power to weight ratio and top speed

[Erunanethiel]

Does power to weight ratio affect top speed? I know drag is the main limit and you use power to overcome it. But in regards to the top speed, is it the total power that you need or the ratio of power to weight?

[henra]

Does power to weight ratio affect top speed? I know drag is the main limit and you use power to overcome it. But in regards to the top speed, is it the total power that you need or the ratio of power to weight?

Weight has very little influence in top speed overall.
Speed is almost entirely dependent on absolute power and absolute aerodynamic drag (dimensionless drag factor Cd times frontal area) alone. You have some mechanical drag and that is losely related to the absolute weight so there is a minor influence but it is mostly negligible.

[Erunanethiel]

Does power to weight ratio affect top speed? I know drag is the main limit and you use power to overcome it. But in regards to the top speed, is it the total power that you need or the ratio of power to weight?

Weight has very little influence in top speed overall.
Speed is almost entirely dependent on absolute power and absolute aerodynamic drag (dimensionless drag factor Cd times frontal area) alone. You have some mechanical drag and that is losely related to the absolute weight so there is a minor influence but it is mostly negligible.

I dont see how that is possible. A heavier object will always be harder to acclerate or keep moving compared to a lighter one.

And bikes, having worse drag coefficients than cars and 200bhp, can go over 300 km/h. A bike has nothing over a car other than power to weight ratio.

[Silent Storm]

You also have to get gear ratio in the equation.. Tall or short ratio too has a impact on top speed.

Drag, power to weight, engine powerband and gear ratio can effect top speed.

[Paul]

Object of the same shape can maintain speed regardless of its mass. It's accelerating, braking and changing direction that becomes a challenge with increase in weight.

Bikes have lower frontal area, that is why they generally can achieve higher speeds than cars.

[Erunanethiel]

Object of the same shape can maintain speed regardless of its mass. It's accelerating, braking and changing direction that becomes a challenge with increase in weight.

Bikes have lower frontal area, that is why they generally can achieve higher speeds than cars.

Yeah, heavier objects are harder to accelerate, and if you cant accelerate, you are not going to pass a certain speed. Given the same CdA and power, gearing etc. the lighter one should have a higher top speed.

Example: it takes the car A 300bhp to accelarate 10 kmh from 200kmh. But car B has 150kg less, so it is easier to be accelerated, meaning it needs less bhp, or if the bhp the same, more top speed.


Or I am severely mistaked somewhere

[Paul]

It is just a matter of time. Sooner or later both cars will be travelling at same speed, rolling resistance notwithstanding.

[bhall II]

EDIT: Paul beat me to it as I was typing.

I dont see how that is possible. A heavier object will always be harder to acclerate or keep moving compared to a lighter one.

And bikes, having worse drag coefficients than cars and 200bhp, can go over 300 km/h. A bike has nothing over a car other than power to weight ratio.

In general, acceleration from 0 to 100kph is dominated by a vehicle's power-to-weight ratio. At speeds beyond 100kph, however, drag power becomes paramount, because drag squares with speed, and the power needed to overcome drag cubes with speed.

In other words: drag increases exponentially, but weight never changes.

Motorcycle performance is down to a lower CdA - drag coefficient factored by frontal area - and substantially less rolling resistance due to slender tires.

[Just_a_fan]

Or I am severely mistaked somewhere

This.

[Erunanethiel]

I dont see how that is possible. A heavier object will always be harder to acclerate or keep moving compared to a lighter one.

And bikes, having worse drag coefficients than cars and 200bhp, can go over 300 km/h. A bike has nothing over a car other than power to weight ratio.

In general, acceleration from 0 to 100kph is dominated by a vehicle's power-to-weight ratio. At speeds beyond 100kph, however, drag power becomes paramount, because drag squares with speed, and the power needed to overcome drag cubes with speed.

In other words: drag increases exponentially, but weight never changes.

Motorcycle performance is down to a lower CdA - drag coefficient factored by frontal area - and substantially less rolling resistance due to slender tires.

http://i.imgur.com/0NPXdI9.jpg

Yes, the weight stays the same, and drag increases with speed. But that doesnt mean we should exclude it from the equation.


So a vehicle with a power to weight ratio of 500bhp/500kg, has a lower top speed compared to a car with 750bhp/2000kg? Everything else being equal.

[Paul]

Yes. Think freight trains, they have worse power/weight ratio than a child on a tricycle, but will still reach respectable top speeds.

[bhall II]

Yes, the weight stays the same, and drag increases with speed. But that doesnt mean we should exclude it from the equation.

The effect isn't excluded as much as it's just greatly diminished.

So a vehicle with a power to weight ratio of 500bhp/500kg, has a lower top speed compared to a car with 750bhp/2000kg? Everything else being equal.

A (very) loose estimate based on those variables gives the 500bhp/500kg car a top-speed of ~335kph, and the 750bhp/2000kg car a top-speed of ~384kph.

[Erunanethiel]

Yes, the weight stays the same, and drag increases with speed. But that doesnt mean we should exclude it from the equation.

The effect isn't excluded as much as it's just greatly diminished.

So a vehicle with a power to weight ratio of 500bhp/500kg, has a lower top speed compared to a car with 750bhp/2000kg? Everything else being equal.

A (very) loose estimate based on those variables gives the 500bhp/500kg car a top-speed of ~335kph, and the 750bhp/2000kg car a top-speed of ~384kph.

How about acceleration? Does a car with 500bhp/500kg accelerate slower than a car with 800bhp/800kg between 200-300 km/?

Thanks for your answers

[Just_a_fan]

An F1 car weighs 750kg and does 210mph on a good day. McLaren F1 weighs 1.1t and does 240mph. The Bugatti Veyron weighs 1.8t and does 250 mph with a similar power to weight ratio to the F1. Thrust2 weighs 4.5t and did 633mph. The Thrust SSC weighs 10.5t and did 763mph.

The heavier cars go faster because they have the power to overcome the drag. The lighter cars tend to accelerate quicker than the heavier cars. The F1 car will be slower to accelerate from 180-200mph than the McLarenF1 or the Bugatti because of its huge drag.

[Just_a_fan]

How about acceleration? Does a car with 500bhp/500kg accelerate slower than a car with 800bhp/800kg between 200-300 km/?

Thanks for your answers

If every thing else is the same (drag, gearing etc) then the two cars should be equal as the power to weight is the same.