Carbon fibre engineer's advice sort after,,,

by petertaylor123   April 18, 2014  

Hi,
I have a technical question regarding my PX pro carbon.
I'm using it for an unsupported JoGLE trip (b&b, not camping) so am wanting a rack for it.

I understand all the arguments that this is an unsuitable bike etc, but I have ONE bike.
It's this one.
It is not a steel or Ti audax'er/tourer.
I can't get another one. So I have to make do.
I love my bike. I don't find the front end 'flexy' as others do. I want to do this ride on MY BIKE, not a hire bike.

It is a carbon frame, with alloy rear mech dropout and an ally seat post.
I am 64kg and plan to carry around 10kg (max) of kit in panniers.
So total weight is not a problem as I see it, when guys of +85kg ride the same bike.
Plus I will be using a 32 spoke Open Pro on a shimano hub on the rear wheel, so I am pretty sure I could carry an elephant from a weight perspective with the wheelset,,,,,

I asked a question of Planet X if a rack that attaches to the quick release skewer and a seat clamp would be ok.
(Such as the Bontrager Backrack Lightweight http://www.evanscycles.com/products/bontrager/back-rack-lightweight-bike... or Axiom Streamliner Road DLX etc...attaching to an M:part seat clamp http://www.ukbikestore.co.uk/product/407/spt102/mparts-seat-clamp-mount-... )...

According to a response from Planet X, any rack is UNsuitable, however it attaches.
But a seat post 'beam rack' would be ok. (Such as the Topeak MTX etc,,,,)

Can someone please explain to me why this would be the case?

I understand that you wouldn't want to 'compress' a carbon seat stay with 'p-clips', but with adapters that attach the rack to the QR skewer and either a seat post clamp or the actual aluminium seat post itself, there is surely no alternate stresses applied to the carbon fibre bits of the frame than in normal riding?
As far as my tiny mind can see, the weight that is on the rack, is being born by the wheel, hub and skewer, with minimal torsional forces applied to the aluminium seat post.
In any case, these forces would surely be greater on a rack that ONLY attached to the seat post, as with the beam rack?

If someone with more idea on carbon fibre frame construction than me can help understand why the beam rack is ok, but the QR and seat post rack is a 'NO-GO,' I would be most appreciative.

I had a look using the 'search' function, and also, Google, but I didn't find anything other than folks saying nothing should attach to a carbon bike etc,,,,

Anyway, if you have made it this far through my question, well done.
Thanks for reading/listening/helping,
Kind regards and happy cycling,
Pete

15 user comments

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Your reasoning seems sound. A rack that fixes to the seatpost and to the dropouts will be better than a beam rack that applys a torsional load through the seatpost alone. Also 10 kg is minimal and your seatpost and frame is designed to cope with a lot more than you will be carrying.

Planet x advice seems a bit numb. Those beam racks are not the best design from an engineering perspective. Having personally seen one swing around on a bike in front of me, I wouldn't trust them. But plenty do use them with no problem.

Although I can't vouch for the other type of rack either as I use a proper rack when I need one.

Oh and I'm an engineer by profession for what that counts.

posted by willerskine [4 posts]
18th April 2014 - 22:26

6 Likes

Why not think about something like the Revelate Designs Viscacha

https://www.revelatedesigns.com/index.cfm/store.catalog?CategoryID=4&Pro...

It took Mike Hall around the world, I think it'll do you for JoGLe

Gkam84's picture

posted by Gkam84 [8859 posts]
19th April 2014 - 0:13

4 Likes

carbon always explodes and everyone always dies instantly... lol just joking.

petertaylor123 I would say your logic is sound. I am doing my masters in motorsport engineering at oxford brookes [just sayin' Cool ] and I am planning on taking my full carbon specialized sl3 tarmac on tour this summer.

The 'rack supported by the quick release' jobby works fine. A couple of mates resorted to it when we rode to Rome and they held up fine. Why wouldn't it? lovely shimano XT 5mm diameter skewer just sucked it up, didn't moan once. Just make sure you do it up tight!

What I would say however, is if you start bombing through potholes with an open pro rim and 10kg on a rack, you might get a few surprises. It's high spec Aluminium alloy, but it's a narrow and shallow rim. A mavic A719 would be better as it's wider, or even better, something with a bit of depth to it. The stiffness of a beam is proportional to the cube of its depth, so even a 30mm deep rim will hold up a lot better than an open pro. Yes - a 50mm deep rim would be even better... yep, an 80mm deep rear rim, 25mm wide, with 36 spokes would be immense, and it might even fit in your frame! I've done a spot of loaded touring (and loaded tandem touring) and it's always the rear wheel that starts popping spokes.

Also you kind of want to be running the biggest volume tyre you can fit in your frame, e.g the new gp4000s II 28mm. (£50 a pair from rosebikes) This will reduce your rolling resistance and decrease your chance of pinch punctures. A wider rim, i.e one seats the tyre beads further apart from each other gives the tyre a wider base and further increases the volume.

10kg on the rack is more aggressive to your wheels than an extra 10kg on your arse... basically because hitting a pothole is a dynamic impact, with a big fat bum getting in the way and reducing the peak contact patch force. Also getting out of the saddle over real nasty terrain really gives the wheels a breather. That 10kg on your rack however, is just going to sit there in waiting, ready to ping your spokes over the first big pothole and there ain't shit you can do about it! (maybe bunny hop your whole laden bike?)

This is my first post on a bike forum as they're usually a bit dull and naff, but your question particularly tickled me, so anyway good luck and let me know how it goes.

Gavin aka rainman onwheels

rainman onwheels's picture

posted by rainman onwheels [11 posts]
19th April 2014 - 1:18

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I have two friends who use sestpost/hub qr mounted racks on their PX procarbons and have done with no issue for over 18 months. One of them uses a bolt through hub rather than qr, only m4 bolts, not sure of the hub make though.

posted by sq225917 [18 posts]
19th April 2014 - 7:39

4 Likes

carradice also make a seatpost-mounted bag that might work for you:

http://www.carradice.co.uk/index.php?page_id=product&under=range&product...

these are definitely a better option than beam racks, might be a neater solution than a QR rack too if it's big enough to swallow the stuff you intent to carry

other option is a frame bag. Alpkit will custom make one to your frame dimensions.

http://www.alpkit.com/products/stingray

Dave Atkinson's picture

posted by Dave Atkinson [7381 posts]
19th April 2014 - 7:44

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Gkam84 wrote:
Why not think about something like the Revelate Designs Viscacha

https://www.revelatedesigns.com/index.cfm/store.catalog?CategoryID=4&Pro...

It took Mike Hall around the world, I think it'll do you for JoGLe

I decided to get one of these to attach to my CAAD8, in the end I got a similar one from Alpkit https://www.alpkit.com/products/koala

They're currently sold out but it might be worth asking when they'll have more. I also bought an airlock extra bag to attach to the handlebars to keep more stuff in. https://www.alpkit.com/products/airlok-xtra

posted by Topcat [20 posts]
20th April 2014 - 12:40

4 Likes

For inspiration, google for images of bikes used on either Paris-Brest-Paris, or our own London-Edinburgh-London. Plenty of riders use carbon. Google for "bikepacking" too. Bikepackers are experts at carrying stuff on bikes.

I agree with others here, that you should use a combination of saddle pack, bar bag, or even a frame bag. Apart from a weight advantage, they make the bike far more aero than panniers on a pannier rack and, since you have a carbon bike, I'm assuming that you're probably keen to consider not just weight but aerodynamics too.

If you're on a tight budget, or you just don't want to spend a load of cash for just a one-off event, replace your bottle cages with ones that mount behind the seaddle, cut two triangles of "coroplast" just slightly bigger than the main frame triangle, cut some small slots, secure with toe straps, and you've suddenly got 5-20 litres of enclosed luggage space (depending on tube diameters and the frame size). Incredibly aero, seriously light when compared to the alternatives, and good for printing on if you're raising cash for charity. Good luck with your ride.

"Hey..... Let's be visible out there."

Neil753's picture

posted by Neil753 [451 posts]
20th April 2014 - 12:46

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I was going to link to the alpkit bike packing range of kit as well.

Yes it probably wont hurt the bike in the short term using p clips but hard point attachments clamped to carbon does not make me feel warm and fuzzy, seat clamp attachment using maybe an M-Parts clamp with eyes might well work and the rack running off a QR mount sounds like an idea also.

posted by MKultra [226 posts]
20th April 2014 - 18:51

3 Likes

Neil753 wrote:
replace your bottle cages with ones that mount behind the seaddle, cut two triangles of "coroplast" just slightly bigger than the main frame triangle, cut some small slots, secure with toe straps, and you've suddenly got 5-20 litres of enclosed luggage space

That is a very clever idea! Chapeau, sir!

harman_mogul's picture

posted by harman_mogul [131 posts]
20th April 2014 - 19:10

3 Likes

Thanks for all of your input. Its been interesting and loads of different ideas thrown in,,,,
Thanks.
Good to know my reasoning isn't massively off the mark!!
Also nice to know this kind of set up has been used 'successfully' before.

I like the alpkit/revelate seat post-cum-saddle bag thingy.
But wont that raise the centre of gravity by a lot?
Combine that with a bar bag and will the bike not handle very waywardly?
Admittedly it is only a JoGLE trip, so I might be over-thinking this. Its not like I'm crit racing, but would quite like to minimise the falling off and the subsequent tarmac rash.

I had a bit of a test build with cardboard, using the frame triangle idea from Neil753.
I'm going to have a bit more of a play with it. Its a GREAT idea.
I need to figure out a way of stopping everything 'sloshing' around inside the area, and how to make it so at the beginning/end of the day it is quickly un/attached to the bike.
(Maybe a wide strips of velcro (hook and loop) similar to the alpkit frame bags?)

I think I'm still heading towards a skewer and seat clamp rack, but there are definitely some other options I need to investigate more before a decision is made.

I will let you know how I get on and post a pic of the final result (or a picture of the scattered bits and pieces of my bike's rear triangle!)
Thanks again guys.
Big help.

posted by petertaylor123 [12 posts]
20th April 2014 - 21:14

3 Likes

petertaylor123 wrote:
I had a bit of a test build with cardboard, using the frame triangle idea from Neil753.
I'm going to have a bit more of a play with it. Its a GREAT idea.
I need to figure out a way of stopping everything 'sloshing' around inside the area.

I recommend using a few plastic bags to solve the "sloshing" issue. Heavy stuff like tools at the bottom, progressively lighter stuff on top. Use different coloured plastic bags so you can find things more quickly during "pitstops" or when you're dog tired. Doing it this way reduces the bike's centre of gravity, making it feel more like a road bike and less like a touring machine. If you carry a couple of those supermarket "bags for life" (the plastic carrier bags not the hessian type) then you can easily put your stuff in those when you're away from the bike. Alternatively, sew a strap to one of those ultra light stuff sacks, and decant into that instead. That way, you can leave the coroplast panels in place for the duration of your trip.

"Hey..... Let's be visible out there."

Neil753's picture

posted by Neil753 [451 posts]
20th April 2014 - 22:22

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only thing I would say, after seeing it happen
once, is the extra load "can" push the wheel out
of the dropout ... true this was only once and
easily fixed but the seed of doubt had been
planted !!!!!

still on the 3rd switch-back of Bwlch !

posted by therevokid [713 posts]
21st April 2014 - 9:29

3 Likes

I used a QR mounted rack (Toba Randy) with a Giant seat collar with rack mount and 2 large axiom typhoon Aero DLX panniers to cycle for two weeks all over Nova Scotia including the Cabot trail on my Giant TCR carbon with no problems. I had the bags loaded to about a total of 25lbs and rode 3-4 day unsupported loops staying at B&B's so I didn't camp but had about 5-6 lbs of tools and parts in case of breakdown. It was amazing how well the bike handled with all that weight on it, and I had lots of room for everything. No damage to the frame whatsoever

John

posted by drbracken [1 posts]
21st April 2014 - 18:24

3 Likes

Yes, used a QR and Brake bolt mounted Axiom 3 point rack on a carbon framed racer no problems for light loads. But do take the advice about a higher volume back tyre, a 23 on on an open pro is pushing it, and changing tubes with one of these racks is quite a lot more painful Smile

Also the Audax fraternity use Carradice campers and nelsons with the Expedition rack year in year out without any problems. The weight being carried high on the frame is less bother than the wind resistance caused by frame bags or large panniers. You can also reach the pockets while riding.

And a bar bag, even a tiddly one, will help a lot. Put your camera, wallet, a few hours of food and extra clothing bits in it and you'll save yourself a lot of mucking about. Practice getting your camera out and taking photos without getting off your bike and you'll save HOURS on your jogle.

alotronic's picture

posted by alotronic [267 posts]
22nd April 2014 - 10:55

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Ok,,, thought I would post a quick update,,,,

I am now 6days into an 11 day JOGLE on a carbon bike, with a bontrager backrack lightweight and a drybag strapped to the top.
The bag weighs around 5kgs, as we are credit card touring, so no tent etc.

I have met a few potholes at quite a pace sometimes, and the bike/rack/bag/wheel have not broken/splintered/suffered catastrophic failure. Yet.
I know I am tempting fate, but this set up does seem to work pretty efficiently for this use.
My friend is using a similar rack, the Axiom Streamliner, which is also working well.

We went from Pooley Bridge to Wigan over the Kirkstone Pass and along the A6 today.
Tomorrow is Wigan to Ludlow along the A49 for most of the day.
Then onto Bristol via Wye Valley.
Then Cheddar Gorge to Chulmleigh, Newquay and finally LE and Penzance.
If you see us give us a wave!

posted by petertaylor123 [12 posts]
24th August 2014 - 23:31

4 Likes