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RSX Suspension Modeling Results

33K views 106 replies 40 participants last post by  XDC5SX 
#1 · (Edited)
I recently took a vehicle dynamics class. We needed to do a class project, so I (and two additional group members) chose to model the RSX suspension. I am attaching the final report so that everyone here can benefit from it. Note, I am posting this here in the autocross forum rather than the suspension forum because it is more relevant to those of us who track our cars or do autocross.

Basically, the car's suspension was measured so that the location of every suspension point was known in x, y, and z coordinates. Next, the suspension was modeled in Solidworks. From this point, lots of info can be measured/calculated.

Anywho, we looked at camber gain, both from suspension compression and from steering input. We also did roll center heights and bump steer. All of these were done for the front and rear. Additionally, aftermarket modifications were 'made' to the model and their effects on the above parameters analyzed. The modifications made were roll center adjusting ball joints, caster/camber plates that move the strut top inward 3/4" and rearward 3/4", raising the inner tie rod ends 1.25", and lowering the inboard side of the rear top link of the rear suspension 3/4".

DISCLAIMER: Every attempt was made to be as accurate as possible during this project. However, it is exceedingly difficult to measure the suspension perfectly. As such, I do not guarantee any values as being 100% exact. They should be pretty darn good though.
 

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#4 ·
Your recommendation to maximize camber using bolts rather than plates is interesting. I theorized the same thing and had the shop max my Ingalls bolts out before touching the plates. In addition, it minimizes the toe impact since you're moving the strut, and therefore the steering arm, less.
 
#7 ·
Thanks guys! This is probably some of the best information I have gotten from this board. As an off road racer, we are more concerned with tuning our suspension to go through things rather than around them! Not being an engineer, I have been kind of struggling with the principals of how our cars work. Discussions like these really help.
 
#9 · (Edited)
Best thread I've come across on this site so far. But then again, I'm biased towards technical gems like this. This should be required reading for anybody who tracks/races an RSX.

Thanks for sharing!

Edit: Do you have any calculated info for motion ratios? I've found some values elsewhere on CRSX, but it would be interesting to see if your modelling produced similar results. Maybe some of the mods you analyzed affect the motion ratios; that would be handy to know.

And again: When you refer to the "nominal location" of the suspension, is that the OEM ride height or something lower? I'm just wondering what ride height actually corresponds to the zero compression reference on the plots.
 
#15 · (Edited)
wait wait wait...

Why does shimming the rear upper link raise roll center?

Did I draw this wrong? Seems like roll center gets lowered.


Also, how does the use of camber plates improve camber gain under compression? Just less compliance?

Lol the results of camber plates + RCA + raised steering rack bracket is alarming. Although I'm going to keep my current setup. Explains why I like the feeling of lots of toe out and the lack of traction is I run 0 toe as opposed to toe out.
 
#16 ·
I was waiting until you noticed that:) It has troubled me too. Unfortunately, I don't have a concrete answer for you. I do know that the rear suspension only vaguely resembles a double wishbone design. The lower control arm 'hinge' is at a huge angle, rather than parallel to the centerline of the car. Plus, the upright/knuckle is attached to the lower control arm at a crazy angle itself. All this 3D nature must be having an effect...I don't know why else the roll center would raise. It doesn't seem intuitive, but I trust the model and calculations.:dontknow:
 
#20 ·
I'm curious though... everything I've read about this chassis for the last 9 years, as well as generally understood behavior of macstrut suspensions, suggests camber loss with compression. Unless I'm reading this wrong, you're suggesting the RSX gains negative camber w/ compression? That's an entirely new assertion. It makes me weary of the model.

I'm not challenging your intent or qualifications, but as a software engineer I know everything, especially the first time around, has at least a few bugs ;)
 
#21 ·
I'm curious though... everything I've read about this chassis for the last 9 years, as well as generally understood behavior of macstrut suspensions, suggests camber loss with compression. Unless I'm reading this wrong, you're suggesting the RSX gains negative camber w/ compression? That's an entirely new assertion. It makes me weary of the model.
I was under the impression most McPherson strut system gained a little negative camber with compression, but just very little camber in comparison to other suspensions(ie: dbl wishbone, etc)...

Positive camber gain during compression would be horrible for handling, be it a street car or a race car. :dontknow:
 
#22 ·
I hear u matt. I've seen those posts too. On my many attempts to maximize neg camber I found the same... but was quickly told I was wrong by some.... Measuring was done at alignment shops. Whe the war was lowered neg camber increased.

Thanks speedengineer... can't wait to read this tonight
 
#25 ·
When people say that it gains positive camber, they are likely meaning that it gains positive camber relative to the ground (body roll included). MacPherson Strut suspensions definitely do have some negative camber gain. They will continue to gain negative camber until the angle between the strut and the control arm becomes obtuse (aka greater than 90 degrees). That will never happen on our cars no matter how much you lower it.
 
#26 ·
Regarding the bump-in that occurs on the front suspension.... Ive been thinking about this for the past few hours.

At what point does the angle of the tie rod cause binding within the balljoint and limit/restrict movement during steering operation??

Also, does anyone have a good approximation of the stock tie rod angle? Or a picture of tie rods(from inside the engine bay) at the stock ride height?
 
#28 ·
Regarding the bump-in that occurs on the front suspension.... Ive been thinking about this for the past few hours.

At what point does the angle of the tie rod cause binding within the balljoint and limit/restrict movement during steering operation??
Yeah, not sure when that would be an issue. I bet it would be pretty huge angle though.

Also, does anyone have a good approximation of the stock tie rod angle? Or a picture of tie rods(from inside the engine bay) at the stock ride height?
It is somewhere around 6.3 degrees. At stock ride height with BC RSD's, it was 6.3 degrees. As far as I know, buddy club and oem struts have the tie rod attach in the same location, but I don't know for sure. Either way, 6.3 gets you in the ballpark.
 
#33 ·
Buddyclub RSD has lower tie rod end mounting points then factory to compensate for the lowered ride height. 6.3 degrees with RSD at stock height would not be the same as stock struts at stock height (essentially the 6.3 angle reading you got is less then what it is stock).
 
#37 ·
So with stock the toe in would be even less severe under compression?

And Mark, since you have the suspension out of the car and an extra RSD bracket, why don't you measure the height difference for us. :)
 
#39 · (Edited)
Note: Pictures taken are of the old version RSD brackets. They may be different, but I believe the steering arm is mounted at the same height and that the only change was beefing up the bracket itself.

I mounted from the top of the steering arm to the middle of the first hole where the bracket meets the knuckle. I got the numbers by trying to keep the measuring tape parallel to the shock and then taking a thin envelope to help provide a visual reference point on the measuring tape to the strut.

The RSD brackets measured in at about 4.25"


OEM suspension measured in at about 5.5"


It would probably be safe to say that the RSD steering arms are about 1.25" lower than OEM.

However, one must remember that the actual angle of the tie rods will be determined by how much the car is lowered.

Edit: I won't be able to check back for awhile since I'm flying out to NYC tonight and will be gone for a week.
 
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