Cam Geometry
06-06-2003, 09:13 AM
#1
Boone & Crockett
Thread Starter
Join Date: Feb 2003
Location: Lehigh County PA USA
Posts: 12,157
Cam Geometry
I am curious as to how an individual, or company, goes about designing cams for certain draw-force curves. Most of the folks that I speak with know the fundamentals of cam design...round wheels, soft cams, hard cams, single cams, etc... and most offer such information as " the rounder the cam then, typically, the easier it is to draw at any given draw weight" . But, I would like to know more about the actual physical geometries of the cam. For example,
1. How does the actual size of the cam relate to the force draw curve, if at all? Lets say a 4 inch wide cam as opposed to a 3 inch wide model.
2. Generally speaking, rounder cams are easier to draw but how does a cam like that found on the Bowtech dual cam bows generate such a stiff draw cycle without being shaped in the fashion of the older style hatchet cams?
Any comments directly related to these type of issues would be greatly appreciated as I am always looking to further my knowledge of some of the technical aspects of the sport.
1. How does the actual size of the cam relate to the force draw curve, if at all? Lets say a 4 inch wide cam as opposed to a 3 inch wide model.
2. Generally speaking, rounder cams are easier to draw but how does a cam like that found on the Bowtech dual cam bows generate such a stiff draw cycle without being shaped in the fashion of the older style hatchet cams?
Any comments directly related to these type of issues would be greatly appreciated as I am always looking to further my knowledge of some of the technical aspects of the sport.
06-06-2003, 01:03 PM
#2
Nontypical Buck
Join Date: Feb 2003
Location: Eastern PA USA
Posts: 1,398
RE: Cam Geometry
Frank:
As I understand it, the force-draw curve is a product of the radius of the string track compared to the radius of the cable track. In the case of the old style round wheels, the string track and the cable track were both round. The cable track loads the limb of course, so a round cable track only reached peak draw weight when the cable track radius from the axle reached its greatest point. The cam track was not at the greatest radius for more than that one point, so the force draw curve was below peak weight for most of the draw, and most people would describe the draw as smooth. When you look at energy wheels, they have a cam track that is not round, but oval, creating kind of a flat area. This " pumps up" the draw before and after the peak, increasing the stored energy.
The string track is also important. When the cam track curves in towards the axle, and the bow is approching letoff, the string track is often increasing in radius from the axle to increase letoff. The old High Country Phantom Extreme (hatchet) cams come to mind as I am thinking about that. Many older cams that stored a lot of energy were oval in shape, and then the hatchet style cam made it easier to get 65% of more letoff and still have the high energy storage, because the bigger string radius was letting more string out as the cable was loading the limb less.
The flat area at the valley of the cam track creates the wall that is felt at the back of the valley. Of course some designs have some kind of stop to make it even more solid. A sudden increase in the radius of the cable track would cause the draw force to increase dramatically and suddenly, creating a wall.
The actual size of the cam (string track) controls the amount of string that is actually let out. I am pretty sure that most of the parallel limb single cams have pretty big string tracks, as the limbs do not move backward significantly, but compress towards each other. Many of those, like the Mathews Straightlines also move their very stiff limbs only a small amount. The draw weight is kept at the same level as a bow whose limbs move more by letting more string out and moving the stiff limbs less.
Example: With Frankenstein, I could only get a 28" draw with the High Country hatchet cams, but with the physically larger Black Knight cams, it will draw 31" easily if I rotate the cams correctly. I can post more later, I have to go right now. Maybe you will hear from a real tech person.
As I understand it, the force-draw curve is a product of the radius of the string track compared to the radius of the cable track. In the case of the old style round wheels, the string track and the cable track were both round. The cable track loads the limb of course, so a round cable track only reached peak draw weight when the cable track radius from the axle reached its greatest point. The cam track was not at the greatest radius for more than that one point, so the force draw curve was below peak weight for most of the draw, and most people would describe the draw as smooth. When you look at energy wheels, they have a cam track that is not round, but oval, creating kind of a flat area. This " pumps up" the draw before and after the peak, increasing the stored energy.
The string track is also important. When the cam track curves in towards the axle, and the bow is approching letoff, the string track is often increasing in radius from the axle to increase letoff. The old High Country Phantom Extreme (hatchet) cams come to mind as I am thinking about that. Many older cams that stored a lot of energy were oval in shape, and then the hatchet style cam made it easier to get 65% of more letoff and still have the high energy storage, because the bigger string radius was letting more string out as the cable was loading the limb less.
The flat area at the valley of the cam track creates the wall that is felt at the back of the valley. Of course some designs have some kind of stop to make it even more solid. A sudden increase in the radius of the cable track would cause the draw force to increase dramatically and suddenly, creating a wall.
The actual size of the cam (string track) controls the amount of string that is actually let out. I am pretty sure that most of the parallel limb single cams have pretty big string tracks, as the limbs do not move backward significantly, but compress towards each other. Many of those, like the Mathews Straightlines also move their very stiff limbs only a small amount. The draw weight is kept at the same level as a bow whose limbs move more by letting more string out and moving the stiff limbs less.
Example: With Frankenstein, I could only get a 28" draw with the High Country hatchet cams, but with the physically larger Black Knight cams, it will draw 31" easily if I rotate the cams correctly. I can post more later, I have to go right now. Maybe you will hear from a real tech person.
06-06-2003, 06:55 PM
#3
Nontypical Buck
Join Date: Feb 2003
Location: Eastern PA USA
Posts: 1,398
RE: Cam Geometry
Sorry for the interruption.[:' (]
If you look at the cable track, you can almost tell what kind of draw the cam will have. When I got home, I looked at the Bowtech Pro 38 cams and the Black Knight cams on Frankenstein. On both, the cable track comes to max radius pretty soon, then flattens out, then drops back quickly to the flat which is the valley, where the cam track radius comes almost right up against the axle. The Pro 38 is actually a little rounder at the start of the draw curve than the Black knight. IMO, the shape of the cable track is the most important factor in determining the draw curve. In comparing the hatchet cams I have with the " Strother cams" , the hatchets have the large sweep of the string track which comes into play when you draw into the valley and the cam track is 1/8 - 3/16" from the axle at its closest point. This give it 65% letoff. The Strother cams do not have as much string track increase as the hatchets, but the cable track is much closer to the axle. On the BK cams, you can actually see the axle in the cable track at the valley. The hatchet cams cable track is generally rounder in shape than the BK, and the maximum radius from the axle is 1.5" . The BK is more square in shape, and has a max radius of 1.8" or more. The
BK' s pull in more cable, move the limbs more, and gave my bow close to 10# more draw weight at similar brace heights.
I know that you can control the draw curve with the cable track alone, because I built a small bow for my twins before they were big enough to pull the shortest kid' s bow available at the time. The bow had concentric wheels, and was like an old Martin Kam-Act, except that it only had one cam, and one limb. I had it set up with various cam " modules" that gave us anything from a 12" round wheel draw, to an 18" hard cam draw. The 18" hard cam was pretty fast for a kids bow, and still had 60% letoff even with the concentric wheels. The cams were made of plexiglass, and we actually had a Seneca bow that had a plexiglass " Max Cam" with a matching idler wheel.
Twins are much simpler of course, since you only need to match the two cams. With singles and hybrids, the whole thing gets much more complicated, at least to design and make.
If you look at the cable track, you can almost tell what kind of draw the cam will have. When I got home, I looked at the Bowtech Pro 38 cams and the Black Knight cams on Frankenstein. On both, the cable track comes to max radius pretty soon, then flattens out, then drops back quickly to the flat which is the valley, where the cam track radius comes almost right up against the axle. The Pro 38 is actually a little rounder at the start of the draw curve than the Black knight. IMO, the shape of the cable track is the most important factor in determining the draw curve. In comparing the hatchet cams I have with the " Strother cams" , the hatchets have the large sweep of the string track which comes into play when you draw into the valley and the cam track is 1/8 - 3/16" from the axle at its closest point. This give it 65% letoff. The Strother cams do not have as much string track increase as the hatchets, but the cable track is much closer to the axle. On the BK cams, you can actually see the axle in the cable track at the valley. The hatchet cams cable track is generally rounder in shape than the BK, and the maximum radius from the axle is 1.5" . The BK is more square in shape, and has a max radius of 1.8" or more. The
BK' s pull in more cable, move the limbs more, and gave my bow close to 10# more draw weight at similar brace heights.
I know that you can control the draw curve with the cable track alone, because I built a small bow for my twins before they were big enough to pull the shortest kid' s bow available at the time. The bow had concentric wheels, and was like an old Martin Kam-Act, except that it only had one cam, and one limb. I had it set up with various cam " modules" that gave us anything from a 12" round wheel draw, to an 18" hard cam draw. The 18" hard cam was pretty fast for a kids bow, and still had 60% letoff even with the concentric wheels. The cams were made of plexiglass, and we actually had a Seneca bow that had a plexiglass " Max Cam" with a matching idler wheel.
Twins are much simpler of course, since you only need to match the two cams. With singles and hybrids, the whole thing gets much more complicated, at least to design and make.
06-06-2003, 08:44 PM
#4
Boone & Crockett
Thread Starter
Join Date: Feb 2003
Location: Lehigh County PA USA
Posts: 12,157
RE: Cam Geometry
Jeff,
Thank you for the thorough explanation. I believe that I was able to follow most of it as I was somewhat able to visualize what you were describing. If I actually had a few different size/style cams in front of me then it would probably be even easier to digest. You and I need to get together again soon.
One question right off the bat though....why is the radius used as opposed to the diameter?
Lastly, I have some dual cams from a Browning Afterburner laying downstairs if you want them to play with.
Thank you for the thorough explanation. I believe that I was able to follow most of it as I was somewhat able to visualize what you were describing. If I actually had a few different size/style cams in front of me then it would probably be even easier to digest. You and I need to get together again soon.
One question right off the bat though....why is the radius used as opposed to the diameter?
Lastly, I have some dual cams from a Browning Afterburner laying downstairs if you want them to play with.
06-06-2003, 09:58 PM
#5
Nontypical Buck
Join Date: Feb 2003
Location: Eastern PA USA
Posts: 1,398
RE: Cam Geometry
Frank;
The radius is important because it causes the cable to pull on the opposite limb. The bigger the radius, the more cable is being pulled in, the more the limb moves. The string or cable distance from the axle is what determines it I think. I have read a little, and played around and experimented, but I am no engineer.
I have two sets of Alpine limbs. If you can find a riser that will work, we could build " Dr. Jeckyl" for you!
The radius is important because it causes the cable to pull on the opposite limb. The bigger the radius, the more cable is being pulled in, the more the limb moves. The string or cable distance from the axle is what determines it I think. I have read a little, and played around and experimented, but I am no engineer.
I have two sets of Alpine limbs. If you can find a riser that will work, we could build " Dr. Jeckyl" for you!
06-09-2003, 07:38 PM
#6
Join Date: Feb 2003
Location: Havertown PA USA
Posts: 232
RE: Cam Geometry
Hey Frank I can' t help ya at all but I was looking forward to the replies forth coming. I thought this one would get more action and be a super dupper techie thread. I guess the techies are out enjoying the spring weather. You remember? Sunshine and all. Something we haven' t seen yet this year. LOL. Good post Joe. Way over my head but informitive none the less. JERRY
