Dropbar MTB, pt 2Comments
by Mark V on Dec 08, 2008 at 2:44 AM
Continuing from Pt1, I want to put a dropbar on an mtb, but one needs a plan to get around the many pitfalls. The first problem is brakes. Since caliper brakes are not an option with an mtb, that leaves cantilevers, v-brakes, or discs.
Once upon a time, mountainbikes had cantilever brakes. With enough fiddling with straddle wire geometry, you can often get acceptable performance with dropbar levers and cantis. But on a bike my size, the rear brake bosses are too close to the seat tube to leave enough room to play with straddle wire height. And of course, who nowadays makes a decent suspension fork with a canti hanger on the fork brace? Scratch cantis.
With V-brake, the geometry of the arms is fixed, and the brake requires a lever with low mechanical advantage and a lot of cable pull to work. And those attributes do not describe dropbar levers. There are some adapters such as Problem Solvers’ Travel Agent that amplify the levers’ cable pull, but they suck. They introduce a lot of friction into the system, are hard to balance the left/right arm spring tension, and require that the pads to be run really close to the rim to get anything near acceptable performance. Plus, I would have issues with the rear cable run on a small bike again because the short housing would over power the spring adjustment. Verdict: not attractive.
Then we have cable-actuated discs. I had tried out the Shimano BR-R505, and I found it’s performance was reasonable. Shop experience suggested that they were more durable than the Avid version, and I like Shimano’s Centerlock rotor fixing system. Okay, we have our brake.
Next is the shifters. Sure, I could put on bar-end shifters for both derailleurs and been done, but where’s the fun in that? Integrated brake/shift levers are too convenient to give up, so bar-end shifters are would be the back-up plan. One derailleur at a time, let’s deal with the rear first.
Full disclosure: it was probably going to be Dura Ace no matter what, but in the end, it made the most sense. I wanted to avoid cable-pull adapters like the J-Tek Shiftmate if possible, so that ruled out Campagnolo. Ergopower shifters work only on Campag cassettes, which don’t fit mtb rear hubs…as in 135mm and disc-compatible. It also ruled out SRAM because SRAM DoubleTaps don’t work with mtb derailleurs and there are no SRAM road triple derailleurs. So it was going to be Shimano, but would it be 9 or 10 speed?
If I went 9sp, I could use an XT 11-34 cassette. There are no Shimano or SRAM 10sp 11-34 cassettes, and anything else out there is crap. On the other hand, I already had a 10sp righthand STI plus an XTR M960 crank that I got on closeout. Since my old mtb had a 2x9 set-up (44/29 and 12-34), I figured that I wouldn’t really need a 22x34 low gear. Besides, in the old days racers used 11-28 cassettes. I’ll run an Ultegra 12-27 cassette with a Tiagra triple rear derailleur.
Okay, the rear shifting is chosen, what about the front? Well, neither SRAM nor Shimano has a solution for running a dropbar integrated brake/shifter with an mtb front derailleur. SRAM DoubleTap is double only, and Shimano road and mtb front derailleurs have a different pull ratio. My bike requires a Shimano E-type mtb front derailleur. In the end, I chose a bar-end shifter for the front shifting duties. It would not be as accessible as an Ergo but it would be simple and guaranteed to work at least. And it would be nearly indestructible. For braking, I would use a Dura Ace BL-7402, the finest aero lever ever made.
What else could I have tried? Well, front shifting leaves you pretty limited if you go with a typical 44/32/22 mtb crank on an external BB. Since the BB spindle is integral to the crank, you can’t do anything to bring the chainline in closer. The chainline is so far outboard that a road front derailleur will be maxed out and swinging upward more than outboard at the end of the stroke. That in turn leaves a big gap underneath the cage to the big ring, which is an invitation for chain drops or at least crappy shifting. Thus, since I’m running an XTR M960 crank, I pretty much have to use some kind of mtb derailleur.
I couldn’t use a road STI like Dura Ace for front shifting duties because the mtb front derailleur needs about 19mm to swing from the granny ring to the big. A road STI triple shifter only pulls about 13mm total.
If I were more of a Campagnolo fan, I would have chosen a left-hand Ergopower shifter (non-Escape type) since it pulls round 19-20mm of cable, which is about the same total as required of a Shimano mtb front derailleur. Since the (non-Escape) Ergopower has many indexed increments, there isn’t a problem with trimming the derailleur. However, I’m not a fan of Campagnolo’s ergonomics, since the thumb button upshift is hard to reach from drops with my small hands. Arguably a bar-end shifter would be still be a step down in ergonomics campared to Campagnolo, but I’ve also seen too many Ergopower levers break on a hard impact. On a road bike, I don’t really plan on taking crash damage, but just about every spirited offroad ride I embark on guaranties at least one biff. Yes, you can rebuild the Ergopower on a new lever body, but you won’t be doing that on the trail.
SRAM DoubleTap shifters are completely useless for a mtb triple crank, since they only work for double chainrings. SRAM mtb front derailleurs have the same cable pull needs as Shimano mtb, so they are interchangeable but don’t help us at all with the shifter issue.
I am somewhat intrigued by the notion of a 2x9 system with a dropbar, but I’m not keen on the need for a Jtek to match a 10sp STI to a 9sp cassette. It works fine on a lot of touring bikes, but mtbs get banged up and filthy quite a bit which might compromise the shifting precision. A Jtek system can be sensitive to tuning even on a very clean system, and they won’t fit the newer Shimano Shadow-type rear derailleurs. I could go with a 9sp STI, but I think the newer 10sp models are more durable over all. But maybe the leftside STI would work on a 44/29 double chainring crank. Actually, it might work out well to use a left-side STI on a mtb front derailleur, since the STI’s total pull of 13mm is almost the same as the first pull on a mtb front derailleur. Maybe the triple STI’s index points would be useful trim positions. It would probably also depend on the chainline of the mtb double crank. Typical mtb cranks by Shimano and others have a chainline of about 50-51mm from the center of the seat tube to the middle of the chainrings, whereas the chainline of mtb double cranks (such as FSA or Rotor) have no established standard. Keep in mind that a cross-country double wouldn’t necesarily have the same chainline as an all-mountain double crank, which is essentially a triple crank with a bash guard replacing the big ring.
Perhaps an older style 3-piece, square-taper mtb crank might make things easier. With square-taper, you could cheat the chainline issue with a narrower spindle. Another possibility would be a front derailleur adapter that would place a braze-on type, road front derailleur a bit more outboard than normal, and thus improve the travel profile of the road front derailleur. However, as this option is beyond my means to investigate, we’ll probably not get an answer on this hypothetical question.
In pt3, I’ll detail the rest of the bike and how it rides, and what I would like to change.