Maximum Weight on a Tourer
- Thread starter HormyAJP
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ch3
New Member
- Location
- London/Athens
On a similar topic, is there any chance at all that a carbon fork may fail because of heavy load, let's say while going downhill and hitting a hole or something? Anyone had a bad experience with carbon parts breaking?
I notice there was a warning on the inside of my fork saying not designed for off road use. I am not planning to ride it off road, but I am planning to go touring with loads of stuff on it and I can't help my self thinking about this video every time I hit a bump going downhill.
View: http://www.youtube.com/watch?v=JLRDRzMWIsg
I know he was pushing the limits, but I bet this bike was probable 10 times more expensive than mine, plus I doubt it had a carbon fork.
I notice there was a warning on the inside of my fork saying not designed for off road use. I am not planning to ride it off road, but I am planning to go touring with loads of stuff on it and I can't help my self thinking about this video every time I hit a bump going downhill.
View: http://www.youtube.com/watch?v=JLRDRzMWIsg
I know he was pushing the limits, but I bet this bike was probable 10 times more expensive than mine, plus I doubt it had a carbon fork.
- Location
- The TerrorVortex
I can vouch that a mountain bike will take 6 cases of French lager, and 6 bottles of cotes du rhone.
It makes the cornering very strange though...
It makes the cornering very strange though...
byegad
Legendary Member
- Location
- NE England
Given I'm 95kg and routinely carry 10kg of stuff you have nothing to worry about except generating the power to climb hills.
A Brompton will carry 50 cans of beer on the back rack. And teenagers will consume said cans in three hours.
In all seriousness I'd ask Madison. Frames do fail, although they are more likely to fail as a result of leaving too little seat post in the frame, splitting the seat tube.
In all seriousness I'd ask Madison. Frames do fail, although they are more likely to fail as a result of leaving too little seat post in the frame, splitting the seat tube.
bike_the_planet
New Member
- Location
- Perth, Western Australia
ch3 said:
Yes.
Carbon has a very high UTS (Ultimate Tensile Strength). But that's measured in tension (ie pulling).
However, unlike steel or aluminium, carbon's strength is not isotropic (ie the same in all directions).
In compression, carbon is pretty weak. Essentially, it is the epoxy that takes most of the compression force.
If you hit a bump at excessive speed then the rear face of the fork blades get heavily compressed and this is the common point of failure. Personally, I've never understood why manufacturers don't really beef up the blades at this point.
In addition, carbon's failure mode is essentially catastrophic. Simply meaning that its yield point (point at which it deforms) is similar to its failure point (point at which it snaps). Steel, for example, will bend well before it snaps.
Carbon forks work great on a lightweight road bike and a smooth road. But I would avoid putting them on a loaded tourer, and then riding through bumps and potholes if I were you.
Cheers,
asterix
Comrade Member
- Location
- Limoges or York
This is a proper 531 steel tourer with about 23kg of load including tent but not counting food and fluids. The bike weighs c.12.5kg.
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It's ten years old and has had several modifications and a new top-tube after I foolishly dropped it against a sheffield stand and dented it. Unfortunately the damage was just before a 2 week tour and I didn't have time to get it fixed, but it was fine. The tyres are 26x1.75 slicks and carry such loads admirably; the descent down the west side of the col was one of the best I have had on any bike. (It rides best with all the stuff on the back although I do have front panniers)
It's ten years old and has had several modifications and a new top-tube after I foolishly dropped it against a sheffield stand and dented it. Unfortunately the damage was just before a 2 week tour and I didn't have time to get it fixed, but it was fine. The tyres are 26x1.75 slicks and carry such loads admirably; the descent down the west side of the col was one of the best I have had on any bike. (It rides best with all the stuff on the back although I do have front panniers)
Tim Bennet.
Entirely Average Member
- Location
- S of Kendal
I think you're getting it confused with kevlar, (see table below).
Simply not true.
There is no common point of failure. Carbon fibre forks are extremely reliable. So much so that they are commonly used on bikes! If manufacturers had any worries about their reliability they would not choose to use carbon fibre for this, the most highly loaded part of a bike. All forks of any material can and do fail. Where is the evidence that carbon forks are less reliable or suffer 'common failures' ?
Carbon fibre forks are not used for touring bikes because the weight saving is not an issue on a loaded touring bike and the inherent conservatism of touring cyclists. But if there was a demand, engineering a touring fork in carbon would be no problem.
Tim Bennet.
Entirely Average Member
- Location
- S of Kendal
How common? Do you have a percentage figure of all carbon forks made? How does this compare to steel fork failures?
I'm not sure about the 'how much you can carry up stairs' rule as you can always take the panniers off and do the lift in two stages (or alternatively, et/drink the contents of the panniers). But 'how much you can drag up hill' would be a good rule of thumb. Unless you are really really heavy or your bike is really really weedy (OK hands up who isn't an engineer) then you're it's much more likely that you'll give in long before your frame does.
Tim Bennet. said:
This is concentrated stress area, and for Al steerer forks, the point of joint stress too. For full carbons, there are failures at the top of the tube, generally due to excessive stack height/poor clamp adjustment/poor internal shimming/parts (wrong nut) etc.
Can't imagine how anyone could collect figures for all carbon forks ever made.
Don't know anything about steel fork failure. Personally, I've never seen one fail.
bike_the_planet
New Member
- Location
- Perth, Western Australia
Tim Bennet. said:
Carbon fibre is based on a weave of carbon filament threads of around 0.5 microns in diameter. Tensile strengths are up around 5GPa in the axis of the thread. However, they are relatively soft and brittle.
The figures you have given are for the fabric weave itself, not the composite carbon structure.
Ever seen it? My local frame repairer has a reel of the stuff. In tension it is incredibly strong. But you can but a piece between two fingernails and squash it. How can a weave based on string fibres be strong in compression? It's the resin body that gives the tube structure its strength
What gives the material tensile strength in particular directions is the weave orientation. That's why the material is non-isotropic in strength. And that's what makes it so useful in structures in industries such as aerospace. You can engineer the strength in whatever direction you want. In impact compression, it is not particularly strong and it's simply not used in that way.
Why are so many carbon parts labelled with maximum torque settings???
Tim Bennet. said:
I don't believe I said that they were less reliable, did I??? What I did say was that they don't bend when they are overloaded. They snap. Steel and aluminium are very forgiving in that their yield point is well below their failure point. You get warning.
My local frame repairer has written several reports and submitted them to manufacturers with regard to carbon forks damaged in impacts with road debris/potholes. In every one, they snapped at the back of the fork where there was the greatest compression. This is an area where manufacturers could improve. Instead of making the fork blades of uniform thickness, they could be beefed up at the back. This might cause the fork to fold rather than snap, a bit more like steel or aluminium.
Tim Bennet. said:
For someone who wants the lightest bike, and is willing to look after it and regularly visually inspect it, carbon fibre is the best choice.
But the average tourist doesn't want to do that. They want a bike they can load up. That might get knocked, bashed, chipped, abused by baggage handlers etc etc. Carbon is not a resilient material when it comes to impact strength.
My local frame builder has given up building frames - instead he makes a handsome living repairing carbon fibre bikes damaged due to either a) accidents or
compression failures where over-zealous mechanics have over tightened seat clamps, headsets etc etc.
Tim Bennet.
Entirely Average Member
- Location
- S of Kendal
Chapter 1 in any Composites handbook will tell you all you need to know.
Really? So the crash cell in a Formula One car isn't made of carbon fibre because it can stand the repeated impacts of flying through the air and tumbling along the track into the armco barrier at nearly 200mph.
Or look at this Youtube clip.
View: http://www.youtube.com/watch?v=__PBiSqFBVY
Seems the carbonfibre passenger cell was pretty resilient to impact damage. The driver walked away.
Repeating popular misconceptions about carbonfibre will never make them fact. Carbonfibre components may fail and their mechanism of failure may not appeal to everyone, but please apportion blame where blame lays. The design, manufacture, installation and maintenance of bike components have at times, all been found wanting, but carbon fibre is just the material. And it's a fantastic material with properties that make it superior to everything else. When we've had as much experience of using and developing CF as we have with steel bike tubing (100 years plus), I'm sure its implementation will be as reliable.
