Please help me wrap my head around this: Why is a low roll center better than a high roll center? Everyone says it's so. But in my mind, the natural tendancy for the body to roll is increased with a low roll center, but counteracted with a high roll center. See diagram below: You are looking at the back of the car driving away from you, taking a right hand turn. The red part is the traditional, HAMB friendly body, the black part is the bias ply wheels. The dark green is the roll center. Wouldn't a high roll center counteract body roll - thus eliminating the need for a sway bar? .
Go drive a Suzuki Samurai and an Austin Heally around the same 15 mph turn at 40 mph and you will learn real quick (HINT: Do the Austin first!)
You also need to consider the center of gravity of the car in relation to the roll center. Throw that in the diagram. and see how that looks. The center of gravity is what is working the forces around the roll center. Ideal is that the roll center is in the same plane as the CG.
For me the easiest way to understand it was that a lower (than CG) roll center transfers weight to the outside tires and a higher (than CG) roll center will push the outside tire. Too low of a roll center means you need higher spring rates and bigger anti roll bars to combat body roll. Some IFS designs put the roll center below ground, that is usually not desirable but there are some exceptions. Past that we are starting to exceed my limited knowledge base. For most hot rod applications with axles on both ends the roll center is determined by the hieght of the panhard bar. The front usually falls where it falls and for the rear you are looking for axle centerline or slightly below.
Sorry. Don't have either of those handy. Are you referring to thier center of gravity or thier roll center?
One of the best I've seen. Lots of math and such but if you read between all that it gives good concise explanations.
swdad's signature: Pessimist: Glass is half empty. Optimist: Glass is half full. Government: Your glass is too big. My wife lives by the philosophy , half empty or half full, there's room for more wine.
OK, I threw the center of gravity into the diagram. I see why you said the roll center and center of gravity should be the same. That would neutralize the leverage created between the center of gravity and the roll center. Still, I still have to ask, why do you want it neutral? Why wouldn't a little loading on outside tires benefit cornering? (The diagram has dramitized it.) That's all a sway bar does. Wouldn't it eliminate the need for a sway bar if you could do the same thing with the angles of your suspension linkage?
It seems like you are looking at roll center height as a way of limiting weight transfer to the outside during cornering. For a given car going a given speed, there will ALWAYS be the same amount of weight transfer...no matter what suspension type or roll center height. It's true that a roll center that is close to the C/G height will limit the amount of body roll, but forces more of the weight transfer to act horizontally (push) through the suspension mounts, rather than vertically (compressing or extending the springs). This typically reduces traction at that end of the car. Another problem is the angle of the suspension links needed to raise the roll center up will cause "jacking" of the chassis and poor camber change on independent suspensions.
What you want is a 'balance' of the cg height in relationship to the roll axis. The higher the rear roll center is to the cgh the looser the car will become because the weight transfer will overcome the adhesion of the tires. Too low and the weight transfer to the rear wheels will be lessened and the front tires will 'take over' (for the lack of a better definition) and cause the car to push.
I think you are forgetting something. The tires "circle of traction" Your questions are all about body attitude and what all the other lads here are suggesting is a gain in grip. The more weight forcing a wheel down the larger its circle of grip the larger the lateral load it can bear. A low roll axis and proper shocks, springs and anti-roll bars to put the body roll into the outside tire is going to get you around a corner much better. But don't trust us, go get yourself a few books. Chassis Engineering by Herb Adams Tune to Win by Carroll Smith and the bible as far as many SAE Engineers claim, Chassis Design by William and Douglas Milliken
Oh, and of note; notice the axis in where your motorcyclist and horse are "rolling" on. Ground level.
The 3 examples you show aren't applicable to what we (I assume) are discussing and that would be a street or track driven car that goes L and R. the mc and horse are completely different animals (literally) and a dirt track sprint car uses a unique rear lateral locating device called a jacobs ladder which, geometrically, causes the rear roll center to rise dramatically with body roll and is the reason they slide sideways through the turn.
The body lean is shown opposite of the way it would actually lean in a right turn. The body wants to keep going straight and the tires are exerting the force that makes the body turn. As the body leans, it would put more force on the outside tires but that takes weight off the inside tire. We try to use all 4 tires to turn the car so we try to maintain enough weight on all 4 to maintain traction. The lower you can get the roll center, the less weight transfer off the inside tires. You can't get a roll center below the lowest point of the vehicle so your second drawing isn't valid.
Driver feedback is a big reason for a roll axis lower than the CG. Cars that corner like a motorboat instill great driver confidence, right up to the point that they fly off of the track.
Yea I guess I was thinking of both center of gravity and roll center and how they relate together. Probably not a fair response to your questions. During several years of competitive RC car racing, it was interesting to play with highly adjustable independent suspension systems to see how dramatically the handling could vary with even minute changes and adjustments. It was always interesting to me how so many factors came together to affect the handling (ride height, CG, Roll Center, unsprung weights, stabilizer bars, shock spring rates, shock damping oil viscosities, caster, camber, adjustable posi units, tire widths and compounds and also the ability to change all of these things differently on each of 4 corners of the car... There is definitely much science involved in going fast around turns!!
An interesting article with a look at the tie of CG and RC... VEHICLE DYNAMICS 2007 PDF http://www.google.com/url?sa=t&rct=...oJb7x0iiWjWXZ9okw&sig2=gp7TRkAqNYLzTKvXmI4d9g