Rear Roll Center Calculations and Questions

[RoadRacer78]

I have been all over the internets looking for a roll center calculator. Everything I have found you have to pay for. And for what I want to do I don't want to pay $150. It's probably a simple question to answer.

I want to figure out what the rear roll center does and how much when you use LCA drop brackets. Anyone have any info on this?

Thanks

 

[Mad Hatter]

Instant Center Calculator for your drag car suspension

Calculate the instant center on your drag car suspension geometry.

www.baselinesuspensions.com

Thats from the "trying to learn squat" thread here on TMO. lots of info in that and other threads here.

 

[RoadRacer78]

Instant Center Calculator for your drag car suspension

Calculate the instant center on your drag car suspension geometry.

www.baselinesuspensions.com

Thats from the "trying to learn squat" thread here on TMO. lots of info in that and other threads here.

I must be missing the part where it shows roll center changes?????

 

[Mad Hatter]

I must be missing the part where it shows roll center changes?????

I know, but in the threads its talked about quite a bit, In particular the LCA angles you mentioned.

 

[RoadRacer78]

I know, but in the threads its talked about quite a bit, In particular the LCA angles you mentioned.

I'm looking for hard numbers.

 

[Fabman]

I have been all over the internets looking for a roll center calculator. Everything I have found you have to pay for. And for what I want to do I don't want to pay $150. It's probably a simple question to answer.

I want to figure out what the rear roll center does and how much when you use LCA drop brackets. Anyone have any info on this?

Thanks

Where the panhard bar crosses the centerline of the car, measured straight down to the ground is the rear roll center.
If you have a watts link, the center pivot to the ground is the roll center.
Drop brackets have no effect on roll center with a 3 link like a s197, but they do if you have a 4 link with opposing angles upper vs lower.
Drop brackets on a 3 link only effect anti squat and roll steer.

 

[Fabman]

Additionally, if you have a diff mounted watts link (pivot on the housing) the roll center stays constant with the ground but the center of gravity rises and falls above it.
If you have a chassis mounted watts link (pivot on a bracket off the frame) the roll center stays constant with the center of gravity and moves up and down relative to the ground.
In either case, what effects handling is the distance from the roll center to the center of gravity. The length of that "Moment arm" is everything.
The longer that arm is (roll center closer to the ground) the more traction is available, but the more body roll you will have.
The shorter it is (roll center moved up) the looser the car will be but will exhibit less body roll.
So, its part of the whole chassis setup. Not everybody likes the same thing, so season to taste.

 

[RoadRacer78]

Additionally, if you have a diff mounted watts link (pivot on the housing) the roll center stays constant with the ground but the center of gravity rises and falls above it.
If you have a chassis mounted watts link (pivot on a bracket off the frame) the roll center stays constant with the center of gravity and moves up and down relative to the ground.
In either case, what effects handling is the distance from the roll center to the center of gravity. The length of that "Moment arm" is everything.
The longer that arm is (roll center closer to the ground) the more traction is available, but the more body roll you will have.
The shorter it is (roll center moved up) the looser the car will be but will exhibit less body roll.
So, its part of the whole chassis setup. Not everybody likes the same thing, so season to taste.

I don't tell you that I love you enough.

 

[RoadRacer78]

Drop brackets on a 3 link only effect anti squat and roll steer.

Addressing that there are almost never absolutes when talking about suspension up front............................Would you agree that drop brackets don't have much effect on the "handling" of a street car > canyon carver (simply can't drive that car that hard in those environments)?

I can absolutely tell when a car has drop brackets......A LOT. But what I haven't been able to tell is if they effect cornering performance on the street. I haven't been able to do a A-B-A test on a track.

 

[Fabman]

I always tell people that if you have a watts link (or a panhard bar that is adjustable for height) go from all the way up to the top hole and drive it, then all the way down to the bottom hole and drive it. (on track)
You will immediately have a crystal clear understanding of what's going on that no one will ever be able to miss inform you about. its eye opening.
This will help you find the best setting for your driving style and also how to adjust for different setups or track layout. Most watts have 4 or more settings.
Its a free education.

 

[Fabman]

Addressing that there are almost never absolutes when talking about suspension up front............................Would you agree that drop brackets don't have much effect on the "handling" of a street car > canyon carver (simply can't drive that car that hard in those environments)?

I can absolutely tell when a car has drop brackets......A LOT. But what I haven't been able to tell is if they effect cornering performance on the street. I haven't been able to do a A-B-A test on a track.

Track is the place for all testing to be done. Eliminates almost all the variables.

 

[RoadRacer78]

I always tell people that if you have a watts link (or a panhard bar that is adjustable for height) go from all the way up to the top hole and drive it, then all the way down to the bottom hole and drive it. (on track)
You will immediately have a crystal clear understanding of what's going on that no one will ever be able to miss inform you about. its eye opening.
This will help you find the best setting for your driving style and also how to adjust for different setups or track layout. Most watts have 4 or more settings.
Its a free education.

This is exactly how I like to test (on track). One extreme to another. Only takes one lap per change. I call it "bracketing".

 

[Fabman]

This is exactly how I like to test (on track). One extreme to another. Only takes one lap per change. I call it "bracketing".

Then you are way ahead of most of your contemporaries.

 

[RoadRacer78]

Track is the place for all testing to be done. Eliminates almost all the variables.

Agreed.

The "problem" I'm currently solving for is a really good street setup. Jack of all trades. Lots of compromises everywhere. How it works on the track is kind of irrelevant.

I'm sick to death of "Muh slammed $1000 coilovers handle great" crap. You know, the ones with triple the spring rates you want and valving from who knows where. And throw some rear control arms with urethane bushings in the mix so the axle binds up all over the place just for funzies and spins.

I'm going to use my POS 3v as a test bed. Put together a killer street combo (which will of course be pretty good on track) that actually rides nice and test it against itself with your average tard Facebook combo. Then make some video's.

 

[RoadRacer78]

Then you are way ahead of most of your contemporaries.

I was pretty hot shiznit when I was doing superbike stuff. I miss it. But I don't miss being at the track 30-35 weekends a year. I don't miss having suspension fluid all over me on weekdays. And you can only make so much money (how many hours you can work). You can't scale the business because if you teach someone they leave with what you taught them and become the competition. The up front cost of setting shop up (or mobile shop) is cheap.

 

[BackInBlack]

When I went from a Whiteline watts link (using lower holes) on a stockish BOSS 302 suspension setup to a factory panhard bar, it felt as if I had swapped in a smaller rear sway bar. After some reading and chatting with TMO folks, I think what happened was that I was feeling the roll center change. So if I understand it right, I slightly lowered my roll center which is somewhat similar to putting on a smaller rear bar. I leave it Fabman to correct me if I'm wrong!

 

[Fabman]

I was pretty hot shiznit when I was doing superbike stuff. I miss it. But I don't miss being at the track 30-35 weekends a year. I don't miss having suspension fluid all over me on weekdays. And you can only make so much money (how many hours you can work). You can't scale the business because if you teach someone they leave with what you taught them and become the competition. The up front cost of setting shop up (or mobile shop) is cheap.

Don't I know it....

 

[Fabman]

When I went from a Whiteline watts link (using lower holes) on a stockish BOSS 302 suspension setup to a factory panhard bar, it felt as if I had swapped in a smaller rear sway bar. After some reading and chatting with TMO folks, I think what happened was that I was feeling the roll center change. So if I understand it right, I slightly lowered my roll center which is somewhat similar to putting on a smaller rear bar. I leave it Fabman to correct me if I'm wrong!

bingo.

 

[Fabman]

When I went from a Whiteline watts link (using lower holes) on a stockish BOSS 302 suspension setup to a factory panhard bar, it felt as if I had swapped in a smaller rear sway bar. After some reading and chatting with TMO folks, I think what happened was that I was feeling the roll center change. So if I understand it right, I slightly lowered my roll center which is somewhat similar to putting on a smaller rear bar. I leave it Fabman to correct me if I'm wrong!

***Provided you actually lowered the roll center center...you'd have to measure to be sure, but that description is correct.
Lower roll center acts like a smaller rear sway bar because the jacking forces are less.
(jacking forces for another discussion at a later time-I need to get some work done)