3 planet hub, more efficient?

[Oran]

So I thought I would continue with removing planets to see if I can learn anything about the efficiency. The only other number of planets that has any chance of working is 3. This will double the load and torque transferred by each planet. If my theory is correct that increasing the torque reduces the efficiency then a noticeable drop should be experienced.

I decided to risk a major failure and use the 3 planet hub for commuting to work. The performance is a bit of a surprise. It's far from the very inefficient hub I was expecting. The 5 planet hub was a different thing being unbalanced and it didn't perform very well with the traction rings getting pulled off center. Amazingly there really isn't any noticeable difference in the efficiency of the 3 planet hub. If anything I'm convinced that it's actually more efficient under some conditions. As if the efficiency of these hub could get any more complicated.
In full underdrive and a low sprocket ratio the "squishiness" is very noticeable and on a really steep hills it's quite extreme making pedaling a bit difficult. Under high torque, low speed situations the efficiency is possibly slightly lower. But considering the amount of "squishiness" it doesn't feel like the hub is eating up as much torque as it could be.

My only explanation for the increase in efficiency or at least no drop is simply the same reason that a 3 wheeled vehicle will have less rolling resistance than a 6 wheeled one. With half the number of contact patches to drag it makes up for the loss due the increased torque. When torque is lower there may actually be a gain from not having so many contact patches.

I'm beginning to think a 3 planet hub has serious potential. The 6 planets are obviously to reduce loads and ensure a long life. I'm wondering if I would probably get more life from a hub if I wear out half the guide slots because I could then switch to the other three. Maybe they'd wear out twice as fast so I don't know. I do know I would probably be risking a major traction ring failure. It would therefore not be a good idea to use the hub for trailer towing, mountain biking or on steep hills. However I think that if I lived in a flat place and installed the hub on a light weight road bike geared for road use the 3 planet hub could be a possibility.
I've done over 40 miles so far with no issues.

[Normand_Nadon]

This is an interesting test.. my only concern is that by having less fricion surface, the remaining balls are getting a lot more stress and I fear that cavitation or surface wear might appear.... Traction fluid reacts in unexpected ways sometimes!

[Oran]

There's no doubt that it will increase the stresses because it's effectively like doubling the power transferred. I know the warranty says not to use these hubs on tandem's. However both power and torque wise a hub will spend only a small percentage of the time under extreme loading. A human endurance curve shows that a topical person can have a peak power output of nearly a kW but after 5 - 10 minutes this will drop to 100 - 200W. So doubling the power topically transferred by the hub 90% of the time isn't even half what the hub is designed for. And torque is very dependent on ratio and sprocket ratio. At higher speeds torque is not an issue and that's when having less planets is a real advantage.

Unfortunately the planets have already been damaged by some sort of slippage whilst the hub was in normal use (used as specified by the warranty). I don't think the traction fluid works very well when the contact patch is stationary. All the damage is on the underdrive area of the planets. It makes the hub feel rough in underdrive especially now there are less planets. Once out of underdrive there is a surge of power as the efficiency suddenly picks up to what feels like near derailleur levels. I haven't experience this before to such an extent with any other hub. It's more fun to ride because it doesn't feel like the hub is consuming most of any additional effect I put in when for example I want to go up a hill a bit faster.
However I sometimes wonder whether a lot of the perception of efficiency is just in the mind. Or maybe I've gained some fitness in the past week. All I know is that I keep ending up going faster than I had been expecting for the circumstances.

[Oran]

I switched back to the 6 planet hub recently and its not as enjoyable to ride. I did over 50 miles with the 3 planet hub and might switch back to it sometime soon. There's no doubt that climbing steep hills is better without all the "squishiness" but otherwise there is noticeably more drag. However I'm not sure if efficiency is totally to blame for this drag.

I was thinking about how efficiency is perceived and how it is really based on the perception of the resistance of the pedals to acceleration. There are other factors apart from efficiency that increase the perceived resistance. Such as a head wind, gear ratio or tired legs. If the "squishiness" is due to auto shifting then the 3 planet hub will drop its ratio much more under load. This means pushing against a lower ratio and a perception of reduced load. The result is a hub that feels a lot more lively.