Hi!
Anyone knows a site where I can read about physics concerning cars. I would like to read more about roll/air resistance and how much or if weight has anything to do with top speed.
That is a good series of articles on the physics of racing.
Im not sure if it discusses that in particular (I havent read them all), but as my understanding goes weight affects top speed very little, or not at all. Greater weight will cause acceleration to be much slower, but the top end will still be there.
Basically top speed is a function of air resistance, rolling resistance, and horspower. As long as the hp produced is greater than the hp needed to overcome the resistance of the air, friction at the wheels, and internal friction in the drive shaft, wheel bearings etc, you will continue to accelerate.
F(net of force produced at wheels and force of the resistance)= mass * acceleration
F/m = a
So... as long as the net force is positive, acceleration will be positive no matter the mass. If mass is greater, the acceleration will be smaller, but it will still be positive.
A car has a top speed because the resistance is proportional to the square of the speed, and at the top end of a cars gearing, its power will probably be decreasing as velocity increases. So that means that Fnet is getting very small which means that a gets very very small to the point that the car seems not to accelerate.
Of course, I may understand this wrong. Your question has caused me to go look it up and try to strengthen my understanding.
Thanx!
Been reading too...And my basic understanding is that weight should not interfere much with top speed but I got thoughts from a friend of mine So now it's time to find some answers.
i read in the Miata files and found this:
"The rolling resistance is approximately proportional to the velocity:
This approximation is probably the weakest one in the model."
I was just reading that section too and was about to come recomend it to you. All he is really saying there is that he isnt really sure that rolling resistance is linear in nature and that his value for a Corvette isnt from a scientific source.
I've seen three types of formulas for approximating rolling resistance.
1. the traditional (archaic) static approach:
rolling resistance coefficient * weight
2. a weird two part formula which first uses a dynamic RRC, which is then fed into the linear equation:
dynamic rolling resistance coefficient = basic coefficient of rolling resistance + (0.0002425 * speed effect coefficient of rolling resistance * (velocity ^ 2.5))
note that was converted from an Imperial formula (worked in mph) to one that works in m/s (that is the equation I use in GRC)
3. another equation, which I don't have to hand, which was something like:
ax^4+bx^2+c
IIRC, don't have any other info on that... I'll have a look and try to dig it up
In short, weight does affect top speed, but is reasonably minor. For most vehicles it is less than 10% of the total resistance of the car at top speed (air drag being the rest, unless you particularly want to work out the friction of the wheel bearings).
Air resistance force behaves quadratic to the velocity:
[HTML]F = k * v^2
with k = coefficient for air resistance:
k = cw * (ρ/2) * As
As = width * height
cw =given value for a specific car, often 0,3-0,4
ρ = density of the air[/HTML]
At high speeds only the air resistance matters because of the quadratic behavor to the speed.
Also the frictional resistance is not easy to estimate, because it's a function of tyre pressure, tyre-road-contact-area, mass, and the µ between street and tyre
And for your question, only the frictional resistance is a function of the mass - so mass isn't really important for high speed. Some time i calculated an example and the top speed decreased only a bit with increasing the mass alot ...
