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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Offset ET28 hole in ET25 for more valve lift
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<blockquote data-quote="timetraveller" data-source="post: 52624" data-attributes="member: 456"><p>Oh Dear! How do I respond? I could use the Mad Max form of diplomacy, which you seem to favour, or the Magnetoman form of trying to explain things as clearly as possible. Your first posting did not mention Terry’s Mk5 cams and asked what offset gave what lift. That is a calculation which is so trivial that it does not need a computer or slide rule. A pencil and paper should suffice. I responded in posting 2 that you could be in for problems doing this with Vincent cams and once it was clear that you were using cams with radiused followers I moved on in posting 4 and pointed out that the exact ratio is not 1:1 but is variable as the point of contact at the lower side of the fork at the end of the rocker moves outwards. Greg, posting 6, Roy, posting 7 and I, posting 8, all pointed out that the potential problem is not the top of the lower guide but rather the bottom of the upper guide. From your posting 9 it seems that you had not understood that potential problem. In my posting 11 I mentioned that the eccentricity of the cam and the variable leverage ratio with the Vincent cam and follower geometry. If you measure the base circle of your cam and then the distance from the base circle to the top of the cam lobe you will find that that difference is much less than the lift of the valve. Similarly, because of the variable ratio of the lever action of the rocker as it tilts one needs to know exactly what lift one is getting, not what one expects. You seem to be altering the geometry of the valve stem to rocker and that is going to make a difference so before anyone can calculate what ratio is needed on the rocker one needs to know exactly what lift you are getting, not what you expect.</p><p>You have assumed that I have no experience of ratioed rockers. That is not so. I have two sets of rockers with different offsets to give different lifts and have helped a friend who has raced for years with ratioed rockers. We do know what happens and it can be made to work reliably but it absolutely no good telling you what offsets were used as it depends upon the cams, geometry etc. I repeat, the calculation of the offset for the pivot point of the rocker is so trivial, and so dependent upon what one is starting off with, that there is no point giving you a value.</p><p>Finally you state in posting 12 that you are only interested in max lift. Let me tell you a story. Several years ago a racing chum had some special cams made that had an extra 40 thou on their noses. The extra was nicely blended in and looked fine. We did the lift profile and I did the calculations for the velocity, acceleration and so on. The value for the acceleration worked out to be about a factor of seven higher that what is normally regarded as the maximum allowable. My chum went back to the cam man and was told not to worry; there were six sets of these cams out there with no problems. A race or two later valve heads pulled off and wrecked a lot of the engine. You want to go from 11 mm to 12.7 mm, an increase of only 15% but now go away and calculate what that does to the acceleration. I think that you might be surprised. I think that you will get away with it but as you point out I do not have measurements of Terry's Mk 5 so the ball is in your court.</p></blockquote><p></p>
[QUOTE="timetraveller, post: 52624, member: 456"] Oh Dear! How do I respond? I could use the Mad Max form of diplomacy, which you seem to favour, or the Magnetoman form of trying to explain things as clearly as possible. Your first posting did not mention Terry’s Mk5 cams and asked what offset gave what lift. That is a calculation which is so trivial that it does not need a computer or slide rule. A pencil and paper should suffice. I responded in posting 2 that you could be in for problems doing this with Vincent cams and once it was clear that you were using cams with radiused followers I moved on in posting 4 and pointed out that the exact ratio is not 1:1 but is variable as the point of contact at the lower side of the fork at the end of the rocker moves outwards. Greg, posting 6, Roy, posting 7 and I, posting 8, all pointed out that the potential problem is not the top of the lower guide but rather the bottom of the upper guide. From your posting 9 it seems that you had not understood that potential problem. In my posting 11 I mentioned that the eccentricity of the cam and the variable leverage ratio with the Vincent cam and follower geometry. If you measure the base circle of your cam and then the distance from the base circle to the top of the cam lobe you will find that that difference is much less than the lift of the valve. Similarly, because of the variable ratio of the lever action of the rocker as it tilts one needs to know exactly what lift one is getting, not what one expects. You seem to be altering the geometry of the valve stem to rocker and that is going to make a difference so before anyone can calculate what ratio is needed on the rocker one needs to know exactly what lift you are getting, not what you expect. You have assumed that I have no experience of ratioed rockers. That is not so. I have two sets of rockers with different offsets to give different lifts and have helped a friend who has raced for years with ratioed rockers. We do know what happens and it can be made to work reliably but it absolutely no good telling you what offsets were used as it depends upon the cams, geometry etc. I repeat, the calculation of the offset for the pivot point of the rocker is so trivial, and so dependent upon what one is starting off with, that there is no point giving you a value. Finally you state in posting 12 that you are only interested in max lift. Let me tell you a story. Several years ago a racing chum had some special cams made that had an extra 40 thou on their noses. The extra was nicely blended in and looked fine. We did the lift profile and I did the calculations for the velocity, acceleration and so on. The value for the acceleration worked out to be about a factor of seven higher that what is normally regarded as the maximum allowable. My chum went back to the cam man and was told not to worry; there were six sets of these cams out there with no problems. A race or two later valve heads pulled off and wrecked a lot of the engine. You want to go from 11 mm to 12.7 mm, an increase of only 15% but now go away and calculate what that does to the acceleration. I think that you might be surprised. I think that you will get away with it but as you point out I do not have measurements of Terry's Mk 5 so the ball is in your court. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Offset ET28 hole in ET25 for more valve lift
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