Bike Forums - New Sheldon Brown article on gear interchangeability

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- - New Sheldon Brown article on gear interchangeability (https://www.bikeforums.net/bicycle-mechanics/151116-new-sheldon-brown-article-gear-interchangeability.html)

Quote:

Originally Posted by Sheldon Brown

Sheldon "Sturmey-Archer Had Indexed Shifters 100 Years Ago" Brown

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Sheldon - thank you for the best laugh I've had all week. :) How true, how very true.

Quote:

Originally Posted by sydney

First, your undertaning is somewhat flawed or poorly understood.And that is not because it was poorly articulated on my part. There are short cage mtb RD that do shift big cogs. Also, there are old RD that shift big cogs because old freewheels with 33 and 34 cogs were fairly common, and there were even old 38 tooth freewheels... Secondly, that cage length business is the common misconception you get about long cage RDs, and it comes not just form LBS guys that should know better.... Duno how many times I have said it, but cage length is about wrap capacity, not big cog, which is in the parallelogram design. All one has to do is look at current shimano shimano road RD specs. The big cog capability is the same for both long and short cage.That's cuz the parallelograms are identical. The 27 tooth spec is conservative tho and most will shift a 30 and sometimes, depending on the application, one can get away with a 32 with them.If it was me and I wanted to use a 32 or bigger cog, I'd just go with a mtb RD, unless I had a road type just lying around and didn't mind spending some time fiddling with it. Campy spec does not tend to be as conservative and one can get away with less with them...A Shimano mtb RD is different in that in order to clear the bigger cogs involved, it tracks a steeper angle as it traverses the cassette, and that applies wether it is a long or short cage model.

Quote:

Originally Posted by 'nother

That is to say: a short-cage derailer will not generally handle a 32 or 34T cog . . . however, not necessarily because of the cage length, but because of the length of the parallelogram/arms.

Now, I'll grant that you can pick on the first part of this as being inaccurate, since there are indeed short-cage derailers that will handle 32+, but to evaluate the entire statement on the basis of that phrase alone is to take it out of context improperly. The bit I've emboldened above is the important part, and I don't think that's in any way contradictory to what you've said, or to reality.

However, relative to Sheldon's article, what I'm referring to is the part that says: Long-cage derailers have greater takeup capacity, and work with all types of cassettes.

Which to me is a bit misleading, based on my understanding and first-hand experience, which is: simply being "long-cage" does not mean a RD will handle any cassette (e.g. any cog size, which is what I assume Sheldon means when he says "all types of cassettes").

I do not think my understanding is flawed as you proclaim, but please feel free to correct as necessary.

Hey - you guys knock it off!!! I need to find out why I'm not supposed to shift from big ring to middle ring, and you guys are clouding the issue with all this other mumbo-jumbo!!! :p

Quote:

Originally Posted by 'nother

Actually I think you are saying the same thing but I think you're arguing from the opposite way. I mentioned you specifically because you have indeed repeated it so many times, and it finally sunk in, through all of the layers of smoke.

Let's review. I said:

Now, I'll grant that you can pick on the first part of this as being inaccurate, since there are indeed short-cage derailers that will handle 32+, but to evaluate the entire statement on the basis of that phrase alone is to take it out of context improperly. The bit I've emboldened above is the important part, and I don't think that's in any way contradictory to what you've said, or to reality.

However, relative to Sheldon's article, what I'm referring to is the part that says: Long-cage derailers have greater takeup capacity, and work with all types of cassettes.

Which to me is a bit misleading, based on my understanding and first-hand experience, which is: simply being "long-cage" does not mean a RD will handle any cassette (e.g. any cog size, which is what I assume Sheldon means when he says "all types of cassettes").

I do not think my understanding is flawed as you proclaim, but please feel free to correct as necessary.

You are right.... That statement as it stands is misleading....I didn't check what context it was in. Maybe it assumes road RD for road cassettes and mtb RD for big cog mtb cassettes... I don't tend to mix applications even there is latitude in doing so with Shaimno road RD due to the conservative spec, and others do it on a regular basis. I got enough stuff in the parts boxes that if I need to use a big cog,there is plenty of stuff availabe to handle the job. Also, it's been stated that a mtb RD(long or short) isn't necessarily a good chooice for a close ratio road cassette with say a 23 or smaller big cog since the mtb RD tracks a steeper angle as it traverses the cassette the upper pulley will be at a great distance from the bigger cogs at the end of it's travel and provides less chain wrap around the cog.Having not tried it I duno how real that issue may be.

Quote:

Originally Posted by Expatriate

They need the clearance for grinding, at the expensive of chain wear.

I'd be surprised if a quarter of the people who run 25/9 need the clearance. But what do I care, it's their faces that are gonna smack the concrete.

Good article BTW, but it doesn't explain why I have a hard time shifting my short cage XC Pro rear mech onto the 28 rear.

Quote:

Originally Posted by shane45

Hey - you guys knock it off!!! I need to find out why I'm not supposed to shift from big ring to middle ring, and you guys are clouding the issue with all this other mumbo-jumbo!!! :p

I think the only good argument against using the middle ring and small cog is the chain angle, the higher the angle the greater the friction and wear in the lateral sense. You've said several times that your chainline is fine in this combination and that's Ok with me. The argument is that it may be fine but not as good as a straighter chainline. If a cyclist is concerned with maximizing the life of the drive train components then maintaining the straightest chainline possible is going to extend the life of those components. But when you are in a situation where a quick one-step shift is more important then I say go for it. Or if shifting to the big ring and back to the middle ring means making multiple shifts just to keep a straighter chainline for a short distance, basically I'm too lazy or just don't have the time.

Sheldon's bike wisdom and advice has been a huge help to a lot of people including me, but I don't always agree with him and I don't think he expects everyone to agree with him all the time.

Al

Quote:

Originally Posted by 'nother

However, relative to Sheldon's article, what I'm referring to is the part that says: Long-cage derailers have greater takeup capacity, and work with all types of cassettes.

Which to me is a bit misleading, based on my understanding and first-hand experience, which is: simply being "long-cage" does not mean a RD will handle any cassette (e.g. any cog size, which is what I assume Sheldon means when he says "all types of cassettes").

I do not think my understanding is flawed as you proclaim, but please feel free to correct as necessary.

Yeah, you are right about that.

Al

Read the article, all of the hulabaloo over shifting from big/small to middlle/small loses significance when you read Sheldon's statement in context. He also explains why it can be bad (see bold print).

Quote:

There is a lot of confusion about the compatibility of narrow 9- and 10-speed chains with older cranksets. Shimano says you should replace the inner chainring(s) with specially designated 9- or 10-speed ones, but then they're all too eager to sell you stuff, whether you need it or not.

These chainrings have the teeth slightly farther to the right than the older chainrings to work a little better with the narrower chains. There is no difference whatever in the crank spiders.

The manufacturers also concerned about clueless users. The worst-case scenario is that you will be riding along with the bike in its highest gear (large front, small rear) and then for some bizarre reason shift down in front before downshifting in the back. (There is no shift pattern in which it is reasonable to shift in this sequence.) If you do shift this way, there's a small chance that the chain might "skate" over the edges of the teeth for maybe half a turn.

In practice this "problem" almost never materializes. Many, many cyclists are using 9- and 10-speed chains with older cranksets and having no problems whatever.

Quote:

Originally Posted by shane45

Jur, you need to go look at your bike.

There is MORE tension when I am in big-front and small-back, then when I front-shift only to middle-front and small-back (which Sheldon says is taboo). <snip some useless stuff>

:rolleyes:

Quote:

Originally Posted by jur

:rolleyes:

Apology accepted.

Thanks to all the posters - some really good points here; hopefully I am absorbing this all...

Quote:

Originally Posted by 'nother

http://sheldonbrown.com/speeds

One of the things I love about publishing on the Web, as opposed to paper, is that when someone points out an error or ambiguity in something I've written, I have the opportunity to immediately correct/improve it.

That passage was, indeed, a bit of an oversimplification, and I didn't define "long" in the context. I've revised it, hope it's more clear now.

In my defense, I'll note that the focus of this article is on interchangeability among different numbers of speeds, not so much on range issues.

Sheldon "Appreciates Constructive Criticism" Brown

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|   Love truth, but pardon error.    |

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Good stuff! I'm thinking about refurbishing an old Parkpre MTB I picked up real cheap. The frame is in good shape but the spokes are shot, and most of the components are lower end. My biggest question was whether a newer, 8 speed drivetrain setup would work. The section on cold setting sounds like a plan. I see a new Deore set-up in my future.

I'm baffled that people still don't seem to understand why it might be a bad idea to run the smallest cog with the middle ring.

Sheldon does not say this because of the fact that the chain isn't straight. He doesn't say it because of any other principle that applies to derailleur bikes only.

imagine you had a derailleur-equipped bike that was designed to use only these two chainring-cog combinations:

56/14
36/9

All three gear ratios are the same. (4 revolutions of the wheel per rev of the cranks)
The 36/9 puts the largest amount of tension on the drive side (top) of the chain. The reason is that the part of the chainring pulling on the chain acts as a lever with a length (from the bottom bracket center) of roughly 3 inches. Your foot is pushing on a crankarm that's 6 inches or so, and if you put 150 pounds of force on that crankarm, you're putting about 300 pounds of force on the chain.

Contrast that with a 56-tooth chainring. (radius of 4.5 inches or something.) Put 150 pounds of force on the pedal, and you end up with about 225 pounds of force on the chain.

It's a matter of leverage. the ground "pushing" back on your wheel can transmit more force to the chain through a small cog. the crank can transfer more torque to the chain through a small chainring than through a large chainring. add up the competing forces and you get more chain wear.

Quote:

Originally Posted by OneTinSloth

only because most BMXers don't realise that 9 teeth in the back and 25 up front is a dumb idea for lots of reasons...cog and chain wear being one of those reasons.

Most BMXers are well aware of it. They're also aware that BMX drivetrains are more likely to die violently than live to old age, and that their chances of survival against concrete ledges and steel coping increase as sprocket sizes decrease. And whoo boy, just wait till you hang your shin on a 44t sprocket for the 47,398th time learning hitchhikers, man...suddenly chain wear is the last thing on your mind.