Roll Centre

[LotusGTR]

Guys, I want to properly learn what this is and how it relates to handling. Some context; it's often discussed that lowering a GTR too much affects the roll centre which adversely affects handling.

Can anyone explain in lay terms, or point me to something on this site or elsewhere would be a good read up on this area. Diagrams would be good too.

Lastly, I hear you can get suspension components that allow an adjustable roll centre. Can you give me some details of what these are preferably with pics / links? This would contribute to my understanding too.

Thanks.

[blind_elk]

There's a bit of info in "How to Make Your Car Handle", by Fred Puhn (available at Pitstop Bookshop - http://www.pitstop.net.au/view/technical-c.../query/plu/1734). The front roll centre of the GT-R will be fairly easy to calculate, not so sure about the rear.

From the book:

(when a car is rolling in a turn) There must be some point in space about which the car body could have rotated to assume that same angle. This point can be real or imaginary, depending on the design of the car. It is called the roll centre. Height of the roll centre above the road is important and it need not be the same at the front as the back of the car

[doo doo]

Get on the net and find 'tune to win' by carrol smith. buy it. Then you might also be interested in 'design to win', and 'engineer to win'. Here's a sample page.

[Equinox]

Yup. Those books introduce the concept as well as any.

[Risking]

Has anyone actually ever calculated factory GTR roll center heights, instantaneous centers, roll axis etc etc???

I can tell you that the factory front roll center of a GTR is typically 80-85mm. Lowering the car by 1 inch normally drops 12-13% off the centers height (my own R&D). So you end up with a roll center of roughly 9-10mm. Still positive but close to going negative.

Quickly showing why excessive lowering screws everything up. All those "roll center correctors" are only a part solution. If you lower a GTR by 30mm and fit a 30mm roll center corrector under the front uprights you correct the roll center by 6-7% generally. The upper arm needs to be corrected to rectify the problem properly.

Bearing in mind this all all static calculations, Roll centers have not even started to move around the car dynamicly yet, roll axis and couple are not even considered either......

Alot of people have the general oppinion that a car set-up with the rollcenter at the same height is the center of gravity should handle well. Couldnt be more wrong, a car with RC and CG at the same height WONT roll at all and will simply slide.

Normally we set a car up so the roll center is lower than the CG. For a good reason. The higher the CG is compared to the Rollcenter the more "leverage" the weight has and the more body roll you will get. This is roll couple. To smaller distance and you create an oversteering pain. To large and you have a body roller that your forever adding more spring and bar to try and correct.

Noramally by lowering roll centers you end up raising the roll couple. Typically and for simplicities sake(so many variables) by the same % you lowered the RC . So lower the RC by 10% you raise the roll couple by 10%. Inducing 10% more leverage and 10% more roll. 10% more spring is now required to get the roll back to how it was before you changed the RC. (Again its not always a linear ammount but too indepth.)

A low roll center induces body roll and vice versa for a higher RC.

If you dont move the Roll centers together it can and will introduce under and over steer.

Lower the front roll center by say 10% you have now got to increase spring rates by 10% to get back to the original ammount of body roll (assuming linear rates/RC for ease)

Lower the rear roll center by 5% and do the same with the springs.

The front of the car now has more tendancy to roll than the rear. So in a corner the front will not resist the body roll as much as the rear. The rear will take on more of the rolling forces and as a result increase its weight transfer. Leading to only one thing, oversteer.

Most people try and dial this out with roll bars a very common problem on GTR's when lowered and not set-up properly, but the solution is to fix the roll center problem you created first.

Ive been setting-up race cars and road cars for a while and have built my fair share. I have my own equipement (C weights, alligner, bump steer guages, Susprog modeling software etc etc) so I have managed to aquire a fair ammount of data and experiance with the GTR and GTS-t chassis.

Im sure someone will sit down and google search the net to blow holes in what Ive said, because someone else uses different terminology etc but there is a little of my knowledge about the concept from first hand experiance. There is little information about roll center effects and so forth so hopefully that helps

I wouldnt normally go into this kind of discussion due to the vast ammount of oppinions surrounding the topic. Every engineer or "expert" if there is such a thing has their own view.

Please do bear in mind what ive written is only just touching on the surface when it comes to geometry and RC's etc. There is FAR too much to consider in writting it down. Dynamicaly everything changes again. I spent an entire day with another forum member last weekend doing practical excersises, calculations and so forth.

If he reads this im sure he will comment on what he took away from the day.

[GTRsean]

That's a good explanation on roll centres and adverse effects.

I for one would like to lower my 34 about 25mm all round, but I like the way the car handles at stock height. It seems to sit quite flat on corners on standard v spec setup. I'm just hoping that it won't be affected handling wise too much, and then have to get uprated sway bars too.

My 32GTR had camber kits, stiffer swaybars and tein ss and wasn't too low, but it was harder to setup, to actually handle mostly due to the changed suspension geometry (rollcentres). I don't want to go down this path again lol.

[sh@un]

great topic

does anyone know what minimum wheel-centre to guard distance on an R34 GT-R is advisable before handling is adversely affected?

[GTRsean]

great topic

does anyone know what minimum wheel-centre to guard distance on an R34 GT-R is advisable before handling is adversely affected?

+1

[bigmikespec]

As soon as you lower it from standard roll centre is moved. It is not a linear scale either, lowering the car 25mm may change the position of the roll centre greater than 25mm. It is also amazing how little Japanese aftermarket "roll centre adjusters" actually do in changing the roll centre considering how expensive they are.

Brad is definitely the man on suspension, everything I have learned has been from a suspension masterclass with him!

[R34]

+2

Brad , sent you a PM.

Would like some advice before I spend money on the wrong mods

Love the way my R33GTR handles but want to lower it without ruining the great handling

[LotusGTR]

OK, I've decided I'm never really going to understand this more than as an overall concept. I've read up and I'm slightly baffled. It is obviously a highly complex area which you need specific knowledge for and by the sounds of things a lot of practical experience too. Simply, too low is bad for GTRs - which we all know. Just need to establish ideal front and rear ride heights (355 345 FR is what I think is right for R32s and I'm not sure how much off std this is), then fiddle with

Track

Spring rates

Damper rates

Antirollbars

All of the above to be determined by above said expert.

Very grateful to you all for your input. I'd be happy to see this discussed further but am not sure that I've got anything useful to add!

[LotusGTR]

+2

Brad , sent you a PM.

Would like some advice before I spend money on the wrong mods

Love the way my R33GTR handles but want to lower it without ruining the great handling

Please share the conversation if you can.

[R34]

Please share the conversation if you can.

Will do mate,or Brad can post it here

[DVS JEZ]

ive been msging Brad lately, definately knows wat he is talking about, biggest problem is that most people want a lowered street car that looks good but then wants race car handling, i am one of these people.

worst part before tackling these setups is having all the adjustable arms and decent coilovers(big expense).

[Risking]

OK, I've decided I'm never really going to understand this more than as an overall concept. I've read up and I'm slightly baffled. It is obviously a highly complex area which you need specific knowledge for and by the sounds of things a lot of practical experience too. Simply, too low is bad for GTRs - which we all know. Just need to establish ideal front and rear ride heights (355 345 FR is what I think is right for R32s and I'm not sure how much off std this is), then fiddle with

Track

Spring rates

Damper rates

Antirollbars

All of the above to be determined by above said expert.

Very grateful to you all for your input. I'd be happy to see this discussed further but am not sure that I've got anything useful to add!

it is a very indepth topic something unless your pushing a car hard and looking for the most out of is best left alone.

The heights you have mentioned are around optimal for a gtr give or take 5mm. Just be sure to set the heights with a driver in the car and an average fuel load.

Or ideally set the heights and corner weight the car at the same time. A slight varyiance in ride height is worth the trade off if it means you can balance the chassis better.

For those worried about handling effects in all honesty unless your pushing the car hard and looking for time, a 30-35mm ride height drop is aceptable 90% of the time. While it will lower the RC and change the chassis chances are you will overcome it with sway bars like 1000's of people have done before. The expense of correcting and modifying a rollcenter height and going even further into it is not really economical when you can get a good result with a well thought out set-up that to the majority of people will drive the same as a car with 100's of development hours. Developed car will be quicker but comes back to how far are you willing to go??

My sports sedan has 1000's of hours put into r&d it hasn't turned a wheel yet. There has been several incarnations of suspension and many geometry changes made. It's still not perfect and honestly I still learn more about it every day.

Your limiting factor will more so be tires etc etc before roll centers and other geometry.

[Kinks]

I for one would like to lower my 34 about 25mm all round, but I like the way the car handles at stock height.

I went through this recently with my R33 GTS-t. I'd be interested to know what the overall handling is like at standard height vs lowered slightly (~20mm).

I went king springs and bilsteins with circlip height adjustment, and I'm running it at standard height (top groove on the shocks). rear has come down 10mm the front is unchanged.

[tarmac]

Interesting topic. Seeing as im about to align my 33 gtst this week with all new arms, can someone point to a good height for a gts25t . I think i have heard 340mm f. 350mm r, , and where is this measured from, from guard to center of hub?

[bigmikespec]

Interesting topic. Seeing as im about to align my 33 gtst this week with all new arms, can someone point to a good height for a gts25t . I think i have heard 340mm f. 350mm r, , and where is this measured from, from guard to center of hub?

http://www.skylinesaustralia.com/forums/R3...r33+ride+height

Standard height as per above. Guard to centre of hub, correct.

[GTS-t VSPEC]

When we lowered my car significantly we used a roll center kit from Whiteline to try and compensate. To be honest I couldn't really tell how much better it was with it or not.

[loffe]

maybe this can help a bit

Here is a picture of close to original setup

under roll condition

Then we lower the car 40mm and see what happens

and make the roll

You can see that the upper-outer link probably is close to bottum out and that the rollcentre is way off, just looking

at the geometry makes it easy to understand that it needs a lower force to make it roll when susp-arms stand

angeled up right from the beginning.

Then you have to test your coilovers workingdistans without feathers to see it can move up and down as it should, not so

nice to ride with coils bottum out at bumpstops.

Maybe it looks cool with a lowered car, but its worthless without setting things up properly i think.

And this is with only 40mm lowering.