Hello fellow riders - I have a carbon bike and I’ve attached a rear rack. 
As background for the curious, I bought this bike (a Giant Avail) years ago and I barely ride it. My ‘real’ bike, a standard commuter with well-used racks, was stolen and I’m determined to make use of this bike before thinking about another purchase. I realise it’s not designed for a rack. 
The rack (a Thule Pack n' Pedal) is tightly strapped to the seatstay - see photo. Carbon is clearly strong material (bikes are made from it) however I’d like to be prepared for any future dilemmas. 
I’d like to know whether the seatstay is likely to buckle as a result of the tight rack straps? And if so, would it happen gradually or suddenly (should I keep an eye on it)?
How long could it  take before I see/experience damage?
Should I carry loads as light as possible to reduce the likelihood of damage? I ride everywhere including the supermarket and sometimes carry heavy loads but I'd wear a backpack if weight carried is an issue. 
I appreciate anything you can offer. I realise nothing can be guaranteed and I do declare that no one will blamed or questioned as a result of comments made. I’m asking far-and-wide to be as informed as possible.  

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Carbon is a very strong material. But Carbon bikes are designed to be as light as possible, which in turn makes them as weak as possible without breaking.

You need to be very careful clamping onto carbon frame parts. So much so, that when working on a carbon bike, they mechanic will put a steel seat post in just to clamp that in the work stand.

If the clamps crush the tube, the tube will fail, and carbon has a brittle failure, so any failure will be immediate and catastrophic (OK technically there is a good chance you'll get micro cracks first, but I believe you need an X-ray machine to see all of these)

Of course, that's all theory, if someone has actually done it and still alive to tell the tale

Carbon fibre has good tensile strength but it is brittle and can splinter. If you must put a rack on a carbon frame, cut up an old inner tube and wrap strips under the contact points. Hopefully lessening uneven pressure points.

I wouldn't tighten the nuts of the rack too tightly.

Warren.

PS, That's a suck-it-n'-see answer.

I would also recommend getting a rack that attaches to the wheel bracket (official name unknown). This bracket is not carbon and is designed to take a lot of forces. Having the rack attached the the bracket will transfer the downward forces directly into the wheel and not the frame.

The attachment pictured above would create torsional forces around the two mount points, the further back the centre of mass of the load, the greater he forces would be. I wouldn't trust the setup above - especially on Sydney roads.

Edit: I'd recommend something like this:

I have used OMM racks for years, on brutal rides. 26" in my case however they have charbon-roadie versions

That said Roisin, your remark about seeking a bike meant for utility work makes sense. I did that, German made Dutch bike does the work and a rack-less MTB does the play.

I have the same rack but it's on an alloy hardtail. There is ample rubber cushioning between the rack and the frame and you can achieve even tension with the straps. Mate, I'd contact Thule and ask them what they recommend.

well - Thule specifically say 'works with any type of frame material' on their website.


As others have said - don't overdo the seat-stay clamps, failure will be catastrophic (not buckling) 

Thule specifically say 'works with any type of frame material' on their website.

Will they put their money where their mouth is and replace a broken frame?

Thanks heaps for your replies. With further googling after your comments I plan to keep an eye on it and tap regularly with a coin to, apparently, ensure a clear hollow sound for intact fibres.

I'm feeling a little uneasy about it (there's no shortage of failed carbon frame stories online) and so I'll also shop around for bike designed for panniers...

Thanks again folks

 

 

Sorry to say I think this is a Really Bad Idea™. Those seat stays are designed to carry loads along their axis, you're introducing shear loads to them, perpendicular to the axis, which they are not made for. The clamping force you can manage fairly easily but it's the overall rack load that I think is an issue. You may get away with it if you're only carrying light loads but every pothole you hit is going to magnify the load applied. Tapping the frame with a coin is not going to help except to tell you it's time for a new frame or at very least hundreds of dollars of carbon repairs. It will not warn you before it happens.

As you know carbon is strong but only in it's designed application. The frame walls tend to be thicker at the junctions where it's needed to handle the stresses and thinner in the middle of the span as it's not needed.

It looks like you don't have a round seatpost either so you probably cannot mount a rack that puts the load onto that. When I ride my nice bike to work I just have to take a backpack or plan ahead for the week and keep a stash of clothes at work. It's a nice bike, I'd aim on keeping it that way.

Good stuff James - thank you. I'm carrying light loads (less than 10kg) and doing my best to avoid every damn hole I see.

Can I ask about my real worry?

I understand bikes are designed for vertical loads and that lateral loads / impacts can be most damaging. My only concern is failure when riding - the bike is headed for the trash heap if it fails.

Do you know whether the seatpost bears a significant proportion of rider-weight? I'm wondering whether I'll have warning to get my feet on the ground before the bike fully collapses. I should add I'm an experienced rider (my bike has felt like an extension of my body for 20+ yrs and can get on/off quickly), I'm a slower rider (only fast on declines) and don't wear cleats - so overall I'm hoping for an incident from which I'll get up and walk away (and my fingers are crossed that should there be cars behind me they'll be are travelling slow).  

The seatpost can bear a good amount of weight but the real advantage of attaching it to the seatpost would have been that it's a readily replaceable item in the event of failure. For a 'normal' round alloy seat post I doubt there would be a problem, they are designed to be clamped and levered on. Your seatpost looks like it may be one of Giant's composite aero ones which I don't think would lend themselves to having a rack hanging on there.

Your rack carries a maximum load of 25kg. Carry more than that will break/crack it.

There`s an QR adapter set for rack without eyelet mounts. Google to find other stores for prices. Wiggle is out of stock at the moment. But it will be a PITA to remove and re-install it when you get a flat rear tyre.

And you`d need the seat post rack mount adapter to fit. But your seat post is aero and I can`t find it on google.

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