FlashBazbo

I'm a long-time BMC rider but I've come across a problem that apparently defies solution. If you've bought a BMC TeamMachine or RoadMachine over the past two or three years, you're familiar with the slick looking "no gap" (concealed) seatpost clamp. I had an early production 2018 BMC SLR01 and neither I nor my LBS could ever get the seatpost to stop slipping. We worked on it for months. We put assembly paste all the way around the seatpost (contrary to instructions, lest it freeze the post permanently), we over-torqued the clamp to nearly twice its value and eventually, after nine months of trying various unsuccessful cures, the clamp pulled through the frame. To be fair, we occasionally were able to get the post to stop slipping for a month or six weeks, but the slippage always returned. BMC warrantee'd the frame and my LBS set me up with a 2019 BMC SLR01 frameset as a replacement.


Now . . . I've put all of 300 miles on the 2019 frameset and . . you guessed it . . . the seatpost is slipping down. When my LBS assembled the new bike, he put assembly paste all the way around the post, but to no avail. I feel as if we've tried EVERYTHING to get the BMC seat clamp to work, but nothing has worked for very long. This is otherwise a GREAT bike, so here are my questions. Maybe somebody out there has found a fix that we don't know about . . .


If you have a BMC with the "no gap" concealed seat post clamp, did you also have the seatpost slippage problem?


If you did, were you able to find a fix? Were you able to retrofit an exposed seatpost clamp? Is the problem the clamp or is it the seatpost (or is it both)?

Kimmo 

Epoxy.

Campag4life

What is your weight OP?
A suggestion...use carbon paste of course...is you need to abraid the clear coat of the post were it sticks inside the frame.
Take #600 grit wet sandpaper, mark the post with painters tape for depth of penetration and likely take a couple thou off the carbon gel coat on the post.
Likewise do the same for the interior of the seat tube which typically is less critical because it doesn't have the same paint/clearcoat coverage.

That, plus carbon paste, torque to spec with a torque wrench and you should be good.

If you have bad build tolerance between post and seat tube ID with carbon I would be loathe to introduce a stress riser to mitigate slippage...a small shim because this can load the carbon creating high stress.
Can you post a pic of the seatpost...and/or tell us if it is proprietary...or generic 27.2mm round post for that particular model BMC?
If you can't resolve it with the above method and the bike uses a proprietary seatpost, I would sell the frame and ride something else.

FlashBazbo

I've thought of that. Epoxy would make it hard to service the Di2 battery, though.

FlashBazbo

I weigh 175 lbs. And the BMC seatpost is proprietary and it is textured from the factory. The cross section of the tube is "D" shaped, so there is no off-the-shelf substitute.

I guess my original post wasn't as clear as it should have been. Of course, we've done carbon paste -- even in areas BMC says not to use carbon paste (lest it seize the post permanently). And we've torqued to spec. We've even torqued to double spec. Didn't work for long. The inside of the tube and the outside of the post are textured already -- but that's not to say the texturing is sufficient. It apparently isn't. But I don't think sanding down the texture is going to help.

Thanks for the ideas.

Kimmo 

Put cling wrap into and over the seat tube, apply epoxy to the post so it forms a ledge against the top of the seat tube.

joesch

I had the same problem with a Cervello R3 carbon seat post and this solved my problem:

KCNC SC13 Twin Seat Post Clamp

KCNC's Twin Clamp is designed to solve the annoying slipping seat post! It does happen - and this clamp is designed to prevent posts slipping down inside the frame by having 2 clamps joined together - one clamps the seat tube as normal but the top clamp grips the seat post itself to provide an extra clamping force without over stressing the seat tube clamp or the seat tube and seat post itself!

https://www.sjscycles.co.uk/seat-pos...clamp/?geoc=US

FlashBazbo

That looks like a good solution for round seatposts. Unfortunately, the BMC seatpost is "D" shaped.

joejack951

Should be easy enough to make one to fit. Contact me offline if you are interested in pursuing something like that. If the problem is common enough I am more than willing to invest some time into finding a proper solution.

ttusomeone

I have the same bike (Teammachine SLR01) and had seatpost slippage after the first couple of rides. I talked to my LBS and they said to torque the bolt tighter. I also applied more carbon paste, but only on the areas BMC recommends. That plus the more torque (it was less than double the recommended spec) and I haven't had a problem. On all my bikes I apply a little paint to the seat post just above the collar to make it easy to visually check if there's been any slippage. FWIW, I weight 140 pounds so the fact that it slipped on me originally using the recommended torque specs shows there is a problem with the seatpost and/or collar design.

Campag4life

Just so you know, texturing isn't the same as high coef. of static or dynamic friction. Since you have tried all the correct things to keep the post from slipping, your next step would be to do as I suggest and abrade i.e. remove the surface finish over the texture to increase 'bite' of the post at the clamp area. If the textured area of the clamp has a common protective carbon gel (clear) cost it is inherently slippery in spite of texturing. I hope that makes sense.
If you want greater clarity, post close up pictures of the seatpost and interior of the seat tube with the post removed exposing the clamping area.

FlashBazbo

Did you get yours at MOAB? (Mark is the one working with me on mine.)

ttusomeone

Yes, I got it at Moab, but the one in Franklin. Jeremy is the one that gave me the go ahead to make it tighter.

Ogsarg

No firsthand knowledge of this issue but given that it has no gap, it would follow that it has less allowable travel. That might indicate that it requires a very tight tolerance on the seatpost diameter dimensions. Since you are now on your second frame perhaps the seatpost is undersized preventing you from getting proper clamping force.

Since it's a proprietary seatpost, you'd probably have to get dims from BMC along with allowable tolerances to verify its within spec. Hopefully your dealer can do this for you. If not, maybe try and swap seatposts with another bike at the shop just to see if you can get it tight.

79pmooney

Not a proper fix but - have you considered a clamp around the post just above the seattube so the post cannot slip down? I'm envisioning perhaps a wrap if innertube around the post that also covers the top of the seatpost, then a pipe clamp or seat clamp around the post. Not having your bike in hand and not being a CF guy, I cannot give you any details on how to do this but it seems to me that done right, it should work.

Ben

Wheever

I would actually go with something rougher, to really give the surface some tooth, and also on the face of the clamp itself, where it bears against the post. Doing this could vastly increase the friction when combined with a really rough carbon gel like Park's. (I have scratches in my clear coat on my bars from that Parks carbon paste/gel just from sliding it around during adjustment with the clamp loosened.)

ingo

I had a similar problem with the Cervelo S3 seatpost which is airfoil shaped. Being a mechanical engineer, the slippage bugged me a lot. The solution was two steps: clean, lube, and retap the threads on the clamp to make sure the proper torque is applied to the screw. The second is to apply assembly paste only where the grip is needed. The surface between the clamping wedge and seatpost must slide to work, even to the point where you could lubricate the wedge. So I applied the carbon paste on the rear/sides of the post, and none on the surface of the wedge or front of the post. That worked.

Dean V

If there is enough clearance I would stick a strip of sandpaper (say, 180 grit wet& dry) to the seat post with some contact glue.
Rough side out.
On the side of the post that gets forced against the frame with the clamp.
If it doesn't work you could remove it as contact glue isn't too strong, then try another solution.

Campag4life

I wouldn't. Likely isn't doable because of lack of clearance between post and seat tube ID. Wouldn't do it anyway.

Campag4life

But not where to start IMO. Never begin more invasive with greater abrasion to the seat post IMO. Gel coat is slippery. Cutting the surface with 600# in my experience has been all that's necessary to create tooth and bite. Removing the gloss coat surface fractionally is all that is necessary...don't have to remove more material with rougher sand paper.

Wheever

FlashBazbo

UPDATE: My LBS received additional information. The theory is that the problem is NOT with lack of abrasiveness, but with lack of lube! BMC's instructions say to use carbon paste on 3 sides of the post but to lube the clamping wedge and the wedge side of the post (counter-intuitive at least at first). The idea is that, if the wedge doesn't slide freely, it binds during the torqueing process. (It tightens at an upward angle to the post.) You torque it to 5 Nm but after a little riding and jarring and shaking, the wedge comes "unbound" and the bolt loses some of its torque. The instructions, then, are to follow the instructions on paste and lube, torque the clamp to value, then do your best to rip the post out of the bike -- crank the nose of the saddle both directions, flex the post back and forth, ride the bike and bounce on the saddle -- really man-handle it -- then check the torque value. If the bolt remains at 5 Nm, all is well. If it drops, you've got to retorque and repeat the process of abusing the seatpost all over again.

Torque value is 5 Nm. First time we went through the routine, the torque value had loosened to 3 Nm. We repeated. The torque dropped again. We repeated and repeated and repeated . . . 8 times. FINALLY, after all the abuse, the torque value stayed at 5 Nm after each of the last three rounds. From here forward, I plan to check the torque on that bolt on a weekly basis. Hopefully, this was the solution. Time will tell.

Cyclist0108

Different carbon paste brands have different granularities. Use the most course you can find, and if that isn't enough, put some sand in it, or micro glass beads.

arizonamed

Campag4life

FB, I know you are a pretty sharp guy and you are stuck with this bike...oh I am sure its a nice bike...BMC makes good road bikes. But the engineers that created that albatross should be taken behind the woodshed and horse whipped. What a bunch of a-holes. I am an engineer btw. When an engineer creates a design like this, they know 'all of the nuances' What you describe which btw, makes perfect sense should NEVER have been created. Grounds for selling the bike and never buying another BMC product. That is really f-ed up. Adjusting seat height, have the saddle stay in place, torque to a target torque of 5 Nm and it staying there should be intrinsic to the design. Whatta PITA. Just had to vent feeling your frustration. Designs get out there that are total BS and this is one of them.