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]

Problems, issues and questions related to combined cornering, braking and ride motions are easily 'handled' by studying them under controlled circumstances using computer simulations making use of mass and inertia data, K&C relationships and nonlinear tire data. Features of cars such as yours are usually designed (as is synthesized) to have specified reactions to race track loading conditions. So, most SPECULATORS worry themselves into Catch-22 nightmares. In fact, synthesis avoids all your worries because of the applied constraints, the magnitude of the parametric changes (how much RC migration under max combined Ay and Fx). When the design is finalized and built, an "inertia relief simulation" can be run on a K&C machine to verify the load transfer distribution. RC is not the primary specification. Front & rear deltaW is. The machine sim will include the engine torque, front & rear steer and camber angles, too. If your not doing this and a competitor is, you lose. It's the current technology ("know how"). Go big or go home.

 

[BUGGAR]

What I understand you are saying, if your car brakes and dives entering a corner, the fall of the front suspension and rise of the rear suspension will cause the suspension motion of each end to create a different roll rate, front to rear. The opposite will happen when accelerating out of the curve. I will leave it to others, but sounds like it could be a tuning tool? Or a bad crutch?

 

[BrianPetersen]

We still don't know what sort of vehicle the original poster (and his apparent alter-ego) is talking about, or what they are planning to do with it.

Typical independent multi-link or double-wishbone or MacPherson suspensions (front or rear) will all raise or lower the instant-center height by more than whatever the height change of the suspension is. If the front end dives 30mm because of braking and the rear lifts 30mm because of braking, the front roll center might go down 90mm and the rear up 90mm (or whatever). It's affected by the length of the relevant arms compared to the width of the vehicle. Short arms = more instant-center motion. But very long arms may interfere with the rest of the vehicle, like the drivetrain, or the interior, or whatever else is in the area.

If you do that ... one would expect that the front has a longer moment-arm with respect to the forces that lead to body roll (the roll center is further below the center of gravity) and the rear has a shorter one. What's the net effect? Presuming that the body is reasonably rigid, it's still going to roll when cornering forces are imposed. Maybe it's a little more, maybe it's a little less. Who knows. Depends on a ton of other factors. How much those forces translate to the contact patches will depend on the springs and dampers.

BUT ...

What the car does in this situation (simultaneous cornering and braking) is going to depend a whole lot on the brake balance, the weight transfer, the toe change due to bump/roll steer (front and rear), the idiot behind the wheel, etc. more than what the instant-centers are doing. Maybe the front wheels lock and the car plows straight. Maybe the unloaded rear end starts swinging out. At a certain point the driver needs to be smacked around "don't do that".

This winter I have established that my own little Fiat has slight lift-throttle oversteer when cornering on slick surfaces with the winter tires currently installed. It has MacPherson front (high roll center migration) and twist-beam rear (roll center close to ground level no matter what). How much does this have to do with roll centers, versus age and condition of tires? I'm thinking next to nothing.

Millions of cars have been built with front instant-centers that move all over the place (MacPherson) and rear instant-centers that also move a lot (multilink with short arms) or which stay put near ground level (trailing arm twist-beam axle) or are a fixed and high height above ground level (solid rear axle with 4-link diagonal uppers).

Suspensions with short lateral arms are going to do nasty stuff when they are far from nominal ride height. If you need long-travel compliant suspension (off-road racing) then use long lateral arms or suspension designs that aren't dependent on this (e.g. twist-beam axle). If you don't need long travel suspension then use any suspension design and don't let it move (stiff springing and damping). Formula 1 is a splendid example of what can be done using a "bad" suspension design and not letting it move. (Upper and lower almost-parallel arms ... it's designed with aero as first priority. They race on relatively smooth tracks, so the extremely stiff springing is of little consequence)

Suspensions with very high camber change are going to do nasty stuff period. See all of the examples that I gave earlier that have rear swing-axle suspension. Also see, Ford trucks with Twin-I-Beam front suspension. If you are stuck with this suspension design then the only way to deal with it is to not let it move (stiff springing and damping). If you are not stuck with this suspension design then don't use it.

Excessive camber change in front suspension not only lets the roll center move around but it also changes the steering axis inclination, the instantaneous scrub radius, side-scrub of the contact patch, etc. The fix ... don't do that. Again, if you are stuck with a suspension design that does this then don't let it move (stiff springing and damping). If you are not stuck with it then don't design in excessive camber change up front. Means the tires will lean over with body roll. (A) so what, (B) use the right tires, (C) don't let it (stiff antiroll bars and/or springing and damping).

Very frequently the things that one is tempted to do to counteract a specific effect have worse side effects than letting the thing itself just happen and dealing with it. Body roll, for example ... at least up to some reasonable point.

It will sure help to know what the original poster wants to do, and with what vehicle, and why.

 

[YKAI]

Hi Cibachrome,

I have heard simulator can do things crazily accurate. For people like me, I only tune and play chassis as hobby. I cannot access to those softwares, and not even tyre data.
From the other side, some people may want a track car that he enjoys, don’t need to be very fast but predictable. If there is a race car that fast, but Hard to drive, It still a winner car, simulation data will tell the drive what the limit is at where. But not for people just want to enjoy their car for weekend.

 

[BUGGAR]

I have seen a some chassis with several alternate locations for the inner pivot points of the control arms (Bill Thomas' Cheetah?) This adjustment would let you play with various locations of instant center and corresponding wheel attitude/roll resistance. Get a car with this adjustment and see what works for you? If you do this, you won't need the expensive computer stuff. I bet the cost of a good suspension analysis program would buy you a car where you could drill some additional inner pivot holes?? Early Mustangs/Falcons could be redrilled to lower the upper arm pivots (memory verification needed here).
I agree with Brian in his suspension nasties. As Colin Chapman said, "Any suspension will work if you don't let it".
Actually, "bad" suspension with "bad" ackermann can work on the "right" race track.

 

[YKAI]

Hello BrianPetersen,

I will try find link of than discussion.
They were either discussing around FSAE car or just general RC motion question itself.
As soon as I started read you comment I realized I forgot something important. Let me talk about what I’m thinking about the RC height during braking first.

During brake, rear end RC comes up. If we think that roll moment is between CG and rear RC, then this moment is getting smaller. Because CG will be the pivot point, so it stay at same place. But mass of the car is not only at CG, it has weight both ends. Rear end comes up during brake means weight of the rear end comes up. So roll moment shouldn’t change too much, because Rc comes up with rear end of the car body. If they come up with different rate, this will change the roll stiffness, and this is what I was worrying about before.
But after you mentioned weight transfer. I realized it’s not that simple. Because under braking, weight transfer from rear to front. So that required roll stiff at rear should be reduced at same time. And it is only rear end, there is front end doing as well.
If we want front rear grip level constant. We need control the RC motion that give right roll stiffness accord to weight transfer for both front rear, and at same time make camber curve right. This it possible?
But now, I Think about driving on track, we all know under brake rear end will be looser..

 

[YKAI]

BUGGAR
I think so. This is what I’m going to do.

 

[YKAI]

Brianpetersen
Forgot to say. For double wishbone, I have found long lower arm with short and steep angle upper arm keeps RC quite stable. Better than same setup with lower camber gain which has same lower arm length but longer upper arm or flatter upper arm.
It’s what 2D software telling me, before I thought long arms and less angle should keep RC stable.

 

[BUGGAR]

If I can get this to post, it's a suspension design with variable inner pivot locations. I have the fabrication drawings for this somewhere.

 

[BUGGAR]

re submit

 

[YKAI]

BUGGAR. This is great thanks for that.

 

[BUGGAR]

Happen to have one under construction right now. Front suspension is identical, except for the axle. Blue plates are for suspension geometry adjustments. I'm using centerlink steering since that has to adjust too. I have a nerdy friend who is redesigning the blue brackets to be slotted and use a trunions attached to stepper motors to adjust the suspension geometry "on the fly". (This is just what old retired Engineers do)

 

[YKAI]

I got exactly same design for the pickups! Any choice to look the picture of adjustable device? Sounds interesting

 

[BUGGAR]

YKAI, I have no photos of his adjustment set up yet since he's still building it. He's using an Arduino computer to run the stepper motors, which weigh about 5 pounds each so there's going to be about 50 pounds of stuff at each end of the car. I may have shot him in the foot when I explained that the outer steering link had to adjust in length as well. He said it would be easier to program a total steer by wire system, especially since my rear spindles are same as front and can be made steerable. What have I started!?

Do you have any photos of your set up?

ps: we're designing the steering links for a load of 2000 pounds, with no impact allowance. This is based on strain gage measurements and is rough. Does anyone have a better figure?

 

[YKAI]

Hi BUGGAR,
I will take some picture after they are made.