rpenmanparker 

Some of you perhaps read my recent thread about a tiny crack in the top tube of my CF Giant. I was riding it today and BLAM, the seat post fixing bolt head snapped off, leaving just the bare post sticking up and my saddle and the clamp parts all over the ground. I was a little wobbly for a few seconds, but no crash. I'm not the type to ride a bunch of miles standing up, so I made the call. Go figure!

Campag4life

Do you use a torque wrench on your saddle bolts? My guess is no.

rpenmanparker 

I would, but I don't have any spec for these discontinued Performance seat posts. Any idea of a good generic torque maximum to use? I would have thought the bolt was metric, but the OD is about 1/4 inch, a little more than 6 mm. Stainless. I appreciate the help.

BillyD

Lazyass

Better than a carbon fiber saddle bolt.

GuitarBob

I imagine the usual would work... you know, tighten the bolt until you break it, then back off a little.

Hell, a torque wrench for steel saddle bolts? Come on now, you just had a bum bolt.

FBinNY 

There are generic torque specs for screws and bolts based on tensile strength (material and diameter) & helix angle. However I suspect that this failure had nothing to do with torque, which usually breaks bolts immediately, but with load in use. Possibly the part itself was under spec'd. or the rider's weight is too far from the post's axis, increasing tilting loads.

rpenmanparker 

Damn! I wish you hadn't brought that up. Yeah, I do ride a setback seatpost with the saddle pushed nearly all the way back. I have always pooh-poohed the idea you have to have your saddle centered on the rails right over the post, but you make a very good point. Something to think about for sure. But there is nothing much I can do about it. I am already using a 25 mm setback. I know they make 35s, but they are not very common, and the selection likely sucks. Maybe holding out for a higher strength bolt is the right answer. The stainless is convenient, but probably not the strongest bolt in the tool box. I will ask as the hardware store for something in high strength steel and just keep it well greased.

FBinNY 

OK, so you want to minimize the dead load on the bolt, leaving as much as possible for the working load. Tighten the clamp by degrees only as tight as necessary to hold it, plus a bit more as a safety margin. Also, if you can source a 316 or non-stainless bolt in CrMo, so much the better, but you'll need to weather protect the bolt as well as possible.

BigJeff

slammed back on a 25mm setback? The frame may be too small. What size bike is it?

Larger frames tend tend to have less seat tube angle as well. How much exposed seat post?

rpenmanparker 

Jeff, what you say seems right on target, but actually no. I am only 5'7" and ride M - 54cm size on all three of my bikes. And the SA on this one is 73 deg. And as near as I can tell, my position is just about exactly KOPS. Not that I think that is necessary, just sayin'. Here is a photo. With a 100 mm stem, nothing looks out of line to me. What do you think?

FBinNY 

Bob,

Things look basically right, though I'd prefer more setback in the post, as was SOP BITD. However the real issue is how long you were riding before the screw snapped. If a while, you might follow the "one is a fluke" rule, and continue without a serious change. If you break another, or if the break occurred relatively early in the scheme, then consider putting more metal under the load.

rpenmanparker 

Thanks. Because I rotate my bike usage among the three, none of my bikes gets a really high usage, but I have been on this seat post for about 5 years, maybe 6,000 miles or so. I'm going to replace the bolt with as high strength one as I can find.

FBinNY 

I wouldn't consider a failure after 5 years an indication of a problem. Get a new bolt, better if possible, and put it out of your mind until 2020.

rpenmanparker 

Yep, just as soon as the laceration on my abdomen from the top of the seat post heals up. Oh, did I forget to mention that? No pictures, I promise.

gc3

Robert has the monkey arms...

FBinNY 

Yes, I've never been a fan of posts where saddle retention and saddle angle could fail together. Some time back there was a proposal in Europe to require that posts would still retain the saddle if a single bolt of the clamp system failed. I don't know if it was adopted or not, bit there's some sense to it.

I don't know your post, but consider some kind of DIY modification to give you a fall back margin in the event of a bolt snapping. Possibly an improvised wedge or fill under the clamp, so it couldn't rock through enough angle to lose the saddle.

Things break, and that's not always preventable, but it can be possible to manage the consequences of critical failures.

rpenmanparker 

That's what's funny. I only wear a 33 inch sleeve in dress shirts. I guess I just like to stretch out Lemond style. LOL That is all we have in common.

Drew Eckhardt 

Yup. Two bolt posts with a pivot in the middle and clamp front loading its bolt in tension can do that.



I broke my first M5 bolt after 3415 miles over 7 months, shrinking from 185 to 148 pounds over that time. Rode home 2-3 miles, dropping the saddle the first time when I tried carrying it nose-first although tail-first worked.

I figured that was a fluke and upgraded from class 10.9 to class 12.9 as in 1000 MPa to 1200 MPa tensile strength, yield at 90% of that.

Broke that one after 1693 miles over 3 months, shrinking from 148 to 140 pounds. Although I only had 5-6 miles left my legs were spent so I had my wife pick me up.

I switched back to a single bolt post (Nitto S65 Crystal Fellow) after that with 25mm of setback. It has a female wedge in the lower cradle which sits atop a male wedge on the post with the bolt in the middle and should not move as much. It uses a 6mm bolt.

rpenmanparker 

Understood, but my situation WAS a single bolt post. Stuff just happens.

bt

how about a frame with a more relaxed seat tube.

FBinNY 

That might have been an option before buying this one, but it doesn't make sense to replace a bike or frame one is happy with over a seat post issue. They do make posts with more setback, and/or better clamping systems, so that would be the "spend some dough" option.

FWIW, at a saddle height of 30" above the spindle, you gain about 1cm for every add degree of seat angle (measured back from vertical). So the OP would need to find a 71° frame to make the same difference as 2cm of added setback.

bt

I've used way flimsier justifications to buy new frames.

FBinNY 

We all have, in which case the OP doesn't need this one.

rpenmanparker 

Also I am not likely to find a stock frame with a slacker head tube or otherwise more set back to the seat position in a racing style. New frame may be in the offing bye and bye, but it won't solve my seat post issue.