conspiratemus1

OK, in that spirit, I will say, sincerely, that your logic around the angular movement of the cage speaks to something I hadn’t thought of. Since there is no guarantee that something not thought of is not decisive—indeed it often is decisive—I’m going to admit I was logically wrong. The true state of affairs requires a test, but since your logic is compelling, the onus is on me to produce the contrary evidence (if it exists.)

Nicely done, and I learned, too.

RobbieTunes

Excellent question.
So, it needs a longer chain, but does that translate to more wrap, or more slack?
Where does clutched fit in the wrap game, besides smashing your fingers the first time you remove a rear wheel?

If you can get the wrap, can you do the wrap? (Name that movie)

hokiefyd 

An interesting twist to this conversation is the old SunTour XC derailleur:

SunTour XC derailleur (3 pulley system)

The "length" of the cage is equivalent to a short cage (draw a line between the upper and lower pulley), in terms of how the lower pulley would rotate forward and rearward during shifts. But it had larger wrap capacity because of a third pulley. I reckon that design had enough downsides (complexity, friction, cost, etc.) that it was dropped without further refinement.

RobbieTunes

So if you bend the cage, that makes no difference?

bulgie 

Exactly. The chain is the same length in big-big whether large or small pulleys, because it is essentially straight. Then in small-small, the bigger pulleys "use" more chain. Total wrap is the difference in length between the straight path and the sinuous path. Definitely larger difference with larger pulleys on the same length of cage. Apologies if I'm repeating myself but I prefer "reiterating".

-mb

hokiefyd 

If you're talking about a traditional 2-pulley cage, and you're talking about bending the cage from what we see as a straight line into a "bent line", then it would make a difference (because it would effectively shorten it).

If you imagine a straight cage that's 4" in length, then the distance between the pulley wheels is 4". The length of chain stored by that cage will be somewhat more than 4", because it wraps around the pulleys, but the C-t-C distance is 4". Now imagine you bent that cage back 90 degrees (if that were even possible) midway between the pulleys. So you'd measure "down" from the top pulley 2", and then straight "back" from there 2"...you'd have a right triangle, yes? A^2 + B^2 = C^2. Instead of a 4" C-t-C distance, it'd be sqrt(8), which is about 2.8". Actual length of chain stored would be somewhat more than that because, again, the chain is wrapping around the pulleys. But we're not changing pulley size, here...we're simply changing the distance between them.

This is the opposite of the original discussion (pulley size), and really does get back to cage length. If you keep the pulley sizes the same, then shortening the cage (or reducing the distance between the pulleys) will reduce wrap capacity.

Anything that increases the length of chain that a derailleur can store (longer cage, larger pulleys, or even adding more pulleys) also increases the length of chain the derailleur can give back to the drivetrain system, which is really what chain wrap capacity is all about, right? (And, of course, the inverse is true about reducing storage.) With big-big and a straight-through derailleur, it's not really "storing" anything in the sense that you could theoretically remove it and make a single speed and the bike would still work. But as you take links away that the drivetrain needs by shifting to lower gears, those links have to go somewhere...and they "get stored" in the derailleur cage. Of course, wrap isn't a literal measurement of the number of links in a cage, but this measurement is a good way to compare the relative storage capacity of two different derailleur designs. And the more links (or length or however you wish to measure it) the derailleur can "store" in its cage system, the increased ability it has to keep chain tension when going towards the small-small combination.

jamesdak 

No, no, no, no. That's not what I meant at all. This is both informative and entertaining while not being confrontational. I'm truly enjoying it.

Sorry if I confused folks. I do have a way of doing that sometime, LOL!

jamesdak 

Me too, I didn't mean to imply this was getting ugly, just truly entertaining. Guess this ol' geezer still doesn't understand all these social medial nuances.

jamesdak 

Man, I go out for a ride and come back to realize I was a buzz kill on an excellent thread. Never intended that, thoroughly enjoying this thread. Don't let my ignorance stop it.

I will say I've always understood things to work the way hokiefyd keeps explaining it.

hokiefyd 

I don't "get" most of them, either. It does seem that the popcorn icons come out right before threads get locked...not that one causes the other, but I think it's sort of an internet "inside joke" that popcorn being hauled out implies the environment is getting toxic and the thread will get closed down.

Nemosengineer 

Both of these wrap the same amount of chain and shift slicker than owl snot, but I think the 7703 looks better doing it.

Dura Ace 7703 GS 9 speed triple

20200418_182918 (1) by nemosengineer, on Flickr

Dura Ace 7803 GS 10 speed triple

20191020_145506 by nemosengineer, on Flickr

: Mike

70sSanO

It’s not chain wrap for big-big. You can always make any chain long enough for the big-big to work with any derailleur.

You need the wrap for the small cog (small-small) so the derailleur doesn’t fold back on itself. That is where you need to take up the excess chain.

John

RobbieTunes

At least it wasn't me, this time. I just couldn't find the angle.

The Golden Boy 

Chain wrap.

Glamorous:

620 Build Derailleurs by Dave The Golden Boy, on Flickr

1985 Trek 720 by Dave The Golden Boy, on Flickr



Then there's the 'triple pulley' theory- which works great- it wraps colossal amounts of chain, it's basically an elbow- the triple pulley isn't engaged until the elbow moves back far enough for it to engage- and you don't have to have a great big ol' arm swinging around and coming 3" off the ground.

I like the look of the red Bullseyes...

Suntour XC by Dave The Golden Boy, on Flickr

Compare that to the Le Tech- which was a triple pivot derailleur- and just happened to have a huge long arm, but normal sized pulleys.

Suntour Derailleurs by Dave The Golden Boy, on Flickr

And then Suntour went all out for the outrageously long cage AND the outrageously huge 15T pulley.

2nd Generation Mountech by Dave The Golden Boy, on Flickr

hokiefyd 

I like that way of thinking. Big-big doesn't need any chain storage, and could be setup to be truly straight through the derailleur. Wrap is to store excess chain given up by the drivetrain so the tension doesn't go slack. The more links a derailleur can store in its cage system, the more links it can take up from the drivetrain...and more capacity there is for wider gearing ranges.

Ferrouscious 

not nearly fast enough

AJI125 

This is fascinating. Love this discussion, the illustration is awesome and bulgie's explanation finally sealed my opinion. If anyone wants to entertain an example...

So I have a ~94 Record 8s RD that I swapped the short Record OR cage onto (cheap, gets a little more wrap, and only one of any length I could find). So if I keep my upper pulley the same, I can get the biggest rear cog size possible. If I increase the size of the lower pulley, I can potentially add a tooth or two of additional wrap, right?

I haven't got this on the bike yet as I need to do some frame de-rusting and then figure out which shifters I'm gonna use (8s Record through and through or 10s Chorus with a 10s Shimano cassette?). But I may just have to play around with this, as I like the lower gearing on my other bike so much. Gotta start hunting on eBay for some 11, 12, or more tooth pulleys, and maybe some spacers to get the width right, and see what I can fit in there. Eyeballing it, looked like lots of room for more teeth.

RobbieTunes

n+1/.5 watt

That's really really fast, Bernie.

madpogue 

Don't worry, you had it right. Going back at least 20 years, has always meant simply "this is interesting, I'm gonna grab some popcorn, sit back and watch (read)". Any interpretation involving conflict is a misinterpretation. Granted, "interesting" exchanges _sometimes_ become controversial, but that's coincidental.

Do the italics on the derailleur on the left have any impact on chain wrap?

RobbieTunes

No, but the bike leans right.

The Golden Boy 

I don't think so- but the one on the right is clearly more aerodynamic.

dazz100

maschwab

I remember seeing an infrared video from this research project www dot georgeron dot com slash 2007 slash 11 slash bicycle-efficiency-boosters dot html . Despite several searches, it doesn't look like it is available anymore. It showed a little heat on the chainring, a little more heat on the cassette, and the most heat on the jockey wheels. The heat generated is directly proportional to the angle the individual chain link has to turn to go around that pulley.

stardognine

Replying to remind myself to re-read this. I think mine is one of those oddball Altuses. 🤔 Whichever it is, it seems fine for touring. 👍

old's'cool

Interesting... While it (i.e. the infrared video) shows the temperature, and you mention a correlation with link angularity, it does not follow that this is a causal correlation. it is highly likely that the chainring, having the highest thermal mass and heat transfer, is quickly dissipating any heat that is generated (and thereby staying close to the ambient temperature). In second place in these properties would be the sprocket (smaller, steel), and bringing up the rear are the jockey wheels (smallest, plastic).