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Old 01-23-2006, 11:18 AM   #1
BJ
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Default Have you worked out your Roll Center?

Hi,

Just been reading through the XXX main book about Roll Centers
After looking at the diagrams shown have you ever taken a photo of yor car and followed what they have said?

I did, I took a picture of my car and tried to work out where the roll center was. I looks like it is very close to the bottom of the chassis.
The thing is that the XXX main diagram shows the Instant roll center at the top, but as my top camber link is angled up ( front c hub ball stud higher than the inner pivot point) and the same for the lower A arm this makes my Instant roll center at the bottom. Then taking the Instant roll center point to the middle of the tire makes the intersection of the center line of the car very low down.

So is my car sitting wrong? ride height 5mm front 5.5mm rear.

I read it that it all depends on your Center of gravity on how much the car rotates around that point, but how do you work out where your center of gravity is?

Hope you followed!!
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Old 01-23-2006, 11:51 AM   #2
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I think I prefer the 'blissfully unaware and don't want to know' attitude rather than buy a book that will mor ethan likely fry your brain than help one bit, lol.

Sorry dude, got better things to expel braincells on than trivia like that and I think there is far more at stake on my car than if I got the correct roll centre, or at least knowing how to find out.

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Old 01-23-2006, 12:02 PM   #3
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Yes I have and at ride height it sits about 7mm under the chassis if that may reassure you, this with rubber tyres of 62mm diameter.
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Old 01-23-2006, 01:16 PM   #4
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Quote:
Originally Posted by TRF415boy
Yes I have and at ride height it sits about 7mm under the chassis if that may reassure you, this with rubber tyres of 62mm diameter.

7mm, how high is your car, can you get a roll center that is that far away off the car?

As to 'DA cookie monster' not really bothered, I have been lead to believe that Roll centers can dramaticaly change the handling of a car. Yes springs, shocks ,wheelbase etc....change stuff but Roll centers are more critical.
Hence I'm trying to learn
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Old 01-23-2006, 01:30 PM   #5
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Quote:
Originally Posted by BJ
Roll centers are more critical.
Hence I'm trying to learn
from what I can remember the closer the RC & the CG gets together the less the car will roll in croners . don't quote me on this


Quote:
Originally Posted by BJ
but how do you work out where your center of gravity is?
good question ! someone pls help here !
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Old 01-23-2006, 01:33 PM   #6
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Let me see if I can get the author of the XXX Main setup guide (Martin Crisp, Las Vegas IIC A Main winner in 19T) in here to answer some questions.

Cheers,

John R
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Old 01-23-2006, 01:39 PM   #7
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You can position roll centres wherever you want to, within reason. In the 1970s, Volvo made a car that rolled INTO the corner by playing with CoG and roll centres.

DCM may not be able to multi-task (!), but ignorance of roll centres is missing a very valuable tuning device. It isn't wise to depart too far from the manufacturer's suspension design, but not knowing the effect of positioning the suspension links that are adjustable to get a good handling car is a waste. One can do more for the handling by adjusting suspension angles than simply playing with springs and dampers.

Put it this way - keeping the tyres in contact with the ground, and sorting out the speed with which the car reacts in roll and bump is one thing, but it's never as good as being able to adjust the amount of roll, camber change, contact patch and roll centre drift, if your really want to get a car to handle.

BJ - the major weight in a car is the motor and batteries. To get an idea where the CoG is, simply draw a line through the middle of those two (looking at the chassis from the side) and measure how far off the ground that is. The roll moment is the difference in height between the CoG line and the instantaneous roll centre, when the chassis is static.

As a guess, let's say the batteries are 23mm diameter (CoG at 12mm) and the motor is 32mm dia (CoG at 16mm) If the cells weigh the same as the motor, then the CoG is somewhere in between, say 14mm. If your RC is at the bottom of the chassis, then from the CoG to the RC is 14mm (as both motor and cells are usuall level with the bottom of the chassis). Don't forget that the CoG and RC are different at each end, and this will take a computer program or a drawing board to calculate accurately!

If you change the height of the inner or outer pivot of the top suspension link you move the RC, and now you can see how much closer, or further away, from the CoG it is. Make a note of driving feel, and lap times, to build up a picture of how these changes affect the car.

For detailed information, read this... http://www.team-orion.ch/faq/car-handling.asp ... as it should keep you testing for the next few years before it is all committed to memory!!! HTH
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Old 01-23-2006, 02:19 PM   #8
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Depending on what car you have the CoG will be roughly between 13mm and 17mm over the bottom side of the chassis, this is without the bodyshell taken into account, which has a big influence on the CoG since it's about 7-8% of the mass of the car with a very high CoG.

The Roll centre being a virtual point which position changes at every moment it doesn't matter wether it is "in" the car or not, what matters is if it's over or under the CoG and what the distance is. Also, when your car rolls the Roll Centre shifts to the side (because the arms aren't angled the same anymore), how much it shifts is relevant as to what the load distribution between the left and right tyre is going to be.

Now for the fun part, there is no way you can really know what the car is going to do while cornering, other than to design it with a CAD software and run a very complex dynamics calculation. Simply because there is no other way of knowing the exact location of the centre of gravity (whose location in space means as much as its vertical location). Not to mention the even funnier part of knowing what's happening in the shocks !

But the good part is, all the work's been done for you ! Just trust the manufacturer's development and work out how your car behaves when you raise or lower it, just don't bother about its exact position as it don't really matters to you.
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Old 01-23-2006, 03:40 PM   #9
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Quote:
Originally Posted by jrabbito
Let me see if I can get the author of the XXX Main setup guide (Martin Crisp, Las Vegas IIC A Main winner in 19T) in here to answer some questions.

Cheers,

John R
That would be sooo cool!!!!!!!!!


Thanks for some of the help, but it's like when new cars have come out for example the Yokomo BD Tamiya MSX etc.. they don't have the holes in the shock towers anymore you just place shims under the camber link ball stud either inner or outer, plus you can add or take away shims under the A arm.
So with the kind of adjustment now available you can have unlimited combinations to get different Roll Centers. So when do you know when to put a 1mm shim under the camber link and the A arm for example, which would be lifting the whole Roll center in general?
When do you say, Ok i can not get the roll center any higher/lower now using the camber link it's time to raise/lower the A Arm.

As everyone knows to try every possible outcome would take a driver a full week to go through everything, as I only get 30mins track time on a sunday, I could be adjusting it for a very long time.

Printing off the orion link as we speak and will read it at work

Thanks guys
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Old 01-23-2006, 03:54 PM   #10
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Measure up all of your pivot points using vernier callipers, distance from the ground, and distance from the centreline of the car.

I plotted all of this in Adobe Illustrator at 1:1 scale, and drew lines through all of the link paths (upper turnbuckles and lower arms)...

a good way to get an understanding of how small changes to your geometry affects the roll centres static height...for isntance raising or lowering the arm mounting blocks has a far greater effect on roll centre height than raising or lowering the inboard end of the camber link (or shimming under the outer ballstud.

I use the link angle for fine tuning of the R.C, and for camber gain. I always tune R.C through arm mount height.

Dynamic roll centre position is another chapter all together, and cant easily be calulated using this method.
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Old 01-23-2006, 08:14 PM   #11
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Color me crazy but I don't think calculating the exact location of the static roll center was the intent of the diagrams in that book. I was under the impression that it was more informational so as to help you understand how CHANGING the links or arms (and thus the roll center) affects the cars handling. I feel that is much more important, and if you understand that then it doesn't matter where the rc is located.
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Old 01-23-2006, 08:37 PM   #12
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Quote:
Originally Posted by JKA
Color me crazy but I don't think calculating the exact location of the static roll center was the intent of the diagrams in that book.
You can say that again...
The XXX book is a bad copy of the original Steve Smith "The Stock Car Racing Chassis"....

Forget about your front roll centers and concentrate on squaring the car to reduce wheel scrub. That's horsepower!

Keep your setup simple and try one thing at a time and test...
Practice driving..Practice. Practice.
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Old 01-24-2006, 02:25 AM   #13
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Somewhere on the web there is a roll centre calculator that someone had created in a spreadsheet. As I recall there were a few errors in it that I never quite managed to overcome though.
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Old 01-24-2006, 03:12 AM   #14
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i play with the roll center adjustment and see which one fits my driving style but so far i never tried to figure out where it actually sits....
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Old 01-24-2006, 04:02 AM   #15
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Quote:
Originally Posted by BJ
That would be sooo cool!!!!!!!!!


Thanks for some of the help, but it's like when new cars have come out for example the Yokomo BD Tamiya MSX etc.. they don't have the holes in the shock towers anymore you just place shims under the camber link ball stud either inner or outer, plus you can add or take away shims under the A arm.
So with the kind of adjustment now available you can have unlimited combinations to get different Roll Centers. So when do you know when to put a 1mm shim under the camber link and the A arm for example, which would be lifting the whole Roll center in general?
When do you say, Ok i can not get the roll center any higher/lower now using the camber link it's time to raise/lower the A Arm.

As everyone knows to try every possible outcome would take a driver a full week to go through everything, as I only get 30mins track time on a sunday, I could be adjusting it for a very long time.

Printing off the orion link as we speak and will read it at work

Thanks guys
You have to be aware of the fact that if you only change for example the upper link's shimming, or only the wishbone's shimming, then you are not only playing with the roll centre but also with the camber change.

As has been said, the effect on the Roll Centre of, say adding a 1mm shim under the wishbones will be far greater than doing the same under the outer ball joint of the upper link. As I see things, by playing on the wishbones the effect on the roll centre is greater than the effect on the camber change, whereas doing so on the upper link has a greater effect on camber change than on the Roll Centre.

So if my car is sluggish and do not change direction quickly enough, I will not bother with the upper link and will raise the roll centre using shims under the wishbones. If direction change is fine but i want more mid corner grip, I'll be angling the camber link a bit more.

As has been said, the exact position of the RC isn't of any importance, unless you want to compare two car's geometry, what is important is to understand the influence of rasing/lowering it on th car's handling and how to achieve it.
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