Center of Gravity

[mep]

For some time now I am thinking about the ideal position of
the center of gravity.
But on longitudional location, not on horizontal location.
It's not so easy because there are several things to take into acount.

1. Best grib in corners at constand speed.
2. Best acceleration on a straigt line.
3. Best grib in corners and best acceleration out of the same corner

I tried to put it in some formels but I hope you could help me here a bit.

Maybe it's best to put as much weight as possible on the rear tires.

[DaveKillens]

THe "ideal" center of gravity for different conditions such as acceleration, cornering, and braking is different for each situation. One good example to study is dragsters. For them, it's all optimized for acceleration. And when they are at maximum acceleration, the best way to be would have the front wheels just barely above the pavement, so that all the weight and shifting of mass due to acceleration passes through the rear tire contact patch. And on some very extreme oval racers, they have as much of the mass located on the inside turn side, while the outside wheels are as far out as can be. Of course, it's not realistic to have the center of gravity move around like that for a road racer, they have to find a compromise position.

[mep]

OK DaveKillens,
you mentioned part 2, best acceleration on a straight line.
Thanks for that.

Now for best grib in corners with a constand speed I think the center of
gravity must be near the midle of the car so the front tires and the rear tires
are both operating at the limit and at maximum grib.
The rear tires have a bit more contact surface so the weight can be
moved a bit to the rear. This can be easily calculated for all circumstances.

But the problems coms up If you want to drive through a corner and
want to accelerate.
Some weight must lie on the front tires.
This is the point where I dont have an idear.

[Tom]

Annoyingly F1 machines are not alowed moveable ballast, however the way I understand it is, weight forward, better brakind and grip under cornering, can induce understeer.
Weight rearwards, better acceleration (RWD most benefiting) more oversteer.

Not sure if this has anything to do with CoG, but I know the lower the weight, the lower the CoG making rolling less likely (why boy racers lower their cars, but they don't understand this and just say it helps)

[mep]

Tom wrote
weight forward, better brakind and grip under cornering, can induce understeer

Yes but more load on the front tires should also increase the friction witch
is needed to steer.

[Tom]

Yeah, but the more weight at the front the more the front of the car tries to push forward in a straight line. Cars don't like going round corners. Their weight is always trying to push them straight.

Understeer: Too much weight at front.
Oversteer: Too much weight at rear. The front makes it round the bend but the heavy read wants to carry on forward so it spins the whole car around.

[Ciro Pabón]

...so that all the weight and shifting of mass due to acceleration passes through the rear tire contact patch...

Like this?

[Ciro Pabón]

Yes but more load on the front tires should also increase the friction witch is needed to steer.

Good point. You can see the article "The Traction Budget" by Beckman. Yes, I know, I recommend it once a week.

Not sure if this has anything to do with CoG, but I know the lower the weight, the lower the CoG making rolling less likely (why boy racers lower their cars, but they don't understand this and just say it helps)

Tom's karting friends have got it right without any formulas: lowering the center of gravity improves your braking, accelerating and cornering ability. The rolling "needs" are not as important for an F-1 as they are for an SUV. The rolling motion also causes weight shifting laterally, but there are mechanisms for improving the rolling attitude, mainly the anti-roll bars. This makes the entire tuning of CoG and suspension an art.

You can see Beckman's article on weight shifting or Wikipedia article on car handling and a link page on vehicle dinamics where it is explained. I also wrote something about this and roads.

There are some figures about the "ideal" weight distribution if you take in account the typical road you will encounter (a point discussed elsewhere with DaveKillens). The Wikipedia article I mentioned (and that I found recently) has a funny section on "Cars with unusual handling problems" that goes a little on this direction. There is also a "quick set-up guide" for race-cars that exemplifies front/rear weight distribution effects.

The front/rear weight distribution is a point argued about since the times of King Tutankhamon which, by the way, died in what could be qualified as a racing accident. Take a look at his fine gold and black chariot (Jhon Player Special?), ironically found in his tomb (it would be as Fangio buried with his car ).

You can even take a look at the egiptian monocoque developed for his Majesty that I cannot resist to post, (yes, I know I am a little off-thread, but for this type of vehicles the CoG distribution was critical ). And, yes, they have monocoques, antiroll bars AND suspension embedded in the chassis itself (the beams of the chassis itself were the springs, something F1 has not think of, yet). They could not integrate the horse and the chassis, but I think they tried...

It has been said that this thing "... surpass all monumental structures of the pharaohs in engineering sophistication."



You can find also a really good acceleration/deceleration simulator byLateral Visions..

The Bugatti 35 and the Lotus Seven are mentioned frequently by racers and designers (not that I have handled one! ) as "ideal" race cars from the point of view of balancing everything, including weight distribution.

Some racer friends think that weight distribution and football (soccer for the americans) forward/defense distribution are like a blanket that it is too short: if you cover your neck, your feet become exposed.

[Ciro Pabón]

BTW: shouldn't this thread go into "Aerodynamics, chassis and tyres" forum?

[Apex]

Ok hang on a sec here!

Some of you people are saying that having a forward based CG will give you more steering where others are saying it will give you less.

I feel thats because you are forgetting about the polar moment of inertia (when viewed from the top of the car), ie the one that matters for change of steering direction.

Take the VW pheaton for example - the W12 understeers like crazy where the other engined modles do not. The W12 is the heaviest engine in that range, so:
The CG has shifted forward but the inertia has increased so much that the added grip provided by the CG shift does not compensate enough to overcome for the added inertia.

If you could keep the polar moment of inertia the same then moving the CG forward would give you more steering.

Also: someone wrote (forgive me i forgot) that lowering the cars weight lowers the CG. This is not necessarly true. It depends where the weight was removed relative to the height of the CG. If you chop the roof off then the CG will be lowered, if you take the remove mass from low down you would actually be increasing the height of the CG.

[Tom]

This might help a little.



Fh is the forces going forward horizontally. (momentum, car always wants to go straight on)

Fv is the forces going vertically down through the wheels (providing grip)

CoG is the way the centre of gravity will go when slowing down for a corner.

If Fh is stronger than the grip provided by Fv (among other things) through the tyres, the car will try to understeer and go straight on because thats where the weight wants to go.
If grip provided by Fv overcomes the weight going forward the car will turn the corner (in theory)

Sorry if this doesn't help. Also my skills on 'paint' are pathetic at best.

[Ciro Pabón]

Ok hang on a sec here!

Some of you people are saying that having a forward based CG will give you more steering where others are saying it will give you less.

I feel thats because you are forgetting about the polar moment of inertia (when viewed from the top of the car), ie the one that matters for change of steering direction.

Not exactly.

Forward = Understeering
Backwards = Oversteering

The yaw inertia does not vary during a curve. The CG and weight of the car are also the same during a curve, but the DISTRIBUTION of weight on the wheels changes when you brake or accelerate. I post the same photograph from my article I posted I few days ago (sorry not to put simply the link, but the photo is "lost" in the middle of the thread).



When you have more weight on the front wheels, the amount of lateral force you can do with the front axle increments and the front wheels will slip before the rear wheels.

Hope this helps. If not, please read the references I gave.

May the best man win! See you after the race.

[Apex]

Sensing a bit of a communication gap here

I never said that the yaw inertia changes during a curve.

I conjure that moving CG (forwards/backwards) from the centre of traction would increase the yaw moment which reduces steering response (parrallel axix theorem)?

[Ciro Pabón]

Sensing a bit of a communication gap here

I never said that the yaw inertia changes during a curve.

I conjure that moving CG (forwards/backwards) from the centre of traction would increase the yaw moment which reduces steering response (parrallel axix theorem)?

Yes, you are right about the yaw moment of inertia being the key element for determining how much will steer the car under a lateral force. But moving the CG ahead will give you understeering, not more steer.

Maybe the gap comes from this: you are also right in that moving the CG forwards will give you more lateral force on the front wheels, because they weight more (more steer). But this means the front wheels will slip before the rear wheels because they are under a greater load (understeer). Am I wrong or simply did not get you? (Damn! This could explain why my seven years old son is starting to beat me at the karting track!)

Besides, some posts said that CG lowering influenced only on the rolling abilities. I was just trying to take your post as a starting point.

[Tom]

Sorry its partly me confusing you.

I'm not entirelly sure what I'm talking about.