I’m sure everyone reading this blog has been on a diesel powered bus. I’m sure almost none of you know what air breaks are – but I’m going to try to make the case that you should know what they are, and that you should care about them.
In a car, breaks are hydraulic (or, if you live in the distant, distant past, cable actuated). What this means is that the friction surface (in a disc break, the pads, in a drum break, the shoes) are pushed onto the other friction surface (the rotor on a disc break, or on a drum break, the drum) with hydraulic fluid that is compressed by pushing on the break (and a hydraulic booster on virtually all cars made in the last 20 years). This gives one very linear control over how much breaking force is applied – in fact, there is a linear relation between breaking force (Fr) and the force perpendicular to the two interfering surfaces (Fp).
However, on air breaks, things are a little bit different. Instead of being pushed together with hydraulic fluid accuated by a break pedal, the friction surfaces are pushed together with massive springs. The break pedal, instead of controling the force pushing the friction surfaces together, controls the flow of air which pushes them apart. The advantages of this system are crucial – if the system fails you don’t have no breaks, but all breaks (full application). Also, massive perpendicular forces can be applied by these springs – far more than hydraulic fluid could manage. However, the real reason is that on large vehicles the temperature of the breaks would simply boil the hydraulic fluid and one would lose breaking entirely – the advantage of air is that it has already boiled.
So, the advantages of air breaks are clear. However, there is a disadvantage, the linearity of the application of breaking force is poor. This is why buses with air breaks lurch about, and have a very sharp transition from “on” to “off” the breaks. Compare this to trolley or diesel electric buses, which can use their electric motors to slow down – on new machines especially they deliver smoother transitions from acceleration to steady state to breaking. However, high speed lurching is not the crucial issue here. The crucial issue is lurching at a stop.
If you know a little bit about physics, you know that the amount of kinetic energy in a moving vehicle increases by the square of its speed. This means, reciprocally, at very low speeds a vehicle has very little kinetic energy. Therefore, air breaks calibrated to apply forces appropriate to slowing down a bus from 50km/h simply cannot apply little enough breaking force to slowly decelerate from 5km/h to 0km/h. Thus, at about 2km/h the breaks lock up, the tires stop, and the bus lurches forward on its haunches. You could replicate the same thing in your family sedan by slowing down to 2km/h and pushing quickly and firmly on the breaks – the wheels will lock, there will be no skid, but the car will rock back and forth.
Now, the reason why this happens on a bus and not a family car has everything to do with the way breaking force is applied to the wheels – air breaks simply do not allow the “feathering” that is required to gently stop a vehicle. However, on a hybrid or trolley bus, this low speed breaking can, and is, done by the eletric motor operating as a generator. This allows a very linear application of acceleration and deceleration, and a smoother ride.
*You might wonder why the same phenomenon doesn’t apply to trains. There are three reasons. Firstly, few trains you ride on use the air breaking systems which are standard on freigh trains. The skytrain uses its LIN motors to break aswell as drive (although it may have some extra emergency breaking). The GO trains in Toronto have disc breaks (yes, exactly – what?). Subways use their electric motors to come to a stop. If you have ridden on a long distance train, you probably noticed that right as the train comes to a stop there is a small lurch – but the lurch is small because the deceleration spike doesn’t stop you from 2km/h to zero, but from 0.1km/h to zero – both because the train is heavy in comparison to the minimum breaking force it can apply, and because the wheels are solid, and because the ration between the spring rate and length of the cars does not allow the forward acceleration to be turned into an up and down rocking – which is what happens in a lunrch.