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Converting Single A-Arm to Double

Converting Single A-Arm to Double

Converting Single A-Arm to Double

Hi all,

I'm new to this forum, but I have read a few of the posts and found your information helpful. So I decided I would attempt to have my own question answered here. Currently, the vehicle I am working on has a rear suspension design similar to this (I apologize in advance for the very basic "paint" drawing):


It is a basic rear suspension with a single a-arm. The spindle is attached to the lower control arm and a macpherson strut which is mounted to the frame. The tie rod is mounted to a lower cradle (mid engine)and then to the spindle. Hopefully that picture depicted it well.
I would like to modify the suspension so that the result is something like this:


This suspension is a double a-arm design with a shock replacing the strut in the original configuration. The lower control arm remains exactly the same and the spindle mounting point for the tie rod remains the same. The rest I'll explain. The supports (purple lines) are coming from the cradle because at the moment the cradle is not solid mounted to the frame. If I am going to add another a-arm, I'd rather it not be solid mounted while the other is under possible influence from the cradle shifted on it's mounts. A simple way to counter that is solid mounting the cradle, but as I need to be able to drop the cradle to perform engine work, I can't just weld it in. I plan on using aluminum bushings to make it as solid mounted as I can, but if there is even the slightest movement, I'd like both a-arms to be under that influence. The spindle will be attached to the upper control arm in a way that will allow it to rotate. The tie rod is leading to a steering rack as I would like to eventually add some sort of four wheel steering system. All the other components are already set up for this, but the use of the strut in the original configuration doesn't allow for movement of the spindle. So that just leaves my questions.

1.) Is it possible to do this and retain the suspension geometry that was the original design? If so, what are the critical things that I should do to retain this? I'd like to improve the geometry if there's room for it, but it's far more important to have at least the same geometry after this endeavor.

2.) In the original design, the camber adjustment is made by the bolt linking the spindle to the strut:


What do I need to do to retain this adjustment point?

The purposes of this redesign are to improve handling characteristics. The overall goal is add in the four wheel steering design, but before designing the four wheel steering system I'd like to ensure that the attachments themselves will work before the system is added. Even if the four wheel steering cannot be developed after this is added, it would allow me to have the same style of suspension in the front and rear. Also allowing my to use the same dampening equipment at all four corners of the car (the front suspension is a double a-arm with a shock so I could just gather four of the same shocks for suspension parts). I'll be the first to admit that I don't have an extended knowledge of suspension design so I'll take your advice and criticisms openly, however if you are giving either, please explain the reason for giving it. Also, if you notice something that I have not considered, please let me know. If you even read this far, thanks already for your attention. I appreciate any help I can get.

RE: Converting Single A-Arm to Double

Is there any advice you can offer in order to avoid improper tie rod location? I imagined the curves would be different, but I don't currently have a modeling software to see how it would change. I'm writing this thread mostly to ensure there aren't things that I hadn't considered.

RE: Converting Single A-Arm to Double

Sure, if your arm hinge axes are parallel to the vehicle x axis then your tie rod location is fairly easy to fix, since essentially you want no bump steer.

so the procedure is - decide what roll centre height and camber gain you want. This gives an infinite set of possible arm lengths and angles. Select one combination. Then place the tie rod to have the same IC as the two arms. Then look at the result and select another possible pairing of arms.

In reality you probably have more constraints than the above, this reduces the number of combinations of arms that can work.



Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Converting Single A-Arm to Double

Any added structure to the upper arm pickup points will need to be evaluated for its own flexibility, which I suspect might be more important to a short knuckle SLA design.  Even if everything is tied to the cradle, the compliance effects do not stop at the bushings.

I'm guessing a little since flickr is filtered out on this computer.


RE: Converting Single A-Arm to Double

Thank you both for your suggestions. I was looking at the design and came up with an idea. Instead of adding an entire control arm, would it be possible to create an attachment for the spindle that allowed rotation, with the shock mounted on top of that retaining the original strut dimensions? I can create a new diagram if it would help.

RE: Converting Single A-Arm to Double


Instead of adding an entire control arm, would it be possible to create an attachment for the spindle that allowed rotation, with the shock mounted on top of that retaining the original strut dimensions?
Are you thinking along the lines of what GM calls a "HiPer Strut" or what Ford calls a "RevoKnuckle"?

About 2/3 of the way down at http://blogs.insideline.com/straightline/MoreCategories/suspension-walkarounds/


RE: Converting Single A-Arm to Double

That wasn't exactly what I was thinking of, but it looks pretty interesting, thank you for the link. I suppose that could be used to do the same thing that I was planning on doing. Can you explain what effects result from changing the kingpin angle like that? Also, do you know how a system like that would retain camber adjustment ability? I attached a picture that shows something along the lines of what I intended to show.

RE: Converting Single A-Arm to Double

There are many, many, many cars with struts designed to allow the wheels to turn. It's not that complicated. I suggest you look at installing a strut like that. Take a look at the front struts from the vehicle this suspension is taken from.


RE: Converting Single A-Arm to Double

The front suspension of the car the suspension came from uses double a-arms with a shock. No strut is used in the front of the car. I did look into what you spoke of though and it seems like the perfect solution to what I need. Thanks for your help. Now it's just a matter of modifying it to fit.  

RE: Converting Single A-Arm to Double

Thanks again for the information given so far. While reading up on steering geometry I came up with another issue, possibly what Greg was trying to inform me of. The original steering geometry is based on a two wheel steering system. I am switching to a four wheel steering system and I believe (what Greg was trying to tell me) that the tie rod locations would be off because of the inherent changes in geometry since the rear of the vehicle is now contributing to the steering. Can anyone give/link me to an explanation of proper calculations and other necessities to ensure proper geometry for the new four wheel steering?  

RE: Converting Single A-Arm to Double

Nevermind, after navigating the site a bit I found that you already have this listed. Thank you very much.

RE: Converting Single A-Arm to Double

There's a few other things too.  Like the compliances that as applied at the front axle give you a little understeer are likely to become oversteerish when applied directly to a rear axle.

Are you considering this rear wheel steering to be in-phase, out-of-phase, or switchable?


RE: Converting Single A-Arm to Double

The rear steering will be switchable between in-phase and out-of-phase. I've been looking into the Honda and Mazda 4ws systems. The differences between them basically amount to the Honda system determining whether in or out of phase by the steering angle. The idea is that at high speeds, low steering effort will be required to make maneuvers, so the system operates in-phase for low steering input and gradually moves back to straight forward position and then on to out of phase positions as more steering effort is used. The system is mechanical and uses connections from the front steering rack to determine the proper angles.  The Mazda system utilizes speed sensors to determine when the rear steering needs to be in or out of phase. There is a control unit that uses input data to determine rear steering angle based on the speed the vehicle is traveling. Essentially these systems do the same things, in-phase at high speeds and out of phase at low speeds, they just achieve it differently. I was looking at using one of these systems as it cuts down on the amount of fabrication I have to do initially. A more professional explanation of each system can be found here:


Is there anything that would make one system have an advantage over the other?

RE: Converting Single A-Arm to Double

May I ask what kind of car you're trying to do this for?
I have a similar project for a Pontiac Fiero.

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