thriftyswift3

Okay, I know as a sheep walking into the wolves' den of mechanics I may lose some fur, but:

Yes, I know about torque specs. No, I haven't ever used a torque wrench on any of my bikes.

Yesterday, I transported our child in one of these iberts for the first time (not a pic of my bike here, but you can see how it attaches):




I had installed it the night before, and in the process, changed the height of the stem. When I tightened the stem bolt I know I didn't tighten it very tightly, as I have a thing where I hate to strip out threads.

Well, riding home with my baby yesterday going slowly up a hill, simultaneously the child seat clamp came loose and the stem came loose resulting in the wheel cocking to the side!

The seat coming loose, I know why it happened and fortunately, it is not a catastrophic failure. The seat and child just flops a bit to the side.

Here is what has me freaked out: The stem coming loose. I've had a love affair of riding bikes for most of my life. Like many of you, I've ridden probably over 100 different bikes in many and varied conditions, including bombing down hills at 30+ MPH. Never once did I realize that all it would take for the wheel to suddenly cock to the side and become independent of the bars is for that one bolt to come loose.

Isn't that a harrowing thought?

Fortunately, I did carry in my pocket two Allen wrenches required to retighen the bolts and continue on home. I have since given the stem bolt some good cranks with the Allen wrench that is about the length of 2/3 the width of my hand (so, not a lot of leverage).

How tightly should I tighten that bolt, and what the freak are the chances of such a failure again? I know, it's not happened before in my 40 year history of riding, but again, the new realization/thought has me freaked out.

cpach

Torque spec on quill bolts is hellatight, or actually 20-30nm. You should always test to see if you can forcefully twist the handlebars with the wheel held in your knees. On some setups, particularly with poor quality stems, it can be actually difficult to make it impossible to turn the bars. Normal best practices are to grease everything, but sometimes I'll put carbon assembly paste on the wedge portion of the stem to increase friction if no sane amount of torque can prevent the bars from turning with force.

cyccommute 

Because of the way that the wedge works on a stem, I don't think there is a torque specification on it. The wedge has to "wedge" in the steer tube and how much pressure is needed is going to depend on several factors.

You are asking a lot from a stem, especially one for a threaded headset. The stem is only meant to gently nudge the front wheel so that you can steer. It's not really meant to carry a load. Putting a load on the stem adds a torque that it really isn't meant to take and in a direction it isn't meant to move. You can try tightening the stem but there is a limit to how much you can tighten it.

The wedge is a wedge (well, duh!) but that means that it can put pressure on the steer tube. Tighten it too much and you risk bulging the steer tube at a minimum. You could even split the tube if you try to tighten it too much. In the worst case, the tube is deformed enough that you can't get it past the cup of the headset.

Frankly, looking at the mechanism, stem movement seems inevitable.

Iride01 

Hmmmm...... and infant/child car seats in automobiles are the subject of safety concerns all the time.

HillRider

Frankly, that seems to me to be a terrible design for a child carrier. First, as noted, it puts a load on the stem it was never intended to take. Second, in the event of an accident, the child hits first.

Wilfred Laurier

That picture seems to show some markings on the stem's quill well above the top of the headset nut. The only markings you will regularly find on a stem's quill are at the 'maximum extension' line.

I suspect that stem was way too high and the fault lies with the person who installed it. In other words, the problem is not a loose bolt int he stem, but a loose nut on the saddle

thriftyswift3

Good observation! As I noted, however, that is not a picture of my bike. I pulled that off the web just to show the setup.

Iride01 

I take a dim view of putting an infant seat up on the handlebars. When I wrecked last year, except for some cosmetic scrapes elsewhere, the damage was concentrated on the handlebar area. Evidence of the force the bars and headset took was a bulge in the steel of the top tube near the lug joining it to the headset.

I'd think a trailer the safest option and possibly over the rear wheel the only other choice. However I don't know for certain, these are only my initial thoughts on something I've never read about or discussed.

dsbrantjr

The way the carrier is mounted puts a lot of rearwards force onto the stem due to the leverage of the mounting arm. Looks kind of scary to me.

AnkleWork

Your stem was grossly under-tightened, but it should not be over-tightened either. The stem should slip with a moderate impact to the wheel or fork. This will reduce impacts to the child.

Still, neither that location nor mounting for a child seat is safe. There are much better options.

madpogue 

In that case, every reply on this thread (no pun intended...) is based on an assumption, the premise of which hasn't been established. The photo shows a THREADED (quill) stem/headset/fork design. In addition, your opening post refers to both a "headset" and "stem" bolt, which is ambiguous. So, significant clarification is needed:
* Does your bike have a threaded/quill stem, or threadless?
* If threadless, was it the bolt for the headset preload (top cap) or the bolt(s) for the stem (side bolt(s)) that came loose?
Torque specs for the above vary, so knowing which equipment you have, and what came loose, will determine the torque answers.

Ultimately, however, the distinction won't make much difference with respect to the setup with the child seat. As said, it's asking an awful lot of the steerer, whichever version it is. Even if it could be mechanically secured, given the high center of gravity, and the nature of the "cargo", it's not exactly dressed for success.

thriftyswift3

It took me while to reply because a few times it seemed this site was having some problems.

It is an old style threaded headset. The bike is a TREK 720 Multitrack that someone on my street had set out with the trash. I figure the bike is probably from the 1980s or 90s.

Russ Roth

Have you ever actually seen any of these situations happen? It all sounds plausible but improbable.

Couldn't agree more. I always refused to install these in the shops I worked in. I liked to point out that most accidents people had were running into things or crashing forward (sewer grate, cracks in the road, pot holes, etc) which basically turns the kid into an airbag, and airbags aren't meant to last more then one hit.
As you just learned things can happen fast, put the kid behind you in a seat or trailer.
Also the torque on that is really frickkin tight. Never used a torque wrench on one, just tighten till the 6mm L wrench started to flex.

cyccommute 

Yes. Not often but it can happen. Attempting to tighten the stem bolt too much could also result in rounding out of the bolt head. Neither situation is easy to deal with.

Unca_Sam

here.
I use 170 in lbs. torque on threaded stem bolts (unless specified otherwise). That ends up as almost 20 Nm before the wrench clicks. The only time a stem has slipped on me was when I neglected this step (while trying to find the right stem height). With the number of stuck quill stems we encounter in the C&V republic, applying grease to the quill and wedge seems like something silly to avoid. I could get behind the CF anti-slip paste, though I don't own any CF handlebars or seat posts.

infamousfaith

Yeah that looks really dangerous to be putting an infant on, I wouldn't risk it. The way it's designed is poor holding onto a tube vertically like that, no matter how much clamping force you apply It'll always have a tendency to move around. Definitely go back to the drawing board on this one and think it out.

2old

I rode extensively for 10 or so years mostly off road or on wide sidewalks with a succession of children (yeah, overdid it) in a rear-mounted child seat and the only accident I recall was when the bike slid out. The sides of the seat protected my son and he wasn't even dinged. Can't imagine ever using any type of front carrier. Also, have tightened stem bolts to the limits of a six mm wrench and never had a problem. I'd need to see an oval steerer tube before believing someone could do it unless the tubing was paper thin.

woodcraft

There are folks around here that freak out over a scratch on the frame,

but that (cast aluminum?) clamp is catastrophe waiting to happen.

I'm surprised the company has not already been sued out of existence.


From Amazon reviews:

thriftyswift3

Well, dang it. I know I'd seen these ibert seats years ago, before we had kids and it seemed like the cool parents used them. Ha.

I got the seat at a Performance Bicycle store that was liquidating. Got it for $20.

The seat says it's made in USA.

All these things made like the seat, but I understand and agree that it's not a great mounting concept. That broken clamp is alarming too!

Yeah, we had a Kettler seat for our first child (behind the rider) but we sold it because we were not going to have another child. I do have a trailer as well – actually two trailers!

zacster

I've returned both a bike child carrier and a child backpack carrier due to safety concerns, this 25+ years ago as my kids are all adults now. On the backpack, the kid could flip right out of it as I recall. The bike carrier just didn't attach to the bike very well. Both were from the better outdoor equipment brands and were supposed to be the best. You could tell they were made to photograph well in the catalogs (we had slow internet but still referred to catalogs back then.) I still have the Blackburn rack from the bike carrier I ultimately used and it is still attached to the commuter bike. The carrier itself attached firmly to the rack and they were sold together as a system and the carrier was removable. I think we discarded the seat portion as it had gotten moldy in our damp basement and we pretty clearly aren't having any more kids! I just looked at Amazon quickly and I don't see anything like it but I'm sure there are somewhere.

thriftyswift3

You know, I still like my Ibert, but here's another feature at which we may chuckle. Ride with the child seat removed and this is waiting to, um, give you the shaft: