Static vs. Dynamic rear suspension

[LowBuckX]

Dont know quite how to word this so bare with me.
'


Hypethetical but useing my static numbers
Static

Anti-Squat 40.81%
Roll Axis Angle 0.00degrees ( - roll understeer, + roll oversteer)Roll Center Height 9.00in
Instant Center X-Axis 102.00in
Instant Center Z-Axis 8.25in

Now I nail the gas coming out of turn 1 ass squats say .750 inches at the front LCA mount point (.875 upper frame mount) and now Im at
Anti-Squat 84.66%
Roll Axis Angle1.67degrees ( - roll understeer, + roll oversteer)
Roll Center Height9.35in
Instant Center X-Axis 59.50in
Instant Center Z-Axis 9.98in

Is this good/bad or is Static measurments all we need be concerned about..

Like I said Ive no clue the correct way to ask this so please shed some lite on this subject even if you are not answering to my specific numbers just static vs dynamic involving a rear link type Pro-touring suspension..... geeze sorry im beeing so vague...

[ArtosDracon]

This is the disadvantage to such a long IC. Your AS rate of increase is HUGE! And your IC has jumped from almost to the front wheels, to just under where you're sitting. Your roll axis angle change is a bit high, but manageable. 45% AS increase across .75" is really really high though, and to get only a .75" rear squat on full acceleration is going to mean 225lb + springs on a 3200lb car, so your back end is likely to buck on any bump and feel even harsher due to that increase in AS. Also, with your instant center at 8-9" and your COG at, say, 20" then your car is going to roll on that axis, and on taht 11-12" difference, which is going to require a heavier swaybar than something with a higher IC or lower COG. It took me months of long nights and brain-ache to get to the understanding level I have, and I'm not a third of the way to what proffesional chassis engineers know, hopefully one of the others can chime in and help more than I can.

Static ride height is far from all you should be worried about.

[Norm Peterson]

Static geometry only means that the upper and lower links are inclined at specific side view and plan view angles. That represents one data point on the anti-squat curve, one on the roll steer curve, and one on a similar curve for roll center height (if you're tracking that). How things change is at least as important as where they are when they do change. To use an analogy, static numbers are a snapshot when what you want is video.

Link lengths (absolute and relative to each other) go to determine what happens when the suspension actually moves (as you'd like it to do, else you'd just make a big go-kart with doors and bodywork by welding everything in the suspension solid and not worry about any of this geometry stuff).

You really want to see what happens at other values for suspension compression (and extension as well). There is no guarantee that what happens in the next 0.75" will be the same as what you have calculated for the first 0.75".


BTW, having roll oversteer on corner exit might discourage you from driving it very hard off the corners more than once or twice.


Norm

[ArtosDracon]

BTW, having roll oversteer on corner exit might discourage you from driving it very hard off the corners more than once or twice.

Norm

I've heard those 360's mean you get more points...

[Bryce]

I am wondering if your calculations are correct. I am going through the same designs right now with the two rear suspension designs I am working on.

With my set up, horizontal LCA and a downward slopping UCA, I lose A/S under compression. So as I squat I get less and less A/S. This situation reduces the force on the rear tires as you accelerate. This reduces the lateral traction of your rear tires in two ways; one you are accelrating and making the tire use some of its available traction to move forward, two you are reducing the dynamic load on the rear tires.

On the other side of that, on my drag car with over 100% A/S as I lift the back end under compression I gain A/S. This puts more and more force into the tire as long as I am accelerating.

[LowBuckX]

BTW, having roll oversteer on corner exit might discourage you from driving it very hard off the corners more than once or twice.


Norm

Since you pointed that out I rechecked and I added height when I should have subtracted


Anti-Squat46.40%

Roll Axis Angle-1.67degrees ( - roll understeer, + roll oversteer)

Roll Center Height8.65in

Instant Center X-Axis68.13in

Instant Center Z-Axis6.27in

These height changes at the frame end at not actual just off the top of my head for discussion purposes only. Im moving my A/s to 60-65% anyway but dont have those numbers yet.

[Mean 69]

Agreed, static only tells a snapshot of the story of importance.

From the large changes in IC migration (and the resulting large changes in the AS, etc), I'd surmize that you are forced to run pretty short link lengths, probably due to packaging? In side view, how long are your uppers and lowers, approximately?

Mark

[LowBuckX]

18 inch upper 26 lower

[LowBuckX]

upper is 18 lower is 26.. Someone pointed something out to me. I was solely taking into account rear drop but not the front riseing as it would so my numbers dynammic mean squat (pun alert)

[Norm Peterson]

Once you start considering the nose rising under acceleration, it gets lots more complicated since the CG does not necessarily remain at the same height either.

I think you'd also have to get involved with all four wheel rates and the actual aceleration in order to determine where the pivot points actually are.


Norm

[ArtosDracon]

upper is 18 lower is 26.. Someone pointed something out to me. I was solely taking into account rear drop but not the front riseing as it would so my numbers dynammic mean squat (pun alert)

You may consider shortening the lower if you can't extend the upper, the disparity you have now is likely amplifying the AS and Roll Axis change.

[LowBuckX]

I think Ill play with what I have. Not running the nurburgring just the Steets and back roads.

[exwestracer]

As noted, one concern would be the amount of IC change for the limited amount of travel, especially with the relatively long static IC you provided. Looking at your height (Z) numbers, it would seem that the lowers are fairly level at static...?

To cut the IC (X) length almost in half with less than 1" of squat says you have a LOT of down angle on the upper links, but the long static length seems to contradict this, unless the uppers are mounted very high over the axle housing?

Probably a stupid question, but are we talking about a Tri-4 setup here? Are you measuring the true side view length of the uppers, or the eye to eye length of the bars themselves?

[LowBuckX]

this is a 3 link. the upper is high But not as high as Lateral dynamics or the Morrison 3-link. . Im at a 5.5 degree down slope and measurement is Eye to eye but I will crawl under and measure Side view length

[ArtosDracon]

This is a parallel three link with an offset upper right?

[Scott Hightower]

When I work on my geometry I plot it through the entire length of travel both in bump and rebound.

From there you can see a trend. Then I make adjustments in overall length, upper and lower length ratio, and mounting points and plot the new results.

In short the answer to your initial question is "Dynamic" you must completely understand the dynamic changes going on so you can de-bugg the system.

Scott
Fab Manager
Welders360.com

[LowBuckX]

This is a parallel three link with an offset upper right?

Correct

I have to get under the car and remeasure everything better because Ive followed all the advice I could find on this site when building and Ive been following it as close as possible but some things have been clearing up through these most recent threads. I know Im in the ball park just need to take better measurments. CAR FEELS 100% BETTER THAN THE LEAFS CRUISING SO AT LEAST iM HAPPY ABOUT THAT.

[exwestracer]

this is a 3 link. the upper is high But not as high as Lateral dynamics or the Morrison 3-link. . Im at a 5.5 degree down slope and measurement is Eye to eye but I will crawl under and measure Side view length

Oh, then that would explain the giant "Follow my 3 link build" sig...haha.

At 5.5deg, you shouldn't have anywhere near that much change in IC length in 1" of travel... As much as I am a fan of computer modeling, sometimes there is no substitute for drawing the thing out full size on the shop floor and connecting the dots...

[LowBuckX]

Oh, then that would explain the giant "Follow my 3 link build" sig...haha.

At 5.5deg, you shouldn't have anywhere near that much change in IC length in 1" of travel... As much as I am a fan of computer modeling, sometimes there is no substitute for drawing the thing out full size on the shop floor and connecting the dots...

That is next plan of action. This program Im useing is a little simplistic. Im a See it ~Learn it ~Do it type.

[ArtosDracon]

Cardboard or Balsa miniatures are a favorite of mine. 1/10th scale makes the math nice and easy.