After much searching and a little guessing, I believe I have captured accurate numbers for figuring out air resistance at a given speed against the E30 body. I've never found all this info in one place, and certainly not with any sort of source or tests for validity.
Drag Coefficient: 0.35
Front area: 1.9417 m^2 (20.9 sq. feet)
Air Density: 1.174 kg/m^3 (happens to be today, where I live)
I then went ahead and graphed the numbers given high speeds, and also plotted M20 torque from peak to redline, assuming a slight drop in torque. The goal was to figure out if my equation and numbers were anywhere close to reality. I decided that if they were, that the force at the wheels should drop below the force of air resistance pretty close to the rated top speed of the vehicle.
Assumptions
So what do you guys thing? BMW rates the 325i at 142mph. Did I come pretty close? I feel this is accurate enough to give me close to reliable air resistance numbers at lower speeds.
Higher Resolution
Notes:
This graph does not take into account rolling resistance, though error in drive train loss and torque fall off probably eclipse any amount of rolling resistance.
I am aware that the maximum speed of a vehicle is not when acceleration reaches 0. That would take near on forever. I am however unsure at what point of acceleration they call it quits and mark the velocity. Is there an industry standard for this?
I wrote everything in metric units, and for the graph I converted it to torque numbers so that you guys could make some quick sense of it. This is why the values are sort of odd, and why peak torque shows up at 100mph (4237rpm) instead of the more accurate 102mph. If you care to see this graphed in Nm and m/s let me know ;)
Calculator for air density.
Drag Force Calculator.
Drag Coefficient: 0.35
Front area: 1.9417 m^2 (20.9 sq. feet)
Air Density: 1.174 kg/m^3 (happens to be today, where I live)
I then went ahead and graphed the numbers given high speeds, and also plotted M20 torque from peak to redline, assuming a slight drop in torque. The goal was to figure out if my equation and numbers were anywhere close to reality. I decided that if they were, that the force at the wheels should drop below the force of air resistance pretty close to the rated top speed of the vehicle.
Assumptions
- 15% drivetrain loss.
- 15% loss in torque from peak RPM of 4300 to redline 6700. This was based on eyeballing stock dyno charts.
- Getrag 260 mated to a 3.73 diff with stock tire size 195/65/14
So what do you guys thing? BMW rates the 325i at 142mph. Did I come pretty close? I feel this is accurate enough to give me close to reliable air resistance numbers at lower speeds.
Higher Resolution
Notes:
This graph does not take into account rolling resistance, though error in drive train loss and torque fall off probably eclipse any amount of rolling resistance.
I am aware that the maximum speed of a vehicle is not when acceleration reaches 0. That would take near on forever. I am however unsure at what point of acceleration they call it quits and mark the velocity. Is there an industry standard for this?
I wrote everything in metric units, and for the graph I converted it to torque numbers so that you guys could make some quick sense of it. This is why the values are sort of odd, and why peak torque shows up at 100mph (4237rpm) instead of the more accurate 102mph. If you care to see this graphed in Nm and m/s let me know ;)
Calculator for air density.
Drag Force Calculator.
Comment