Author Topic: Wheelbuilding myths debunked  (Read 20248 times)

GraemeMcC

  • CaptainContours
Re: Wheelbuilding myths debunked
« Reply #75 on: 17 August, 2012, 12:54:51 pm »
Apologies folks - been away. But I'll defend my opinions:

Engineering fact: stiffness = strength.
Er, wtf?  Is that meant to be a joke?

No: it is a fundamental engineering principle.
As I stated, the load running along a spoke will take the most direct loadpath available to it which is its stiffest.
Stiffness in any structure, whether it's a laced wheel or a multi-storey building or whatever, will attract load to it until such time as deflections occur and load-sharing becomes possible when the initially stressed elements deform to a stage that their effective stiffness reduces to the level that other elements can attract some of the load.
PBP 2011: 1234 km by Nr. 5678 in < 90 hrs. Most auspicious...

Re: Wheelbuilding myths debunked
« Reply #76 on: 17 August, 2012, 01:01:21 pm »
Engineering fact: stiffness = strength.
Er, wtf?  Is that meant to be a joke?

No: it is a fundamental engineering principle.

Erm, you're having a laugh mate.
Simple google for school textbook type explanations of the difference between stiffness and strength produces:

http://www.vendettacycles.com/vendettacycles/stiffness.htm
http://www.bledsoebrace.com/education/cp030004.htm
http://www.matter.org.uk/schools/content/YoungModulus/stiffnessexercise.html

<snip slightly unnecessary rudeness>

GraemeMcC

  • CaptainContours
Re: Wheelbuilding myths debunked
« Reply #77 on: 17 August, 2012, 01:28:31 pm »
Let me get my answers out, mate...

Quote
So, the tying and soldering can only affect lateral stiffness. By tying and soldering the cross-overs, a node point is created at each tied cross-over which forms a series of stiffer triangulated sub-frames in the rim/spoke/hub structure.
Stiffer how exactly?
An untied spoke has an effective length running from rim to hub flange. In any direction.
So under tensile strain, it will stretch slightly and deform evenly over the full spoke length (assuming plain gauge uniform thickness spokes for this explanation).
Under lateral forces, the rigid system is from flange A - spoke A - rim - spoke B - flange B.

An untied crossed spoke is only partially restrained at its crossover: it can move in one direction by slipping along its partner at its lace point.
So the effective subframe size is still the overall spoke length relationship from rim to hub as above.
Thus the lateral rigid system is also still flange - spoke A - rim - spoke B - flange B.

However, a tied/soldered crossed spoke will have directional restraint at its crossover. So the spoking becomes two sets of triangular subframes: a larger outer set from rim to X-over to rim, and the smaller inner set from hub to X-over to hub.
The fixed node at the X-over rmeans that any lateral flexural movement of from a spoke must mobiliize the other spoke at the cross-over. So that movement must induce a force into that other spoke (induced tension). So that is developing strain energy which is taken up from the total energy in the wheel. So the overall lateral energy is being mobilised and transferred from deflections in one spoke into two spokes. So, forces are being shared. So, overall lateral force effects in one spoke are reduced - wheel is effectively laterally stiffer.

Does that help explain?

PBP 2011: 1234 km by Nr. 5678 in < 90 hrs. Most auspicious...

GraemeMcC

  • CaptainContours
Re: Wheelbuilding myths debunked
« Reply #78 on: 17 August, 2012, 01:34:48 pm »
Nobody is saying GraemeMcC (or anyone else) should cease to tie and solder if they so choose.   Nor, I hope, is anyone trying to impose tying and soldering on us mere mortal wheelbuilders who can build a wheel that is satisfactory for our own purposes without doing so.

I'm not advocating the benefits of soldering cross-overs - I'm trying to add to the pointlessness of it to further debunk the myth - from a structural engineer's p-o-v.
PBP 2011: 1234 km by Nr. 5678 in < 90 hrs. Most auspicious...

Re: Wheelbuilding myths debunked
« Reply #79 on: 17 August, 2012, 01:46:36 pm »
I ain't arguing with your analysis of a wheel, but, stiffness=strength?

A 5mm rod made of glass is very stiff compared to a 2.5mm spoke, but it doesn't take much force to break it.
<i>Marmite slave</i>

Re: Wheelbuilding myths debunked
« Reply #80 on: 17 August, 2012, 01:58:01 pm »
Two points:
The lateral force being applied is on the rim, not one spoke.  That means that when one spoke is being moved by a lateral load, the ones adjacent are also.  The spokes which cross over are only two holes apart.  They're moving the same way at the same time.

When you load the rim laterally, the point of the spoke at crossover moves laterally.  i.e perpendicular to the crossover contact, not along it.  So they don't slide along one another.  To get a movement which would be stopped by the tying but not otherwise you'd have to have a sufficiently flexible rim and localised force that the rim would bend laterally enough for the spokes to separate and get air between them without tying (not gonna happen, I think).  Otherwise there is going to be no relative lateral movement of the crossover anyway, even without tying. 


The only way you would get the transfer of forces you are talking about would be if there was sliding between spokes otherwise.  But I'm looking at a wheel in front of me and I can't see a mechanism for it to happen with lateral loading. 

Difficult to prove who is right with words rather than measurements of course, but Brandt appears to have done the right experiment for us.

GraemeMcC

  • CaptainContours
Re: Wheelbuilding myths debunked
« Reply #81 on: 17 August, 2012, 06:13:13 pm »
Fair points, Tom-e.

And my apologies to others - by "stiffness = strength" I was referring to structural system stiffness.

These were lunchtime-in-the-office posts and my brain was still in its "analysis and design" mode. Of course, a glass rod is a brittle structural material with good compression and tension qualities but, in the presence of surface micro-cracks, has poor flexural strength (unless specially treated to remove all micro-cracks). A steel spoke is a tensile component with great tensile properties but little flexural or shear strength and too slender to offer much in the way of compressive strength. A spider's web strand has fantastic tensile structural strength, but little else. Apples & pears...

But, another point to ponder, as I was coming home and thinking about posts from rogerzilla [#74, p5] and andrew-s & PeteJ [on p3] :-
  • a spoked bicycle wheel is a tensile structure;
  • when subject to imposed vertical load (i.e. rider on bike) the predominant imposed load path is from the axle to hub, transmitted via those spokes in most direct tension to the rim, then transferred around the circumference of the rim to acting onto the ground. (If you doubt that, consider a wheel with loose spokes and how that transmits the load. The bottom spokes would push out through the nipple if it weren't also for the fact that normal steel spokes have negligible compressive strength, and the side spokes have no flexural shear strength ability to transfer any imposed load).
  • But, the rim is a hoop structure which will bend by flexing outwards at "3 & 9 o'clock" (like an egg) because it is being pulled downwards at the top from the imposed loaded spokes and is pushing up off the ground.
  • This induced load bending is resisted by the spokes in tension (max induced tension at 3/9 o'clock, reducing elsewhere as a function of the rim hoop flexure).
  • The resistance to the rim's hoop flexure is also a function of spoke elasticity (i.e. cross section area) & numbers of spokes.
So, I accept Rogerzilla's point, although these flexural hoop induced spoke forces are not going to be as severe as those directly involved in hanging the hub.
But, does this mean that a rigid (i.e. stiffer = stronger :facepalm: ) deep aero section rim builds a better wheel that doesn't develop much in the way of these hoop effect stresses or, should we have thinner section rims that do flex more thus share more of the tensile spoke load? (My apologies if this debate exists elsewhere...)

I could see that a wheel with a deep rim would have a lower degree of secondary hoop-effect stress fluctuations but its spokes would be subjected to a higher level of imposed load stress fluctuations per revolution. So, the fatigue life of the spokes may be compromised but the fatigue fluctuations in an alloy rim from local bending effects would be less, so the rim may be less susceptible to fatigue cracking.
Whereas, a more flexible rim and with fewer spokes would have higher hoop flexibility but lower peak imposed load variation in its spokes, so its spokes could last longer (at the expense of reduced flexural fatigue life of the rim)? And be more comfortable to ride?

Oh, my head hurts now...

PBP 2011: 1234 km by Nr. 5678 in < 90 hrs. Most auspicious...

Re: Wheelbuilding myths debunked
« Reply #82 on: 17 August, 2012, 07:40:50 pm »
You have a fundamentally different view to the widely accepted understanding of the bicycle wheel explained by Jobst Brandt.  If you haven't read it, it is well worth it.

And I always thought that hoop - effect stress fluctuations were what you get when you experience lock- to-lock shimmy during a 40mph descent..




GraemeMcC

  • CaptainContours
Re: Wheelbuilding myths debunked
« Reply #83 on: 17 August, 2012, 07:59:23 pm »
Cheers T.

I obviously haven't read through all of that yet, but I don't disagree with Brandt's basic description - a wheel is a (pre)tensioned structure and the key to how strong it is likely to be and its fatigue resistance is predominantly going to be down to how much prestress is built into the system and how those stresses vary when the wheel is subjected to additional loads when it is loaded by a rider sat on the machine.

The greater the pre-tension in the wheel, then the lower the stress variations will be due to rider loading. It's these stress variations that need to be dealt with. At the lower end of the pretension effect, then rider load effects will dominate. At greater pre-tensions, then rider imposed load will be less influential.

Sadly, in our office sit a team of stress analysts with just the right software to play with and to look at the variables such as rim stiffness, spoke positions, spoke stiffness, spoke stresses, etc. However, no way could I get them to apply their technology to this...  Would cost more than I earn...
PBP 2011: 1234 km by Nr. 5678 in < 90 hrs. Most auspicious...

Re: Wheelbuilding myths debunked
« Reply #84 on: 18 August, 2012, 12:09:42 am »
Quote from: GraemeMcC

The greater the pre-tension in the wheel, then the lower the stress variations will be due to rider loading. It's these stress variations that need to be dealt with. At the lower end of the pretension effect, then rider load effects will dominate. At greater pre-tensions, then rider imposed load will be less influential

And that is why wheels with loose or inadequately-tensioner spokes break spokes.
<i>Marmite slave</i>

rogerzilla

  • When n+1 gets out of hand
Re: Wheelbuilding myths debunked
« Reply #85 on: 30 January, 2013, 09:33:26 pm »
Which is what I said at the start of the thread  :smug:
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

slope

  • Inclined to distraction
    • Current pedalable joys
Re: Wheelbuilding myths debunked
« Reply #86 on: 21 February, 2013, 04:24:47 pm »
On the CTC forum bits for sale

http://forum.ctc.org.uk/viewtopic.php?f=32&t=73083

"Built by one of the best wheelbuilders in England (and the world!)"  ::-)

How many bests constitute a whole pile of myths?  :demon:

Reminds me about that add from SJSC - asking punters to call them and speak to one of their GURUS about anything! Track mitt gurus? Seat tube angled gurus?  ;)

MalRees

  • Hayes - centre of no known universe
Re: Wheelbuilding myths debunked
« Reply #87 on: 21 February, 2013, 07:38:30 pm »
Well Hewitt is indeed a damned fine wheel builder. Wonder what the world wide experience of the seller actually is though  ::-)

rogerzilla

  • When n+1 gets out of hand
Re: Wheelbuilding myths debunked
« Reply #88 on: 21 February, 2013, 07:57:39 pm »
The only wonky wheel I have is a Hewitt wheel.  It hasn't actually got any worse, though.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

Re: Wheelbuilding myths debunked
« Reply #89 on: 21 February, 2013, 08:40:05 pm »
Well Hewitt is indeed a damned fine wheel builder. Wonder what the world wide experience of the seller actually is though  ::-)

I very much doubt Paul Hewitt builds the wheels he sells.

Re: Wheelbuilding myths debunked
« Reply #90 on: 21 February, 2013, 11:11:24 pm »
Well Hewitt is indeed a damned fine wheel builder. Wonder what the world wide experience of the seller actually is though  ::-)

I very much doubt Paul Hewitt builds the wheels he sells.

He was truing up a pair for me when I arrived to collect them although it could have been a quick QC check.
All the wheels that I've had off him save one (the Rohloff) has been perfect.


Re: Wheelbuilding myths debunked
« Reply #91 on: 22 February, 2013, 05:20:53 pm »
So, having read through all this thread once more and not understood half of the arguments:
1. If wheels are necessarily so stiff that they don't move with weights hung off them (or otherwise), why aren't you all using solid disc wheels in your tourers and audax bikes; no spokes to break and after all is said and read it's the tyre that provides the comfort, not the spokes. ???
2. Why don't wheels just go oval, instead of going pretzel?

I am still going to carry on building my wheels as I have done for the last 40 years. I don't sell them so it's nobody's business but mine

slope

  • Inclined to distraction
    • Current pedalable joys
Re: Wheelbuilding myths debunked
« Reply #92 on: 22 February, 2013, 05:44:28 pm »
So, having read through all this thread once more and not understood half of the arguments:

That's because a simple bicycle wheel brings out the devils, the myths and the blinded like no other :demon:

Can you imagine Any (Cycling) Questions with Dimbleby anywhere in the country, with four guest speakers and some bloke asks about bike wheels? There'd be a riot ;D

slope

  • Inclined to distraction
    • Current pedalable joys
Re: Wheelbuilding myths debunked
« Reply #93 on: 22 February, 2013, 07:01:14 pm »
I'd go as far to say that drug testing should be applied to wheelbuilders. All that RSI and arthritis, not to mention back pain sitting at the jig day in day out.

According to my mate fast Eddy Fisher, who lived around the corner from J D Whisker when he had a shop in Kilburn in the 70s, there was a chap called Depressing Derek (name changed just in case the impossible could happen and he is STILL alive - doubt he had any family) who built wheels and moaned about customers and his bad back from building too many wheels.

As a semi professional unmythical lacer and truer myself, I have taken drugs. Mainly Cocodamol, alcohol, caffeine and an Ibuprofen rub gel - I own up! Bring on the truth and reconcilliation. Imagine the force us global spoke twizzlers would have? I wanna  stand proud with my Chinese and Taiwanse sisters. We HAD to do it to compete on a level playing field/pay the bills/feed the kids.

Biggsy

  • A bodge too far
  • Twit @iceblinker
    • My stuff on eBay
Re: Wheelbuilding myths debunked
« Reply #94 on: 22 February, 2013, 07:14:07 pm »
1. If wheels are necessarily so stiff that they don't move with weights hung off them (or otherwise), why aren't you all using solid disc wheels in your tourers and audax bikes; no spokes to break and after all is said and read it's the tyre that provides the comfort, not the spokes. ???

Because they're bad in cross winds and are more expensive and heavier.

Quote
2. Why don't wheels just go oval, instead of going pretzel?

The wheel is so much weaker laterally than radially that it only takes the slightest difference in tension between the left and right side for the rim to go sideways.  Left and right spoke tension is never perfectly matched, especially after the event that caused the wheel to start to pretzel.  The rim deforms in the easiest way for it.  (Spokes pull the rim sideways as well inwards).
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