Performance Sensitivity

[Bogie]

I've often wondered how the sensitivity of our vehicle's performance (0-60, quarter mile, etc.) is affected by changes in the variables (atmosphere, street/track composition, aerodynamics). Not finding the information online, I decided to bite the bullet and figure it out myself. Assuming my calcs were performed correctly, here's what I found:

A 10% change (or error) in aerodynamics, which can include a 10% pressure change, a 10% change (or error in the area), a 10% change (or error) in the coefficient of drag, or a combination of any of the above adding up to 10%, while running a SKY in fifth gear, near the aerodynamic limit, equates to about 23 lb-ft of torque at the wheels.

I ran asphalt vs concrete and came up with 28 lb-ft difference, in fifth gear, at the aero limit.

Collectively, a 10% aero error and road surface change can map into the equivalent of about 50 lb-ft of torque, at the wheels, in fifth gear, at the aero limit. That's more than 20% of the torque delivered to the wheel. And that's just from F=ma. I didn't even consider how the pressure change or temperature affect the ability of the turbo to deliver torque to wheels. Nor did I factor in a host of other error sources.

Suffice to say, the ability to lock in a top speed is highly sensitive to the variables, be they equation variables or the real life variables handed us by mother nature.

Of course, I know some folks here already new this intuitively. But I had fun running actual numbers....and I can sleep better at night now that I know.

[Lakeland]

Originally Posted by Bogie

I've often wondered how the sensitivity of our vehicle's performance (0-60, quarter mile, etc.) is affected by changes in the variables (atmosphere, street/track composition, aerodynamics). Not finding the information online, I decided to bite the bullet and figure it out myself. Assuming my calcs were performed correctly, here's what I found:

A 10% change (or error) in aerodynamics, which can include a 10% pressure change, a 10% change (or error in the area), a 10% change (or error) in the coefficient of drag, or a combination of any of the above adding up to 10%, while running a SKY in fifth gear, near the aerodynamic limit, equates to about 23 lb-ft of torque at the wheels.

I ran asphalt vs concrete and came up with 28 lb-ft difference, in fifth gear, at the aero limit.

Collectively, a 10% aero error and road surface change can map into the equivalent of about 50 lb-ft of torque, at the wheels, in fifth gear, at the aero limit. That's more than 20% of the torque delivered to the wheel. And that's just from F=ma. I didn't even consider how the pressure change or temperature affect the ability of the turbo to deliver torque to wheels. Nor did I factor in a host of other error sources.

Suffice to say, the ability to lock in a top speed is highly sensitive to the variables, be they equation variables or the real life variables handed us by mother nature.

Of course, I know some folks here already new this intuitively. But I had fun running actual numbers....and I can sleep better at night now that I know.

I got a lump in my throat reading this post....lol....spoken like a true fellow Engineer.

[Gerry]

So if you are anticipating a high speed run, try to do it at altitude -like over 40k feet, since density is lower. Traction is kind of poor up that high, though.

[mos119]

40,000 ft.??? what mountains are you driving on?? Olympus Mons?

[jwolfpack02]

40K feet? Sky high?