hisey'shomework

K, so we have this insane teacher (Mr. Hisey) we have to work on researching skills and get the answer to certain stuff from a real person rather than just a website. so here it goes...Question: How do the frame and wheels of a bike keep it from collapsing under the weight of a person? And do bikes always having triangles in the frame have anything to do with it. If anyone knows PLEASE help me. I need this for tomorrow, so I need the answer tonight. THANK YOU SO MUCH!!! -herBert

Paul L.

Well, not all bikes have triangle frames. My bike has a Stick style frame and it is a recumbent.




The frame of this particular bike has creates a platform between the centers of the two wheels that is stiff enough to support the weight of a rider.

Keith99

Yes.

Though bikes do not always have triangles, look at the so called womens bikes. To understand look take some sticks and lay them on the ground to make a triangle. To change the shape you would actually have to break one of the sticks. Now take 4 and make a quadralateral. You can change the shape without breaking a stick! Same for bikes and other structures. The triangle can not get bent, other figures with more sides can.

hisey'shomework

thank you so much. If anyone can elaborate a little more as well that'd be beyond awesome! I need as much detail as I can get. Anything about recumbent frames or triangular frames. Doesn't matter. Just how they support the weight. OH! And I think we're supposed answer like if how the wheels are thin w/long spokes (I think that's what their called?) helps any as well. AHHH THANKS! -herBert

Retro Grouch 

If it was me, I'd confine my answer to wire bicycle wheels. Explain to Mr. Hisey how all of those skinny little spokes, because they're in tension, can hold the hub in the center of the rim. Spokes that worked in compression to hold the wheel together would have to be much fatter.

FarHorizon

the bike "hangs" from the spokes that attach to the top of the wheel. The wheel axles are the transfer point of the bike's weight to the wheels. The "triangles" of the frame are just to provide support for the seat, the steering tube (with the fork), and the rear "chain-stays." In fact, the seat stays are in compression, the chain stays are in tension, the seat tube is in compression, the top tube is in compression, and the down tube is in tension.

badkarma

From the standpoint of a frame, there are HIGHLY educated people paid a lot of money to develop these frames - it all comes down to load distribution, hence frame geometry. The frames used are hollow tubes made from many materials, steel, aluminum, carbon, titanium, etc.

The wall thickness for most of the tubes on a bike is on the order of a fraction of a millimeter. There are various failure modes that designers consider, buckling loads (a concern for the seat tube), lateral deflection, etc. These designers use FEA (Finite Element Analysis) programs to aid them in ensuring the frame will withstand a variety of loads.
I wish I could sit here and explain it all to you in a forum, but this is a very complex topic, as engineers are paid a lot of money to be able to develop the lightest and strongest frames.

jim-bob

Technically, I think the OP is supposed to talk to a real live human being, not just post to a webforum.

Anyway, the answer is 'magic'.

ETQC

The frame works like a bridge and it's trusses would, each node transfering the weight and forces into tension and compression trough the tubes. Most of the weight sits on the bottom bracket, so the downtube and chainstays work in tension, while the seatube, seatstays and top tube work in compression.

jur

According to Jobst Brandt, this is false.

KingTermite 

If you need the answer so quickly (e.g. you waited until the last minute), then it seems your teachers efforts on improving your research skills didn't work so well.

FarHorizon

Since I'm sure that Mr. Brandt knows more about bicycle wheels than I, what is his explanation? I've read his book, I'm pretty sure, but don't remember his disputation of this...

jph6t

https://www.sheldonbrown.com/

FarHorizon

Yes, I've read Sheldon, and I don't see anywhere that he says the bike doesn't hang from the spokes either.

supcom

Ha ha ha. Kid gets an assignment to go talk to a human to learn something. He waits until that night and hasn't thought to call or visit an LBS to ask one of the wrenches the question so he comes here to ask a 'real person'.

I'd like to be in that kids class when he announces that he got his information from people named BadKarma and FarHorizon.

Since the point of the exercise was not so much to learn about how a bike works as it was to figure out who to ask to get that information, I'd say he failed. Not that there isn't good info here. The problem is that the kid has no way of determining who, if anyone, knows what they're talking about.

FarHorizon

Just so. "Caveat Emptor" should be the motto of all "open" forums such as this. At least on e-Bay or e-Pinions, a feedback system exists to rate the posters. Here, any idiot (mea culpa!) can argue with even Sheldon Brown, and a noob has no way to know who's the idiot!

cooker

My kid was assigned a similar task this week on Monday, due Tuesday, so we can't assume this kid left it to the last minute. I think his research strategy is pretty good. He was supposed to ask a real person, not use a website. He found a forum full of people interested in bikes and asked a group of us for the information. If one person gives him poor information, for certain someone else will correct it so he can be reasonably assured of valid answers.

Robert

RabidCyclist

K..so like..what part of not going to a web site didn't Mr. Hisey make clear? BikeForums.net is a web site. I guess insane teachers are hard to understand.

Cromulent

Your name wouldn't be Mr. Hisey, would it?

cooker

A. he's probably not insane

B. As someone else already pointed out, the triangular frame of a bike is much more rigid that a square frame would be. You can deform a square into a parallelogram, but you can't as easily deform a triangle into a different shape. Some bikes don't have triangles but their frames have to be made of stronger or thicker material than a standard bike

C The high pressure inflated tires spread the weight of the bike out around the wheel rim so no one part of the rim gets bent or crushed.

D The spokes share the load of holding the bike and rider's weight, and they are adjusted to a certain tension so that, like the tires, they spread the weight around the rims, so even though they are individually very thin, together they can support the rider's weight. Also, because they attach to both sides of the axle and are simultaneously pulling the rim to one side and the other, they make the wheels stronger and more resistant to buckling or folding.

I will ask others to chime in and agree or disagree so you won't just have to take my word for it

Robert

jur

I have seen only one vague remark of his about this. I don't have the bookmark here with me now, but it does not give any explanation either, it just says that the claim that bicycles are kept upright by gyro forces is as false and oft-repeated as the one that the hub hangs from the top spokes.

Interesting that it is not mentioned in his book. I'll see if I can get a response on the alt-bic-tech forum.

Nicodemus

I happen to sympathise with most other opinions here, but Robert - you da man. Although this is an internet site and therefore a serious fudging of the instructions, I think they were meant more against going to technical information and reference websites.

In any case, I would bet my money that I'll get more reliable information from a bunch of diverse opinions from which I can think for myself and decide which ones are stupid, or just go to the local LBS and rely on the opinion of one or a few people, who could well turn out to be drooling vegetables anyway.

I give the benefit of the doubt and let the OP decide if what he/she's doing is right.

I also like your explanation, Robert. This makes the most sense so far.

Nicodemus

kid's probably already gone, why am I still posting?

cooker

Thanks