Rollcentre Movement - fix to chassis or ground??

[markocosic]

Q:

Is it better for the rollcentre to be fixed with respect to the chassis, or the ground?

If I were to build a vehicle with a live axle, located with two upper links and a lower a-frame, would you attach the point to the axle (rc fixed relative to ground)

http://www.sandsmuseum.com/cars/seven/assembly/sevenm.gif
http://www.sandsmuseum.com/cars/seven/assembly/sevencut.gif

(mounts to the bottom of the diff pan)

or attach the point to the chassis and the two legs to the axle? (rc fixed relative to chassis)

Suspension travel in this instance 6" total with track 48".

I have my thoughts, but will keep stum until others have spoken :-)

[js10coastr]

... I'm pretty sure there is a thread on this somewhere way down in the pile.

[J. Vinella]

Its a good classic thread,

http://fsae.com/eve/forums/a/tpc/f/125607348/m/47110403711

[Composites Guy]

I'm not a moderator... but I am a daily reader/contributor and have been for a good while... and to me things seem to be deteriorating.

I'm getting a bit tired of NEW discussions getting shot down on this board. Sure there are classic threads discussing most topics relevant to fsae, BUT contained therin is a lot of miss-information, general rules-of-thumb taken out of context without thorough understanding, and ommitted perspectives of interest.

Please quit killing relevant discussion... you are ultimately hurting the longevity, freshnesss and interest of this forum. I suggest as an alternative that you referrence the classic thread with a simple link... and then give your take on things that were wrong, right or downright interesting about the classic thread. Then add some new thoughts of your own.

[exFSAE]

I didn't read that thread back in the day. I'm all for starting a new discussion.

My thought would be...

Having the roll centers stationary relative to the chassis will keep the elastic weight transfer distribution fixed. Since the RC's will move relative to the ground though, and more importantly relative to each other as the car pitches, the direct weight transfer distribution will change.

As a rule of thumb I would say elastic weight transfer is dominant for these cars, so having the RC's fixed relative to ground will change the balance of the car more. This isn't necessarily a bad thing.

It really depends. As you trailbrake into the turn the car will shift towards oversteer (or less understeer). By playing with that roll center migration you could probably adjust how much that balance shifts. Depends on your tire. If the tire naturally wants the car to rotate a lot, you might be able to minimize it. If the tires dont react much to load, maybe you could get the roll centers to move a lot and get more weight transfer distribution on the rear tires so the car tucks in better, or has that extra ability for the driver to trail off the brakes and get the car to yaw around.

[Composites Guy]

Ok... "practicing what I preach" I read that thread and the Mitchell article, and I have to admit it gave me more questions than answers. Lateral roll center meaning and usefulness has always been a mystery to me.

1. If we draw the classic kinematic (Carrol Smith) diagram of the roll center, then a roll center displaced laterally from the center of the car will induce roll as the car pitches. Its unclear to me if that is a bad thing, or which direction you'd want, for instance, the front end to roll under the pitch forward during braking.

2. A roll center outside the track of the vehicle is bogus. An infintesimal roll of the chassis about this point must put all suspension springs into compression or extension at once... which defies the newton's law (sum of the forces = 0) in the vertical direction. This makes me doubt the validity of using lateral roll center movement.

3. Bill Mitchells article http://www.neohio-scca.org/comp_clinic/hand_out_reprint...e%20Dynamics2007.pdf (http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Vehicle%20Dynamics2007.pdf) makes sense... however, in the design tent with many judeges a large lateral roll center movement will lose you points.


Another point of interest. We race in parking lots with lots of cracks and lumps. If you are going around a turn at steady state lateral acceleration and your outside (heavily loaded) tire hits a bump you would like the wheel to move upward and INWARD (releaving some of the lateral force). Otherwise that tire may break free. The only way for this to happen is with an instant center which is below ground... thus with a roll center below ground.

[murpia]

Marko is interested in opinions on live axle 'roll centres'...

I would say it doesn't matter very much, as your centre of gravity height doesn't change very much. Yes the car pitches and roll, but that's not the same thing.

With a live axle you're much better off concentrating on the compliances in the setup. Either your system has no redundant constraints, in which case look carefully at roll-steer effects, or it is over-contrained and you need to pay close attention to your bushing compliances and friction effects.

For a car with independent suspension, if I had the time and resources I'd model everything properly in ADAMS or SIMPACK and prepare a presentation for the judges on why 'roll centres' are an unecessary concept.

If I was up against it I'd just do a good job of designing a low-compliance, low friction, easily adjustable racecar with good loadpaths and test the hell out of it...

Regards, Ian

[B Hise]

According to regular old engineering principles, the "roll center" or "pitch center" is merely the vector along which tractive forces are applied to the chassis through the suspension. Its an important concept and it warrants some study if you're designing a car.

ADAMS, SIMPACK, SimMechanics or a lot of paper and ink can tell you what axes the sprung mass moves about. And in that context the roll center is unimportant.

Generalities aside, I'd fix it to the chassis/CG.

-Bryan

[rjwoods77]

Mark,

I think it is worth mentioning, while off topic, that certain live axle configurations exhibit much less roll center migration than others. It is highly dependant on where your side view swing arm ends up for the lateral force constraining pair. In that config the rc is a the ball point on the diff of the car so you will have a 1:1 ratio of wheel movement with rc vertical movement. If you desire less that I would look at a satchell link with slight roll understeer built into it. That is what we use on the Buffalo FSAE car and it is shocking how the rear rc practically doesnt move at all. Some people say that rc's dont matter and I am too uninformed to agree/diasgree so from a stone age perspective of looking at them there other options out there than the lotus setup and I will unabashedly say that the satchell link is one of them. So once you figure out what rc movement you want I would be more than happy to help you lay one out for a car. I am looking to do one in the Toyota Starlet I am about to buy. Look at the VIR comp thread and you will see pics of it

[J. Vinella]

When thinking about "actual" roll center location on live axle configurations I find myself wondering if this application is where a force based method really becomes more accurate.

Marko think about this, when lifting that inside wheel where is the roll center? Center of pressure of the contact patch??
Ian's point of the roll-steer effects is correct. I would focus on that more than where your kinematic roll center is.

[rjwoods77]

http://zzyzxmotorsports.com/library/roll-center-myths-and-reality.pdf