Messing about with seatpost clamps

Manc33

I got a 31.6mm 2 bolt seatpost with 25mm setback where the bolts go left and right of the saddle rails, it has infinite adjustment. I tightened it up on my carbon saddle rails and the saddle kept tilting back on the clamp. It wasn't moving on the rails, even when tilting back. It was one of those "no matter how tight it is" it is going to move and with it being carbon rails, I shyed away from making it super tight. Its tilting back because the stupid clamp is made from 6061 and the ridges on the two washers have long since been flattened. They only ever pressed onto a smooth metal surface anyway so served no real function.

So I dug out another seatpost I have got, a "Brand-X Inline 6061" thing, but it has 0mm setback so I have the saddle right on the limit as far back as it will go (cue "I always said that frame was too small for you") and while I just tried it for about 7 miles and its fine, I wouldn't want to always ride sat right over the rails that way. The great thing about this seatpost though (and the point of this thread) is its clamp - it has a bolt front and rear of the rails, so it cannot possibly tilt forwards or backwards. The back screw holds it and stops it tilting forwards, while the front screw stops it tilting backwards.

I hunted for ages for a 31.6mm + 25mm setback + this exact bolt design and they are few and far between.

Those "not infinite adjustment" seatpost clamps with only one bolt can work when brand new, but if its 6061 again, the ridges on the clamp will just get crushed again (even at a normal tension) and you're going to get that tilting.

The carbon railed saddle I had on before this one did not tilt back by the way. :roll:

Just a heads up in case any beginner finds all the various clamps confusing. There's those sideways clamps too... I wouldn't use one of those even on metal rails but that's me. On carbon rails its a big no-no.

I only have each bolt on 5 Nm and its pretty ruddy tight on a 4" allen key. Honestly I have found this "2 bolt front and rear" setup the best, it eliminates any chance of tilting and with carbon rails, you can't start tightening it up more and more.

Ended up getting a new one thats got it all, with a solid platform for the rail to rest on (a lot of posts now seem to have a thin clamp where it meets the rails, urgh no thanks, might be alright on metal rails).

imposter2.0

That's nice 'n clear now...

letap73

Translation:
Saddles with carbon rails need an oversized rail clamp and alloy rails usually need a normal sized rail clamp.

Manc33

letap73 wrote:

Translation:
Saddles with carbon rails need an oversized rail clamp and alloy rails usually need a normal sized rail clamp.

No, its to do with saddles tilting backwards because the clamp material is 6061 aluminium and the clamp's ridges (or washers with ridges) get crushed and become smooth themselves. It might work if you only ever make a few adjustments and never change your saddle, but as you put saddles on and off the metal becomes worn down. I would expect 7075 aluminium to be perfectly fine.

Solution: Use one with 2 bolts front and rear, eliminating any possibility of tilting. Clamp sizes don't enter into it except the risk of clamping a carbon rail to 5mm of metal as opposed to the clamp bottom having a solid platform.

letap73

Okay, from experience and on multiple bikes with multiple seat posts and multiple saddles I have two one bolt seat posts - both with Aluminium saddle clamps which do the job effectively. One of the seat posts has had four different saddles mounted (Prologo - Aliante - Antares -Romin) and I have continually fiddled about with adjustments and saddle swopping - the current saddle is clamped exactly how I want it - no tilt. Admittedly I go for alloy railed saddles and not carbon rails - which I believe are mostly fitted with over sized clamps( clamp sizes are important!).

darkhairedlord

chuck it all on ebay and buy a bike that fits.

Manc33

letap73 wrote:

Okay, from experience and on multiple bikes with multiple seat posts and multiple saddles I have two one bolt seat posts - both with Aluminium saddle clamps which do the job effectively. One of the seat posts has had four different saddles mounted (Prologo - Aliante - Antares -Romin) and I have continually fiddled about with adjustments and saddle swopping - the current saddle is clamped exactly how I want it - no tilt. Admittedly I go for alloy railed saddles and not carbon rails - which I believe are mostly fitted with over sized clamps( clamp sizes are important!).

This is the first time a saddle has ever tilted back on me in that way. I could understand the rails sliding with it being carbon, but that tilting back is strange. It only slides on the rails when it gets really tilted back. My last saddle (also with carbon rails) didn't slip anywhere on this exact same setup, but one saddle has like a sandpaper texture to the rails, the other is smooth, too smooth. Someone said "rough up your clamp" but I don't want to lol.

Do I really need to take pics of those squashed washers? You get what you pay for. Pretty much everything I ever buy is cheap. :oops:

This:





I reckon you can't make parts like these out of 6061, or you can but this will happen (or not, if you're lucky).

I used that clamp with a Brooks Colt, a cheap Avenir saddle, a different carbon railed saddle and all were fine, its just this certain saddle can't be clamped tight enough. I still blame the clamp because - look above.

apreading

If the Ti rails are slightly thicker then it may be that the clamp is stopped by them before it builds up enough tension to adequately grip the washers.

Are they 8mm when the clamp is for 7mm?

Manc33

No its 7x9mm carbon rails not titanium - on a 7x7mm clamp, but like I said, the last saddle with carbon rails didn't tilt backwards. Neither saddle slips on the rails themselves, just one of the saddles tilts back. It will be fixed once my new post arrives. 8) Admittedly I did have it on about 4 Nm and it was safe to tighten slightly to 5 Nm but it won't cut it, a different bolt system entirely is the only real fix.