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Multi-link car suspension assembly drawing - rotational joints binding

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Message 1 of 13
Archie.
10492 Views, 12 Replies

Multi-link car suspension assembly drawing - rotational joints binding

IRS screenshot.jpg

 

Hi,


I've been teaching myself inventor over the past couple of months so I can design the rear suspension setup for my car.


I've got the IRS modelled with enough detail to show my engineer the concept, but I want the suspension to "work", ie each link will move as the suspension flexes.


I'm attaching the arms (blue, dark green, red and yellow) to the cradle (orange) using the joint command, and selecting "rotational".  This works fine and the arms can swivel about a single axis as they should.  But as soon as I attach the knuckle (lime green) to the other end of all the arms using the same type of joint (except the joint with the blue lower control arm, which is a ball joint), nothing will move. 

 

I understand that in real life, each joint has a bit of "slop" in it, allowed by rubber bushings, which prevents the whole setup from binding.


To my uneducated mind, the solution would lie in one of 3 methods:
Convert all the joints to ball joints (I'd like to avoid this if possible)
Add some kind of flexible rubber bushing to all of the joints (probably not possible)
Get rid of the rotational joint constraints altogether and somehow mate all the parts to each other using work points, rather than a cylindrical bolt and eyelet, so that each arm has full range of movement in both the X and Y axes.

 

I hope this was more-or-less understandable.  Any pointers would be much appreciated.



Thanks for your time,

Archie

12 REPLIES 12
Message 2 of 13
CAG_DRAFT
in reply to: Archie.

Have you studied/researched bar-linkage design?

Are you aware of all possible 'Lock positions' in your design?

Are any of those lock positions within the normal operating range of motion?

Is it an acceptable design practice to have a lock position within the normal range of motion? (and rely on slop to break the locks...)

 

Edit:

upon actually looking at your picture I'm thinking that you will need to change joint types and set limits for the off-axis deflection somehow.

 

 

Message 3 of 13
Archie.
in reply to: CAG_DRAFT

"Edit:

upon actually looking at your picture I'm thinking that you will need to change joint types and set limits for the off-axis deflection somehow."

This sounds exactly like what I want to do. "Off-axis deflection" was the term I was looking for 🙂

As for design of the actual suspension. What you see in the picture is a slightly simplified representation of a standard S13 Nissan Silvia IRS, which is what I'll be putting in my car. So I wont be designing the suspension as such (and thus, I know that it already works in the real world without binding). What I will be designing is a chassis to mount the "off the shelf" suspension under, so I need this model to see what shape my chassis will be and if and where the control arms and link bars are going to make contact with my custom chassis as the suspension flexes. I will be adding springs and such soon, but for now I just want a working model of the stock Silvia suspension.

What joint types do you recommend, and how do you set limits of off-axis deflection?
Message 4 of 13
blair
in reply to: Archie.

Generally the difference from "Street" to "Race" is bushing. "Street" suspensions use bushing to add compliance and absorb/dampen the road vibration. "Race" is a straight solid bearing arangement. You may want to suppress some joints/constraints and verify your basic geometry. If it's in a lock-up state it would sound like you have some geometry thats amiss.


Inventor 2020, In-Cad, Simulation Mechanical

Just insert the picture rather than attaching it as a file
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Delta Tau Chi ΔΤΧ

Message 5 of 13
Archie.
in reply to: blair

Thanks Blair. Ive checked the geometry and everything is square, as in eyelets and pins all line up perfectly. If i attach a single arm, I can rotate it no worries, but as soon as i attach a second arm to the knuckle (lime green), regardless of what order I connect the arms in, the assembly locks up. Im pretty sure this is because of what CAG_DRAFT said, there is no room in a rotational joint for off-axis deflection, because I have not got any flexible bushings. Is there a way I can add some "slop" to my pin and eyelet connections without having to physically model ball and socket type joints into every arm? Can I add a virtual rubber bushing that will allow for deflection?
Message 6 of 13
CAG_DRAFT
in reply to: Archie.

Do you understand what I/we mean by a 'lock' position?
(if no;t google 4-bar linkage and start reading)

...I'll be very surprised if the Nissan suspension designers relied upon slop & bushings to avoid them.

If you are certain the geometry is correct, then remodel the parts & joints to allow for the kind of movement they exhibit. (probably with mated work points and lots of min-max/limit conditions)
But that is a last resort, if it locks up it is most likely wrong somewhere else.
Message 7 of 13
Archie.
in reply to: CAG_DRAFT

Thanks for your replies and your patience CAG_DRAFT, I had a read in wikipedia and it did help. 

 

Just thinking aloud here, but as my suspension is from a road-going car, the amount that this setup is designed to articulate is relatively small (compared to say an off-road vehicle), so to my uneducated mind, the amount of deflection would be extremely small, and only a tiny, but essential degree of freedom would be required of each joint.  Surely this degree of freedom could be provided by a rubber bushing? 

 

However, if what you say is right (it probably is) and the suspension is designed so each link pivots without any deflection, my methods of measuring the physical object (using rulers, protractors and calipers) are not accurate enough to reproduce an exact model that doesn't bind. 

 

To get back to solving my problem, I will probably just have to accept that my available methods of reproducing the suspension geometry are only ever going to be "close enough" and use ball-joints to allow a degree of freedom so it wont lock up.

 

Any information you could give me on these mated work points with limit conditions would be useful.  Would that mean I wouldn't have to physically model all those ball joints?  I know how to create work points but I don't know how to mate them or add limit conditions. 

Even if you could just point me to some tutorials, Id be grateful.

 

Thanks again.

Message 8 of 13
CAG_DRAFT
in reply to: Archie.

I dont have 2014 so I dont know about the full extent of constraint tools available to you.
Start off by looking at all the options available in the constraints/joints menu, you should be able to set some max/min limits. (so instead of a perfectly mated axis-pair they can move to out of alignment by the amount you set)

work through your model allowing a little bit of slop and see how it reacts, you might not need to completely change how the model is constrained.

but once again, I think you must have mis-measured something as I can't bring myself to believe that suspension geometry would have lock states in the normal range of motion.
Message 9 of 13
vamador1379
in reply to: Archie.

I had a similar problem when I got started with Inventor. You probably figured it out by now, but have you enabled Contact Set for all the elements of the suspension?

Contact set allows you to move the elements and keeping them connected together without having to over-constrain the assembly.
Message 10 of 13
ecomunky
in reply to: vamador1379

HI I just started with inventor. I accidentally removed the origin history box tool bar usually on the left hand side of the work space. I cant find where to click so it goes back on the screen. I needed it to see the highlights. email me @ ecomunky@yahoo.com. not much spam please, but idc.

thank you.

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Message 11 of 13
blair
in reply to: ecomunky

You can turn the visibility of the browser on and off. Click the ribbon View tab Arrow Windows panel User Interface drop-down list, and select or clear the Browser check box. You can dock the browser on either side of the application window.

Inventor 2020, In-Cad, Simulation Mechanical

Just insert the picture rather than attaching it as a file
Did you find this reply helpful ? If so please use the Accept as Solution or Kudos button below.
Delta Tau Chi ΔΤΧ

Message 12 of 13
JDMather
in reply to: blair

...here.

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Message 13 of 13
rmerlob
in reply to: JDMather

Actually I think there is some suspensions that require bushing play to move around, my sideckicks suspension comes to mind, if you recreate it in a simple 2d sketch it locks up, I suspect all macpersons have that too.

 

Of course thats a ****ty car and also not a four-link suspension but still.

 

RM

 

SIDEKICK FRONT SUSPENSION: http://image.fourwheeler.com/f/33806431+w600+re0/131_1110_03%2Bsuzuki_roundup_sleeper_suzy%2Bfront_s...

 

See that the shock is completely fixed to both the chassis and the knuckle, and the a-arm at the bottom shouln´t be able to rotate (because rotating up would cause some sideway´s movement that the shock on top doesn´t allow.

 

The suspension that the OP posted has too many linkages, Is that the actual suspension or the modified one?

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