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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.

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