Roll center migration 1

[Bluefoxy]

Hello,

I was wondering how roll center migration affects suspension performance during roll movement.

I have carried static suspension calculation, and I did not see any releveance of non migration roll center suspension, but my calculation are static not dynamic.

Do someone had experience on no roll center migration suspension and how it performs?

Regards

 

[Bluefoxy]

At least, recognize FBRC-CG stay constant near the ground!

 

[BrianPetersen]

You're acting like you're trying to use that as a selling feature. I'm trying to explain why you won't have any success with that.

 

[Bluefoxy]

I m not trying to sell anything.

I find a way to design a twin arm with Rc not moving, which many author claim it is a good thing. honestly I have no clue if it is good not or not, so my initial question on this thread.
I never give an attention to FBRC concept, because as far as you deal with force the law in static sum of force = masse x acceleration, and then you calculate every force on every compenent and you are able to conclud something or not. FBRC only deals with lateral.
In my point of wiew, for a steady state corner, a suspension is a mechanical system wich mass is summit to vertical and lateral acceleration, on this mecahnical system, only spring are deformable, once you have calculate every force on each component, and end up with the force on the spring and thus their deflection, you could have an acceptable wiew of what happen in roll.
For having done this, the suspension does not roll around roll center definitely as long as you get jacking effect, and as long as the track alter between static and roll.

What I dislike in the Mitchell paper, it start from static to a roll position, then include force, then include FAP. the question is how his value of roll is done (I imagine symetric, which is rarely the case due to jacking effect).

You wrote 'Bear in mind that the FAP is most relevant for the side that is more heavily loaded (the outside wheel) because of the way the FBRCH is calculated', I was thinking it was important for you, so I notice RC-COG constant means FBRC-Cg constant.

If you mention the nowadays F1 suspension, you have to keep in mind, that wishbone are place this way only for aero reason, It is not me who say that, it is written in Adrian Newey book.

'I'm trying to explain why you won't have any success with that.', I will be pleased to know.

 

[cibachrome]

Much of the attention paid to roll center position and height is due to the excitation and transmission path. Roll is not induced buy steer (OK a bit by caster) but by lateral acceleration and lateral force. We refer to it by the mathematical term Convolution. It is a process by which steer induces lateral forces / accelerations followed by the roll response as a secondary characteristic. The timing of all this is worth studying because placement and movements of the unsprung mass(s) force points defines the initiation of events. In closed loop control (driver-in-the-loop), the lateral position and heading angle implied to the driver modulates their steering and throttle/braking action. Movement of the nose is part of this signal conditioning feedback. That's why a 'hood ornament' makes a good driver training and engineering feedback aid. It implies a heading correction to be anticipated. If sprung mass front end is orbiting the desired path instead of staying tangent to it, then excess control and poor steering judgements result. It's all about perception. That's why even seat position has a great influence on a driver being able to 'get up on the wheel'. Routine ISO testing doesn't reveal this because the steering is open loop (no feedback correction).

It DOES show up on the ISO Weave test (sine steer), though. With simultaneous path, frequency, and g-level constraints, some vehicles are very difficult to drive. The indications of this are in the variation computed during multiple steering cycles. We measure it, integrate it and produce a score to rate he vehicle. You should not be surprised to see a list of best to worst vehicles and their measured K&C data for roll, friction and tire relaxation properties. AND why this variation does NOT show up with a mechanical, robotic driver. Check it out !

https://www.youtube.com/watch?v=S4O7b0-epBo

 

[BUGGAR]

Graphics attached.
I'm attempting to load a pdf file and it won't load! It is my treatise on the "roll center".

 

[Bluefoxy]

Sorry but nothing is attached

 

[NormPeterson]

Much of the attention paid to roll center position and height is due to the excitation and transmission path. Roll is not induced buy steer (OK a bit by caster) but by lateral acceleration and lateral force. We refer to it by the mathematical term Convolution. It is a process by which steer induces lateral forces / accelerations followed by the roll response as a secondary characteristic. The timing of all this is worth studying because placement and movements of the unsprung mass(s) force points defines the initiation of events.

In other words, this really needs to be looked at using a time history approach rather than 'before' and 'after' snapshots.


Norm

 

[Bluefoxy]

'In other words, this really needs to be looked at using a time history approach rather than 'before' and 'after' snapshots'
You are rifght but my calculation are static not dynamic.

What mention Cibachrome is tested to see the dynamic of car. First those test are for production car, all the suspension are silent bloc and they use MC Pherson full of friction, tire etc. For sure you will see time delay, hysteresis and so on. It is not the case on race car where suspension are rigid joint and so, only the tire are rubber. I understand the point of wiew of Cibachrome, roll center is irrelant when you test, because some other point are more much important.

I asked yesterday to Claude Rouelle: Do design a upper and lower wishbone suspension, with invariant distance of roll center to COG , and with minimum lateral displacement of roll center, on roll and suspension travel, is relevant?

He replied: Of course. What is important is not only the initial position of the roll centers but the variation of their vertical and lateral coordinates.

My question was: I find a way to design a twin arm with Rc not moving, which many author claim it is a good thing. honestly I have no clue if it is good not or not, what do you think?
I should add: if you have no experience in race car or you are thinking 'Mac pherson works well', please do not reply. If you reply, please use clear technical explanation (drawing calculation, measure) (not photos of old suspension, and not picture of car on 2 wheels). I have a mechanical engineer diploma, I m 47 year old, I m working for 20 years in motorsport, and I m still learning, but it is not a reason to waste me my time, by reply 'out of subject'

 

[NormPeterson]

You may be asking too much from static-only analyses. I'm coming up on 8 years retired, and I remember that certain types of analysis in my career were best handled by time history methods. Even if the analyses themselves between what I did and what you're trying to do now are different, the approaches can be more similar across disciplines than you perhaps realize. I see a time history approach being of value here, unless you're making everything so rigid and the "roll moments" so low that the kinematic roll center hardly moves off its static location at all and all the forces are either zero or maximum.

At least keep the matter of time in mind. LLT through the roll centers (using that approach) builds more rapidly than LLT through the elastic suspension elements (roll takes a finite amount of time, assuming some sanity in choosing component stiffnesses). Even within the 'elastic' portion, damper forces (being velocity sensitive) don't peak at the same time as spring or bar forces (displacement sensitive). Meaning that over time even the amount of understeer isn't a constant.

You might also want to get further clarification on "the variation of their vertical and lateral coordinates" - is it only the coordinates themselves that matter, or is their variation over time also important? I'm a firm believer that the driver responds to subliminal-level effects, and while everything in the suspension is still moving everything is changing at least slightly.


I've already stated my thoughts regarding the MacStrut, and FWIW I've more or less been my own vehicle dynamics guy (off and on since the early 1970's; Structural was what I got paid to do). It's for sure been better than taking the word of everybody who'd sell me something that their solution was going to be my best solution.


Norm

 

[BUGGAR]

Here's another attempt to submit my "Roll Center" math.

 

[BUGGAR]

Next page

 

[Bluefoxy]

Buggar,

Nice job. but:

1. If you isolate a wheel with upright, sum of the force apply to W+U must be zero. This mean line force upper link, line force lower link, and line force from the tire contact must be concurent on a single point, otherwise your system is not static.
More clearly you have a degree of freedom (z).

2. How do you determine your jacking effect?.

 

[Bluefoxy]

Brian,

I assume you mention this kind of graph

So if I well understand built up of geometric force (partially depend to Rc-CG) come fisrt with no movement of the suspension (no roll), then built up of elastic force (mainly dependent of spring, etc) with movement of roll.

As soon as no movement of the suspension is happens relative to geometric force, the movement of RC during roll (elastic force) is no relevant.

Do I well undertsand?

 

[GregLocock]

"It is not the case on race car where suspension are rigid joint" While it would be possible to build a racing car that has a structure as stiff as a ball joint, it would not be competitive.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Bluefoxy]

"It is not the case on race car where suspension are rigid joint" While it would be possible to build a racing car that has a structure as stiff as a ball joint, it would not be competitive.

I meant rose joint not silent block like production car

 

[GregLocock]

So did I. And I have rubber bushings that are stiffer than rose joints.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[cibachrome]

The evidence for a dynamic influence on 'roll center migration' ought to consider the entire roll axis location trajectory because it does show up in frequency response tests and simulations of frequency response tests. First, here are some time traces from a simulation of a 'real' racecar on real (tested) tires and real vehicle parameters. A realistic step steer input based on driver steer velocity capability is used as input. The signal traces are normalized by their steady state values.

 

[cibachrome]

So, once you can get your arms around the physical mechanics with migrating roll centers (actually a migrating roll axis). You can play sandbox with the sim(s). The fundamentals are simple: Roll frequency is roll stiffness and roll inertia modulated by damping.. Stiffness is not the issue. Roll inertia is. Usually roll frequency is fixed: it's an inverted pendulum: No matter what the speed is.

But, yaw velocity and sideslip (hence lateral acceleration) IS speed dependent. It is possible (and GM did it) to configure a vehicle such that the roll frequency overlays the yawrate or sideslip frequency. When this happens, the 'Q' of the system can get very high and can self actuate unwanted responses. There are some other cool direct results from this analysis but you will have to guess what they are. No need to debate over the fence, just do the math.

I can see how a roll axis departure from its static position drives up the roll inertia via the change in distance to the sprung mass cg. I suggest you expend your energies recreating this analysis to make and prove your points.

 

[BUGGAR]

Can't figure out what you want and can't waste the time

 

[gruntguru]

BlueFoxy. Minor point. In your diagram. Rear tyre delay period should begin when front lateral forces begin.

The "roll axis" clearly has some significance - eg when calculating the roll moment (as seen in cibachrome's last link).

je suis charlie