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Before starting to design the buggy, or choosing individual components, it's necessary to consider the sort of environment it will be required to operate in, the sort of performance that will be required of it in each environment and how the design can maximise the buggy's performance in each situation.
The primary objective of the design is to produce a buggy that can travel to almost anywhere on the face of the earth that vehicles of some sort can access. I've added the last part of the sentance because there are many regions which, as well as having no roads, have no fuel supplies and so are only accessible on foot or by some form of self fuelling transport e.g. horses, yaks, etc., and no amount of good design will make the buggy run on thin air!
Click here to see a slide show of Baja Bugs that give an idea of the sort of thing I'm aiming for.
The majority of the buggy's time will be spent on metalled roads and motorways, traveling at reasonable speeds on good surfaces. I hope to be able to achieve a top speed of around 80/90 mph under these conditions. There doesn't seem to be much point in aiming any higher than this, as it will become too uncomfortable due to the buggy's poor aerodynamics, resulting in wind noise and buffeting. In order for reasonable distances to be covered without too much fatigue, it will need a comfortable cruising speed of about 70 mph at reasonably low revs, say 3000, in top gear, without too much wind or road noise and vibration. It will need reasonable, but not excessive torque/power to maintain speed up inclines and against head-winds, especially when fully loaded. It will also need good fuel economy to keep costs down, minimise the frequency of fuel stops and to maximise the range.
I would like to achieve a range of about 1000 miles without having to stop for fuel, to enable large unpopulated areas to be crossed, e.g. deserts. Using nice round numbers to make things easy, assume that the buggy will give about 25mpg, so 1000 miles @ 25mpg will require 40 gallons, which could be carried in 4 x 10 gallon tanks; one in the front, one in the rear and one on each side.
Getting slightly more off the beaten track will take us onto dirt roads and tracks, or possibly grasslands. As the surface quality deteriorates, the speed will need to come down and more will be required of the suspension in order to keep the wheels in contact with the ground. These conditions will require more suspension travel, good damping and low unsprung weight in order to maximise the suspension's responsiveness. Under these conditions I hope to be able to maintain a speed of about 40/50 mph, probably at high revs in 3rd gear. When traveling with other vehicles (local trucks etc) good driver protection will be required to prevent injury from flying rocks, good windscreen clearing (washers and wipers) will be required to maintain visibility and the whole vehicle will need to be protected against the ingress of dust, dirt and water spray, e.g. good air, fuel and oil filters and sealed electrics. Good torque/power will be required in order to quickly and safely pass slower vehicles. Traveling in these sorts of conditions will cause a lot of vibration and so all components will have to be protected against working loose.
Traveling on dirt roads and tracks in dry conditions is likely to throw up a lot of dust behind the buggy. Dust generated by the front and rear wheels will then be drawn into the turbulent air directly behind the car, where the engine is located! Since the engine is very low down and completely exposed in a buggy, this could pose a significant problem for engine wear and reliability. In order to protect against these problems, the engine air intakes (both for the induction system and, to a lesser extent, the cooling system) will need to be located as high up and far forward as possible and may even be located inside the car, where the air should be much cleaner. The induction system air intake will need to be protected from the ingress of abrasive dust particles by use of the biggest, most efficient, air filter possible, which will need to be of the multi-stage, washable, oiled element type (rather than paper element) so that it can be cleaned frequently in extreme conditions. Other dust ingress routes into the engine are via the fuel system and crank case breather vents, which will both require efficient filters. The VW flat four engine also has a rear crank shaft oil seal, in the form of a helix, which is designed to constantly draw escaping oil back into the crank case. Whilst this system works well under normal conditions, in extremely dusty conditions the helix also draws dust into the crank case. This problem can be obviated by replacing the stock rear crank case seal with a 'sand seal', which comprises a replacement crank shaft pulley and conventional oil seal. In order to protect the engine from damage caused by abrasive dust particles that do manage to get inside, a high capacity, full-flow oil filter will be fitted. Other areas vulnerable to dust ingress and wear are exposed rotating components like the distributor and generator, which must both be sealed as far as practicable.
As the road or track deteriorates still further, or there is no road at all, the surface will become very rough and potholed and the demands on the buggy will increase. The main problems under these conditions will be getting stuck and damaging the buggy. Negotiating these conditions successfully will require very low speed in 1st or 2nd gear. This in turn will require low gearing and an engine which will run smoothly at low revs and pull strongly from tick-over and throughout the rev range, i.e. the engine must be designed for smooth running and good torque, rather than maximum power. In order to negotiate obstacles, the buggy will need large ground clearance, which will be obtained by using large diameter wheels and raised front and rear suspension with long travel and good damping. Whilst the wheels need to have a large diameter, they don't need to be wide for these conditions and narrow wheels will perform better due to their lower (unsprung) weight. The major cause of both becoming stuck and damaging the buggy on this type of ground will be lack of ground clearance and axle articulation, particularly at the rear, where the driving wheels are located. If the buggy is grounded with one or more driving wheels off the ground, then a lot of digging, possibly followed by repairs, may be required before further progress can be made. The likelihood of damage to the underside of the buggy, particularly the engine, transaxle and front/rear suspension components, will be minimised by fitting substantial front and rear skid plates. The VW floor pans will also require strengthening, particularly around the perimeter, where they join the body to protect against impact damage. The overall strength and rigidity of the chassis will also need to be increased in order to prevent failure due to stress cracking caused by large torsional loads that the chassis was not designed to cope with by VW. Loss of rigidity caused by removing the VW Beetle body will be compensated for, and improved upon, by the addition of a multi-point, space-frame, roll cage that will tie into the front and rear suspension. Since the buggy will only be two wheel drive I would like to fit some form of limited slip differential (LSD) or differential locking device to ensure that both wheels continue to drive the buggy under all conditions, even when one is lifted off the ground. VW did make several LSD transaxles but these were not very durable so are unlikely to be available now. A proper diff lock also doesn't seem to be an option on a VW transaxle. However, the same effect can be achieved by using 'fiddle' (aka 'steering') brakes, which brake each rear wheel independently. This will enable drive to be restored to a wheel with grip by manually braking a spinning wheel and will have the added benefit of giving the buggy a significantly reduced turning circle. Finally, tyres with an aggressive tread pattern will be required on the driving wheels to maximise grip and a recovery winch will be fitted to enable self rescue when stuck.
Ground clearance is the distance from the ground to the lowest part of the underside of the car and is, in effect, the height of the tallest obstacle that the car can pass over safely. It is also the maximum depth of the ruts that the car can travel in before its underside rests on the ground and it has a significant impact on the maximum fording depth. Ideally, the underside of the car should be flat and smooth so that the ground clearance is the same all over, with nothing protruding below this level to be damaged by obstacles.
The issues involved in traversing rocks are much the same as for rough ground except that they can be far more damaging to tyres, wheels, suspension and the underside of the buggy in general than softer obstacles. In order to minimise the risk of rock damage to the wheel rims and tyres, the buggy will be fitted with heavy duty 4x4 tyres, with a high aspect ratio (70-100%), inflated to reasonable pressures, which will be fitted on the narrowest possible rims. This will minimise the chances of the rocks contacting the rims provided that speeds are kept very low.
Soft Ground - gravel, sand, mud, snow
Gravel - ground clearance, large diameter wheels, wide tyres, aggressive tyres, weight over driving wheels, light weight, good torque, high power, speed, momentum
sand - ground clearance, large diameter wheels, wide tyres, aggressive tyres, weight over driving wheels, light weight, good torque, high power
mud/boggy - ground clearance, large diameter wheels, wide tyres, aggressive tyres, weight over driving wheels, light weight, good torque, high power
snow and ice - aggressive tyres, snow chains, ground clearance, weight distribution
ruts - ground clearance, flat smooth bottom, skid plates, aggressive tyres, same track width as vehicles which made the ruts
fording - ground clearance, sealed body tub, high exhaust and engine air intake, high electrics, fully sealed electrics, water deflection under/around car, keep water out of cooling air intake/fan
steep inclines - up/down/sideways - Approach Angle, Departure Angle, Ramp Breakover Angle, traverse angle, wide track, low C of G.
ascent angle - weight forward, grip, torque, fuel starvation, carb flooding, oil starvation
descent angle - weight rearward, rear brake bias, fuel starvation, carb flooding, oil starvation
traverse angle (aka Lateral Angle) - wide track, low c of g, fuel starvation, carb flooding, oil starvation
When viewed from the side, this is the angle between the ground and a line running from the bottom of the front tire to the lowest-hanging body part directly in front of it, which is usually the front bumper. This angle gives an indication of how the vehicle can approach a steep incline without damaging the front bodywork.
Also viewed from the side, this is the angle between the ground and a line running from the bottom of the rear tire to the lowest-hanging body part directly behind it, which is usually the rear bumper or towing bracket. Similar to the approach angle, this angle indicates how the vehicle can depart from a steep incline without damaging the rear bodywork.
This angle is a measurement of a vehicle's ability to drive over a sharp ridge, hump or ramp without touching its underside on the ground. The "included" ramp breakover angle measures the angle inside the ramp, whereas the "excluded" ramp breakover angle measures from the "included" angle to the vehicle's horizontal. A short vehicle with large tyres will have the best (largest) ramp breakover angle. Most SUV's run around 20 degrees "excluded" Breakover Angle (stock).
The consequence of insufficient ramp breakover angle is shown below...
high temperatures-low temperatures - cooling, warm up, overheating, carburation, demisting
sun - light colours, hood/sun shade, sun visor
rain - hood, windows, sealing leaks, windscreen wipers, demisting, sealed electrics
sea level-high altitude - carburation
poor quality fuel - carburation, compression, timing, fuel filter, multiple fuel tanks
load carrying - spare wheel, roof rack, side racks, trailer