Timj219

I know this must be a stupid question but I don't get it and haven't found an aswer.
The signal is sent to the derailleurs through a programmable control box. That means changes from gear x to gear x+1 can be accomplished without needing the rider to remind the controller the required change needs derailleur moves at both ends. It seems to me logic can be programmed that chooses the best front back choices even if the same gear-inch value is available using more than one combination.
So what's up with the extra shifter?

Maelochs

Not sure i understand ... but i wuld imagine, just as with mechanical shifters, there are times (hillclimbs come to mind) where the quicker shift up a the cogs is better than the slower shift down the chainrings.

If I know the route I might start a hill on the big ring, but if I don't know the hill it is usually better to be on the small ring ... so I can shift really quickly and not lose much momentum.

I know when I used to ride the same MTB trails regularly there were times I'd shift down up front and up in the back a while before a hill so I wouldn't have to change chainrings on short, sharp climbs.

Cyclist0108

That is an insightful question.

I think the answer is to make it as similar to the mechanical shifting process as possible, so people don't have to learn completely new shifting protocols.

Timj219

Considering the cost of electronic shifters why is an integrated cadence/force sensor not included? Then the control box coukd be programmed for the rider's preferred values and make all the shifts automatically?

Timj219

I guess that makes sense. The shifters can't see what's up ahead.

I've never needed to worry much about the speed difference between rear and front changes so it didn't occur to me. I do understand the ease of one chain ring chage instead of multiple sprockets. But then almost all my riding has been done with friction shifters. It seems that with programmable electronic shifting "ease" is pretty much guaranteed.

dsaul

Fairwheel bikes has a fairly extensive explanation in their blog here. Shimano Di2 Synchro Shift for road bikes? - Fairwheel Bikes Blog

blakcloud

That would take one of the fun parts of cycling out of the equation. There is an art to shifting or some would call skill and it is part of the experience. If the computer did all this for me, I might as well go single speed.

dedhed

Because electronics can't read minds.

TimothyH

What the OP describes already exists. Shimano Synchro Shift.

https://bike.shimano.com/content/sac-bike/en/home/technology0/technology/synchoshift.html

You shift up or down and it figures out whether to shift the front, rear or both.


-Tim-

jefnvk

I'm guessing because such systems are designed for competition, and as demonstrated by even the most expensive race cars still utilizing manually controlled gearboxes, top level competitors don't like computers making decisions for them.

I bet you see something exactly like the aforementioned fully automated system, if the technology ever trickles down to recreational riders.

Marcus_Ti

Uh....Ultegra Di2 6800 shifters cost $200ish USD. A power meter costs $500. AKA a power meter costs 2x what the shifters do.

Cyclist0108

Then we would see posts like "why does my bike insist I climb my hill in gear X, rather than gear Y?", where X is a much harder gear that someone at Shimano thinks you really ought to be capable of pushing.

It is bad enough the Di2 designers took away my triple.

BobbyG

Last year I bought my first car with an automatic transmission in 30 years; a Nissan Rogue with a CVT (continuously variable transmission). Traffic has gotten too busy out here for a stick. I actually like the CVT, as I now drive in a relaxed fashion and the CVT handles things in a smooth, relaxed manner with no discernable shifts (because there are none!).

People have asked me if I will miss shifting. I tell them, no, I still shift gears on my bikes, and seeing how I ride to work 3-4 days a week, there's still a lot of shifting.

My buddy just bought a recumbent trike with a CVT in the rear hub. I haven't had a chance to ride it, but as long as I can shift, instead of a computer, I might like it.

indyfabz

If I ever get to the point where I want my bike to tell me which gear I should be in it will be time got me to hang up my cleats.

wphamilton

Why not? The roads are pretty well mapped out.

But on the original post, I think it's a good question. Electric shifting could shift both gears as easily as one, and I'm skeptical about a delay or lag on the front being a significant drawback. I'm inclined to says that it's purely market resistance and not technical.

Slash5

I believe on MTB setups that you can program automatic front shifting.
However, I can see that may not be the best option. Under load, a front shift is always less reliable than rear shifts.
If I was climbing steep short sections I would probably cross chain on a short flat section for a few seconds rather than shift the front, knowing I'll be back in a steep climb in a few yards. However, automatic front shifting would shift the front to keep me in the center of the cassette and then shift back down on the next climb - just asking for trouble.

Even on the road, it's common to shift up on a hill and stand, then shift back down and sit - you wouldn't want a front shift when you are doing that.

BikeLite

one for front derailer and one for back? I know nothing about elec shifting ; )

TimothyH

OP isn't asking about automatic shifting.

He is asking why the bike can't shift to the next higher or lower gear with one shifter instead of two, why the bike can't figure out on it's own whether the front, rear or both derailleurs need to be actuated to get to the next higher/lower gear.

This already exists. Shimano Synchro Shift.

Synchro Shift is not automatic shifting but linear shifting - you go up and down the gears and the bike decides whether to shift front or back derailleurs for you.


-Tim-

Cyclist0108

One simple reason is that you might not want to go through up to 21 shifts. Separating front and rear shifting gives you a shortcut from say 50x11 to 34X32 that only involves 11 shifts, not 21.

Timj219

Great article thanks!

Timj219

My bad I was just seeing the total cost of the electronic system and assumed power sensor cost would be trivial in comparison. I didn't realize a torque meter cost anywhere near that much. Certainly the cost of logic to integrate torque/cadence data with electronic shifting seems trivial. And once you've combined torque/cadence/curret gear you have all that's needed for auto transmission.

Timj219

Yes that's what I wondered about. All the information I was coming across indicated a need for 2 sfifters. I managed to miss this somehow.

Timj219

Thanks for the answers I hadn't realized that the one shifter system does exist. I also hadn't considered that electronics are mostly a pro thing for now. Linear (thanks Tim didn't know there was a name for it), and certainly automatic, shifting will probably have greater appeal for recreational riders. I'm not sure how big the recreational market for $2k bike transmissions is. But you have to figure when the price of the electronic groupsets drops then single pushbutton shifting will be common and maybe even auto shifting.

I'm not sure I'd be interested but then again I'm still not positive I need indexed shifting and aluminum or carbon frames so the bike companies would go broke focusing on people like me

markjenn

Not mentioned yet is how typical 2x gearing systems on bicycles aren't well setup for linear shift schedules which shift both derailleurs.

Take a typical 11-32 cassette and 34/50 chainring combo and write down all the ratios and how one has to shift the bike to go through these ratios in a linearly increasing fashion. It's a total mess....you say on one chainring for a few gears at the very top and bottom, but otherwise, you're shifting back and forth between chainrings on nearly every shift right in the 13-18 mph range that most of us ride. And the gaps between gears vary widely - often a double-shift will result in only a tiny change in gearing while at other times the jumps are just as big as if you were shifting through the cassette staying on one ring. Do you want to move the shift lever one gear and have it shift both derailleurs on nearly every shift while giving you a 6% change in gearing on one shift and a 0.5% change the next? And having no control over cross-chaining?

You could potentially optimize the cogs in a somewhat different way to avoid this problem but you can only go so far as any cassette which spans a wide range of gears will naturally have breaks where the step in gearing has to change from, for example, 1-tooth to 2-tooth. This plays heck with any system where you're trying to keep double-shifting gear changes uniform.

Most folks prefer to stay in one chainring as much as possible and enjoy the relatively smooth and regular gear steps you get by just using the RD to step through the gears. Then go to the bigger/smaller chainring if they run out of gears. Folks who ride pacelines or race may have dissected their gearing to know they need to double-shift to find an "in between" gear for a particular situation, but these folks are few and far between. With 11-speeds the increments on just the RD are actually pretty small, so any advantage you'd get with a system that started including the FD in the shift schedule is very marginal. Most riders have two front chainrings to improve the overall range of gears they can choose and to allow the cassette to have smaller steps while they stay on one chainring.

And there's another much more rationale way to get single-lever shifting that has a whole host of benefits apart from the single lever: 1x. It has some overall gear range limitations at the moment, but they're less with every iteration.

- Mark

Marcus_Ti

The problem with 1X....to get as wide a gearing range as a double, you necessarily have to have gigantic cassette spacing. An 11-46 for example; 11-13-15-17-19-21-24-28-32-37-46, 2T jumps minimum (not fun for roadies)...up to 9T (uck)?


Di2 Synchro offers the best. You set when the FD trips over to get as many desired ratios as possible. Doubles have ratio overlap necessarily (or pretty close), but you'll have much close spaced gearing with a minimum of fuss....and you get the simple/stupid up/down shift interface. Although I don't understand why it is so hard to use your FD even on a cable-actuated transmission nvm a Di2/eTap/EPS one. It isn't rocket surgery