Shimano nexus 8-speed internal gear 7-speed hub planetary nexis nexus gears hub gear eight speed seven four speed roller brake rollerbrake disc brake commuter commute commuting city bike town
The Shimano Nexus Seven and Eight Speed Technical Tips
Note: This page was written about the Nexus 7-speed, but most of the information also applies to the Nexus 8-speed
and 4-speed models.
Lubrication
| Sprockets
| Upgrading Existing Bikes
| Vertical Dropouts
Nexus is a family of parts, but the most exciting part is the Nexus 8 speed hub.
Shimano Nexus 7-speed hub Gear Ratios
The overall range is 244%, in jumps of:
| 1st | 17.2% | 2nd | 13.8% | 3rd | 17.3% | 4th | 15.8% | 5th | 16.6% | 6th | 15.7% | 7th |
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For comparison, your old 3-speed had a range of 177%, in jumps of:
A typical 1970's "10 speed" had 8 usable gears, a range of 247%:
| 42/28 | 16.6% | 42/24 | 20.0% | 42/20 | 3.1% | 52/24 | 14.0% | 42/17 | 5.2% | 52/20 | 17.7% | 52/17 | 21.3% | 52/14 |
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Sprockets
Nexus hubs use the same 3-spline sprocket system as other internal gear hubs from Sturmey-Archer and SRAM/Sachs, and also used on most coaster brake hubs.
These sprockets are readily available in a range of sizes from 13-24 teeth. By changing the sprocket, you can raise or lower all of the gears at once.
The sprocket is held in position by a spring circlip (snap ring). The circlip can be pried off with a thin flat-blade screwdriver, and the sprocket can then be lifted off.
Most sprockets made for this system are "dished" so you can adjust the chanline by flipping the sprocket over.
The circlip snaps on, also most easily by levering it into position with a flat-blade screwdriver.
After re-installing the sprocket, it is a good idea to seat the circlip by going around it and tapping with a hammer and punch. This is espcially important on coaster brakes, because the brake will become inoperative if the sprocket slips off.
Other features of the Nexus system:
Nexus 7-speed internal gear systems also incorporate all-weather braking systems. They are available in two forms:
- Coaster (back-pedaling) brake
or
- Hand operated "Rollerbrake ®" at the rear hub.
Both of these systems provide quite a good rear brake, which works as well in rain and snow as it does on a sunny spring afternoon.
There is also a front roller brake available, but, in my opinion, a conventional cantilever brake is better for the front. I would not particularly recommend the Nexus front brake.
Cable Installation
Cable Adjustment
Gear Adjustment-all Nexus 4-, 7- and 8-speeds:
The adjustment is made witht the shifter in 4th gear. On the right side of the hub, just outboard of the sprocket, there is a "cassette joint pulley" which the cable moves back and forth as the gears are changed. Next to this is the "cassette joint bracket" which is stationary. Both the pulley and the bracket have red index marks, and gear adjustment is correct when the marks on the two parts align with the shifter in 4th gear.
There are two sets of these red marks, one on top, the other on the bottom. This lets you see one set of marks whether the bike is right side up or upside down.
This procedure is the same for the Nexus 4-, 7- and 8-speed hubs, using 4th gear as the reference in all cases. In the case of the 8-speed, however, the marks you need to line up are yellow, not red.
Anti-rotation (non-turn) Washers
 Internal gear hubs always require some system for preventing the axle from rotating, as it is used as part of the gear train. Like most other internal gear systems, Shimano uses tab washers that engage the dropout slot to keep the axle from turning.
Because the shift cable linkage ("cassette joint unit" in Shimano-speak) is also keyed to the axle, they make different types of anti-rotation washers to fit different dropout angles. You need to select the correct type of washers for your particular frame.
(Earlier Nexus hubs only used an anti-roatation washer on the right side, but newer models use a pair of them, each a mirror image of the other.)
The washers are color coded as shown in the chart.
Vertical Dropout Issues
The Nexus hubs were not originally intended for use with bikes that have vertical dropouts. The anti-rotation washers provided with the hub have the wrong orientation for use with vertical dropouts, and originally, Shimano said they could not be used with vertical dropouts.
Shimano has reconsidered, however, and now makes available an optional set of anti-rotation washers that work with vertical dropouts. Note that you will need either a pulley-type chain tensioner (or a rear derailer) or an eccentric bottom bracket, because vertical dropouts do not permit moving the axle back and forth to adjust the chain tension.
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Converting an Existing Bicycle:
In some cases, it is possible to convert an existing bicycle to the use of a Nexus hub.
Ideally, the frame should to have a drop-out spacing of 130 mm. Older bicycles are typically narrower than that. In the case of steel (Cromoly) frames, it is usually possible to spread the rear triangle to the needed 130 mm width. Any good bicycle mechanic will be able to do this, or you can do it yourself.
If you leave off the Rollerbrake ®, you can get the spacing down to about 126 mm. This is what I did on my Nexus bike. The Rollerbrake ® is a separate module, and when you buy the hub it is not even installed. (The brake unit would get in the way of fitting the spokes through the hub flange, so it has to be installed after the wheel has been built.)
Vertical Dropouts
Ideally, frames for use with the Nexus system should have horizontal drop outs. Bikes with vertical drop outs don't provide any way to adjust the chain tension, so you would need to install a chain tensioner (or a rear derailer) to regulate the chain tension.
Chain tension is normally adjusted by moving the axle back and forth in the slots of the horizontal dropouts/fork ends. With vertical dropouts, you can't do this.
This problem can be solved by use of a spring-loaded chain tensioner or a rear derailer. Note, this will not work with the coaster brake version of the Nexus hub.
Internal geared hubs need to use special keyed washers that resist the tendency of the axle to rotate under pedaling loads. These keyed washers have a tab that is supposed to fit into the slot in the fork end. If your bike has vertical dropouts, you need to order a special set of anti-rotation washers designed for vertical dropouts, or the cable routing will be incorrect.
Lubrication
The Nexus hubs are packed with a very special grease. They are quite well sealed, and should only require service at very rare intervals. The great majority of mechanical problems that I've encountered or heard of have been the result of "preventive maintenance" including re-lubrication with incorrect grease.
The hub must be disassembled into its major components, solvent cleaned, dried and re-greased with Nexus hub grease (not to be confused with the similar Nexus brake grease.
Dismantling the hub is not as difficult as you might think, because of its modular construction.
If you unscrew everything that is screwed onto the left end of the axle, the whole mechanism can be pulled out of the right side as a unit.
The brake is lubricated without any disassmbly--there's a small rubber access plug on the side of the brake unit, just pop off the plug, put the nozzle of the grease tube up against it, and squeeze.
Shimano Nexus 8-speed Hub Dimensions:
Links:
Shimano Nexus lubricants. |
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We carry Nexus-equipped bicycles
from several manufacturers, including: |
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Come on in and test ride one soon,
here or at your local dealer...
you're in for a pleasant surprise! |
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If you would like to make a link or bookmark to this page, the URL is:
http://sheldonbrown.com/nexus-mech.html
