Can Weight be a Bad Thing? The Other Side of Added Grams

by Emile Abed

Let me introduce myself, I’m a Physics teacher with an engineering background and teach 11 – 18 year olds in the UK the wonders of the physical world. This doesn’t make me a diecast guru but I do have a nerdy interest in making sense of one of world’s most chaotic sports.

The first thing most of us do to try and make our cars go faster is to increase the weight. This is because when trying to make anything go fast you want the forward force, in this case the weight, to be as high as possible giving you enough acceleration to only ever see your fellow competitors as ever-decreasing dots in your rear view mirror.

What would you say if I told you weight makes no difference to acceleration? You’d think I’m crazy. But if there is no friction this is true. If you’ve ever watched the grainy footage of an astronaut dropping a feather and a hammer on the moon you see them both hit the ground at the same time. There’s a much clearer video of Professor Brian Cox doing something similar in a giant vacuum chamber with a bowling ball and feathers.

Without air resistance everything drops with the same acceleration. It all seems unreal as we only ever experience objects traveling through air. So what does this mean for diecast cars going down the track? Is weight as important as it seems and, to really turn things onto its roof, can weight slow you down?

Now, I can’t go through a Physics “lesson” without talking about energy. With diecast racing we normally start with gravitational energy, the stored energy of a object at a height, and we want this all to turn into kinetic energy, the stored energy of a moving object.

As all the cars start at the same height the only variable that effects its energy is the weight. This is where a student normally turns round and shouts, “Hang on a minute, that means a heavier car has more energy that’s why it’s faster!”. But it takes more energy to move a car with more weight. If you do the calculations, when all the gravitational energy is converted into kinetic energy, a change in the mass has no effect on final speed. (Don’t worry I’m not going to bore you with calculations here!)

Now, I could tell you weight makes no difference and give your competitors an advantage in a world-shattering diecast conspiracy. But weight does make a difference.

The important thing here is the percentage of energy loss a car has from its initial energy. This is where weight can make you faster. The same loss of energy will slow the lighter car down more than the heavier car. So what I’m going to look at here is what could make a heavier car lose so much energy that it becomes slower than a lighter car.

To try and understand what happens to a modified car with extra weight imagine it was possible to make a car very heavy, I mean so heavy it’s a minor weightlifting session just to lift it onto the track. What would happen? Although this is unlikely to be possible, the factors that effect this neutron star of a car will, to a much lesser extent, affect a modified car with additional weight. So let’s get started with a selection of the issues that might arise.

Possibly the most obvious disadvantage to increasing weight is stability. By increasing the weight it is likely that the centre of gravity of the car will increase in height.

With any increase in the height of the centre of gravity the likeliness of the car tipping over goes up (fig. 1). This is why taller vehicles are inherently less stable. If you have ever watched or tried to race anything like food trucks you’ll know what I’m talking about. This is particularly important when there is a chance of anything that may destabilise a car. (Bumps, curves, jumps or some friendly paint swapping.)

fig 1. Weight raises the centre of gravity of a vehicle. How is it positioned can have an effect on your speed.

It is possible to add weight and distribute it well enough to make the car more stable. So let’s make sure the weight in this sumo wrestler of a car is very well distributed. So well that we can assume it has the centre of gravity similar to a child’s inflatable bopper toy. The toy that no matter how hard you punch it, the resilient clown always comes back up.

What other factors would this incredible weight affect to slow the car down?

Our imaginary car will be so heavy it is likely to bend its axles causing the wheels to rub in the wheel arches or worse ground out in the start gate. With lots of racing cars having low ground clearance and large wheels, that just about fit in the arches, this problem becomes much more likely. If the axles survive this abuse there would be an increase in friction on the axles with the added weight again preventing the wheels from moving. What we have now is a very cool paper weight.

If our car doesn’t suffer this level of failure there is still an additional strain on the axles, resulting in less movement to soak up any bumps in the track, and could result in a very bumpy ride losing precious energy.

Everything would be intensified on a banked curve as the additional G- force would increase the force acting downwards on the car.

Okay, so what if we modify the car further with good clearance, larger arches and reinforced axles that are polished to a mirror shine with the best lube available to keep alloys spinning indefinitely.

Surely, this will be the fastest possible diecast racer of all time?

Well, there is one more thing I want to mention, and unfortunately a diecast modifier has very little control over it. The track.

fig. 2 -- The car moving on the track applies downward pressure, and energy is lost in the process.

I know my own track is likely to collapse if this behemoth of a car was to be positioned at a start gate. If the straight could take the weight it would bend, absorbing precious energy (fig. 2), and when coming to a corner it is likely to just keep going in a straight line through an unsuspecting barrier, requiring special digging equipment to dig it out of a football pitch size crater.

This is because the faster an object goes and the greater its mass the more it will resist any change in direction or change in speed (fig. 3).

fig. 3 -- Inertia of a heavier object will tend to move the car in a straighter line, making cornering more difficult.

This inertia is advantageous when an unwitting challenger attempts to nudge you out of the way and finds themselves surrounded by sparks as they attempt to steer an inverted car. But if the car needs to change direction it could result in a significant loss of energy or a much faster route down the mountain!

So with this limited look at whether weight can slow you down the answer is yes. However, used correctly and in the right amounts weight could make your car an unwavering, challenger-crushing, finish line-seeking, speed beast.