What part of an MacPherson suspension reacts to the braking/longitudinal forces?

[Sebastian Viancha]

Hello! A question that I have had for a long time while studying automotive design is that, if you see the suspension in a lateral view and a static moment, almost all the vertical load goes to the shock absorber and almost all the lateral force to the lower control arm, but what about the longitudinal (braking one) Should it be shared between the two parts? Who should react more or is it equal?

 

[gruntguru]

The ARB and the lower arm combine to form a wishbone which reacts the longitudinal force. Because the contact patch is lower than this wishbone, a smaller force in the opposite direction is also generated at the top mounting point of the strut.

je suis charlie

 

[GregLocock]

As grunt says. You need a side view of the thing to make sense.

Not many people use the original MacP layout, using the ARB (sta bar, sway bar) as a suspension member is cheap but not refined, and hard to develop for the NVH/dynamics tradeoff. So normally we use an A or L arm and decouple the ARB via a droplink.

Cheers

Greg Locock


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[Sebastian Viancha]

I see, Thanks for the quick reply. I thought the ARB only worked for lateral loads and for curve control, since that's what you hear most of the time, so if I only see it as a quarter model vehicle, would it look something like this?

This is the MacPherson suspension of the vehicle I'm guiding, a quite cheap Alto 800.

 

[GregLocock]

Yes, that's it. The sta bar is being used as longitudinal control arm. Fx is mainly reacted in the D block of the sta bar, which connects the sta bar to the body.


Cheers

Greg Locock


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[BrianPetersen]

In the original MacPherson concept, the lateral link was just a link, and the trailing/diagonal part of the ARB also served fore/aft location of the lower ball joint (it connected to the lateral link with a bushing near the end with the ball joint).

Most newer designs use a lower L shaped arm with two bushings on the chassis side, so that it does both the fore/aft and the lateral guidance, and the ARB no longer does anything other than being an ARB.

 

[GregLocock]

Yes, but the Alto being the Alto I can believe they reverted to the original design. Worked OK on Mk 1 Escort.

Here you go, Suzuki's illustration looks just like Earle drew it. I've never had the pleasure of designing one of these, the rubber bungs at 24 and 25 have busy lives.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Sebastian Viancha]

Thanks for the replies, guys! Now it makes sense why this specific suspension doesn't have a drop link. Yeah, I was a little confused because now-a-days vehicles have a lower control arm with 3 points of connection as Brian says and you kind of see where the braking force goes.