cracked seat post!

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MB2 said:

I installed a luggage rack on my Trek 1.5 (Bontrager carbon seat post) - the type that cantilevers off of the seat post.

I commuted to work for a week, 45 mins each way, with my bag weighing 6 kg (the weight limit of the rack was 10kg)

A week later I hear some cracking around the bike and discover that the seat post was cracking around where it fits into the frame (picture attached)

So what is the deal? Your opinions? Does the fact that I fitted the part out of the factory mean that I may not be able to claim under manufacturer's warranty

Hell, I'm just glad that the post didn't crack.

I am a fairly heavy rider, but I only noticed the cracking in the last week - and I had removed the post to fit the rack and did not notice any cracking. I torqued the bolt up nip tight

What are your thoughts?

I would definitely recommend against using this type of luggage rack. The shop I bought it at recommended this type since, in the opinion of the sales rep, the frame would be too weak to accomodate the other type.

My thoughts are it's not a rack problem but attaching it to a carbon seatpost problem. Get an aluminum one and use it.

I don't think putting one of those seat post racks is ever recommended on a carbon post..... but - if you took your seatpost out and put it back in you may have overtightened the collar when you reinstalled it. Did you use a torque wrench? Carbon can be cracked if you are overly zealous..... (and 5-6nm, though more than finger tight is not cranked down as hard as a big guy can muster)

I would bet that both the added weight of your rack and stuff, plus overtightening your seatpost collar both contributed to its demise. Like Peter said - if you want to use that kind of rack get an aluminium seat post (and buy a torque wrench - you can still ruin a metal post if you crank down too hard on it!)

I second what Eden says. I think that you overtightened the installation and that is the main cause of your problem. Get a strong alu post, install as per spec and you should not have any troubles.

Powerful Pete said:

I second what Eden says. I think that you overtightened the installation and that is the main cause of your problem. Get a strong alu post, install as per spec and you should not have any troubles.

I agree with all of your posts and will proceed with AL post and careful torquing. Thanks every1 for your participation.

Any ideas how to measure the torque applied to the allen key fitting (as it is on my bike) for the seat post - the required torque is 9 Nm - I have a cheap auto torque wrench but that is designed to measure much larger torques - hence I do not trust its accuracy at the low numbers...

What do you guys use?

MB2 said:

I agree with all of your posts and will proceed with AL post and careful torquing. Thanks every1 for your participation.

Any ideas how to measure the torque applied to the allen key fitting (as it is on my bike) for the seat post - the required torque is 9 Nm - I have a cheap auto torque wrench but that is designed to measure much larger torques - hence I do not trust its accuracy at the low numbers...

What do you guys use?

You need to rent, borrow or buy a low range torque wrench. The ones used for automotive just aren't accurate at low torques, if they go that low at all. I found one at a rental shop. I have a set of 3/8" drive allen wrenchs to put on it, but you can certainly just buy the one you need. After using that, I found out a friend who's a professional mechanic has a suitable torque wrench that he said I could borrow next time, but he also told me not to bother looking to buy one because a decent one is very expensive.

As for the post, I agree with all the others. Being one that bought a carbon post in a moment of weakness, I feel I can state with authority that the carbon post saves little if any weight and does not add to comfort over a good quality (and much cheaper) aluminum post.

PS: if you're clever 🙂 and know how to calculate inch-lbs (or the metric equivalent), you can fairly accurately torque with a digital fish scale pulling on a measured length. I did that and it was within tolerance when I re-checked with a torque wrench.

A decent torque wrench is not that expensive. The dial torque wrenches are but the micrometer handle torque wrenches are fairly accurate and are not expensive at all. I wouldn't trust a deflecting beam torque wrench on a $1500 and up bicycle. If you are going to spend that much on a CF frame bike though, you can afford a good torque wrench as insurance.

kdelong said:

A decent torque wrench is not that expensive. The dial torque wrenches are but the micrometer handle torque wrenches are fairly accurate and are not expensive at all. I wouldn't trust a deflecting beam torque wrench on a $1500 and up bicycle. If you are going to spend that much on a CF frame bike though, you can afford a good torque wrench as insurance.

Yeah fair enough. The frame is not carbon, just the seat post (and the fork)

Well I just got back from the LBS - picked up an Al fork for AUD $50.00 to give me some confidence in riding with the luggage rack - which I'm assured will be pretty safe.

What is a fish scale? I thought of using a spring-type scale, but if I couldnt apply the force at 90deg I cannot be bothered doing the calculations - I hate being at work at home!

Get this though - I asked the LBS and they do it by 'feel' - Far out, what a dodgy LBS. Not taking my bike there.

They did make a very good suggestion about upgrading my wheels frmo Bontrager select to race -I will do this soon, should be good.

Thanks all.

MB2 said:

Yeah fair enough. The frame is not carbon, just the seat post (and the fork)

Well I just got back from the LBS - picked up an Al fork for AUD $50.00 to give me some confidence in riding with the luggage rack - which I'm assured will be pretty safe.

What is a fish scale? I thought of using a spring-type scale, but if I couldnt apply the force at 90deg I cannot be bothered doing the calculations - I hate being at work at home!

Get this though - I asked the LBS and they do it by 'feel' - Far out, what a dodgy LBS. Not taking my bike there.

They did make a very good suggestion about upgrading my wheels frmo Bontrager select to race -I will do this soon, should be good.

Thanks all.

PS now that I have an Al post I can torque up all I want? (within reason) -correct?

MB2 said:

PS now that I have an Al post I can torque up all I want? (within reason) -correct?

I would still not torque too much. Al is somewhat soft unlike steel. So, be careful.

MB2 said:

PS now that I have an Al post I can torque up all I want? (within reason) -correct?

Look for instructions or ask the shop. I personally would't torque it any moe than is needed to keep it in place.

Camilo said:

Look for instructions or ask the shop. I personally would't torque it any moe than is needed to keep it in place.

Yeah I asked and the reply was that I can torque it up much more than the carbon post, to the extent that I don't need to measure the torque, however - I shouldn't crank away for hours...

Crank it tight enough that the seat post does not slip. No need to give it the torque of death routine...

FYI, a decent low range torque wrench can be had for something in the range of 90~100 Euro (IIRC) here in the Old World. Syntace makes quite a nice one, if you are in the market...

I have a Topeak rack on my Trek with a aluminum seatpost and no problems. In fact I constantly put twice the recommended wieght on it. Bungeed a milk crate to make it really useful. Mine attaches to the seatpost via quick release, same as the seatpost to the frame.

MB2 said:

Yeah I asked and the reply was that I can torque it up much more than the carbon post, to the extent that I don't need to measure the torque, however - I shouldn't crank away for hours...

The problem with overdoing the torque on the seatpost bolt is not only damage to the post, but also to the frame. The big risk is stripping the thread in the frame. The general rule of just enough torque applies to alu posts as well.