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RSX Suspension Modeling Results

29545 Views 106 Replies 40 Participants Last post by  XDC5SX, 
speedengineer
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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speedengineer
You're welcome.

I just went ahead and attached the raw suspension dimensions too, just in case anyone wants them.
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speedengineer
AER0 said:
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.
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Thanks!

Actually, I have been meaning to calculate the motion ratios as a function of suspension compression (they likely change a little bit as the wheel moves up). I'll report back after I do.

The nominal location refers to the stock/oem ride height, as the car sits normally (aka, zero on all the graphs).
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speedengineer
Temptazn_Mark said:
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.
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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:
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speedengineer
lando said:
I definitely agree it's horrible, but I always understood that that was what happened, and a primary reason for maximizing static neg camber in our cars. If I'm wrong then great :)
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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.
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speedengineer
JamesL said:
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??
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Yeah, not sure when that would be an issue. I bet it would be pretty huge angle though.

JamesL said:
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?
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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.
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speedengineer
Temptazn_Mark said:
Do you think stock tie rod angle is optimal? Or more angle?

If I lower my car 1.75 inches but the steering rack bracket raises the inner rods by 1.25 inches, doesn't that mean my tie rods should be about stock angle or maybe a little greater?
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Yeah, that sounds like your angle should be right about stock or a little more then. I would tend to think that you certainly don't want more toe-in than stock. I have no clue how much of a difference it will really make to lap times though...
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speedengineer
Temptazn_Mark said:
So because my angles are a little bit greater than factory, it should have a little less toe-in under compression than factory right?
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Yes.
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speedengineer
Importtuner said:
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).
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Temptazn_Mark said:


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Shit. :( In light of this information, looks like I'll have to remake the bump steer plot for the front. I measured the nominal angle at the buddy club height, not the oem height. Basically, add "with BC RSD" to all the lines on that plot. At stock, all those lines will come down a little bit then... sorry guys.
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speedengineer
Ok, thanks for the info. Have a good trip!
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speedengineer
Importtuner said:
While on the topic of toe. When you did the toe plot against compression for the front, do you know if plates that added just positive caster (staight forward and back, not angled or horizontle) had an effect on the toe under compression? Or was it only when you messed with camber it had an effect.
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I'm not sure because I only modeled the one situation. If I had to guess though, I would say that if you moved the strut only in caster and not in camber/kingpin, it would not affect toe much. However, if you were to increase camber/kingpin and not caster, it would have more of an effect. This is because the greater angle (kingpin) the strut is at, the more the attachment point for the outer tie rod moves inward with suspension compression, thus creating more toe in.
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speedengineer
matrix22 said:
bump steer: happens going over bumps, pot holes? tracks and autox's are generally not that bumpy.
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Ha, not quite. "bump steer" as it is called, simply refers to the fact that the wheel changes toe angle under suspension compression. For example, under braking, the front dives/compresses causing the wheels to toe in (rather excessively too).

I'm not sure where you are driving at, but most tracks are plenty bumpy enough, at least in the US.
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speedengineer
Ironically, my power steering FAILED a week ago on track. The high pressure output line blew and it spewed fluid ALL over the engine bay. Good news was I was able to get it fix that night (2am...) and return to drive again Sunday.

Personally, I would not even have driven the car on track had I not been able to fix it. Far too much effort to steer and it really didn't seem safe.
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