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Old 1st March 2010, 21:59   #61
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Originally Posted by ex670c View Post
Hi Guys,

I would request Sutripta and Spike to explain to the Lay man;

i) Torsional Rigidity

ii) Effect of Cross-Members (Tubular/X/K) on Chassis.

Regards,

Arka
Hi,
Being a 182 post newbie layman, wouldn't dream of mixing it up with the big boys I agree with Spikes suggestion below. I however retain the right to free speech (read kibitz).

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Originally Posted by SPIKE ARRESTOR View Post
@Arka please do the honours,...
Spike
Actually, never thought anyone was interested in this rather dry thread, and so was treating it as a private conversation.

Am sure the basics (and far more) can be answered by Google far better than we can. Maybe we can pick up from there.

The interesting thing will be to get ballpark figures for unit construction sedans, convertibles, and body on frame designs. (Surprisingly, (very casual) googling did not yield any such list.

I have a more philosophical question:- How important is torsional rigidity in a Jeep?

Regards
Sutripta
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Old 2nd March 2010, 08:54   #62
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@Sutripta the chassis of Load carriers are designed more for bending moments than torsional rigidity, whereas it is opposite in the case of small vehicles (cars,jeeps)

Spike
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Old 2nd March 2010, 09:26   #63
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Originally Posted by SPIKE ARRESTOR View Post
@Sutripta the chassis of Load carriers are designed more for bending moments than torsional rigidity, whereas it is opposite in the case of small vehicles (cars,jeeps)

Spike
Hey Spike - A silly question - Whats the difference between bending moments and torsional rigidity ? Are they not similar?

Torsion is twisting force on a plane - right?
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Old 2nd March 2010, 09:47   #64
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Hey Spike - A silly question - Whats the difference between bending moments and torsional rigidity ? Are they not similar?

Torsion is twisting force on a plane - right?
Headers,

Below definitions are related to ship stability and ship construction but i guess most of the principles are the same here too. Spike please correct me if it does not apply here or if it is different.

Bending moments and torsional rigidity/force are NOT the same.

You are Right, torsional forces are twisting forces. Eg: Imagine a scale/ruler hold it at both the ends and twist it, it is torsional forces being applied on to the scale.

Bending moments is like "HOG" or "SAG" bending in a straight line. Eg; Hold the same ruler/scale and bend it in a straight line like center up and both ends down (HOG) or center down and both ends up (SAG). this is bending force/moments being applied to the scale.

Cheers

Nishanth

Last edited by hgnishanth : 2nd March 2010 at 09:50.
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Old 2nd March 2010, 10:29   #65
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@Nishanth you are correct, the definition of Bending moment and Torsional forces does not change if you move from ship to automobiles just like Gravity which remains unaffected,only its magnitude changes when you move to moon. For the sake of forum members i am attaching a few pics from Schaums outline on Mechanical Design showing Axial,Bending,Torsional loads and a combination of all these

@Headers going by your comments i presume you are not a Mech. Engineer right?

Spike

OT-Perhaps there is something which becomes more noticeable in ships than in an automobile while they are taking turns, may be Nishanth can tell this better, any guesses guys?
Attached Thumbnails
Various types of chassis sections used in Ladder frame construction-1.jpg  

Various types of chassis sections used in Ladder frame construction-2.jpg  

Various types of chassis sections used in Ladder frame construction-3.jpg  

Various types of chassis sections used in Ladder frame construction-4.jpg  

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Old 2nd March 2010, 10:41   #66
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Let me give it a try in automotive terms.

Torsion: Imagine three wheels of the car/SUV/Jeep/etc. are fixed and held tight. On the fourth wheel apply a force to push it up (Simulating a Bump). This kind of force will make the vehicle to twist. The amount of twist that the vehicle bare without failure or within predefined safe limitswill define the torsional rigidity of the vehicle.

Bending: In this case all the wheels are held tight and then force is applied downwards at the B pillar when viewed from side of vehicle. This force will tend the vehicle to sag between the front and rear wheels. The amount of force that the vehicle can bear without failure or within predefined safe limits will define the bending stiffness of the vehicle.
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Old 2nd March 2010, 13:05   #67
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Originally Posted by SPIKE ARRESTOR View Post

@Headers going by your comments i presume you are not a Mech. Engineer right?
A perfect giveaway eh? Yes you are correct - I'm NOT a mechanical engineer

And Thanks for the definitions - understand it better now.

The C axis force is called torsion whereas the Y axis force is bending..
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Old 2nd March 2010, 19:09   #68
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@Headers you are right, imagine holding a pen in your hand at one end (this is called as cantilever action) and trying to twist the other end with your other hand (this is torsion) the ability of the pen to resist this twisting motion is called its torsional stiffness

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Old 2nd March 2010, 22:24   #69
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Quote:
Originally Posted by SPIKE ARRESTOR View Post
@Sutripta the chassis of Load carriers are designed more for bending moments than torsional rigidity, whereas it is opposite in the case of small vehicles (cars,jeeps)

Spike
Agreed. (Maybe you should explain why!).
My question was different. Let me rephrase. If you were designing a new Jeep (Thar!), where on your list of priorities would be torsional rigidity? Which of your other priorities would compromise torsional rigidity?


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Originally Posted by hgnishanth View Post
Headers,

Below definitions are related to ship stability and ship construction
Nishanth
Naval architect/ engineer/ deckside?

Quote:
Originally Posted by ranjitss View Post
Let me give it a try in automotive terms.

Torsion: Imagine three wheels of the car/SUV/Jeep/etc. are fixed and held tight. On the fourth wheel apply a force to push it up (Simulating a Bump). This kind of force will make the vehicle to twist. The amount of twist that the vehicle bare without failure or within predefined safe limits will define the torsional rigidity of the vehicle.
Force (couple) per unit (degree) deflection, I would think.
Also, for laymen I think it is easier to visualise if you say one end clamped rigidly, both corners at other end are deflected, one up and the other down, with the centreline remaining where it was.
Think of the vehicle as a towel which you are wringing to expel water, and the towel does not want to be wrung!

General question: Why is torsional rigidity taken as a figure of merit, and engineers/ teams/ companies which have achieved a high figure go about tomtoming that fact.

Regards
Sutripta
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Old 3rd March 2010, 10:24   #70
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Originally Posted by Sutripta View Post
I have a more philosophical question:- How important is torsional rigidity in a Jeep?
Hi Sutripta,

Since you have given me a non-engineering way into this discussion, I'll try to contribute.

1) Torsional Rigidity is the tendency of the Chassis to straighten itself, when either wheel is lifted.

2) The chassis also dictates how much the body and suspension will flex and the axle articulate.

3) In the C-Section Chassis the body and chassis flex very well, but as the torsional rigidity of this chassis is not as good as a box section chassis, the suspension and axles are not pushed down into the ground as truly/efficiently.

This is also very important in road handling.

4) the position and type of cross-members add to torsional rigidity, through a trussing action, i.e a chassis is a truss.

Regards,

Arka
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Old 3rd March 2010, 11:14   #71
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OT-Perhaps there is something which becomes more noticeable in ships than in an automobile while they are taking turns, may be Nishanth can tell this better, any guesses guys?
OT: Well in ships the bending moment is more noticeable (and by noticeable i mean visually noticeable) than the torsional forces. Yes if you compare it to the automobiles both the torsional force and bending moment are very much more noticeable in ships.The difference is that in automobiles/jeeps it is more applicable like in off roading etc where due to the nature of the terrain the stresses are created on to the chassis compared to the amount of weight loaded, but in ships it is due to the way we load the ships puerly due to the sheer weight of the cargo being loaded, and also the seas, especially in rough weather!

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Naval architect/ engineer/ deckside?

Regards
Sutripta
Chief mate / Deck officer


Cheers

Nishanth
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Old 3rd March 2010, 14:52   #72
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@Arka please elaborate on "trussing action" as referred to chassis

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Old 3rd March 2010, 15:02   #73
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Originally Posted by SPIKE ARRESTOR View Post
OT-Perhaps there is something which becomes more noticeable in ships than in an automobile while they are taking turns, may be Nishanth can tell this better, any guesses guys?
I don't know the correct term for this - but i'll try to explain my guess -

The same way as HOG and SAG might apply for pitching (say in the case of a airplane)

The same sort of flexing when the ship is making a turn (yawing) would probably affect the ship's hull much more than the chassis of an automobile making a turn, due to the nature of the material it travels in.


Super thread. Loving the discussion.
cya
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Old 3rd March 2010, 15:29   #74
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yawing action in a ship has more pronounced effects as this also dictates which side the ship will pull to (port side or starboard side), although the phenomena of yaw is applicable to automobiles too but it is not easily discernible, why does yaw happen in the very first place? Simple, when a body rotates and translates about the same point a component of the force creates this effect, this point is often referred to as the Instantaneous centre (I centre)

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Old 3rd March 2010, 20:53   #75
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Quote:
Originally Posted by ex670c View Post
Hi Sutripta,

Since you have given me a non-engineering way into this discussion, I'll try to contribute.

1) Torsional Rigidity is the tendency of the Chassis to straighten itself, when either wheel is lifted.

2) The chassis also dictates how much the body and suspension will flex and the axle articulate.

3) In the C-Section Chassis the body and chassis flex very well, but as the torsional rigidity of this chassis is not as good as a box section chassis, the suspension and axles are not pushed down into the ground as truly/efficiently.

This is also very important in road handling.

4) the position and type of cross-members add to torsional rigidity, through a trussing action, i.e a chassis is a truss.

Regards,

Arka
Actually question was slightly different, as I've explained in post # 69.
@ Spike: I'm really interested in that answer!
Also, does MM have a 4 post rig? Have you used it?

@ Arka: Lots of things to discuss, which I think is best left to Spike (Spike, are you fresh out of college?) Suffice it to say that chassis flex is bad news. Always. The purpose of the chassis (term used in a very general way, including unit bodies) is to provide a stiff stable platform to attach all the other aggregates which make up an automobile. I don't think suspension engineers use chassis flex to increase suspension travel!

Can't really understand what you are saying in point three, so no comments. However, your comment on handling is the key. How important is handling in a Jeep in the scheme of things?

Point four Spike has taken up.

Quote:
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OT: Well in ships the bending moment is more noticeable (and by noticeable i mean visually noticeable) ....
Nishanth
Korean Tankers?

Regards
Sutripta
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