HowellGC

Today on a ride the seat bolt broke on the stoker saddle. We were going about 20 mph when the bolt securing the saddle to the rails broke. My poor wife fell back with her backside rubbing on the rear tire before she was able to lift herself up and I steadied the bike to a stop with saddle and seat rail clamps crashing to the road. She has some respectable road rash from the tire on her backside. Needless to say she (we) was very frightened. Thankfully she was holding onto the handle bars when it occurred and we did not crash.

This is the second time a bolt has broken on the stoker saddle. The 1st bolt, broke about two years ago, and was the original bolt that came with the seatpost (woundup) and the 2nd bolt was from the hardware store. My wife weighs 135 lbs, so I don't think her weight is an issue. We ride a Co-Motion Supremo.

My theory is that she likes to sit back on the seat at times, as she is a little cramped in her riding position, when we are not riding hard and the pressure of being so far back on the seat eventually leads to the bolt breaking. Also, she sometimes sits up without hands on the bars to enjoy the country side as staring at my back does get a little tiresome.

Your thoughts on what might be happening?

My thought is to get a set-back seatpost to give hear a little more space, but it obviously won't be much.

Abram

Could you be over-tightening the bolts? In what way are they breaking?

Monoborracho

That it has happened twice it deserves some serious attention. Can you tell from where the break started? How good was the hardware bolt? What was the torque? Lots of questions to answer here.

This could easily have had a very bad ending. Thankfully stoker did not get seriously injured.

HowellGC

Thank you for replies. Unfortunately, I am not sure how to answer the questions precisely. The bolt was tightened to the point that the seat did not move. Hate to think over-tightening was problem -- definitely will consult LBS. It's hard to say where the bolt broke as only 1/2 was "recovered." It appears the bolt broke in the middle -- eye balling the broken bit to the bolt in my seat.

After the 1st bolt broke, LBS didn't have replacement, but said bolt from Home Depot would be of same quality. Perhaps not.

waynesulak

My guesses,

I doubt it was quality of two separate bolts purchased from separate vendors.

possiblly saddle with long rails slammed all the way back gets a lot of leverage on a single bolt seat post. I have had good luck with setback salsa shaft which has a long rail contact point and separate bolts for adjusting tilt and clamp. Amazon out of stock so it may not be made any longer. I suggest a set back post and not pushing the saddle all the way back. Also something with more than one bolt.

Amazon.com: Customer Reviews: Salsa Shaft 27.2 x 350mm Seatpost Black

could be over tightening. Use torque wrench.

Also Insure stoker is 135 lbs and not 135 kilos. Just kidding. Good luck.

zonatandem

Definitely would give the setback seatpost consideration.
If you have an adjustable stoker stem suggest giving her a bit more space by moving it forward a bit.
A fraction of an inch here and there can make a difference and hopefully alleviate the situation.

Monoborracho

Since you have broken two, it would seem to be something in the way the seat post's saddle rail clamp is working on the bolt. If that is all you can point to, then change seat posts.

I like the Thompson seat posts we have on our tandem. Two bolts which spread the load/stress - easily adjustable. They have a setback post also. Stoker is worth it.

Thomson Elite Setback Seatpost | Competitive Cyclist

mstyer

Sorry to hear about your mishap, and hope that your wife heals up soon.

There are different grades of bolts, and there's a chance that you may have used one that is not appropriate for the loads placed on it. Also, overtightening and causing the bolt to stretch or undertightening and allowing cyclic loading of the bolt could have contributed to the failure.

Has your wife been professionally fit to either the tandem or a single bike? Pushing back on the saddle is often a sign of either the saddle being too far forward or too narrow. Once you know exactly where the saddle should be, you can determine whether a straight or setback seatpost will put the clamps closest to the middle of the saddle rails.

The only references I can find to a Wound-Up seatpost are from several years ago and show a modest setback. Does the photo below look like your seatpost?

I'd strongly consider replacing the post regardless of whether it's the right fit, just for your wife's peace of mind.

DubT

I would suggest that you use a grade 8 bolt, if a grade 8 is not available then get at least a grade 5. Determine what size bolt you need and then see if Lowes or one of the other big box stores has it, if not do a google search and find it on line. You may have been using an inferior grade bolt.

merlinextraligh

I think the odds are pretty high that over tightening played a part in this. It's easier to snap small allen bolts than people imagine.

You tighten the bolt too tight, and it's close to failure, then hit a bump which puts a load on the already stressed bolt and it snaps.

I'd second getting a torque wrench.

Also use carbon assembly paste to lower the torque necessary to hold things snug.

Given that this has happened twice, there are really only 2 likely possibilities: defective seatpost, or over tightening. Buy a new seatpost to rule out No 1, and a torque wrench to avoid No 2.

Carbonfiberboy 

If the photo in this thread is your seatpost, it looks to me like there's more than the usual amount of leverage on that bolt. Different seatpost. I like the kind with the two bolts fore-and-aft which allow micro-adjustment and is very strong.

HowellGC

Thank you for the replies. Appreciate the thoughts and suggestions.

Yes - that is the Wound Up seat post. My (captain) seatpost is the same. Hmmm. Also, her seat was as far back on the rails as recommended.

I am now in the market for a setback post with a 2 bolt design and a torque wrench.

This could have been really bad, obviously. Thankfully it occurred on a straight road and we weren't going fast, but fast enough for some unfortunate but burn and a bad scare. (Incidentally, pretty amazing how the rubber on the tire can burn the skin in a flash without making a whole in clothing -- if you one didn't know otherwise you'd think it was real 'road rash' from pavement.) I'd best not show any pictures....

Dean V

I would give Wound Up a call and see what they say.

DrMarkR

Excellent idea.....and CoMotion as well. Not that either of them would have any ideas on this matter...... ;-)

MR

merlinextraligh

I think it's reasonable to call both of them. There may be some known design defect or recall they would know of.

Assuming there isn't a known defect, I'm pretty sure how I can guess what they'll say. Dollars to donuts, both of them will raise the issue of torque of the bolt.

Also given that it appears that Wound Up no longer makes seatposts, and Co-Mo no longer uses Wound Up seatposts, I bet both of them will recommend replacing both seatposts with something newer, as a precaution.

If they don't give you both of those answers, they need better lawyers.