6

Do the large custom pulley wheels and cages increase the capacity of a derailleur significantly? There are reasonably priced ones on E-bay with 16t pulley wheels.

Every demo of these I've seen says you'll need to install a longer chain when you switch, but does that really imply that the capacity of the derailleur has increased?

I am mostly interested in this question because I'd like to run an 11-40 cassette on my gravel bike and use Campagnolo components if at all possible.

  • 1
    You fail to define what you mean by "capacity". There are basically 4 measures -- number of cogs, minimum cog, maximum cog, chain length delta. – Daniel R Hicks May 7 at 16:58
  • 2
    The only definition I am aware of is total tooth diff count. big-little cluster + big-little chainrings. – Fred the Magic Wonder Dog May 7 at 17:40
  • Large pulleys have been the hallmark of cheap and nasty derailleurs, particularly on kids bikes, for at least couple of decades. Amazing how this industry regurgitates stuff with different spin and sells it as new, shiny and expensive. Must go an get me an Altus M310..... – mattnz May 7 at 19:47
  • FWIW, I have done the experiment using a Digrit 16t campy cage from Ebay and the answer is "yes it does increase capacity". I used the original 11t pulley on the top and the capacity is significantly increased, both due to a longer cage and a larger pulley on the bottom. – Fred the Magic Wonder Dog Jun 11 at 0:14
  • BTW, I can't really recommend Digrit cages in the Campy version, the tolerances don't allow the tension screw to engage the threads on the derailleur so I had to glue the serrated ring in place on the cage. – Fred the Magic Wonder Dog Jun 11 at 0:43
3

Longer cages will increase capacity as Andrew says.

If I have or move to a long cage derailleur will one or a larger pulley increase capacity?

To be transparent - I have no experience with oversize pullies.

I'm offering a summary of: "Do Oversized Derailleur Pulleys Really Help?" parts 1 and 2. Both articles are interesting but part 2 addresses the derailleur capacity issue.

The 46 tooth large cog on the XX1 did not offer a large enough range for everyday riders, forcing them to choose between pushing their bike uphill or getting dropped on fast downhills. Problems were solved by introducing a massive 50T cog as a twelfth gear. This large cog requires a longer chain in order to make smooth shifts. Instead of increasing the length of the derailleur cage, Sram increased the lower jockey wheel size in order to prevent the cage to run close to the ground and be susceptible to rock strikes. My wild guess is that they left the top pulley untouched at 12 teeth to increase shifting accuracy.

So, SRAM is building derailleurs that have a larger lower pulley (14 teeth on the bottom, 12 teeth on top) to increase capacity. (Linked as an example, not an endorsement)

It was difficult to find information on people moving to a larger pulley to increase capacity. Most of the articles went on and on about energy savings.
As a very skeptical person I'd like more information before making a change on my bike.

One thing that makes me more open to the possibility of this working is my experience with the triple pulley Sun Tour derailleur in the distant past.
enter image description here

It's not exactly the same thing but the two bottom pulleys are a little like a single larger pulley. The triple pulley did increase capacity. It seems logical that if a derailleur can handle more chain it can handle more capacity.

So, given my skepticism I'd be willing to try a larger pulley solution if:

  1. My current derailleur was already a long cage.
  2. I could find a larger pulley or pulley/cage combo that was not too expensive

"Too expensive" would be a price that would make me feel bad about throwing the parts away if the experiment didn't work.

Like SRAM I'd only put a larger pulley on the bottom - not the top - for the reason Andrew gives in his answer:

It also seems reasonable to assume that increasing the size of the jockey wheel closes to the sprockets without changing its location would result in a smaller maximum usable sprocket size.
I'd say you'll get more capacity, but you may not be able to use a larger cassette to take advantage of that larger capacity.

Let us know what happens if you take the plunge.

  • There are oversize pully cages on Ebay for a little over $100, I might pull the trigger and see what happens. I am already using Wolftooth roadlink to successfully shift an 11-34. – Fred the Magic Wonder Dog May 7 at 17:38
  • I'm too cheap for that kind of experiment :) – David D May 7 at 17:41
  • From cyclingpowerlab.com/DrivetrainEfficiency.aspx "The efficiency difference between an equal gear that involves the 24 sprocket and the 13 sprocket can be worth 1-2 watts when riding in the 200-400 watt range." -How does going from 11 to 14 tooth on unloaded chain get double those savings? "Cheap derailleur pulleys compared to high end equipment can cost 1 watt." give an idea of the order of magnitudes involved. If it really was 2.5W, this would have been done decades ago. I will need to see if the experiment that lead to these claims is repeatable before I believe them – mattnz May 7 at 19:44
  • 1
    I am not interested in the watts available, If I had enough watts I wouldn't need a 40t in the back... – Fred the Magic Wonder Dog May 9 at 15:39
  • @FredtheMagicWonderDog no one ever has enough watts :) – David D May 9 at 18:57
5

All else being equal, a longer cage will increase the capacity of the derailleur.

For example, the only difference in Shimano's long- and short-cage 105 5700 rear derailleurs is the cage. See https://si.shimano.com/pdfs/ev/EV-RD-5700-3042A.pdf

Per https://si.shimano.com/pdfs/si/SI-5X90B-003-ENG.pdf that the short-cage SS models have a 33-tooth capacity while simply changing to a longer cage gives the GS model a 39-tooth capacity.

But note that the largest supported sprocket size remains unchanged, but that appears to be because the location of the jockey wheel closest to the sprockets also remains unchanged with the cage swap.

It also seems reasonable to assume that increasing the size of the jockey wheel closes to the sprockets without changing its location would result in a smaller maximum usable sprocket size.

I'd say you'll get more capacity, but you may not be able to use a larger cassette to take advantage of that larger capacity.

I also suspect that larger jockey wheels is likely to result in degraded shifting performance since the larger wheel could allow more side-to-side movement than a smaller wheel would. "But the marketing says it's stiffer and the bearings have tighter tolerances." They would say that, and it might even be true. When it's all clean and new and unworn - how durable and free-spinning will those ceramic bearings be when they get grit and grime inside:

“Ceramic bearings are beneficial in environments not requiring grease lubrication,” says Lew. But a bicycle is expected to cope with a vast range of conditions, rain and dirt, and where maintenance schedules may be less than optimal, and the last thing you want is to ride bearings without grease. It’s this requirement to cope with the conditions common to cyclists that offset the promised lower rolling resistance of a ceramic bearing, according to Paul Lew.

“The rolling resistance of a ceramic bearing compared to an ABEC 3, 5 or 7 steel ball bearing is offset by the resistance of the grease,” he says. “In order for a ceramic ball bearing to out-perform a steel ball bearing, grease is not an option. Does this mean I should run my ceramic ball bearings dry or with light oil? Yes, but you won’t like the result in an environment where the bearings can become contaminated. If you run your bearings dry they will feel gritty and rough.”

Hope’s Alan Weatherill concurs with Paul Lew’s conclusion that ceramic bearings are not suited to the demands of cycling and says their suitability to industrial machinery doesn’t necessarily provide the performance benefit for cyclists that many people and companies claim they do.

“Another issue with using them [ceramic bearings] on bicycles is their hardness,” says Weatherill. “While this again is an advantage in many industrial applications, it's a major drawback on bikes. The shocks from hitting potholes and other road blemishes impact the hard ceramic balls into the softer steel races commonly used. This dent in the race is then felt when the bearing is rotated, giving you rough bearings.”

0

No.

Imagine that your, e.g., 2x10-speed bike is actually twenty superimposed single-speed bikes, each with a correctly sized chain for one combination of chainring and rear cog. Derailleur capacity is a measure of the difference between the shortest and longest chain that's needed by those twenty bikes. Using larger jockey wheels just means that all of those chains need to get a couple of links longer; it doesn't change the differences between them.

  • I don't understand the -1, this is a good explanation to me. – Gabriel C. May 7 at 17:50
  • @GabrielC. Perhaps the point is that larger pulleys require a longer cage, and that gives more capacity. Would be helpful if the downvoter would explain, but that's not mandatory. – David Richerby May 7 at 17:51
  • Maybe if you add something to dissociate chain length from pulley size it would help? Since it's the former that determines capacity, long cage + small pulley VS shorter cage + larger pulley wouldn't change anything if chain length remained the same. – Gabriel C. May 7 at 17:56
  • 1
    It is conceivable a larger pulley may increase the capacity: the chain wraps only partly around the lower pulley. How far the chain wraps around it depends on the angle of the chain to the cage. It is the shortest when the cage is nearly parallel to the chain (largest cog) and farthest when the cage points down (smallest cog). This will add about a fourth of the difference in pulley circumference. Which is not all that much. – gschenk May 7 at 18:54
0

Normally 'capacity' of a rear derailleur means the total capacity specification: basically the ability to take up chain slack between the 'longest chain' and shortest chain' conditions. It's dictated by the cage, a-pivot and b-pivot geometry.

The capacity needed by any drivetrain setup is: (difference between smallest and largest sprocket size) + (difference between smallest and largest chain ring size), measured in number of teeth.

Larger pulley wheels are not going to increase the total capacity, they may even decrease it by hitting a sprocket when the cage is in it's most rearward position.

'Capacity' might also mean the min and max sprocket size range that can be accomodated. This is linked to a derailleurs total capacity, but is also a function of the angle of the parallelogram the center plane of the bike. Obviously, a larger upper pulley wheel is more likely to interfere with the largest sprocket.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.