Pb of elementary mechanics on the springs

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AXEAU
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by AXEAU » 13/10/14, 21:15

Hello grelinette. The shock absorbers are not prestressed. Look carefully, the top one dampens the vertical movements and the bottom one the horizontal movements. They really have two different functions

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by Grelinette » 14/10/14, 10:26

AXEAU wrote:Hello grelinette. The shock absorbers are not prestressed. Look carefully, the top one dampens the vertical movements and the bottom one the horizontal movements. They really have two different functions

jlg

Hello AXEAU,

I don't quite understand your explanation: "top -> dampens vertical mvts, bottom -> dampens horizontal mvts" ??? " : Shock:

We are moving away from the subject a little but to return to the trailer-motorcycle system in question, if I understood the principle correctly, here is how it works:

Image
According to my diagram, the distances D1 and D2 (fixing points of the shock absorbers) lengthen and shorten with acceleration and braking.

For example, the D2 in Fig. 2 lengthened during acceleration, the same for the D1 in Fig. 3 when braking.

However, as I think, if the shock absorbers are initially at rest, they cannot lengthen because they are in abutment: the internal piston (blue) is in abutment with the top of the chamber, as on the following sketch:
Image

Besides, if there were no prestressing, we could only put one shock absorber that connects the top attachment point of D1 and the bottom attachment point of D2 ... but it would not work no more !

That said, I repeat, a simpler assembly with a single shock absorber would work better.

Unless, one thing escapes me in the functioning of the system we are talking about ...
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by Ahmed » 14/10/14, 21:09

A very classic leaf spring arranged transversely, would limit the overall length and absorb shocks in both directions ... the icing on the cake, it easily adapts to the width of the horse (in fact the two lateral traction lines ).

Note: the solution I had proposed above included two coil springs, but it can easily be simplified to count only one, which would therefore sometimes work in compression, sometimes in traction.
Personally, I find it more in line with mechanical orthodoxy to work only in compression, hence my proposal.
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by AXEAU » 14/10/14, 21:38

Hello, I'm coming back to this trailer which I find a bit fanciful.
let us admit that its weight is at the limit of leaving the amortings in stop relaxed. If the motorcycle encounters an obstacle a little high, the frame will lift, the shock absorber. D1 will compress. D2 would like to stretch but it is already in abutment. The moment the trailer crosses the obstacle, same reaction.
If the pilot brakes abruptly before the obstacle, the trailer will push, D1 remains in a stretched stop and D2 compresses by giving a small diving movement. The motorcycle clears the obstacle and D1 compresses. For a fraction of a second the two benefits can be in action.

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by Forhorse » 14/10/14, 21:51

In your analysis you forget that the attachment to the motorcycle is a ball joint (a ball ...) the top axis therefore does not remain horizontal.
The shocks do not "stretch", and there is no preload either. At rest, and depending on the loading and balance of the trailer, they are simply in abutment.
In the event of acceleration or braking, there is compression of one or the other, and as a result, a modification of the "attitude" of the top bar around the coupling ball.
No need to look more complicated.

(if you still don't understand, it's time to bring out the good old box of mechanics ... or lego-technic for the less old)
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by AXEAU » 14/10/14, 23:40

I take good account of this ball joint.

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by Grelinette » 15/10/14, 10:54

Ahmed wrote:A very classic leaf spring arranged transversely, would limit the overall length and absorb shocks in both directions ... the icing on the cake, it easily adapts to the width of the horse (in fact the two lateral traction lines ).

Indeed, a leaf spring, possibly doubled with an air or oil damper, should solve the problem of damping in both directions.

Ahmed wrote:Note: the solution I had proposed above included two coil springs, but it can easily be simplified to count only one, which would therefore sometimes work in compression, sometimes in traction.
Personally, I find it more in line with mechanical orthodoxy to work only in compression, hence my proposal.

Can you specify specify the 2 points of your Note:

1) "... can easily be simplified to count only one, which would therefore work sometimes in compression, sometimes in traction": This is precisely the initial and fundamental question of the subject. What montage do you suggest?

2) "more in accordance with mechanical orthodoxy to work only in compression": If I understand correctly, you recommend the use of a Compression spring (or damper).

Forhorse wrote:... The shock absorbers do not "stretch", and there is no preload either. At rest, and depending on the loading and balance of the trailer, they are simply in abutment.
In the event of acceleration or braking, there is compression of one or the other, and as a result, a modification of the "attitude" of the top bar around the coupling ball. ...

We are moving away from the question but the mechanics of this system, although complicated, is more interesting than it seems. It should be modeled or see a video to fully understand how it works in motion (braking and starting). I'm going to bring out my old lego box ... : Cheesy:.

Admittedly the axle of the trailer and the ball joint on the motorcycle are also axes of rotation (which I omitted in my sketches) which intervene in the deformation of the system, but I find it difficult to explain how this system cannot operate without ever opening an angle (drawbar drawbar and the one attached to the motorcycle) to never put one of the shock absorbers in traction. Certain angles will inevitably open, if only in the event of impacts: holes, or speed bumps on the road.
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by Ahmed » 15/10/14, 11:55

Without drawing it is not too easy ...

Basically, on the same plane slide three * square profiles: two connected to the towed load and that of the middle to the lifting beam.
They are joined by two distal flanges and between the ends of these two sets a compression spring is inserted; a fork is integral with each end of the profiles (therefore, two since a profile on one side and two on the other): these forks respectively surround part of the last turn of the spring, so that they allow a support in both ways.

That's it and the conditions are met! 8)

* This arrangement is adopted so that the forces remain well in the axis and do not induce torsional forces.
(is there a simple way to display a drawing, if my explanations seem too incomprehensible to you?)
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by Grelinette » 15/10/14, 21:10

Ahmed wrote:Without drawing it is not too easy ...

Basically, on the same plane slide three * square profiles: two connected to the towed load and that of the middle to the lifting beam.
They are joined by two distal flanges and between the ends of these two sets a compression spring is inserted; a fork is integral with each end of the profiles (therefore, two since a profile on one side and two on the other): these forks respectively surround part of the last turn of the spring, so that they allow a support in both ways.

That's it and the conditions are met! 8)

* This arrangement is adopted so that the forces remain well in the axis and do not induce torsional forces.
(is there a simple way to display a drawing, if my explanations seem too incomprehensible to you?)

Houlala, not all inclusive ... a drawing is essential!
and I admit to being very curious to see how your system works!
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by Ahmed » 15/10/14, 22:53

In my first proposal with two springs, the axis slides between 2 yokes fixed on the drawbar; with a single spring, the principle becomes simpler but the assembly a little more delicate.
Instead of a fixed axis (drawbar) and an offset axis, I proposed above a configuration with two square axes, on one side and another axis in the center of the first two, on the other.
It is however possible to remain on the first configuration, it will perhaps be simpler to understand: it is also necessary two support copings, but unlike the first case, they do not play any role of stop.
Each axis is provided with a fork which pinches the last turn of the spring, in order to constitute a double stop which can thus work in both directions.
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