MKII Suspension Analysis: Lowering, 91vs93, Graphs, ect

12/29/07: Added analysis for the front, see this thread


EDIT DEC 10th: Replaced 93 graphs with graphs using 93 crossmember dimensions supplied by twoina2. Rescaled Y axis on all toe graphs. For refrence, I run roughly .1" of toe in on my 91, so the max toe changes we see in roll for either suspension are roughly 10% of the total toe. Comments related to the reacent edit are shown in blue.


I have been doing a lot of work these last couple of weeks with a suspension analysis program called Optimum K on our Formula SAE car. If anybody else wants to try it out they have a free fully functional demo available, only limitation is you can't save.

I decided to try to analyse the MR2 Suspension, hopefully I can shed some real light on the age old debates about the differences between the 91 and 93 suspensions, the effects of lowering, ect. At this point I have only analysed the rear, and I wouldnt exactly call it a "full" analysis, but I think its a good starting point.

Please note:
1). I got the input data from my own car and a tape measure, so accuracy is +-1/4" maybe. That being said, these results arn't perfect, and shouldnt be used as the final word, especially where the exact values are concerned. However, they should be good enough to spot trends and get a general idea.

2). The 93 is assumed to have a 1/2" lower ride height than the 91. The lowering ammounts are based off of my car with Tien S-techs, and added onto from there to approximate the heights for a more modest spring, the 93 stock springs, and the 91 stock springs.

3). The body roll used is to the right. AKA, the car is in a left hand turn. Therefore, the right wheel is the outside wheel, the really important one. The outside (right) wheel is the dashed line on the graphs. The inside (left) is the solid line.

4). Heave (vertical chassi motion) is defined positive up, negative down.

5). I analyzed 3 different ride heights. Stock (Green), a modest lowering spring such as the Pro Kit or the TRD (Blue), and a more extreme drop such as the S-Tech, the Sportline, or an aggressive coilover drop (Red). This coloring scheme is used throughout.

Now, some results.

First, the 91 suspension in roll. On the lowered graphs you will notice that the curve "freaks out". This is due to extreme roll center migration, to the point that the program doesn't know what to do with it. Basically, ignore the plots after about 1* of body roll. Because of this I think that anyone lowering with performance in mind should STRONGLY consider stiffer sway bars as well, to limit body roll and keep the roll center under the chassis.

One thing to note is that lowering has very little impact on camber curves (which, for what its worth, pretty much suck). Moderate lowering has minimal effect on toe characteristics, however extreme lowering really makes the toe go nuts. I'm not sure if it was the rescale or if I did something different this time, but lowering, even extremely, does not seem to have as big of an effect anymore. If it was a change in my method I believe it to be for the better, as this time I actually setup different models for each state of lowering instead of adjusting the same model over and over. Much less chance for error, hopefully.

Next, the 93 in roll. Again ignore the curves past about 1* of roll. The choppy graph is due to the roll center migrating far outside of the cars wheel base, causing the program to produce errors.


Heres the 91 in heave.


And the 93 in heave.


The heave characteristics are much less interesting, IMHO, although they do show the increased toe in character of the 93.

Some new analysis: Neither rear suspension changes toe MUCH in roll. Both maintain nearly constant toe. The 91 still responds less drastically to lowering, however the change suggested by twoina2 severely reduced the negative impact of lowering on the 93.

It appears now that at stock ride height the 93 toes in on the inner wheel and out on the outer wheel, effectively causing slight oversteer. Severe lowering reverses this, effectively limiting oversteer. The changes are very slight however, maybe not noticeable.

Stock the 91 appears to toe in the outer wheel and toe out the inner wheel (VERY SLIGHTLY). Lowering the 91 has even less impact than on the 93, although in the 91's case extreme lowering should slightly increase the tendency toward oversteer, as it makes the outside wheel toe out slighly more and the inside toe in slightly less than stock.

In conclusion, I might reverse my statement about lowering. The effects on toe characteristics appear to be minor, such that a little bit of static toe in can easily compensate. However, the issue of roll center migration is still present, and should be mitigated by larger sway bars on lowered cars.

SO! Discuss, rip apart my methods, tell me what else to test, tell me what I missed, whatever. I just hope the community can benefit from this in some way. The more thoughtful discussion we have on the topic the better.