What's the efficiency of hub gears compared to derailleurs? I know that hub gears are not as efficient. Apart from being heavier, how much will I lose in performance if I use hub gears?
A lot depends on the rider and what you mean by efficiency.
It is easy to keep a hub gear running well for years, but an unmaintained derailleur will become inefficient very quickly. A hub gear allows the chain to be fully enclosed, for all but the most dedicated cyclist; an enclosed chain will be more efficient as it will be cleaner and better oiled.
An enclose chain allows you to arrive at work, with clean cloths and no need to change, this can save lots of time. Not having to spend time adjusting a derailleur or cleaning your chain also increase the efficiency of your life. However it takes longer to remove the wheel if you have hub gears and an enclosed chain.
Gear shifting efficiency. This is most important in cities with many traffic lights to stop at. With a derailleur, you must shift before you stop, or you'll be starting in a high gear. With an IGH, you can forget about shifting while you are braking, and just select the right gear for the next acceleration a second before you start pedaling. It's a huge difference for a commuter, irrelevant for a racer.
Personally I think on a road bike:
- Hub gears are more efficient if you wish to use a bike to make lot of short trips as part of your day to day life.
- But derailleurs are more efficient if you consider going fast on a bike to be very important and are willing to put the work into the bike that is needed to keep it in a very good state of maintenance.
(I also like hub brakes for the same reason)
In 2001, Kyle and Berto published a comparison of the mechanical efficiency of several configurations of derailleur and internally-geared hubs in Human Power, which you can find here. Among the systems tested were a Shimano MTB derailleur system, a Sturmey-Archer 3-speed hub, the Shimano 7-speed Nexus hub, and the 14-speed Rohloff hub. MTB derailleur systems are (now) known to be less efficient (more lossy) in terms of power transmission than "road bike" derailleur systems. In particular, note fig. 12 in the cited link, which compares average efficiency at three different levels of power for the internally-geared and MTB derailleur system. Spicer (2000) has measured the power transmission efficiency of an ideal chain driven system at about 98%, so that is what you would expect the maximum efficiency to be.
It depends on the model you get, but the efficiency is generally comparable. Derailleurs that are in really good condition and properly lubed will be more efficient, but marginally, and will often be less efficient due to real world conditions.
At least that's what the wiki says:
I have one road bike with a 3x9 derailleur, a smaller bike with a 7 speed nexus, and a brompton that actually has a hub AND a derailleur for a low/high gear. The other considerations (tire size, pressure, etc) probably matter far more than the gear. For easy shifting and maintenance, hubs, no question.
Newer hubs are generally considered more efficient (Shimano Alfine vs Nexus, that sort of thing). Also, Shimano has an 11 speed coming out that has a different type of lube than usual, and is supposed to have improved efficiency, but I don't think we'll know much till its out.
Then there's the Rohloff. Way too expensive to buy, but shows what a good hub can pull off:
I don't know how much is lost, but there is definitely some. I lose speed more quickly coasting with my hub (Nexus 7) than with a freewheel design. This is confirmed by turning the crank on a tuning stand and seeing how much more quickly the wheel stops turning than with a freewheel.
Similarly, on the tuning stand, if I slowly spin the crank backwards, the wheel will start to rotate backwards which never happens with a freewheel (or at least to much less an extent).
Both of these tell me that the hub is creating a bit more internal resistance than the bearings and ratchet mechanism on a freewheel do.
The chart posted above would indicate that internal hubs maybe lose 2.5% of efficiency compared to a derailleur.
That and the slightly higher weight are probably why racers all use derailleurs.
There was a recent study done on this by FahrradZukunft magazine, which is nicely summarised in this excellent article. The results are summarised in the following graph:
The dark pink line represents a singlespeed transmission which is around 97% efficient (a typical figure quoted for a derailleur system is slightly lower at 95%).
As you can see the efficiency of the hub gears depends which gear you're in, as a different number of moving parts can be engaged for each. Overall the Rohloff speedhub is the clear winner, with the Nuvinci CVT hub coming in last. As the article says, at a 200W power output this equates to a difference of 22 watts, which is pretty significant.
Personally I've found the drag from hub gears to be noticeable if not huge, but a price worth paying on an all-year commuting bike for example.
I can only compare the basic Shimano Acera and Deore (3x8) deraileurs to the Rohloff 14-speed hub, but these are my observations relating to efficiency. There's a distinction here between pedalling efficiency and gear-shifting efficiency. I'm going to mention both as I don't know which one you meant.
On the Rohloff hub
- In the majority of gears it doesn't feel like there's any loss of pedalling power compared to a direct-drive fixed gear bike (subjective, I know). Gears 7 and 12 make some noise and seem a little less efficient.
- Changing gears is always impossible if significant force is on the drivechain.
- A short pedalling pause is required to shift gears.
- It is possible to change to/from any gear when stationary or coasting.
- Gear inches change between gears is simple and predictable (roughly the same percentage per click).
On a derailer
- A small amount of pedalling power seems to be lost to the jockey wheel. This is significant only if it gets dirty or one of the shifters is misaligned. Several gears are noisy/inefficient due to chain angle.
- Changing gears is sometimes tricky if there a large force on the drive chain. It depends on which change is being made, some are really easy and smooth, others need some care.
- No pause to fast, gentle pedalling is required to shift gears.
- Only possible to change gears when pedalling.
- Two shifters means that shifting quickly to a predictable-length gear a skill that requires a bit of thought.
YMMV, but I find the Rohloff to be efficient and simple to use. We use it on a tandem, so it may be that the pause needed when shifting is less distruptive on a solo bike.
However, I also expect that an experienced racer would be able to shift their high-spec gears much more quickly and efficiently than any hub gears and that this might mean that deraileurs are the only option for racing. That and the fact that you can't seem to connect them to brifters.
In my ~1 month experience with the Deore (11 speed?) internally geared hub compared with well-maintained derailleur systems of comparable quality and newness, the internally geared hub has a noticeable amount of internal loss. Note that this test was carried out about a year ago. I ended up returning the bike.
First test (idle loss): turn the crank a few times and get the wheel to spin, see how long it continues to spin. This will tell you how much energy is lost in the hub. In my experience the loss is noticeable.
Second test (loss under power): using a similar gear ratio, turn the crank at a fixed speed for about a minute, you will get an idea of how much energy input is needed to keep the wheels turning at a fast, steady pace. Compare the two energy inputs.
I set the same electric bicycle up using a Bafang 1kw mid drive anf a thumb throttle. I ran.3 different gear set ups with same final drive ratios. 1st set up was with a Shimano Nexus 8, top speed was 43mph. Sluggish. 2nd was with a Sturmey Archer. Top speed 45mph I ran it with a derallier and got 49.5mph. Max rpm on the moyor was the same for all 3 with a full battery. This is actually quote a big % change. While it only lools like a 4.5mph increase, the energy required to.go.those extra mph is quite significant. Remember to double your speed neefs 8x the power...so an extra 4.5mph is probably over 10 % more power to the wheel rather than absorbed by the gear system. I agree they are nice and.tidy but I now build all my electric.bikes with derallier. I get noticably more miles per battery...about 10% more coincidentaly !
There are various studies of this. Some have quoted them. The objective answer is that hub gears are generally less efficient than derailleurs.
My bike last year was a Ridgeback World Journey, a heavy steel tourer with an Alfine 8 hub and drop bars.
This summer I made up a tourer using a Ridgeback Panorama Deluxe. Still a heavy steel tourer. I didn't pay much attention to the component weight.
Both bike are very similar, but the Panorama is a 3x9 derailleur, the World Journey is Alfine 8. So effectively the same bike with different transmissions.
Here is a numberless, but still objective answer. With the bike in a stand, I can spin the back wheel and let it go. The derailleur wheel will continue spinning for a very long time, the hub gear will slow quickly.
Subjective answer - the derailleur bike rides a lot better. Exploring a Scottish island this summer I could pedal a little and then coast for a long time, even on the flat. I could never do that on the hub gear bike. I think I could feel the drag.
The overall difference in weight of the two transmission systems is not that great, apparently. But with the hub gear all the weight is at the back end. I swapped the tyres between the two bikes, so at one point had both back wheels off. Again numberless, but there was a huge difference in weight. This heavy back end makes the bike 'feel' sluggish, I think.
I've always had hub gear bikes in the past. It seems like a great idea, and changing gear while stationery is useful. Combined with roller brakes it could be the ultimate commuter (I've got a Carrera Subway 8, that has exactly that setup).
But they have many drawbacks:
The gear range will be narrower than a derailleur set. Might not matter where you are, but bottom gear isn't really low enough on my Alfine.
They feel, to me, 'sluggish'.
The back end is heavy, and if you have rear panniers, VERY heavy, so increasing chances of pinch flats and needing higher tyre pressure.
Changing the rear wheel for flats is difficult. You can't have a QR, and you need to carry something to unhook the cable (at least with Shimano). So you have more tools to carry, specifically a 15mm spanner and a 2mm allen key. On the bike stand it's a bit awkward. In the dark/rain it would be a horrible job. Is a horrible job, I've done it.
You have very limited choice of changers if you want to use drop bars.