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Old 01-26-2010, 11:37 PM   #1
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Cool How how lowering your suspension affects suspension geometry

This subject came up in my D23 race engine build thread, so I needed to spin-off this subject to the suspension forum!

While I'm waiting to get my hands on the valve seat cutting tools from a friend, and the block is still out at the machine shop, we might as well cover this whole suspension drop issues and how it impacts the suspension geometry. There are tons of books and internet references to this, but I'll take a stab at it on my own here.....

Here's why lowering a Focus (or any car)can "tamper" with the suspension geometery, and specifically its impact on camber.

FYI -

Camber is the tilt of the top of the tire as viewed from above, tilt the top outwards for positive camber, tilt the top inwards, you have negative camber.
Camber "gain" is the change created in this angle as the suspension moves through it's range motion. If the top of the tire angles outwards as the suspension compresses, this is "positive camber gain", and when the top of the tire angles inwards, this is "negative camber gain".


Ususually you want to have a "negative camber gain" as the vehicle rolls onto the outer wheels and compresses the suspension, to help offset the body roll and keep the tire sitting a flat as possible.

if the LCA on a mac strut car is angled down when static, (like it is on our Foci) as the car turns, rolls and compresses the suspension, the LCA swings through an arc that moves the Lower Ball joint a small distance in the outwards direction. This changes the angle of the strut/spindle/wheel increasing Negative Camber. (I.E. Negative Camber Gain)

If you drop the suspension so the LCA sits horizontal when static, as the car turns, rolls and compresses the suspension the LCA will continue to swing through it's arc. But it is now moving the ball joint inwards as the arc is curving up and inwards away from its static horizontal position. As the ball joint is moving towards the center, so is the lower portion of the tire. This causes the top of the tire to angle away from the car producing "positive camber gain" making it more difficult to keep the tire flat on the track.

as wrc_fan suggests, most drivers will never worry about the little differences with these dynamic angles etc, but on the track or autocross its as important as having a few points more of horsepower.

While you can just crank up the static camber to an extreme negative number to keep the tires flat on turns, this dramatically reduces your braking abilities, and can make it difficult to keep tire temps where you want them. It also affects your ability to put power down mid corner becuse the inside tire is just touching on a small area on the inside edges of the tread.

Our simple single LCA and strut design of the front suspension does not offer any "tricks" of geometry provided by more advanced multi-link front suspension like seen on honda, acura, BMW, and countless other cars that have moved on from a simple strut design. So the best you can do (when allowed by the racing class) is raise the inner pivot points to set the LCA in a slight "droop" condition when sitting at static ride height. This helps recover that little camber gain effect as the suspension rolls and compresses the suspension so the LCA is horizontal when the turn/roll/compression is at it's peak, and you get the most camber gain possible.

This allows a milder static camber adjustment, producing better braking effectiveness/stability, and a bit more rubber in contact with the track on the inboad wheels to help power out of a turn.

And all of these benefits help keep tire temps more uniform etc.


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Old 01-26-2010, 11:48 PM   #2
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Excellant write up. Stickied
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Old 01-26-2010, 11:56 PM   #3
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Quote:
Originally Posted by norcalfocus01 View Post
Excellant write up. Stickied
That didn't take long! Thanks......
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Old 01-27-2010, 12:17 AM   #4
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How do you move the pivot points on the front LCA?
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Old 01-27-2010, 08:31 AM   #5
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Sure makes a lot more sense when you explained it here. Nice.
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Old 01-27-2010, 09:14 AM   #6
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Nice writeup! I love hearing from people that knows what they are tallking about. Unless of course your copy and paste skills are as good as mine....
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Old 01-27-2010, 09:44 AM   #7
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One other thing to note is that the body roll of a vehicle also adds positive camber to the outside tire while you are in a turn. So we have suspension compression and body roll contributing to the camber angle of the tire.

Stiffer sway bars can help to reduce this body roll impact, but the tradeoff is lift up and spin of the inside front tire on corner exit (even with a torsion style differential) from the weight transfer by the sway bars. Stiffer springs can help reduce the body roll, but you can compromise suspension motion.

Long story short, suspension modification is a constant battle of trade-offs.
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Old 01-27-2010, 06:35 PM   #8
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I've been dealing with these tradeoffs too. my car is quite low, probably too low, but it looks good dammit.

My plan to deal with the loss of camber gain was huge static camber. I bought LCR plates to do this. I found, as you mentioned above, that i had lost too much acceleration and braking traction.

My solution was to swap the camber plates from driver to passenger (and vice versa) and rotate 270. What this did for me was to give me enough negative camber adjustment for my needs, but also gave me appeciable castor.
So now, when i auto-x. i have good straight line accel and braking, and when i corner, the castor provides more camber than I lose through the position of the control arm.
win-win in my opinion.

I dont have any alignment printouts from this setup, because I do my alignments (mostly toe adjustments) in my driveway, and now I'm on my winter springs. So in the spring when i go back to coilovers and LCR's. i can put it on a rack and let you know.
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Old 01-27-2010, 10:08 PM   #9
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A good, clear discussion. Well worth the sticky. Thanks, and rep given!
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Old 01-28-2010, 12:17 PM   #10
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The reason MacPherson strut equipped cars have this problem is because the angle of the lower control arm relative to the chassis changes as you lower the car. Lower too far and the arm tilts the "wrong" way (up), lower on the chassis side and higher on the strut side--and the rolling of the car has a greater mechanical advantage on the strut causing it to compress the spring easier as the weight shifts to the outside. That change in geometry is what changes the roll center.

When you lower a car with MacPherson struts the roll center lowers farther/faster than the CG. So if at the stock height the roll center is, for example, 4 inches below the CG, when you lower the car 2" the CG drops 2" but the roll center drops say 6". The actual tendancy of the car to lean during cornering then increases. To counter that tendency you need MUCH larger springs. Not ordinary lowering springs in the 200-400lb range--but 500-600lb race springs.

"If you can imagine picking up a barbell with a weight on just one side---if you pick it up close to the weight it's easy to lift/handle, but if you pick it up at the opposite end of the weight (which is a great forearm workout) it becomes more difficult as you lose mechanical advantage. The length or arm length creates a greater "polar moment of inertia" (distance from roll center to Center of Gravity). So with stock geometry your CG and roll center are close together (like grabbing the bar next to the weight which requires less effort or less springrate to control the roll) but as you lower the car the roll center drops quicker than the CG creating more mechanical advantage (makes the car top heavy)/longer arm/requiring more spring rate"---like I said above, quite a bit more.

Here is a good illustration:


http://www.modified.com/tech/0508_sc...t_3/index.html

Here is a real-world example http://buildafastercar.com/tech/Roll-Centers : ...go back to the example earlier where we found that lowering a Subaru WRX STI drops the front roll center by 2.6 times as much as the chassis height (which also means 2.6 times as fast as the chassis's center of gravity). As it turns out, a one-inch lower ride height actually increases the vehicle's roll couple by about 18%, which means body roll will also increase by 18% unless firmer springs are used to resist this force. The springs would need to be 18% firmer just to maintain stock-like chassis roll numbers. Again, that's 18% just to break even!
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