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Old 07-18-2011, 08:13 PM
Wheelman Wheelman is offline
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Default Auto-X suspension tuning, 240Z, want feedback

This last winter I made major upgrades to the suspension of my 73 240Z and now it's developed a mild case of understeer.
I want to run some ideas for tweaks past you guys, see what you think.

First, here is the current setup.
350lbs/in springs front
325lbs/in springs rear
Koni Yellows, at softest rebound damping front and rear
MSA anti-roll bar set 1" front, 7/8" rear.
Energy suspension poly bushings every where there is a bushing
R200, 3:54 CLSD
1" roll center spacers
-2 degrees front camber (have to use adjustable LCAs to get more)
-1.5 degrees rear camber
+3.5 degrees caster
~0 toe in front and close to stock in the rear.
Stock control arms (for now)
ride height about 4" at frame rail, basically level front to rear, at least as much I can tell.
sectioned strut tubes (not hitting bump stops)
New Kumho V710s, 265/45-16, 22psi front, 20psi rear

The car's weight distribution is about 52/48.

The goal is to get the car to a more neutral to slightly loose setup
This is for auto-x, not high speed, lots of quick transitions
Here are my ideas
1. Move the heavier springs to the rear.
2. Raise the rear ride height (already raised it 1/4" which helped some)
3. Run more rebound damping in the rear
4. Use adjustable LCAs in the front to get more negative camber
5. Remove camber from the rear
6. Play with tire pressures
I don't really want to as the pressures I'm running are to get the tires to roll over all the way to the edge of the tread, big tires on a light car. Tried going lower in the front but turn-in became vague.

The understeer is not horrible, this is more of a fine tuning exercise than anything. The car puts power down very well exiting elements but will push if I apply too much mid-corner, I'd rather it rotate a bit instead.
This is why I'm thinking the heavier springs may work better in the rear, there's not a lot of difference between them, it would soften the front a little and at the same time stiffen the rear.
I also have a set of 250lbs/in and 225lbs/in springs I took off.

Feel free to ask questions and make suggestions.
Future plans are to add adjustable control arms to the front and rear, a splitter and most likely a wing for higher speed stability
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73 240Z, LT1, 5-Speed
92 240SX SE, KA24DE-T

Last edited by Wheelman; 02-17-2013 at 03:41 PM..
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Old 07-18-2011, 08:52 PM
Bubba Bubba is offline
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Tried playing with toe in the front?
In my head, because I visualize everything, if you did toe-in up front it would make the front turn faster. Just a thought though. I don't mess with my suspension enough to give you a solid answer.
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Old 07-18-2011, 10:47 PM
Wheelman Wheelman is offline
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Originally Posted by Bubba View Post
Tried playing with toe in the front?
In my head, because I visualize everything, if you did toe-in up front it would make the front turn faster. Just a thought though. I don't mess with my suspension enough to give you a solid answer.
Yes I tried running 1/8" toe out but found the inside front tire in tight turns had too much angle so was pretty much sliding. I'm not 100% sure why that setting produced that behavior, maybe I'm dealing with some bump steer.
I reset the toe back to about 0 and it felt much better but toe out in a front steer car like the Z is supposed to induce some ackerman affect so I gave it a try.
I've always been under the impression that toe in makes the front more stable in a straight line but tends toward under steer, thats why most manufactures design it into the suspension geometry, it's supposed to be safer.

I should try some toe in just to see how well it works. If I do have a bump steer issue it may work well to have the outside tire turned in a little more and the inside toed out by the bump steer effect.
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Old 07-19-2011, 05:40 AM
Jonty Jonty is offline
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Quote:
Originally Posted by Bubba View Post
Tried playing with toe in the front?
In my head, because I visualize everything, if you did toe-in up front it would make the front turn faster. Just a thought though. I don't mess with my suspension enough to give you a solid answer.
to visualise it you need to have the outside wheel at the desired angle, and then think what the inside does ref that, not the other way around.

This is because the outside wheel is most loaded and therefore providing most of the grip, so as a driver your butt accelerometer will feel getting the outside tyre to the best slip angle gives best cornering speed, and any less angle (i.e. when because of ackermann steering and tyre loading characteristics the inside is at its best slip angle) you won't be turning as fast. One thing some race cars have is zero or anti-ackermann steering, which causes the inside wheel to turn less than the outisde as less load means peak grip is found at a lower slip angle, so the inside wheel contributes more to the overall cornering effect.

HTH and makes sense!

Jonty
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Old 07-19-2011, 06:58 AM
Matt93SE Matt93SE is offline
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I'd vote on a bit more camber and caster if you can get it, and go to a bigger/stiffen the front sway. and see what happens.
can you slot the upper strut mounts in your class? it's a free way to get more camber..
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Old 08-10-2012, 08:30 AM
Empire Empire is offline
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it is awesome to find a forum where people actually care about this stuff. Most of the other forums wouldn't even talk about anything like this. They are all more concerned about who can scrape up the most reflectors when going down the high way.

good stuff.
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Old 08-10-2012, 12:48 PM
josh18_2k josh18_2k is offline
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Hi Ken!

More camber!!! And probably more front spring and/or bar. Conventional wisdom says more front roll stiffness = more under steer, but if roll stiffness fixes your alignment the opposite can be true. With a Mac strut car, you're always going to need your roll angle in camber as a minimum. I highly doubt you roll less than 3*

Compliance is a bitch, and takes up a sizable amount of your static camber. If you can use bearings in your flca, def do so. I use Delrin in mine. Urethane and oem are unacceptable on a racecar.

Regarding front toe, it really depends on the car and tire. And since none of us have tire data, it's all guessing (testing?) anyway.

And BTW, outside tire slip angle is just an output of normal force, camber, and lateral force. It's not from a steering input. The inside slip angle is dependent on the rest, and that's what you tune with Ackerman. If your inside tire doesn't do anything (steady state + accel on lots of Mac cars), it doesn't even matter.
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Old 08-10-2012, 07:54 PM
josh18_2k josh18_2k is offline
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Oh crap this thread is a year old lol
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Old 02-17-2013, 04:13 PM
Wheelman Wheelman is offline
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Doing some searching I ran across this old thread I started and figured I'd update it, thanks for all the input guys.

Here is what I ended up doing for last season:
1. Added adjustable tension/compression rods and dialed in ~6* caster.
2. Adjusted front for as much camber as I could get ~-2.5*.
3. Set rear camber to ~-2.0*.
4. Modified rear lateral link uprights so I could set rear toe to stock setup (it was offset to the left due to an accident).
5. Set front toe at 1/16" out.
6. I loosened the swaybar end links to reduce the stiffness and delay they're action.
7. Replaced solid front rotors with slotted units and eliminated the rear drums in favor of solid disks and Maxima calipers. Replaced the stock prop valve with an adjustable unit.
8. Replaced 3/4" brake master cylinder with a 1" unit. Allows better brake modulation but requires more foot pressure.

After the original post I found out the tires where rubbing on the TC rods in tight turns (basically turn arounds). This is what caused the sensation of the inside tire dragging but it wasn't sliding on the ground, it was on the TC rod. Strange sensation and reason for it. To eliminate this I installed limiters on the steering rack (its from a Subaru Forrester).

These changes produced the following:
The car now has very good turn in and slow speed tight corners are my friend. I used to hate turn arounds but this car now easily handles them. I have a small amount of understeer in fast steady state sweepers. Being able to adjust the front/rear brake bias has made it possible to tune out most of the brake lockup I was experiencing and allow better trail braking control.

I have yet to install adjustable front and rear control arms, this will probably wait for a year or so as I've decided to park the Z this season and build the 240SX instead. The Z is forced to run in our local prep/mod class (SCCA XP class) due to the LT1 in the engine bay. This class is demanding more than I want to invest in the Z to remain competitive so I'm going to concentrate on the 240SX for SM instead.
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92 240SX SE, KA24DE-T
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  #10  
Old 02-20-2013, 06:53 AM
Def Def is online now
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Needs more camber and caster up front IMO.
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