Usable range for roll center height in racing conditions 1

[sierra4000]

Hello ,
How is defined the usable range for roll center height in racing conditions?
How is possible to fight against bad RCH ? (how effective is?)

Thanks for opinions
Radek

 

[BrianPetersen]

Don't fall into the trap of attempting to eliminate all body roll in the interest of attempting to keep the tire upright at all costs. Unless you are using tires with excessively low aspect ratio, they will tolerate being leaned over more than a lot of people think. It is possible to establish the relationship between camber and lateral force ... if you are serious about this, find out what that relationship is.

Attempting to eliminate body roll will result in excessive spring and antiroll rates, which will make the vehicle less able to keep the tires in contact with the ground over bumps and dips. It is ALL a tradeoff. Once again ... Formula 1 cars can use extremely high spring rates that will not work in WRC. Different conditions.

 

[NormPeterson]

"What extra roll stiffness do you need to fulfill your roll per g target, given the roll per g from springs ?"
i understand

"then its front and rear bar sizes that are not heavy, easy to change, efficient and fit."
this probably i not understand, this mean for ARB stiffness no exist limit? (or maximum % stiffness contribution?)

What you need to do with those two thoughts is tie them together. Once you've chosen your spring rates (on whatever basis), how much more roll stiffness do you need? After that, then it becomes a question of how you should distribute this additional roll stiffness.

Unless you're looking to define some maximum/minimum value envelope of RC heights to work within, I'm not sure how productive chasing a single aspect of suspension design - and having to make substantial compensation for the associated downside(s) - is really going to be. Even chasing a lower sprung mass CG height for its value might not be as productive as many people think. Smaller changes with smaller compensations might work better overall.


Consider that the tires you're going to be running - which may be dictated or otherwise limited by the class rules - will define an upper limit on lateral g. So there may not be much point in holding roll down to something really low like 1°/g or so if the tires aren't good for much more than 1g.

Maybe use damping to improve things like subjectively feeling that the car has taken a set.

Don't forget that after you've gone through all of these seemingly endless calculations, you'll still have to drive the real car rather than your simulation of it to see if the results lived up to the predictions.


Norm

 

[sierra4000]

OK,
Thank you for all your valuable comments!!
now some supplementary questions

What difference between vertical force from RCH or ARB? (both works against roll, but exist difference for contact patch vertical load over road bump when different % contribution is used?)
What transitional behavior difference if car controled his roll with RCH or ARB different % contribution?(slow versus fast load transfer),
We can expect problems when RCH axis have high angle?

another ideas?

Radek

 

[gruntguru]

There is a factor which tends to make a car with excessively high RC "skatey".

A car is cornering and a transient lateral load is applied - lets say a sudden increase. This could be a sudden steering input or sliding tyres suddenly finding grip. Lets compare two cars - one with high RC and low roll stiffness and the other with low RC and high roll stiffness, both with the same roll in deg/g. Although both will settle at the same roll angle in steady state cornering, the car with high RC will roll more gradually - call it "roll inertia" if you like. On the high RC car the transient is laterally transferring vertical contact patch load instantly, whereas the low RC car transfers load in proportion to roll angle. This sudden load transfer compromises the tyre's ability to maintain grip in these circumstances and make the car slower and considerably more difficult to drive.

je suis charlie

 

[GregLocock]

It is informative to plot the build up in forces in the various load paths as the car enters a corner. Milliken has a useful but limited series of plots of this. these show that RCH is a much 'faster' way of transferring forces than ARB. I have done this and written a report about it. Sadly it is behind a firewall.

Cheers

Greg Locock


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[sierra4000]

I can imagine as the car enters a corner, then high RCH build vertical forces faster "similar" like a hard damping" (for example extra lower front RCH will be give corner entry oversteer)

But when car already in steady state and hit road bump, this have similar effect?

 

[GregLocock]

Damping is a very slow path for forces, in cornering.

Cheers

Greg Locock


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

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[BrianPetersen]

Thought experiment.

Bear in mind that with a suspension design having a high instant-center, the contact patch moves "out" as it moves "up".

If you are cornering then the tire will be at a certain slip angle in top view with a certain force being applied due to the lateral acceleration. What the relationship is between that slip angle and lateral force will depend on the tire characteristics, the inflation pressure, etc.

Now, combine the two statements above. If it goes over a bump, it forces that tire to a greater slip angle and it applies a greater normal force and it applies a greater force in the direction of the lateral acceleration. Going down the other side, the opposite happens.

Maybe that results in a "jolt" laterally, felt by the driver. (I hate the ride motions of Jeeps with solid axles because of this.) Maybe it results in the tire momentarily losing grip (on either side of the bump). Who knows.

 

[NormPeterson]

But when car already in steady state and hit road bump, this have similar effect?

Why wouldn't it? Forces due to kinematics always build more quickly than forces that develop via displacements involving elastic components. Relative magnitude would be a separate matter.


Norm

 

[cibachrome]

As long as this thread still hangs on, I suggest you make yourself up a mechanical model (I was raised on Erector Sets) to OBSERVE the effects of low and high roll centers. We even used on in a legal case to help explain to a judge and jury some odd things about roll induced 'whatever'. Allow the model to be manipulated laterally (glass top table is nice) and in induced roll from a rolling road. You will need a couple of sets of springs (soft and hard) to try to maintain the same steady state roll gradient. Have a stop watch on hand, too.

The reason you should have a course in Mechanics is because the transient behavior for the 2 'roll center' options is usually (and sometimes even deliberately overlooked). Yet it's a player in this discussion. Especially so with a mid-80's Corvette.

You see, the yaw velocity peak frequency of a vehicle (let's just says it's 1 Hz) is the result of it's dimensions, mass distribution and cornering compliances (axle side-slip gradients). It's also speed dependent. One of the reasons we test vehicles at multiple speeds ranges is to observe these key critical metrics. Now roll peak frequency is a product of it's roll stiffness and inertia tensor, blah blah, blah. (OK, its a matrix with cross terms). Speed is not a player in roll dynamics for just about all vehicles. So what happens to this matrix when you move the roll axis up or down ? You have NOT changed the sprung center of gravity. What HAVE you changed ?

If you are so unfortunate as to have your roll frequency line up with the inherent yaw rate frequency at some speed, then typical roll/yaw coupling terms will light up, as in front and rear roll-steer and roll-camber (i.e. steer by roll for the vehicle I mentioned. This means that for example, rear roll understeer intended to reduce the rear cornering compliance and improve transient response, gets it's sign flipped as speed increases and a whole lot of bad shitT starts to happen.

One school (The Mechanics) throws a LOT of damping at the vehicle to calm it down while the other school (The Nerds) sets the roll-steer to roll oversteer to prove beyond all reasonable doubt that Roll Oversteer is good for you and the only way to do business. I wish I could get Louis Black to do a comedy sketch on this subject. It would be amazing, funny as Hell and technically correct. Watch him on YouTube. You'll get the idea...

"And so, Your Honor, the Idiot who read a book on vehicle dynamics and drew up some really pretty colorful lines on some pretty white Vellum had only book learning passed on by previous horse and buggy designers ..."

 

[sierra4000]

More or less i understand
Thank you all
only Cibachrome........ why "gets it's sign flipped" ?
something i overlooked?

 

[sierra4000]

probably I understand at last
only just don't understand why Roll Oversteer can be good

restore steady state balance?

 

[cibachrome]

Look up the meaning of "convolution of cascaded transfer functions". Roll does not get it's input from the driver/operator in a flat road turn. Yes, you can be a weeee bit from large caster settings on steered wheels, but for now, get your head around this notion: The driver creates a yaw velocity and a sideslip velocity by steering some tires. Lateral acceleration results from the yaw rate and the sideslip acceleration. Roll happens when the sprung mass wakes up and gets the message. So in the mathematical sense, an Ay by Steer mechanism (transfer function) is input to a Roll by Ay mechanism (transfer function). Transfer functions for dynamic systems involve complex variables to simplify the math. This multiplication in the complex frequency domain is call 'convolution'. There "can be/is" coupling feedback by means of the suspension designer's intent to have 'Roll Steer'. (Don't call it 'bump steer', that's a term used by wrench heads. This is New School). The convolution process feeds back steer from the roll 'black box' to the chosen axle with a phase angle resulting from its net frequency and damping traits. Since yaw rate frequency increases with speed, the phase angle for roll steer can change sign if the peak frequencies are pretty close and yaw rate peak is below that of roll at low speed. As speed increases, the yaw rate peak will climb and overtake that of roll and keep going. This will eventually result in reduced damping, high overshoot, long settling times and even an instability at VERY high speed when the driver's own transfer function starts leaking soft brownish material. Even the most simple controls models of Vehicle Dynamics show you this if done correctly.

This overall mechanism is also a boundary constraint for your roll axis and sprung mass center of gravity locations. And the R.A. inclination angle is a means of altering the convolution process to favor the dynamic *transient response, especially if the vehicle is intended to carry large differential payloads (passengers, building materials, and old school vehicle dynamics text books.

 

[sierra4000]

I think i can imagine well
(I have very good imagination...........mainly for conditions close to leak soft brownish material :-D )

If i understand, roll axis and sprung mass center of gravity locations are in priority relation,

Seems that roll axis inclination value is MOST important for transient events before only roll axis height which extra low height can be more or less corrected with ARB

We can set general scheme? (60/40........50/50.......40/60 % front weight distribution)
You can give some specific examples or even some recommendations?