Weight vs Braking Distance


If you’re building a camper conversion, this article might be the most important one you read; it’s not about colour coordinating splash backs or which insulation is better for you no, it’s about what you can do for free to improve your safety and braking distances.

Braking in textbooks

Remember at school we learned of Newton’s formula of F=m*a  or Force is equal to mass multiplied by acceleration?

It’s a universal that applies to everything from solar systems to snooker balls, from big bangs to Bacteria, and basically means that energy, weight, and acceleration are all interlinked.

Why are we talking about this when we really want to know about adding extra payload to our van or motorhome?

Well, the most critical consideration to the weight of our vehicles is how they are going to still be able to brake safely.

Rearranging the above formula gives us (a=F/m ) or acceleration is equal to the force applied divided by mass or weight.

So if we visualise a sports car on the starting line, the acceleration is a result of the force it can apply between the wheels and the track (grip) and the weight of the vehicle; a light weight sports car is going to be faster than a pickup truck with the same power right? Simple.

But what if we look at slowing down again? After all braking is deceleration or negative acceleration.

Ok, so a heavy vehicle is going to take much longer to slow down, as it has more force (momentum) due to its higher weight.

This all makes perfect sense right? and would tell you that if two identical vans are braking on the motorway, and one is empty and one is full, the empty one is going to brake much sooner than the full one? Common sense, and this is what all braking requirements are based on.

But funnily enough that’s not true, and I’m going to try to explain why below.

Braking in the Real World

Back in 2019 (or as I prefer to call pre-2020 “the good old days”) a bunch of scientists in Slovakia named Skrucany, Vrabel & Kazimir decided to challenge this conventional wisdom and after performing pages and pages of calculations decided to buy a Peugeot Boxer L4H3 van and do some real world testing.

They connected sensors, and calibrated distances, and factored in things like wind speed and the effect of the sun's gravitational pull on the steering wheel (I might’ve made that last bit up) before going out on the test track. They got the van up to 75mph and braked to a stop and measured the braking distance.

They then repeated this experiment with some dummy weights raising in 250kg increments from 0kg up to 1,000kg, but here is the important bit, they also repeated the braking test with the weights in 3 different positions too; right up to the bulkhead behind the driver, in the middle of the load floor, and right at the back of the load floor.

In graph 1 is the results of the braking tests, and the dotted line is what you’d expect to see, a nice constant of “more weight = longer stopping distance” but that isn’t reflected in reality and the stopping distances appear all over the place.

However, if we group these into weights, and change the bottom axis to the position of the load in the back of the van, the results make a lot more sense, and the graphs tell us some interesting stories.

So the blue line at the bottom is the empty van, and it can stop in 29.28m.

When the van is loaded with a 250Kg dummy weight right behind the bulkhead the stopping distance increases. We expected this with our “common sense” didn’t we; more mass means longer stopping distance, and this is reinforced with the 500kg and 750kg weights in the front too.

However, when the loads were moved rearwards in the load bed an interesting phenomena occurred, as you can see the braking distances actually reduced.

As you can see loading the van with 750Kg right at the front of the van resulted in the longest stopping distance of 31.05m, but shifting the exact same load to the back of the van resulted in a braking distance of just 29.75m.

In the case of the 1,000Kg load (which could not be mounted in the front position without overloading the front axle weight limit) the results are even more surprising; when it was located in the rear position, the braking distance was shorter than every other load carrying test at 29.33m, just 50mm off of the empty vehicle braking distance. That’s probably shorter than your thumb!

So the old common sense of “more weight equals longer stopping distances” isn’t really true at all, as what is actually more important is where the weight is located.

Brake Force

The reason behind this is effective brake force distribution.

By loading the rear axle the additional cargo weight increases traction available to the tyres and the road surface, thus greater braking force can be applied to the rear wheels without the risk of a skid.

So if we go back to those sensors we fitted earlier, and have a look at what the brakes are doing.

Below (Table 1) are the Braking Forces of each axle, and we’re comparing the worst performing test load (750Kg in the front position) and the best performing (1,000Kg in the rear position).

As you can see the 1,000kg load over the rear axle is able to produce 2,207N of additional braking force, but it does this without overloading the front brakes, it does this by being able to load up the rear brakes with 5,368N of additional braking force at the rear.

But what does this all mean?

If we remember the formula from the beginning (a=F/m ) or acceleration is a result of Force over Mass, well instead of thinking about the whole vehicle as single calculation, and instead look at it from the perspective of 4 individual wheels, we’re increasing the braking force, and thus increasing the rate of deceleration, meaning we lower our braking distance.

So in conclusion, when we’re building campervans, or uprating motorhomes we need to keep an eye on the total weight of the vehicle, but even more importantly we need to keep an eye on how we distribute that weight across the vehicle.

Spreading your weight out, or aiming for a 50/50 weight distribution might be useful for a race car, but for a heavy vehicle it’s actually better to shift that weight to the back wherever possible.

Getting a weight change specialist involved even at the beginning of your project is definitely going to help, and it might just mean you can keep to your original braking distance even with extra weight on board.