Custom Control Arms
Posted: Sat Dec 09, 2017 4:40 am
Didn't want to clutter up the Mirage "build thread" with this, as it's just ideas at this point.
I've lowered the car 2". I need to take some measurements and do some calculations, but general wisdom says that I've probably lowered the roll center more than the CG, which is bad. (more distance between roll center and CG = more leverage for your body roll to work with)
This happens because the angle of the lower control arms changed. They started out level. Now, because the body of the car is lower, the inner pivot of the control arm is lower. The solution to the problem is to bring that control arm back to level. The common fix is either a trick control arm that repositions the ball joint, or just a "tall ball joint" that effectively moves the end of the control arm down.
Of course, this is a Mitsubishi Mirage. Nobody makes trick control arms or non-standard ball joints. (in fact, the ball joint is theoretically not replaceable separate from the control arm)
Assuming my measurement and calculation indicates that I should fix this problem, I'm thinking about modding the control arms. I know it sounds scary. I used to freak out about it, too. But, I've looked closer at factory steel control arms, the fact that they are welded from the factory... and those welds don't always look that great! Plus, an awful lot of folks are driving around on control arms that are at least marginally compromised by rust. Bottom line: They are an overbuilt part. If they are modded in an even more overbuilt manner, no problem.
And, I've already played with modding a set of Miata control arms. Worked out great. My fear is gone. (mind you, I won't be welding this with my low-precision stick welder)
If I'm going to be hacking up control arms, might as well make the ball joint replaceable. And if I'm going to do that, I want it to be a dirt cheap, common, and durable replacement. There are ball joint "sleeves" available for both standard press-in style or thread-in style ball joints. Could get a set of those and just weld them in. Or, the ol' S10 uses the same suspension as the GM J-body (that's 70's-80's Camaro, Monte Carlo, etc) and is very popular in the hot-rodding world. Stock S10 ball joints can be had for less than $15 each, and would be plenty strong enough for a Mirage. Bonus: The S10 ball joints simply bolt in with four bolts. Would be really easy to fab a flat plate onto the control arm to attach it to. (but, the thread-in Mopar ball joints are cool, too... and equally inexpensive)
Before I get too carried away... I need to know the dimensions of the stud taper of the Mirage ball joint. I'll still be going into the same spindle, so I need the same size and taper. Since the Mirage doesn't have a replaceable ball joint, the only way to get those dimensions is to take the car apart and measure it. (or get a spare control arm... which... I probably should do, anyway)
So, yeah. Just a little mental exercise tonight thinking about how to solve the roll center problem (if it exists). Seems pretty simple once I've done some more measurement. The end result should be to get the control arms back to level, and maybe extend the control arm a bit. Need to do some math on that, too. Extending the control arm changes the angle of the strut ("steering included angle"), which is one of the angles used to derive the roll center... I need to consider the pros and cons of moving it.
This might all sounds completely nuts... but, check around the modified classes with cars that have better aftermarket support. (like BRZ/FRS) You'll find that if the rules allow it, and they are available, custom lower control arms are not at all uncommon. I just have to make mine.
I've lowered the car 2". I need to take some measurements and do some calculations, but general wisdom says that I've probably lowered the roll center more than the CG, which is bad. (more distance between roll center and CG = more leverage for your body roll to work with)
This happens because the angle of the lower control arms changed. They started out level. Now, because the body of the car is lower, the inner pivot of the control arm is lower. The solution to the problem is to bring that control arm back to level. The common fix is either a trick control arm that repositions the ball joint, or just a "tall ball joint" that effectively moves the end of the control arm down.
Of course, this is a Mitsubishi Mirage. Nobody makes trick control arms or non-standard ball joints. (in fact, the ball joint is theoretically not replaceable separate from the control arm)
Assuming my measurement and calculation indicates that I should fix this problem, I'm thinking about modding the control arms. I know it sounds scary. I used to freak out about it, too. But, I've looked closer at factory steel control arms, the fact that they are welded from the factory... and those welds don't always look that great! Plus, an awful lot of folks are driving around on control arms that are at least marginally compromised by rust. Bottom line: They are an overbuilt part. If they are modded in an even more overbuilt manner, no problem.
And, I've already played with modding a set of Miata control arms. Worked out great. My fear is gone. (mind you, I won't be welding this with my low-precision stick welder)
If I'm going to be hacking up control arms, might as well make the ball joint replaceable. And if I'm going to do that, I want it to be a dirt cheap, common, and durable replacement. There are ball joint "sleeves" available for both standard press-in style or thread-in style ball joints. Could get a set of those and just weld them in. Or, the ol' S10 uses the same suspension as the GM J-body (that's 70's-80's Camaro, Monte Carlo, etc) and is very popular in the hot-rodding world. Stock S10 ball joints can be had for less than $15 each, and would be plenty strong enough for a Mirage. Bonus: The S10 ball joints simply bolt in with four bolts. Would be really easy to fab a flat plate onto the control arm to attach it to. (but, the thread-in Mopar ball joints are cool, too... and equally inexpensive)
Before I get too carried away... I need to know the dimensions of the stud taper of the Mirage ball joint. I'll still be going into the same spindle, so I need the same size and taper. Since the Mirage doesn't have a replaceable ball joint, the only way to get those dimensions is to take the car apart and measure it. (or get a spare control arm... which... I probably should do, anyway)
So, yeah. Just a little mental exercise tonight thinking about how to solve the roll center problem (if it exists). Seems pretty simple once I've done some more measurement. The end result should be to get the control arms back to level, and maybe extend the control arm a bit. Need to do some math on that, too. Extending the control arm changes the angle of the strut ("steering included angle"), which is one of the angles used to derive the roll center... I need to consider the pros and cons of moving it.
This might all sounds completely nuts... but, check around the modified classes with cars that have better aftermarket support. (like BRZ/FRS) You'll find that if the rules allow it, and they are available, custom lower control arms are not at all uncommon. I just have to make mine.