New Sheldon Brown article on gear interchangeability
 

I get a lot of email questions about compatibility of "n-speed" doohickeys with "n+1-speed" frammises.

I've put together a Web page that tries to summarize these issues in a clear and concise manner.

See: http://sheldonbrown.com/speeds

Sheldon "Speedy" Brown

What a great reference with no barnyard waste. Will sure save time and bandwidth.

Thanks Sheldon.

The only issue I have is with your comment:

"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.)"

I assume you are speaking of road bikes specifically? On MTB's, it is quite common (ie. not "bizarre" at all) to shift from the big chainwheel to the middle chainwheel when in the highest (smallest) cog on the back, then downshift through the gears out back.

No, this is not good practice on- or off-road. Arguably it's even worse on MTBs because they generally use smaller chainrings which are more stressful, and 11 tooth small rings which wear faster.

Sheldon "Chainline" Brown

Ok, one burning question, re rear derailers: "Within a given brand/style of rear derailer, all 'speed numbers' are generally interchangeable. This applies to all indexable models, basically everything manufactured since the around 1990." -- So what *is* an indexable rear derailer? It seems to me that if a rear derailer is able to shift through all the gears using a friction shifter, it should also be able to do so with an index shifter.... The barrel adjuster on newer rear derailers I'm sure helps make it much more reasonable to keep things in adjustment, but other than that, is there really a difference?

Thanks again, Sheldon. You & your website continue to be invaluable cycling resources.

Hmmm...I'm not sure I'm understanding this.

My chainline is fine using my big chainwheel and the highest (smallest) cogs. My chainline is also fine using my middle chainwheel and the highest (smallest) cogs.

Why is it bad if I am in big (up front) and small (out back) and decide to shift my chainwheel only from big to middle? Again, my chainline is still fine with this combination (using outboard XTR BB bearings).

Theoretically all derailers are "indexable" but before indexed shifters became the norm for derailer gearing in the late '80s, the travel ratio (cage/cable) was not rigorously kept constant accross different models, since it didn't matter before the advent of indexing.

In addition, derailer and sprocket design has gradually improved over the years, so the newer models require less skill to shift, making them more compatible with indexed shifters. With many older derailers, you needed to slightly "overshift" the lever to initiate the shift, then move it back a skosh to get the chain to run straight once the shift had been completed.

I have successfully indexed various older derailers, but some of this sort of technique is still needed occasionally.

My first ever indexed derailer system used original Dura-Ace S.I.S. 6-speed levers, a Sun Tour Cyclone derailer, modified Regina Oro 12-22 6-speed freewheel and Sedisport chain. I drilled and tapped the Cyclone derailer to permit me to install a cable adjusting barrel.

I rode this system for many years, even though the 52/42/28 Biopace chainrings greatly exceeded the capacity of the Cyclone derailer. (I could only use the two or three largest rear sprockets with the 28 chainring, but those were the ones I wanted with it anyway)

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

Simple lever-arm length fact: The smaller your gears that you are in, the higher the tension in the chain (because the chain is running on a shorter "lever-arm"). Higher tension, higher wear. AVOID. get out of the back small cog way before shifting to middle ring. Always run the biggest cogs practical.

Again, shifting from the big chainring to the middle ring REDUCES my chain tension. If it is OK for me to run big front and little cog, then it is EASIER on the chain if I shift to middle chainring without touching the back cogs at all.

How is this bad???

The opposite of BMX.

I wouldn't think it's so bad as long as the shift is made smoothly. You're the best judge of whether or not this shift is causing any accelerated wear on your bike. I think your points are well made.

Al

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.

I suppose you are referring to the tension set up by the read derailer? This tension is negligible compared to the tension on the top part of the chain that is under load as you pedal - that's the one that buggers chains and cogs.

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

No shift pattern? I run halfstep + granny gearing so I somtimes shift the front down while on the smallest cog, it is the next gear from the top. It a 7 speed cassette, an 8 speed chain, and a TA Cyclotouriste triple crank so there is no issue with the chainring width.

Nice work, another Sheldon link I'll be referring to frequently in responses to NooB Q's
Thanks,
Adam

I need to know why Sheldon indicates that it is not OK to shift the front der. first, from the big to middle chainring - before shifting down on the rear cogs. That is my ONLY question.

If I am in the big front and small rear, why is not OK to shift to middle front without touching my rear??? My chainline is still fine, and the tension is fine. Sheldon seems to indicate that this is a no-no and we should be shifting down on the back cogs before switching the front chainring. What is wrong with going from big front / small rear to middle front / small rear using only the front derailleur?

Thanks, Sheldon.

I have a question regarding short-cage vs long-cage. My understanding (from the inimitable first poster to respond to this thread) is that the cage length has nothing to do with cog capacity. 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. But neither will any old long-cage derailer simply by nature of it being long cage. While I don't think what you have contradicts this, it might be nice to see this issue explained further and clarified.

Specific case of why I bring it up: A while ago I was shopping for a new RD and a guy at a LBS sold me a long-cage Shimano 105, telling me "yeah, it's long-cage, it should work" for my 32T cog. It didn't. Even with an extra-long B-screw adjusted to the maximum, I was unable to adjust it to prevent the jockey pulley from hitting the largest cog, not even close. I exchanged it for a "mountain" derailer and had it working within about 30 seconds. The derailers had the exact same cage length, but different parallelogram size.

Thanks. Now back to carpet-smoking.

Thanks Sheldon, great stuff! especially all the solid info on freewheel/cassettes. Great reference.

It is the assumption that you would use that particular gear combination - ie middle-front & small-back - until shifting again. Purely shifting is fine, of course. But using that combo - you are accelarating your chain and cog wear compared to using the same gear ratio with bigger cogs due to the chain tension being higher.

[edit]:
I can only suppose Sheldon calls this shift pattern "bizarre" because experienced riders would a) downshift at the rear because this gives a smaller jump and b) know about the wear issue and avoid this. I myself shifted in this "bizarre" pattern when I got my first bike, and rapidly buggered the chain and cassette's smaller cogs, as well as wearing the middle chain ring. Lesson learned the hard way, in the hip pocket.

Of course I would use that combo. As I have already stated, the chainline is fine. The tension is fine. I can't possibly understand how this is "bizarre". :(

I just don't understand what the harm is in simply dumping the front from big to middle without shifting any cogs first - and yes jur - even pedaling :eek: in that combination after a front shift only.

"If a person does not understand something, repeat it a bit louder."

THE TENSION IS NOT FINE.

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).

If, what you are trying to imply is that if I shift from big-front and small-back to middle-front and small back, that my bike will implode because of the pedaling tension, you need to get real.

I understand that my big 44-tooth distributes load better (has more teeth), but don't give me that BS that my 32-tooth middle can't handle running with my smallest cog, especially if my chainline is fine with this combination.

My tension is just fine, thank you. And there is no need to yell.

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.