Spoke tension under load (hanging vs standing)

[TNT1]

 

http://mamarant.blogs.com/mamacooks/files/ham_pineapple_pizza.jpg

http://www.proteinpower.com/drmd_blog/wp-content/uploads/2007/09/hot-buttered-popcorn.jpg

 

TNT12008-12-11 01:51:11

[ZeroPlay]

...

 

By analogy:

 

Consider a bow (as in "bow and arrow") with a completely rigid frame and a bowstring under tension' date=' orientated as you would hold it when shooting.  If one pinches the string at some point along its length and pulls the string down (while the frame is held in place), is there only a decrease in tension of the bowstring below the point where you are pinching and no corresponding increase in tension above the point where you are pinching?

[/quote']

 

Is my analogy valid, or does a bowstring behave differently to a spoke?  Can someone answer the question I asked above?

 

Thanks.

[JA-Q001]

You need to answer the following for your self:

Can a spoke transfer a compression force to a rim? Yes/No

Are all the spokes always (Normal operating conditions) in tension? Yes/No

[Gman1834]

 

Ok let see.

The drawing shows two cases:

1) spoke pulling (tension) the nipple stops the movement tuns inducing a tension force .

2) Pushing on the spoke (no glue between rim and nipple) The nipple moves into the rim noting (except the rimstrip and tube) to restrict the movement thus no compressive force can be induced in the spoke.

 

 

 

[Minion]

 

 

 

 

The drawing shows two cases:

1) spoke pulling (tension) the nipple stops the movement tuns inducing a tension force .

2) Pushing on the spoke (no glue between rim and nipple) The nipple moves into the rim noting (except the rimstrip and tube) to restrict the movement thus no compressive force can be induced in the spoke.

A slight addition to show what's actually happening:

 

3.) Initial pretension (red) pulls spoke. 'Compression' from rider/bike weight (green) reduces initial tension to produce resultant remaining tension (black). Load is carried by a reduction in spoke tension, but spoke remains in tension due to correct pretensioning.

Edman2008-12-11 11:15:20

[The Guy in Pink]

I've read the claim that when a downward force is applied to a hub in a spoked wheel (e.g. when you're sitting on your bike)' date=' the tension of the spokes above the hub is not increased, but the tension of the spokes below the hub is decreased.  The hub is "standing on the lower spokes".  It is not "hanging on the upper spokes" or equally distributing the downward force by both "hanging" and "standing". [/quote']

 

Spokes only act as tension members, and not in compression, (being too slender to withstand buckling )  For design purposes it will be assumed that the spoke cannot resist a bending moment.

 

With to-days very rigid rims the spokes and high spoke tensions  the spokes are always in tension no matter the position under normal loads.  So the load is always "hanging" on all spokes

 

By analogy:

Consider a bow (as in "bow and arrow") with a completely rigid frame and a bowstring under tension' date=' orientated as you would hold it when shooting.  If one pinches the string at some point along its length and pulls the string down (while the frame is held in place), is there only a decrease in tension of the bowstring below the point where you are pinching and no corresponding increase in tension above the point where you are pinching?

[/quote']

 

No, the tension in the string is equal at every point in the string.

[Johan Bornman]

 cut cut cut cut

 

With to-days very rigid rims the spokes and high spoke tensions  the spokes are always in tension no matter the position under normal loads.  So the load is always "hanging" on all spokes

 

 

 

No.

 

Have a good look at Edman's explanation. The hub never hangs on any of the spokes. It "stands" (I use this terms reservedly) on the bottom spokes which remain under tension but under less tension that spokes elsewhere in the wheel.

 

It's just like banking without an overdraft. You can only take out of the bank what you have in. If you exceed that, the spokes go out of tension, into compression and obviously start to buckle.

 

 

How many spokes does the wheel "stand" on? That depends on the stiffness of the rim or in engineering terms, the stiffness of the beam.

 

The stronger the rim, the more spokes find themselves in the load-affected zone.

 

Assume that we put about 1200N tension in a typical rear wheel's spokes. If only three find themselves in the load-affected zone, you have 3600N to play with.

 

That's why you only exceed their prestress in a car accident or similar.

[Johan Bornman]

 And I was under the impression we discussing wheels not composite concrete structures.  And yes I know how a how a lintal works but a spoke is not a composite structure. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

You need to answer the following for your self:

Can a spoke transfer a compression force to a rim? Yes/No

Are all the spokes always (<?: prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Normal operating conditions) in tension? Yes/No 

 

OK, explain to us how a lintel works.

 

Note that a lintel is strictly speaking not a composite structure but a prestressed structure. The fact that it is made of concrete and steel is not relevant in the composite sense of the word.

 

 

I ask this in a sincere effort to help you join us on the other side.

 

You want answers to your questions?

 

Can a spoke transfer a compression force to a rim? Yes

However, your question is vague. It can transfer a compressive force to the rim but it cannot put the rim in compression. By transferring a compressive force to the rim that's smaller than the tension in the spoke, the spoke remains in tension and therefore doesn't buckle or pop through the rim tape.

 

Some rims like Ksyriums can actually receive a small (very) compressive force from the spokes since the nipples and spokes (when seized up) are rigidly fixed to the rim. That's neither here nor there, since they still have to work in tension only.

 

 

 

Are all the spokes always (<?: prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /><?: PREFIX = ST1 />Normal operating conditions) in tension? Yes

 

 

 

[Groot Lem]

f*&k. So what do you suggest for the Argus??

Hanging, pushing, compression, standing?

[Ox_Wagon]

Groot lem, I don't think the wheels cares....

 

Edman, I agree 100%. So the hub will see the following (two spoke system): <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

F_top(top spoke tension) = F_bottom (Black arrow spoke tension) + W(Weight on the hub)

 

Cool, if we agree that spokes are always in tension we can move on to the next self explanatory pic.

 

[Gman1834]

Thanks Edman for adding the third picture, i agree the tension in the bottom spokes will go from a positive (tension) force to zero as soon as it reach zero the spoke will loose contact with the rim but will never go into compresion.

If you have a spoke or a ruler on your table please hold it vertical and press from the  top what happens (it buckle) now pull it from side to side you need more force to make a difference.

 

Some rims like Ksyriums can actually receive a

small (very) compressive force from the spokes since the nipples and

spokes (when seized up) are rigidly fixed to the rim. That's neither

here nor there, since they still have to work in tension only. It is just the opposite for the ksyrium sl were the nipple is fixed but the hub side will loose contact.

 

 

[TiBones]

Hey TNT more popcorn please!

[Johan Bornman]

f*&k. So what do you suggest for the Argus??

Hanging' date=' pushing, compression, standing?

[/quote']

 

Right. Hang in there, keep pushing, don't let any compression from last night's carbo loading escape and keep standing up Smitswinkel.

 

 

[Johan Bornman]

It is just the opposite for the ksyrium sl were the nipple is fixed but the hub side will loose contact.

 

True, I forgot about that side. But I hope this doesn't present a diversion from the real world scenario.

[droo]