Developing a Proper Suspension Model

[Shankenstein]

It's a multi-link, so everything interacts with everything else.

Given how far forward the toe link attaches to the upright (white arrow in your pic), it does most of the toe management.

Caster isn't a big deal for rear suspension, since you're not steering.

The remaining links (long lower-rear arm, short lower-front arm, and upper wishbone) manage camber and transfer most of the suspension forces. You can consider the two lower links to be an "H arm" with one side disjointed to improve anti-squat.


The relative lengths of the upper and lower arms will dictate camber, but generally as you compress the suspension we will see more negative camber with an exponential curve. I'd expect it to be similar to this (A = rebound, B = compression):


Regarding your question, the geometry of your front lateral link has a minor contribution to toe, moderate contribution to camber, and lots of impact on anti-squat.

[Tristor]

Has anyone produced a single downloadable/importable model that we can use in any of the available software to model the suspension geometry ourselves to see what the affect of various suspension modifications is? This thread has a ton of useful information, but I had not yet seen a finalized document. Was this abandoned?

[Butterman]

What's the datum for the suspension coordinates? I've got optimum K and happy to make a model and share it.

[DIY]

I am currently considering converting the front end to a double wishbone.

I plan to use knuckles from a car that comes standard with this suspension type. From my research there are many options but once you mandate a 5x114.3 stud pattern this list gets smaller.

Some options are late model Mazda MX5, Honda S2k and Mazda RX8.

From these options the RX8 has the most similar dimensions to my BRZ. In fact I have been able to figure out that the HKS TRB-03 is in fact running the RX8 factory lower control arms, I have not been able to find photos showing the knuckle they are using but I would not be surprised if it is also from the RX8.



I have found some rough dimensions of the control arms and knuckles on Locost forums although I will fabricate my own control arms and only use the knuckles from the RX8 ideally to keep the standard track width of the BRZ.

The tricky part for me will be the upper control arm length and mounting them in the optimum position. Working this out using some measurements and software is where my skills are lacking and maybe you guys can help?

I'm guessing the procedure would be to take the figures from this current suspension model and then feed them into the "double wishbone" version and play with the upper arm length and chassis mount points until you get the desirable camber change with roll etc. all within the size constraints of the chassis.

The rear end of my car is already done and uses the standard geometry with the exception of lowering the trailing arm chassis attachment point a little and it also has multiple control arm mounting points for different ride heights, the diff can be raised too.

[Butterman]

Found some great points of info from Car and Driver Via Morse Measurements. RCs height, anti-squat and anti-dive per axle, roll stiffness, etc. Useful stuff in general to understand the dynamics of the 86.


These could used to help verify the suspension coordinate estimates.



@DIY I'm looking to develop a suspension model for the 86, just for fun at this point. Once this is done I can help you develop a double wish bone setup. Why do you want a double wishbone setup? Is there something about the stock macpherson that you're unhappy with?

[DIY]

Originally Posted by Butterman

Why do you want a double wishbone setup? Is there something about the stock macpherson that you're unhappy with?

The main appeal of the double wishbone is better control of camber. As you can see my car is a ground up track car build and I believe double wishbone is the best choice for this purpose.

[Butterman]

Originally Posted by DIY

The main appeal of the double wishbone is better control of camber. As you can see my car is a ground up track car build and I believe double wishbone is the best choice for this purpose.

Ok, just wanted to make sure your expectations were reasonable. I think it would be a relatively straight forward process to design a double wishbone setup with stock handling characteristics but with more dynamic camber gain.

Lets PM so we don't clutter this up with our back and forth.

[wparsons]

Is there enough room for a long enough UCA under stock fenders? I love the idea, but it's pretty tight in there up front.

[DIY]

Originally Posted by wparsons

Is there enough room for a long enough UCA under stock fenders? I love the idea, but it's pretty tight in there up front.

Look at the photo I posted above of the HKS 86, they were able to make it work.

[wparsons]

Originally Posted by DIY

Look at the photo I posted above of the HKS 86, they were able to make it work.

Those look like at least 6" flares... your comment about standard track width made me assume you wouldn't be running huge flares.


*edit* - looking at this angle, it looks like it's even more than 6" wider per corner

[DIY]

Originally Posted by wparsons

Those look like at least 6" flares... your comment about standard track width made me assume you wouldn't be running huge flares.

*edit* - looking at this angle, it looks like it's even more than 6" wider per corner

Yes I would say the extra width on that car is on the wheel offset though and they are running at least 295 width tyres. You can see in my photo they are still using the factory Mazda aluminium lower control arms so the extra width must be happening with the wheels.

[jamal]

That assumes the mazda links grafted onto a subaru are the same distance apart as they would be on an rx8. Seems that perhaps the brz, with it's much wider engine, has the chassis rails a little farther apart?

You would not want to add that much track just with wheel offset. Scrub radius would be huge.

[DIY]

It runs 18 x 11 +30 wheels, so that's 69mm extra extension on the outside edge per side compared to stock wheel size.

The RX8 is only 19mm less front track width than an 86 although it does use a slightly wider wheel which would have a small effect on the arm position.

I will be purchasing the entire front end of a RX8 very soon so would be happy to share some measurements.

[wparsons]

Originally Posted by DIY

Yes I would say the extra width on that car is on the wheel offset though and they are running at least 295 width tyres. You can see in my photo they are still using the factory Mazda aluminium lower control arms so the extra width must be happening with the wheels.

Unless you've already measured and know that the attachment points are the same distance apart as on the FRS/BRZ (or have measured the arm lengths), seeing the stock arms doesn't tell you anything. If the attachment points are further apart it'll be wider with the same length arms too.

Originally Posted by DIY

It runs 18 x 11 +30 wheels, so that's 69mm extra extension on the outside edge per side compared to stock wheel size.

The RX8 is only 19mm less front track width than an 86 although it does use a slightly wider wheel which would have a small effect on the arm position.

I will be purchasing the entire front end of a RX8 very soon so would be happy to share some measurements.

69mm = 2.72", that's way less than the extra width.