Bicycle Mechanics - Broken seat post bolt, again?

Not sure what Im doing wrong. Im on my 4:30am ride and I snaped the bolt that holds the seat on. Its a Cannondale hybrid and I dont think Im riding it really hard though the morning rides are my workouts, for my legs not the bike. Ive been riding a limestone trail which does cross a few curbless streets. Since I broke one back in Feb, Ive been careful and got out of the saddle when crossing the streets as I thought the minor bouncing @ 14mph might have done it. I've been riding the same style and trails with this bike for 4 years without any problems but to have 2 break in months... I know the seat was not loose when I started either.
Im also 20lbs lighter (now 230) than when I first got the bike. Its kind of a shock to have the seat fly off with 5 miles left to go.

On the other hand, its quite a workout on the legs to stand & ride with no seat.
any ideas?

Is the frame using an integrated clamp and a seat binder bolt? Like thishttp://www.lickbike.com/0897.gif
Or is there a clamp around the top of the seat tube with a threaded bolt in it? If it is the latter then I would suggest using a better seat clamp like the salsa liplockhttp://www.salsacycles.com/images/liplock.jpg

IF you are instead breaking the bolt at the top of the seatpost which holds the clamps onto the seat rails then I would suggest a new seatpost altogether. Try a quality Kalloy or other seatpost that retails for 30 dollars or so. If you want to eliminate the problem completely then go for the Thomson, although at 85$ they arent cheap.

-Ryan

Ryan, I think he is talking about the bolt on the post that holds the seat on the post.
If you have broken two bolts on the same post in two months, I would suggest getting a new post with a bigger clamp area. Pull you post out and get the size off of it, should be near the max line.

Well, honestly, you are not the lightest person, so special attention should be given to parts. What exactly broke, the bolt itself in a half, the tread on it, or the clamp?
If you are sure that everything was adjusted properly and you havent contributed to the failure by jumping or similar suvere stress, then you might want to think about inproving the structure... Maybe getting bigger bolts/clamps/ seatpost tube...

It was the bolt that clamps the seat to the post, busted right in half. Im just amazed as I put thousands of miles on the bike without a break and then to snap 2 so fast.
I guess I can look at a better/durable post...I have to hit the LBS anyway. Is there a big difference in post strength or clamps?

If it was OK for a long time and then broke suddenly and then broke another right after that I wouldbet what happened is that it worked loose or deformed and put extra stress on the bolt, which fatigued and broke. Then you replaced the bolt and the situation was ripe for another failure due to preexisting problems.

edit: oops, yeah, what he said ^ :)

If you want to try again with your existing post, go to a large hardware store and find a high-grade bolt, like a 12.8-grade or 14.4-grade. If you want a new post, the American Classic (http://www.amclassic.com/Seatposts.html) has a really long cradle like Rev. Chuck is recommending, and the cradle is supported under its rear by an "elevator" bolt so the main bolt isn't experiencing a lot of torque. They are no longer made in America, but I would expect they're no worse than other Taiwanese-made posts.

I happen to have a brand-new 27.2mm x 350mm American Classic of the Taiwanese-made variety sitting right next to me. The main bolt is a healthy 8mm x 1. I think your LBS would probably be selling these for $45 or so, and probably would need to special-order it. Incidentally, my postal scale is calling it about 9.3 ounces, or 260-270 grams.

Rev,
After I read your post I took a closer cleaner look at the seatpost and found that it has lost a bit of the grip. Looks like it sheared down over time so its probably time for a new post huh? Guess if it shifted and wore down, that would explain the snap of the bolt.
I'll research a new post and replace the bolt for now.
Thanks for all your imput.

I too am a larger rider. They call us over 200 lbs, Clydesdales. I have had numerous problems with post of the single bolt variety. I think that due to my weight, I then to shear the bolt and wear out the detents on the clamps like you have.

I would recommend a double bolt system for you. It's the type found on Thomson Posts, and many others. The other advantage, is that they "Micro" adjust instead of "getting close" with the detents on the two parts. You simply loosen either the front or the rear and tighten the other to adust to your preferred seat angle.

When I was first into cycling, I thought spending $85 for a seatpost was ridiculous. Then I bought a Thomson. I'll now never use anything else. I have these on every single one of my bikes, (my wife's too!).

Once you get them on, you forget about them. I never have to touch mine, except when changing out the seat.

L8R

BTW, if you've got a Cannondale. The seatpost diameter is 27.2 mm.

I've busted my share of seat bolts, too, at 250+. They wear out, it happens. I use Thomsons now, too, and have not had a problem since. They can be had on ebay from time to time.

Ok, you guys sold me. Im looking up Thomson posts now, $70 for the 27.2X330. Ebay would be nice but I kinda need it now so I'll take the plunge.
Thanks to all for the advice.

How much does your current post stick out of the frame. If it's a lot, Thomson makes a 410 length that a lot of mtn bikers use. 330 is standard and there is also a very short one at 250mm.

Believe me, you can't go wrong with a Thomson. And NO, I don't work for them, but I feel that strongly about the company!

L8R

I stopped off at a LBS to replace the bolt and they actually had one Thomson in stock, the 330 (same as my old one). It came out about 3 bucks more than online but its a local business and now I'll ride in the morning. Looking it over, I should not have any post problems again. A really nice piece of equipment.

Clydesdales..eh? well, I've been called worse.
Thanks.