## Estimating maximum velocity of a car

Hi

I've been working on a speadsheet to estimate the top speed of a car and am running into difficulties. I'm trying to take as much into account as possible, for example aerodynamic drag, drivetrain loss, rolling friction, etc, which probably isn't helping the issue. I've made a fair amount of progress but it's certainly not correct yet.

I've found a few web pages that do something similar, but they don't take as much into account and they tend to use all kinda of weird and wonderful imperial units that make it a nightmare to work anything out with.

If anyone could take a look and give me some pointers, that would be awesome.

Bob
Attached Files
 Top Speed estimation.zip (10.1 KB, 26 views)
 Recognitions: Science Advisor You should use the actual torque curve of the car that you're looking at rather than just the peak power. Also, drivetrain efficiency is a complicated thing that really can't be estimated very well a priori (it is highly dependent upon speed, gear choice, load, temperature, lubrication types, etc). You would probably want to find a chassis dyno measurement of the car, which will already "take into account" a lot of the losses. A problem with that is that they aren't necessarily measuring brake power (i.e. power delivered to a "brake" allowing zero acceleration). Most dyno's allow the car to accelerate fairly quickly, so you also have "losses" that go into spinning up the various drivetrain components. The acceleration in these tests has nothing to do with what a car would experience in real life, so this is problematic. Anyway, near top speed, you want measurements that are as close to brake power as possible. Also, since you seem to like a lot of detail, take into account changes in the air density. This affects both drag and power output. Tires also "grow" a bit at high speeds. I also don't think your ram air calculation is correct. It probably scales as v^2, and I doubt it would be significant on a car without specially designed intakes (such as an F1 car). "Ram air" intakes on street cars are really just meant to bring in colder air from outside the engine compartment, which is a completely different effect.

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