This is the first in a sequence of articles exploring motorcycle exhaust basics and diverse basic dynamic characteristics of the handling demeanour of motorcycles. general this is a very complex subject and needs a good grade of numbers and physics to correctly realise what's happening. although, in these items I'll try and interpret the basics with the unconditional minimum of numbers, but where this is unavoidable I'll not go after easy trigonometry. For those that are unhappy with any mathematics at all, don't worry, just skip those components and the rest should still prove helpful. I'll try and illustrate the mechanics with many sketches and graphs. |
It appears unbelievable that just two small communicate patches of rubber, can support our machines and manage to consign large amounts of power to the road, while at the same time carrying cornering forces at least as much as the heaviness of the two wheeler and rider. As such the tires use perhaps the single most significant influence over general management characteristics, so it appears befitting to study their characteristics before the other various aspects of chassis conceive. When Newton first expounded to the world his ideas of mechanics, no doubt he had on his brain, things other than the interaction of motorcycle exhausts with the road surface. Never-the-less his suppositions are identically valid for this position. In particular his third law states, "For every force there is an identical and converse force to resist it." or to put it another way "Action and answer are identical and opposite." pertaining this to exhaust activity, means that when the exhaust is pushing on the road then the road is impelling back identically hard on the exhaust. This concerns identically well regardless of whether we are looking at carrying the heaviness of the two wheeler or opposing cornering, braking or going by car burdens. What this specific law of Newton does not anxiety itself with, is which force is the originating one nor really does it issue for numerous purposes of analysis. although, as a guide to the understanding of some personal schemes it is often useful to brain distinct the activity from the reactivity. The forces that occur between the ground and the tires work out so much the demeanour of our machines, but they are so often taken for allocated. exhausts really present such a multitude of distinct jobs and their apparent simplicity hides the degree of engineering sophistication that goes into their design and fabrication. primarily pneumatic exhausts were fitted to advance comfort and decrease burdens on the wheels. Even with modern suspension systems it is still the tires that supply the first line of protection for soaking up street shocks. To explore carcass building, tread compound and tread pattern in large detail is beyond the scope of this book. Rather we are worried here with some basic values and their effects on management characteristics. heaviness Support The most conspicuous function of the exhaust is to support the weight of the appliance, whether upright or inclining over in a corner. although, the genuine mechanism by which the air force and tire passes the wheel burden to the street is often misread. Consider fig. 1, this sketch comprises a slice through the bottom of a rim and exhaust of unit thickness with an inflation pressure of P. The left hand edge displays the wheel unpacked and the right hand edge shows it supporting the heaviness F. When loaded the exhaust is compressed vertically and the breadth increases as shown, possibly surprisingly the interior air force does not change significantly with burden, the interior volume is little changed. At the broadest section (X1) of the unpacked tire the interior half width is W1, and so the force usual to this section due to the interior pressure is easily 2.P.W1 . This force actions upwards in the direction of the wheel rim, but as the pressure and tire breadth are evenly circulated around the circumference the general effect is completely balanced. This force furthermore has to be opposed by an identical stress (T) in the exhaust carcass. The loaded exhaust has a half width of W2 at it's widest part (X2) and so the usual force is 2.P.W2 . Therefore, the additional force over this part, when laden, is 2.P.(W2 - W1) but as the tire is only broadened over a little piece of the bottom part of the circumference, this force carries the burden F. The overhead describes how the inflation pressure and exhaust width boost make forces to fight against the vertical wheel stacking, but does not absolutely explain the minutia of the mechanism by which these forces are moved to the rim. The bead of a fitted exhaust is an interference fit over the bead seat of the wheel rim, which places this locality into compression, the in-line constituent of the side-wall stress due to the inflation force decreases this compression rather. This component is shown as F1 on the unpacked half of F1 = T.cos(U1). The larger bend U2 of the side-wall when loaded means that the in-line constituent of the stress is reduced, thereby furthermore refurbishing some of the rim to exhaust bead compression. This only occurs in the lower part of the exhaust circumference, where the broadening takes place. So there is a nett increase in the compressive force on the smaller rim acting upward, this carries the two wheeler heaviness. The nett force is the difference between the unpacked and laden in-line forces, F = T.(cos( U1) -cos(U2)) The left hand edge shows half of an inflated but unpacked tire, a tension (T) is created in the carcass by the internal force. To the right, the compressed and broadened form of the loaded exhaust is shown. Suspension activity In performing this function the pneumatic exhaust is the first object that feels any street shocks and so actions as the most important component in the machine's suspension system. To the span that, while painful, it would be rather feasible to ride a two wheeler round the streets, at sensible races with no other form of bump absorption. In fact rear suspension was not at all widespread until the 1940s or 50s. while, regardless of the sophistication of the conventional suspension system, it would be rather impractical to use wheels without pneumatic exhausts, or some other pattern of tire that permitted considerable bump deflection. The burdens fed into the wheels without such tires would be enormous at all but slow speeds, and continual wheel malfunction would be the norm. A couple of figures will illustrate what I mean:--Assume that a two wheeler, with a usual dimensions front wheel, strikes a 25 mm, pointed edged bump at 190 km/h. This not a large bump. With no tire the wheel would then be subject to an mean upright acceleration of roughly 1000 G. (the peak worth would be higher than this). This means than if the wheel and brake assembly had a mass of 25 kg. then the average point burden on the rim would be 245 kN. or about 25 tons. What wheel could stand that? If the wheel was shod with a usual exhaust, then this would have at ground grade, a spring rate, to a pointed brim, of approx. 17-35 N/mm. The maximum force then transmitted to the wheel for a 25 mm. step would be about 425-875 N. i.e. less than four thousandths of the preceding number, and this burden would be more evenly spread round the rim. Without the exhaust the shock loads passed back to the sprung part of the two wheeler would be much higher too. The upright wheel velocity would be very much larger, and so the bump damping forces, which depend on wheel velocity, would be marvellous. These high forces would be conveyed directly back to two wheeler and rider. The following five charts display some outcomes of a computer replication of accelerations and displacements on a usual street motorcycle, and illustrate the tire's significance to comfort and street retaining. The bike is traveling at 100 km/h. and the front wheel hits a 0.025 metre high step at 0.1 seconds. Note that the time levels alter from graph to graph. Three situations are considered:
• With typical upright tire stiffness and usual suspension jumping and damping. • With equal tire properties but with a suspension spring rate of 100 X that of the preceding.
• With tire stiffness 100 X the overhead and with usual suspension jumping. So fundamentally we are considering a usual case, another case with nearly no suspension springing and the last case is with a effectively rigid exhaust. Structural stacking, comfort and roadholding would all be adversely influenced without the primary cushioning of the tire. Note that the overhead charts are not all to the same time scale, this is simply to better show the appropriate points. This displays the upright displacement of the front wheel. There is little distinction between the greatest displacements for the two cases with a usual exhaust, for a small step the front exhaust soaks up most of the shock. However, in the case of a very rigid exhaust, the wheel action is increased by a component of about 10 times. It is conspicuous that the exhaust departs the ground in this case and the setting down bounces can be seen after 0.5 seconds. These bends display the upright action of the C of G of the bike and rider. As in Fig 1 it is clear that the rigid tire determinants much higher two wheeler movements, to the conspicuous detriment of solace. illustrating the different accelerations conveyed to the two wheeler and rider, these curves show the upright accelerations at the C of G. Both of the stiffer exhaust or stiffer suspension situations show alike values of about 5 or 6 times that of the normal case, but the shape of the two bends is rather different. With the rigid suspension there is little damping and we can see that it takes a couple of cycles to resolve down. The second bump at round 0.155 seconds is when the rear wheel strikes the step, this back wheel answer is not shown on the other graphs for clarity Royal Enfield Brass Parts . Front wheel vertical acceleration for the two situations with a usual exhaust. The early part is alike for the two situations, the suspension has little effect here, it is tire deflection that is the most important for this height of step. As in Fig 5 the lack of suspension damping permits the tire to rebound for a couple of cycles before settling down. As in these bends are of the wheel acceleration, the standards of the normal case are overwhelmed by the rigid exhaust case, with a top value of close to 600 G contrasted with almost 80 G normally. Again note the consequences of the setting down
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