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Old 06-10-09, 02:52 PM   #19
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Yes, and if you have two cats and are making anti-gravity device, you need super glue.
Zip-ties are just as effective IMO,and allow for disassembly and possible restoration at a later date.
If permanence is desired, a metal bracket may be fabricated and attached to the rear mounting hole after drilling and tapping (this may require the use of a vise), or a 10mm bolt may be forcefitted with care. This may also require the use of a vise.
Brazing or welding are not recommended due to the possibility of heat distortion.
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Old 06-10-09, 02:55 PM   #20
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If the grease, paste, and anti-gravity cats don't work, I'm with RG and you may need a post 0.2mm larger.
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Old 06-10-09, 03:08 PM   #21
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Don't overlook the simplest solutions either.

Grease the QR lever threads and the cam interface (if it has one). Much of your force will be resisted by friction at the lever and threads. Greasing eliminates the friction and increases the clamping torque.
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Old 06-11-09, 07:15 AM   #22
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btw:

"How to keep seat from going down?"

In my experience ... marry her.

[sorry]
Ha!!!
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Old 06-11-09, 04:28 PM   #23
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Originally Posted by Retro Grouch View Post
Hmmm.

If it was my bike the first thing that I would do would be to look at the slot in the top of the seat tube. If the sides of the slot taper noticeably inward at the top I'd try to find a cheap seat post that's 0.2mm larger diameter. You'll probably have to work a bit to round out the top of the seat tube again so that the larger seatpost will fit.
+1
A properly sized metal seatpost should fit slightly snug in the seattube (unclamped) and should not need any special concoction or super-tight clamping. It just sounds to me like the current post is slightly undersized. If you replaced the seatpost using the old one for size reference, then you didn't really change anything.
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Old 06-11-09, 04:39 PM   #24
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Yeah, what Retro said. Make sure the two ears at the top of the seat-tube where the post-clamp goes through doesn't touch. If they do, you can crank on the clamp as hard as you want, but it's not gonna clamp down on the post any harder. Someone may have inserted a smaller seatpost in there. Verify that the slot on the back of the seat-tube is parallel and the ears aren't touching.
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Old 06-11-09, 08:57 PM   #25
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Or you can go the super cheapo route(as long as the ears on the seattube aren't touching) and cut a 1" wide strip of metal from a beer can and use that as a shim in between the seatpost and seattube, then close your clamp. If the ears on the seattube are touching, take a dremel and cut a slot between them, allowing them to be compressed again, from the force of the seatclamp.
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