Getting your Force Based Roll Center's

[GSpeedR]

I think simply determining jacking coefficients for each side is more valid and more informative. Others may disagree.

I agree.. but my point is, those two jacking coefficients can equally be expressed by themselves as ratios, or as a (Y,Z) coordinate for the intersection of those two vectors (in an opposed force compliance test on a K&C rig for example).

I think a lot of this comes down to the term 'roll center' being a poor description, and even worse how much attention is paid to it. I don't particularly care about the point about which the sprung mass is rolling at a given point in time. Big deal. The force coupling is where it's at.

If you want to construct an intersection between the force-lines (defined from tire patch to IC) and then deconstruct the intersection back into the force-line slopes before calculating individual jacking forces then that is fine. It's the further analysis done (whether you have a KRC or a FBRC) applying it as a singular joint that is very sketchy, in my opinion.

ubrben: In the heat of practice, a race engineer must work with what he/she has. I agree that a full ADAMS analysis isn't feasible, but obtaining new jacking coeffs is (by definition) faster than obtaining a new KRC. However, when the driver's bitching, whatever keeps you from hiding behind the toolbox is a good thing.

[fastback33]

I think simply determining jacking coefficients for each side is more valid and more informative. Others may disagree.

I agree.. but my point is, those two jacking coefficients can equally be expressed by themselves as ratios, or as a (Y,Z) coordinate for the intersection of those two vectors (in an opposed force compliance test on a K&C rig for example).

I think a lot of this comes down to the term 'roll center' being a poor description, and even worse how much attention is paid to it. I don't particularly care about the point about which the sprung mass is rolling at a given point in time. Big deal. The force coupling is where it's at.

Good, this is the kind of answer I have been hoping/looking for and makes me feel a lot better about this. I was afraid maybe there was some concept beyond statics/dynamics that i hadn't had yet, but im glad that the classical method makes sense.

[customcreationsllc]

Sorry I'm not an F1 guy, I'm an offroad guy I just don't have F1 around here nor the money to race F1 but I do enjoy watching it.

I've been trying to do my homework on RC have it be Kinematic or Force Based which I haven't found how to actually calculate force based just people talk about it.

I've been searching both ends of the internet and back I have the RCVD book it doesn't have force based RC, I know everyone says Jacking Effect, and to stay away from it like its the plague. Like your going to get a jacking effect and roll, but no one gets into the what actually happens. Even RCVD just states your going to get a jacking effect end of sentence

Would you be able to explain to me what really happens when the RC is above the CG?

Correct me if I'm wrong but this is the way I'm thinking of RC vs CG when your go around a turn you can think of everything creates a moment around the outside tire contact patch.
So you have the weight of the vehicle at the CG pushing down.
Then you have the jacking effect due to the RC CG heights and lateral forces, either pro lean or anti lean will add/subtract to the moment around the outside tire contact patch.

Like I said I could be wrong.

Now for offroad tires are pretty round I don't care so much about camber, the road is rough mostly dirt so I couldn't image getting even close to 1 G turn so how much could the jacking force really effect?


The reason for all this.
I want to be able to maxims my travel and steering on my new offroad race buggy.
IFS suspensions have a problem that the CV joints get compound angle due to the suspension angle and the steering angle so you loose a lot of steering.
I was thinking about going with single A-arm IFS.
If the a-arm mounts are int he pivot of the then outer CV angle doesn't change per suspension travel and I can use it all for steering.
I already made a rough CAD model and found the point where I can get 0º of bump steer so I'm not worried about steering right now.

The problem with Single A-arms the RC gets high with offroad rigs they CG is pretty high to begin with do to ground clearance requirements but the model get shows the RC could get a couple inches higher than my guesstimated CG when the suspension droops, so if I hit a bump in a corner.

The offroad forum just says it won't work because the jacking effect will cause it to handle like crap and cause me to roll. So I'm looking outside my community.
That statement doesn't make too much sense to me. Doesn't the jacking effect actually do the opposite and cause the outside wheel to lift making the car slide out or push? I cannot see making it roll.
So I've been diving into this deeper and deeper, and I just cannot find anything that explains what happens when the RC is above the CG.
I don't think the Force Based RC is changes with Single A-arms.

I'm not looking for world class handling I'm just looking for something that isn't scary to drive at speed under 70mph. The rest of the rig due the single a-arms design I think I can still be competitive with loosing a little handling since there are other benefits I'm getting such as able to turn the wheels further, lighter, just simpler, and my most important reason being able to see better.
Nothing harder than driving over rocks you cannot see.

I'd really appreciate the input and just clarify what I've been finding.
Thank you
~Justin

[mep]

Hi customcreationsllc
and welcome to the forum.
Maybe a picture of your suspension would help to make clear what you are plan to do.

[Jersey Tom]

Net jacking forces are NOT inherently bad.

They can be quite good.

[bill shoe]

Net jacking forces are NOT inherently bad.

They can be quite good.

Interesting comment. I understand how net jacking forces could be good for aero (especially with minimum static ride height requirements, etc). Are there potential benefits that do not depend on aero?

[thisisatest]

i think the real problem with the swing arm suspension is that the roll center height will be varying significantly with suspension travel. imagine you're in a corner, the outside wheels are loaded up, the arms are almost level with the ground. the car will be leaning away from the corner. then there is a dip mid-corner, the front swing arm extends, increases the jacking tremendously, and instead of the spring and damper being able to compress on the other side of the dip, it's now locked into a fully extended position until you straighten out past the turn.
that's not so good.
vw beetles set up for offroad would use straps to prevent overexension of their rear swing arms. they also would employ a Z-bar, basically a pro-roll bar. This would keep the outer wheel from being able to extend too much when the inside wheel was already extended. maybe with a Z-bar, your swing arm front suspension idea would work?

[fastback33]

You know what, I actually think there is a very simple visual explanation to what jacking does on a car. Look at a modern kart turning during a corner, it has a solid rear axle, which looking at it form a dynamics case tells us that the instant center is located on the axle somewhere. This causes the inside wheel to lift off of the ground which helps the karts yaw rate.

I can see where jacking can also be a good thing. Looking at it form the tires perspective, the more total force you have pushing down on it, the more the Cf is reduced. At speed the faster you go, the more negative lift you're going to get, which I believe is good to a point. However if you are clever about this you can get your jacking forces to sort of counter act the excessive downward force on the tire.

This is how I've come to view jacking, and the kart example maybe correct or not, but I do think it still illustrates how jacking forces affect the RC.

[Jersey Tom]

Remember though, that vertical transients aside the total load on all four tires must equal the vehicle weight plus aero loading. As such, jacking forces cannot add to the net load on the tires.

As such, if you have a net downward jacking force at an axle... the springs will have to unload a bit to get back to equilibrium - which will make the ride height go up a bit. That is the real effect of net jacking forces.

(Incidentally, by net force I mean the left and right summed).

[customcreationsllc]

Net jacking forces are NOT inherently bad.

They can be quite good.

That's what I wanted to hear, especially for a very high CG vehicle.

i think the real problem with the swing arm suspension is that the roll center height will be varying significantly with suspension travel. imagine you're in a corner, the outside wheels are loaded up, the arms are almost level with the ground. the car will be leaning away from the corner. then there is a dip mid-corner, the front swing arm extends, increases the jacking tremendously, and instead of the spring and damper being able to compress on the other side of the dip, it's now locked into a fully extended position until you straighten out past the turn.
that's not so good.
...

That is what I was my initial thinking but then I started thinking why.
The net down force is still the weight of the car (Jersey Tom explained) acting at the CG so the springs still needs to hold the weight of the vehicle.
So the RC/CG moment arm effecting on the swing arm now just cancels out a little bit of the CG weight moment arm.
I don't think it will lock it out until the RC/CG moment overcomes the weight of the vehicle either very high G turn or very large RC/CG moment.
I just couldn't imagine it being a switch get above the CG and it locks out.
But the way everything is worded.
I was looking for the real answer.

Here is a few pictures real basic models I didn't want to spend a lot of time just laying things out. I just made the models easy to move.

I sketched the RC line form the center of the tire through the center of the swing arm pivot so it gives me the height RC height at the chassis center line.
This is not in the pictures I added it later. If I have time I can work on some more pictures. If I remember correctly I was messing round with the model a bunch the RC moves about 10" say Diff center line to the top of the bell housing.
Most people use the top of the bell housing as a CG guesstimate so if my RC goes above the CG it shouldn't be by much.


I actually cannot remember what is in the pictures as work blocks all the photobucket sites I just grabbed them off my offroad forum thread.









http://www.pirate4x4.com/forum/showthre ... 738&page=4


I appreciate all the input
Thank you
~Justin

[customcreationsllc]

The Force Based Roll Center
http://www.neohio-scca.org/comp_clinic/ ... cs2007.pdf

Says the tie rod takes more load than people realize and can normally be ignored because it is usually close tho the RC anyways.
But I can either put the steering down low in front or up high in the back
Which would help lower my roll center.
I'm only talking about 1" off the swing arm pivot which gives my zero bump steer.

Though maybe a little bit could help
But I cannot find the equation Should I just be looking at the tie rod as it's own swing arm give the tie rod its own RC?
Then the lower the Tie rod frame is on the frame side it will help lower the net RC.

Thanks again
~Justin

[fastback33]

Just out of curiosity, it looks as though your upper and lower control arms are one control arm? Why would you effectively want a swingarm suspension?

[customcreationsllc]

Just out of curiosity, it looks as though your upper and lower control arms are one control arm? Why would you effectively want a swingarm suspension?

...

The reason for all this.
I want to be able to maxims my travel and steering on my new offroad race buggy.
IFS suspensions have a problem that the CV joints get compound angle due to the suspension angle and the steering angle so you loose a lot of steering.
I was thinking about going with single A-arm IFS.
If the a-arm mounts are int he pivot of the then outer CV angle doesn't change per suspension travel and I can use it all for steering.
I already made a rough CAD model and found the point where I can get 0º of bump steer so I'm not worried about steering right now.

...

It is a single a-arm wishbone swing arm whatever you want to call it suspension.
Basically it is just to get me more steering radius which is critical off road.

So I just want to know if I can get away with basic a swing arm suspension.
I don't car much about camber change offroad with roundish tires.
So the other issue is a high RC causing a jacking effect I just wanted to know exactly what jacking effect is and if I can get away with a little.

Like I also said I don't need the best handling rig ever the other benefits will probably be enough like great vision light weight etc.