Derailleur Gears
Strictly speaking derailleurs are not gear systems, just being a choice of sprocket pairings which can be selected dynamically to give a variety of ratios. As such they are nearly 100% efficient, the only losses apart from the chain, being in the chain path diversions. Gear changing can only be accomplished while pedalling, maintaining the drive almost continuously. For these reasons derailleurs are dominant in cycle sport.
In their simplest form they have a set of from 5 to 10 rear sprockets which can be selected individually by a rear mechanism arm guiding the chain to a selected sprocket. The rear set of sprockets are normally spline mounted on a freewheeling cassette centre, which is designed for standard 10 mm bike spindles, but some hub motors with a spindle cable entry need a larger 12 mm spindle. These have to use a screw-on freewheel with multiple sprockets instead, which is less strong.
To multiply the number of gear combinations, 2 or 3 chainwheels are added at the front with a simple metal arm used to guide the chain from one chainwheel to another. In this way up to 30 gears can be achieved with different front to rear sprocket combinations, though in practice some combinations are near duplications. Electric bikes with some makes of crank-mounted drive motor cannot have multiple chainwheels, so are restricted to rear wheel gear mechanisms only.
Gear changing can be done by thumb and finger levers, with separate left and right controls for the front and rear mechanisms, or by using twistgrip changers which can be twistgrip and lever combinations to give a single handed control.
Derailleurs are intrinsically high maintenance due to the need to regularly clean and lubricate all the exposed wearing surfaces and the propensity of the lubricated surfaces to gather road dirt, which impairs their operation.
Hub Gears
This naming is restricted to hub gears which consist of a hub shell containing epicyclic gears to give ratio changes. They can have from 2 to 14 gears at the time of writing. They are intrinsically less efficient than derailleurs, especially when the drive is sent though a combination of more than one epicyclic gear to achieve a particular ratio. However, the efficiency is of much less importance on an e-bike since it’s easily compensated by the rider and motor power combination.
The big advantage of hub gears is that they are low maintenance, some needing only a re-lubrication at two yearly intervals on heavily used bikes. Many moderate mileage riders just use them for several years to the death, when a new gears core can often be installed into the hub shell.
Hub gears are less strong than derailleurs and have much lower torque limits, but the only time this troubles e-bikes is with crank-mounted drives where both rider and motor power passes through the gear hub. At least one maker of these crank drives reduces the motor power slightly for use with hub gears, so if a high power crank drive is contemplated, hub gears should not be used with it.
Though some makers claim hub gear changing can be done when pedalling normally, in practice it’s usually necessary to ease off pedal pressure or even stop pedalling momentarily to achieve clean gear changes. This means that it’s best to anticipate the gear needed before starting a climb, since pausing pedalling while climbing can lose speed. However, gear changes can be made when stationary, handy for selecting a low gear to standing start from traffic lights etc.. This benefit and the low maintenance needs makes hub gears ideal for daily in-town commuting and utility use.
As with derailleurs, gear changing is normally accomplished with either lever or twistgrip controls. An automatic gear change system is made by Shimano and available with some of their gear hubs, but since it’s electrically operated and powered by their hub dynamo, this is usually only an original bike model option. There are also some automatic changing 2 speed gear hubs available.
Dual Drive
Only from one maker, SRAM, their DualDrive cleverly incorporates a gear cassette with 8, 9 or 10 sprockets onto a 3 speed gear hub. The function of the gear hub is that of the three front chainwheels on conventional derailleurs, enabling from 24 to 30 gears to be available when a triple chainring cannot be fitted, as on some crank drive units and some recumbents. The operation of all the gears is single handed by a twistgrip and lever combination. The downside is that the disadvantages of both derailleurs and hub gears are present, but offset by SRAM’s 3 speed hub being independently tested as the most efficient of any hub gear from any maker.
NuVinci
Again from one maker only, Fallbrook Technologies, this system uses a set of six spindle mounted steel balls between the dished surfaces of steel tracks on either side. By tilting the spindles, the radii of the upper ball surfaces presented to the tracks is changed to give shorter orbits on one side track compared to the other. This results in gains or losses of rotation speed on the output track, depending on the direction of tilt of the steel balls. To understand this visually, visit fallbrooktech.com and select NuVinci Technology, Overview, How it Works.
This form of ratio changing means the gear ratios are continuously and smoothly variable across the whole 360% range and operated by a twistgrip and therefore a form of CVT.
The grip of the steel balls on the tracks is ensured by running them in a special liquid which compresses to near solid when squeezed between a ball surface and track, giving secure grip. This system has the highest torque handing of any gear hub, making it ideal for crank drive units and easily able to handle any legal motor and rider power combination. That grip system inevitably means some loss of efficiency due to continuously compressing the fluid at twelve points, but once again inconsequential with e-bike motor and rider power combination.
The continuous gear range for any purpose, the high torque handling able to cope with crank drives and the essentially maintenance-free nature are its major benefits. The downsides are the greater weight and cost compared to other gear hubs. Also the twistgrip gear change operation can stiffen when under high drive loading, but that’s easily compensated by slightly easing the drive pressure, so of little consequence.
Two Speed Bottom Bracket
At the time of writing only one make of these is available, the 2 speed systems made by Schlumpf Innovations. The gearing system is a chainwheel epicyclic operated by heel kick buttons on the centres of the bottom bracket spindle. There are three forms, the Mountain Drive which gears down 2.5 times, the Speed Drive which gears up 1.65 times and the High Speed Drive which gears up 2.5 times. These can be used in conjunction with any other gear system to extend their ranges and are available for chain or belt drive. As with any epicyclic there is some efficiency loss detracting a little from the pedal effort.
Gear System Ranges
When choosing a gear system, as well as the factors of the number of gears and the efficiency, an important factor is the gear range, this being the degree of difference between the lowest and highest gear and expressed as a percentage. Put simply, a gear system with a high ratio twice that of its low ratio has a 200% range, three times as high is 300% range.
Those needing to tackle very steep hills and also attain high road speeds at other times need the greatest gear range, while those in flatter areas and/or having moderate speed requirements have a lesser gear range need. In a totally flat riding area, gears need only be very narrow range or even dispensed with altogether. A 300% gear range meets the needs of most riders.
Derailleur gears and the Dual Drive can have gear ranges exceeding 600%, more than enough to cope with any need. Hub gears range from 200% on three speed hubs through Shimano’s 307% eight speed and 409% eleven speed to the 526% of the Rohloff fourteen speed hub. And as stated above, the NuVinci has a 360% range so directly comparable to Shimano’s eight speed.
Our thanks to Flecc for this article. Flecc is a long-time contributor to Pedelecs. If you have a question you’d like Flecc to answer please contact us here.
