It's due to inertia. If your unsprung components have a high mass they are harder to accelerate/decelerate and thus it is more difficult for the suspension to maintain a consistant tyre load. (Wheel frequencies are reduced with higher unsprung mass)
Mass is an enemy in racing, the less the better. So in both cases, the engineers try to reduce mass as much as possible.
Let's examine an example of high mass to unsprung wieght (eg, a Cadillac of the 60's with 3 tons of sprung mass). The chassis, because it has so much mass and inertia, resists movement, and this is why in a luxury car where ride comfort is important, mass can be found to be of a high value.
Take that same car, reduce it's mass and sudenly it becomes more sensitive to pitching and other road distrubances.
In racing driver comfort takes a back seat to almost everything, but that excess movement can alter the optimum airflow, as well as posess added and unwanted movement of the chassis.
Please keep in mind that wheelbase also is relevant for the same reasons.
A slightly different approach.... it turns out that for a typical road car, the selection of spring rate is most influenced by the mass of the car at that end. You want a decent ride, minimal bottoming and some resistance to dive, roll and squat. Ironically the unsprung mass connected to the car through the suspension and the spring, has a natural frequency (undamped) of around 800 cpm. This just happens to correspond to 100 kph. If you have ever driven a car with a broken shock, you will know that below this speed the wheel will stay on the road. Above it, regardless of how well balanced it is....look out because the wheel will literally jump off the road. This is why wheel balance machines all run at around 800 rpm.
What does this have to do with unsprung mass you say.... if you can reduce the mass of the wheel and tyre, you raise this resonant frequency and make life easier for the suspension and damping system. The tyre stays on the ground better at low and high speeds. Increasing spring stiffness will do the same thing but this is about unsprung "weight".
Personal motto... "Were it not for the bad.... I would have no luck at all."
Also the rotating mass affects accleration and braking. So, the car will respond quicker to inputs. I have often heard it said the 1lb off a rotating mass (wheels or flywheel etc.) is worth taking 10lbs off the car in terms of acceleration/braking. Accurate or not it illustrates the importance of reducing rotationg weight.
Before your engine can accelerate car it had to start the flywheel and wheels turning...........
The ones who know me in the forum knows I hate tunning, in fact writing against it was some of my first posts.
But this thread (and the energy hungry world we live in) got my thinking, what would be colateral effects if I modify my car but just to achieve less fuel consumption? (not to make it look like a DTM)
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Loads of things you can do to improve the energy efficiency of a "normal" road car.
Aero stuff works well if you drive at high speed much of the time. Bigest thing for slower driving is turning the AC off if you have it. Now don't tell me you always run with it off because it will come on automatically with the de-frost on most cars. Fords are partuicularly bad for the AC running when you least expect it. You can easily get 10 to 20% improvement if you run with it off.
Simple aero mods are extending front air dams, fairings between the front bumper and the front wheels, take the flipping roof rack off, check under the rear bodywork to see if there is a little dam hanging down to reduce the dirt that flips up on the back of the car, very common on estate models and vans.
This is worthy of it's own thread....
Personal motto... "Were it not for the bad.... I would have no luck at all."
I used to perform calculations during the degree to calculate an effective vehicle mass (which included the effects of accelerating the rotating components) so that the acceleration could be modelled. It was suprising how much effect the rotating masses had. The wheels, engine internals and gearbox/diff internals and flywheel etc.
The rotational component it not the main driving force though behind trying to keep UNSPRUNG mass down - It is reducing the intertia of the moving parts of the suspension.
