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While Jake brakes are common on large trucks, we don't have them on cars.
Why is that?
They allow extra braking power, they don't upset the balance of the vehicle and they reduce wear on brakes.
I understand that they require extra parts on the valvetrain and that they create more noise, but I believe that these two problems could be solved on mass produced vehicles.
There are many reasons:
Car brakes are much larger in proportion to the fully-laden car as truck brakes are to the fully-laden truck. There is very little risk of brake overheat and brake fade in a car.
An over-speeding car is easy to slow down when going downhill. An over-speeding truck is much more difficult.
The cost of running a truck is of high importance to the truck owner. Longer lasting brake material is a selling point.
Jake brakes do nothing to improve CO2 emissions, and add weight the engine and would not be seen as an improvement to a buyer. They are not a priority to the car engineer.
Jake brakes are very noisy. Some residential areas prohibit the use of Jake brakes while driving.
Retarders (electrical technology with the same purpose as a Jake brake) on the other hand, are already used by cars and trucks to generate electrical energy for a hybrid battery and slow the vehicle without using brakes. They are also near-silent.
Well, the extra weight and cost of the equipment are two points, small in comparison to the weight of the trucks. The accountants (bean counters) for the car manufacturers are always trying to keep car manufacturing costs to a minimum as saving a penny on 50,000 cars makes a difference.
Not only that, the brake shoes and pads are much more expensive on trucks due to the sizes involved - stopping 30, 40 or 60 plus tonnes takes some doing.
So, jake brakes, or eddy current braking systems as used on coaches mean that the running costs of the trucks and coaches are kept down. Also the engine braking systems are common as well.
Also, the drivers tend to be watching the road conditions and slowing gently earlier than "some" car drivers to minimize their braking anyway.
