Camshaft center support bearing for 9000RPM ultra short stroke

One could stiffen up the cam on the tach drive side by making the shaft diameter mucho large between the RH can journal and just up to the RH exhaust lobe.

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When I went to Megacycle for the custom cam in this picture I stated I did not need a tach drive but it was included. I suspect they had the billets in inventory with tach drive already made sans the final lobes. All of this costs money to do so before anyone rushes off to try all of this, make sure you understand exactly what is happening lest you have a solution hunting for a problem.

Dances with Shrapnel said:

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?

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I believe its a shortened cast iron cylinder. Aluminum pushrods, recommended lash clearance is around .006" (maybe more would help?). Don't know the journal clearances but I'm assuming they're average.

I've communicated with Roger and he's thinking about it. Its his motor and he will decide what to do.

I'm wondering if a full circle center bearing would work just as good or better than two half moon supports.

Here's why I think a full circle bearing might be necessary:

Do a crude test - lay a piece of metal on an immovable object such as an anvil. Smack it with a big hammer and notice the metal piece bounce upward. The metal piece never moved downward at all that you could see but it bounced up in the air along with the hammer. Something similar might be happening with the cam.

It appears that the issue of camshaft flex is focussed on the negative benefits when camshaft flex results in valve bounce - but is it safe to assume that the effects of valve bounce are negative ?
This may well be the case where longevity and reliability are the main concern, but where actual power is primary interest the results may be positive.

Well developed engines rely to some extent on induction ram charging over the intake valve closing period, even though the valve is closing and the piston is rising within the cylinder. It is difficult if not impossible to determine at what point pressure reversal will take place, because loss of charge can only occur when that state has been reached, and because the valve bounce is minimal at best, how much charge might be lost?

The worst cause of camshaft flex is not coming from the intake cam load, but from the exhaust.
Over the engine speed range where maximum power and torque are developed, the in cylinder pressure at the time of exhaust valve opening is many times greater than any valve spring force, and the exhaust pushrod will be subjected to a compressive force far greated than will be levied on the intake pushrod, as the cam opens the valve against the combined force of spring and in cylinder pressure.
Although the exhaust cam is closer than the intake to a bearing, it is possible that it is the trigger for the resulting undesirable camshaft flex.

JS, your hammer blow may well be a useful illustration of a very large force applied to the camshaft, but it will be the exhaust cam that carries the greater load, not the intake.

Snotzo
I no longer have the cams with me but as far as I can tell from the photos of the impact wear mark on the left intake lobe heel that I suspect is caused by cam flex happens - when the Right intake valve is lifting and when the right exhaust valve is being slowed before it hits the seat (before exhaust valve bounce).

Slowing down the right ex valve just before it closes also adds to the pressure at the same time the intake valve is opening. This slowing down of the right ex valve is what I think causes the cam to flex and then rebound a bit later to push the ex valve off its seat - causing valve bounce. Those combined pressures could all be building up at once to push down the right side of the cam while tilting up the left side (because of the center bearing) - forcing the left side to bump up against the intake lifter and causing the wear mark on the soft heel of the left side intake lobe.

I'll admit this may be guesswork, its reaching and the exhaust valve bounce might be happening too early in the intake opening ramp cycle but its worth considering.

It might be interesting to compare the best top end Commando motor with the best which is built to deliver more torque, on various race circuits - when both are geared to their optimum. When I built my Seeley 850, I deliberately stayed away from all the tricky stuff. I've had enough of that. What I have got, I know from experience is fast enough. The biggest problem has been the gearing. The standard gearbox is hopeless anywhere and four speeds close ratio are great but hopeless in a clutch start. Five speeds - 4 close ratio,but with a low first might be good.

Dances with Shrapnel said:

One could stiffen up the cam on the tach drive side by making the shaft diameter mucho large between the RH can journal and just up to the RH exhaust lobe.

View attachment 14724

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Yes, this will reduce deflection significantly. I calculated an example using a centerbored beam (Ø25mm, bore Ø6mm) with an increased end support diameter of Ø35mm. For a test load of 8 kN and a length of 300mm, the maximum deflection was reduced by 25%. Naturally, the result is more favorable if a center bearing is fitted, or an increased shaft diameter is feasible at both ends. I guess the casting doesn't allow a larger bearing on the LH side.

-Knut

I think you might be trying to make a silk purse out of a sow's ear. But good luck anyway. I have always believed that big twin cylinder British bikes were never developed to their full potential. Although the Qantal might have gone close.