# I Lighter equals faster acceleration

1. Jun 29, 2016

### Mohd Abdullah

Why did motorcycles always have less horsepower than cars? If a lightweight, say, a 300 pound dragbike or car have 7000 horsepower which is the same as Top Fuel dragsters, it could easily be the fastest accelerating machine on Earth thanks to its very light curb weight. Maybe it can do 3-4 seconds in quarter mile straight drag race. Thoughts?

Last edited: Jun 29, 2016
2. Jun 29, 2016

### FactChecker

If you could get 7000 HP from a small engine motorcycle, that would be impressive. One disadvantage of motorcycles is that they do not have the aerodynamic down-force at high speeds that a top fuel dragster does. The dragster tire traction is higher at the high speeds, but that is not true for motorcycles. Both can easily lay rubber at the beginning, which limits the acceleration they can do, but the dragster can get more traction and accelerate more at high speeds. So a motorcycle will never get the times that a top fuel dragster can (unless you put an upside-down wing on it).

A jet-exhaust-powered vehicle would be different because it is not counting on tire friction for its acceleration.

3. Jun 29, 2016

### Mohd Abdullah

I see. But how about, say, a 300 pound mini car? If this small car with very light curb weight have the same horsepower like the Top Fuel dragster, can it or will it acquire the same elapsed time in quarter mile drag race like the Top Fuel dragster always does? Other people said that 300 pound car will simply fly away over the drag strip because of its very light weight but I'm not sure about it. Thoughts?

4. Jun 29, 2016

### FactChecker

That would give you a lot of flexibility for putting down-force wings on it. Remember that F=MA, so as long as a larger engine gives more force/pound, it will be better. Also, there is a lot of weight even before the engine is added, so comparing two engines with the same force/pound would give an advantage to the heavier engine. (The vehicle with the heavier engine would have a smaller percentage of non-engine weight. As an extreme example, a 1 oz, 50 HP engine has a great engine HP/weight number, but would not even move the driver very fast. ) I am not sure if top fuel dragsters have weight restrictions in the rules. If not, you will probably find that every single pound more than pays for itself and that their weight is at the optimum.

5. Jun 30, 2016

### Svein

Remember that the maximum force you can "push" the car with is limited to the friction between the tires and the surface. Since the friction depends on the weight of the car, a light car will probably spin - at least until the rubber in the tires has melted enough to increase the friction coefficient.

If you try accelerating too fast on a motorbike, the bike will probably flip over backwards ("backwards somersault"). This is due to the torque from the motor and the light weight of the front wheel (draw a diagram and you will see it).

6. Jun 30, 2016

### Mohd Abdullah

There is a weight restriction on Top Fuel. Top Fuel dragsters at least must weigh 2320 pounds.

So if I understand you correctly, even a 300 pound mini car that have the same horsepower like the Top Fuel dragster, can't beat the Top Fuel dragster in a quarter mile drag race. Right?

7. Jun 30, 2016

### Mohd Abdullah

So, the Top Fuel dragster, despite weigh at 2320 pound, still beat the 300 hundred pound mini car with 7000 horsepower (which is just the same like the horsepower of Top Fuel dragster) in a quarter mile drag race?

8. Jun 30, 2016

### Svein

I do not know. What is the friction coefficient between the tires and the ground?

9. Jun 30, 2016

### FactChecker

No, that is not what I meant to say. Sorry for the confusion. All else being equal, the lighter car will accelerate faster. What I meant is that greater weight of the engine that gives it greater horsepower may make it faster. The "dead" weight of the frame, suspension, etc. does not add horsepower. So the less % of "dead" weight, the faster. Of course, there is a reason for the frame, suspension, etc.

Just considering F=MA: If two dragsters have the same amount of "dead" weight and their engines have the same horsepower per pound of engine, then the heavier engine dragster will be faster. The heavier dragster will have the higher horsepower per vehicle pound.

10. Jun 30, 2016

### Svein

Thought experiment: Assume that the track is slippery ice. Which car will move at all?

11. Jun 30, 2016

### Mohd Abdullah

Top Fuel dragster uses Chrysler 426 Hemi "Elephant Engine" which is very heavy around 700-800 pounds. So, assuming if someone want to make the 300 pound mini car faster by installing that kind of engine, that mini car would weigh around 800-900 pound with the weight of the engine included.

But if you are going to see the records of the Top Fuel, Top Fuel drag bike which is lighter than Top Fuel dragster, still can't beat Top Fuel dragster in a 300 m drag race. The Top Fuel drag bike weigh at around 1000 pounds and making around 1000 horsepower while the Top Fuel dragster weigh around 2300 pound but makes 7000 hoesepower.

12. Jun 30, 2016

### Mohd Abdullah

I guess the lighter one? Because the lighter car will give less friction.

13. Jun 30, 2016

### Svein

With no friction, none of them will move. Less friction means less force to move the car forward.

To quote "Top Gear": Thousands of horsepower does not help if you cannot transfer the torque to the track!