Roll Center height

[Rafael Sequeira]

hello, i'm designing a new suspension and i don't know typical values for RC height in front and rear suspension or typical relations between front and rear. Besides that, what do you think about RC height variations with Wheel travel be positive in droop and negative in bump, for exemple.

[Pawel Roszyk]

Hi, Rafael. Which team are you in?

RC is typically between the ground and CG, but not necesserily. RC Height divides your weight transfer into "elastic" and "geometric".

1) Low Roll Centre -> Little track width variation, less tire temperature, bigger Roll Moment is acting on your springs and dampers ("elastic weight transfer") therefore you've got more inertia in roll and car respones slower
2) High Roll Centre -> Big track width variation, more tire temperature, lesser Roll Moment is acting on your springs and dampers and you have more responsive car

As grip is generated by the front axle, weight transfer starts from there. That causes a time gap between front and rear axles weight transfer. In order to decrease this gap many designers increase the amount of "geometric" weight transfer on the rear (which acts faster - isn't damped) by setting higher RC on that axle.

With higher RC you can also have softer springs which will affect your damping. It all depends of your design goals.

[Rafael Sequeira]

FST Novabase and you? thanks for your answer!!
Could you please tell me more about it? for example, if we have a negative RC height at 20 bump and a positive at 20 droop, what's his effect?

[Pat Clarke]

Rafael,

Roll centres have been discussed on this forum many many times. Do a search and you will find out all you need to know...and more!

Pat Clarke

[Rafael Sequeira]

thank you!

[Ahmad Rezq]

Rafael,
There are some golden discussions here where many guys have added a lot to them.

Jacking Force
http://www.fsae.com/forums/showthread.php?4063-Jacking-force

Roll Center Migration
http://www.fsae.com/forums/showthread.php?4390-Roll-Center-Migration

[dynatune]

Technically speaking the RCH as is being used in modern suspension design is "only" valid at the very beginning of lateral vehicle movement. As Wolfgang Matschinsky quite extensively demonstrated in his book, the theory of how to calculate the vertical roll center height or lateral roll center migration is only valid if we assume that the vehicle would actually run on rails which would allow a rather precise location of a contact patch point. Since we do however have a tire on a car that is far from consistent as a railway steel wheel the actual position of that contact patch becomes more and more less defined with more and more lateral acceleration. Hence, we do not know where the rollcenter is, neither in height nor in lateral position. We do know however that having a low roll centre is reducing jacking forces so that is a good thing. Also we know that "if the roll center migrates" it should migrate towards the inside wheel, for the very simple reason that - still assuming that the vehicle body is rolling about the roll axis - an increase of roll angle would lead by definition towards more wheeltravel on the outside wheel than on the inside wheel and thus work against jacking. This info should help you avoiding to make the most common mistakes.

Cheers
dynatune

[BillCobb]

If that's the case, perhaps you'd like to explain just how it is possible that, of these almost 700 K&C tests of production (only) cars from around the world, I can predict the roll gradients of them measured by BOTH step response, frequency response, and constant radius tests to within 0.30 and 0.40 deg/g in an average population of around 5 deg/g ???? That's a roll in the sack with Honey compared to a stiff FSAE car. Sometimes we see that much of this 'error' is due to considerable flexure between the roll xducer(s) and the reference position at the driver's location. For example in a pickup truck, there is a roll gradient at the front end, the body/cab and the pickup bed, and they ain't nearly the same critters. And the body is fully clamped in a 'normal' K&C test. Not so is 'special' configurations.

These roll center heights have been determined during lateral force compliance tests with the body fixed. If anything is amiss, it would be the CG height estimator which is a statistically based methodology used in the absence of an actual CG Height test. ALL types of suspension architectures are in this population: strut, wishbone, solid, twist, and elastic. Plus, the estimates from ADAMS, NASTRAN, and home-brewed suspension analysis programs are very good at calculating front and rear roll centers (and roll stiffness, too).

This scatter plot only includes production cars, There's almost 2000 experimental prototype, race, and military vehicles in this pool, all of who's predictions of roll gradient seem to jive with measurements very well. In order to get the simulated roll gradients correct, you need to include the effects of the tire MX moments acting via the roll by camber mechanisms. If these vehicles rolled 15 - 20 deg/g they may be an argument that migration off-center is plausable. But, NOBODY would ever want to drive, ride in , or BUY such a vehicle unless it was a commercial truck. If they reach 0.3g Ay, their roll gradients come out to be 90 or 180 deg/g.

BTW: every one of these dots has a name, so if you have any questions...

[DougMilliken]

Neat -- thanks Bill. Let me guess...the original BMC Mini (1959-2000) had a pure trailing arm rear suspension, so perhaps it is one of the 4 points where rear roll center height ~0.0?

[BillCobb]

Doug: don't have any Mini data in the collection, but a list of vehicles having rear roll centers near the ground includes the Audi Quatro, Suburu Impreza WRX Lexus RX300, Saturn Vue and it's Equinox step brother . Slightly higher up are Volvo S60, Chrysler Pacifica and the 'new' Camaro.

[Claude Rouelle]

Bill, Where is the Mercedes Class A? Claude

[BillCobb]

I have 16 Benz models, some repeats of different years. Model year change did not produce geometry changes (as expected). On the scatter plot, only 'outliers are the SLK350 and the Freightliner trucks. R,S, and C 350s are usually all in the cluster 140mm F / 150mm R. That may be an element of MERC ride and handling dogma.

I'm limited to tests for which ALL required parameters for a full scal simulation were requested. If and engineer only wanted rear or front data, then my database won't contain it. You can't believe how frustrating it is to buy or rent a car and then have somebody ignore, omit, or forget to chack all the boxes.

[dynatune]

Bill,
I have asked myself that same question many times. I am equally guilty of using the "simple" formulas and they all work quite well with your indicated range of confidence level. However when I do look into details like how a tire deforms under lateral force, changing the contact patch and vertical tire rate, how camber compliance (both due lateral force and also on some suspension due to vertical force) also affects the location of the contact patch I am still "amazed" about the simple formula being "wrong" for only 10%. In fact that is why I am passionate about the "basic" approach, but I do have my doubts that it is all correct in every detail. But if it works ...

[BillCobb]

... But if it works ...

The reason it works is because you stop treating it as a science project and put an engineering hat on. It's pretty easy to get the last 10% of roll issues by using some common sense (right brained stuff for right hand rules). Where was the roll transducer placed, did you cascade the simulated sideforce induced roll by the roll by steer effect to get a more complete response, and did you measure roll angle or left and right ride heights. And are we talking a 2.5 deg/g 'race' car or an 8 deg/g truck at GVW. While we are at it, I believe roll velocity control is perhaps more important than steady state roll angle control.

My goal in all the tire, chassis and steering specification projects I ever worked on was to 1). was to eliminate 90% of the configurations determined not to be worth building. 2) Make use of all available statistical information on variability of parts and subsystems, and 3) to hand off to a 'customer' a vehicle still needing refinements and polishing because 'they' are the final receivers of the product. And, these folks will usually have little or no current science or engineering education/skills and could not care any less for what went into the vehicle dynamics recipe. As long as its 'safe', durable, competitive, within budget and 'fun', the design represents a quality product. Some people never finish a job because their science fair project is still incomplete. Some get to finish it, retire and get a life that revolves around things other than springs, tires, lap times, roll center heights and fuel economy.

[dynatune]

I agree. But next to putting on my engineering hat - which is always focused on getting to that 90% as best as possible (as can be seen in some of my tools) - there is still the science hat, that wants to be worn every now and then. The problem with that hat is, that when you put it on, it tends to try to stay on.

For sure roll velocity is very important, I spent two years developing robust objective methods for defining customer related metrics for "initial roll".