I just bought a new Shimano Alivio RM-410 rear derailleur (long cage), to replace an old beat up XT rear, circa '96. This is on an Xtracycle build using my old '94 Stumpjumper Pro as the donor bike. When I was searching for an affordable, sturdy derailleur, I was struck by how many seem to sport enormous pulleys compared to derailleurs from 15+ years ago, when I was last wrenching on my bikes (a bit of a hiatus of over a decade when I didn't do any cycling). So, what's the advantage to these pulleys which seem to be a third larger in diameter then what I remember pulleys looking like back in the old days?

If you look at current XT's, the size of the pulley wheel hasn't changed since 1996, or before than.

The big change in higher end MTB derailleurs is the clutch system, which XTR had last year (Shadow Plus), and has now filtered down down SLX, if you ride off road, it's a nice thing to have.

Larger pulleys bend the chain links less as it passes through them. It might save a bit of wear?

Larger pulleys increase the effective cage length compared to standard pulleys, but that could have easily been done by increasing the actual cage length. IMO, we see larger pulleys on some derailleurs these days simply so makers have something different than before.

Likewise those bright red pulleys. There's no inherent difference in plastics based on color.

Larger pulleys rotate slower putting less wear on the bearings.

Absolutely true. Another brilliant solution to a non-existent problem.

I can see increasing the effective cage length (without actually increasing it), along with less wear on bearings to be a couple of reasonable sounding design benefits. Considering one review has a guy racking up over 20,000 miles it must be a robust design.

20,000 is nothing to pulley bearings. 30-50,000 miles is not an unreasonable expectation. I've never had pulley bearings wear out before the pulley itself had excessive wear.

I'm not saying that there's any disadvantage to larger pulleys, but given that pulley size was about the same for about a century, I find it hard to believe that after all this time, some light bulb lit up with a brilliant discovery that bigger was better.

Similar discussion on C&V has the definitive answer in post #10:

https://www.bikeforums.net/showthread...ar-derailleurs

Another similar discussion on RBR:

https://forums.roadbikereview.com/com...rd-268328.html

FYI, only the top pulley is a jockey pulley. The proper name of the bottom pulley is a tensioner or toggle pulley. Increasing the diameter of the toggle pulley wraps wraps about 50 -66% more chain than increasing the length of the cage by the same amount. The actual percentage will vary depending on the cage geometry. Thus you can increase wrap without decreasing ground clearance as much (an issue with ATBs in off-road conditions). A dirty chain is also less likely to bind and skip on a bigger pulley. It also reduces the deflection of a longer cage. Finally, it looks different and this must not be underestimated.



Now this makes the most sense of all, and frankly since this is on a longtail cargo bike, anything that helps the chain behave is all good!

That edit of yours with the extra link helps explain some more and also reiterates that point about shorter derailleur cages. However, in the case of the Alivio derailleur I have, the cage is just as long as the old junk XT that it is replacing. So, the shorter cage argument goes out the window. I guess there could be less stress on the drivetrain due to the larger pulleys but I don't know how much difference it would make in this setup. Or, maybe it's all marketing hype - some pseudo science that makes something seem like it's a good idea when really it isn't. Remember all those Biopace chainrings? I don't think larger derailleur pulleys are in the same league as Biopace at all, but in a long cage design I wonder what the benefits are supposed to be. In both of the links you posted, the info is pretty much all regarding large pulleys in short cage derailleurs. I'm still as mystified as when I started this thread.

Air resistance.

The larger pulleys result in cleaner airflow around the cage.

And gravity.

They're bigger and the increased gravitational forces attract dirt away from the chain.

OCD overthinker? Me? Never.

Larger pulleys and/or longer cage lengths, are used to increase the "total capacity" of the dérailleur (this "total capacity" determines the ability of the dérailleur to take up chain slack). A higher "total capacity" dérailleur can be important when used with chain rings and/or cogsets "abnormally" greater size differences between the largest and smallest sprockets and cogs.

Thus, dérailleurs that offer a larger "total capacity" give them the ability to handle larger differences between the largest and smallest sprockets and cogs. However, there are many important benefits to running as short a cage length as possible with your particular chain ring/cogset (improved shifting being the main benefit).

Another positive factor in running larger pulleys is that they tend to operate smoother than smaller pulleys (pulleys and sprockets that are under 17 teeth don't run as smoothly as those with between ~20 - 67 teeth).

So it is just a guy thing then like "bigger is better"? Turns out most ladies want larger also apparently...and most component OEMs would love for there to be a reason for lots of otherwise adequately-thinking folks to envy a new set of expensive bits they just happen to sell to replace the older bits that rarely wear out naturally. It has to be a win-win-win and thus its unavoidable apparently. So how long will it be before we see a fully Scandium-alloy component set with carbon nanotube little bits holding it together and provide a 15-speed cogset with quad cranksets to achieve 1 tooth increments top to bottom.... geez the topic is as boring as the gram counters isn't it?

the bigger pulleys don't come on the top tier derailleurs, there the difference is 1 tooth..
Campag, used 10t, Shimano and others may have 11 tooth pulleys..

but with Shimano the answer lies in Osaka, the designers, at Shimano HQ.

Odd number teeth wear better with even numbered links and vice versa?

We should probably observe where the big pulleys (13t) show up.

1. They are on Alivio and lower classes like Altus, Acera and maybe 2300 RDs. So the standard higher end RDs still sport 11t.

2. Lower end pulleys use BUSHINGS. Not bearings. Even higher end are still using bushings for the most part. But we're talking simple, cheap, bushings. Maybe after-market has bearings. Regardless, wear was never a problem here on the bushings. The problem was tooth wear in the face of cheap plastic. The higher end RDs can sport expensive polymer pulleys. But on the lower-end, with thinner teeth to accommodate 8/9 speeds, this is a cost factor. Solution: More cheap plastic to reduce individual wear so there are 18% more teeth and therefore 18% longer wear, which is lost because now the teeth were thinner. But we can now get the same wear life with thinner teeth using bigger pulleys, which are still cost effective because of cheap plastic and the see-thru interior which reduces the amount needed to cast these.

3. Bigger pulleys effectively pick up more chain because the diameter of the pulley increases the effective radius of the RD arm. It's the same effect as a longer cage. In fact, the amount of chain pickup is pretty much dependent on the cage length of the main lower arm, and partly on the upper body B-rotation to some degree. The pulleys don't have much to do with this except to increase slightly the effect lower cage length and the effective upper body extension. So no real effective difference. And note, with the shadow Deore/XT/XTR RDs with small pulleys, and long arms, they have slightly greater capacity than the Altus, Alivios, etc. Because they have longer effective cage length despite smaller pulleys.

4. Less chain angle, more distribution of load on more cogs, yes. But the wear isn't less on the cassette or the chain per se. That was never the issue. If we're talking less friction in the drive train, I don't think that was important either because if so, why not have them on the higher end? The chain is under massive tension when riding on the top portion of the chain. The friction going through the pulleys is second or third order if at best and not significantly different between 11t and 13th. The bigger pulleys require more wrap around the cassette - more friction. But are gentler on the radius when the chain bends around. Less friction. It's a wash that's 4th order if any.

Personally, I like the big pulleys. It's looks beefy psychologically. It makes feel like the derailleur is more expensive than the $15 I paid, and the big plastic pulleys with the spoke wheels do make it more aerodynamic until mud clogs them up, but then I feel like I know when I've cleaned them when I hose them down and I see daylight.

By strange coincidence, I bodged an 8sp Alivio MC20 w/ the pulleys & backing plate from a (damaged) XT M739 not two weeks ago. The teeth on the Allivio pulleys had been worn down pretty far (and btw, both derailleurs are listed as being compatible w/ an IG70 chain). The plate was exactly the same length (nicer milling on the XT); the Alivio pulleys were 11t & the XTs are 10t, but the XT pulleys most definitely run with less noise & chatter. Granted, part of that might be down to one having bushings and the other ceramic bearings or maybe just simple wear & tear, but while tuning the shifts, it seemed to me that the chain ran more smoothly through the whole apparatus with the smaller pulleys.

700 wheels are faster than 20 Inchers all things being equal,,

same same..

AND I got a 1.3567 MPH Increase In UP HILL speed when I put red pully's in my bikes derailleur !!!

I also have this wax,,it makes your bike faster if ya rub some in yer ears,,

nvmd

... Increase In(Florida?) UP HILL speed.. where, Overpasses?