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how mechanical force affects the motorcycle and the rider

how mechanical force affects the motorcycle and the rider

You’re riding along a rugged Cornish coast road and suddenly see an opportunity to overtake the car in front. You pull on the throttle and whoosh! There’s a rush of blood to the head and your adrenaline kicks in.

This is the exhilarating beauty of riding a motorbike. But while it may feel exciting, it also comes with hidden dangers: accelerating and decelerating affects the mind and body in various ways, some good, some not so.

The dynamics at play

When you look at the forces that act upon a rider and bike, you have to first take into consideration the physical dynamics of riding a motorcycle. Your average sportsbike weighs in the region of 200kg, so when you consider a 6ft adult male usually tips the scales at around 84kg – nearly half the weight of the bike – you can see why the rider plays such an important part in the overall dynamic.

Even at low speed such a weight ratio can have a noticeable impact on how the bike behaves but at high speed, impacts can be much more pronounced. And it’s not a one-way ticket either: rider and motorbike influence how each other behave.

When you compare this to a sports car such as a VW Golf GTI, which weighs 1350kg, you can see why the rider’s behaviour is far more critical and why the forces that are acting upon a motorcyclist are so noticeable.

That’s only half the story

To get a vehicle moving (accelerating) you first need to overcome its weight (mass), which on a car or bike is done through the motor’s power output. And this is where bikes totally out perform cars.

Going back to your GTI, which weighs 1350kg and makes 210bhp, it has a power to weight ratio of 0.16bhp/kg. Compare this to a Ducati 1299 Panigale, which weighs 190kg yet makes 200bhp, and the bike has a massive 1.05bhp/kg, which is why it accelerates so much faster than a normal road car. Each kilogram of weight has more than 1bhp to shift it, where on the GTI it only has 0.16bhp. Similar power outputs, but less weight to get moving and that makes a massive difference in terms of performance.

To put this into context, an F1 car weighs 700kg and makes an estimated 900bhp, giving it a 1.29bhp/kg ratio, which isn’t that far off the power to weight ratio of the £21,000 production motorcycle you can buy from your local dealer. That’s why on acceleration tests, motorcycles always beat cars and why, when you crack open the throttle on a bike, you really need to hang on!

But what about stopping?

When you hit the brakes, you are trying to convert all your momentum (mass times velocity) into other forces such as friction and heat, which is why brake discs get so hot. You may think a bike would out-decelerate a car at this point as it has less momentum at a given speed due to the fact it weighs less, but this isn’t true.

A car will almost always stop faster than a bike from high speeds, which is something you need to consider when riding on the road. The reason for this is purely down to motorcycle design.

When you brake, it is the tyres’ grip that stops you skidding and cars have far more rubber on the road than a bike. When you brake hard on a bike, the weight of the bike and rider is shifted forward as the suspension compresses, putting all the force through the front wheel and tyre, squashing the rubber into the road for grip.

MotoGP riders don’t use the rear brake to actually stop the bike, it is all done on the front as the rear is often up in the air. With a car, all four wheels are used to brake, dramatically increasing their stopping ability because there is so much rubber on the road providing grip.

Head over wheels

Physics also plays its part as there is a limit to just how hard you can brake on a bike before the whole thing rotates forward over the front wheel. With a car, your only limit is the tyres’ grip and it is staggering just how hard an F1 car on slicks can brake. This amount of rubber also influences how cars and bikes corner.

When it comes to cornering on a motorcycle, the first thing you need to overcome is gyroscopic force. When a wheel is spinning, this force wants to keep it going in a straight line, which obviously isn’t ideal when a bend is approaching. The faster the wheel spins, the more effort is required to overcome this force, which is why it is harder to turn a bike quickly on a motorway compared to riding in town.

You can reduce this force by fitting lightweight wheels to a bike, which is why race bikes turn faster than road bikes, but sadly it’s just a fact of two-wheeled life and is why you need to put a lot of force through a bike’s bars when entering a bend and even move your bodyweight to help it corner. Bikes also suffer from a lack of grip when you’re in a bend, making it even harder.

Sticky tyres

Just as with braking, cars will always corner faster than bikes as they physically have far more rubber on the road, providing grip. Even a sportsbike with its fat tyres only has a rubber footprint of roughly the size of a CD, which is why you can’t take on a GTI in the bends. Motorcycles have to use very sticky rubber on their tyres and therefore will wear tyres out faster than car tyres.

A sportsbike tyre will last 4,000 miles if you are lucky, a sports car tyre will do 15-20,000 miles easily. You could always fit harder tyres to your bike, but they’ll still be bound by the physics of cornering forces. Curse you Sir Isaac Newton!

Bending the rules

We humans are always looking for a way to circumvent nature’s rules and on vehicles this is done via electronic assists. Modern sportsbikes now come with anti-wheel and traction control devices that allow them to accelerate even harder as the rear tyre’s grip, and front wheel’s lift, is constantly monitored by onboard computers. If you electronically optimise the rear tyre’s grip, and stop the front lifting, then you’ll accelerate at the maximum amount possible.

The same is true for braking

ABS now allows riders to exploit every possible bit of front tyre grip safe in the knowledge that if they do exceed the tyre’s grip levels, the ABS will kick in. One thing electronics can’t overcome, however, is cornering and that still relies on manual input and bravery, which is why MotoGP riders, who are incredibly fit and strong, continue to amaze and astound us mere mortals.

The finishing line

There’s so much more to cornering, braking and accelerating than meets the eye. Your own body weight can affect how the bike will respond in relation to the bike’s design, power, weight and type of tyres. Similarly, as this animation shows, how the bike responds in reaction to the rider will affect the rider’s body and mind.

During rapid acceleration for instance, the heart can reach 180bpm: 95% of its capacity. Blood gets pulled away from the brain and moves toward the lower half of the body. This gives an exciting, but temporary, sensation of light-headedness and heightening of all your senses.

That’s why riding is such an exhilarating experience. Thanks to the laws of physics and the twist of a throttle, there is a powerful dynamic at play that can’t go away.

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