Advice with MTB wheel/rim

[Johan Bornman]

Dear Johan

 

You are not doing your reputation any good by continuing to show your ignorance about structural design and force distribution. Having just spoken to another professional mechanical engineer to confirm, I can safely say you don't know what you are talking about. I certainly won't let you build my wheels !

 

Can I kindly suggest you make an appointment with a professor at a reputable engineering university (not college) to explain or demonstrate to you how forces in a spoked bicycle wheel are distributed.

 

Amen

 

Ok, now that you're backed into a corner, you revert to insults.

 

It seems to me you realised you are losing face and had to use the only weapon left to you. Blustering and bullshitting.

 

You have chosen not to respond to my analysis of your theory but trusted the casual observation of a colleagued who like you, never gave it one inch of intelligent throught until now.

 

I have given detailed examples, photos and scenarios and all you've done is sniped from behind your shield of anonymity.

[Topwine]

Sigh ! ...

 

Actually, its got nothing to do from where you start the analysis. That's the beauty of force vector analysis in equilibrium. All the forces need to balance out, or else there is breakage !

 

Regards

[Wyatt Earp]

Ok, now that you're backed into a corner, you revert to insults.

I did not get the impression that Topwine was being insulting, straightforward yes, or is this a character trait which only you are permitted to posses ?

Just a quick question Johan, what is your engineering background or do you simply use applied logic ?

The Mavic X 819 is a good rim, a very good rim.

The fact that a layman explained why it's not, should not HAVE to be taken as concrete proof.

There is perhaps some merit in that explanation, but not everyone has to be bullied in to the same belief.

I have also discovered out that in the 13 years I worked in civil engineering I was too blinkered to see the similarity between a lintel and a hub.

Interesting discussion that will be debated to death with the usual this side and that side of the fence groups.

It's human, we will never agree and reach a consensus as long as we have so many variables that could affect the outcome.

[Johan Bornman]

Cut cut cut cut

 

 

As the rim is subjected to a load from the ground, be it your weight or a rock at the bottom of a drop, the rim deforms and displaces inwards towards the hub, relieving some of the pretension in the spoke affected by the deflection zone of the rim. And the stronger the rim, obviously the smaller the deflection zone and less spokes are affected.

 

Small correction.

 

The stronger the rim, the LARGER the deflection zone and MORE spokes are involved in supporting the load.

 

Perhaps you were thinking about smaller deflections. Yes, the deflection is smaller but the zone that deflects gets larger as strength of the beam increases.

\

[Tieffels]

Sigh ! ...

 

Actually, its got nothing to do from where you start the analysis. That's the beauty of force vector analysis in equilibrium. All the forces need to balance out, or else there is breakage !

 

Regards

 

I agree with you 100%, you cannot change the law of physics, no matter where you start, but your understanding is a different matter. And that is what I meant by when I said your understanding of the matter is affected by your mental approach to the problem..

 

I suggested an alternative to your analysis and in the process pointed out errors with your analysis (i.e. the deflection of the rim playing a massive role in the analysis) and tried to bring in the correct (according to me at least) observations and conclusions of previous posts. Feel free to comment on the analysis side. PS you have to define your system that is in equilibrium because the wheel is not in equilibrium in most instances.

 

@ JB. My bad, you are indeed correct re the rim strength issue

 

@ The Mavic 819 supporters: I went through great effort to import 2 rims from the USA as they are insanely expensive here. My rear rim cracked down the middle of the rim, in the plane of the spokes, in 15 months. Internet search and talking to my LBS revealed it was not uncommon. Wanting to stay with the tubeless sytem I got another one. Also did not last, somehow bent it way out of wack. This is only my personal experience, yours will differ.

[Topwine]

 

 

I suggested an alternative to your analysis and in the process pointed out errors with your analysis (i.e. the deflection of the rim playing a massive role in the analysis) and tried to bring in the correct (according to me at least) observations and conclusions of previous posts. Feel free to comment on the analysis side. PS you have to define your system that is in equilibrium because the wheel is not in equilibrium in most instances.

 

 

 

Before we start to debate about the correctness of my or any other analysis, lets just get the first point of argument out of the way. This is the "fact" stated by Johan: "Jumping your bike doesn't put strain on the spokes at all."

 

Do you agree with this or is he plain wrong?

 

Regards

[Topwine]

Small correction.

 

The stronger the rim, the LARGER the deflection zone and MORE spokes are involved in supporting the load.

 

Perhaps you were thinking about smaller deflections. Yes, the deflection is smaller but the zone that deflects gets larger as strength of the beam increases.

\

 

Johan, you are talking absolute rubbish. I am sorry that I have to say this, but it is true. It is clear that you have no engineering background whatsoever. This above quote will prove it to ANY qualified engineer. Please state any verifiable sources of any qualification you have. Just don't tell me you have read a book !

[Wyatt Earp]

Before we start to debate about the correctness of my or any other analysis, lets just get the first point of argument out of the way. This is the "fact" stated by Johan: "Jumping your bike doesn't put strain on the spokes at all."

 

Do you agree with this or is he plain wrong?

 

Regards

I will have to say it's just plain wrong and not a fact.

 

Johan, you are talking absolute rubbish. I am sorry that I have to say this, but it is true. It is clear that you have no engineering background whatsoever. This above quote will prove it to ANY qualified engineer. Please state any verifiable sources of any qualification you have. Just don't tell me you have read a book !

 

 

Eventually somebody had the balls to stand up and didn't fall for the usual bullsh1t baffle brains theories, my main reason for avoiding all contribution to the tech shack.

And yes I am contradicting myself by posting here and no I am not picking on the lord of the shack because I just feel the need to do so.

 

 

Don't even get me started on lintels, I have built quite a few manholes in my time, lintel's don't have any relevance here.

[Tieffels]

Before we start to debate about the correctness of my or any other analysis, lets just get the first point of argument out of the way. This is the "fact" stated by Johan: "Jumping your bike doesn't put strain on the spokes at all."

 

Do you agree with this or is he plain wrong?

 

Regards

 

 

Before I start where you want me to start, let me start by saying thank you for being a big enough man to admit that you were wrong. Or sort of implying it, as it is the hub way of saying sorry.

 

I do not agree with JB's statement 100%, but so far I'd say he's addressing more of the picture than some other arguments. In than rhetoric of his that makes him loved by all on thehub. The spokes affected by a big impact to the rim, with the number of spokes affected determined by the stiffness of the rim, will loose tension, but I suspect than the tension the rest of the spokes need to gain to "take up the slack" as it were, is considerably less because this taking up the slack is shared across more spokes across the top arc of the rim.

 

Cheers

 

The way I see it is that what both camps are arguing for is taking place..

[Tieffels]

Johan, you are talking absolute rubbish. I am sorry that I have to say this, but it is true. It is clear that you have no engineering background whatsoever. This above quote will prove it to ANY qualified engineer. Please state any verifiable sources of any qualification you have. Just don't tell me you have read a book !

 

I suspect you misinterpreted the deflection zone, as in the length of the beam (or rim) that is affected by the deflection, with deflection, as in the amount of movement in the radial direction.

 

What is implied, and what I agree with, is that a weaker rim will deflect across say 2 spokes as it is not strong enough spread the load across say 4 spokes if it was stronger. This number of spokes involved is what JB referred to as the deflection zone.

[Bike Lore]

This is a video of a FEA that i run showing the effect of impact on a wheel has.

The hub is held fixed and the force is applied at the bottom

To simulate the pretension in the wheel i placed a pressure on the inside of the rim. this is wrong and should be noted that the results may very. if i get the opportunity to us a more sophisticated FEA application that can set the pretension i the spokes i will verify this results.

 

http://www.youtube.com/watch?v=hHUMm0-zsnc

 

Note the increase of tension in the other spokes as the bottom spokes tension are reduced due to the deflection of the rim

[Tieffels]

This is a video of a FEA that i run showing the effect of impact on a wheel has.

The hub is held fixed and the force is applied at the bottom

The simulate the pretension in the wheel i placed a pressure on the inside of the rim. this is wrong and should be noted that the results may very. if i get the opportunity to us a more sophisticated FEA application that can set the pretension i the spokes i will verify this results.

 

http://www.youtube.com/watch?v=hHUMm0-zsnc

 

Note the increase of tension in the other spokes as the bottom spokes tension are reduced due to the deflection of the rim

 

Lekker hannes

 

Is the pressure in the inside of the rim + ve or -ve?

 

Also it would be interesting to see the effect of allowing the hub to displace downwards with a force representing a rider + impact acting on it and keeping the contact point on the rim fixed...

[Wyatt Earp]

 

 

Also it would be interesting to see the effect of allowing the hub to displace downwards with a force representing a rider + impact acting on it and keeping the contact point on the rim fixed...

This could change things quite dramatically.

[Topwine]

Lekker hannes

 

Is the pressure in the inside of the rim + ve or -ve?

 

Also it would be interesting to see the effect of allowing the hub to displace downwards with a force representing a rider + impact acting on it and keeping the contact point on the rim fixed...

 

Ah, at last you are actually getting to a true analysis! But at least Hannes has now proven beyond any reasonable doubt that the spokes DO carry increased tension from a jump by a rider !

[Bike Lore]

This could change things quite dramatically.

 

what is the difference between applying the force in the middle and holding the contact aria fixed or other way around? IMO it will be exactly the same except that the hub will move down where now the contact aria is moving up.

 

chancing the reference will not effect the result. example: if you are on a boat and you pull on a rope that tied to the shore do you pull the shore closer or do you pull your self closer to the shore... it all depends on how vain you are and where you put your reference. but the result is that the distance between you and the shore shrinks. well it depends if you are strong enough the counter the huge whirlpool that is sucking you in.. MWAHAHAHA DIE DIE DIE.

[Topwine]

what is the difference between applying the force in the middle and holding the contact aria fixed or other way around? IMO it will be exactly the same except that the hub will move down where now the contact aria is moving up.

 

Hannes, Yes you are right. It should make no difference.