The theory and practice of derailleur chain tensioners, vertical dropouts, horizontal dropouts, slotted frame ends and Rohloff OEM slider dropouts, with reference to power and braking torque resolution, and to suspended and unsuspended bicycles, with a critical path decision tree through the complications
Every drive chain wears over its service life. This wear is misnomered “stretch” because the visible effect is growth in the operating length of the chain. This increase must somehow be adjusted out in every bicycle transmission of whatever type. This article is about the technical ins and outs and aesthetics of “somehow”, to serious cyclists a matter of importance third only to sex and hydration.
ONLY TWO CHOICES, REALLY
On a derailleur-equipped bicycle, with more than one chainring and most likely many sprockets in the rear cluster, the chain tensioner, a spring-loaded arm, does what it says on the tin, adjusts the chain for wear (“stretch”), and it does it automatically. It is a well-understood, proven system, albeit crude in conception; it has the additional advantage of offering flex for the varying chainline between different sized front and rear cogs at different distances from the
centreline of the bike.
On a fixie, single-speed or internal hub bicycle there is a fixed, hopefully straight chainline because there is only one cogged ring on the bottom bracket axle and one cogged ring on the wheel axle, but chain wear over the chain’s service life must still be allowed for, and in this case too a chain tensioner is simple to design and execute with widely available components in a wide range of qualities and prices.
However, with a fixie, a single-speed or an internal hub gearbox, there are several advantages over the derailleur system — which are lost by the installation of a spring tensioner. The advantages lost in what we generically call “single-speed bicycles” include the “clean appearance” of a simple chain between two cogs without appendages, easy adjustment and service, cleanliness, longer service life of the chain and all other transmission parts, a very long list. Perhaps the greatest loss brought about by the thoughtless, reflex fitting of the chain tensioner is the important ability in all but the crudest bicycles of fitting a full chain case with its own advantages in attracting a wider class of rider and putting the bike to a wider class of service, or in intensifying the other advantages of doing away with the chain tensioner.
A SERIOUS COMPLICATION
When you replace the ugly, dirty, imprecise, wear-inducing derailleur system and its chain tensioner with a tidy, self-contained, longlasting hub gearbox, you don’t want the gearbox to turn around in the frame. So, whatever design you put in the place of the chain tensioner must not only provide adjustable drive length (the centre to centre distance between the bottom bracket axle and the rear wheel axle), but must react torque for the drive power.
Regardless of which transmission system you choose, disc brake torque too must be reacted and ditto the torque of the very effective roller brakes available as an integral fitting with Shimano hub gearboxes (Nexus, Alfine).
SOFT TAIL: CHAIN TENSION ON REAR SUSPENSION BIKES
Rear suspension on bicycles are perforce swing arm systems. There are only two possibilities:
In one type the swing arm contains only the rear wheel, with the swivel before the bottom bracket on the forward part of the frame, in which case a chain tensioner is inevitable whatever the transmission type preferred.
When the swing arm contains both the rear wheel and the bottom bracket, with the swivel to the main frame placed forward of the bottom bracket, the swing arm forms a rigid brace as on the tradional diamond frame, and all the other chain tension possibilities canvassed below open up.
ADAPTING A DERAILLEUR BICYCLE TO BE A FIXIE, SINGLE-SPEED OR INTERNAL HUB GEARBOX BICYCLE
First we’ll look at the most difficult installations, in which a traditional derailleur-equipped bicycle is turned into a fixie, a single-speed or a hub gearbox bike. The thorough Germans have of course thought the matter through exhaustively, and Bernd Rohloff supplies kits of his hugely admired Rohloff Speedhub for every configuration of frame imaginable. This is the opening page of Rohloff’s Speedhub Finder, which purports to simplify a complicated decision tree:
You can play with this decision tree but basically, unless you have horizontal dropouts 25mm or longer (illustration B), to fit a Rohloff, you will need either a chain tensioner (illustrations A, C D, E) or a custom frame or at least custom frame ends brazed/welded on (illustrations F and G).
“Frame ends” is the proper name for rear dropouts that don’t drop out… I don’t know what front dropouts with lawyers’ lips, which don’t drop out easily either, are called. Long horizontal slots that open rearwards are called “track ends”, if you want to be fancy. Stick to “dropouts” and you can’t go wrong.
I have long horizontal slots on bikes fitted with Shimano hub gearboxes and, together with tug nuts, and in conjunction with serated axle locknuts which chew up the aluminium frame ends, they work a treat. My Rohloff hub gearbox is fitted to the full katootie OEM slider dropouts on a custom frame, so I cannot say from personal experience how well the adaptations of the Rohloff to standard derailleur frames work, but they appear to work for tens of thousands of riders and mud pluggers.
CUSTOM FRAMES FOR FIXIES, SINGLE-SPEEDERS AND HUB GEARBOX BICYCLES: BACK TO TWO CHOICES
So, if you’re resigned to having a derailleur frame altered, or to buying a dedicated frame for your fixie, single-speed or hub-gearbox bicycle, what choices do you have for adjusting chain tension? A chain tensioner is possible but we’re assuming that you’re taking a more expensive route because you’ve already ruled out the crude, ugly, dirty chain tensioner.
Again, there are only two choices, an adjustable bottom bracket, and some kind of movable dropout.
THE ECCENTRIC BOTTOM BRACKET
An EBB is a bottom bracket set off-centre (ex-centrically — English is not always intuitive!) in an aluminium plug sized to fit inside the bottom bracket shell. The bottom bracket shell may be larger than standard, so the plug is bigger than a standard bottom bracket and will then take a standard bottom bracket. Or the bottom bracket shell may be a standard bottom bracket diameter, in which case a smaller than standard bottom bracket is required to fit the plug; this is a uncommon option.
Chain length adjustment is achieved by rotating the plug in the bottom bracket shell so that the bottom bracket axle comes to rest nearer to or further from the rear axle as required. A special peg tool (a pin wrench) is required but it is usually combined with another tool useful in a touring kit so that excessive weight is not added.
The plug is fixed in the bottom bracket in a variety of ways, the most common being by pointed- or rounded-end bolts entering the soft aluminium some short way, which have the disadvantage that eventually they ruin the EBB by wearing grooves in the aluminium plug and then will no longer hold it in position.
An alternative is splitting the bottom bracket shell at a pair of lips that bolt together and clamp the EBB in place, a method hated by many frame designers as compromising the strength of the frame at a critical concentration of loads. You take your pick and pay the consequences.
The Bushnell EBB does not require a split shell; it is fixed in position by turning a screw which expands the eccentric within the bicycle’s bottom bracket shell. There are also external bearing EBB, such as the Trickstuff Excentriker or the Phil, which screw into standard bottom brackets but whose adjustment is external; they use modern cranks with integrated axles. Most of these “special” EBB have the disadvantage that they’re priced for plutocrats.
Note that the aficionados of the EBB who use hub gearboxes (and that’s almost all of them) must still design and construct a special dropout for torque reactions, or make do with a kludgy arm, so the EBB choice isn’t necessarily the cheaper construction option (though it is often presented as such). Furthermore, replacements of inevitably ruined EBB (for those who choose the cheap option of fixing the EBB by dimpling bolts) could over time make it a more expensive option than sliders. But for most designers cost appears to be an afterthought in this choice, an extra justification, as many have a visceral dislike of the only alternative to an EBB, some kind of slotted frame end in which the axle can slide.
COMMUTER AND UTILITY BIKE SLIDER DROPOUTS
Slotted horizontal dropouts, long familiar from hub gearbox practice on Dutch-style city, commuter and continental holiday bikes, are slider dropouts, open at one end.
The axle is retained by friction between the axle nut, often with a serrated mating face, and the frame material, usually but not invariably assisted by an adjustable tugnut. For torque control, flanged washers fit over flats on the axle.
Adjustment is by loosening the axle nuts and the tugnut screws (often wing nuts or other finger-operable fasteners), sliding the axle until the chain reaches the required tension, and then locking all four fasteners.
The horizontal open slot has worked well for decades on tens of millions of installations. However, with anything more complicated than a simple rim-braked bike, it soon becomes complicated, though Shimano managed to integrate their rear roller brake with only a single extra bolt on the torque reaction arm braced to the chainstay to be undone-retightened for chain adjustment. Disc brakes are also possible but more complicated.
Long horizontal slots also allow beautifully clean installations without any unsightly dangly bits like chain tensioners when a standard hub-gearbox frame is converted into a fixie or a single-speeder.
THE ROHLOFF OEM DROPOUT FOR FAST, POWERFUL, COMPLICATED AND SPORTING BIKES WITH HUB GEARBOXES
Though the open-ended horizontal slot beloved of Continental commuters works well within its limits, which is basically for bikes with 300% or so range in their hub gearboxes, and for fixie and singlespeed conversions, the Rohloff, with 526% range, that is torque multiplication, is altogether a different kettle of fish. A serrated nut, especially on the sort of steel frame often used for Rohloff installations (rather than the aluminium common on Dutch city bikes), and a pressed steel tugnut in an open slot is a recipe for high maintenance and frequent breakages, and very likely painful incidents (which is what we sensitively prefer to “accidents”). The Rohloff concept is anyway high quality and low maintenance, so tugnuts and open slots, such obvious choices for an internal gear hub, were also obviously out.
Bernd Rohloff solved the conundrum by designing his own frame end and dropout to suit the particular strengths and needs of his gearbox, adapted his designs for every possible application, and then put the designs in the public domain so that today you can buy a frame with frame ends to his design, suitable for socalled “Rohloff OEM dropouts” made by a variety of manufacturers in a variety of decorative patterns.
The Rohloff slider frame ends consist on each side of two closed slots in a line at a shallow angle to true horizontal, in the same way that traditional open “horizontal” slots are at a slight angle to horizontal.
The dropouts are two entirely separate machined aluminium plates. The plates are tapped to accept two M6 bolts, one through each slider slot. Dropout plate and frame are further located to each other by a precisely machined tongue and groove system.
The dropout has a long vertical slot that does double duty as a torque reactor by holding both the axle and a rectangular stud the width of the slot, protruding from the gearbox. The non-driveside dropout can be shaped with or without ears for a disc brake caliper, which then moves in correct relation with the axle.
Custom frames often have additional strengthening triangulation between the chain- and seat-stays if a disc brake is intended, or even just in case a disc brake is added later, because that is so easy.
To adjust chain length with the Rohloff slider frame ends, both slider bolts on each side are undone, the wheel is slid to the desired chain tension, and all four bolts are tightened. This takes less than a minute, much, much faster than resetting an EBB or the somewhat fiddly open horizontal slot system with tugnuts. However, large movements may require rim brakes to be adjusted to suit, and if the bike is assembled to fine tolerances, other adjustments may need to be made. It’s never happened to me, and my bike is constructed to 1mm clearances between moving parts, but it is theoretically possible.
There no reason that less puissant gearboxes cannot be hung on Rohloff sliders, or a fixie or a single speeder rear axle.
WHICH IS BEST?
Many bicycle designers really hate one or another aspect of every one of these systems.
The chain tensioner is an aesthetic bicycle killer, the devil’s work.
Nor do I much like the idea of the expensive eccentric bottom bracket being a consumable part, but then I’m proud of having developed a virtually zero-service, near-zero-replacement bike, which is not everybody’s ideal.
Of course I dislike like the possibility of cascading adjustments flowing from a chain tension adjustment in the horizontal slot or the Rohloff slider systems, but it has never happened to me on my several bikes with these systems.
So, recognizing that all these systems are compromises, I’m happy with the Rohloff-designed slotted frame ends and “OEM” dropouts as the least evil, and in any event vastly superior to derailleurs and an accompanying chain tensioner.
Your mileage may vary!
Andre Jute is a novelist and painter who also keeps a bicycling page.
Copyright text and images © Andre Jute 2015. This text may be freely reproduced on not-for-profit sites as long as it is complete and unaltered, including all the links, illustrations, and this copyright notice. Commercial, print, broadcast, other use, contact the author.