Needle and roller bearings are awesome from a friction and load carrying standpoint, but the problem is longevity - rolling element bearings hate eccentric loads more than a fluid bearing with proper pressure.
Even worse, the higher the RPM, the more severely the lifetime is reduced (Bearing companies have lots of batch testing and have come up with some empirical formulas to predict average lifetime). Bearings have to be massively overspecced for loads to meet the lifetime requirements.
Rolling element bearings are awesome for small, light engines, and for efficiency, but for a high performance engine that sees high crank loads and high RPM, the bearings will would have to be very robust, and having lightweight and incredibly well balanced internals is a must.
It varies by manufacturer, but bearings are rated to a standard load for 10% failure after a million revolutions (Timken tests at 500rpm for 90 million revs and back calculates)
Here's a good
Primer
The general rules of thumb for lifetime in number of revolutions are:
Doubling the load reduces life to 1/10
Doubling rotational speed reduces life to 1/2
The reverse is also true. Furthermore, the speed calculation is for speed only, and does not include the effects of eccentric imbalance. If it were the only load (not including piston loads), because of centripetal force (increases as the square of RPM), doubling the speed increases the force by a factor of 4, which reduces the bearing life to 1/100th, plus the speed increase which results in a total lifetime reduction to 1/200th.
A 4 cylinder crank properly balanced has an alternating eccentric load on each journal equivalent to half the weight of the piston and the small rod end that never goes away, and can be fudged as lasting half of each rotation.
Consider that at 9000rpm, our engines will see as much or more loading on the rods from rotation as from cylinder compression , and that that peak load is applied directly to the bearings instead of somewhat tangentially, and you can imagine the problem involved in even calculating what size and rating of bearings to use. Don't let me discourage you from trying, but it's not as simple as picking a bearing based on the load rating, unless you don't mind a 10% failure chance after a little under one engine hour at full throttle (at an average of 3000rpm say, on a trackday, not even including the effect of the eccentric load). Say less than 10,000 miles on the odometer before the bearings start failing? I pulled that out of nowhere, but you get the idea.
Fluid bearings do not have longevity issues. As long as the oil is fresh and clean, and the pressure is high enough to carry the peak load, a fluid bearing will basically run forever barring contamination or oil starvation issues. This is why we use oil bearings in automotive engines. Motorcycle engines that run high RPM but low inertial loads can get away with it.
Roller bearings have a place in high efficiency automobile engines of the future, but don't expect to see them in high performance street applications any time soon. (sorry to rain on your parade)
Edited for truth:
Quote:
Yeah that 45mm is for the rod to crank, not crank to block. If we could get the 4g63 FULL roller bearing then these motors would be
stoppable!!! and rev out like a lambo...
-Greg
