4 link suspension question

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coaster
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Joined: Sat Jun 30, 2012 4:10 am

4 link suspension question

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In side view to a solid rear axle you have your standard 4 bar arrangment usually with bars angled to the centre of gravity, aiming a bit higher or a bit lower gives you anti squat or squat depending on the type of effect you desire, wheelspin or grip.

Another variation is the 'z link' 4 link, the top link in the opposite direction much like a watts link.

Question is the 'z link' will always point the bars to a centre of gravity at infinity, will this have a detrimental effect on traction or braking?

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Tim.Wright
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Joined: Fri Feb 13, 2009 5:29 am

Re: 4 link suspension question

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The orientation of the linkages just sets the amount of jacking effect coming from wheel centre (usually traction) and the contact patch (usually braking). If the instant centre is at infinity it just means that you have the same jacking effect at the contact patch as at the wheel centre.

There's no reason why this will cause traction problems. Several sports cars have this characteristic on independant suspensions too.
Not the engineer at Force India

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coaster
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Re: 4 link suspension question

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Thanks Tim, I am familiar with the watts link requiring parallel bars, if I tilt the plane of the parallel bars towards to the centre of gravity, I should be good? Maybe only 2 degrees below horizontal seems to look good on the cad.

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Tim.Wright
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Joined: Fri Feb 13, 2009 5:29 am

Re: 4 link suspension question

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That angle sets your anti-squat/raise. So you need to decide how mauch squat and raise you want and then set the angles based on that.

P.S. if you can post a sideview of the geometry it will help the discussion.
Not the engineer at Force India

Greg Locock
Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: 4 link suspension question

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Usually when designing 4 bar rear suspensions the criteria of interest to a circuit car are : roll steer, anti squat, and perhaps UJ angles. I wouldn't be mad keen on pointing them at the cg, more parallel seems better. I'm not really sure that antisquat is a good idea, as it adds an 'impacty' sort of loading to the tire, whereas the aim is to be nice to the tires.

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coaster
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Re: 4 link suspension question

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https://freeimage.host/i/20200601-160535.JNSZCX


This is a generic image very close to what I plan, except the bars are going to be paralell at mid stroke.
Sorry no cad, its very rough, its embarassing, i only polish the worthwhile stuff and the rest just gets scrapped to a zip file.

Greg Locock
Greg Locock
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Re: 4 link suspension question

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Neat, odd. OK so what's your question?

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coaster
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Re: 4 link suspension question

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The rate of squat is constant yes?
Like motorcycle forks, follows one path.
I guess if transposed the theory of anti dive for motorcycle forks, i would have the solution for for anti squat on a rear axle with this z link, yes?

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coaster
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Joined: Sat Jun 30, 2012 4:10 am

Re: 4 link suspension question

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That foils my plans, the only way control dive and squat on a motorbike is to uncouple the torque loads from the suspension and transfer them to the chassis with a link, the drive axle would experience both squat and dive loads from braking and acceleration.
This might be a situation for 'cut and try', build it as best i can and make alterations/attempts to improve.
This is for a locost style of car, i vaguely remember a z link de dion with 3 points of linking, 2 lower trailing and 1 upper leading.
I wish i could get some chat time with that locost builder.

Greg Locock
Greg Locock
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Re: 4 link suspension question

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Surely the free body diagram of the Z link is almost identical to that of a conventional 4 bar axle?

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coaster
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Re: 4 link suspension question

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My bad, brainfade.