# Top-tube carriers and carbon fiber fatigue?



## snovvman (Jan 4, 2010)

These are very similar to the type of bike rack I have. It's old school. While I believe that it is best to carry the bikes supported by their wheels, I chose it because it is [relatively] light and has a simple design.

http://www.bikerackshops.com/HRBOOMER.html

http://www.autoanything.com/Generic...oupID=undefined&ImageID=106901&ProductID=1570

http://www.summitracing.com/parts/THU-A30604/?rtype=10

I have read, because of its elasticity, carbon fiber is less prone to fatigue than aluminum. At the same time, it is very strong.

With the type of rack shown above, are there any concerns with carrying my carbon fiber-framed bike on it all the time? Keep in mind that the bike, along with the car, is bouncing around all the time and the stresses are where the top tube meets the front tube and the seat post tube.

With my aluminum bike, I don't really care. It's cheap and the top tube/welds seem strong. With the carbon fiber (expensive) bike, I don't want to damage the frame and/or cause stress fracture or anything.

Your input please. Thanks.


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## trailville (Jul 24, 2006)

I personally wouldn't want to have to put a carbon bike on a carrier like those. Any time you clamp a carbon tube you have to be very careful. Another risk with carbon is abrasion, so any type of rack where you end up strapping the bike, you risk abrasion that can eat into the carbon.


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## snovvman (Jan 4, 2010)

trailville said:


> I personally wouldn't want to have to put a carbon bike on a carrier like those. Any time you clamp a carbon tube you have to be very careful.


Thank you.

What is the risk of clamping? CF can crack/fracture from the crushing forace? How is this addressed with seat posts and handle bars?


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## skyliner1004 (Jan 7, 2010)

i was told there wouldn't be a problem clamping a CF tube for long periods of time, if the clamp pressure is good

i mean look at a carbon seatpost. that thing is clamped 24/7 and it doesn't give in; if its clamped with good pressure. i've hung my bike off the seatpost for 36 hours on my Park bike stand with no problems.


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## kapusta (Jan 17, 2004)

snovvman said:


> These are very similar to the type of bike rack I have. It's old school. While I believe that it is best to carry the bikes supported by their wheels, I chose it because it is [relatively] light and has a simple design.
> 
> http://www.bikerackshops.com/HRBOOMER.html
> 
> ...


I would not have any concerns about clamping a CF top tube in one of those. I mean, don't clamp the sh!t out of it, but that is true for an aluminum frame as well. Scratching or scuffing the finish is not going to weaken the CF, either. I have never heard of any warnings from a CF manufacturer against using a rack that holds the tt, and they are pretty common.


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## Bikinfoolferlife (Feb 3, 2004)

While I moved away from a hanging type rack long ago, I can offer a bit of experience in that I had a carbon bike when I did have that kind of rack. No harm done, at least not to that bike. Just make sure it's not going to move around. If the bike slides and works its way so that it sits at one end of the top tube or the other, the imbalance can create quite a bit of leverage and kill a bike (like we did to a friend's aluminum bike on one lengthy road trip, which is why I got rid of the rack and switched to a Sportsworks wheel tray type).


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## kapusta (Jan 17, 2004)

Bikinfoolferlife said:


> Just make sure it's not going to move around. If the bike slides and works its way so that it sits at one end of the top tube or the other, the imbalance can create quite a bit of leverage and kill a bike


Never thought of that. Yeah, that would be pretty hard on a bike.


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## snovvman (Jan 4, 2010)

Thanks for all the feedback. In thinking about it, all of the weight is on the bottom bracket. The top clamp simply secures the bike. Both sides have thick rubber padding. So long as I keep enough pressure to keep the bike secure and make sure the weight is properly distributed, I suppose there won't be any problems.

In any event, I doubt that I would be able to clamp hard enough with the small knob and rubber paddings in the way.


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## trailville (Jul 24, 2006)

snovvman said:


> Thank you.
> 
> What is the risk of clamping? CF can crack/fracture from the crushing forace? How is this addressed with seat posts and handle bars?


The difference is that the portions of the tubing that are in the areas that will be clamped by a seatpost clamp or stem are designed to be clamped (they are thicker), whereas other portions of the tubing are not. You'll notice that some carbon bars specifically designate the safe area for clamping shifters and brake levers as well. 
I'm not saying that clamping your frame in a rack will break it, only that you need to be very careful because that type of force is dangerous with carbon tubes. It's dangerous with aluminum too, but not quite the same.


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## Bikinfoolferlife (Feb 3, 2004)

snovvman said:


> Thanks for all the feedback. In thinking about it, all of the weight is on the bottom bracket. The top clamp simply secures the bike. Both sides have thick rubber padding. So long as I keep enough pressure to keep the bike secure and make sure the weight is properly distributed, I suppose there won't be any problems.
> 
> In any event, I doubt that I would be able to clamp hard enough with the small knob and rubber paddings in the way.


Only one of those racks had a bottom support of any kind, and I wouldn't say that it puts the weight on the bb on that one. Balancing the bike in the cradle before clamping is the best you can do on the hanging rack IME, and having as long a cradle as possible is good. Clamping force from those clamps isn't great enough to worry about IMO, but like I said before, if it starts to rock in the cradle, bad things can happen; just the rocking alone creates forces at each end of the cradle greater than at the center, let alone it being able to move in the cradle. None of those racks look particularly secure from swaying, especially the Swagman. Me, I'd go for a Thule T2 or similar, especially for suspension bikes with odd top tubes or shock mounts.


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## Frozenspokes (May 26, 2004)

IMHO those racks are made to hang huffys or bottom end Treks off the back of campers. But, i'm sure that if your carefull, You could have great luck using one of those on the TT of your nice CF bike. Just don't let it anywhere near mine.


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## snovvman (Jan 4, 2010)

Thanks again guys.

My rack is most like this one:
http://www.bikerackshops.com/HRBOOMER.html

The bottom rod is simply a tie to help stablize the bikes. The bottom bracket/suport is longer than the top clamp.

I had a Hollywood folding rack that had trays/channels on the bottom that held the bikes naturally by their wheels. I ended up selling it because it was very heavy. The above shown rack is much lighter and simpler mechanically. I have only had cheap bikes before and never worried. Now I am reexamining my options...


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## Bikinfoolferlife (Feb 3, 2004)

snovvman said:


> Thanks again guys.
> 
> My rack is most like this one:
> http://www.bikerackshops.com/HRBOOMER.html
> ...


Frankly, the lighter the rack, the less worthy it is...it's not an attribute.


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## snovvman (Jan 4, 2010)

Bikinfoolferlife said:


> Frankly, the lighter the rack, the less worthy it is...it's not an attribute.


Now if they only made a carbon fiber bike rack


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## Bikinfoolferlife (Feb 3, 2004)

snovvman said:


> Now if they only made a carbon fiber bike rack


You might be on to something there for the high performance vehicle crowd...I can see it now as an option for Mercedes/BMW/Porsche vehicles....


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## mik_git (Feb 4, 2004)

Read an interview with a guy from kestrel (actually a bunch of manufatures easton etc) but the kestrel bit is pertinant, he said that the major problem they had with carbon frame failures is from people putting them in bike racks...


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## Bikinfoolferlife (Feb 3, 2004)

mik_git said:


> Read an interview with a guy from kestrel (actually a bunch of manufatures easton etc) but the kestrel bit is pertinant, he said that the major problem they had with carbon frame failures is from people putting them in bike racks...


I think you can still narrow the problem down not to the frame material but to the "people putting them in" part....


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## mik_git (Feb 4, 2004)

yeah, I used to put my steel frames in, they wewre always wobbling around... but have never put any aluminium frame in there... but mine have all been very very thin walled...

I wouldn't see a problem with carbon, so long as your very carefull... but i'd never do it myself... all my bikes live inside the car or on the roof racks


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## drz400sm (Nov 12, 2008)

if your gonna spend that much on a rack that carries the frame like that, why not just invest in one that holds it by the wheels?


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## kapusta (Jan 17, 2004)

snovvman said:


> Thanks for all the feedback. In thinking about it, all of the weight is on the bottom bracket. The top clamp simply secures the bike. Both sides have thick rubber padding. So long as I keep enough pressure to keep the bike secure and make sure the weight is properly distributed, I suppose there won't be any problems.
> 
> In any event, I doubt that I would be able to clamp hard enough with the small knob and rubber paddings in the way.


My issue with the racks shown is that they will not work with some designs out there. They need a straight, unobstructed section of top tub to work, which (I know from experience) can be a real deal-breaker for some bikes, especially FS bikes.


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## kapusta (Jan 17, 2004)

drz400sm said:


> if your gonna spend that much on a rack that carries the frame like that, why not just invest in one that holds it by the wheels?


I think he already owns the rack.


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## snovvman (Jan 4, 2010)

kapusta said:


> My issue with the racks shown is that they will not work with some designs out there. They need a straight, unobstructed section of top tub to work, which (I know from experience) can be a real deal-breaker for some bikes, especially FS bikes.


Very true. I ran into many bikes during my bike shopping process that would be incompatible to the rack of scope. Some do not have a large enough top-tube section, or the bottom tube is in the way, or the diamater of the top tube is too large.


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## siwilliams (Jul 23, 2009)

snovvman said:


> Very true. I ran into many bikes during my bike shopping process that would be incompatible to the rack of scope. Some do not have a large enough top-tube section, or the bottom tube is in the way, or the diamater of the top tube is too large.


You can get an extension bar that fits between the stem and top of the seat tube. Effectively a false top tube for transport. My wifes Giant has an awkward curving top tube so this was the solution.


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## snovvman (Jan 4, 2010)

siwilliams said:


> You can get an extension bar that fits between the stem and top of the seat tube. Effectively a false top tube for transport. My wifes Giant has an awkward curving top tube so this was the solution.


I actually have one of those for k-framed and girl/women's bikes. At the same time, I can see excessive loads on the front stem/bearings and seat post/seat post recepticle. As I think about the bike shifting about while traveling suspended by the stem and seat post in the loose adapter bar, I'm not sure if that is actually better...


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## siwilliams (Jul 23, 2009)

snovvman said:


> I actually have one of those for k-framed and girl/women's bikes. At the same time, I can see excessive loads on the front stem/bearings and seat post/seat post recepticle. As I think about the bike shifting about while traveling suspended by the stem and seat post in the loose adapter bar, I'm not sure if that is actually better...


I did think about the loads applied to the headset and seat tube/seatpost and they will be nowhere near as high as that applied by someone riding on single track. When riding the bike is dealing with your full body weight. On the rack it's only the weight of the bike involved.

With regard to shifting around... I always attach the first bike to the vertical section of the rack (to which I've attached foam padding) with a spare toe strap. I do this even when not using an extension bar. When carrying two or more bikes, they are all connected at the cranks/pedals with straps to prevent movement and damage. Not worried about scratching the cranks, they're already battle scared from rocks etc. Note, my second bike doesn't need an extension bar, if it did I might need to rethink the movement issue.

I quite enjoy belting down dirt and gravel roads in the car and despite the corrugations and pot holes the bikes barely move. They get a better ride than my dashboard which tends to flop around a bit!


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## Lambdamaster (Nov 5, 2008)

skyliner1004 said:


> i was told there wouldn't be a problem clamping a CF tube for long periods of time, if the clamp pressure is good
> 
> i mean look at a carbon seatpost. that thing is clamped 24/7 and it doesn't give in; if its clamped with good pressure. i've hung my bike off the seatpost for 36 hours on my Park bike stand with no problems.


carbon seat posts are engineered for that. carbon frame tubes are not.
carbon is extremely brittle, and surface imperfections and abrasions are catalysts for cracking


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## skyliner1004 (Jan 7, 2010)

Lambdamaster said:


> carbon seat posts are engineered for that. carbon frame tubes are not.
> carbon is extremely brittle, and surface imperfections and abrasions are catalysts for cracking


really, so if a carbon bike were to... fall over on the trails or crash, that $4000 bike would need to be replaced at cost to the customer because crashes are not covered under the warranty?


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## Lambdamaster (Nov 5, 2008)

skyliner1004 said:


> really, so if a carbon bike were to... fall over on the trails or crash, that $4000 bike would need to be replaced at cost to the customer because crashes are not covered under the warranty?


I don't quite understand what you are trying to say here.. most manufacturers don't cover crashes under their warranty. Some have crash-replacement programs. I don't see why a company would pay to replace a product broken by the consumer's own fault.

just because it is a carbon tube does not mean it has the same properties. material choice is hardly the only determining factor.


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## skyliner1004 (Jan 7, 2010)

Lambdamaster said:


> I don't quite understand what you are trying to say here.. most manufacturers don't cover crashes under their warranty. Some have crash-replacement programs. I don't see why a company would pay to replace a product broken by the consumer's own fault.
> 
> just because it is a carbon tube does not mean it has the same properties. material choice is hardly the only determining factor.


you're saying abrasions/surface imperfections are catalysts for cracking. what happens when a rider goes for a crash? a surface abrasion.

so if you combine those, 1 crash = surface abrasion = frame crack. something doesn't fit right here, since mtbers fall all the time. not all of them pay $4000+ for a new bike everytime they fall


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## Lambdamaster (Nov 5, 2008)

skyliner1004 said:


> you're saying abrasions/surface imperfections are catalysts for cracking. what happens when a rider goes for a crash? a surface abrasion.
> 
> so if you combine those, 1 crash = surface abrasion = frame crack. something doesn't fit right here, since mtbers fall all the time. not all of them pay $4000+ for a new bike everytime they fall


Just because abrasions lead to cracks does not mean every scratch is going to cause a crack. Perhaps you need to brush up on the definition of catalyst?? Saying abrasions are a catalyst for cracking does not infer causality.


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## snovvman (Jan 4, 2010)

siwilliams said:


> I did think about the loads applied to the headset and seat tube/seatpost and they will be nowhere near as high as that applied by someone riding on single track. When riding the bike is dealing with your full body weight. On the rack it's only the weight of the bike involved.
> 
> With regard to shifting around... I always attach the first bike to the vertical section of the rack (to which I've attached foam padding) with a spare toe strap. I do this even when not using an extension bar. When carrying two or more bikes, they are all connected at the cranks/pedals with straps to prevent movement and damage. Not worried about scratching the cranks, they're already battle scared from rocks etc. Note, my second bike doesn't need an extension bar, if it did I might need to rethink the movement issue.


Thanks. You've given me something to think about. I always thought that the top-tube would be stronger and better to put the carrying load on it rather than using an adapter. In my mind, an adapter would put the load of the bike (and the up/down motion while in transit) on the two 5nm bolts/friction fit of the front stem and one bolt on the rear seat stem collar.

With my older, cheaper bikes, the tube is welded and appears much more reliable than those friction-based points secured with realatively small fasteners tightened to relatively low torque.

I know the adapters work in practice. Nevertheless, what do you think?


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## siwilliams (Jul 23, 2009)

snovvman said:


> Thanks. You've given me something to think about. I always thought that the top-tube would be stronger and better to put the carrying load on it rather than using an adapter. In my mind, an adapter would put the load of the bike (and the up/down motion while in transit) on the two 5nm bolts/friction fit of the front stem and one bolt on the rear seat stem collar.
> 
> With my older, cheaper bikes, the tube is welded and appears much more reliable than those friction-based points secured with realatively small fasteners tightened to relatively low torque.
> 
> I know the adapters work in practice. Nevertheless, what do you think?


Mate, if your top tube is up to it I'd use that first. The adapter only comes into play with light weight top tubes or top tubes with awkward shapes.

I hear what your saying about the bolts and it's definitely another opportunity for things to go wrong if they're not tight enough. The thing is that you need to check those (and other bolts) regularly anyway if you don't want a nasty accident on the trail. A crash from seat or stem failure could both be pretty horrible. The strength of both items, when properly torqued up, is more than up to the rigors of bike racks.


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