Front & rear camber gain and RC migration difference 2

[JOMMMM]

Hello everyone,
I’m new in suspension design. I have been looking for the answer for my equations, but I couldn’t find anything really helpful. And it has been in my head for while. If anyone can help me please:

I can understand the location of the initial roll center can affect the weight transfer and grip level. I set rear RC little bit higher than front as most car dose to give better transition.
People saying you need to minimize the RC migration. I guess the reason is if RC changes too much, the grip level will constantly changing and make the driver hard to predict the car? But if grip level of front and rear are changing at same rate , I don’t think it is too bad for driver? Am I right?
Camber gain effects RC migration. If we have a car with different front and rear camber gain, will this make the car behave very weirdly? For example, we start with initial RC of rear a bit higher than from, and because the camber gain is different between front and rear, during cornering rear RC drops to a point which lower than front RC. And the car will have different oversteer and understeer characters all the time. Will this be possible?
If I had any English mistake or if my understanding is wrong. Sorry about this.

Thank you
Yikai

 

[cibachrome]

Better to specify what type of vehicle, what lateral capability of vehicle, how much roll is expected and axle configuration (independent, solid, etc.), weight distribution and range of tire properties. for example, if roll per lateral g is low and tire camber stiffness is low, worry about some other aspects of the suspensions. 3 other combinations of these terms. If it rolls a LOT and tire camber stiffness is very high, then you might start to consider getting more serious about it, IMHO. I suppose your understeer budget ought to be considered first.

 

[JOMMMM]

Hello Cibachrome,
For example, we have double wishbone suspension RWD high performance road car such as BMW m3. Or 911(maybe not double wishbone). They have fair amount of roll compare to full race car, but still not as much suspension tralve if we compare to things like off-road buggy. For those car, do we need to worry about the RC changing between front and rear?
Maybe it is not good example, but I am trying to say if there is a on road track day which has amount of roll and maybe because package, it has short double wishbone arms. Will Rc migration difference between front and rear make driver feel harder to control and we need to beware of?

Thanks

 

[cibachrome]

Well this seems to be the BMW recipe for roll camber specs for M, X, C class vehicles.

 

[cibachrome]

Their ingredients for roll centers..

 

[cibachrome]

Roll Stiffness ...

 

[cibachrome]

And one more key ingredient: the tire's size dependent reaction to the roll control issue. Try to figure out why this is meaningful. BTW: I've never been a fan of the roll center migration religion because cars like these roll 4 - 5 deg/g at most. 1 - 1/5 of this might be the tires alone. If your analysis is 2D and kinematic only, you ought to get out of playing in a sandbox and see what 3D workings with real suspension compliance elements does to your storybook analysis. If this was so important, then simple simulations with fixed roll center lateral locations would do a terrible job of predicting load transfer and understeer out to the limit. Instead, they do VERY well if the car structure is any good. Welcome to the real world!

 

[YKAI]

Hello Cibachrome,
Thanks for these informtion. I was having Logging In issue at this website for while.

From graphs I can see OEM cars like to have larger static camber at back and lower camber gain at back. I think the reason is to make sure a little under steering under emergency for safety.
But for race car I think we need front and rear camber gain to be as close as possible? so if there are RC changing, they will change at same rate? Be honestly, I have never had any experience of how RC migration will affect the handling..

What I have seen at race track. Formula car has almost no camber gain at all, upper arm is shorter at wheel side, I think only because they want to keep scrub radius lower.Also, some Touring cars has short arms and almost no camber gain. But,some of track cars like Radical has much larger camber gain.(maybe not Raical, But kind of car)
Those cars roll so little during turning, they will not have much help from camber gain. But why some of cars have big camber gain, some of have almost no camber gain. As you said RC migration is not big problem. Otherwise everyone will run small camber gain for track car.

 

[BrianPetersen]

What's your example of a car that has "big camber gain"?

Chevrolet Corvair? Fiat 126? VW Beetle before 1967?

Ford Mustang S197?

There are reasons why I gave those specific suggestions ... can you see why I suggested them?

 

[cibachrome]

You presume that any sort of camber by roll or camber by side-force has some contribution to make. If a turnip has no blood, why would you bother squeezing it ? Teams know what their tire camber stiffnesses are, including the effect of slip angle and vertical load on it. If they have any camber stiffness to work with, they monitor predicted Mx to know when the sidewall will buckle (snap-thru) or chatter.

 

[YKAI]

Hello BrianPetersen,
I am not sure, because they are old?

Recently I saw a front suspension picture of “Factory five 818” track car. That upper arm is so much shorter than lower one.

 

[YKAI]

Hi Cibachrome:
Its making sense

 

[Tmoose]

"Looks" can sometimes be deceiving.

The first Ford Mustangs had front suspension that looked like this -

http://image.mustangandfords.com/f/...03_+frontend_alignments+alignment_diagram.jpg

A relatively slight relocation of the upper control arm inner pivot ( Arning/Shelby Control Arm Drop) did some very good things for "handling."

https://www.mustangirs.com/pdf_articles/mustang_monthly_1983-09.pdf

 

[NormPeterson]

Teams know what their tire camber stiffnesses are, including the effect of slip angle and vertical load on it. If they have any camber stiffness to work with, they monitor predicted Mx to know when the sidewall will buckle (snap-thru) or chatter.

I'm assuming that 'chatter' produces a juddery feeling. What would that indicate, and would it be associated with an independently sprung drive "axle"?


Norm

 

[cibachrome]

"Chatter" is not associated with drive axle designation. It is more like a "shimmy" within the tire as lateral sections of the tread in contact with the road excite an nth mode standing wave in the carcass. Certain combinations of pressure, camber, slip and Fz create hot (as in melted) spots which generate a stick-slip phenomenon and this forcing function excites a normal mode. It is/can be very destructive to the tire as well as the suspension elements depending on the mode number. More evident with pressure below minimum recommended.

All that being said, the chatter phenom is more widely noticed in motorcycle racing bikes under full power in a turn with heavy camber angles. But the 'feature' of this tire resonance is also seen in stock car front tires (that I'm familiar with). The tread is reckoning with the peak of the mu-slip curve. One side is over and one under. There is an Mz reaction from the inner and outer tread band longitudinal forces which 'shimmy's the wheel. There may be some videos around showing chatter on a tire Force & Moment machine. When running at speed, the vibration will be severe and make you look around to see if the building is going to self destruct ! I've even seen it on a race tire being tested at 3.5 kph. It doesn't sound like frying bacon anymore (like it should).

Some folks talk about 'power hop' in the context of 'chatter', too, and this is a similar resonance but different degree of freedom from the one I think we are talking about here.

 

[NormPeterson]

Thanks. I suspect I've gotten into a little 'chatter' in the Subaru (Legacy 2.5GT) in a corner taken unusually, ummmm, enthusiastically. OE tire and wheel sizes. Can't remember ever having experienced it in any other car, and the S197 (significantly wider wheels and tires - but the same mfr & model tires as the Subaru) sees road course track time.


Norm

 

[cibachrome]

From a tire usage standpoint ('what the ducklings call Optimization'), a simple but capable cornering simulation can show you that, as is often the case, the pair or tires on the steered axle is not coordinating their Fy generation. One of those tires will be very unhappy and complain about it loudly. If you have the nonlinear tire data and some Ackermann function values, you can investigate to find that the outer (higher vertical load) tire is actually fighting the inner one. So, to get more front grip you need to set up a better Left - Right steer angle pair. It can be very puzzling as the 'best' function is tire construction specific (The old "Do I need positive or negative Ackermann ? question). I think I can show you an example of what a ridiculous toe advance that might require. But, as you give the front more grip, you are dramatically lowering understeer even to the point of closed loop instability at a high lateral g-level (i.e. the driver can't save it no matter what). So, you need to lower the rear cornering compliance a bit too, to save the day. Also the large toe-in or toe-out angle differences at that steer angle can make the vehicle nearly impossible to drive straight. (Ackermann is hard to package, so they change static toe instead). Obviously tire wear and aero and braking and wheelhouse clearance concerns come along with the deal, too. just like in-laws. Hence the popularity of 'parallel steer'.

 

[cibachrome]

Here's a cheezy example from a premier mid-sized Euro Sled simulation. A streamlined 4 term Pacekja4Lite tire model for the car's tires. You will need to know or iterate how much load transfer is apportioned to the front meats. In the curves shown on the plot, the actual amount is one of those dashed lines. In this case, the tires want some toe-in as you approach Max-Lat (at say 5 degrees of slip angle to give the front end some jump. But the car is already pretty balanced so there is not much to be gained. However, by adding the 1 degree or so of toe-in, we've cut about 0.6 deg/g of understeer out of the balance act, which is no small amount for this car. My numbers aren't exact because I've only considered Fy and not any Mz or Mx or camber influences, but you get the picture and the right tools easily hand you the in-law compromises.

Put a slick on or a VERY large size and width tire and that plot may mysteriously flip upside down. Cars with 2 different tire/wheel recipes can really muddy the water because the rear may get into the act if it gets lonely.

 

[YKAI]

I’m not Sure what “chatter” is. My Alfa has special front suspension which moves upright position forward and back while you turn the wheel. At parking speed, front tyres jump at full lock. Is it chatter?

Is it possible to tune Ackerman or Toe without data? Some people measure front tyre temperature and compare both sides after skid pad. But Weight transfer based on speed as well as Ackerman, skid pad test shouldn’t be useful

 

[YKAI]

Sorry those thing above are not related to the topic.
Anyway, I saw discussion on SAE forums about Rc motion yesterday. Someone mentioned that If RC position is changed during squat or lift or roll. Roll stiffness may change at same time. If front and rear change at different rate, we may having problem of alternating understeer and oversteer?
Rc point shift around is fine, but we have to watch the other side of the car.

Dose this make any sense?