Vol 49 Page 3 Vol 73 Page 9 Allan George’s Gems Why are disc brakes not usually fitted to caravans? They’ve been around for years, but you might ask, ‘Why are disc brakes not usually fitted to caravans?’ Given the widespread use of disc brakes in our towing vehicles you might wonder why the technology isn’t used widely in caravan hardware. It’s rare to find a van straight from the factory with disc brakes and as drum brakes have been working fine for most people since the 1970s, why change? Certainly, disc brakes offer better braking performance, but that comes at a cost, one that’s not readily accepted by the industry that needs to keep manufacturing costs down and, in general, consumers want cheaper vans. The drum brakes found in most caravans are electrically operated. Any van over 750kg needs brakes, over-ride or electric, and anything over 2000kg needs brakes on each wheel, which is usually electrically operated drums but occasionally more complex hydraulic/air disc brake systems. The electric drum brakes are simple in design and operation, but they’re also basic in the either fully on or fully off status they deliver. That’s why the operation can be clunky or jerky, and of course, there’s adjustment and maintenance that we’d wager isn’t as thorough as it should be in RAAF Radschool Association Magazine. Vol 73. Page 9 many cases. Drum brakes may be adjustable, but do many owners set them up correctly with their electric brake controllers, or do they dial up the controller for simply more stopping power? Essentially, what it comes down to is force and the force required to operate the hydraulically- operated brake callipers used in disc braking. When cars first appeared on the road and had foot- operated drum brakes, you could push your brake lever and you’d stop. When discs arrived, with a calliper, the force required was beyond that of your average human leg, so the brake booster was born. A brake pad has a much smaller surface area in contact with the brake disc than a brake shoe within a drum and so when it receives its electrical signal to activate it brake duties it does so well. So does the calliper but it requires a much greater amount of force that is provided by the booster/actuator set-up and that’s the expensive and more complex tech that stops most manufacturers from opting for hydraulic discs on their vans right from the drawing board. Not to mention that the vacuum pressure required to activate a brake booster, runs directly from your vehicle’s engine. It’s also worth remembering that braking systems are installed in line with prevailing manufacturing design standards, VSB1, so it’s not like you’re relying on technology that’s not up to the job. In any event, if you’re not invested in your braking performance due to lack of knowledge, then it’s worth finding a skilled brake technician. The drum brake also doesn’t enjoy water crossings and if you’re a full-time off-roader then the maintenance for these terrain-supplied contaminants such as dust, water and mud may direct you to the disc brake option (or a better maintenance program). B RAAF Radschool Association Magazine. Vol 73. Page 9 You need to work out whether the cost impost is worth the benefits. What are the benefits of a disc brake system on your van? Well, improved brake feel and operation, over what can be a bit harsh drum-brake bite. The disc set-up copes better with off-road use and is definitely a better brake in pure stopping power. Importantly, discs are less prone to brake fade in the way that a drum might on long downhill sections or arduous conditions. And fade is caused by heat, so heat dissipation, especially on very heavy trailers, is vital and ventilated disc brakes do this very well. If you own a light van or trailer, then the financial considerations mean the heat dissipation from your wallet might outweigh the need for discs on your 1500kg camper-trailer. And the downsides? Unless you can buy a second-hand van with disc brakes already installed you’re going to be up for a price penalty. Indicative hardware pricing alone is about $2000 for a single axle and $3500 for a twin-axle hydraulic disc brake set-up. The system is much more complex and requires hydraulics and different electrics, and there are, as with drums, maintenance items and consumable brake pads to consider (don’t forget drums and shoes are consumables too). And of course, there’s the brake fluid that must be flushed every two years. Discs at some point will require machining plus you still need a mechanical handbrake, but some disc-brake kits supply this part. A disc-brake system will lock on as soon as the Anderson plug is disconnected. Conclusion. Essentially, the bottom line is don’t under-brake your trailer. For the most part (and budgets), the tried and true electric drum brakes will do the job just fine. Make sure your brakes are maintained and adjusted to suit the loads you’re carrying and cleaned after off-road use. If you’re on the very heavy end of caravan models, carry a lot of gear and do all this off-road a lot then you may want to specify disc brakes from the outset. The Bacon Tree. Two Mexicans are stuck in the desert after crossing into the United States, wandering aimlessly and starving. They are about to just lie down and wait for death, when all of a sudden Luis says........."Hey Pepe, do you smell what I smell. Ees bacon, I theenk." "Is, Luis, eet sure smell like bacon. " With renewed hope they struggle up the next sand dune, and there, in the distance, C RAAF Radschool Association Magazine. Vol 73. Page 9 is a tree loaded with bacon. There's raw bacon, there's fried bacon, back bacon, double smoked bacon ... Every imaginable kind of cured pork. "Pepe, Pepe, we ees saved! Ees a bacon tree!" "Luis, maybe ees a meerage? We ees in the desert don't forget." "Pepe, since when deed you ever hear of a meerage that smell like bacon...ees no meerage, ees a bacon tree!" And with that, Luis staggers towards the tree. He gets to within 5 metres, Pepe crawling close behind, when suddenly a machine gun opens up, and Luis drops like a wet sock. Mortally wounded, he warns Pepe with his dying breath.... "Pepe... Go back man, you was right, ees not a bacon tree!" "Luis, Luis mi amigo... what ees it? " "Pepe.. ees not a bacon tree. Ees.... Ees..... Ees.... Ees..... a ham bush...." Radial Engines. Why do most radial engines use an odd number of cylinders? Actually it’s not true that radial engines have odd numbers of cylinders. For example, the Pratt and Whitney R-2000, as used on the Caribou, has 18 cylinders and the Pratt & Whitney R-4360 which powered the post-war B-36 Convair Peacemaker, had 28 cylinders. What is true is that each bank or row of cylinders has an odd number. It’s important to make sure that any internal combustion engine is well balanced both from the standpoint of minimizing internal stresses in the engine and to minimize discomfort to any passengers. One primary factor in achieving this is to arrange the engine so that the intervals between power strokes of the various cylinders are equal. Any engine operating on a 4-stroke cycle, two revolutions of the crankshaft are required for each power stroke. The following applies in general to most engine configurations, inline engines, opposed engines, V engines and so on, but not to radial engines. As an example, look at a six cylinder engine, such as the air-cooled Porsche 911 flat six. The pic at right shows the 911’s engine crankshaft with rods and pistons attached. You can see that each piston has its own crankshaft throw, so by varying the relative angles between the crankshaft throws, it’s possible to get equal firing intervals distributed over the two revolutions in the crankshaft. In this case, power strokes occur at 120 degree intervals of crankshaft rotation. The firing order is 1-6-2-4-3-5. In some instances pairs of pistons on opposite sides of a v or flat engine may share a crank throw, but in the most general case each has its own. D RAAF Radschool Association Magazine. Vol 73. Page 9 Here is a top view of the cylinders of this engine. You can see that the left bank is offset from the right, to accommodate the spacing between the various crank throws. However, in a radial engine, each bank of cylinders lies in a single plane and so all the pistons in that bank share the same crank throw. Let’s look at how a 9 cylinder radial engine would work: Here’s top dead centre for cylinder 1 (above). As the crankcase revolves, 40 degrees later, cylinder 2 hits TDC, and so on. So one way to arrange this would be to have the cylinders fire in order: 1-2-3-4-5-6-7-8-9 but now you’d need a second revolution of the crankshaft with no power strokes to complete the cycle, not exactly even spacing of the power strokes. E RAAF Radschool Association Magazine. Vol 73.