That's a misconception. Tensile strength has nothing to do with stiffness against deflection, which is governed by Young's (or Hook's, for europeans) modulus. This modulus hardly changes for most alloyed steels.
-Knut
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Camshaft center support bearing for 9000RPM ultra short stroke
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That's a misconception. Tensile strength has nothing to do with stiffness against deflection, which is governed by Young's (or Hook's, for europeans) modulus. This modulus hardly changes for most alloyed steels.
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Not only would I rev it to the max but if I missed a gear it would rev to the sky in neutral (before I had a rev limiter). The valves were rubbing each other and hitting the pistons - it was not OK.
At first glance the vid I was referring to looks like the left side and if its true then it throws everything into question. But I didn't make the vid and its possible the vid got flipped.
The cams with stock lifters would get beat to shit in a season and need to be replaced. The radiused lifter cams looked much better and could be re-used. So I started using radiused lifters.
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Knut
you say a 'normal' size drilling, this is comparable to acotrell's 'ANY' steel !
The sizes you indicate are as you say, likely to have little effect on the camshaft stiffness, nevertheless, they DO have an effect, and the effect will be increased where the distance between the bearings becomes greater, or the thru hole size is enlarged.
I have before me as I type a steel camshaft of 24.5 mm outside diameter, and a thru hole of 18.5 mm diameter. The stiffness comparison to the same camshaft but with no thru hole, and at bearing distances the same as for the Commando, the drilled shaft is some 29.4% less stiff.
A further issue which thankfully we don't have to worry about is camshaft torsional wind-up (only wind-up we have to worry about is here on the forum, some irritating, some highly amusing)
you say a 'normal' size drilling, this is comparable to acotrell's 'ANY' steel !
The sizes you indicate are as you say, likely to have little effect on the camshaft stiffness, nevertheless, they DO have an effect, and the effect will be increased where the distance between the bearings becomes greater, or the thru hole size is enlarged.
I have before me as I type a steel camshaft of 24.5 mm outside diameter, and a thru hole of 18.5 mm diameter. The stiffness comparison to the same camshaft but with no thru hole, and at bearing distances the same as for the Commando, the drilled shaft is some 29.4% less stiff.
A further issue which thankfully we don't have to worry about is camshaft torsional wind-up (only wind-up we have to worry about is here on the forum, some irritating, some highly amusing)
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True but this goes to the crux of the assertions being made by JS on need for a middle cam bearing and cam deflection and I will get to that in a minute. I have gone over Jim Comstock's excellent spintron work and in my opinion, the case has not been made for significant valve bounce due to cam rebound. I can speculate on what is being observed but will not go there now.
True, Young's Modulus is one governing factor in simple beam deflection analysis but a fundamental guidance on beam flexure is to also "check extreme fiber stress". In a simple static beam, this is a point calculation as a simple pass/fail for failure and appropriate safety factor and is dependent upon the ultimate strength of the material. With cam flexure, one needs to determine extreme fiber stress and put it into the context of a cyclic reversing load/stress which goes to the heart of this matter (based on my first hand experience in this particular application). As you may already know, below a certain threshold stress, steel exhibits an infinite durability; above that threshold, the steel exhibits a finite life. Above the threshold the greater the magnitude of the load reversals at the extreme fiber and/or number of load reversals, the shorter a steel components life.
Below is a cam we used for the 500 Norton USS phased for a 180 degree crank. The profiles were developed by Prof. Gordon Blair, the middle bearing was a Herb Becker concept implemented maybe 15 years ago (which may have been done before - e.g. Jim Comstock) from way back and the cam fabrication was by Megacycle.
Viewed from the timing side this cam rotates counterclockwise.
The cam breakage problems we were experiencing (pre middle bearing support) I now attribute to more or less two cam lobes (more or less mid beam) on the cam opening the RH EX and LH IN concurrently while coming off the LH exhaust. The double valve opening coupled with much greater spring rates and acceleration loads of the extremely high speed pushrod valve train resulted in a few caput cams. The middle bearing decisively resolved the cam breakage problem. We started the 500 USS with a 360 crank and did not experience cam breakage.
In contrast, rather than post a picture of a standard 360 cam, visualize the above cam picture looking from the timing chest, visualize the two RH cylinder lobes being rotated 90 degrees clockwise and you should be able to see that with the 360 phased (stock) cam, at no time is the cam lifting more than one valve at a time. So whatever high speed demands we were placing on the Commando camshaft, with the 180 degree cam we were more than doubling it. Again, the middle bearing fixed the breakage problem associated with the 180 degree phase and we did not experience this with the 360 phase crank.
With the 180 phase I am certain there were other goblins at work (snatch, vibration and harmonics) which were giving us fits. We kicked this can all the way to the point (e.g. gear driven cam) that the crankshaft timing pinon was the weakest link.
As best as I can tell, there's no direct nor indirect measurement of cam deflection (other than VR correlation) in Jim C's spintron work and from what I have seen it is open to interpretation as to exactly what role cam deflection plays in valve seat bounce as there's more than the cam in the valve train. It would be great if someone could reproduce/replicate the spintorn work with different valve head profiles and materials of construction. My hunch is it would be an eye opener. It would also be nice to be able to get the deflection data from 4StHead modeling software but my hunch is there are bigger fish to fry.
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Hello Snotzo,
I specified my understanding of "normal size drilling" .... please read again.
You will not prove a point by using unrealistic figures. No one in their right mind would field a camshaft constructed like a thick tube for the Norton engine, which is in need of high bending stiffness! Which purpose would a tube-like camshaft serve otherwise? Maybe decrease the hardening cycle. Your stiffness figure may be correct, but it would be more helpful to all if you applied the bore figures I proposed.
-Knut
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The discussion (or at least Alcotrel's posting which I responded to) was about valve bouncing and to which extent camshaft deflection has a bearing. Now, that is a problem which can be handled in a quasi-static (for the deflection part) and dynamics analysis (for inclusion of masses and the remainder of springs).
There is no direct relationship between maximum bending stress (at the outer fibres) and deflection, even though L (distance between bearing supports) and Jx, Jy (the shaft's moment of inertia) are factors needed to calculate both. Furthermore, fatigue strength is not proportional to UTS (Ultimate Tensile Strength), and the parameter requirements to achieve fatigue strength is an entirely new discussion.
-Knut
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Apparently I failed to make the points which are, in the case of the Norton twin cam, from what I have read and seen on Jim C’s spintron posts, the case for cam deflection being a significant contribution to valve bounce is at best indeterminate at this time.
I did not say extreme fiber stress was a factor in deflection, only that complete and proper analysis includes assessment of extreme fiber stress as it is pertinent to when a cam support is needed and I did not say the UTS is a factor in deflection but a factor in the complete analysis (the check) for the same reasons stated above. Based on my experiences, in certain cam applications, durability is the issue.
The point is (from first hand experience) the middle bearing was employed by us on the 500 Norton USS to eliminate cam breakage, therefore, UTS and extreme fiber stress due to deflection were THE problem. In my opinion, from what I have read on Jim C’s spintron work as well as past experiences, the jury is still out regarding cam deflection being a significant contributor to valve bounce.
Alan’s suggestion of a larger diameter camshaft to mitigate deflection and increase durability has merit though as I stated in an earlier, note, it would certainly have it’s challenges.
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I s that a fuel injector on your bike?[/QUOTE][QUOTE="jseng1, post:
Those are hand make guillotine slide carbs. They were crude but they worked great. That bike and I took 2nd at the Laguna Seca AMA BOTT National in 1984.
Dances
Even though the stress on your cam was doubled - the fact that they were breaking says a lot about cam deflection. So what would you like to see in a test? Measuring cam deflection is difficult. The only practical way I can think of is to mount a tab under the center of the cam, set the clearance and then see if there is evidence of interference.
Have you calculated the peak lobe pressure at extreme RPM? What about putting a cam in a press and measuring the deflection?
What spring pressures were you running?
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JS
At elevated engine speeds where the valve is opened with sufficient velocity for the inertia to overcome the spring force, lofting can result, and when this occurs the spring force on the nose of the cam may well be zero.
Even if lofting does not occur, the force on the nose of the cam will usually be much less than when measured static.
Spring force that will place max load on the cam will be when the load on the opening flank is loading the valve train to the max, and at this point it's possible that some pushrod buckling will occur.
Thus far all attention has been directed at camshaft flex being the cause of valve bounce, and it certainly may be the case, but other components within the valve train are more likely to be the culprits, particularly pushrods and valve springs.
Both pushrods and valve springs will each have a natural harmonic frequency of vibration. If the natural harmonic frequency of vibration of spring and pushrod are very close, this is an ideal setup for the two to resonate together in a way that jeopardises all valve train operation, and multiple small valve bounces can be one of the end results.
I don't say this is what we are seeing when looking at the spintron film, but neither do I disregard the possibility.
The camshaft flex may well be the main instigator of the valve bounce as seen in the spintron film, but spring and pushrod harmonics may also be playing a part, and as yet we have not included rocker flex in the discussion.
Dances photo of the camshaft from his high revving engine shows the additional centre bearing, noticeably of a larger diameter than the rest of the shaft, so Herb Bekker had his options pretty well covered with an extra bearing, and a boost in shaft strength to go along with it.
At elevated engine speeds where the valve is opened with sufficient velocity for the inertia to overcome the spring force, lofting can result, and when this occurs the spring force on the nose of the cam may well be zero.
Even if lofting does not occur, the force on the nose of the cam will usually be much less than when measured static.
Spring force that will place max load on the cam will be when the load on the opening flank is loading the valve train to the max, and at this point it's possible that some pushrod buckling will occur.
Thus far all attention has been directed at camshaft flex being the cause of valve bounce, and it certainly may be the case, but other components within the valve train are more likely to be the culprits, particularly pushrods and valve springs.
Both pushrods and valve springs will each have a natural harmonic frequency of vibration. If the natural harmonic frequency of vibration of spring and pushrod are very close, this is an ideal setup for the two to resonate together in a way that jeopardises all valve train operation, and multiple small valve bounces can be one of the end results.
I don't say this is what we are seeing when looking at the spintron film, but neither do I disregard the possibility.
The camshaft flex may well be the main instigator of the valve bounce as seen in the spintron film, but spring and pushrod harmonics may also be playing a part, and as yet we have not included rocker flex in the discussion.
Dances photo of the camshaft from his high revving engine shows the additional centre bearing, noticeably of a larger diameter than the rest of the shaft, so Herb Bekker had his options pretty well covered with an extra bearing, and a boost in shaft strength to go along with it.
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I have some new interesting information.
Awhile back, Roger showed me a couple cams he's been running with the center bearing support. One was a JS1 cam ground by Megacycle and the other a JS2 smoothramp ground by Web cam, both had hard weld on the lobes. The strange thing was that both cams had a very small flat spot on the heel of the left side intake lobe. He returned the Megacycle cam to Megacycle but they had no explanation. Both cams looked very good otherwise. I didn't have an explanation for him either and I've spent months thinking about it and coming at it from all directions. It just seemed impossible. Then last night it finally hit me.
Neither cam is hardwelded on the heel so they are a bit soft there but it never seemed to matter before. Nothing could be causing harsh contact between the heel of the cam and the lifter. Or so I thought... What's happening is that when the right side intake is ramping half way up the flank at high pressure - the cam is deflecting downward on the right side of the center support bearing. The center bearing holds the middle of the cam in place so the cam actually bends, tilts and lifts the cam on the left side of the center bearing and when this happens the soft heel of the left intake lobe crashes momentarily with the hard surface of the lifter (past the point when valve bounce would occur). It happens over and over again at extreme RPM until a tiny little flat develops on the heel of the cam.
It seems fantastic that this could happen and that there could be so much flex in the cam to make this possible. But there is no other explanation I can think of. Strange things are happening at 9000 RPM - stranger than I expected.
Below is an image of a high lift Norton race cam (asymmetrical with easier exhaust closing ramps). As Snotzo mentioned in his post above - "max load on the cam will be when the load on the opening flank is loading the valve train to the max".
The blue line below is the jerk graph and the black line is the acceleration (loading the flank of the cam lobe).
Note - the Megacycle cam is softer on the heel and the flat mark is plainly visibly - about 1/16" wide. The Web cam is a hard welded and reground original cam core and the mark on the heel of the left intake lobe is harder to see but with magnification you can definitely see it and its at the exact same spot as on the Megacycle - opposite to when the right side intake lobe is about 1/2 way up the opening ramp.
Awhile back, Roger showed me a couple cams he's been running with the center bearing support. One was a JS1 cam ground by Megacycle and the other a JS2 smoothramp ground by Web cam, both had hard weld on the lobes. The strange thing was that both cams had a very small flat spot on the heel of the left side intake lobe. He returned the Megacycle cam to Megacycle but they had no explanation. Both cams looked very good otherwise. I didn't have an explanation for him either and I've spent months thinking about it and coming at it from all directions. It just seemed impossible. Then last night it finally hit me.
Neither cam is hardwelded on the heel so they are a bit soft there but it never seemed to matter before. Nothing could be causing harsh contact between the heel of the cam and the lifter. Or so I thought... What's happening is that when the right side intake is ramping half way up the flank at high pressure - the cam is deflecting downward on the right side of the center support bearing. The center bearing holds the middle of the cam in place so the cam actually bends, tilts and lifts the cam on the left side of the center bearing and when this happens the soft heel of the left intake lobe crashes momentarily with the hard surface of the lifter (past the point when valve bounce would occur). It happens over and over again at extreme RPM until a tiny little flat develops on the heel of the cam.
It seems fantastic that this could happen and that there could be so much flex in the cam to make this possible. But there is no other explanation I can think of. Strange things are happening at 9000 RPM - stranger than I expected.
Below is an image of a high lift Norton race cam (asymmetrical with easier exhaust closing ramps). As Snotzo mentioned in his post above - "max load on the cam will be when the load on the opening flank is loading the valve train to the max".
The blue line below is the jerk graph and the black line is the acceleration (loading the flank of the cam lobe).
Note - the Megacycle cam is softer on the heel and the flat mark is plainly visibly - about 1/16" wide. The Web cam is a hard welded and reground original cam core and the mark on the heel of the left intake lobe is harder to see but with magnification you can definitely see it and its at the exact same spot as on the Megacycle - opposite to when the right side intake lobe is about 1/2 way up the opening ramp.
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Yes, fantastic and absolutely incredible deflection considering a Commando cam with a middle bearing. Absolutely incredible!
A flat spot on the heel of a cam lobe is odd and in my opinion, more indicative of a long term harmonic which one would expect from a fixed rpm engine such as a large stationary Diesel engine. A 500 Norton USS does not spend that much time exactly at any given rpm. Excursions through a race rpm range, yes, but fixed rpm, no.
If the “S” curve bending is occurring to such a degree as you allege then wouldn’t one expect this phenomena to manifest on more than one cam lobe heel?
We never noticed any cam heel flat spots on any of our 500 Norton USS builds with or without a middle bearing so I can only suggest you look elsewhere. My opinion is: this is confirmation bias. If you are convinced of your conclusions, that is also fine. As I stated earlier, your results may vary.
Regarding an apparent flat spot forming on the heel of a cam lobe, if you are correct and it were a road bike expected to be reliable for tens of thousands of miles then yes, there is a problem. If the cam/valve train components get you through a race season or two, not really a problem - change out components. We do it with valves and valve springs.
A flat spot on the heel of a cam lobe is odd and in my opinion, more indicative of a long term harmonic which one would expect from a fixed rpm engine such as a large stationary Diesel engine. A 500 Norton USS does not spend that much time exactly at any given rpm. Excursions through a race rpm range, yes, but fixed rpm, no.
If the “S” curve bending is occurring to such a degree as you allege then wouldn’t one expect this phenomena to manifest on more than one cam lobe heel?
We never noticed any cam heel flat spots on any of our 500 Norton USS builds with or without a middle bearing so I can only suggest you look elsewhere. My opinion is: this is confirmation bias. If you are convinced of your conclusions, that is also fine. As I stated earlier, your results may vary.
Regarding an apparent flat spot forming on the heel of a cam lobe, if you are correct and it were a road bike expected to be reliable for tens of thousands of miles then yes, there is a problem. If the cam/valve train components get you through a race season or two, not really a problem - change out components. We do it with valves and valve springs.
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Sometimes what happens in practice is a long way from the theory. I agree that if you want ultimate horsepower, dramatic cam, high revs and bigger inlet ports and megaphone exhausts are the way to go. But the bike which is good for land speed records, it not necessarily good for road-racing. A motor with a heap of torque is usually better than one with loads of top end and not much torque. - Depends on the circuit. I don't like large power circuits.
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All things that happen in practice are firmly grounded in theory. You just need to understand the applicable theory.
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Dances - I would be happy to find another explanation for the flat spot on the heel. Remember that this happened with two different cams and two different builds. Your center bearing cam had a thicker center section which makes it different and your unsupported cam broke. The right intake lobe is the weakest and most flexible area of a Norton cam that's the reason the cam bends/tilts when the right intake side is ramping up on the cam lobe.
The center cam support does not prevent upward flex. If there is some weird harmonic happening then that harmonic/mark should be showing up on the other lobes as well. The only valve bounce that could be happening at this time would be the right side exhaust and if the bouncing valve is bouncing the lifter against the cam then it would contribute to cam flex - but the oscillating/flexing cam is what is causing the right side ex valve to bounce in the first place.
Whatever is causing the flat spot (as you say "absolutely incredible" and I agree) it is occurring during the left side power stroke and robbing HP if the left in valve is being bumped off its seat. So its a problem that needs to be corrected and changing parts hasn't solved it.
I've checked into and exhausted all other possibilities I can think of and haven't been able to come up with another reason.
Snotzo - got any ideas?
Remember that this motor spends time at 9000RPM. The flat spot is only about 1/16" wide.
The center cam support does not prevent upward flex. If there is some weird harmonic happening then that harmonic/mark should be showing up on the other lobes as well. The only valve bounce that could be happening at this time would be the right side exhaust and if the bouncing valve is bouncing the lifter against the cam then it would contribute to cam flex - but the oscillating/flexing cam is what is causing the right side ex valve to bounce in the first place.
Whatever is causing the flat spot (as you say "absolutely incredible" and I agree) it is occurring during the left side power stroke and robbing HP if the left in valve is being bumped off its seat. So its a problem that needs to be corrected and changing parts hasn't solved it.
I've checked into and exhausted all other possibilities I can think of and haven't been able to come up with another reason.
Snotzo - got any ideas?
Remember that this motor spends time at 9000RPM. The flat spot is only about 1/16" wide.
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Here's photos showing evidence that the cam is flexing/bending/tilting.
The intake lobe to the left of center shows the mark on the soft heel when the cam flexes upward and bumps against the lifter. This happens when the lobe to the right of center is being pushed downward under pressure from opening the intake valve. The cam rocks or tilts because of the center bearing support. Whats important to notice is that the mark does not occur all the way across the lobe because the cam is out of alignment with the lifter while the cam is bent.
In the next photo you can see the arrow pointing to where the left edge of the lifter has been digging into the soft heel of the Megacycle cam (top cam in photo below). This happens because the left lobe is flexing upward and going out of alignment with the lifter. The lower cam is a Webcam and you can see again that the arrow points to where the mark is more pronounced on the left side because of misalignment while bending (you can see where the left edge of the lifter is starting to dig in). Otherwise the lobes look pretty good.
The right side of the cam is still weak, unsupported and subject to flex even when there is a center support bearing. The solution to this extreme RPM cam flex is to add a 2nd cam support bearing close to the outermost lobe on the right side. That would prevent the cam from flexing and bending out of alignment when the right intake lobe is ramping up. Its more trouble to go through but it might be necessary in extreme RPM cases with high lift cams. But it might be worth the trouble if the left intake valve is being bumped off its seat during the power stroke and losing HP.
The lifters below don't show misalignment wear because of the hard stellite pads.
The intake lobe to the left of center shows the mark on the soft heel when the cam flexes upward and bumps against the lifter. This happens when the lobe to the right of center is being pushed downward under pressure from opening the intake valve. The cam rocks or tilts because of the center bearing support. Whats important to notice is that the mark does not occur all the way across the lobe because the cam is out of alignment with the lifter while the cam is bent.
In the next photo you can see the arrow pointing to where the left edge of the lifter has been digging into the soft heel of the Megacycle cam (top cam in photo below). This happens because the left lobe is flexing upward and going out of alignment with the lifter. The lower cam is a Webcam and you can see again that the arrow points to where the mark is more pronounced on the left side because of misalignment while bending (you can see where the left edge of the lifter is starting to dig in). Otherwise the lobes look pretty good.
The right side of the cam is still weak, unsupported and subject to flex even when there is a center support bearing. The solution to this extreme RPM cam flex is to add a 2nd cam support bearing close to the outermost lobe on the right side. That would prevent the cam from flexing and bending out of alignment when the right intake lobe is ramping up. Its more trouble to go through but it might be necessary in extreme RPM cases with high lift cams. But it might be worth the trouble if the left intake valve is being bumped off its seat during the power stroke and losing HP.
The lifters below don't show misalignment wear because of the hard stellite pads.
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Interesting stuff there JS. Can you tell us in both instances what materials were used for the cylinder barrel and pushrods as well as what the cold valve lash was and hot valve lash if you have it.
Also, any assessment on cam journal clearances?
Also, any assessment on cam journal clearances?
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I would think that the degree of thickness in the instance you cite was based on the perceived need for greater torsional stiffness the closer you approached the drive side of the cam- it would make some sense. It could also have something to do with harmonics. Just speculating here.
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