carguy123 PowerDork
April 23, 2012 12:11 p.m.

Wiki has this to say:

"Center of mass

In steady-state cornering, front-heavy cars tend to understeer and rear-heavy cars to oversteer, all other things being equal. The mid-engine design seeks to achieve the ideal center of mass, though front-engine design has the advantage of permitting a more practical engine-passenger-baggage layout. All other parameters being equal, at the hands of an expert driver a neutrally balanced mid-engine car can corner faster, but a FR layout car is easier to drive at the limit.

The rearward weight bias preferred by sports and racing cars results from handling effects during the transition from straight-ahead to cornering. During corner entry the front tires, in addition to generating part of the lateral force required to accelerate the car's center of mass into the turn, also generate a torque about the car's vertical axis that starts the car rotating into the turn. However, the lateral force being generated by the rear tires is acting in the opposite torsional sense, trying to rotate the car out of the turn. For this reason, a car with "50/50" weight distribution will understeer on initial corner entry. To avoid this problem, sports and racing cars often have a more rearward weight distribution. In the case of pure racing cars, this is typically between "40/60" and "35/65".[citation needed] This gives the front tires an advantage in overcoming the car's moment of inertia (yaw angular inertia), thus reducing corner-entry understeer."

Can someone give me a simple way to demonstrate to a teenager that keeping the engines inside the wheelbase and the weight more centered makes the car feel lighter and rotate better?

Duke UberDork
April 23, 2012 12:17 p.m.

Tape a weight to a yardstick. Have them pinch the yardstick at the midpoint and hold it flat, then rotate it left/right to simulate yaw. Move the weight to various points on the stick and feel the difference in effort it takes.

If you have a simple fisherman's scale you can even measure the difference. that would require some kind of consistent bearing for rotation, though.

MrJoshua PowerDork
April 23, 2012 12:32 p.m.

Does said teenager have a bike with "pegs" on the front and rear axles for doing tricks or hauling passengers? If so have a passenger ride on the various positions and let the "driver" feel the difference in handling. The same could be done with a grocery cart and a 50lb child hung off of the front, in the cart, or hanging off of the back.

Curmudgeon MegaDork
April 23, 2012 12:38 p.m.

Another way: hold a heavy object in your hand, a 5 pound barbell weight is ideal, then turn it with your hand held vertical (twisting motion).

Now use a small barbell with a 2 1/2 pound weight on each end, the total weight is 5 pounds just like the first weight. Hold the barbell in the middle, then use the same twisting motion. The barbell is a LOT harder to stop turning than the solid object. That's PMOI.

Mass centralization is how PMOI is controlled. In the mid 1990's Honda, Yamaha etc spent a TREMENDOUS amount of R&D time and money on their motocross bikes to do just that. That's why on the 4 stroke bikes you see things like the cylinder being as vertical as possible, radiators mounted low and rearward, the fuel cells dropping down almost around the engine, the two strokes have the expansion chambers as near the center of the bike as possible, airboxes that nearly wrap around the carb, electronics at the rear of the engine and mounted low, stuff like that.

Not an MX bike engine, but this will give you an idea of how much they work at this whole thing:

akamcfly HalfDork
April 23, 2012 12:40 p.m.

Shopping cart's great. Throw a few cases of carbonated corn syrup beverage in it and move them around to demonstrate the effects of mass on the front or rear wheels

carguy123 PowerDork
April 23, 2012 1:06 p.m.

Thanks guys surely one of those items will get the point across. He doesn't have a bike with pegs now, but maybe he did in the past.

tuna55 UltraDork
April 23, 2012 2:46 p.m.

I am still curious as to this, because I understand it from a physics point of view, but I have to see anyone actually measure it. Mostly I hear about it from fanbois hating on Corvettes or 911s. Seems odd that nobody publishes any actual number - maybe it's less important, or varies less, than some of you think.

Curmudgeon MegaDork
April 23, 2012 4:03 p.m.

Yeah, PMOI is definitely important. Ask anyone who's ever put too much weight behind the axle of a trailer.

Well, there's also the day I had a deep cycle marine battery in the back of my GTi. Long story short, it slid across the back of the car and came to an abrupt halt against the inside rear fender. The back of the car came around like you would not believe.

carguy123 PowerDork
April 23, 2012 4:16 p.m.
tuna55 wrote: Mostly I hear about it from fanbois hating on Corvettes or 911s.

Well it's why it took Porsche about 30 years to make the car handle well without biting the drivers.

Salanis PowerDork
April 23, 2012 5:08 p.m.

Two weights on a yardstick. Show that if they're equal distance from the center, the balance point (CG) stays the same, but closer to the center and the stick takes less force to rotate.

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