How Much Slower Is A Heavy Racecar?

You’ve a heavy racecar. Its over the minimum weight, but is it really a problem?

Here is how you can make answering that question easier – with Maths!

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Your Race Cars Infinite Jobs List!!

Ever get the feeling your car’s job list seems never-ending?

As soon as you tick off one job, another five appear!

It is so hard to prioritise…

…even worse, when members of your team have (very) different opinions on what is critical (or not)!

Even working on these on your own is hard.

What should you do?

What is more important… Now?

A team I work with asked me to help add a little objectivity to a “discussion” they were having on this.

They had a heavy racecar – over the minimum weight for their class.

The debate wasn’t about whether that was a bad thing (clearly it is) but about how bad it was – in the context of the infinitely long list of other “to-dos” on the car.

Some felt the weight was a priority, others (strongly!) felt it wasn’t.

What would You do?

Ever found yourself in a position like this? What did you do?

With multiple different things on the board, the heated debate isn’t normally about the jobs per se, but about how much should be invested in solving them now versus later.

Below, I’m going to show you the effect of two different masses on racecar acceleration. But don’t forget, the actual problem here is prioritisation.

I’m happy to help you too [link].

Where is a heavy racecar slower?

Perhaps surprisingly, the weight of your car does not affect your potential top speed – in simplistic terms at least. For a given power, your top speed is determined by losses – some mechanical but the majority aero.

I’ve written more about this in the gearing articles [link].

Your car’s weight becomes an issue in acceleration – i.e., the time it takes you to get to that top speed.

A car that weighs more is going to accelerate slower – I think nearly everyone would agree with that mental model? I hope so.

How much slower? Doing The Maths

For this race team, I built a simple model to show them their car’s acceleration profile. I’ve changed the numbers to my race car so I can share this, but the result is below:

How much slower is a heavy racecar

I ran two cars in my model: 650kg and 700kg, with 145hp and a 0.7 Cd. Starting out at 50 mph – as these guys don’t do standing starts – you can see a couple of things.

The top speed is the same, and the lighter car is getting to each speed faster.

The thing is, the lines look pretty similar. If you were the guy who said, “don’t bother with the weight reduction now,” you’d be feeling pretty good.

But look what happens when you look at another view.

Comparison view of the data

I decided to compare the time it took for each car to reach the same speed by calculating the difference.

The difference in time for two heavy racecars compared
How Much Slower Is A Heavy Racecar? 9

This chart surprised me.

What it says is that the difference in the time it takes for one car to go from 50 mph to 100 mph is about 0.6 seconds.

Not great, but perhaps not the end of the world. The big difference is at the high speeds.

And thinking that through, it makes sense. As you get closer to your maximum speed, you’ve got less force available to accelerate the mass. Your top speed is the point where the power available can’t accelerate the car anymore.

The Upshot?

Well my feedback to the team was a question: What tracks have you got coming up? If you’ve lots of slow, twisty tracks park the mass for now.

If its all high speed, then what can you do to take mass out of the car in easy steps, now!

More Power…

When you have models like this, you can’t help but play around… so what if you had 5hp more power? Sure:

Comparing the time to accelerate to a speed for two heavy racecars with different power
How Much Slower Is A Heavy Racecar? 10

This is two cars, both 650kg, one with 145bhp and one with 150bhp.

The more powerful car goes about 1mph faster top speed and gets there quicker – in a very similar way to taking 50kg out of the car!

I’ll have both please!

Hope you found this interesting? As I say, if you’ve a study I can help with get in touch [link].


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