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PostPosted: Thu May 10, 2012 7:12 am 
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If I understand things right.

1. You want both control arms to be level with the ground, when the car is at ride height and at full weight,IE. ready to drive down the road.

2. Upper control arm length to lower control arm length typical ratio? If the upper control arm is 12" long from mounting point to ball joint, then you want the lower arm to be around 18" long. then build in adjustments for caster and camber.

I under stand all this is just for a basic guide line, but I am almost ready yo start working on the layout of my front suspension


What would happen if the lower control arm is level and the upper control arm in built to be in an angle downwards from the mounting point to the ball joint?

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PostPosted: Thu May 10, 2012 8:15 am 
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Hi Rebeca-
You're gonna get lots of answers on this... My .02 is below, in colors.

1. You want both control arms to be level with the ground, when the car is at ride height and at full weight,IE. ready to drive down the road.
You want the lower control arms (LCAs) level with the ground. Uppers typically slant downward slightly from hub to chassis.

2. Upper control arm length to lower control arm length typical ratio? If the upper control arm is 12" long from mounting point to ball joint, then you want the lower arm to be around 18" long. then build in adjustments for caster and camber.
I'm not aware of a "ratio" per se. That does NOT mean there isn't one. Typically, you experiment using a "string computer" or a software package like the ones mentioned in various threads in here (Wishbone, for example) to define control arm lengths based on desired track width and camber gain.

I under stand all this is just for a basic guide line, but I am almost ready yo start working on the layout of my front suspension


What would happen if the lower control arm is level and the upper control arm in built to be in an angle downwards from the mounting point to the ball joint?
I don't know exactly what this would do to suspension geometry, but I think it would do strange things to your roll center height. Other folks can likely answer this better.

You might want to "study up" a bit on suspension geometry before you go to much further. As soon as I remember the title of the book I used as my main reference, I'll edit this post and put it in here. You need to figure out the geometry of the front end, calculate the instant centers and roll centers and such. The design of the front end, especially the angles of the control arms, will make a lot more sense then.

:cheers:

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PostPosted: Thu May 10, 2012 8:47 am 
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Quote:
What would happen if the lower control arm is level and the upper control arm in built to be in an angle downwards from the mounting point to the ball joint?
I don't know exactly what this would do to suspension geometry, but I think it would do strange things to your roll center height. Other folks can likely answer this better.


If the upper control arm is pointed downward, then as you progress through a bump, the camber will begin to go more positive, then switch to the negative direction. Most drivers prefer the feeling of an monotonic negative camber gain.

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PostPosted: Thu May 10, 2012 9:35 am 
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rx7locost wrote:
Quote:
What would happen if the lower control arm is level and the upper control arm in built to be in an angle downwards from the mounting point to the ball joint?
I don't know exactly what this would do to suspension geometry, but I think it would do strange things to your roll center height. Other folks can likely answer this better.


If the upper control arm is pointed downward, then as you progress through a bump, the camber will begin to go more positive, then switch to the negative direction. Most drivers prefer the feeling of an monotonic negative camber gain.

I think that would feel unsettling. The lower arm would be moving away from the chassis until it reached the horizontal position, then it would be moving toward the chassis. I'm trying to think what that would feel like to the driver, but I bet it would be strange. Sort of a bump steer strange. Would it not rock the car from side to side?


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PostPosted: Thu May 10, 2012 10:49 am 
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Ok, so I want to have both control arms level with the chassis bottom plane at ride height, to allow for smoother travel.

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PostPosted: Thu May 10, 2012 11:26 am 
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Long story short: Lower arm longer than the upper arm. Lower arm close to level. Upper arm angled down from spindle to chassis.

The method for determining the exact specifications gets quite a bit more complex, with a lot of trade-off's and compromises, and only a few generalizations that can be made with absolute certainty. So much like the quote in rx7locost's signature, pretty much any suspension can be made to work. There are some great threads on this site where we have delved deeper into suspension design theory, and I would recommend doing as much searching and reading on the topic as you can. There are also a number of good books out there we could point you towards as well, if you would like. With the level of performance you're looking at, I would strongly recommend a suspension that is as well engineered as your powertrain components will be. Attached is a pretty good suspension design primer that might serve as a good starting point.


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suspension101.rar [1.62 MiB]
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PostPosted: Thu May 10, 2012 12:46 pm 
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Driven5 wrote:
Long story short: Lower arm longer than the upper arm. .



FWIW there has been a number of successful designs with equal length upper and lower arms (generally non-parallel), Citroen probably the most famous for it.


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PostPosted: Thu May 10, 2012 1:32 pm 
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Of course there have been...There have been "successful designs" of just about every suspension layout ever conceived. The MacPhearson Strut has also proven to be a highly successful and winning design...But that alone does not generalize it as an ideal solution either.

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PostPosted: Thu May 10, 2012 6:55 pm 
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Try Race and Rally Car Source Book by Staniforth, he explains the "string computer" in detail. I substituted a cheap 2D CAD program. You will learn a lot using either method. All useful.


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PostPosted: Fri May 11, 2012 12:38 am 
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Driven5 wrote:
Long story short: Lower arm longer than the upper arm. Lower arm close to level. Upper arm angled down from spindle to chassis.
.



For example, here is a double A-arm with longer top arms that provides appropriate camber gain (determined by arm length and angle and pivot separation). This layout was seen on a number of Formula race cars before aero became all important and can still be found on various race cars .. there's some good reasons for home builders to orientate their suspension more in this direction than parallel (left the sample dimensions for people to play with it themselves).

Attachment:
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blah.JPG [ 26.42 KiB | Viewed 1571 times ]


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PostPosted: Fri May 11, 2012 11:16 am 
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Neat...Another suspension design that can get impressive camber curves, and has its share of success in racing, is the swing axle. :wink:

Realistically though, the frame design to get the upper frame pivots that close together would be a bit more challenging when trying to fit a V8 sitting at least partially between the front wheels, and I believe the OP's intended Corvette spindles have somewhere around 9* of KPI...Both of which tend to work against the upper arm being equal length or longer, without artifically shortening the lower arm as well. So in looking at the world of eternal compromises regarding both geometry and packaging, I believe most people designing a suspension will naturally gravitate towards upper arms that are shorter than the lower arms...But yes, that is just my humble opinion.

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PostPosted: Sat May 12, 2012 2:08 am 
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I will have WIn7 Pro installed on my IMAC over the weekend then I can try to use some of the suspension programs suggested to fet what I want for my suspension set-up.


Will I need to know the type and style of shocks I plan on using for the programs as in length, bore adjustable, remote resivour and coil over spring size and rate or weight for the springs. I already know my upright / spindles I plan on using. Or at least i think I know what I am planning on using at this point. I may change my mind depending on the suspension layout and if I keep the car open front wheel design or not.

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PostPosted: Sat May 12, 2012 10:33 am 
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RebecaLynn wrote:
If I understand things right.

2. Upper control arm length to lower control arm length typical ratio? If the upper control arm is 12" long from mounting point to ball joint, then you want the lower arm to be around 18" long. then build in adjustments for caster and camber.



From Chassis Engineering by Herb Adams, "In general, it is best to make the lower control arm as long as possible." and, "In general, the upper control arm length will be between 50% and 80% of the length of the lower control arm."

Of course these are just starting points and it all depends on the camber gain and suspension characteristics you are after but it's a starting point.


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PostPosted: Sat May 12, 2012 3:21 pm 
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I have WIN 7 Pro installed on my MAC now, so I will down load the suspension program mentioned and give it a try.

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PostPosted: Sat May 12, 2012 6:58 pm 
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I tried to download wishbone and there was a problem with it installing. I posted the issue on that thread.

I almost have everything in stalled now. So I will be spending the next few days trying things and going from there.

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