Maelochs

This is prima facie wrong. Go shift and watch the motion of the derailleur cage. The derailleur does not only push from side to side, it also swings up and down. Up. and Down. "One way of another" implies there is only one plane .... in fact the derailleur moves in an arc through space, in both the horizontal and vertical directions.

When I say "In Fact" I am not speaking rhetorically. You can actually See the derailleur move and you can see that in the "low" position it is indeed closer to the BB and farther away from the BB in two planes -- it both higher and further outboard---and this is VISIBLE. You can see it. You can measure the relationship between the BB and derailleur cage in each position.

VISIBLY, he front derailleur both lifts the chain vertically and pushes the chain horizontally.

Again I say, the line "Who your going to believe, me or your lying eyes" was meant as a Joke.

smd4

Of course it moves up. It has to clear the chainring. That movement imparts ZERO upward or downward impetus to the chain. I guess since some FDs also move slightly forward or backward, that you're going to tell me this makes the chain's motion faster or slower?

I am actually starting to believe a lot of people here have no idea what they're talking about.

Maelochs

So .... the derailleur cage goes up and down, but the chain does not?

I guess the derailleurs on my bikes---and I have owned several dozen---were all different than all the rest of the derailleurs in the world.

Cool.

smd4

How do you explain how old FDs, like Simplexes work--where the FD literally just moved side-to-side, with zero upward movement? Those must baffle you.

tomato coupe

The front derailleur can only push the chain sideways. The up and down motion of the chain comes from its interaction with the chain rings.

rm -rf

That 25-622 tire size is for road bike 700C wheel. (I wondered if this was for a small wheeled bike...no.)



Speed ranges for 11-40! and 34/70! Ha, two more shifts above 30 mph. Charts clipped from Mike Sherman's Gear Calculator
At typical road bike cadences:


At very low cadences, note the graph stops at 40 mph here.


~~~~~

12-25 and 34/70! No overlap at all between the two chainrings. That's really annoying. Needs electric shifting, the front rings will be shifted often, and the rear will go two or three clicks at a time.

genejockey 

While this is certainly true of those FDs where the cage is made up of two flat plates, more modern derailleurs often incorporate some kind of shaping of the inner cage, which looks kind of like a ledge. Now, I will grant you that the bulk of what shifts to the big ring is the chain simply being shoved into it and then lifted by the pins and ramps on the inside of the big ring. But IIRC, Shimano made claims BITD that their derailleurs actually lifted the chain to enhance shifting. And as we all know, no bike company would EVER make hyperbolic claims that don't really comport with reality!

genejockey 

I'm trying to imagine what you'd have to put on the inside of the big ring to lift the chain from 34 to 70 teeth.

tomato coupe

Ledge or no ledge, the amount that a front derailleur moves vertically is tiny, so it could never raise the chain a significant distance. Your statement in bold pretty well summarizes how front derailleurs work.

Trakhak

I'm not sure he was just BSing. It appeared from what tellmethetruth wrote that he saw a lot of skinny guys riding around in packs and genuinely concluded that a guy with actual muscle could ride much faster than they could. Apparently he was built more or less like a linebacker.

Some of us told him that it wasn't the gearing that was the problem, it was aerodynamic drag. He was sure we were wrong. I even provided the famous Robert Forstmann toaster video to show that there were track cyclists with big legs but that even they wouldn't be able to hit 50 mph riding alone, regardless of the gear. He wasn't dissuaded.

livedarklions

On an unfaired upright bike. That's not what's being contemplated here.

I also don't think anyone would expect him to hit 70 mph. I think the experiment of how fast it could go is interesting.

livedarklions

Here's the post that made me convinced he was just lying about everything:

https://www.bikeforums.net/20845741-post96.html

He claimed without proof that he was able to sustain 40+ mph in the flat, no wind assist. for 5 minutes using a 53x11 gear combination. He then refused to answer anything about faring

Trakhak

Yes, I forgot about that post. Wonder how he came up with the 40+ mph, since he apparently didn't have a bike computer---he decided to order one after his thread gained momentum so that he could prove it to us doubters when he hit 50 mph.

There were some exchanges in the thread about the test run conditions: would it be captured on video? Would an independent witness---preferably someone already known on Bike Forums who lived in his area---be present? His replies began to be a bit vaguer and more intermittent right around then.

I, too, am sorry that he disappeared from his own thread. I also miss the posts from his "Anyone can be a winner!!" boosters.

livedarklions

He wouldn't post a picture of his bike because the computer didn't work! At that point, someone knows they are busted.

He didn't just disappear from the thread. He hasn't posted anything since.
I think the entire thing was a piece of fiction.

genejockey 

A reasonable assumption, but an assumption nonetheless, given that the OP has not been especially forthcoming about it.

I didn't pull that number outta my ass, you know. It was mentioned above and determining the power you'd need under the best circumstances on an unfaired upright bike to achieve that seemed like it might be of interest. You're welcome to be uninterested. That's what the scroll wheel's for.

livedarklions

I have no problem with you posting the number, but I thought it was fair to put it in context.
He mentioned that he will be doing something to reduce drag, I'm assuming that's fairing. My impression is that he's just starting on this, and is at the "thinking out loud" stage. He did throw out the 70 mph number, and it's almost certainly improbable.

I like big gears and I will not lie. I hope he builds it. Don't know how fast he could go, but I'd like to find out.

smd4

Just as I expected: Crickets.

A simple question that cannot be answered by people who think front derailleurs somehow "lift" the chain upwards (and presumably also "pull" it downwards in the other direction) because they can "see it" move up and down. A simple question asking why front derailleurs that move side-to-side on a horizontal shaft can somehow shift a chain "up" cannot be answered. Or, more likely, is simply ignored because it does not comport with their false understanding about how they think front derailleurs work.

Charlatans.

The fact that some cyclists, even those who bikes, cannot grasp how a front derailleur functions is simply astounding. They claim such things like or when, in fact, they have no real understanding of exactly how the part works to begin with. If you cannot understand how probably the simplest of bike components works, and when it is explained to you call it "wrong" or "not correct," then you have lost all credibility.

I bet even a lesson from the late great Sheldon Brown can't convince them.
https://www.sheldonbrown.com/front-derailers.html

pdlamb

I'm imagining something like a wooden clockwork, except the ring of pins coming off the big ring would spiral outward. It'd be hard to do it with a ramp pressed into the side of the ring (if anyone would dare bring to market a mostly solid ring that big) without pinching the chain and jamming it between the ring and the derailer.

For a grin, think of the possibility for massive chain suck and jams on a rig like that!

livedarklions

It's a word quibble that really is a red herring and I wasn't going to waste any more time on it, but since you have this little class, I'll go ahead. At best, your statement that all that the derailleur is doing is pushing the chain back and forth is grossly over-simplified. It's accomplishing the lift by precisely squeezing the chain against the chain ring such that the lift is caused. FDs' role in this is not passive. You want to say the ring is causing the lift, and I want to say that it's the coordination between the derailleur and the ring that's causing the lift. Either way, the way you described it is grossly incomplete if you don't note that there has to be fairly precise coordination between the FD and the chain ring, and that switching to a much larger chain ring makes that more complicated.

I get that your justification for making a sarcastic comment to the OP is that you think you're the smartest guy in the room, but again my main point was that you were acting like a jerk when you said it to him. If it makes you feel better if I say that technically, at an absurd level of abstraction, you were "correct", fine. I apologize for not recognizing your complete mechanical genius. You still were acting like a jerk and you should apologize to the OP.

smd4

It's certainly not a "word quibble." It's a complete misunderstanding about how a front derailleur works on your part. Which you fail to acknowledge.

It's a simple mechanism, and my description is absolutely accurate. Didn't you read Sheldon's article?

Exactly. If by "squeezing" you also mean "pushing." Exactly as I described. Didn't you give me a lesson on synonyms earlier?

What the front derailleur does is not complicated in the slightest. Though they do have their limits. A 70 tooth chainring will certainly push them.

Again--I wasn't being "sarcastic." The OP said he wanted to understand how a FD operated. I explained it to him in not only the simplest terms, but also the most accurate. To suggest it as "sarcasm" again shows that you simply do not grasp how one of the simplest components on a bike operates. But thanks for recognizing my mechanical genius. Though a hollow compliment from someone who doesn't understand how a front derailleur works.

tomato coupe

Your posts are wrong more often than they’re right. You’re celebrating this one like you hit a 9th inning home run to win the World Series, when you really hit a bloop single in a preseason game.

livedarklions

Think you better get your alibi straight, pal.

smd4

Base hits often result in runs.

smd4

Didn't you see the post where I reconsidered? Pal?

livedarklions

Considering I have frequently described myself as the world's worst mechanic, I honestly don't care if he takes the trophy. He had no business in talking to the OP like that. Now he's pretending it was supposed to be helpful?