10T Cog & Does Aero Lose/Beat/Draw Friction? Questioning Josh Poertner's Methodology

Triathlon Forum
61

here’s another way to look at it. pedal your bike backward while it’s on the workstand. a really slippery drivetrain, combined with a hub that has tight seals and heavy grease, the freehub may not break loose, and the chain may just sag and gather between the crank and the cassette. the friction in the drivetrain overcomes the friction between the hub and the cassette assembly, traditionally.
Friction in the drivetrain isn’t what overcomes the freehub stiction. The freehub gets pulled backwards by the chain being pulled - after traveling through the pulleys - into the bottom of the chainring. Increasing the drivetrain friction while keeping everything else equal would actually make the freehub stiction worse, since it would decrease how forcefully the chain pulls on the back of the cassette. (Another way to visualize it is: it’s harder for the upper run of the chain to feed into the cassette if it’s harder for the lower run of the chain to feed out.)

If the upper run of the chain sags due to chainring rotation, the lower run of the chain must un-sag by a similar amount. In practice, this means that the tension cage on the rear derailleur releases that amount of chain by rotating forwards slightly, and the tension on the lower run of the chain increases. When the tension on the lower run of the chain becomes great enough to overcome freehub stiction, the freehub starts rotating backwards.

So, when the upper run of the chain sags, the issue is that the freehub stiction is high relative to the chain tension.

Now, there is a correlation here: lower chain tension does reduce drivetrain friction, so a lot of low-friction drivetrains will have this issue, because they achieve their low friction by using a really lightly-sprung tensioner.

Probably, anyway, maybe. I’m not actually an expert on drivetrain physics.

imagine pedaling your bike backwards, with it on the stand. yes, if the chain sags as it travels from the crank back to the cassette, the RD cage is put on stretch.

something must break the cassette free from its own stiction. you think it’s chain tension. i think it’s chain (or system) friction. i’ll have to think about that. i believe i’m right. but maybe you are. i don’t think you are. but i need to think about it some more.

62

here’s another way to look at it. pedal your bike backward while it’s on the workstand. a really slippery drivetrain, combined with a hub that has tight seals and heavy grease, the freehub may not break loose, and the chain may just sag and gather between the crank and the cassette. the friction in the drivetrain overcomes the friction between the hub and the cassette assembly, traditionally.
Friction in the drivetrain isn’t what overcomes the freehub stiction. The freehub gets pulled backwards by the chain being pulled - after traveling through the pulleys - into the bottom of the chainring. Increasing the drivetrain friction while keeping everything else equal would actually make the freehub stiction worse, since it would decrease how forcefully the chain pulls on the back of the cassette. (Another way to visualize it is: it’s harder for the upper run of the chain to feed into the cassette if it’s harder for the lower run of the chain to feed out.)

If the upper run of the chain sags due to chainring rotation, the lower run of the chain must un-sag by a similar amount. In practice, this means that the tension cage on the rear derailleur releases that amount of chain by rotating forwards slightly, and the tension on the lower run of the chain increases. When the tension on the lower run of the chain becomes great enough to overcome freehub stiction, the freehub starts rotating backwards.

So, when the upper run of the chain sags, the issue is that the freehub stiction is high relative to the chain tension.

Now, there is a correlation here: lower chain tension does reduce drivetrain friction, so a lot of low-friction drivetrains will have this issue, because they achieve their low friction by using a really lightly-sprung tensioner.

Probably, anyway, maybe. I’m not actually an expert on drivetrain physics.

imagine pedaling your bike backwards, with it on the stand. yes, if the chain sags as it travels from the crank back to the cassette, the RD cage is put on stretch.

something must break the cassette free from its own stiction. you think it’s chain tension. i think it’s chain (or system) friction. i’ll have to think about that. i believe i’m right. but maybe you are. i don’t think you are. but i need to think about it some more.

Or…rather than doing a “thought experiment”, you might be able to figure out how to actually test it :slight_smile:

63

here’s another way to look at it. pedal your bike backward while it’s on the workstand. a really slippery drivetrain, combined with a hub that has tight seals and heavy grease, the freehub may not break loose, and the chain may just sag and gather between the crank and the cassette. the friction in the drivetrain overcomes the friction between the hub and the cassette assembly, traditionally.
Friction in the drivetrain isn’t what overcomes the freehub stiction. The freehub gets pulled backwards by the chain being pulled - after traveling through the pulleys - into the bottom of the chainring. Increasing the drivetrain friction while keeping everything else equal would actually make the freehub stiction worse, since it would decrease how forcefully the chain pulls on the back of the cassette. (Another way to visualize it is: it’s harder for the upper run of the chain to feed into the cassette if it’s harder for the lower run of the chain to feed out.)

If the upper run of the chain sags due to chainring rotation, the lower run of the chain must un-sag by a similar amount. In practice, this means that the tension cage on the rear derailleur releases that amount of chain by rotating forwards slightly, and the tension on the lower run of the chain increases. When the tension on the lower run of the chain becomes great enough to overcome freehub stiction, the freehub starts rotating backwards.

So, when the upper run of the chain sags, the issue is that the freehub stiction is high relative to the chain tension.

Now, there is a correlation here: lower chain tension does reduce drivetrain friction, so a lot of low-friction drivetrains will have this issue, because they achieve their low friction by using a really lightly-sprung tensioner.

Probably, anyway, maybe. I’m not actually an expert on drivetrain physics.

imagine pedaling your bike backwards, with it on the stand. yes, if the chain sags as it travels from the crank back to the cassette, the RD cage is put on stretch.

something must break the cassette free from its own stiction. you think it’s chain tension. i think it’s chain (or system) friction. i’ll have to think about that. i believe i’m right. but maybe you are. i don’t think you are. but i need to think about it some more.

Or…rather than doing a “thought experiment”, you might be able to figure out how to actually test it :slight_smile:

part of my thought experiment is thinking about how to test it. it’s not a trivial test. you’d need a wheel with a fair bit of stiction, and then you’d need the capacity to test out different drive train parts. but according to darkspeedworks there’s a question as to whether friction-reducing parts actually reduce friction. so you’d need to first replicate what friction facts did. you’d need to test components to demonstrate the friction “value”.

which i’ve been thinking about doing anyway. i just haven’t figured out an easy way, yet, test the value of chain tension and of drivetrain friction.

64

so you are concerned about the amount of energy needed to break the pawl lock when first pedaling backwards…why? do you pedal backwards when riding? it is not even a relevant comparison to pedaling forward, or are you concerned about the amount of chain suck the pawls allow when pedaling forward and you begin coasting abruptly? these are the only “stictions” I seem to experience and I could “reduce” them by cleaning the freehub internals more, but it really isn’t costing anything when actually pedaling and moving, and pretty sure it is not related to cog size other than the latter happens to me only at high speeds, ie >30 mph

65

Wouldn’t a 10T help in some specialty races? I do downhill 20 mile TT and i use 58/48 w/ 11-28…helped a lot c/w previous 54-11, passed a dozen folks in a tuck.
Wouldn’t one by w/ ?60 and 10T in back help? I still spin out w/ 58-11. And pedaling feels more stable. BTW, spend majority of time in 11-15…

66

Crazily enough, all other things being equal, the amount of top chain sag does depend on what gear you’re in. This can be tested too. If you go down the same hill at the same speed (being at the same speed in any comparison test of this is important of course) with a kind of sticky freehub, the chain sag will happen a lot more when the chain is in the 11 tooth cog vs when the chain is in the 34 tooth cog (the size of the chain rings does not matter if you’re holding the drivetrain with your legs).

67

are you concerned about the amount of chain suck the pawls allow when pedaling forward and you begin coasting abruptly?

you got it right there. the question is whether it’s an issue in 1x applications, not just because of drivetrain striction/friction but because of the mass of a fast turning cassette that’s got a lot more mass than a typical cassette (say, an eagle cassette).

these are the only “stictions” I seem to experience and I could “reduce” them by cleaning the freehub internals more

unless the freehub is already pretty thick, because of heavy grease and seals.

68

are you concerned about the amount of chain suck the pawls allow when pedaling forward and you begin coasting abruptly?

you got it right there. the question is whether it’s an issue in 1x applications, not just because of drivetrain striction/friction but because of the mass of a fast turning cassette that’s got a lot more mass than a typical cassette (say, an eagle cassette).

In the abrupt coasting scenario, l think freehub stiction is the main issue. But cassette mass might be a factor too, especially when the chain is on the smaller cogs. But, l am not familiar, how much heavier is a big eagle cassette vs, say, an 11-25 ultegra cassette?

69

are you concerned about the amount of chain suck the pawls allow when pedaling forward and you begin coasting abruptly?

you got it right there. the question is whether it’s an issue in 1x applications, not just because of drivetrain striction/friction but because of the mass of a fast turning cassette that’s got a lot more mass than a typical cassette (say, an eagle cassette).

In the abrupt coasting scenario, l think freehub stiction is the main issue. But cassette mass might be a factor too, especially when the chain is on the smaller cogs. But, l am not familiar, how much heavier is a big eagle cassette vs, say, an 11-25 ultegra cassette?

i think they’re all an issue. it’s like a teeter totter. what happens on one seat is not the issue. there is no single seat on a teeter totter that’s the issue. or, this isn’t an issue at all, and i’m just making something up. but what i’m hearing in the field is that i’m not making it up.

70

If your chain sags when you coast the freehub/hub needs attention.
The drag is caused by worn out bearings, too much preload on older bearings or missing shim/spacer and too much grease.
Grease between the hub bearing and the freehub bearing can cause viscous drag and is caused by over enthusiastic greasing when servicing.
Worn out hub bearings or freehub bearings can cause contact between the hub bearing and the freehub bearing shields that the shim/spacer is meant to prevent and cause frictional drag.
Similarly, if the shim has accidentally been left out, then any lube between the bearings will cause viscous drag or even contact between raised printing on the seals.

The pawls themselves will NEVER cause a freehub to follow the chain.
Their action quickly disperses any grease that makes its way into their path and if you have ever had a freehub apart you will quickly realise that the springs on the pawls are extremely light, to the point that if you completely remove them, the hub still drives and freewheels, just that it does not always engage all pawls reliably.
Most freehubs require oil only and if greased used very light grease like Slick Honey.

I have recently had to make up a hub spring using guitar wire, whilst trying to find the right gauge wire to use I found that even with a wire that was several orders of magnitude too strong the freehub would not follow, even without a chain fitted. The normal seals in the freehub provide enough friction to stop the cogs turning, it just ticked much louder.

So if you are having trouble with chain sagging on the top span when coasting, fix the dam thing.

71

One of the local reps of a ceramic bearing company posted a proud video of his “friction-optimised” drivetrain, showing the crank being turned by his pixie-dust chain, oversized pulleys and ceramic BB. Guess what? A Zipp Super9 freehub with the QR done up too tight will do that even with the cheapest touring drivetrain…

If there was a difference FrictionFacts has made, it’s not improving on the best practices. Those “in the know” waxed their chains (still optimal) and ran light-tension and low-friction bearings forever (maximum wattage to be saved ~0.6w, according to FF). It’s the education of the general public - you and I now know what lube (still wax) and bearings (I think Tacx ceramic are best value/money on that chart) to use.

72

are you concerned about the amount of chain suck the pawls allow when pedaling forward and you begin coasting abruptly?

you got it right there. the question is whether it’s an issue in 1x applications, not just because of drivetrain striction/friction but because of the mass of a fast turning cassette that’s got a lot more mass than a typical cassette (say, an eagle cassette).

In the abrupt coasting scenario, l think freehub stiction is the main issue. But cassette mass might be a factor too, especially when the chain is on the smaller cogs. But, l am not familiar, how much heavier is a big eagle cassette vs, say, an 11-25 ultegra cassette?
I got this issue on my gravel bike when I went from an 11-28 to an 11-36 in the 46x11 combo. The slack is so back it slaps the chainstay if I abruptly stop pedalling from 110rpm. This is with a shimano 2x with the RX clutch RD and a generic formula 3 pawl freehub

73

are you concerned about the amount of chain suck the pawls allow when pedaling forward and you begin coasting abruptly?

you got it right there. the question is whether it’s an issue in 1x applications, not just because of drivetrain striction/friction but because of the mass of a fast turning cassette that’s got a lot more mass than a typical cassette (say, an eagle cassette).

In the abrupt coasting scenario, l think freehub stiction is the main issue. But cassette mass might be a factor too, especially when the chain is on the smaller cogs. But, l am not familiar, how much heavier is a big eagle cassette vs, say, an 11-25 ultegra cassette?

i think they’re all an issue. it’s like a teeter totter. what happens on one seat is not the issue. there is no single seat on a teeter totter that’s the issue. or, this isn’t an issue at all, and i’m just making something up. but what i’m hearing in the field is that i’m not making it up.

You should try the same scenario, but before you do your abrupt coasting or pedaling slowly while flying downhill, put your chain on the biggest rear cog. Does your drivetrain have the same chain sag problem as when the chain is on a very small cog?