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Angle of control arms?

24K views 13 replies 6 participants last post by  Mutt's37Buick 
#1 ·
My project is a 37 Buick Roadmaster.
I have the components together to do a front suspension upgrade as described in:
http://www.crankshaftcoalition.com/wiki/1937-1957_Buick_Oldsmobile_Pontiac_suspension_upgrade

Somewhere I recall reading that the lower control arms should be level from inside to outside.
I do not see anything in the article about best angle of the upper control arms from inside to outside.
Is there a rule of thumb for this angle on a street cruiser?
Thanks
 
#2 ·
I am going to risk getting in an argument, but go ahead and say that regardless of what you hear, the lower control arm angle shouldn't automatically be anywhere. It's angle has a lot to do with where the vehicle "roll center" ends up (height-wise), and you might want that to be in different places for different-type cars and trucks. Ideally the roll center normally should be at some percentage of the distance from the ground to the vehicle center-of-gravity...so a truck that sits kinda high or has a high c/g will likely handle best with the lower a-arms at a different angle than with, say, a Corvette or such. Beyond that you might want to "tune" a little either way, a higher roll center gets you (obviously) less tendancy for the body to lean in corners (desirable with trucks), a lower roll center will deliver better handling over rough surfaces and need to be made-up for with sway bar stiffness, something that works well with sports cars.

Upper control arm angle is usually non-adjustable relative to the lowers unless you change parts or pivot points. Cars through the seventies with designed-in understeer will have upper-arm angles that are close to parallel with the lowers at ride height so that with the uppers being shorter than the lowers things get screwy (intentionally) with body lean, an improvement is to have them angle upward so the tires, inner and outer, stay more perpendicular to the road as the body leans. But again, there's no simple adjustment for that.

So anyhow, jus' saying it's not that simple.
 
#4 ·
Roll Centers are an argument everyone like having yet no one can win because it's just theory. Like suckers, there's one born every minute. My advice is to throw the Roll Center theory out the window and build around camber gain and bump steer. With a typical street rod set up the lowers are level, and the uppers are about 10' down hill to the frame mounts.
 
#5 ·
A bizarre thing to say but, this is the internet. The above is, however, a good place to start as far as angles go. I would advise anyone interested in the real reasons that links and angles go where they do to just pick up a book on the subject from an experienced author/builder and go from there. If you look into what kind of effort that drag racers put into rr end "instant center" placement and tuning for different situations and tracks for acceleration loads (easy to see at your local track), you may understand how the same-type tuning can affect an a-arm or other suspension in cornering. Anyhow, off-to-work...
 
#7 ·
Yup 10 degrees down hill should about do it.. However if you wish to understand the why and how then Steve Smith autosports has the books and computer programs to help with the understanding. In the alternative you can get the Design to Win book by Carrol Smith and play with the paper dolss like was done in the ancient days..

Sam
 
#9 ·
Yup 10 degrees down hill should about do it.. However if you wish to understand the why and how then Steve Smith autosports has the books and computer programs to help with the understanding. In the alternative you can get the Design to Win book by Carrol Smith and play with the paper dolss like was done in the ancient days.. Sam
Not sure about Design to Win, but I did the paper doll exercise years ago according to Tune To Win by Carroll Smith and built an IFS from scratch.
Tune to Win: The art and science of race car development and tuning: Carroll Smith: 9780879380717: Amazon.com: Books

And by the way, the late Mr. Smith is the only author who made sense to me. I read several others that didn't make sense.
 
#8 ·
Thanks everyone for your feedback.
I found an interesting article on the subject at:
Finding Your Center – Finding Your Front and Rear Roll Center | OneDirt
I have my suspension modeled in Solidworks and a front view is attached.
Currently the front suspension components are:
58 Pontiac lower control arm modified for 67 Camaro lower ball joints
67 Camaro drop spindles
87 Chevy C10 upper control arms
Currently my model has:
* the lower control arm is angled up from ball joint to pivot at 4.1 degrees
* upper control arm angled down from ball joint to pivot at 1.4 degrees
This arrangement:
* puts the instantaneous center 199 inches from center of car
* puts the roll center 3.3 inches off the ground
The article says I want the instantaneous center inside the wheels.
Not sure this is possible, but could move in if I can use taller ball joints and/or lower the pivot shaft of upper control arms.
I want to minimize body roll for this car which will be a cruiser.
kso says I may want the roll center higher on a vehicle with a high center of gravity. May be able to increase lower control arm angle with taller/stiffer coil springs which may help this.
1) Am I on the right track?
2) Any other suggestions or insights anyone can offer?
Thanks
 

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#11 ·
The article says I want the instantaneous center inside the wheels.
Not sure this is possible, but could move in if I can use taller ball joints and/or lower the pivot shaft of upper control arms.
I want to minimize body roll for this car which will be a cruiser.
kso says I may want the roll center higher on a vehicle with a high center of gravity. May be able to increase lower control arm angle with taller/stiffer coil springs which may help this.
1) Am I on the right track?
2) Any other suggestions or insights anyone can offer?
Thanks
I do think you're on the right track, and can't see getting the inst. center inside the wheels on a street vehicle...the camber change through the suspension travel would be nuts. It could however be brought in closer some to good effect, if the upper control arm inner pivots can't be moved, then the idea w/ taller upper ball joints or the taller Camaro spindles now available (which have 2" drop also) would be a cure. That would raise the r.c. a little too, I think that wouldn't hurt. (I'm assuming your car is "rear-steer"...if it were from the front you could switch to 2nd-gen F-body& similar which are taller but have to front-steer).

Love the Solidworks graphic! I bought Solidworks a long time back yet it still sits there on the computer w/ me at kindergarten-level.

You know, I think making a "paper doll" as mentioned might be a fun project for later this weekend. It wouldn't hurt me to refresh-up the intuitive (ancient) understanding a bit.

My reason for ever believing a roll-center mattered started when I first lowered a car, and found it handled worse. Suddenly I was chasing cars not leading them when the corners got dippy, and watching them handle it better...that wasn't what was supposed to happen...

On dirt, it seems to be a different world where the r.c. should be...and with a car that will never be driven near it's limits on asphalt, I doubt it's so important. Still, when you're getting into designing/building something yourself, why not try to do the best job possible?
 
#10 ·
Roll centers and the roll center migration sure sell a lot of magazines. It can be a tuning tool, or a tool used to quantify results, however building a well working front end geometry around an arbitrary RC is foolishness. The first question that needs asking here is "Where do you want it? Why do you want it there?"
 
#12 ·
I'm not trying to build a race car, but like you say, may as well do best I can, within constraints of my frame. I moved the pivot of the upper control arms down as far as possible and improved set-up. Now my upper control arm slopes down at 4.8 degree, my instantaneous center moved to 46.9" from center, and my roll center move up to 5.5". See attached.
The C10 upper control arms are 10.55" long and have ball joints .5 taller than stock Camaro.
In order to make more improvement may have to go .9" tall Chevelle ball joints and 9" long tubular control arms. This will increase the upper control arm angle, move the instantaneous center inward, and move the roll center up.
 

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#13 ·
The coil springs have a large effect on the RC locations as do the tires and wheel rates. Spring hysteria will affect the RC migration patterns. Castor gain also changes the desired static RC locations. You might want to consider those too when deciding where you want the RC and Why you want the RC there.
 
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