depends on who you ask about crank failure...one builder I talked with recently says he's built 100 hotrod '07 and later big twins without a single issue, but some of the dealers he builds for are reporting crank issues with the '07 cvo 110...the man said he believes it was a bad batch and that the issues have been corrected...of course there will always be a low incidence of issues, which are greatly exaggerated here on the internet...most builders recommend bulletproofing as a cover your azz kinda deal...no one wants the customer to have problems, and then things become a finger-pointing match
Two popular concerns regarding the bottom end when considering performance mods are flywheel shift (scissor) and the ’03 & up left main bearing. Flywheel shift occurs when torque at the crankpin is great enough to overcome the friction between the flywheel and crankpin. Often the first thing suspected of causing a flywheel shift is reduced friction resulting from an inadequate interference fit of the crankpin and flywheel. Another item that doesn’t get a lot of attention in the field, but the manufacturer seems to have their eye on, is managing the torque spikes at the crankpin.
A scenario that subjects the crankpin to extreme torque load is when the engine is lugged down in high gear, to a very low RPM, at high throttle settings. These conditions result in a rapid and abrupt loading and unloading of the drivetrain. This drivetrain “snatch” exceeds the torque capacity of the compensating sprocket, transferring load directly to the sprocket shaft and left flywheel. The inertia of the right flywheel opposes the forces acting on the left, resulting in sharp torque spikes at the crankpin. These torque spikes are analogous to the torque spikes created by the Technician’s hammer during a flywheel truing operation from days gone by.
I was chatting with the girls around the water cooler a while back, and the topic of crankshafts and torque load came up
(cut it out). My memory is a little fuzzy on the details, but my recollection of the conversation is that the interference fit of the Twin Cam’s crankpin and flywheel assembly will normally withstand somewhere around 1,300 ft. lbs. of torque. Testing reveals that torque spikes approaching 1,600 ft. lbs. can occur when the machine is operated under the above described scenario.
It is my understanding that Milwaukee’s “torque smoothing” calibration was designed to reduce torque spikes at the crankpin by reducing the engine’s output when running at very low RPM’s with aggressive throttle settings.
A couple of other things that Milwaukee appears to have done in an effort to reduce crankpin torque load are: the cushion drive sprocket and the new high capacity compensating sprocket. The new “accessory” comp sprocket is advertised to have 700% more capacity than the OE component. These items become more important in the longer stroke engines. As the crankpin’s center is moved away from the crankshaft centerline, the leverage against the crankpin increases, reducing the relative force required to generate high torque load at the crankpin. It seems to me that the new accessory comp sprocket will go a long way to reduce the extreme torque spikes in high output or other extreme applications. This is especially true since the advent of the Cruise Drive. IMHO, the 6-speed’s larger comp sprocket doesn’t have the energy absorbing capacity for performance applications. Under certain conditions it isn’t unusual to hear the new comp sprocket mechanism hammering against its stops.
IMHO, the highest risk OE scenario is a 2007 Cruise Drive 110, without the torque smoothing cal, cushion drive sprocket and P&A comp sprocket. This machine has high overall gearing, long stroke, high output, and none of the later countermeasures designed to reduce torque spikes at the crankpin.
Regarding the left main bearing, IMHO the ’03 & up bearing is more likely to experience durability issues when it is run in an engine with a sprocket shaft that is not running parallel to its bearing. The off axis operation concentrates load on the end of the rollers, resulting in bearing overload. The most common cause of off axis operation is flywheel shift; although it may be possible that in some applications, under max load, there may be enough flex and distortion to cause an uneven loading of the bearing; although that is speculation on my part.
The Timkin left main bearing design is better suited to handle off axis operation. The question is whether the ’03 & up bearing is reliable in high performance applications where the sprocket shaft remains reasonably parallel to the bearing. The issue still somewhat subjective; although based on just over 6 years of operation, it does not appear that there is growing evidence of durability issues with the late style left main bearing.
As always, this is just my humble opinion.
djkak
Author
Topic: Andrews 54H or SE 251 for the 110 (Read 16987 times)