Getting to the Bottom (Bracket) of Things
All of the other units tested here are currently manufactured. But it has been a couple of years, as far as I know, since the last bottom bracket generators were sold in the US. They were victims of the success of dynohubs in a way that nothing else could be, because—in many respects like the dynohub—bottom bracket dynamos were invisible.
Well, let’s consider what a bottom bracket dynamo is. It’s based on a small metal platform that fits between the chainstays ahead of the rear wheel. Extended behind and below the platform is a metal roller, often but not always coated with rubber. A lever, or in some cases a cable arrangement, allows that roller to be brought into contact with the tread, rather than the sidewall, of the tire. From the drive side of a given bicycle, a bottom bracket dynamo is invisible. From the NDS, it looks something like this:
(Sorry for the photo quality–shot the day I was testing for wet-weather slippage.)
Pressed one way, the lever at the front (projecting WSW in the photo above) releases a catch and a spring presses the roller against the rear tire. Push the lever the other way, and the dynamo moves forward and locks away from the tire. You can see the white, rubber-insulated wire that connects to the ”hot” side of the generator coil snaking its way around the mount.
The bottom bracket dynamo is similar to, but superior in two important respects to the sidewall dynamo.
First, unlike the sidewall unit, it presents no risk to fragile sidewalls. It runs on the same tread that the weight of bike and rider applies to the road.
Second. It is tucked out of the way and is, as described above, practically invisible.
However, those advantages come at a cost: more than any other generator, the bottom bracket unit is exposed to the worst that the road can throw at it. It’s more likely than any other to encounter guck from the wheel and from the road. Ride through puddles? The bottom bracket dynamo is likely^H^H^H^H^H^H GOING to get wet.
Interestingly, some users figured out ways to mount the dynamo in other places—for example, near the fork or at the front end of a front fender. This prevented slippage due to the roller getting wet and (in the case of top-mounted units) allowed for some attempts at integrated light-and roller units.
The heyday of the bottom bracket generator was probably the 1980s (I say probably because I haven’t found a good historical source; one may exist, but if so, I don’t know of it. This means that if you want to try something like this, you’re going to have to buy a used unit. Take heart—bottom bracket dynamos, the most common of which appears to be the Sanyo Dynapower shown in the photograph above, are fairly common on eBay and can generally be had for less than $50.
Costs Associated with Bottle Dynamos
If you want a bottom bracket dynamo, you’re looking at finding one used or, alternatively, “NIB.” Either way, there’s really only one place to go these days: eBay. Sometimes they’re out there, and sometimes they’re not, but bottom bracket dynamos seem to turn up nearly as reliably—if not as commonly—as downtube shifters. They are far less common than bottle dynamos, however.
You can either go cheap, or expensive. Cheap will mean used, $50 or less, and that’s fine. If you want expensive, that means you’ll be buying an NIB—“new, in box”—unit, and those can run up to $200 (which is, solely in my opinion, kind of ridiculous).
The Sanyo NH-T6 unit I have, complete with its cast alloy mounting base, weighs 254 grams. The NH-T10 that Peter White used to stock looks like it might be a tad bit lighter. So a bit more than a bottle dynamo (at least than the Dymotec) but no additional mounting equipment is required.
Here are three reasonably common bottom bracket generators, so you have a better sense of what I’m talking about:
Sanyo NH-T6 (also shown in the slippage photo above):
Sanyo NH-T10 (note the rubber cover on the roller):
There’s only one bolt—the one that clamps the upper and lower portions of the dynamo to the chain stays—but that doesn’t mean that this can’t be tricky. Bottom bracket dynamos are designed to sit in the “V” formed by the chain stays as they come together forward of the chainstay bridge (if one is present). Some bikes, I am told, have a mounting plate (probably the same one used for a kickstand) to which you can mount the dynamo. Mine fit just fine on two frames I tried, but I could see with oddly shaped stays, you could have a problem. And you wouldn’t want to clamp this onto carbon stays, it should go without saying, and, possibly, on some aluminum stays. Further, part of the clamping process penetrates the finish on the stays to ground the dynamo to your frame, so be aware!
It’s important to get the roller aligned properly with the tread of the tire, which the V-shaped nature of the stays makes comparatively easy. When it’s released, a spring will hold the dynamo’s roller against the tread. The only decent way I could find to adjust tension was to move the mount forward or back between the stays, varying the distance between the disengaged roller and the tread. That’s probably not ideal, but it worked well enough. I may be missing an adjustment somewhere. The NH-T10 can be remotely activated, in which case it would probably be cable tension holding the dynamo in place rather than a spring, but I cannot verify this; it’s possible the cable is set up simply to release the dynamo.
Bottom bracket units generally provide a single wire connection, since they ground the other side of the circuit to the frame. The weird thing about connecting them is that, of course, the bottom bracket is nowhere near either the headlight or the taillight. You can just run both head- and taillight connections to the bottom bracket and hook them up there in parallel, or you can run a wire up the seat tube (that’s how I did it) to connect things near the seat tube cluster. It’s not difficult, in any event, you just need a little more wire.
Overall, I was quite pleased with the performance of this dynamo, especially given my earlier experience with an identical model (some ten years before). It may be a matter of finding just the sweet spot for mounting. The dynamo ran very acceptably with 28mm Pasela tires.
My previous experience with bottom bracket dynamos was that they slipped, and badly. This one did not. I suspect several factors here. First, my prior experience was with incandescent halogen lighting, which needs a fairly constant flow of power (and a lot of it). With such a light, any slippage becomes instantly noticeable as flicker. With the eDelux headlight, on the other hand, a capacitor is charged by excess power from the dynamo and can “cover” small slippages. Further, when I rode in Wisconsin, a good part of my commute was over trails covered in limestone screenings—essentially dust, which turns to slippery mud when wet. By comparison, most of my wet miles during this test were run on asphalt surfaces, including extensive dips into puddles. I would guess that a combination of good surfaces—surfaces that didn’t put a lot of muck on the tire tread—and good adjustment were key. Again, I did not ride this unit into a downpour, but in moderate rain, with fairly extensive puddles, I had no issues.
Like the non-limited bottle dynamo, the bottom bracket unit—correctly adjusted— generates plenty of power.
While, as noted above, some units could be handled via a cable-and-lever assembly, with the NH-T6 you really need to dismount to turn the power on and off. There’s no other good way to reach under the bottom bracket!
If you’re not on the bike, the bottom bracket unit can appear very noisy. But once you’re riding., its placement makes it very quiet in operation. It’s about as far away from your ears as anything can be on a bicycle, and it’s not quite, but almost, behind you. It’s easy to leave the noise behind. Other riders, however, may suspect that you’re being boosted by a small electric motor.
I didn’t notice any significant vibration with this unit. Perhaps a little at very low speeds, but overall, anything that was there was drowned in road surface “noise.” Most bottom bracket dynos have either a light tread pattern somehow impressed into the metal roller, or else a rubber cover over the roller, and since this is running on virtually the same surface as the rest of the tire tread, it’s hard to pick it out even if you’re trying.
Does the dynamo cause resistance, and can you feel it?
The situation is similar to that of a bottle—more than a dynohub. In addition to the magnetic resistance of the device itself (which is lower than that of a dynohub) there is mechanical resistance as the roller turns against the tire.
Can you feel it? Yes. This is particularly true at low speeds, but diminishes pretty rapidly as you spin up—likely because a greater proportion of your work is going into the contact between tire and road than between tire and dyno.
Like the sidewall dynamo, the bottom bracket unit can be completely disengaged when you don’t need it, so there is no wear and tear on tire or dynamo, and not resistance.
One thing worth mentioning at this point is that a bottom bracket dynamo frees you from concern about tire sidewalls. You can have concerns about sidewall fragility, but you generally trust your tread. The bottom bracket dynamo exploits this—both tust and tread. However, you are limited to tires with invert, slick, or minimal tread patterns. A bottom bracket dynamo will not do well with knobby cyclocross or MTB tires—for those uses, you need to find something else.
No parts are available. When you deal with a bottom bracket dynamo, you are dealing with “obsolete” technology and you’re on your own. If you’re good mechanically, you can carefully clean the roller and surrounding mechanical parts, but that’s about it.
Bottom bracket dynamos are tough. They have to bit, given where on the bike they live, and their comparatively higher weight probably reflects that. Given the number that are available on eBay, I would venture to say that most of us will be outlived by these units. That’s not to say that they can’t be damaged, but that they seem to have established a long service life.
If there is any dynamo less visible than a hub dyno, it’s the bottom bracket generator. From the drive side, invisible, and from the NDS, all but. Nor are they likely to invite “tweaking” from a malevolent passer-by; the dynamo is tucked, safely, well below eye level.
So, what’s not to like?
The bottom bracket dynamo is in a great position from the perspective of the rider, but a poor one from the perspective of the mechanism. It’s exposed to water, dust, mud, and—really—anything that the front tire throws up or that the rear drops in (the latter especially a factor if you’re using fenders). During testing, at one point the dynamo started to make a really awful noise. It turned out that a bit of twig was caught between the roller and its built-in “mini fender” and it sounded like the bike’s rear fender had been bent into the rear tire! (The mini-fender is there, by the way, both to provide a little protection for the bottom bracket generator from materials thrown back by the front wheel and the protect the bottom bracket area of the bike from any mess that the generator throws back.
Bottom bracket dynamos limit you in terms of tire selection, and they may—depending on the type of riding you do—be more likely to slip than any other kind of dynamo. This is especially a consideration if you ride on dirt or limestone (or other loose) paths.
Finally, they can make it harder to insert or remove a rear wheel, especially if you have horizontal dropouts, since they impinge–like fenders–on the tire’s space to move.
The final unit I will be reviewing is a philosophical and physical reworking of the bottle generator, manufactured by Velogical. I’ve been riding with this unit for several weeks, and I have an interesting relationship with it. Stay tuned!