Carbon forks on a hard core tourer ?

[spoon boy]

I saw recently on ebay a bike that I would call a hard core tourer and it had carbon forks on it..first time I'd seen such a fit

Now many of the hardcore touring bikes stress and stress again that steel is the only choice as one, the steel can be flexed to repair and steel can be repaired almost anywhere in the world so You can imagine then why I was puzzled when this bike from a world renown hard core touring bike company had carbon forks with steel inserts for a front rack

anyway I truly understood the leightweight issue certianly but the strength and application?

I would value any inputs/opinions for future use

[stutho]

I think these are the forks your refer to

http://www.sjscycles.co.uk/product-Thorn-Thorn-26-Inch--559-1-1-8-Inch-Carbon-Blade-Fork-with-Alloy-Steerer--to-fit-MKI-Thorn-Raven-Tour-and-MKI-Sport-Tour-10735.htm

I memory serves (and it is dodge at best) these were a cost option on the mk1 RT & mk1 RST. I think the forks were aimed more at the Audax rider than the tourer.  Where the reputed extra comfort of carbon and the lighter weight they may well of being of use.

I think in the order form it used to say about a restricted load on the forks - but again this is from memory so don't quote me.

On a touring bike, being used as a touring bike,  I don't believe there is any place for carbon forks. This is my personal opinion. 
Carbon can break without warning especially if loaded in an unexpected direction.  Touring bike tend to have a much rougher life than a normal road bike - even mountain bikes tend not to have to put up with baggage handlers!

Stutho

 

[spoon boy]

Certianly Stutho I agree or at least I currently agree

the bike I mentioned could have been any bike and as such carbon forks on a road bike would seem to make a bit of sense but not a lot. You will lose more pounds in cash than in weight saving and to be honest if you want to save a few pounds in weight try laying off the friday vindaloo for a few weeks

I did notice it was an option on some of the thorns and avaliable for other bikes with the thorn being recognised as a hard core tourer I was surpised to see this option

What did puzzle me was they are offered as an upgrade for a hard core touring bike and they have the steel inserts in them to fix racks I genuinely thought this was a contradiction but not claiming to know everything I thought it worth putting out there

Saying that, I know a chap who bought a pace and has never ridden it and also has a whyte prst1 and never ridden that..they hang on his wall so not all hard core tourers will be bought to world or continental tour. Also not all tourers will have front racks or front luggage so in those instances perhaps carbon is a personal non functional thing?

[middlesprocket]

Its the steel inserts and their tensile strength that interests me, along with the capacity of the bonding process to resist the tensions placed upon it even under a light load

[spoon boy]

I'm sure they were engineer stress loaded and tested but the thought in my mind is one of hanging a heavy weight against a plasterboard wall with plasterboard rawplugs..I'd always be waiting for them to fall out

[stutho]

spoon boy,
>steel inserts in them to fix racks I genuinely thought this was a contradiction
Personally I agree.  But looking at it from SJSC point of view (And no I don't work for them) You could say that it would be a  contradiction for them to make a fork that fitted a tourer and not include a rack mount!  By adding it at least it gave the user the option of using it.

[back to my opinion]
What would worry me most is the kind of load placed on the forks if the rack / pannier was to catch on anything and the torsion (twisting force) this would generate.  I am not a expert on carbon but I think that this could be a big no no.  (carbon experts please correct me here)  

[middlesprocket]

spoon boy,
>steel inserts in them to fix racks I genuinely thought this was a contradiction
Personally I agree. but looking at it from SJSC point of view (And no I don't work for them) You could say that it would be a contradiction for them to make a fork that fitted a tourer and not include a rack mount! by adding it at least it gave the user the option of using it.

[back to my opinion]
What would worry me most is the kind of load placed on the forks if the rack / pannier was to catch on anything and the torsion (twisting force) this would generate.  I am not a expert on carbon but I think that this could be a big no no.  (carbon experts please correct me here)  

It probably is a contradiction without a rack mount but this has been bypassed with the use of p clips to mount low loaders to forks without eyelets for sometime, and the torsion (twisting force) is exactly what I have in mind as to a barrier to using these for light loads.

Is it possible to get some info from those directly involved in the thorn design dept

[expr]

I have a cycle with carbon fiber rear seat stays, which was made by a leading cycle maker, the pr#urchase of this cycle took much thought as it was a xc bike and at the time I didn't know very much about carbon. I took it upon myself to find out about carbon and what it's uses and limitations are. I went into it in quite a depth and with that knowledge I will try to pass on that information.

Carbon is its strongest when a tensile stress is applied to it by the very nature of the material, but it can be manipulated in different ways to react to different stresses ie torsion / sheer and compression. The neat strand that you see on the top layer just under the lacquer is only for aesthetic purposes and serves no purpose at all, the clever weaving of the material underneath is where its strength comes from. By using different methods of weaving and alternating the lamination's different types of characteristics can be achieved.

There is a cyclic yield point with carbon as well which means it reacts similar to steel in it's longevity as a material.

Its poor points are that when its bonded to differing materials for connection to the bike ie if it were a set of carbon forks and the steer tube was steel bonded to the fork crown then this creates issues with regards to differential thermal coefficients, because the two materials expand and contract at differing rates then the point of which they were joined can become parted, If you go for a fork or complete bike then it's much better to have a solid peice of carbon then it becomes as strong as it was intended to be.

Carbon has a nasty habit of breaking with exceeded stress, and can't be repaied easy if at all. Its primary intention was to take alot of the road shock out of racing bikes and i beleive that's were its best kept.

It can look nice but in my opinion there are many downsides to the material than up.

The inserts that are boded in around caliper bosses etc suffer the same expansion problems as the in different material bonding process .

If it were me I would only use steel for it's strength, repairability  dependability etc etc 

[spoon boy]

true the tempreratures of the metal inserts and carbon forks will be different and as such will react differently and I wonder if they will react differently enough to cause issues

My feeling is if you are going to have a dedicated hard core tourer then avoid carbon forks

If however you intened not to use the front forks for weight carrying or front racks then it's a personal choice

I think ?

[geocycle]

I'd not use carbon for forks for touring.  This discussion on CTC forum is cautionary: http://forum.ctc.org.uk/viewtopic.php?f=5&t=24386

[expr]

Yes there have been cases where the detachment of the two halfs have happened for that exact reason, thats why if you do have carbon forks even on a road bike then you are much better suited to getting a full carbon fork as appose to bonded to a steerer tube but then that has its issues with regards to the fitting of bearings and the forces that they impose on the carbon.

[middlesprocket]

Excellent post Expr, I can understand fully now the negatives against using them for touring and the use of racks for the reasons pointed out, but the amount of  ''stresses ie torsion / sheer and compression.'' that a rack would produce would not be as much as that exerted by the brake caliper bosses would they?

Sorry, Geocycle an informative link, thank you,

[expr]

Absolutely right middlesprocket, the callipers will give the most strain around that area by far and the carbon content therefore is beefed up for that reason, and if its designed right by the manufacturer they  will endure such strain for considerable time, however as I say the main concern I think as far as touring and / carbon especially anything frame related could end in catastrophic failure through several known facts which is either forced beyond its limit of flex, detachment of bonded parts through expansion contraction or even poor bonding in the first place or if the carbon has taken an earlier knock then this will severely add to any counter damage that occurs in ort round that area.

Then the most important fact of all, you are in a remote area with little means of repair and a broken fork, who you gonna call , well it may as well be  ghost busters because your going to need plenty slime around that joint to make it work.

[middlesprocket]

Ok, bear with me on this.

Where I work I got into a discussion with a few metalwork dept technicians a while back on this subject and they suggested the same weakness as to the bonding as you did. However they also suggested if the steel thread mount was attached to a steel sleeve and the carbon fork was fabricated around this the sleeve then would disperse the load. So you would gain from the weight of the carbon and gain from the strength of the steel inner sleeve

[expr]

I would agree with that, by the vey nature of fitting it that way you would be shifting the load away from the area, again by using clever angles and smoothing the area away from the load then this would also help, similar to a brazed frame, if the braze is gently tapered away from the joint then you are encouraging the stress away also but if you finish almost on a butt joint then this is asking for trouble, some manufacturers of frame have tried to adopt the same tecnique instead of using an expanding mandrill to give a double butted / tripple butted frame they have tried variable taper technology to draw away the strain from the joint.