SRAM G8 hub, freewheeling version
There are also articles on this site more generally about internal-gear hubs,
and about Sachs/SRAM internal-gear hubs.
Sheldon Brown's Internal-Gear Calculator
The G8 8-speed internal-gear hub was introduced in 2013, and the G9 in 2014. Production was discontinued in 2015. The G8 was offered in three versions:
The G9 was offered only in a disc brake version (which may also be used without the disc). Drive ratios (in italic for the G9 only) are:
The internals of the G8 and G9 are substantially the same. The G9 offers one additional low gear which is theoretically possible but not implemented in the G8. The G8's total range is narrower than that of 8-speed hubs from Shimano and Sturmey-Archer, 259% with an even series of small steps. The G8 is, then, best suited to urban cycling without steep climbs and to cyclists who are picky about cadence. The G9's range is 292%, still slightly narrower than that of the Shimano 8-speed.
The shell of G8 and G9 is somewhat larger than that of the discontinued i-Motion 9, but the internal mechanism is much simpler, and the G8 and G9 weigh less.
The following specifications have been released, or measured:
The G8 and G9 are grease-lubricated. These hubs require periodic cleaning and relubrication. Cleaning and relubricating these hubs when new is a good idea, even though it voids the warranty, because factory lubrication often doesn't stand up to bad weather conditions. The coaster-brake version of the G8 requires high-temperature grease for the brake mechanism, like earlier SRAM coaster-brake hubs; otherwise lubricate with Phil Wood oil for the gears and boat-trailer grease in the bearings for the least drag consistent with good resistance to contamination. Unlike with the i-Motion 9, the disc-brake mounting holes do not extend all the way through to the inside of the hub, but you could drill one hole through (with the hub disassembled so you can clean out metal chips) to use it as a lubrication port. Mark which one you drilled. See the information on lubrication in the main article about internal-gear hubs on this site.
There are installation instructions for both hubs online, and also a document giving frame fit dimensions for both hubs. The SRAM 2015 Spare Parts Catalog lists replacement parts on page 13, but the internal mechanism is only available as a unit. Aaron Goss, of Aaron's Bicycle repair in Seattle, has provided illustrated rebuilding instructions.
A wheel with the G8 or G9 should be built cross 2 or cross 1, due to the large spoke hole circle diameter. Low cross numbers require careful calculation of spoke length.
Axle nuts and anti-rotation washers are the same as for the i-Motion 9 and different from those of other SRAM internal-gear hubs. As the G8 and G9 are rotary-shifting hubs, different anti-rotation washers are needed depending on the angle of dropout slots. The SRAM 2015 spare parts catalog, page 18, shows which anti-rotation washers work with which dropouts. Shimano Nexus anti-rotation washers can be made to fit with some filing.
The axle flats for the anti-rotation washers are aligned vertically when the cable attachment faces forward. Except with vertical dropouts, the washers do not, then, hold the axle flats parallel with the dropout slots, and so an older bicycle with narrow slots will need to have them widened. File the underside of each slot only, so as not to misalign the wheel. The overlocknut distance of this hub is 135 mm, and so many bicycles need to have the frame respaced -- possible, though, only with steel frames. It appears possible to get the overlocknut distance of the disc-brake version down to 130 mm or less by leaving out the brake disc and its locknut.
The axle should be secured and chain slack adjusted before securing the reaction arm of the coaster brake to the left chainstay -- as with any hub brake that has a reaction arm. If the coaster-brake version is used with vertical dropouts, the chain must be adjusted using an eccentric bottom bracket rather than a chain tensioner.
The sprocket is the same as for the i-Motion 9, and supplied only in sizes of 18 through 22 teeth. As it is flat, it can be flipped over to double its wear life; also, its life can be greatly prolonged by replacing the chain frequently, or grinding away the hooks which a worn chain wears into sprocket teeth.
The G8 uses a proprietary indexed twist shifter which fits only a flat handlebar but an owner reports that this shifter does not work properly. Very unusually, the G8 is compatible with an indexed derailer shifter, SRAM 3.0 series -- see article decribing how to use one. An aftermarket shifter for drop bars may be available. The G9 uses a two-lever trigger shifter.
A German blogger who has tried a G8 reports that it runs quietly and shifts smoothly. Reader Brian Sorenson reports:
I took the plunge and have been using the G9 on my Surly ECR for the better part of a wet, Pacific Northwest year.
A few days ago I took it apart to check for wear and lubrication issues. I was somewhat surprised to see that it looked new inside. The internals look very well machined, and it's pleasant to look at. I'd say it's no more complicated to work on than an old AW hub.
The only area that makes me scratch my head is the drive-side dust cap, actually attracts grit and presses it against the hub shell. There was a bit of rust underneath it, but the bearing it protects was just fine, so I suppose it works.
Other than that, it does what it says on the box. Adjustment is dead simple and it has a nice quiet sound to it. Efficiency seems on par with similar hubs, and shifting even under load is smooth.
The G8 and G9 have a first stage with three decrease ratios, followed by a second stage with direct drive and two increase ratios.
The illustration below, from SRAM, (in my translation from the German) is a schematic diagram of the gearing. "Z" in the diagram stands for "Zähne", which is German for "teeth." The coaster brake is driven independently of the gearing, good because then the brake works the same in every gear. A coaster brake in an internally-geared hub is, however, a bad idea even though it is popular in some parts of the world.
The internal mechanism of the coaster-brake G8 is shown in the view below. The parts are in the same orientation as in the diagram above. The 48-tooth sun gear (at the gap in the middle of the hub) and the brake shoes have been removed. Sectors of the shell, ring gears, planet cages and driver have been cut away. Plastic dropouts as shown are recommended only for light service!
The photo at the right shows the axle assembly with some additional parts. The left end of the axle is at the top. The 54- and 57-tooth sun gears of the first stage are in place; the remaining sun gears have been lifted off. Just above the 54-tooth sun gear is a pawl to block rearward rotation of the (absent) 48-tooth sun gear. Pawls which block forward rotation of the two sun gears of the second stage are nearer the top of the picture. It may look as though there are more pawls, but each pawl is grooved in the middle for its pawl spring.
The relatively large diameter at which the axle pawls engage the sun gears promises greater reliability than with other hubs (particularly, all but the newest Shimano 7-speeds) where axle pawls engage at a small diameter and are subject to high force. On the other hand, the G8 and G9 have some 12-tooth planet pinions (though the i-Motion 9 has 9-tooth planet pinions). Gearing down before gearing up increases stress on the mechanism, but reduces drag.
The G8 has no direct drive (1:1) ratio, and all but the two lowest gears (three lowest for the G9) use both stages, so efficiency can be expected to be lower than with some other internally-geared hubs. Three additional drive ratios would be possible if the first stage had direct drive-- but with the gear tooth counts of this hub, all but the highest ratio would fall close to others, and the jump to the highest would be large. The lack of direct drive may be to avoid an awkward shift, as with the Shimano 7-speed.
I thank Brook Fowler for passing along technical specifications, Taras Bereznyak for analysis and calculation of gear ratios, Torben Finn Laursen for photos of the G8's internals and the schematic diagram, Zack B for pointing the way to additional information about the G9, Brian Sorenson for his feedback and Aaron Goss for publishing rebuilding information.
Installation instructions for the G8 and G9 hubs
Frame-fit dimensions for both hubs
User's Manual for the G8 and G9
SRAM 2015 Spare Parts Catalog
Using a Derailer Shifter with the G8
Aaron Goss's rebuilding instructions
Radtouren magazine article about the G8 (in German)
Radmarkt article about the G8 (in German)
Last Updated: by John Allen