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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Camshaft
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<blockquote data-quote="Deleted member 3831" data-source="post: 105301"><p>TT</p><p> if you had 1 degree increments for your measured data, your graphs would probably shown up a few discrepancies. I don't know because obviously I am not privvy to your design details, but that's my guess.</p><p>I start from measured cam data when I want to work out what it will be when realised at the valve. </p><p>Cam data in a somewhat different form is also used for manufacturing.</p><p></p><p>The data resulting from a valve lift design exercise has to be converted by running it backwards through the valve train to find what the cam profile must be. Before this point is reached some design subtleties have to be indulged in, for dynamic events are not the same as the static ones. </p><p></p><p>To give the exhaust opening as an example. If the engine is turned over slowly by hand (is this possible?) the exhaust cam has to initially load up the valve train components, and increase loading to the point where the seated pressure of the exhaust springs are neutralised, and only at that point will the next movement of the cam initiate the first fraction of valve lift. This will be some small distance along the first opening (or quieting) ramp. The moment the engine fires this situation is drastically altered, for now there is also the residual cylinder pressure resulting from combustion that adds to the resistance opposing the cam. </p><p></p><p>This additional pressure is considerable, and the forces involved place severe loads on the valve train that most always results in some pushrod buckling, thereby delaying the actual point of exhaust valve opening by as much as 20 camshaft degrees or more. The buckled pushrod is now a form of spring, and eventually it will spring back to more or less it's original form, quite often as it does so pushing the valve to open further than the cam dictates.</p><p>This is not the whole story, just the exhust valve opening given as a typical example of the situation that exists with any pushrod valve train system</p><p>What can one do? As I mentioned earlier to Greg, its a choice between a rock and a hard place. </p><p>For example a steel pushrod as large in diameter as can be accommodated and you have the difference in expansion rates to worry about. A solid steel pushrod will give the optimum stiffness but carry the maximum weight penalty.</p><p>This above applies mainly to a racing engine, touring requirements usually involve running at considerably lower engine speeds, so the problems are reduced in their magnitude, but they are still there.</p></blockquote><p></p>
[QUOTE="Deleted member 3831, post: 105301"] TT if you had 1 degree increments for your measured data, your graphs would probably shown up a few discrepancies. I don't know because obviously I am not privvy to your design details, but that's my guess. I start from measured cam data when I want to work out what it will be when realised at the valve. Cam data in a somewhat different form is also used for manufacturing. The data resulting from a valve lift design exercise has to be converted by running it backwards through the valve train to find what the cam profile must be. Before this point is reached some design subtleties have to be indulged in, for dynamic events are not the same as the static ones. To give the exhaust opening as an example. If the engine is turned over slowly by hand (is this possible?) the exhaust cam has to initially load up the valve train components, and increase loading to the point where the seated pressure of the exhaust springs are neutralised, and only at that point will the next movement of the cam initiate the first fraction of valve lift. This will be some small distance along the first opening (or quieting) ramp. The moment the engine fires this situation is drastically altered, for now there is also the residual cylinder pressure resulting from combustion that adds to the resistance opposing the cam. This additional pressure is considerable, and the forces involved place severe loads on the valve train that most always results in some pushrod buckling, thereby delaying the actual point of exhaust valve opening by as much as 20 camshaft degrees or more. The buckled pushrod is now a form of spring, and eventually it will spring back to more or less it's original form, quite often as it does so pushing the valve to open further than the cam dictates. This is not the whole story, just the exhust valve opening given as a typical example of the situation that exists with any pushrod valve train system What can one do? As I mentioned earlier to Greg, its a choice between a rock and a hard place. For example a steel pushrod as large in diameter as can be accommodated and you have the difference in expansion rates to worry about. A solid steel pushrod will give the optimum stiffness but carry the maximum weight penalty. This above applies mainly to a racing engine, touring requirements usually involve running at considerably lower engine speeds, so the problems are reduced in their magnitude, but they are still there. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Camshaft
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