With electric cars, the classic performance metric of 0-60 time has gotten much faster, approaching the theoretical limits of available traction. Yet some cars show 0-60 times even faster than this, which seems impossible. Accelerating faster than available traction requires thrust that doesn’t depend on traction, like jet or rocket engines.<\/p>\n
I suspect that the 0-60 times being quoted for some of these cars are not real, but just theoretical projections based on power to weight ratio. Here’s a way to sanity check them.<\/p>\n
Braking is already traction limited. So when acceleration is also traction limited, the car should accelerate 0-60 in the same distance and time it takes to brake from 60-0. These might be slightly different, due to the car’s uneven front-rear weight distribution and different sized tires front and rear. But it’s still a good rough guide and sanity check.<\/p>\n
Braking 60-0 is usually given as distance rather than time. But assuming constant acceleration (not exactly true but a decent approximation) it’s easy to convert. Remember our basic formulas:<\/p>\n
v = a*t\r\nd = 1\/2 * a * t\u00b2<\/pre>\nThe best street legal tires have a maximum traction of about 1.1 G. You can get up to about 1.3 G with R compound racing tires, but most are not street legal and the ones that are, don’t last more than 1,000 miles.<\/p>\n
Here’s how we compute this for 1.1 G with English units:<\/p>\n
60 mph = 88 fps\r\n1 G = 32 fps\/s\r\nv = a*t --> 88 = 32 * 1.1 * t --> t = 2.5 secs\r\nd = 1\/2 * a * t\u00b2 --> d = 1\/2 * 32 * 1.1 * 2.5\u00b2 --> d = 110 feet<\/pre>\nBraking from 60 to 0 at 1.1 G takes 2.5 seconds and 110 feet. If you look at the highest performance cars, this is about equal to their tested braking performance. So, that same car cannot accelerate 0-60 any faster than 2.5 seconds because no matter how much power it has, that is the limit of available traction.<\/p>\n
Some cars claim to do 0-60 in 2 seconds flat. This is 1.375 G of acceleration and takes 88 feet of distance. It might be possible with R compound racing tires, but not with street tires. Any car that actually does this in the real world, must be able to brake 60-0 in 88 feet. If its 60-0 braking distance is longer than 88 feet, then it takes longer than 2 seconds to go 0-60.<\/p>\n
Note: there’s rule of thumb for cars whose 0-60 time is power limited (not traction limited). Divide weight in lbs. by power in HP, then take half that number. For example, a 3,000 lb. car with 300 HP has a ratio of 10, and will do 0-60 in about 5 seconds. This of course is only a rough approximation, but it’s usually close; it works because acceleration depends on power to weight ratio.<\/p>\n","protected":false},"excerpt":{"rendered":"
With electric cars, the classic performance metric of 0-60 time has gotten much faster, approaching the theoretical limits of available traction. Yet some cars show 0-60 times even faster than this, which seems impossible. Accelerating faster than available traction requires thrust that doesn’t depend on traction, like jet or rocket engines. I suspect that the … Continue reading Sanity Check: 0-60 Times<\/span>