Roll Centres

[T04GTR]

ok.. for all you suspension gurus..

weres the perfect roll centre position on a r32 gtr. front and back.

ie.. below the road, on the road, or at the middle of the wheel etc etc. (at a sitting position,not tilting(static roll centre)

cos im gonna start having a play with it to get some more corner speed and reduce my spring rates..

or.... should i basicly return it to where the factory had it ? (before i lowerd it)

any input is welcome is a think thisa area is almost always overlooked

[T04GTR]

ok... i did a search and revealled that yess it changes when you lower the car.

well darrr

i need to know WHERE is a happy meduim for roll centre placement front to back.

as in front 20mm above the ground, rear..50mm aboove he ground etc etc

hope this makes sence.

the reason for this topic is that we (my work) are partly preparing a s14 for "production sports" and as it sits 50mm lower than stock the front roll centre is 1 METER BELOW THE GROUND..

at stock height it is pretty much at the surface. (without puttling on a 100% leval surface with laser angle guages etc etc)

not at all desirable

thanx

so summing up what IN THERORY is a good position for the centres

[T04GTR]

ps.... i know where to make the adjustmants

[djr81]

Theoretically the best location for the roll centre is slightly below the centre of gravity.

When you change the ride height, you change the c of g, however.

When you change the ride height, you change the suspension geometry which changes the roll centre.

When the car rolls, you change the suspension geometry, which changes the roll centre.

The front roll centre will be at a different height to the rear roll centre to help transfer load away from the front tyres when cornering.

If someone has set a roll centre to be a metre below the ground they will be cornering the car on its door handles.

What you need to do is measure up & model the suspension geometry as a first step...

[Sydneykid]

ok... i did a search and revealled that yess it changes when you lower the car.

well darrr

i need to know WHERE is a happy meduim for roll centre placement front to back.

as in front 20mm above the ground, rear..50mm aboove he ground etc etc

hope this makes sence.

the reason for this topic is that we (my work) are partly preparing a s14 for "production sports" and as it sits 50mm lower than stock the front roll centre is 1 METER BELOW THE GROUND..

at stock height it is pretty much at the surface. (without puttling on a 100% leval surface with laser angle guages etc etc)

not at all desirable

thanx

so summing up what IN THERORY is a good position for the centres

SK, rule of thumb for McPherson struts. Space the lower control arm from the upright by the amount you have lowered the car. This returns the lower control arm to the same angle as it had before you lowered the car too much.

SK tip, don't forget to correct the bump steer.

What are doing about the rear?

cheers

Edited July 10, 2006 by Sydneykid

[T04GTR]

djr81. yess im aware of all this thanx but i need a xxmm etc etc not "its differant.. higher roll centre at the front ,lower ????

yeap. s14 front end is crap. (for lowering) that was doing the corect mesurements and using a angle guage and making a scale drawing..

of the top of my head the arm centreline sits at 6 deg up (noooo good) following directions from a book called "how to make your car handle" i forget the author

sk.. i was thinking of doing that to the front. easy as. (thanx phuil ryal:P)

rear.. hmmm well i dont want to have to buy ikea formular lower arms c

as there REALLY EXPENCIVE but it would be easy to just make up a variety of spacers for them. i think replacing the outer joint with a sperical ball joint seems the only practical way.

but yeah i think lowering the outer lower balljoint seems to de the go. but.. il have to draw them to see if a upper arm pivot adjustment is more feasable

yeah bump steer corection goes with out saying i spose. front and rear?

im gonna do some drawings and see if i have to lower the outer arm end the same amount as the cars been lowerd?

[djr81]

Dave,

I am a little confused - are you asking after the S14 or the R32? Because answers to the question will be different depending on which car you are looking at.

On the assumption it is the R32:

Mostly making suspension adjustments amount to trying to reduce the worst effects of the limitations of the stock suspension eg too little camber compensation in roll on the front end of the GT-R and too much on the rear.

If you are talking about the GT-R don't worry about the rear end too much - focus on the front as that is where the gains are to be made.

With regard to the roll centres (Note plural). The idea is to incline the roll axis (an imaginary line plotted between the two centres) forward. This allows a transference of load away from the fron end of the car and hence reduces understeer. This is the reason R32's are sensitive to rake.

How are you going to measure your centre of gravity? I ask because it is the roll couple you are really chasing rather than just the roll centres.

Edited July 11, 2006 by djr81

[T04GTR]

yeah im talking about the 32.

so i should aim for making the roll centre of the front higher than the rear?

this is my main question.. ^^^^^^^^^^^^^^^^^^^^^^

the purpose for me carieng out this adjustment is purly to reduce roll. and soften it up so i can carry more corner speed. rather than stifening it up so it goes slower

[T04GTR]

(ive got the front sorted, well more than the rear)

[T04GTR]

yay. annswers

[T04GTR]

should i return the centres to factory spec?

[T04GTR]

but that would be a litle boring. i wanna make this a usefull test

[Sydneykid]

Run it at 355 mm centre of wheel to guard at the front and 345 mm at the rear. Use a 24 mm solid front adjustable stabiliser bar on the softest setting. A 24 mm solid rear adjustable stabiliser bar on the middle setting. The rear spring rate needed is ~250 lbs per inch (4.5 kg/mm), the front is ~400 lbs per inch (7.2 kg/mm). Set the rear subframe alignment bushes to the "all round setting". Try for 6.5 degree of positive caster on the front, with 3.5 degrees of negative camber and 2 mm of toe out each side. On the rear, 1.5 degrees of negative camber with zero toe out.

Then adjust the camber for even tyre temps. If it needs more than 4 degrees negative camber on the front, then you don’t have enough bump valving in the shocks.

Once you achieve even tyre temps, then you can adjust the stabiliser bare to achieve the handling balance you like. If it still understeers on corner entry, try another 2 mm of toe out on the rear. If you get mid corner understeer and the front tyre temps are even, then increase the rear ride height, 5 mm is usually enough. If it power oversteers and the rear tyres temps are even, then back off the rear bar I hole. If it still power oversteers then change the rear subframe setting to “max traction”.

“Take 2 panadol and call me in the morning”

cheers

Edited July 13, 2006 by Sydneykid

[T04GTR]

ic.

thanks for that.

heres my curant setting.

front=

9deg caster

3deg neg camber

1mm toe out each side

8kg springs with bilsteins (nismo kit.old one)

whitline 24mm adj bar. softest setting

rear=

2deg neg camber.

2mm toe in each side

7kg rear springs with bilsteins (old nismo kit)

whitline rear bar softest setting

5mm rake (front lower)

the car is taily as f**k. removed the rear bar and it slightly improved.

turn-in is really sharp. mid corner oversteer

[T04GTR]

changing the springs is not an option yet.

its easier for me to adjust the roll centre, well i wanna get it in the ball park. but noone seems to know or have actually done this on a skyline

FRONT ROLL CENTRE HIGHER OR LOWER THAN THE REAR ROLL CENTRE?

sorry for caps. but this is ALL i need to know at the moment. as im ready to adjust

[Sydneykid]

changing the springs is not an option yet.

its easier for me to adjust the roll centre, well i wanna get it in the ball park. but noone seems to know or have actually done this on a skyline

FRONT ROLL CENTRE HIGHER OR LOWER THAN THE REAR ROLL CENTRE?

sorry for caps. but this is ALL i need to know at the moment. as im ready to adjust

The rear spring rate is way too high, hence the oversteer. Changing the roll centre will accomplish nothing. It’s not a geometry issue, it’s a tyre contact patch issue. The tyres are skipping from bump to bump. Changing the roll centre will not affect this.

cheers

[djr81]

changing the springs is not an option yet.

its easier for me to adjust the roll centre, well i wanna get it in the ball park. but noone seems to know or have actually done this on a skyline

FRONT ROLL CENTRE HIGHER OR LOWER THAN THE REAR ROLL CENTRE?

sorry for caps. but this is ALL i need to know at the moment. as im ready to adjust

Lower.

Sorry.

LOWER.

[T04GTR]

COOL. thanx

[AzzurrA]

*subscribes*

[T04GTR]

dammit. doing the rear is harder.

its soo much easeir to make up an alloy billit for the front ball joint :