This will be a living document to collect the advantages and disadvantages of internally geared hub and derailleur drivetrains.
Advantages of gear hub drivetrains:
Less maintenance because of a sealed system. Lets divide the whole drivetrain for better understnding into two major parts. The first one is the internal gearing hub itself. The second part is the drivetrain which consists out of a chain, the front chainwheel and the cog which is mounted on the right side of the internal gear hub. The internal gear contains the gears and is well sealed against water and dust. The sealing function is done by a more or less good seeling construction and the hub shell. The second major part, the drivetrain, can optional be capsuled by a chainguard (e.g. Hebie “Chainglider”). This will protect the chain, cog and chainwheel from dirt. This chainguard option is not available for derailleur drivetrains because of the changing chainline while shifting between the different cogs and chainwheels.
Less maintenance efforts and costs than derailleurs (Straight chain line, optional wider and stronger chain and cogs available compared to small 7,8,9 or 10 speed chains). The wider and stonger chains are optimized for force transmission and not for flexing ability which is necessary for derailleur systems. Another big point which has an effect on maintenance and efficiency is the chain path. A chain of a gear hub drivetrain has two flex in to direction forward around the cog and back to the chainwheel. The chain of a derailleur drivetrain has to flex more often because of the two derailleur pully wheels. These little wheels will force the chain to turn in a small radius. This has the effect of additive friction in the chain. The friction in the chain is indirect propotional to the size of the cog. This means a smaller cog will effect a higher chain friction. To reduce maintenance to a minimum there are complete capsuled drivetrains (e.g. Hebie “Chainglider”) available. Result: A wide capsuled straight chain of a internal gear hub drive-train lives 4 to 6 times longer (according time and km) than a thin messy flex chain of a derailleur drive-train
Easy to adjust than derailleurs.
No dirty trousers by using the Hebie “Chainglider” for example. Important for commuting!
Higher reliability. This is a requirement for all bicycle applications!
Less fragile to crashes or when bicycle falls to one side. There is no distant derailleur (except Srams “Clickbox”). On children bicycles and freerider this is a problem!
Easy to use with one shifter. The number of a gears for a internal gear hub complies.
There are no ratio overlaps and ineffective “cross” combinations of chainwheels and cogs on the derailers side. For beginners and inside high traffic areas this is a benefit!
Less and simpler parts: Single chainwheel, single cog, shorter chain, no rear derailleur, no front derailleur, no chain tensioner (When using horizontal dropouts), no slack adjuster, no front shifter, no front bowden cable. What´s not there cannot break! Higher reliability for all applications.
Shifting without pedaling. Important for city rides and trials!
Clean and pure apperance. The derailleur is the PC and the gear hub the MAC drivetrain according to design!
Combinations with coaster brakes are possible. This will reduce the overall bicycle maintenance to a absolute minimum. With the coaster brake no (frozen) bowden cables are necessary (possible!). All year commuting requirement!
No chainsuck! When riding off road or bad roads!
The chain cannot jump from chainwheel and cog because of missing rear derailleur. When riding off road or bad roads!
Shifting under force is possible for Sram i-Motion, Shimano Nexus Inter 8 / Alfine and Rohloff Speedhub. When you ride in a sportive way or uphill this should be the hubs to look for!
Constant gear ratio steps in one shifting row (Rohloff Speedhub, Sram i-Motion 9). This is a benefit for long distance riders (20km +). You´ll find easy the right gear to achieve your preferred cadence.
Low shifting force because of pedaling power system (Shimano Nexus Inter 8 / Alfine and Sram i-Motion). This is interesting for children bicycles and when riding in the rain (with reduced grip on the handlebar).
Stronger and longlasting rear wheels. There is only one cog on a internal gear hub. This has the effect that the spoke flange of the hub can be placed more on the outside. That means the spokes from the left side and the right side (cog side) of the hub can be tensioned more homogenius because of a similar spoke angle. This gives a wheel builder the possibility to increase the spoke tension. A higher spoke tension will result a longer lasting stiff wheel.
Picture of a gear hub drivetrain
Disadvantages of gear hub drivetrains:
A lower overall gear ratio. A derailleur drivetrain comes with 550% overall gear ratio. This means with one turn in the highest gear you´ll cycle a distance which is 5.5 times longer compared to the first gear. A Shimano Inter 8 hub comes with a overall gear ratio of 306%. So for a very mountainous region you should go for a high gear ratio. That means you have gears for climbing and for fast downhill in your backpack. An expensive internal gear alternative is the Rohloff Speedhub with a overall gear ratio of 526%.
Heavier than high end derailleurs. Necessary for sportive cycling!
More expensive than derailleurs. This is also a question of quality. But in general the ratio Price/Gear is a advantage for derailleurs. For bikes which are parked poutside and could be stolen.
More complicated than derailleurs. The internal mechanism may be difficult to understand. But normally it´s not necessary to open a gear hub.
8% efficiency loss against derailleurs (except Rohloff Speedhub). Internal gear hubs have a efficiency of 92% compared to 98% for derailleurs. These numbers are valid for new and perfect trimmed drivetrains! There are three friction loss sources inside a internal gear hub drivetrain:
a) Inside a internal gear hub are planetary gears. The planetary wheels cogs into the sun wheels and the ring gear wheel at the same time. This system results a friction loss of 4%
b) Also the friction bearings of the planetary wheels are a source of friction loss.
c) Finally the friction loss of the chain which is also relevant for the derailleurs.
This is the most important disadvantage for sportive riders. But compared to the power loss because of air resistance of the system rider + bicycle this is not worth mentioning!
Inhomogeneous weight allocation between front and the rear wheel. This can be a point for sportive applications (e.g. MTB, Freeride, Downhill)
Less minute adjustment to gear ratios than derailleurs. The gear steps are defined the overall gear ratio can be adjusted by other cogs and cahinwheels. This is again a point for sportive cyclists.
Difficult wheel removal. With the new i-Motion Sram hubs the differences fade. But it´s still an issue to tight the chain. This is not necessary for derailleurs.
The input momentum into the internal gear hub is limited. This means low overall gear ratios cannot be realized (except Rohloff Speedhub). Important point for uphill cycling.
Picture of a derailleur drivetrain