OK guys I'm no math genius. my highest math was trig. but I use it a lot. I'm very good with geometry.... the whell diameter does matter but not by much. on a model engine you can overlook it and get good results. biggest factor being that it doesn't change too much from start to Finnish on a multi cylinder engine. even then you are not likely to know the difference.
if you want absolute repeatability then worry about the wheel size but the relationship between the cam diameter and the wheel diameter makes the wheel look like a nearly flat surface. if you have a 1/2" can diameter and a ~4" wheel the worst case scenario puts the contact from the lobe to the wheel in a range that could never deviate from the centerline more than 1/4" drawing a 1/2" chord on a 4inch diameter gives a height of about .008" that's a good amount but its not the significant value we are looking for... if you swamped in a 50% larger wheel the chord height would change but it only drops to .005 a .003 difference so a massive change in wheel diameter made a noticeable but still small difference in the chord. remember this is a worst case scenario. the chord you are worried about in reality would be much much less than the cam diameter, in fact to avoid the lifter from scraping the lobe off the cam lift needs to be slow enough that the contact point always lands in an area smaller than the lifter diameter which on a pushrod american engine is often smaller than the camshaft. as the wheel gets bigger and bigger the change to the chord height gets smaller and smaller and thus the effects on geometry. remember the rocker ratio is not constant or linear, the lifter radius can vary, and the thing will wear, not to mentioned lash. a .003 change in lash will change timing more than a 50% change in wheel diameter if the cam is small enough.
the important part is that the valve accelerates open decelerates to the max lift and does the same closing so it doesn't release energy on direction changes and bounce or jump. the lift curve on a well engineered cam should resemble a section of a sine wave. the cam may look different from a sine wave wrapped around a radius because of how the contact point of the lifter moves across the lifter surface but if you have flat lifters and a large grinding wheel and you plot a single sine wave in g-code across a section of rotation equal to the advertised duration you will be in good shape. if you use roller lifters you can't ignore the difference in wheel radius to lifter radius when you design the lobe and a lobe design becomes mathematically complex to translate into g-code but if you are going off a blueprint that describes the lobes dimensions you can treat the wheel like a flat surface as long as the wheel is large enough.
even if you don't exactly match the ideal cam as the blueprint or your own design sees it, it's not the end of the world. I mean people successfully do this with a file and never plot the valve lift curve on the assembled engine or check that the lifters aren't binding.. its only a model after all.
if you want absolute repeatability then worry about the wheel size but the relationship between the cam diameter and the wheel diameter makes the wheel look like a nearly flat surface. if you have a 1/2" can diameter and a ~4" wheel the worst case scenario puts the contact from the lobe to the wheel in a range that could never deviate from the centerline more than 1/4" drawing a 1/2" chord on a 4inch diameter gives a height of about .008" that's a good amount but its not the significant value we are looking for... if you swamped in a 50% larger wheel the chord height would change but it only drops to .005 a .003 difference so a massive change in wheel diameter made a noticeable but still small difference in the chord. remember this is a worst case scenario. the chord you are worried about in reality would be much much less than the cam diameter, in fact to avoid the lifter from scraping the lobe off the cam lift needs to be slow enough that the contact point always lands in an area smaller than the lifter diameter which on a pushrod american engine is often smaller than the camshaft. as the wheel gets bigger and bigger the change to the chord height gets smaller and smaller and thus the effects on geometry. remember the rocker ratio is not constant or linear, the lifter radius can vary, and the thing will wear, not to mentioned lash. a .003 change in lash will change timing more than a 50% change in wheel diameter if the cam is small enough.
the important part is that the valve accelerates open decelerates to the max lift and does the same closing so it doesn't release energy on direction changes and bounce or jump. the lift curve on a well engineered cam should resemble a section of a sine wave. the cam may look different from a sine wave wrapped around a radius because of how the contact point of the lifter moves across the lifter surface but if you have flat lifters and a large grinding wheel and you plot a single sine wave in g-code across a section of rotation equal to the advertised duration you will be in good shape. if you use roller lifters you can't ignore the difference in wheel radius to lifter radius when you design the lobe and a lobe design becomes mathematically complex to translate into g-code but if you are going off a blueprint that describes the lobes dimensions you can treat the wheel like a flat surface as long as the wheel is large enough.
even if you don't exactly match the ideal cam as the blueprint or your own design sees it, it's not the end of the world. I mean people successfully do this with a file and never plot the valve lift curve on the assembled engine or check that the lifters aren't binding.. its only a model after all.
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