OK Gage, thanks for the demonstration of your maturity level. I think you proved my point quite nicely. Can you please go and inflict yourself on another thread now that you've done your best to kill this one?
Now back to the topic:
angib wrote:That Cd figure doesn't sound plausible ... According to my Tatra book, the 77a did 150km/hr on 75bhp and the 87 did 160km/hr on 75bhp, so the 77a probably has a higher Cd than the 87's 0.36.
Using the data furnished by both Andrew and George, I thought I'd take shot at calculating what a realistic Cd for the Tatra 77a would be. Sure, everyone quotes the 0.212 Cd, but this could have been a mistaken piece of information that just got passed down over the years...
I used the following assumptions for the two cars (based on Andrew's statements and George's website references:
Tatra 77a (1935)
- maximum speed: 150 km/hr
- engine type: 3.4 litre V8
- maximum power: 74 bhp (75 bhp based on Andrew's and George's citation; 69-74bhp (51.4-55.2 kW at 3500rpm from the Czech website))
- weight: 1,800 kg (from the Czech website)
Tatra 87 (1936-?)
- calculated Cd: 0.31 (based on top speed)
- measured Cd: 0.36 (based on VW wind tunnel measurement and also quoted in the websites)
- maximum speed: 160 km/hr
- engine type: 2.97 litre V8
- maximum power: 74bhp (75 bhp based on Andrew's and George's citation) 74bhp (55.2 kW at 3500rpm based on the Czech website)
- weight: 1,370 kg
Assume standard air density, temperature, etc., and that the following are the same with the 77a and the 87:
- frontal area (A)
- coefficient of rolling friction (Crr) for the tires
- transmission/drivetrain loss %
I came up with the following numbers for the Cd of the 77a based on the given top speed of the 87 and 77a (and based on the assumption the max. speeds are controlled by max. power, not by gearing):
- Even if we use the lower Cd=0.31 figure for 87, the 77a would have a calculated Cd of 0.36
- If we use the 69bhp figure for the 77a (and keep the 87 at 74bhp) and use the Cd=0.31 figure for 87, the 77a would have a calculated Cd of 0.34
So, that's an estimated Cd range of 0.34 to 0.36 for the 77a. To put it another way, if the 77a actually had a Cd of 0.212, it would only need ~50bhp of max. power to reach a maximum speed of 150 km/hr. Or if we assume 74 bhp max. power, it would have a maximum speed of ~ 175 km/hr.
My conclusion is that the 0.212 number is wrong, or that the specs given for top speed and max. power are wrong.
Rik