Custom Control Arms

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Postby blacksheep-1 » Thu Dec 21, 2017 12:06 pm

I don't have those answers, Loren, I know when you lower the Fox body cars, the roll center actually goes underground so that's why there is far more to be gained working on the front of the car than the rear, since those cars are so nose heavy. I noticed in your other post that you were worried about tires coming off the ground and whatnot. for us, which is of course not your car.. that is the best cornering attribute the car could have, in a RWD car it allows an increase in forward bite and to be able to get on the gas faster. There are many articles by a guy name dBolles,
who writes for Circle Track magazine and has a setup shop on the east coast of Florida. You might google his articles. In the meantime I'm going to post a pic, this was at VIR, the car was setup to run an endurance race, it was running against Porsches, BMWs Miatas and whatnot. Many who've seen this started giving chassis recommendations, the fact it, this is how mustangs handle. Note at the back of the car the driver has already left 2 tire marks from the corner center, he was on the button early. At this point, the car is sitting on the bump stops.


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Postby blacksheep-1 » Thu Dec 21, 2017 12:11 pm

Also..found this, scroll down a ways to "instant center"

http://www.onedirt.com/tech-stories/sus ... ll-center/
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Postby Loren » Thu Dec 21, 2017 12:58 pm

High powered RWD will lift a front for the save reason FWD will lift a rear. Maximum weight transfer to the driven end.

If I've already lifted the rear of my FWD car, and the front starts to come up... That's an issue!
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Postby Loren » Thu Dec 21, 2017 11:47 pm

Finally got around to plugging all of my info into this calculator. It's not bad. Kinda fun to play with.

Here's a link to my Current Setup (it's a really long URL, no guarantees it will work with the forum)

Interesting things:
My estimated roll centers from Photoshop approximations were pretty close.
My current ride height yields about a 0.4" below ground Roll Center.
Just a half inch lower (which I was thinking about doing) drops it to -1.7
If I lower the ball joint position by 1.75" with the same ride height, roll center goes to +3.5
If I go a half inch lower at that point, roll center goes to + 2.4

So, for now, I'm leaving the ride height alone. I could lower it a half inch after repositioning the ball joint.

What's REALLY interesting that this calculator lets you play with is what happens to the Roll Center (and camber angles, etc) with body roll.

For instance, as it sits, with the Roll Center just barely below ground, 2 degrees of body roll moves the Roll Center up by 4" and OUT by 85". I'm not quite sure what effect that has, but it's pretty extreme movement of the roll center. At 4 degrees of body roll (which is probably about what we're getting at max), the roll center goes up to about 14" and OUT 140". Seems like with a roll center that far out, it would be almost impossible for the car to roll over.

If I drop the ball joints by 1.75" (same ride height), Roll Center is at +3.5. With 2 degrees of body roll, it goes to +3 and IN by 10. With 4 degrees body roll, it gets down to +1.2 and goes IN by 23 (still well within the track width). So, Roll Center will stay a lot more centered, and actually moves in the opposite direction. Will that make things feel more stable? Idunno. At 4 degrees roll, the roll center is very close to the contact patch of the inside tire. With that being the roll axis, I can't imagine it wanting to lift that tire. That's got to be a good thing.
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Postby Loren » Fri Dec 22, 2017 12:05 am

Just for kicks, at stock ride height:

Roll Center is at 5.75"

At 2 degrees roll, it's 5.4 and moves IN by 6.9
At 4 degrees roll, it's 4.1 and moves IN by 14.5
And since it rolled a shit ton more with stock suspension:
At 6 degrees roll, it's 2.1 and moves IN by 22.4
At 8 degrees roll, it's -0.7 and moves IN by 30.3 (right at the edge of the inside tire's contact patch)

Pretty good control of the roll center throughout its intended range of motion. And you wouldn't know it by looking at those crazy pictures of how much body roll it had in stock form, but it actually FELT stable... just had so little grip that it carried no momentum through a turn)
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Push Harder - Drive as close to the limit of your tires as possible.
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Postby mymomswagon » Fri Dec 22, 2017 9:14 am

great stuff thank you.
my control arms look horizontal at stock height so thinking I won't lower until I get to the bottom of things.
no off-the-shelf correction for the wagon as of now.
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Postby Loren » Fri Dec 22, 2017 9:35 am

I was searching for something last night, don't even remember what. But, this came up.

If you poke around enough on serious racer or race car engineering forums, you'll hear about a book called Race Car Vehicle Dynamics by Milliken & Milliken. You'll hear of some other books, too, but this is "the one". It's out of print, hard-to-find, and expensive.

Well, lo and behold some college has Chapter 17 - Suspension Geometry up on their website as a PDF! I haven't read it all yet, and so far it jives with things that I've read elsewhere, but it IS very well written, and not too hard for a non-engineer to understand. (I always assumed that it would be "thick")

I was just snooping around on the website. It's in a professor's webspace at the University of Texas. I was hoping to find the rest of the book, but no. However... the entire syllabus for his course is out there and it looks like it might include a lot of useful material for those looking to learn. Vehicle System Dynamics and Control
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Suck Less - Drive something resembling a proper racing line.
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Postby Loren » Thu Jan 04, 2018 7:30 pm

Well, I blew another half day or more trying to come up with a workable design for a ball joint extender/spacer. I really wanted to go that route. Something simple that I could just bolt onto the car to achieve the desired effect. But, I'm back to "it can't be done".

"Can't" is a strong word. It's just too "iffy". I could exceed my design parameter of 2" and get it done a little easier. But, as sure as I did that, I'd end up having to modify the control arms for wheel clearance. Not modding the control arms was the whole point of this exercise!

There are a couple ways I could build the spacer.

The "C" channel idea would work. But, unless I offset the ball joint pin, it would be REALLY tight clearances. Offsetting the pin by .5 to .75" would make it workable, but then I'm adding 2-3 degrees of camber. Which I could pull back out with the strut-to-knucle adjustment... but, I'm just not sure about moving those forces out of line like that. Alternately, I could offset it to the front side for 2-3 degrees more caster. That might be a good thing, but I wouldn't want to do it in that manner. I'd want to be able to adjust it down if it ended up being too much. So, I reluctantly abandoned that idea.

The other method is the "tube" method. I thought about welding my 1-3/8" round tapered hole mounting widget into a piece of tube. A couple ways to do that, as well. Could do a half-tube, leaving one side open for wrenching. But, I measured (with an actual wrench!) and an open-end wrench is too fat to fit. And a closed end wrench would fit, but there wouldn't be enough clearance above the ball joint stud to remove the wrench! (see "exceeding the 2-inch limit" above) So, that won't work.

The last method WOULD work. Use a full piece of thick-walled tube, no cut-out. That means we need access to the top to put a socket on it. That means that the top piece that has the pin to fit into the hub/knuckle would have to be a bolt-on removable piece. I would absolutely work. But, that's a bit more persnickety fitting and machining than I'm willing to do for this.

So, there ya go. Bolt-in spacer idea is done. Back to modifying the control arms. To commit myself to that idea, I just ordered a pair of control arms from eBay. They were only $95. Not bad.

Now I have to decide if I want to mod them to accept a commonly available bolt-in "tall ball joint", such as what's on my S10, which limits me to only 1" taller... or if I want to go full adjustable heim-joint to get more than that.

Decisions, decisions.

BTW, the pitfalls I'm hitting with my spacer idea have a lot to do with the type of ball joint configuration this car uses. The ball joint is mounted to the control arm, and the tapered pin goes into the knuckle. The only way to get more space ABOVE the ball joint is to extend the ball joint pin in some manner. Pretty awkward thing to do, as I've noted. But, this isn't the only configuration. Some cars (like some older VW's and Hondas, for sure) have an inverted ball joint. The tapered end goes into the control arm, and the ball joint itself is bolted to the knuckle. With THAT style, all you need is a simple spacer to bolt between the ball joint and the knuckle! Why can't my life be so easy? :bangwall:
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Postby twistedwankel » Fri Jan 05, 2018 3:46 pm

Loren wrote:Why can't my life be so easy?
Because you have an inquiring mind and a small wallet.
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Postby Rpwolf » Sun Jan 07, 2018 10:37 pm

thanks for the links, I'll throw it into the pile of bookmarks I have yet top catch up on
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Postby Loren » Sun Jan 07, 2018 10:50 pm

More hours of research and headscratching... but, here we are...

Looking at "Tall Ball Joints", the tallest available is always 1" extended. Nobody goes further than that. I kinda wanted to go to 1.5 or 1.75", but the fact that nobody makes that part has me thinking that maybe it's not such a good idea. And, looking at the half-inch diameter of a ball joint stud being that long and that spindly... it doesn't even LOOK like a good idea. I think I'm best to stick with something "tried and true". Plus, it will be cheaper than doing something completely custom.

So, here's the plan: Instead of going with a change of 1.75" that would put my roll center at 3.5", I'm going to stick with an off-the-shelf 1" extended ball joint that would put my roll center at about 2". And, since I'm modding the lower control arm, I'm going to extend it by .5", and I'm going to RAISE the ride height by a half-inch. All of that combined will put my roll center at a healthy 3", and my "roll couple" (the distance between the roll center and the CG) a right about the same length it would be if I just did a 1.75" extended ball joint. (I don't know where my CG actually is, so I can't quote a number for it)

Added bonus, extending the lower control arm will make the front track 1" wider, which will also help things. I was going to do the same thing with wheel spacers, so now I don't have to.

Exactly how I'm going to implement this solution is pending getting my hands on my spare set of control arms and looking at them. (and also verifying that the stock ball joint has a 7-degree taper) But, I'm leaning towards using Howe Racing rebuildable ball joints because they have a solid reputation and appear to be well-built. And I can choose between the Chrysler style threaded "screw in" style (which I can get a threaded ring to weld onto the control arm for), or the GM style which used 4 bolts to secure it to the control arm.

I'm kind of excited to see how much of a difference this will make in how the car feels, as well as how much the body rolls and if it tames that front wheel lift. I've got plenty of experience with the effects of making a car lower and stiffer, and with the effects of alignment changes. This will be the first time I've made a change that was purely a "geometry correction", leaving everything else the same.
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Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
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Postby Loren » Thu Jan 11, 2018 3:43 pm

By the way, I've been thinking about this, but haven't mentioned it here.

Bump Steer. Probably deserves its own topic at some point.

But, by fitting shorter springs to lower the car, we've changed the angle of the lower control arm. We know that. Sort of most of the point of this thread. We ALSO changed the angle of the steering tie rod.

The tie rod (like the control arm) "should" be near level in its static condition. This makes "bump steer" or "roll steer" minimal as it is right in the middle of the range of motion of that control arm. When level, it's at its longest point in the arc. Plus or minus an inch or so, it's still mostly at its longest point. That's how things are designed.

Lower the car, now the end of the tie-rod is pointed up. It's already past its longest point, thus it's no longer "hovering" in that vicinity. ANY additional suspension compression is making that tie-rod significantly SHORTER. If your steering rack is forward of the axle, this results in toe IN. If your rack is behind the axle, you get toe OUT. When all you did was hit a bump with one wheel, you don't want EITHER of those things to happen much. You want the car to keep steering straight!

So, while I'm addressing the control arm and trying to get it more level (to get all of the Roll Center benefits that go with it), I should also strongly consider addressing the tie-rod end to correct that angle and eliminate the bump steer that has been induced by lowering the car.

AND... because I'm modding the lower control arm to move the ball joint OUT further, I should incorporate that into the correction that I'll be doing for the tie rod. It needs to be made a half-inch longer to allow enough adjustment for proper toe. Failure to do that could result in either the tie rod end being unthreaded from the tie rod too far (dangerous!), or to alignment being limited to a possibly significant Toe-OUT condition (on my car). Since I'll be fixing the tie rod angle, might as well be sure to fix that, too.

Nothing is ever as simple as it seems.

Another factor to consider is that by moving the lower ball joint out by 1/2", I'm adding about 2 degrees of negative camber. But, I've got at least that much adjustment in my strut, and it's currently maxed out giving me -3.5 degrees of camber. So, I should be able to compensate for that without issue. (and if I can't get it there... I can always adjust at the top)

Along with that... if I get the camber corrected to exactly where it is now, the net effect of moving the lower ball joint out by 1/2" is that the entire wheel and tire will be moved out by 1/2". Do I have clearance for that? Why, yes, yes I do!

As far as I know, I'm working within safe parameters for what I'm intended to do. "What could possibly go wrong?"
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The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.

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