[elcee]

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.

[elcee]

@mstone 10739 wrote:

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

[elcee]

@CCrew 10724 wrote:

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.

[elcee]

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.

[elcee]

I just biked through this area and Arlington must have added arrows and extra striping since the first post. I really like the new design, particularly the following:

– On westbound Lorcom Lane there are now 3 distinct lanes as you approach Nelly Custis. There’s a straight ahead car lane, a straight ahead bike lane, and a right turn lane. If you’re biking and want to go straight, then there’s no ambiguity, for either cyclists or drivers.

– On eastbound Lorcom Lane approaching Fillmore, there’s a dedicated left turn lane and a straight/right turn lane. Bikes and cars will have to share the latter; I think the lane is wide enough, but others may be more comfortable with sharrows. In the old design, cars turning left onto N. Fillmore would block cars trying to go straight, and people further back in the queue would simply pull out onto the right-most lane.

P.S. The restriping reminds me of Military Road when it was converted from 2 car lanes to 1 car lane plus a bike lane several years ago. It took a while for people to get used to the new pattern, but I no longer see cars straying into the bike lanes.

[elcee]

Be very careful. Fifty lbs of lead and acid could make for a very bad day. Remember that if you drop the battery, you’ll probably shorten its life by a significant amount.

[elcee]

There’s a Bruegger’s Bagels in Ballston, on Quincy and Wilson, across from Conte’s/Freshbikes.

[elcee]

Cantilevers … what can I say? Too many degrees of freedom, meaning that it’s all too easy to mess things up.

It helps to have another set of hands to hold the brake arms or pads in position while you tighten the bolts.

Believe me, when you get those cantilevers adjusted right, you’ll feel such a warm glow of satisfaction that you’ll want to do it all over again.

[elcee]

@DismalScientist 9854 wrote:

Am I to interpret this as saying that I must anticipate potential bonehead maneuvers by drivers before entering the crosswalk even when the light indicates I have the right of way? Furthermore, these anticipated bonehead maneuvers can commence even after I entered the crosswalk? Ouch… Ride defensively everyone.

Actually, that’s my mantra when I drive as well.

[elcee]

If the forum can’t agree what proper behavior is at this intersection, what chance do ordinary drivers and cyclists have?

This is a real problem with roads where explicit (“Stop” sign, crosswalk) and implicit (courtesy stop) signals conflict. Same case as Lee/Lynn – what exactly does a “Stop” sign mean for pedestrians and cyclists when there’s a traffic signal as well? Adding more signs isn’t the answer. Maybe taking them away is: witness the experiments in Europe where some intersections have no signs or lights at all, and drivers, cyclists, and pedestrians all have to negotiate who goes when.

Can’t work in America? Just look at Manhattan, where cars routinely make right turns through crosswalks filled with pedestrians. Everyone manages to make it through. I don’t have statistics, but I’ll bet the accident rate is pretty low.

Of course, this depends on cars slowing down to a pace where everyone can make eye contact. It would probably require some significant road re-engineering and major driver education.

[elcee]

I have a new-in-box Campagnolo Veloce crankset, 172.5 arms and 34/50 chainrings, square taper. Let me know if you’re interested.

[elcee]

This may be overkill, but the first time you change a flat you’ll probably want to be in a place that’s well-lit, where you won’t be rushed, and where you won’t mind getting your hands, arms, and legs a little dirty.

If the tire is tight, you will spend a lot of time trying to muscle it off the rim. At some point it may be useful to step back, take a deep breath, grab a suitable beverage, maybe mutter some curses or prayers.

Don’t forget to look for the cause of the flat – road debris, pinched tube, torn valve, etc. – and then figure out whether you need to do anything about it.

I think 15-30 minutes for your first few flats is a reasonable expectation. With practice, you can change a tube in a couple of minutes – really!

[elcee]

It looks like Garmin is trying to tempt us with its new Edge 200. SRP of $149.99 and available now. This might just push me over the edge.

https://buy.garmin.com/shop/shop.do?cID=160&pID=90675

[elcee]

@pfunkallstar 9124 wrote:

When I was working at Spokes I remember seeing a guy come in with tri-bars on his lefty with bar ends. He liked to “keep all his options open.”

I would have pooh-poohed that solution until I watched “Ride the Divide.” All the competitors rode mountain bikes with both flat bars and tri-bars. They all needed to vary hand positions and stretch out the back over long distances.

Back to Riley’s original post – I too looked at alternate bars for my rigid mountain bike, as the flat bars made my wrists hurt after riding for an hour. I settled on Salsa Cowbell bars with a higher-rise stem, Cane Creek brake levers, and bar-end shifters. It’s a familiar position to this primarily-a-roadie, and I can ride on the hoods, the flats, or the drops without bending over too far.

[elcee]

Apparently the “no right turn on red” option has been studied, and would cause too many backups on I-66.

http://www.engagearlington.com/archive/2011/10/06/the-rosslyn-meeting.aspx

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