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JustinW
11-11-2011, 09:47 AM
Hi all,

Considering a fairly inexpensive used bike to add to the fleet. Primary use would be commuting. Looking at a 1996 Trek with a CF frame.

Are CF frame from that era considered worthwhile at this point? I am a panniers kind of guy, so I would likely use a rear rack that clamps onto the seat post. Would that create too much stress on the frame at the seat post area for an older Cf frame?

Any thoughts are welcome.

Thanks, Justin

Dirt
11-11-2011, 10:36 AM
I'd suggest having a pro look at it before building up the bike. Factor into the price having an experienced person checking out the frame to make sure that it is sound. I was a serious skeptic about carbon frames... especially in my size and with my weight. I got hooked on the ride and haven't looked back. Each year I send my Cervelo to Craig Calfee's shop in California (http://www.calfeedesign.com/) to have it checked out. I do that in the off season, when I'm riding other bikes. For less than $100, he checks it out and gives it a once over to see if it is in good shape for another year of riding. This year I purchased a 1998 Colnago C40 and had his team check it out before I rode.

His web site has been redone this summer and I don't see Frame Inspection offered as a service anymore. I dropped him a line to see if they still offer that service. He may have stopped. I'll let you know when I hear back from him.

The cool thing is that if there is a problem with the frame, Calfee is the shop to fix it. They've been building carbon frames since 1987 and know more than pretty much anyone. They can repair carbon frames and they've got a great track record of doing good work.

Hope that helps a little.

Pete

pfunkallstar
11-14-2011, 08:05 AM
I love carbon - light, flexible, and WAY more comfortable than aluminum for the under 160 lbs group. Like Dirt said, have someone competent check it out, but 96' was a perfectly fine year for Trek frames. The ones to avoid in my experience are the aluminum lug-carbon tube bikes that they were building around that time (I think they stopped in 95' - not sure). I've seen those things do some pretty crazy things - tubes popping out, joints flexing, etc. Something to take into account when examining any carbon frame is that surface scratches rarely impact the integrity of the frame. You have to get at least two or three layers down before you have to start worrying about lamination issues or direct failure.

americancyclo
11-14-2011, 08:57 AM
I am a panniers kind of guy, so I would likely use a rear rack that clamps onto the seat post. Would that create too much stress on the frame at the seat post area for an older Cf frame?

The mechanics at my shop always balked at installing a seat post rack on a CF bike. I think there are concerns about clamping pressure on the carbon. Th pressure you need to secure the rack might be more than should be applied to the seatpost.

I'm of the school of thought that you shouldn't augment your bike too much, unless it was designed for it. If there are rack eyelets, then you should get a rear rack. if there aren't, you should keep the weight on your body, or look for a bike that better suits you needs.

That being said, my bike is way too racy for my needs, but I enjoy it, and I carry a messenger bag for my stuff.

Dirt
11-14-2011, 01:30 PM
I still haven't heard back from Calfee yet. I'll let you know when I do. I actually have a frame to send them, so they should be getting back to me soon.

elcee
11-14-2011, 07:19 PM
While researching a completely different subject I came across this fascinating article about carbon forks:

http://velonews.competitor.com/2002/12/bikes-and-tech/technical-faq/technical-qa-with-lennard-zinn-carbon-forks-2_3270

Separate question: Justin, are you talking about clamping the rack to the seat post (presumably alloy) as opposed to the seat tube? If so, I'd expect the forces on the seat tube to be similar to that of the rider's weight.

CCrew
11-14-2011, 07:29 PM
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Separate question: Justin, are you talking about clamping the rack to the seat post (presumably alloy) as opposed to the seat tube? If so, I'd expect the forces on the seat tube to be similar to that of the rider's weight.

Err, not really, because a rack hanging off of the post is going to work more like a lever whereas the rider's weight is pretty much centered over the post.

elcee
11-14-2011, 09:15 PM
Err, not really, because a rack hanging off of the post is going to work more like a lever whereas the rider's weight is pretty much centered over the post.

Let me clarify. The seat tube is angled, not vertical. From the perspective of the seat tube, the rider's weight has two components: one parallel to the seat tube, and another at 90 degrees, trying to bend the seat tube backwards.

There are other forces as well: when a cyclist pushes down with the right foot, the whole bike tries to tip over to the right. There must be an opposite force applied (through the seat and handlebars) to counteract the pedaling force. So the seat tube ends up carrying loads in many directions.

A rack that clamps to the seat post exerts the same kinds of forces as a cyclist's weight on a saddle, however unintuitive this may be. Both are levers that exert forces parallel and perpendicular to the seat tube. Yes, the rack has a longer lever arm, but also less weight (hopefully!) than the cyclist. The contribution of rack vs. rider would vary depending upon where you measure, i.e. at the seat clamp vs. the bottom bracket.

That's what I meant by "similar forces." (By the way, I'm very open to counterarguments, which is what engineers do.) Obviously in the real world, Justin would not load 200 lbs of stuff (5 lead-acid car batteries?) onto his seat post rack! My quick back-of-the-envelope sense is that a reasonable rack + load wouldn't overload the seatpost because its forces would be similar to replacing Justin with a somewhat heavier rider.

JustinW
11-15-2011, 07:25 AM
"Obviously in the real world, Justin would not load 200 lbs of stuff (5 lead-acid car batteries?" -- unh, why not? Sounds fun, as long as you are going down hill... ;)

I think I am not pursuing the CF frame route due to rack concerns. This is the kind of rack I'm talking about (http://www.performancebike.com/bikes/Product_10052_10551_1023820_-1_1552500_20000_400104) -- works well on my current urban assault vehicle (one without other rack attach points on the frame) and so I'm thinking of doing the same on the next acquisition as well.

From what I've seen one does not use a CF seatpost with these, as the clamping force may make the seatpost unhappy. I suspect (as another forum person noted) that such a set up may put bad twisty forces on a CF frame (that a non-CF seatpost is stuck into).

Anyway, thanks all for the inputs!

mstone
11-15-2011, 08:23 AM
much of the rider's weight is on the pedals and arms, whereas the rack has a bunch of weight on a long lever that responds to every bump.

elcee
11-15-2011, 10:46 AM
much of the rider's weight is on the pedals and arms.

Much to my dismay, I could not find a freely-available article that discussed forces at the pedals, saddles, or handlebars.

For those of you who like to play with analytical tools, follow this link:

http://www.analyticcycling.com/ForcesPower_Page.html

I plugged in the following: bike+rider weight=90 kg (200 lbs), speed=7 m/s (about 15 mph), level surface, asphalt road, no wind. The biker needs to generate 77 W to sustain his speed, with an effective pedal force of 140 Newtons, which is about 32 lbs of force, exerted over 70 degrees of pedal rotation. (This is the net force needed to keep the bike moving, and is not the total force on the pedals, which would include the weight of your legs.) It's not a lot of force and it's what makes bikes so efficient.

It's a complex subject and I don't claim to have all the answers. Lots depends on whether the cyclist is seated and coasting, sitting upright on a Dutch bike or bent over on a racer, etc. All I wanted to point out is that the forces imposed by the rack and its load are in the same ballpark as those imposed by the rider's weight and pedaling action. That being said, I don't disagree at all with Justin's decision. I myself decided not to buy a carbon fiber bike out of concern for long-term durability (though I must say that it was really fun shopping for one).

P.S. As your old professor would say, there's optional reading: Keith Bontrager's essay on bike fitting. (There's a qualitative discussion about pedaling forces near Fig. 3.) Interesting article, overall.

http://sheldonbrown.com/kops.html

americancyclo
11-15-2011, 10:54 AM
What about the forces of loaded panniers (laptop, lunch, change of clothes, groceries?) on that rack, and the stress that accompanies riding over bumpy trails, tree roots, and potholes that we encounter in the regular commute? That's what would worry me the most, and I don't think are usually taken in to account in a standard 'performance oriented' overview of cycling forces and energy.

elcee
11-15-2011, 11:41 AM
An in-depth analysis would take a lot more time and knowledge than I have. What I did was a quick free body diagram of the forces on a seat post with some reasonable assumptions, e.g. seat post length around 6 inches, rack center of gravity about 6 inches away from the middle of the seat post, rack load about 1/10 of rider weight. That's the static analysis, and it didn't raise any danger flag for me. (Please tell me if I've gotten it wrong!)

Your question, a very valid one, concerns dynamic forces, and is far more complex. For a quick look at the kind of analysis involved, see this article:

http://www.sheldonbrown.com/rinard/fea.htm

The authors model steady-state pedalling, front and rear braking, and various impacts. Figures 5 and 6 are especially illustrative. Note that this article dates from 1986, and there have been tremendous strides in finite-element analysis and the understanding of materials properties, but you get the idea.

I guess my gut feel is that the rack would break far sooner than a carbon fiber seat tube, assuming the seat tube is in good condition. With a 1996-era bike, you may not be able to see whether the glue has failed. On the other hand, older CF bikes were overbuilt, with more CF layers than modern bikes.

P.S. I know this discussion is really straying from the original question. I assume Justin was looking at a seatpost rack because the bike lacks threaded eyelets on the dropouts. I believe there are racks that mount to the axles, like this:

http://www.axiomgear.com/products/gear/racks/rear-racks/streamliner-road-dlx/

I don't have any experience with these, so other members, please chime in.

JustinW
11-15-2011, 03:11 PM
http://www.axiomgear.com/products/ge...iner-road-dlx/ -- coolness. Anyone have experience with such a set up?

americancyclo
11-16-2011, 08:51 AM
http://www.axiomgear.com/products/ge...iner-road-dlx/ -- coolness. Anyone have experience with such a set up?

Not personally, but here's a few reviews from Amazon (http://www.amazon.com/Axiom-Streamliner-Road-DLX-Racks/product-reviews/B003UWESMQ/ref=dp_top_cm_cr_acr_txt/190-9352957-6472244?ie=UTF8&showViewpoints=1).

Dirt
11-17-2011, 01:27 PM
I heard back from Calfee. They've simplified their inspection service a bit. They now do a visual inspection only... it costs $50 + shipping. I still think that is a good thing to go through... Having the most experienced company with carbon frames look over a frame and fork for signs of damage or weakness is worth the money to me.