Clamping a Dropper Post

[Mountain Bru]

Having stripped and serviced a number of posts I can confidently say that it is more than a just a rubber seal supporting the pull force on the post. 

These posts are built very well. 

 

I'm not a mechanical engineer though and my decision to clamp the way I do is based on extensive research and my own experience with using and servicing the posts. 

 

I've never stripped a dropper, so I have no real idea whats going on inside there, but if the post is mid travel, am I right in assuming that the only thing keeping the post from extending is the seal between the hydraulic chamber and the air? What else keeps it from extending?

 

This is where my statement comes from that the way the forces are supported is completely different. In compression, the force is supported by the hydraulic fluid being compressed into a chamber. In tension, to my knowledge, only the seals that keep air from entering that chamber keep the post from extending? I have no idea though, so please help me if I'm wrong

 

 

what 12kg pull? I would not advocate that the dropper be clamped in any way other than the post fully extended. 

 

I guess the "12kg pull" is the weight of the bike trying to extend the dropper post. This could probably be more if you're working on the bike. 

 

Clamping it fully extended makes sense to me though. Is there typically a mechanical stop at the end of the travel that would then be taking the tension forces?

[Alouette3]

From the Rockshox Reverb user manual

[Meezo]

From the Rockshox Reverb user manual

for the Reverb I'm not suprised!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

...yes i am trolling

[TNT1]

Wouldn't it be simpler to just buy a used seatpost, and pop that in when working on/washing the bike?

[DieselnDust]

From the Rockshox Reverb user manual

 

 

the Reverb has hydraulics all the way to the top of the post. The stanchion is a thin walled cylinder so clamping it can deform it.

 

Posts with a cartridge have a thicker walled stanchion that's more resistant to deforming

[BMXER]

How wide is the clamping area on your modified jaws? I though of making an insert for my jaws that only clamps between the non-stanchion are, but I only have about 3cm of room there.

I have 11cm between the seatpost clamp and the dropper point in my seatpost. So I cut the workstand clamps to 10cm. Fitted silicone tape and done. I did this as I also have 3 kids with low seat heights so needed a smaller clamp regardless.

I run the RSHOX axs dropper, at that price i was not going to risk my dropper in jaws...

Edited February 24, 2021 by BMXER

[Steady Spin]

I've never stripped a dropper, so I have no real idea whats going on inside there, but if the post is mid travel, am I right in assuming that the only thing keeping the post from extending is the seal between the hydraulic chamber and the air? What else keeps it from extending?

 

This is where my statement comes from that the way the forces are supported is completely different. In compression, the force is supported by the hydraulic fluid being compressed into a chamber. In tension, to my knowledge, only the seals that keep air from entering that chamber keep the post from extending? I have no idea though, so please help me if I'm wrong

 

 

 

I guess the "12kg pull" is the weight of the bike trying to extend the dropper post. This could probably be more if you're working on the bike. 

 

Clamping it fully extended makes sense to me though. Is there typically a mechanical stop at the end of the travel that would then be taking the tension forces?

 

Edited my post. I don't always explain my thinking all that well.

Edited February 24, 2021 by Steady Spin

[MTBeer]

Not sure if you're trolling.....

 

The technical answer though is that the carbon layup design used in the tubular bike sections and components (like handlebars and seat posts) isn't made to resist much stress in "clamping". So they're super strong in bending and tension/compression, but you can easily crack a carbon seatpost just by over tightening the clamp bolt with a normal allen key. Clamping the top tube is the same. You can ride off huge drops and the frame will be fine cos it's designed for those forces, but then slightly overtightening your work stand clamp can crack the frame.  

no I wasn't trolling. I always clamp mine on the top tube - I've just never felt the need to clamp it very tight so I guess that's saved me.

[The Ouzo]

no I wasn't trolling. I always clamp mine on the top tube - I've just never felt the need to clamp it very tight so I guess that's saved me.

I dont think its just the clamping force but also the torsional force when you're torqueing up a particularly tight not/bolt.

[BMXER]

no I wasn't trolling. I always clamp mine on the top tube - I've just never felt the need to clamp it very tight so I guess that's saved me.

Top tube on my giant is the only option with a isp. Just make sure it's no too tight and no major force through the frame

[Mountain Bru]

Wouldn't it be simpler to just buy a used seatpost, and pop that in when working on/washing the bike?

 

I guess that's an option, but it's a bit of mission to have to to fiddle with cables, especially internally routed ones, and then also have to get the saddle back in the right position afterwards. I kind of just assumed that this must be a common problem, so hopefully there's a simple solution

[droo]

Pull on a compressed dropper only if you want to replace the cartridge or rebuild the post.

 

The vacuum will cause the oil to aerate, which causes the squishy feeling at the top of the stroke.

 

When it's fully extended there's a mechanical stop.

[Mountain Bru]

no I wasn't trolling. I always clamp mine on the top tube - I've just never felt the need to clamp it very tight so I guess that's saved me.

 

On alu frames, thats not a problem. But on carbon, replacing a seat post is much cheaper than replacing the frame, so most people rather take the risk by clamping the post. Buying a torque wrench is also much cheaper than replacing something that's cracked, or falling because something wasn't tight enough or broke due to the bolts being overtightened and damaging the part they attach.  

[Mountain Bru]

Top tube on my giant is the only option with a isp. Just make sure it's no too tight and no major force through the frame

 

I guess this kind of stand could be an option if you have an isp, but that's only until you take your bike to a shop. I've never seen these at a bike shop, only the clamping type

 

[Mountain Bru]

Pull on a compressed dropper only if you want to replace the cartridge or rebuild the post.

 

The vacuum will cause the oil to aerate, which causes the squishy feeling at the top of the stroke.

 

When it's fully extended there's a mechanical stop.

 

Thanks droo. Word on the street is you're the suspension expert around here, so I guess that settles it

[BMXER]

I guess this kind of stand could be an option if you have an isp, but that's only until you take your bike to a shop. I've never seen these at a bike shop, only the clamping type

 

It would be but taking my front wheel of amounts to hours of disc brake rub. 3rd world problems.