Look, either way, it would be a while until major bike manufacturers adopt internal gearboxes. It's sad to say, but they just wouldn't take that next step into custom building a frame around a gearbox. It's too risky. Personally I think it would be a risk worth taking, especially for one of the big boys, to just make one model with a proven gearbox. Make it a popular model, advertise the hell out of it and get one of your famous riders to race it. Isn't that how all the new innovative technologies make it to the public? Oh, and make it affordable.
Internal vs External
In some ways it's a no brainer. Having a hanging gear changer on a very vulnerable section of your bicycle seems counterintuitive. It dangles, it moves back and forth and it moves the chain about. Doesn't seem practical, does it? This is what is moving you forward; this is what drives the entire system, and to have all that exposed is risky.
There's that whole conspiracy theory (of which I subscribe) that says that manufacturers from all industries -- from cellphones to cars -- purposefully build-in expiration dates into their products. So basically, whatever the product, it will only be so long until it needs repair or replacement. You can't entirely blame manufacturers for this. How else would they make money? If every BMW or iPhone or Trek lasted 20 years with minimal maintenance they would go out of business -- quickly.
It still screws the consumer though. It actually gives meaning to that term: consumer. And the bicycle industry consumes derailleurs on a mass scale. Not to mention chainrings, cassettes and chains. These items are not made to last.
But let's put that argument aside for now and look at the actual benefits of the derailleur drivetrain.
In a nutshell, they are what works. Whether it's just that manufacturers persevered to make it work or it's actually what should be driving a bike, it doesn't matter. What matters is that they are found on almost the vast majority of bicycles, from road racers to kid's department store cheapies to downhill monsters.
In most recent times, Shimano and Sram have been making some pretty damn well-working drivetrains. Derailleurs can actually last more than one season and free-wheels are significantly less maintenance. I haven't broken off any teeth from a cassette in a long time too. It's a good thing, and about time.
In terms of weight, there's no question: derailleur drivetrains are the lightest option. Gearboxes will probably never be as light as derailleurs; it's the nature of it. The cassette gears don't need to be so robust, so thick, and the oil necessary to keep everything running smooth like in a sealed internal system isn't needed.
They're expensive too. To the average rider, a mid-range derailleur drivetrain will work just fine, if not as well as the high-end, albeit with some weight penalty. High-end equipment isn't necessary for outrides or casual weekend races. But gearboxes are only high-end. You show me a mid-range gearbox and I probably wouldn't go near it, not now. If it were to fail, then that's your whole system gone.
Unfortunately, there isn't enough of a market to support such a thing and make it an actual option. So few people actual subscribe to gearboxes, or can afford them in the first place, so that no cheaper option is viable to build. If you look at Alutech Cycles, for instance, they offer the same frame with or without a Pinion P1.18 gearbox. The Pinion option basically doubles the price. It's extraordinary. But then again, it's about the same as Sram's XX1 groupset. Shimano does do a cheaper hub gearbox option called the Alfine that has been used on DH bikes like on the Zerode G2, but this hasn't gotten much attention outside touring and cruiser bikes.
Technology surrounding derailleur drivetrains has grown in leaps and bounds. Chains, cassettes and derailleurs are stronger; chains don't fall off and those silly flimsy as hell hangers are all but replaced by stronger and often replaceable dropouts. They finally work. But for how long? And how far can they go up the evolutionary ladder? Isn't it just time to push to that next step.
Logical or over the top?
If I can be so frank and perhaps a bit crude: it's almost like the difference between male and female genitalia. Sure, men can stand up, but it also leaves everything exposed to the elements, exposed to potential punishment; just dangerously exposed in general. And guys know all too well the pain involved when things go wrong.
If you've ever smacked a root / rock / traffic sign / chicken, whatever; you know the pain of having to push your bike home. Your day has been ruined. That's the biggest downfall; the most obvious. Smack your gearbox on that same hard object and you're fine; you get away scott free.
Having the entire drivetrain in a sealed, contained package makes sense. You don't see motorcycles or cars with dangling clutches or gears hanging off the wheels, and bicycles more often go through more rugged conditions. The entire drivetrain in a simple package, in one place, protected from the elements by a metal housing. Sounds good, no?
Yes, it's heavy. But where is the weight? Speaking of Rohloff, the Speedhub still sits at the wheel, replacing the rear hub, so not ideal weight distribution. But with gearboxes like the Pinion P1.18 and effigear, the weight is kept exactly where you want it: in the centre and low down, helping with the overall balance of the bike. Unsprung mass is also kept to a minimum, allowing the suspension to move more freely.
It really does seem like an obscure idea hatched many decades ago, derailleurs, and not much thought was given to the practicalities of them. And this is mostly the manufacturer's fault, I think. Let's look into the efficiency of both systems. With a 2x9 speed setup, there are at least four redundancies, four gears that repeat themselves. A 1x11 setup solves this, mostly, but with the effigear you can choose how many gears you want and what the ratios are to be, while with the Pinion you have 18 gears, all spaced equally apart with no redundancies.
What are your options
The only thing stopping me, admittedly, is expense. They are just too damn much! I would jump at the opportunity to ride one -- any one -- of the gearboxed bikes out there just to see exactly how they feel.
Gear changes are smooth, there's no need to constantly adjust everything and maintenance is minimal. I mean, the Pinion P1.18 is said to be good for at least 60 000km. That said, what happens when the 60 000km is up? As far as I can see, there are only three options that are worth buying: the Pinion P1.18, the effigear and Rohloff's Speedhub.
This is a gem. It looks fantastic and from what has been said about it, works just as well as it looks. On a technological level it trumps almost every system out there. The total ratio of its 18 gears is 636%, way more than the typical derailleur system, which averages around 500%, and you can change gear whenever you want: no peddling is required.
The gears sit in a sealed steel housing and operate in an oil bath system, keeping everything running smooth. It may have too many for gravity use, but it is perfect for enduro. For hardtails there is a chain or Gates Carbon Drive option, whereas a full suspension frame needs an idler to account for chain growth as the suspension moves. Pinion lists a number of frame manufacturers using its gearbox, but they are mostly boutique brands doing custom titanium hardtails. For more on Pinion go here.
Similar to the Pinion, the effigear has a slightly different take. Not quite as many gears, but lighter; I could see it being used on DH bikes more so than the Pinion because it doesn't have the full 18 gears like the Pinion, but a selection: from 6-9, depending your application. The total ratios depend on the gearing, ranging from 260% for the 6 speed, to 444% for the 9 speed. Effigear would surely argue that there is no need for more than 9 if you spaced them properly, but some people might find it lacking.
It uses a three-axle system, two holding the gears with the BB acting as the third. Interestingly, effigear works on a sequential-type shifter. It looks like a grip shift but instead of twisting it moves only a little, changing one gear at a time. Effigear fitted system to a Cavalerie DH bike, more details of which can be found here. The bike uses a Gates Carbon Drive belt, which increases efficiency is because the belt does not flex as much as a chain.
Known for reliability and heavy weight, this is still the most tried and tested internal gearbox available. While it usually acts as the rear hub -- especially on commuters and the like -- like I mentioned above, Nicolai also has two frames with the Speedhub situated above the BB. This fixes the weight distribution issue of having it at the rear. Nicolai is not the only one to offer this setup: a few downhill bikes over the years have played with it; though, it is very unusual to have the chain on the left.
The Speedhub has 14 gears with a total ratio of 526%, right between the effigear and Pinion. The gears are evenly spaced at 13.6% ensuring a smooth transition between gears. If reliability and aftermarket servicing is your main concern, then this might be the option to go for.
The MTB industry is full of fads, but technologies that work are often held onto. The Rohloff Speedhub is one of those items and has so far stood the test of time. The others are full of promises and whether they make good on them only time will tell. Hopefully they will catch on, though I think it may be wishful thinking. Fact is, the derailleur is still king and won't be ready to relinquish it's crown for many years to come.