Low r.p.m. urban myth

[StewartD]

There was a comment in a recent post regarding ‘slugging the motor at low r.p.m. I think a new thread is justified to discuss this. Here goes:

The contention that slugging a motor at low r.p.m. is particularly bad is something that I have heard many times and it seems to be linked to Ducati big end failure. I don’t believe there is any truth in this. I think it is no more than an ‘Urban Myth’.

Any roller bearing big end is subject to loads that increase with r.p.m. These loads are the direct force of the piston that is forced down by the petrol vapour explosion; this force is higher at the r.p.m. where maximum torque of the motor is made, at near idle speeds this load is negligible in comparison.

When the pistons travel is decelerated on the down (power) stroke and accelerated for the upward (exhaust) stroke there is a force, due to the piston and connecting rods inertia, transmitted through the big end bearing. This force is proportional to the r.p.m. of the engine, but again, negligible forces at low r.p.m.

Another force that is destructive of roller big end bearings is the acceleration/deceleration of the bearing itself. Consider the connecting rod swinging back and forward as the crankpin moves from the 3 o’clock position to the 9 o’clock position. The effect of this conrod swing is that the big end bearing’s rotational speed fluctuates wildly during one revolution of the crankshaft.

Phil Irving’s book ‘Motorcycle Engineering‘ explains this on pages 232 - 233. He gives the example of an engine, (of typical conrod length to stroke proportions), running at 8000 r.p.m. At this speed the instantaneous speed of the big end bearing is 10,000 r.p.m at top dead centre and 6,000 r.p.m at bottom dead centre. Since the bearing cage rotates at half bearing speed, this big end cage must change angular velocity from 5,000 to 3,000 r.p.m. 133 times a second (8,000/60). This will tend to cause the rollers to skid and cause higher rates of wear and heat buildup.

Once again an effect that is proportional to the engine r.p.m. and negligible at low r.p.m.

Phil Irving’s book goes into detail of uncaged roller big end bearings as well. The centrifigal force experienced by the big end causes the rollers to fling outwards and crowd against each other. This is interesting, but not so relevant these days.

All the forces that are destructive of big ends increase vastly at high r.p.m. or at the r.p.m. of maximum torque, compared to the forces in play at close to the motor's idle speed. I hope this explanation will de-mystify this subject. I think the idea that low r.p.m. is bad for a Ducati single may have appeal to people who have been brought up on a diet of silky smooth Japanese fours. At near stalling speed, any single shakes a fair bit. If the motor is under load, then the shaking is even more noticeable. This may not be too pretty, but it is not destroying your big end.

Cheers,

Stewart D

[Jordan]

I believe large throttle openings at low engine speeds can be bad for bigend bearings.
As a thought exercise, imagine a big sudden thrust on the con rod at zero rpm. Surely it wouldn't take many of them to cause damage?
Better to spread the load over a larger portion of the bearing's surface, as would happen at higher engine revs, during the ignition phase.
I have crowded roller bigends on some of my old bikes, so it's still relevant for some.

[machten]

I'm with Jordan. You also need to consider your final drive gearing. To my mind, it is not so much the RPM, more the change in RPM you ask for at low revs if you put the throttle down. Force is about acceleration. At low revs, the rate of change of force demanded is proportionally greater and consequentially has the greatest possible impact on the wear components of the engine that handle the load (bottom end)

That's how I see it anyway.

Kev

[Nick]

RPM is probably not the only factor to consider. More likely the rpm / load relationship. Any authorities care to chime in?

Too, while rollers don't need much oil, as far as Duc singles are concerned I wonder if the big end gets enough oil under extended low-rpm conditions.

[Harvey]

Its the time that the load is applied to the roller contact area. At low speed it is longer than at high speed.

[m82a1]

There is another thing to be considered as well. I was always told that the problem was little/no oil pressure at low rpm. Big load and combined with very low oil pressure are most likely, not a good pairing. Maybe this is an urban myth as well. Can't say as I have ever measured the oil pressure at various rpm to see if there was anything to it, but it makes sense.

Ken

[Harvey]

Post by StewartD » Fri Apr 11, 2014 8:02 am

Any roller bearing big end is subject to loads that increase with r.p.m. These loads are the direct force of the piston that is forced down by the petrol vapour explosion; this force is higher at the r.p.m. where maximum torque of the motor is made, at near idle speeds this load is negligible in comparison.

This bit is where the theory falls down. You are assuming that the torque loading on the piston is less at lower rpms. Sure max torque is made at higher speeds, but if the throttle is open, the torque loading on the piston is almost as high at lower rpms.
The combustion pressure lasts for a set amount of time, the speed of rotation counts how many rollers will take that torque load, if the speed is low enough it could be as low as 3. At higher rotation, more rollers will take that load in the same time.
So it is not a myth, on roller bearings you should not run full throttle lower than 3000 rpm.

[StewartD]

Jordan,

An imaginary sudden thrust at zero r.p.m. is an interesting concept. It would be difficult for this to really occur as the engine relies on the high speed of inlet air in the carburettor venturi to atomise the petrol and the engine speed to compress it. Of course this is only a thought exercise but what happens at idle speed is worth while discussing.

The motor at idle speed is only producing enough power to overcome friction and to maintain enough kinetic energy in the flywheel, so that after the exhaust stroke there is enough energy for the inlet stroke and compression stroke to be completed. It must maintain high enough speed so that the air in the inlet tract is fast enough for the petrol to be vapourized. The combustion process is very inefficient at idle compared to the r.p.m. where maximum torque is produced. That is why it is easy to stall an idling motor.

It is not possible to have a 'big sudden thrust' at idle speed. The combustion is inefficient and the force exerted on the piston is feeble. Just whipping the throttle open does not cause the force on the bearing to be more sudden. On each combustion cycle the the forces acting on the big end go from zero to maximum. This may be considered sudden, even at idle speed, but it is a lot more sudden at normal running speeds.

Kev,

Acceleration is caused by force*. The wording you use -
'Force is about acceleration. At low revs, the rate of change of force demanded is proportionally greater and consequentially has the greatest possible impact on the wear components of the engine that handle the load '
- suggests that you think the opposite and that the acceleration of components is causing undue wear. It is the force transmitted through a bearing that causes wear, not the acceleration experienced by the bearing, due to the force. This is why bearings are rated by force. If you look in any bearing catalogue, you will see that alongside the main dimensions, there are listed dynamic and static bearing load ratings. They are rated in units of Force; i.e. pounds force, kilograms force or kiloNewtons (kN). There is no rating in the bearing catalogue for acceleration.

*The physics equation Force = Mass x Acceleration can be transformed to Acceleration = Force / Mass
In other words a greater force results in a greater acceleration, (if the mass is the same); or acceleration is greater for a lesser mass (if the applied force is the same).

Nick,

In Phil Irving's book 'Motorcycle Engineering' he confirms that roller bearing big ends require very little oil but that too much oil will result in churning action and the bearing becoming very hot. Since the oil pump is directly run by gear drive from the crankshaft, it will always deliver the same quantity of oil for one revolution of the crank, whether the speed is high or low. If the pump were not sized correctly, then delivery would be either too high at all speeds, or too low at all speeds.

Harvey,

Please note my comments about the bearing table load ratings; the tables only give static and dynamic load ratings. There is nothing about duration of force.

It is the force that a bearing is rated on. At low revs the force is very low compared the forces at high revs. A low force occurring for a long time is irrelevant to a designer who has a high force occurring for a short time. The designer knows that the highest force is the one that is going to damage an under sized bearing

Ken,

It is not a big load; see my original post. At near idle speed, all the big end loads are negligible compared to what they are at high speeds, which is what the engine is designed to cope with.

Harvey,

Dynamometer testing of any petrol engine shows that maximum torque occurs at elevated rpms; for a Ducati single, I think about 6000 rpm would be the maximum torque rpm. You nearly contradict yourself in saying - 'max torque is made at higher speeds, but if the throttle is open, the torque loading on the piston is almost as high at lower rpms.' Dynamometer (taken with throttle full open), readings give the maximum torque at all points in the rev range and do not lie. There would be no point to Dynamometer testing otherwise.

Cheers,

Stewart D

[Nick]

There may be one other factor to consider. At lower rpm, peak combustion pressure occurs with the piston further down the bore than it does at high rpm. Since ignition timing on our old bikes is fixed, the mixture is always going to light off at the same crank angle. However, rpm determines at what crank angle peak combustion pressure occurs. This is why detonation occurs at low rpm under high throttle openings. Remember Daytona and the Triumph/BSA triples? Hailwood's and all the other triples blew up due to detonation, only Romero's finished. When asked why, he said he kept the revs up to prevent detonation. In other words, he reduced the load on the rods/pistons/crank bearings by moving the moment of peak pressure to a point where the piston was further along in its stroke. The same excess combustion pressure that can cause detonation will also cause heavier loads on the rod bearings.

Perhaps the amount of oil is not the determining factor.

[StewartD]

Nick,

I don't think any Ducati singles lack an automatic ignition advance and retard. Certainly my 1965 Mach 1 has a mechanical unit and my 1974 450 had an electronic advance and retard. I'm sure the machinery at the Daytona event you mention would have all had automatic advance and retard. What year was this particular race?

I'm not sure of the point you are wishing to make regarding detonation. All the information I have seen about detonation and pre-ignition does not suggest there is any danger of dangerously high loads on the big end bearing. The danger is that the piston will be eroded by detonation and possibly overheated. Pre-ignition is likely to overheat a piston and possibly cause it to melt.

Cheers,

Stewart D