Heavy lifters and valve bounce (2015)

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jseng1

jseng1

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There’s a lot of talk about springs and valve bounce on this forum. Something important has been missed so I’m starting a new thread with a different approach to solving the problem of Norton valve bounce at high RPM by looking at the lifters. If you study Jim Comstocks excellent video below showing a high RPM PW3 cam causing valve bounce and look at it very carefully you can see that the rocker arm generally follows the valve tip during the bounce. Most of the time the rocker arm seems to return to push down the valve and help cause it to bounce off the seat after the cam lobe has already passed the lifter. Nortons have a 1.14 to one rocker arm ratio which means that the heavy stock Norton lifters are traveling very fast and all that kinetic energy is not going to stop all of a sudden. The Norton cam is flexing like a whip and the heavy lifter will be throwing itself back up at the pushrod helping to increase the dreaded valve bounce.

[video]https://youtu.be/8M3DW8F2CA8[/video]

You can barely see the rocker arm in the interesting video below that Jim Comstock (thanks Jim) made of Sir Eddies valves with Honda springs at high RPM (9000?) If you GO TO THE END of the video and observe carefully you can barely see the corner of the rocker arm tappet as it tracks the valve tip during bounce – the tappet adjuster stays pressed against the valve tip and appears to be pushing the valve open during the bounce (assisted by the pushrod and heavy lifter).

[video]https://youtu.be/L0_pud6vcuw[/video]

Honda springs with a high seat pressure of 140 lbs were used to get the valve bounce under control in the video immediately above. But a better way to get rid of the bounce would be to get rid of the cause - those heavy stock lifters. This is why Factory sponsored Nortons used lightweight BSA lifters back when Nortons were competing in highly tuned dirt track AMA races. Ken Canaga is the one who hipped me to this in the 1st place. Stock Norton lifters weigh about 75 grams. BSA lifters are only 38 grams. THAT’S HALF THE WEIGHT. True you have to install bronze lifter blocks and BSA lifters as below. You can make the blocks yourself or buy them. You can find and clean up used BSA lifters or wait for new ones that are working their way through production in the UK.

Photo of a brass lifter block made many years ago by an unknown engine builder.
Heavy lifters and valve bounce (2015)


BSA lifters
Heavy lifters and valve bounce (2015)


The choice is between higher spring pressures & stresses or lighter valve train.
 
This problem is with the racers not the street people, right? For me as a street rider, this is a problem that doesn't exist.
 
illf8ed said:
This problem is with the racers not the street people, right? For me as a street rider, this is a problem that doesn't exist.
Click to expand...

Yes - valve float is not a problem with a lower RPM street bike and you can safely use flat lifters. But there is an advantage with radiused cam/lifters when you consider reliability and longevity. Street Nortons with flat lifters use a pointy cam that wears out faster than a more rounded nose like you see on the BSA/Triumph radius cam lobes below. The stock Norton lifters also have a wear issue and develop a pronounced line or groove that we are all familiar with. These problems are reduced with the radius geometry that allows for rounded against rounded instead of pointed against flat.

Heavy lifters and valve bounce (2015)
 
JS
I agree with your comments re the advantage of the radiussed follower against the flat, but there are also some disadvantages too that one might wish to consider.
A radius follower will have a lower contact stress with the cam, but it will also squeeze the oil film to a greater degree, will require more power to turn the cam, and the effort required to do so will be at the expense of more torque. Small as these considerations may be, never the less they are present and should not be totally ignored in favour of a less stressed contact.
Simply replacing the stock flat follower with a radius will inevitably have the effect of reducing the area under the valve lift curve, although the duration and lift will remain the same. As far as performance goes this may be beneficial or it may not, but if the wish is to retain the same area under the lift curve, this will require the cam to be re designed to include the radius of the follower in place of the original flat.

Regarding the effect of the weight of the follower, within the limits of the weights given, i.e 75 grams for a flat follower and 38 grams for the BSA type, the results from the use of either type will be marginal, the advantage being with the radius, but only just. The greatest benefit will result from any reduction of mass on the valve side of the rocker pivot. A Titanium valve replacing one of steel will bring with it major benefits, and accompanied by light weight retainer and keepers give the best possible results.

The most difficult aspect to overcome is the limitation imposed by the design of the cam. Few people are in a position where they are able to undertake such a task, most have to rely on a manufacturer having done a decent job with whatever is on offer. That, and finding a suitable valve spring to match the rest of the valve train components can be both time consuming and frustrating. That some such as yourself have made available both cam and springs as a package is a great service to those who have neither the race experience and/or the overall expertise to make an informed choice of components.

The camshaft whip you refer to does not materially either add to or detract from valve bounce. While shaft bending does occurs, and more so with a cast shaft than an steel one; with a twin cylinder engine, as one lobe is opening, there is an equal opposite lobe engaged in a closing action. There are also two other lobes at some stage of opening/closing. This combination of forces in direct opposition to the line of thrust makes the camshaft adopt a bowed aspect when the engine is running, and as such cannot impart any force against a follower to cause it to rise and interfere with valve closing.

The problem on the camshaft side of the rocker pivot is mainly coming from the pushrod buckling under load. Jim Comstock ultimately will get to see this in detail with his spintron. He will also see the pushrod buckle and then spring back once the peak load is passed, and it is this springing back that can push the valve train into separation momentarily.

Any of the above will only be of concern to a racer or someone seeking maximum performance for whatever reason. A normal motorcyclist (is any motorcyclist a 'normal' person?) whose enjoyment is derived from pleasure riding and touring, will be well served by the standard items readily available.
 
Snotzo said:
JS
I agree with your comments re the advantage of the radiussed follower against the flat, but there are also some disadvantages too that one might wish to consider.
A radius follower will have a lower contact stress with the cam, but it will also squeeze the oil film to a greater degree, will require more power to turn the cam, and the effort required to do so will be at the expense of more torque. Small as these considerations may be, never the less they are present and should not be totally ignored in favour of a less stressed contact.
Simply replacing the stock flat follower with a radius will inevitably have the effect of reducing the area under the valve lift curve, although the duration and lift will remain the same. As far as performance goes this may be beneficial or it may not, but if the wish is to retain the same area under the lift curve, this will require the cam to be re designed to include the radius of the follower in place of the original flat.

Regarding the effect of the weight of the follower, within the limits of the weights given, i.e 75 grams for a flat follower and 38 grams for the BSA type, the results from the use of either type will be marginal, the advantage being with the radius, but only just. The greatest benefit will result from any reduction of mass on the valve side of the rocker pivot. A Titanium valve replacing one of steel will bring with it major benefits, and accompanied by light weight retainer and keepers give the best possible results.

The most difficult aspect to overcome is the limitation imposed by the design of the cam. Few people are in a position where they are able to undertake such a task, most have to rely on a manufacturer having done a decent job with whatever is on offer. That, and finding a suitable valve spring to match the rest of the valve train components can be both time consuming and frustrating. That some such as yourself have made available both cam and springs as a package is a great service to those who have neither the race experience and/or the overall expertise to make an informed choice of components.

The camshaft whip you refer to does not materially either add to or detract from valve bounce. While shaft bending does occurs, and more so with a cast shaft than an steel one; with a twin cylinder engine, as one lobe is opening, there is an equal opposite lobe engaged in a closing action. There are also two other lobes at some stage of opening/closing. This combination of forces in direct opposition to the line of thrust makes the camshaft adopt a bowed aspect when the engine is running, and as such cannot impart any force against a follower to cause it to rise and interfere with valve closing.

The problem on the camshaft side of the rocker pivot is mainly coming from the pushrod buckling under load. Jim Comstock ultimately will get to see this in detail with his spintron. He will also see the pushrod buckle and then spring back once the peak load is passed, and it is this springing back that can push the valve train into separation momentarily.

Any of the above will only be of concern to a racer or someone seeking maximum performance for whatever reason. A normal motorcyclist (is any motorcyclist a 'normal' person?) whose enjoyment is derived from pleasure riding and touring, will be well served by the standard items readily available.
Click to expand...


I always thought that the contact stress was increased with the radius follower (using the hertzian contact stress calculation for a line contact) the contact point will move though.
 
Is it likely to get valve bounce with a standard 850 cam and followers etc. doing 6,500 RPM ? I always change up below 7000 RPM and don't rev the motor over 7000 RPM on the way down through the gears. My bike is tuned to pull rather than rev high, and the overall gearing is extremely high, and the box internals are very close. I suggest that when you start revving commando motors over 7000 RPM without light pistons, billet cranks and thick cases, you are asking for the big bang and there is probably no real need to ever go there. Without talking about horsepower - how many racers are successful without pouring the big bucks into their motors ? I suggest a lot of this stuff is only bragging rights. From experience, I've found that a very light, really good handling package with moderate top end power and loads of torque is more important than all the razzamatazz. Especially on tight circuits, if you are on the gas way back in the corners and have a good run on the others coming out - that counts for a lot. If the bike has a top end motor, what I've just described gives you the hi-side.
 
Next time you go to a race meeting, move to one of the tighter corners and see where the fast guys get on the gas. Plenty run wide to the ripple strips and do 'point and squirt'. It's not necessarily the fastest way around.
 
Cheesy
I always thought that the contact stress was increased with the radius follower (using the hertzian contact stress calculation for a line contact) the contact point will move though.
Click to expand...

You are correct for a situation where a direct line contact is the consideration, but the cam profile is not a sphere, and the contact has an angular component as the cam profile passes from one side of the follower to the other.
Generally speaking, for a given valve lift design, the smaller the follower radius, the more rounded the nose of the cam

In the case of Commando followers with different foot shapes, the flat follower has Hertzian stress of 1040.7 Mpa.
A follower having a foot radius of 1.125" has Hertzian stress of 916.8 Mpa.
If a roller follower of 4" radius is used, the Hertzian stress is 959.9 Mpa.
These all at a simulated camshaft speed of 3000 rpm and a single beehive valve spring.

It would appear that the smaller the radius the lower the stress, but now substituting a 3/4" diameter roller as the follower, and the Hertzian stress
rises to 1058 Mpa.
This latter is just an exercise to illustrate a point. To grind a PW3 profile for such a follower would require a grinding wheel radius of around 35 mm radius - a job for a specialist shop with a high speed spindle and small wheel attachment.

Many contributing factors combine to determine the stress, and no one stress figure is applicable across the range of Commando cams and followers. Each individual combination would require a one off calculation if stress was the specific interest
 
JS
Further to your original post.
An interesting aspect to the standard lifter with a weight of 75 grams.
Running a simulation with a PW3 intake cam at 7000 rpm, and a beehive valve spring, showed harmonics across the higher frequency range to be such that some vibration within the valve train could be expected to become a problem.
Increasing the follower weight to 175 grams showed no discernable difference in either separation or bounce at the same rpm, but the harmonics in the higher frequency range were significantly reduced.
This would appear difficult to believe, and if such a situation arose in the Nascar teams, the spintron would be employed to verify the simulated results.
Should be an easy one for Jim Comstock, only needs some lead in a standard follower.

High frequency harmonics can be troublesome and be the source of vibration within the valve train that can cause lock nuts to loosen and such like. In the case of the PW3 the profile itself is the initial source of these unwanted harmonics, but within it's operating range it gives good results so the negatives tend to become acceptable when the positives fall nicely into place.
 
Snotzo

Thats some pretty heady stuff. I don't get that deep into the calculations.

My philosophy:

The lighter the valve train from end to end (including spring and lifter) - the better.

My research:
The cam lobes I use are Triumph/BSA lobes that have already been worked out. In my research I was surprised to find that many of the Norton cams (such as D+, PW3, Axtell/Megacycle NR etc have equivalents in the Triumph/BSA world and its a good bet that most if not all of the Triumph/BSA cams came 1st and were copied.

You can see some of the radius tappet cams roughly compared to the flat tappet cams here (right column):
http://www.jsmotorsport.com/technical_CamLifters.asp

My real life experience:
The BSA lifters and rounded nose cam set up last much longer than the flat tappet variety. This becomes obvious pretty quickly when you're racing and have to replace Norton flat tappet cams each season whereas the radiused variety still looks good after a season.

Looking at Comstocks vids above and seeing the rocker arm tip follow the valve through its valve bounce indicates to me that there is a lot more that pushrod buckling going on. For a pushrod to buckle that much it would probably be bent or rubbing adjacent parts. The Norton cam is only supported on the far ends with the intake lobes in the middle where there is the most flex. Increasing the lifter weight to 175 grams would almost certainly break the cam. You can't seriously be suggesting that increasing the lifter weight would be a good thing.

My point is that the rocker is following the valve tip during its bounce. Something is pushing or flexing the rocker. The stock lifter is by far the heaviest item in the entire reciprocating valve assembly and its moving nearly as fast as the valve. More weight and kinetic energy savings can be made there than anywhere else. Think about it - if you lose the (weight of) lifter and pushrod entirely then you have an ideal overhead cam situation.
 
JS
I take the comments you make, and generally agree with your methodology. That you have achieved such race results speaks well for your attention to detail and development.

I wasn't intending to imply that a 175 gram follower would be something suitable for inclusion in any engine, but that the weight of such was not in any way responsible for valve bounce.

The cam designs as you say, in all probability owes much to early BSA designs of the 1950's, specifically those intended for use in the Gold Star models. The designer at that time was an ex Norton trials rider Jack Amott. He was something of a mathematical genius in many ways. The designs were made without the help of any modern day aids such as calculators or computers. Even slide rules were useless. Amott had to wotk his designs out using long hand calculations, and are all notable for the long constant velocity ramps. Nowadays computers make short work of any calculations.

My in detail look at the followers and valve bounce was done using a sophisticated suite of computer programs written by Professor Gordon Blair. He wrote his first valve lift design program after being pressed to do so by an American cam manufacturer, and other programs quickly followed to complete the suite. The first interest was the American Nascar engine developers, and the programs were designed to aid the development of these engines, in particular to replicate by simulation the results determined by spintron tests. As one of Prof. Blair's beta testers I have used the program for more than twenty years, and in all that time have never seen computed results which were not replicated by these tests.

The above does not mean that the program is infallible, far from it. Neither am I as a user infallible either. If the follower behaves as you surmise, then it will ideally need to be ratified by tests, and a visual test is not on it's own sufficient. The Comstock spintron is improving all the time (I am amazed at how much time and effort he is putting into it) and with his laser equipment now functioning a simple test of a lead weighted follower would be sufficient to either prove or disprove a point.

Whatever the outcome of such a test, there is nothing at all wrong with your approach via lightest possible valve train components.
 
So far my testing has done a pretty good job of confirming Snotzo's prediction.

I have not tried any BSA followers yet as I have not received my shipment of suitable cams.

I have used the laser and camera and found that with the 86C cam, conical springs, stock lifter and pushrods, the valvetrain starts to loose control at around 7500 rpm. I start to see both excessive valve bounce and oscillations around peak lift at about the same rpm.

I have substituted a radically lightened stock lifter and seen no improvement in bounce -but a delay in the start of oscillations around peak lift.

I have substituted a [heavy] steel pushrod and seen the oscillations at peak lift increase with no change in bounce. This surprised me as I had suspected the oscillation to be from the springiness of the aluminum pushrod.

I have tested with the un-lightened lifter and steel pushrod and seen a decrease in oscillation around the peak but the valve loft did start to become evident in the laser sensor trace just after peak positive acceleration. [over the nose]
This is the only time I have seen any real deviation between a low speed sensor trace and the high speed trace -other than a slight delay on the opening ramp.

The oscillations I am referring to are not visible with the camera and at first I had thought they must be caused by the mounting of the laser sensor.
I have since moved the sensor from above the valve spring, where it sensed the retainer movement -to inside the cylinder where it measures at the center of the valve face. I still see the oscillations which seem to be at near the 3333 Hz limit of my sensor. I have noticed that I can hear an increase in the noise "rattle" of the valvetrain at the same time I see the oscillations in the sensor trace.

I have seen that the RPM that these anomalies begin can be raised from about 7500 rpm to 8000 rpm by installing the dual coil "Honda" spring.

I have some "ultra -high frequency" IRL springs coming that I will be comparing with the Honda springs next.

I know that when I was racing I would always see peaks of 84 or 8500 rpm on the logger after every race -even with the longstroke 750 with a messaged stock crank and stock rods. Jim
 
Really good info and details from both Comnoz and Snotzo. Its got me going.

I'm still stuck on the heavy lifter being a cause of valve bounce and here's why.

The profile below shows that as the intake lifter is descending down to its closing ramp - the exhaust lobe on the opposite cylinder has just peaked. This means that all that upward pressure on the exhaust will suddenly reverse and at high RPM the exhaust lifter has probably flown off the cam lobe on its way back down. That means that the pressure on the cam has just disappeared and so the cam has probably rebounded and flexed in the direction of the floating exhaust lifter. So now you have both the Intake lifter hitting bottom to push down the cam, make it flex and then re-bound upward to push up the intake lifter and help cause intake valve bounce while at the same time the exhaust lifter has just jumped off the cam letting the cam spring back upward to help push up the intake lifter and contribute to the valve bounce. When you consider how flexible the cam is its easy to see how this could occur. It seems like a very likely scenario. It wouldn't take much of a springy push to make the lifter jump even further and transfer that energy to the valve resulting in bounce.

Maybe I'm letting my imagination run away with me - I often have that problem.

Heavy lifters and valve bounce (2015)
 
So your theory is that the cam flex is causing valve bounce and even if it were a Desmo situation or mondo springs installed, you would still bet a bounce.

It seems to me that they call it bounce for a reason. The valve is simply bouncing off the seat, a high impact deflection. Although adding more spring pressure may help this, we are knocking at the door of the limitation of this Push Rod head. 6500, 7000, 7500 rpm? Of course its going to bounce, it is at its design limits.
 
Copmnoz > I *know* that when I was racing I would *always* see peaks of 84 or *8500* rpm on the logger after every race -even with the long stroke 750 with a messaged stock crank and stock rods. Jim
Click to expand...

REAlity Check ALERT!!! Long before ANY 'puters and spintron or exotic materials and cryogenics I personally experienced and many others from Axtel, Woods, TC and many drag flat track and desert racers All Report Commonly hitting 8500 to good advantage and not blowing up. Some like my P!! were meant-marked to exceed red line and others like TC intended to shift at 8grand but rpms rising so fast in lower gears and missed shifts etc often exceeded 8 grand regularly but stil won races w/o blowing engines often or at all.

Point being valve bounce is always expected even in NASCAR and Engine Masters tests so they try combo's to limit bounce to < .015" but can still make wining power for time enough with bounces over .020". I am in JMS camp that most significant spring is the bow string plucking of the unsupported cam shaft that then shoves on the other parts with higher frequency spring harmonics than cam. There are a few example of center support cam attempts but don't know the results. I also think there is torsional springs not being accounted for in cam and chain drive. Point being so far nothing has been learned or offered that ain't already been exceeded routinely in past so eagerly waiting to see if future much better.
 
jseng1 said:
Really good info and details from both Comnoz and Snotzo. Its got me going.

I'm still stuck on the heavy lifter being a cause of valve bounce and here's why.

The profile below shows that as the intake lifter is descending down to its closing ramp - the exhaust lobe on the opposite cylinder
Click to expand...

I only have lifters and valves installed on the LH cylinder of my spintron -so far.
 
In company with Comnoz, I am only able to simulate a single cylinder at present, so the combined effects of multiple cams on a shaft create forces that await further investigation. Certainly the Commando camshaft flexes when the engine runs. The amount of flex is to a great extent determined by the stiffness of the shaft and the load applied. Comparing a cast iron camshaft to a steel item, size for size the steel item is some 23% stiffer than its cast iron cousin, and if anyone is rash enough to drill the centre out of either, the stiffness will be severely compromised.

Torsional wind up and unwind of a camshaft in a multi cyclinder engine is a known feature that can result in a variation of valve timing of some several degrees between cylinders where the windup or unwind is greatest. This situation is present to a greater or lesser extent throughout the engines operating speed range, and greatly affects engines such as V8's, inline 6's and similar, but is at worst only likely to have a very miniscule effect in a Commando engine, due to the location of the cam lobes in close proximity to each other. Torsional windup effects can therefore be dismissed.

The whip and bow of the camshaft is another matter, and is not the result of a continuous load application, but a sucession of loads at intervals as each lobe lifts it's follower and valve against the force applied by the valve spring, and in so doing imposes a load on the camshaft. Said load will shift along the camshaft from one lobe to another, so the end result is a complicated source of vibration which will play a part at some point within the engines operating speed range.

The effect of this will be undoubtedly interesting to observe as and when the Comnoz spintron is able to run as a complete engine,
 
It would be interesting to see some really industrious person cast some new beefy cases with central cam support, and build new high grade hardened steel cams with a center main journal.
 
grandpaul said:
It would be interesting to see some really industrious person cast some new beefy cases with central cam support, and build new high grade hardened steel cams with a center main journal.
Click to expand...

I figured you know Comnoz from the track. When I met him his bike had a modified VW diesel crank with center main bearing to run higher rpms. A split center bearing on the cam should be relatively easy. We'll find out soon I'm sure.
 
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