tyrion

If the FD cage was just a bit wider than the chain then the cage would have to change angle to accommodate changing chain angle. Jockey wheels don't have this issue, they can handle an angled chain without having to change angle themselves. FD cages can't be too tight on the chain and they require trim functions.

Pushing from the outside is more crude than pushing from the inside.

edit: do the modern high end electric FDs change cage angle?

cyccommute 

The front is slow and clunky because it is operated wrong. The front derailer depends on a spring to knock the chain off to the lower gears under torque. Shimano’s Rapidrise suffered from the same problem. The spring just isn’t strong enough to knock the chain off under high torque situations and, thus, the derailer just clatters against that chain until torque decreases enough that the spring can knock off the chain. If front derailers worked the same way as rear deraiers…i.e. the cable drags the chain off the larger gears to the smaller ones…the shifts on the front would be as lightning fast and efficient as the rear. Suntour made a few of the high normal front derailers (as did Shimano) that worked really well back in the days before pins and ramps on the chainrings.

We have developed the tech. There weren’t enough of them on bikes because people thought it was “weird” or the high normal front was a cheap unit. Then Shimano screwed everything up with RapidFail.

In the low normal world, SRAM made some really good fronts because the spring was stronger. Shimano went all “light action” on front and rear derailers but SRAM’s X9 front really “KACHUNKS!” when shifted and will knock the chain off under high torque situations far better than Shimano’s does.

Nope. You can get four. It requires friction shifting but it does work. Of course, these come from the old days when a 32 tooth freewheel was about the lowest you could go. An 42 tooth cog with a 22 tooth inner ring gives a gear that about the same as a 17/32.

Weight is an issue with internal gearboxes and internal gear hubs. Not a problem if you have several horse power. Big problem if you are working with 1/4 horse power.

mschwett 

it doesn't look like it does. it trims for each gear combination as appropriate, so there's never any contact, but doesn't look like it has the ability to rotate. this trim illustration seems to support that.

mschwett 

i do not think that's the entirety of the issue. a modern electronic front derailleur has no spring, and you're not fighting against anything. the motor can move either way, just like the RD or a desmodromic derailleur. ultegra or dura ace di2 front shifts definitively but still way slower than the rear. like, not even a comparison.

big john

If you can't find it, grind it.

spelger

i was thinking just a small amount wider, not as wide as it it. i measured mine as best i could: 14/32" for the width of the inner cage distance vs 8/32" for chain width. i'd not call that a lot wider. but tyrion makes the case for why that would not work and it makes a lot of sense.

i'm really just spitballing here. i am an engineer actually, just not an ME. we have discussions on how to do things differently, some ideas are good and some are not, right? if we don't try to do things differently then we are only doing the same thing and that is not engineering, that is stagnation.

i have Ultegra Di2 and i don't notice the FD changing angle at all. but i think it would be a very small change were it to do so and so probably not noticeable. what i do notice is that when shifting from small to big the FD over shifts then a second or two later backs off a bit. i can see it and hear it. not sure if that happens when going from big to small.

Maelochs

The basic premise ,... that it is easier to shift down up front than in back ... bespeaks to me of an inability to use derailleurs properly. Therefore .... you know ...

If the initial premise is flawed, no amount of reasoning thereafter will lead to the correct solution because the problem is not what one thinks it is.

Width is the main reason i see for not having more front gears (chain rings.) An 11-speed cassette is about two inches wide, and there would need to be a little clearance for the crank arm--at least to allow for the chain to slide on and off the outer ring ... plus the width of the arm itself ... so the drive train and BB would be at least eight inches wide ...

As far as operation .... I don't mean to demean anyone, but I am pretty clumsy, and I can shift even under hard load .... it does involve lifting off on the pedal pressure when the chain is unwrapping and moving, but that is not hard. If people think shifting is like pushing buttons (except for people with electronic units) then maybe they might think that the problem is the derailleurs .... but anyone who has driven a stick shift understand lightening load and shifting ratios ...... Or anyone who has ridden a bike enough to notice that shifting involves the chain and the pedals are connected to the chain.

Sure, shifting under load is loud and abrupt, and could lead to a snapped chain .... but it works ... and shifting in the rear, with modern mechanisms, is about as thoughtless and effortless as can be.

I'd suggest the OP practice. Also, good on him (her) for thinking outside the box and all ... but sometimes the box contains exactly what is needed.

njkayaker

So, you mean narrower than it is. Why isn't it narrower already?

The thousands of other engineers who have spend years on this already aren't "spitballing". You really have to do more than "spitballing". A lot more.

Some ideas are better thought out and some are not. You'll tend to get better ideas from experts than those who are not. Compared to people in the industry (who you keep insulting), you are not an expert. You have to do more work to convince people to consider your ideas.

You are making a big assumption that people (referenced above) are "doing the same thing" or "stagnating" and don't really have any evidence that is going on.

njkayaker

It's not a lot different. Shifting the rear tolerates load better than the front. That's the basic premise of it being easier to shift the rear.

This is something you don't really need to do in the rear. It's something that many (most?) people have to work at making a habit.

Rear shifts tolerate load and happen quickly enough that you really don't have to do this for rear shifts.

All this stuff means rear shifting is easier. This shouldn't be controversial (it's pretty clear). Yet here you arguing that it's equivalent.

You are saying rear shifting is easier here!

=========================

EDIT: never mind. I misunderstood what he said. (We pretty much are agreeing.)

Maelochs

You said : Apparently I was not being clear … What I said was I said this because the first post said: See ….the OP in the first post advanced the premise that shifting up front was mechanically easier than shifting in the rear …. Which I think both you and I think is completely incorrect.

Which is why I said: Which is why I think there must be some misunderstand, based on you saying Pretty much, reading is fundamental, and all that.

Maelochs

YES!!!! In fact that is what I said in ever single post. I said it because we all know .... IT IS A FACT.

It is a fact we have (most of us) learned over decades and tens of thousands of miles of riding and shifting.

Read back carefully over what I have posted. pretty sure it will be clear.

SurferRosa

Only $550 and looks like ****.

kyselad

I’ve biked for plenty of years and plenty of miles, and I shift under load both front and rear just fine without dropping my chain. My question was why the function of the FD didn’t develop more along the lines of the RD (e.g. putting the FD on the unloaded side) given the mechanical advantage of downshifting in front, though I appreciate there are various reasons this may be impractical.

Regardless, I do in fact consider it pretty demeaning for someone to tell me there’s essentially no basis for my question and I just need to learn how to ride a bike. I’m also reminded of why my participation on this forum trailed off several years ago.

PeteHski

As a real professional ME I observe this trait quite often. Armchair engineers are everywhere and it is extremely rare for any of them to come up with a better engineering solution. Especially for mature tech like a derailleur bike drivetrain.

There are genuine alternative technologies to replace derailleurs, but there is no valid case for a back-to front Nx2 derailleur system.

Let’s forget about front vs rear shift quality and start with the very basic issue of gear ratios. If you want a 12x2 with the same range as a 2x12 then you need for example a 50T large front ring and 34T small ring with another 10 front rings in incremental steps between them. At the rear you need a 10T small cog and say a 34T large cog. Do I need to go any further with this ridiculous proposal? Or is it obvious that it won’t be a viable solution?

Leinster

yes, this version from a small company nobody has heard of has an expensive, crappy-looking version. I’m saying I’m surprised SRAM/Shimano/Rivendell haven’t tried to make a slicker, more affordable version.

PeteHski

I don’t believe this for one second. We are now well into the age of electronic shifting and front shifting is only fractionally improved. It is still very much limited by the inherent mechanical properties ie chain tension and relatively low chainring cadence.

PeteHski

It’s heavy and reduces drivetrain efficiency and the industry is moving toward 1xN drivetrains anyway. So I guess the big players don’t see a viable case.

Maelochs

Front shifts are inherently less rapid because the distance the derailleur must move is greater. Whether the big jumps were up front or in the back, so long as the two- (or three-) chain ring design in used, one derailleur is going to have to move the chain a relatively larger distance.

I am not sure how a bike could be designed so that the front derailleur would shift a lightly-loaded chain .... there is not enough space up front (ahead of the chain rings, between the chain and tire) or under the BB for a jockey-wheel arrangement (safely (as I see it)) and frankly ....I don't see the need.

Modern derailleurs are amazing compared to what we had a few decades ago .... and they worked well enough back then. Perfect? Nothing is. Useful? No front derailleur has even stopped me from riding.

njkayaker

No, not quite as clear as you could be. (Consider not using the excessive/extraneous periods and spaces.)

I think his conclusion is a result of the springs (and lower friction) that make shifting to a smaller thing (cog or ring) easier than shifting to a larger thing (within the same set of things). If he's talking about shifting to the biggest cog in the rear, it could be easier to drop to the smaller ring in the front than shifting to the biggest cog in the rear. (Note that some mountain bike front derailleurs are backwards regarding how the spring works).

It's not really that. It's pretty quick to move the front derailleur.

It takes longer for the chain to reposition after the derailleur moves. The reason for this is that the gear is (generally) larger in the front. The pins and ramps to get it to the larger ring need time too to get into position, which can only happen in one (or two?) places on the ring. Dropping to a lower gear can happen anywhere along the ring.

njkayaker

It doesn't quite work this way.

Going to the small ring, the derailleur move the chain over and lets it hang over the smaller ring (so the teeth on the larger ring can't catch it). The rotation of the ring causes the chain to drop on the smaller one,

Going to the larger ring, the derailleur move the chain over so it can catch on the pins and ramps, which lift the chain up so the teeth pick up the chain.

Because there's more friction, more force (and enough) is needed. It might be more reliable to use muscle power for that since the amount of force can be varied. A spring can't vary its force, which means a spring that happens to be or get weak might not work.


I know about those. They are so rare that it's not exactly wrong that 3 is the limit (what the vast majority of people will ever see).

They aren't being made any more either. So, saying you can "get" them is misleading.

njkayaker

It's common all over the place.

Nonexperts have a "harebrained" (warning: an exaggeration) idea and can only imagine the pros but no cons. They only see the cons of established solutions (developed by actual experts) they have an issue with but none of the pros.

To do this "right", they should see the pros/cons of both sides.


I thought of that but didn't want to put the effort into illustrating it.

spelger

Seriously? did i hurt your feelings too? what on earth did i say that was insulting to the ME profession? just the mere suggestion of something different? you guys have some pretty thin skin.

PeteHski

Hurt feeliings? Insulting? Thin skin? Not me. Just a simple observation I have made.

My profession is F1 engineering with a lot of keen armchair engineers telling us how we could do it all much better. So quite a few years ago we decided to set up a company wide forum where literally any employee (outside of formal engineering) could propose ideas about how to improve our performance. About once a month we used to review all the suggestions and I don't remember a single one that made it past initial review. But it was pretty entertaining!

So I very much doubt this thread will spark a major push toward Nx2 drivetrains. They are a non-starter in terms of the derailleur concept as soon as you start specifying real world gear ratios as I did a few posts ago.

PeteHski

I was in two minds about being bothered myself.

njkayaker

This.

You are overly dismissive of the work that other people did. It's weird arrogance from an internet nobody.

Note that the "we" doesn't include you because you are the one saving the day with your "great" new idea. You have no basis for accusing other people of "doing the same thing" and "stagnating" just because you came up with some idea. "Bad ideas" aren't not "doing something different" anyway. Unless, you execute the bad idea.

No, not "just" that. The problem is that you don't seem to know enough about what you are talking about and the rest of us are stuck having to read it.