Has anyone ever calculated the suspension roll center for stock and/or lowered J-bodies? If I can get the gumption up to do it, I'm going to try to. I understand how to calculate the roll center, just never done it in a real-world scenario (always been given the measurements first).
Well on stock suspension its a roller coaster, lowered or more stiff suspension no body roll. JK i dont know man i dont even know how to measure it
Well yeah, that's the basic theory behind the roll center. For good handling characteristics you want to minimize roll center movement, but going too low can actually lower your roll center to a point where extra roll is induced due to lower roll center versus CG location, increasing the torque induced on the roll center by the cornering forces applied.
Generally optimal lowering for handling is around 1.5"
juicedz4 did on his 2nd gen. He also corner weighted it before he swapped to coil-overs.
The optimum lowering is when the control arms are parallel with the ground, or around 1.5" like CTS said.
Actually I havent quite calculated it yet. Im waiting to get my new setup all together so I can take some (hopefully detailed) measurements etc.
And yes, anytime you lower a car with McPherson front suspension the roll center moves lower much faster than the center of gravity. As this difference grows, so does the car tendency to roll. This is due to the "lever arm" created by the distance between the RC and CG.
For sure you never want the control arms to point up. However, higher than level may actualy work quite well too. I know that the VW guys who race dont lower much if any. They increase spring rates for sure. Their setups are almost identical to ours.... MK3 GTI ( I own one
etc.
Dan
YEP, my 60' times suck
247HP/267ftlbs at 13psi with built motor
Project updates and stuff:
www.clubhousecustoms.com
email: dan@clubhousecustoms.com
Yeah, that whole "arms parallel to the gound" thing, that applies to our cars. That's not some general rule of thumb. But for our cars the optimal height is where they are just about parallel, and that's an easy way to describe it.
I'll break out the math book tonight and see if I have it written down somewhere...
juicedz4 wrote:And yes, anytime you lower a car with McPherson front suspension the roll center moves lower much faster than the center of gravity. As this difference grows, so does the car tendency to roll. This is due to the "lever arm" created by the distance between the RC and CG.
For sure you never want the control arms to point up. However, higher than level may actualy work quite well too. I know that the VW guys who race dont lower much if any. They increase spring rates for sure. Their setups are almost identical to ours.... MK3 GTI ( I own one etc.
Dan
This is exactly what I was trying to ask about. I know the theory behind the roll center and the effect of roll center height versus CG height. The hard part is actually finding the roll center and CG.
C.T.S wrote:Yeah, that whole "arms parallel to the gound" thing, that applies to our cars. That's not some general rule of thumb. But for our cars the optimal height is where they are just about parallel, and that's an easy way to describe it.
Do you have some method of proof? Not trying to be inflammitory or anything, I just want to know. Lowering the ride height does affect both the CG and the roll center, and since stiffer springs and struts are used, the roll center doesn't move around as much as it would at stock height. I would think that leaving the arms at a positive angle with respect to the ground parallel would be more beneficial than a completely parallel arm. However, I haven't done any measurements myself to experiment with.
it definitely would be nice if someone had the ability to take accurat emeasurements and figure out where the roll center is and how it moves.
Calculating the roll center is the "easy" part. Figuring out where the CG lies is another thing. Finding the roll center is about useless without knowing where CG is. Would probably have to estimate the GC height (which is different for the front and rear suspensions).
Dan
YEP, my 60' times suck
247HP/267ftlbs at 13psi with built motor
Project updates and stuff:
www.clubhousecustoms.com
email: dan@clubhousecustoms.com
Take a look at my CG thread, I just posted how to find the complete CG.
Your CG height is the same throughout a whole plain. Your GC point is the intersection of three planes of CG.
The concept of CG in a plane is, on a piece of physics string (infinitely small, infinitely strong), you could run a line on the intersection with the body, and balance it perfectly.
Thus, if you find the CG along your three axes, any force applied to the car can be equated to a force applied to the CG.
Basically, your CG height is your CG height... it's at one spot and doesn't move (assuming no mass moves within the body).
Your roll center, however is different between your front and rear which means you have different torque arm lenghts acting upon the suspension at the front and rear.
^^^^ your right.
It must be too early for me to type about suspension stuff.
Maybee I will try to do the CG measurement. I think i could make up a solid strut without a ton of work. it would be fairly easy to do now before I bolt my new front coilovers in for good...
Ill see how things go once the back is all together. My springs are due today, so I could probalby get to the measurements next week.
I also need to measure my bump steer, for which I made my own gauge.
So much to do....
Dan
YEP, my 60' times suck
247HP/267ftlbs at 13psi with built motor
Project updates and stuff:
www.clubhousecustoms.com
email: dan@clubhousecustoms.com
