Those great things called "roll centers"

[Exar-Kun]

OK, so...upon request, I've been asked to define the roll center thing....

first off, everoyne nees to understand that the roll center depends completely on the geometry of the suspension, and that it DOES change duing cornering(since the geometry of the suspension changes during cornering)..

now, onto business....

the roll center is the point about which the body will try and roll whenever a cornering force is applied. It normally lies on the cnter line of the car, but its height depends upon the gemoetry of the suspension(like I said above) it can be above, on, or below the centerline of the car (and in some cases beneath ground level!)

cars have two roll centers, one front, one rear. if you connect the two points(they may be at different levels!) they form a "roll axis", the line about which the body's entirety rolls when cornering force gets applied.

now, onto the fun stuff.... WHAT DOES IT MEAN TO YOU?!?!

any force which causes the body to roll depends on the distance between the roll axis and the center of gravity of the car. the less the distance, the less the movement(bringing the two closer together...IE lowering the car if the roll axis was lower then the CG of the car) !

*race cars usually run a roll center a VERY slight bit lower than the CG(running them identical brings out other problems...like hitting bumps)*

you can slope, via different suspenions systems(and ride heights) the roll axis nose down or tail down....

now, the CG(center of gravity), like the roll centers, may be different front and rear(think like a dog bone...)

now, the cliff notes version of what adjusting roll centers does to a car and why

tilting the front roll center down(nose down) makes the front springs oppose a far greater force than the rear...but because the body must oppose the roll movenent across its entire roll axis, the rear springs and sampers find themselves compressed/extended just as much, exerting the same anti roll forces on the body and the same opposing roce on their wheels. in effect the body becomes a large torsion bar/beam, transfering stress/load from front to rear

this causes understeer!

thuslyinclining a roll axis nose down creates overtseer/reduces understeer andinclining a roll axis tail down created understeer/reduces oversteer..

(side note, this is why some springs lower the s13 more in the front and rear, such as my RS*R down springs, I would assume)

more later on how roll bars and things come into the equasion.-chet

[turtl631]

Sweet I'll try to understand that...

[veilside180sx]

a good link i picked off some M3 guys...http://e30m3performance.com/my...2.htm

[Nismo_Freak]

...IE lowering the car if the roll axis was lower then the CG of the car) !

Some items Chet didn't address, and from another prospective, and with a 240SX application.

When you lower the car you also have to keep in mind that the the COG (Center of Gravity) is typically lowered at a slower rate than the RC (roll center). This causes the car to gain excessive pitch (roll) due to the now greater distance between the two. However, this is where weight management and suspension resistance come into play.

Say we have a stock 240, we all notice the roll in a corner and we all can say the ride quality is soft. Well the roll comes not from poor suspension geometry, but from the soft factory springs being compressed. So naturally people decide to get sway bars or lower the car to reduce the roll. All they are really doing is increasing either the static or/and the dynamic spring rate, which is what prevents roll.

Where does roll center come into play? Simply remember what I said before, when you lower the car the RC lowers at a faster rate than the COG. Now theoretically if we had the exact same spring rates as before we would have even GREATER amounts of roll. This is because we are taking Roll 1 and adding the addition amount of Roll 2 (which comes from the greater distance between COG and RC), BUT, when you reduce a spring's length (ie. cut it) or you buy lowering springs they increase the spring rate. From what I said before we can deduce that this is why the car seems to have less roll and handle flatter. When in actuality the car can have the same amount of roll it's just the driver's mind telling him that it's vastly improved.

thuslyinclining a roll axis nose down creates overtseer/reduces understeer andinclining a roll axis tail down created understeer/reduces oversteer..

This would have been better said as "can induce" rather than "creates".

I say that because lowering the front more than the rear also creates more static front negative camber, moves more static weight to the front for the crossweight analysis, and also must use a slightly higher rate than one that is sits the car slightly taller. Most of which can induce understeer as well.

[crzycav86]

I say Nismo and Exar should have a battle to the death for the title of "Mr know-it-all".

Good stuff guys

[Nismo_Freak]

I say Nismo and Exar should have a battle to the death for the title of "Mr know-it-all".

Good stuff guys

Negative... he doesn't wish to go there

[ceniack]

this would make a good article or sticky or something.