Angle of Control arms (Suspension theory thread) - Page 3 - Mazda3 Forums : The #1 Mazda 3 Forum
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post #21 of 512 (permalink) Old 06-11-2013, 12:44 PM
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Nuse, as luck would have it, I have a stock 3 (2007 hatch) to get measurements from.

It'll take me some time to reacquaint myself with all the measurements, but I could probably lend a hand in this regard. Once I get some good baselines, I can tell you what angles/measurements I need from your car to do the math and figure it out. As you've mentioned, your higher spring rates will limit the suspension travel, but there's still a reasonable amount of travel you need to keep in the suspension to account for roll before your control arms start increasing positive camber which is washing out your grip. My guess is you're experiencing this in the front versus the rear because Macpherson strut suspensions (front) do not gain much camber in travel versus multilink/double wishbone (our rears are multilink) which gain a fair amount. This is more than likely the reason you're not running into a limitation on the rear.

I'll get engaged in this. It may be till next weekend before I can get too much done (recovering from a nasty stomach flu this past weekend, but at least I GOT MY A/C FIXED!!!) but I'll lend what knowledge I can. I just have to translate all this Miata specific knowledge to a new platform, and get rid of some of my old preset suggestions as they don't translate well to a 500lb heavier FWD car.

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post #22 of 512 (permalink) Old 06-11-2013, 01:00 PM Thread Starter
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(about damn time; it's getting hot for sure here in GA!)

I can get very accurate measurements as I have access to every part imaginable thanks to Mazmart.
That said, that won't allow me to get angles of said parts (junk yard with half the cars taken apart) but I do have access to knuckles, control arms, etc.

It would be really freaking nice to get a very accurate reading on everything for the sake of the community.

I was reading through a section of VWVortex and they had done some math with their setup (slow day at work; can't complain). And oddly enough they began to see quite a large negative camber gain between a 1-2" drop (theoretically of course; dimensions were incorrect BUT... it goes to prove what I was asking about), and less negative gain at 0" drop, and of course decreasing after 2".. so the MacStrut can probably have a good negative camber gain, question is... where.

What would also be interesting is to see if I should go to a stiffer rear spring setup; otherwise I can keep my rears just as stiff and go to a softer spring up front no problem. (I just don't know right now).

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post #23 of 512 (permalink) Old 06-11-2013, 02:09 PM
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I mean I guess what I'm curious about (because I dont' know), is like I said there a point where the arc decreases camber and if so, where (within reason of course).
This is going to be when the LCA is perpendicular to the line from the strut top to the lower ball joint. That's guaranteed from basic geometry. What you'd need to measure then is the angle and length of this line at stock ride height.


You're right, everybody would love to know where the CG really is! Only way to be absolutely certain is with a crane that you can dangle the car from, but there are ways of doing a decent job of estimation. The fore/aft position should be deducible from the weight balance on the front/rear wheels. The height should be deducible in one of two ways:

1) tilt the car nose up/down while it's on scales. The height of the CG will dictate how quickly the front/rear balance shifts.

2) figure out the roll stiffness and roll center height, then collect data on lateral acceleration and body roll while driving. That should let you find the rolling moment, and thus the CG height.

The second method is nearly impossible to get correct, since you need to know the instant roll center during the turn, which is hard to be sure of. You also need to know the other numbers very accurately. I think method 1 is more reasonable, since it only requires some ramps and scales.

I hadn't even thought about scrub radius. As long as it's within reasonable limits, what does it impact, other than steering feel?
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post #24 of 512 (permalink) Old 06-11-2013, 02:22 PM
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1) tilt the car nose up/down while it's on scales. The height of the CG will dictate how quickly the front/rear balance shifts.........I hadn't even thought about scrub radius. As long as it's within reasonable limits, what does it impact, other than steering feel?
With accurate measurements of the suspension arms, and accurate corner weights (at a minimum front and rear axle weight) I believe we can mathematically approximate all the information that's being called for. Of course, the only accurate way to do this is with a Five Post Machine, but unless anyone has one lying around (pretty much only OEMs or an F1 team) we'll have to settle for math and approximation. It'll be close enough to at least make some recomendations from, and trial and error from there.

Let me get the math together, and then I'll peg Nuse for the measurements since those are often easier whith the hardware off of the car. I can get the angles and ride height from my car. Let's try to line up someone on a Pro Kit for comparison. Any angle finding app on an iphone can produce accurate enough info for our purposes, as there's going to be some margin for error.

The scrub radius affects steering feel, and to some extent brake torque/torque steer/bump steer. As a general rule, I try not to deviate too far from OEM as 0 scrub radius is damned near impossible to achieve outside of a purpose built race car, and increasing scrub radius serves no productive purpose. There are limits when widening the wheels, however, and you often have to compromise with some scrub. It's al give and take at that point.

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post #25 of 512 (permalink) Old 06-11-2013, 02:28 PM
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You won't be able to find the vertical position of the CG without tilting the car, physics simply doesn't allow for it. You should only need F/R weight balance at a few different pitch angles though, so its not impossible to get the data. Just measure it level, measure it with the front on ramps, then with the rear on ramps.

For the other stuff, I agree -- simple measuring tools are quite good enough to measure the the lengths/angles of the suspension links.
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post #26 of 512 (permalink) Old 06-11-2013, 02:50 PM
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Problem with transferring weight by lifting the car is that the suspension will compress/unload and skew your results.

You'll have to lock the suspension solid to find the CG.


Oh and: http://farnorthracing.com/autocross_secrets.html
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post #27 of 512 (permalink) Old 06-11-2013, 04:10 PM
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Good point! I hadn't thought of that. Still, it's easier to fabricate some rigid surrogates for the dampers than it is to find somebody willing to dangle your car from a crane
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post #28 of 512 (permalink) Old 06-11-2013, 04:22 PM
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Quote:
Originally Posted by brandini View Post
Problem with transferring weight by lifting the car is that the suspension will compress/unload and skew your results.

You'll have to lock the suspension solid to find the CG.


Oh and: http://farnorthracing.com/autocross_secrets.html
We could also see how much suspension flex will affect the outcome by measuring fendergap/ride height during the corner weighting. If the change is neglible, it could be ignored. It will skew the results, but if we're talking a very small percentage then I vote it's not worth locking down the suspension. We're modding a production car, not racing F1 here.

Nuse's spring rates are already so high it shouldn't move much. On a stock car, that difference may be quite a bit. It's a valid point though.

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post #29 of 512 (permalink) Old 06-11-2013, 04:34 PM Thread Starter
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About to say mine is stiff; BUT my car is FAARRRR from stock (gutted, roll bar, battery relocation etc. etc.)

I dangle my car from a crane every week YO!

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post #30 of 512 (permalink) Old 06-11-2013, 07:21 PM Thread Starter
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Here's the silly angle of my LCAs.

I believe those that buy BCs and lower them all the way (stock setup) this is about where it will put the LCA angle at.

With that assumed; the Tein S-techs and Eibach Sportlines are probably near this angle as well (a bit less, but not much)

And I definitely need to sweep out my garage floor I know lol.




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