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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Camshaft
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<blockquote data-quote="timetraveller" data-source="post: 105237" data-attributes="member: 456"><p>It is always interesting to communicate with an expert and someone who has designed cams which won TT races etc. is certainly an expert. However, referring to the above graph from Grey One which is taken from the face of the cam with no cam follower and no allowance for Vincent timing gear geometry, then it seems to this untutored producer of 'worthless data' that although this might be the correct way forwards to manufacture cams, it lacks something vital when it comes to valve movement etc. And before Grey One asks, yes I have designed cams and had them manufactured and, as an aside, I found it difficult to get people to work to one tenth of a thou (fourth decimal place) never mind an eighth decimal place. My worry about the relevance of the above graph to the behaviour of the valves in an engine is the geometry of the Vincent valve gear. The flat based lever follower imparts a variable lever ratio between the cam face and the valve movement. Once again, writing as a non professional cam person, it seems to me that it is the movement of the valve which is of importance, not just the cam profile. I have taken the liberty of tracing the profile above on to tracing paper, inverting it and then putting it on top of the original to see just how asymmetrical this displacement curve is. The answer is ' not very'. This seems strange to me as Vincent cams are clearly very different from one side to the other.</p><p>Below I am going to show a graph of the valve motion and the cam displacement for a Vincent cam.</p><p>[ATTACH=full]26283[/ATTACH]</p><p>The blue points are the valve motion and the red the cam displacement. One can clearly see the differences in the ratios as the cam surface moves along the flat lever follower, giving a variable leverage ratio. The horizontal scale is cam degrees and the vertical scale is inches.</p><p>The next graph shows the way in which this ratio varies Between about 1:1 and 2.8:1.</p><p></p><p>[ATTACH=full]26282[/ATTACH]</p><p>Please note that the shape of this graph would change if one were to index the lift profiles at a different point. Moving the point at which the two maxima coincide makes a big difference to the ratio values but they always vary considerably. Consider that the inlet valve starts to lift when the cam is contacting the lever follower towards its pivot, high ratio, and the exhaust starts to lift when the cam starts to contact the lever follower away from the pivot, low ratio.</p><p>What I do not understand and which it would be a pleasure to be educated about, is why the velocity and acceleration curves from the cam surface look sensible when the above ratios need to be allowed for when it comes to valve movement, which also has to look sensible.</p></blockquote><p></p>
[QUOTE="timetraveller, post: 105237, member: 456"] It is always interesting to communicate with an expert and someone who has designed cams which won TT races etc. is certainly an expert. However, referring to the above graph from Grey One which is taken from the face of the cam with no cam follower and no allowance for Vincent timing gear geometry, then it seems to this untutored producer of 'worthless data' that although this might be the correct way forwards to manufacture cams, it lacks something vital when it comes to valve movement etc. And before Grey One asks, yes I have designed cams and had them manufactured and, as an aside, I found it difficult to get people to work to one tenth of a thou (fourth decimal place) never mind an eighth decimal place. My worry about the relevance of the above graph to the behaviour of the valves in an engine is the geometry of the Vincent valve gear. The flat based lever follower imparts a variable lever ratio between the cam face and the valve movement. Once again, writing as a non professional cam person, it seems to me that it is the movement of the valve which is of importance, not just the cam profile. I have taken the liberty of tracing the profile above on to tracing paper, inverting it and then putting it on top of the original to see just how asymmetrical this displacement curve is. The answer is ' not very'. This seems strange to me as Vincent cams are clearly very different from one side to the other. Below I am going to show a graph of the valve motion and the cam displacement for a Vincent cam. [ATTACH type="full" alt="26283"]26283[/ATTACH] The blue points are the valve motion and the red the cam displacement. One can clearly see the differences in the ratios as the cam surface moves along the flat lever follower, giving a variable leverage ratio. The horizontal scale is cam degrees and the vertical scale is inches. The next graph shows the way in which this ratio varies Between about 1:1 and 2.8:1. [ATTACH type="full" alt="26282"]26282[/ATTACH] Please note that the shape of this graph would change if one were to index the lift profiles at a different point. Moving the point at which the two maxima coincide makes a big difference to the ratio values but they always vary considerably. Consider that the inlet valve starts to lift when the cam is contacting the lever follower towards its pivot, high ratio, and the exhaust starts to lift when the cam starts to contact the lever follower away from the pivot, low ratio. What I do not understand and which it would be a pleasure to be educated about, is why the velocity and acceleration curves from the cam surface look sensible when the above ratios need to be allowed for when it comes to valve movement, which also has to look sensible. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
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