Developing a Proper Suspension Model

[Racecomp Engineering]

I had some more things to say but I'm totally blanking. I just came back from lunch.

- andy

[fika84]

Thanks for all that Andy! Have you played around with the roll center correction kits at all?

[Racecomp Engineering]

Originally Posted by fika84

Thanks for all that Andy! Have you played around with the roll center correction kits at all?

Yes and they are helpful. I can tell if one is installed in a back to back drive but the effect isn't super noticeable unless you're at an inch of drop or more. I've only tried the Whiteline kit which doesn't relocate the outer pivot a ton but it's easy and includes the tie-rod extenders for bumpsteer.

With the bigger drops on these cars you NEED roll center correction and/or some very firm spring rates especially up front (and that's pretty obvious whether you're looking at the KRC or FAPs or just the camber curve). A lot of people JUST try to pick spring rates on these cars by picking suspension frequencies and designing for a flat ride at 80 mph or whatever and IMO that's leaving a lot on the table and pretty shortsighted.

- Andy

[RBbugBITme]

I think this analysis from post #216 may be the most informative. The saw tooth at the far right of this analysis is at OEM ride height. You're looking at camber and FAP height and from other images I've posted, the kinematic roll center is also under good control here, which is not a coincidence since it is also the area with the least camber gain.



What I see is at steady ride height, the FAP heights vary greatly with roll angle. The FAPs also pass through the ground plane at lower ride heights and you can see from the list on the right JackCGL is negative while JackCGR is positive when FAPheightL is negative and FAPheightR is positive. I would think just like I stated with the kinematic roll center, you wouldn't really want the FAPs to pass back and forth through the ground plane every time you turn the car.

Additionally, camber gain with some steering is quite large even at OEM ride height and you guys now have that quantified. I think this is also why Jarrett at Raceseng suggested up to 1deg more caster made a big improvement.

[Thorpedo]

Originally Posted by Racecomp Engineering

With the bigger drops on these cars you NEED roll center correction and/or some very firm spring rates especially up front (and that's pretty obvious whether you're looking at the KRC or FAPs or just the camber curve).
- Andy

Care to toss out an approximate value on what a "bigger drop" is? Also, I saw you mentioned the whiteline kit doesn't do a lot of correction. What one would you recommend to make a bigger difference? I see Megan makes a really affordable RC correction option, but I haven't yet emailed these companies to inquire about the actual measured differences vs OEM. Any help or input is appreciated. Hopefully I don't seem lazy in asking this :/

[fika84]

Originally Posted by Racecomp Engineering

Yes and they are helpful. I can tell if one is installed in a back to back drive but the effect isn't super noticeable unless you're at an inch of drop or more. I've only tried the Whiteline kit which doesn't relocate the outer pivot a ton but it's easy and includes the tie-rod extenders for bumpsteer.

With the bigger drops on these cars you NEED roll center correction and/or some very firm spring rates especially up front (and that's pretty obvious whether you're looking at the KRC or FAPs or just the camber curve). A lot of people JUST try to pick spring rates on these cars by picking suspension frequencies and designing for a flat ride at 80 mph or whatever and IMO that's leaving a lot on the table and pretty shortsighted.

- Andy

Thanks for the input. I'm a little over an inch drop on ST Suspension coilovers. I don't do much tracking, but as my daughter gets older (only a month old at the moment) I'm sure I will venture to the track more! Either way, something else to add to the list of mods!!

[Racecomp Engineering]

Originally Posted by Thorpedo

Care to toss out an approximate value on what a "bigger drop" is? Also, I saw you mentioned the whiteline kit doesn't do a lot of correction. What one would you recommend to make a bigger difference? I see Megan makes a really affordable RC correction option, but I haven't yet emailed these companies to inquire about the actual measured differences vs OEM. Any help or input is appreciated. Hopefully I don't seem lazy in asking this :/

IMO an inch is a "bigger drop" for some of the same reasons Ryan said...past that drop things start to change more drastically. Not insurmountable without the RCK but it is nice to have at that point.

I do recommend the WL kit even though it's not a ton of correction...it's just a pretty well designed complete kit at a good price. With just the ball joint extenders you have a bumpsteer problem and a lot of the more expensive kits don't account for that.

- Andy

[ZDan]

Originally Posted by RBbugBITme

You're right but the roll center for this car isn't that controlled. Depending on your ride height you can see RC heights during roll from -1" to +10" or +1" to -10". That is a big swing worth noting.

That's exactly the group of curves that I don't believe. They are greatly exaggerating RC height movement with roll the nearer static RC height gets to 0.

Its also worth noting that RC movement is fairly well controlled at stock ride height so I would guess a properly designed roll center correction kit would be worth while.

Looking at those curves again, if far left is -3" and steps are 0.1", black vertical line is -1", but the far right is more like +0.5", not +2".
Either way, at OEM height this curve family still suggests a lot of movement of RC height with roll, about the SAME as for -2" lowered! Curves show that at OEM height the RC height is getting lower with roll, so as you corner you lose roll stiffness, whereas at -2" your gaining a similar amount of roll stiffness. But I don't think that's what's really happening. For sure it's not happening to the extent suggested for the -1" lowered case!

Originally Posted by RBbugBITme

I would think just like I stated with the kinematic roll center, you wouldn't really want the FAPs to pass back and forth through the ground plane every time you turn the car.

Whatever points are "passing through the ground plane", it is still a (nearly) linear and continuous change. It's not like a sudden discontinuous change in handling behavior.

Nothing magical or spooky happens when these points pass the ground plane.

Is it better to have the roll center somewhat above the ground plane? To a point, yes. But in most cases, lowering a car with no geometry-"correcting" mods will still give handling benefits despite the lowered roll center height.

I would worry more about usable suspension travel when lowering.

[Captain Snooze]

Originally Posted by Racecomp Engineering

IMO an inch is a "bigger drop" for some of the same reasons Ryan said...past that drop things start to change more drastically. Not insurmountable without the RCK but it is nice to have at that point.

I do recommend the WL kit even though it's not a ton of correction...it's just a pretty well designed complete kit at a good price. With just the ball joint extenders you have a bumpsteer problem and a lot of the more expensive kits don't account for that.

- Andy

Can you please tell me what the consequence/s is/are of fitting a RCK only at the front?

[fika84]

I'm pretty sure its because the rear is double wishbone and not quite as sensitive to lowering. Just my guess. I'm sure Andy will know best.

[Calum]

[Racecomp Engineering]

Originally Posted by fika84

I'm pretty sure its because the rear is double wishbone and not quite as sensitive to lowering. Just my guess. I'm sure Andy will know best.

Mostly that. It's much more necessary at the front. You can look at the plots Ryan posted and basically with a 1 inch drop you'd have a 1 inch lower COG but the geometry and camber curve of the stock ride height. That's good. Or 20 mm, I can't remember the exact correction the WL kit gets you and I'm at the bar now.

It would increase front grip and decrease understeer.

- Andy

[7thgear]

[Calum]

Originally Posted by Racecomp Engineering

First a good link on FAPs (Force Application Points) vs. Kinematic roll centers that might help everyone that's interested in this discussion:

https://drive.google.com/file/d/0B3p...ew?usp=sharing

It's worth reading...and rereading a couple more times.

- Andy

I downloaded that to my phone the other day, expecting to read it at work. I tried to open it today but it's blank. I've tried to open it on my work computer aswell, nothing. Is anyone able to read what Andy linked to?