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Does this make sense to you?(19 posts)

Does this make sense to you?nee Spoke Wrench
Jan 22, 2002 4:45 PM
Garret Lei, in his current article in "Bicycling" regarding wheels states that the bottom spokes of a bicycle wheel hold up the hub. He says this is possible because the tension on the spoke exceeds the compression weight so the spoke stays straight insted of flopping over like spaghetti.

What he doesn't explain is how the rim can push on the spoke since it doesn't have anything to bear against in that direction.

I don't want to pick on Garret because I've heard this theory before from more than one source. It just never made any sense to me and still doesn't. Am I missing something?
what an idiot!supercorsa
Jan 22, 2002 8:27 PM
they must be givin' out free crack cocaine over there at bicycling. his idea re/ "compression weight" is really additional tension on the upper spokes. as you have correctly surmised, you can't push on a string. as well, if his theory was correct rim strength wouldn't be such a big deal, especially with low spoke count wheelsets.
While I agree with most of what you said.....sprockets
Jan 23, 2002 3:04 PM
I have had the experience of fooling around with a wheel that I was taking apart and I did get the bottom half of a wheel to support a hub. I don't think the hub would have supported much weight at all, but it physically does what the writer said.

The bottom line is as you stated: the lower spokes do not seem to be able to do much to hold up the bike. While you are not pushing a string, you are pushing on a rather flexible object.
I think so...guido
Jan 22, 2002 8:33 PM
I've heard this before and wondered myself. In other words:

Weight forces the hub downward, pulling the spokes above and unloading the spokes below. As the rim rolls around, it is only the tension in all the spokes that prevent the rim from flattening at the point of contact on the ground. At any given time, the one spoke right above this contact patch will be unloaded, while the others are being pulled by the rim trying to ovalize. That's if the spokes are snug, but not tensioned.

Tension the spokes, though, and none of the spokes really unload, because, yes, the pulling force of all the spokes overcomes the ovalizing effect of the load.

This pulling force is obvious if the wheel hits something sideways and the rim "pretzels." It has no ability to stay round and true unless all the spokes are pulling on the rim all the time. Once a few of the spokes are truly unloaded, that is, made slack, the rest of the spokes actually pull the rim so far out of true, it can't be repaired.
IncredibleLeisure
Jan 23, 2002 12:39 AM
I can't believe stuff like this gets printed in a national-level publication. Not only did someone go through and think about how to articulate this, but probably two or more editors downstream read over the thing and didn't see anything wrong?
What a lousy magazine ...pmf1
Jan 23, 2002 4:47 AM
I remember when Bicycling was actually a good cycling magazine. Lately, they've gottedn so desperate that they send it free to anyone who has a Team Performance membership. This is why it shows up in my mailbox.

Here's a good one --- in this issue, someone writes in saying that the advice to put armor-all on your tires (apparently given in an earlier issue) to make them look good is bad advice. Putting this stuff on your tires would simulate ridng on ice. I can't believe someone hasn't been hurt and sued them.
Bicycling is a great magazine ...allervite
Jan 23, 2002 9:19 AM
for the; what is a 700c tire, where can I buy an interval so I can get faster; who is that Zabel guy and what type of mountain bike does he race, cyclist.
Yeah, you're right ...pmf1
Jan 23, 2002 12:30 PM
They should offer it in 3 month subscriptions. That way people new to biking can read 3 issues as they gain the necessary knowledge. At that point, it becomes boring. Even a bike pervert like me (who can't speak Italian, but will pour over a BiciSport) finds not enough content in Bicycling to last one bowel movement.
I'm telling you, the last issue would last ... well, more thanbill
Jan 23, 2002 12:57 PM
that, anyway.
What magazine do you fellers like?
Velonewspmf1
Jan 23, 2002 1:34 PM
Its not always the most exciting, but I subscribe to it. It defintely lasts more than 1-2 movements. Its the only real cycling mag left in the U.S. and is therefore worth supporting. I personally do not like the Brit ones (e.g., Cycling Plus). There used to be a mag called Road Bike Review that was decent, but it went under and was morphed into some horrible magazine named Bike.

Bicycling used to be a decent magazine. You'd think they could rescue themselves. In this issue, they compare sports drinks. This could be useful. The problem is that the biggest section on the box is "ask the bartender" where someone says "this tastes great with a shot of xxxx vodka". Gimme a break.
yeah, they do shoot for cute more than common decency allows,bill
Jan 23, 2002 1:57 PM
but so do a lot of American mag's. Trying to keep it young and real and all that bloody rot.
I've picked up Velonews a couple of times. Pretty cool. Maybe I'll get a subscription, because I kind of need a newe fix. I'll probably not give up my Bicycling subscription just yet, as much for some sort of possibly misguided sentimental loyalty as anything. And, let's face it, it is the (ulp!) premier cycling magazine in the US right now.
Premier?pmf1
Jan 23, 2002 2:03 PM
Maybe the most widely read, but surely not premier. Is USA Today (the McPaper), the premier newspaper in the U.S.?

Get a Team Performance membership for $20, and they'll give it to you free.
free-body diagrams (nm)SamDC
Jan 23, 2002 8:14 AM
Physics Student???biknben
Jan 23, 2002 9:31 AM
When studying Physics, my fellow students and I joked that a "Free Body Diagram" was the answer to all of life's questions. At least that what our professor led us to believe.
Not an Original Ideagrzy
Jan 23, 2002 9:41 AM
Jobst Brandt in his classic book "The Bicycle Wheel" makes the same arguement and probably spends much more time developing it - I haven't read the Bicycling mag article (nor is it likely that I will). The concept is that all the spokes are in roughly equal tension w/out and load on the wheel. Once the wheel is loaded the spokes above the rim don't increase in tension, instead the tension on the lower spokes is decreased since the rim deforms in this area. So it's a bit misleading to say that the lower spokes "hold the wheel" up, but it is a true statement. The trick is that the lower spokes don't really go into compression nor could they since they'd buckle due to their high slenderness ratio (ration between diameter and length). It's just that there is less tension in these spokes - so in a strict analysis (free body diagrams) the lower spokes are supporting the hub/bike/rider.

Even though Brandt is a Stanford trained mechanical engineer the book is quite readable and helps sort through the tremendous amount of bunk and junk science that surrounds bicycle wheels. Even if you never plan to build a wheel it's good info for the layman. Brandt's appendicies detail some of the finite element analysis results (FEA) for those that want to see the data.
yup/yep nmgtx
Jan 23, 2002 9:58 AM
This BS never ceases to amuse meCT1
Jan 24, 2002 6:47 AM
I still can't believe this guy graduated from Stanford. Oh, I weap for the future.

JohnG
here is an articleDog
Jan 23, 2002 10:10 AM
http://hea-www.harvard.edu/~fine/opinions/bikewheel.html

I agree. The hub hangs from the upper rim.

Doug
I remember going round and round (so to speak) on this somebill
Jan 23, 2002 10:47 AM
time ago. As a lowly liberal arts grad, my qualifications to discuss it are rather thin, but the inordinate amount of science I took, anyway (which completely trashed my GPA), corroborates my interest if not my credentials.
I think that this is an instance where lay semantics overtakes scientific analysis. Or vice versa. That is, I don't think that it is any more wrong to think of the the hub's being held up by the lower spokes than it is right to think of the hub's hanging from the upper spokes. Depending on your perspective, I think that they are both right, sort of.
Think of the wheel as a system. Systems tend to seek the lowest, balanced energy level, and excess energy in one area will drain off into a lower energy portion of the system if possible (if there is a method of communication or translation). Well, we know that gravity is going to drive the hub downward. What happens? The force of the hub reduces the tension on the lower spokes (which also are already pulling the hub down). The spokes directly opposite react by increasing their tension. The result is that the wheel wants to become squished into an oval. But, because there are spokes on the side, and the rim's (theoretical) ability to deflect provides a means of communicating the energy from spoke to spoke, the spokes on the side balance the system by holding the wheel in a circle.
So, the spokes all work together. You couldn't hang the hub from the top spokes any more than you could perch it on top of the bottom. If you had only top and bottom spokes, it wouldn't work either. You need all of them.