NS F355 Project (Page 65/73)
Bloozberry JAN 25, 08:31 AM

quote
Originally posted by Yarmouth Fiero:
Can anyone offer a layman's interpretation for the "suspension challenged" ie: me



Well, your static roll center is located at the point where the two lines that come up from the center of each tire patch intersect each other. At ride height (with both L&R suspensions level), those lines intersect at the chassis centerline at 9" below the center of gravity.

The next things you want to look at are camber change in roll, and roll center migration in roll. You might be able to do it longhand, but here's where a software program like Lotus Suspension Analyzer comes in handy. Try PM'ing Zac88GT to see if he'll lend a hand... he's helped me and fieroguru in the past.

Your configuration is going to produce some roll center migration, simply because it's unavoidable with a strut design. Whether it's going to be worse than the stock car or not, only an analysis can tell, though I'd say you'll find your roll center migration will be better than the stock Fiero's simply because at ride height, your lower control arms have a long way to go before they ever come close to becoming perpendicular to the strut. To get an appreciation for why it's an issue with a strut design, I'll paraphrase a discussion I had on page 13 of my build thread:

Bear in mind that the roll center location depends on the intersection of two lines, one perpendicular to the strut and the other through the lower control arm axis. Since in a strut-type suspension the strut angle doesn't move appreciably throughout the range of suspension travel, this leaves the change in angle of the control arm as the major variable responsible for the change in roll center location. In double wishbone style suspensions the rigid strut axis is replaced by moveable upper control arm which changes in angle as well as the lower control arm so it can be designed to hold the roll center location relatively constant. On the Chapman strut suspension, as the angle of the lower control arm approaches perpendicularity to the strut in jounce, the instantaneous center for that side of the suspension dives away towards infinity, taking the roll center on a nose dive and sending it out laterally as well. Do this for both sides and where the lines from the tire contact patches to their respective instantaneous centers intersect with each other, that's where the rear roll center will be located.

[This message has been edited by Bloozberry (edited 01-25-2015).]

Yarmouth Fiero JAN 25, 08:59 AM
Thanks for the feedback Blooz. I'll digest what you typed.

Until then, here is the chassis rolled 6 degrees with the corresponding suspension geometry. This was a helluva lotta work.



Edit: to add a dimension x 2

[This message has been edited by Yarmouth Fiero (edited 01-25-2015).]

Bloozberry JAN 25, 09:52 AM
You're still missing a dimension. You listed the height of the rear roll center (5.182") but not the lateral migration from the centerline. From the looks of things, it should be around 19.25".

[This message has been edited by Bloozberry (edited 01-25-2015).]

Yarmouth Fiero JAN 25, 09:53 AM
Crap........ I may have kept my upper strut bearing in the camber plate rigid when I rolled the chassis. That may be why my outside wheel has such a large angle and my inside wheel has such a small angle. I think allowing the top of the strut to pivot would let the outside wheel pivot down and the inside wheel pivot upwards.

Back to the drawing board.

Edit: Fixed that. You were close Blooz

[This message has been edited by Yarmouth Fiero (edited 01-25-2015).]

Bloozberry JAN 25, 10:15 AM
OK, so now if you add in the effect of the slight change in strut angles, you'll get the position of the rear roll center in the worst possible state. It'll change the roll center a bit, but not by much so it's looking pretty good. The stock '88 suspension at 6 degrees of body roll places the rear roll center at 82" from the centerline, and drops it 18.5" below static. Currently, your design shows a 23.2" lateral move and a 3.8" drop. Definitely a significant improvement over even an '88!
Yarmouth Fiero JAN 25, 10:21 AM
Thanks Blooz. I've gone back and looked at my struts and I did allow my struts to pivot. Because they are fixed to the spindles, the whole assembly pivoted about the ball joint center. My mistake is that I pivoted the whole camber plate as opposed to just the spherical bearings. I realized this when I turned my strut towers back on and saw that my camber plates were no longer sitting flat on the tower mounting surface.

I'll fix that and repost.

Edit to add: Is it correct to assume that since my struts did pivot as they should, I can glean from the drawing with 6 degree body roll that the camber in this worst case condition would be +3 degrees on the outboard wheel and -1.2 degrees on the inboard wheel. Is it also correct to assume that since both my struts are probably bottomed out at 6 degrees of body roll, that there is no need to calculate jounce and rebound conditions while rolling?

[This message has been edited by Yarmouth Fiero (edited 01-25-2015).]

Yarmouth Fiero JAN 25, 04:01 PM
Well I managed to capture the results of Body Roll vs Camber. That is tricky data to collect as the fixed points of the control arm pivot and the strut upper bearing are now each following their own unique scribed circle about the CG as the body and everything attached to it moves.

So here are the results, again plotted along with stock '88 data from Blooz. The results seem to indicate that while there is camber change during body roll, it seems to be significantly less than the stock suspension. Am I interpreting this data correctly?



You may notice that the final data points at 6 degrees of body roll don't quite match the earlier drawing I posted of the rear suspension and body with wheels rolled at 6 degrees. This is possibly due to the fact that the earlier drawing was just that, a drawing rolled to 6 degrees and I tried to represent the suspension moving accordingly in one jump. It's likely that my drawing was wrong. The actual data I collected in 0.5 degree increments should be the more accurate results so please take the picture of the car rolled with angles dimensioned with a grain of salt. In the end, its just a pretty picture. I will try to redraw the rear view of the car using the data collected at 6 degrees of body roll.
wftb JAN 25, 11:01 PM
I have been watching this thread since it started .The drawings amaze me and your work is amazing .But one thing I would like to suggest is that you move the inboard pivot mount for your rear lower tie rod to the bolt that holds the control arm and get it off the arm .Since your outer mount is on the same plane as the ball joint , bolting the inner mount to the arm mount puts it on the same plane .No calculations involved , it eliminates all possibility of toe movements top to bottom .
Yarmouth Fiero JAN 26, 07:17 AM
Thanks wftb. I have considered what you are suggesting as it would make bump steer zero. The only problem is that I would have to move the rear inner control arm mount almost 2" further forward which would essentially have the rear rail of the control arm swept backwards slightly as opposed to swept forward as it is now. So I may have to move the rear inboard control arm bushing farther aft and mounting the tie rod on the forward side of the bushing. I just have to make sure the tie rod doesn't want to pass through the 1" diameter cross brace of the control arm. I am sure before I actually build the control arms, I'll modify the design to improve it as you have suggested.
wftb JAN 26, 08:31 AM
I think your bracket to keep the toe in place is the best idea I have seen yet for a strut based Fiero suspension .No matter what the final design of arm ends up being , that ball joint to tie rod bracket is an easy solution to a big problem .Keep up the great work .

[This message has been edited by wftb (edited 01-26-2015).]