Gael

I'm replacing and old cotter crankset with a square taper one. I want to buy a 52/42 crank. I thought I had it all covered by making sure I had the appropriate spindle length and taper (JIS) in order for my crank and BB to match. But now, in the specs of the crank, it is written that it is for "8/9 speed". Another one for sale is said to be for "6/7/8 speed". I don't understand what it means and how a crank can be defined by the number of sprockets on the freewheel (or cassette).
Sheldon Brown seems to say that it's nothing but marketing (sorry, I can't post the url, I'm too junior on this forum). Here is a copy paste:

Component manufacturers like to sell you lots of new parts, even if you don't need them. This has led to much confusion as various parts are labeled as if they are incompatible with other parts even though they are not.The following parts ONLY are "speed specific":

• Indexed Shifters These need to have the correct number and spacing of detents ("clicks") to match the system they'll be used with.
• Cassettes It is really the cassette that determines how many speeds you have in back.
• Chain As you go to more sprockets on the cassette, you need a narrower chain. However, using a narrower chain with an older system rarely presents any problem.

Can anyone give me an explanation ?
Thanks!

PS: I don't know if it matters but the bike has friction shifting.

dabac

Crankycrank

Very generally chainrings rated for different speeds have slightly different thicknesses and spacing between each other to accommodate wider/narrower chains. The fewer the cassette/freewheel cogs the wider the chain and wider the spacing between cogs and chainrings. With cranksets there is a lot of fudge room to use rings designated for certain "speeds" and still have it work fine. Most likely if you currently have a 5-7 speed setup the cranks mentioned above will work fine (I'm making the assumption that that is what you have). If you find that your chain gets slightly stuck on the inner chainring when shifting you can just add some thin washers inside one of the chainrings to increase the spacing. Most shops will have these and not expensive. With friction shifters all of this is even less important.

fietsbob

If the spread between the adjacent chain rings is wider than the narrow chain,
to such an extent that it falls between 2 chainrings , when you shift , that would not be best , right?

...

Andrew R Stewart 

Having friction shifting the rings' center to center dimension is of lesser concern then if you had an indexed system. But too narrow a ring to ring dimension can cause chain rub, when on the inner ring, when in the small rear cogs. Especially if you use a ramped and pinned large ring. How many rear cogs do you have? How wide (by measurement, not by "speeds" is your chain? Andy

alcjphil

If your bike has a cottered crankset, it is pretty likely that your freewheel has 5 or fewer cogs. What happens when freewheels or cassettes have more cogs is that they have to be spaced closer together. When cogs are closer together, chains have to be narrower. Chains that are too narrow for the cranks can cause problems. Chains too wide for the cassette also cause problems. You can fudge things to some extent, but past a certain point a crank has to come close to the number of "speeds" indicated by your freewheel or cassette. If you have a 5 speed freewheel, an 8/9 speed crank may be stretching things. I currently run an "11 speed" crank on a 10 speed system and all is good, at least as good as the previous "10 speed" crank was. That was a very small change to the front shifting. However, proposing to go from a cottered crank to an "8/9" speed crank may be a bridge too far

zacster

I tried running a 6 speed or thereabouts crank with a 10sp chain and cassette and found that to be noisy as the chain was narrow over the teeth. Otherwise I've never had a problem. I use a 7 speed crank with a 10sp mtb setup and it works just fine, except that I replaced the outer chainring recently because the other was worn, and used a 9/10 version.

Gael

Hi Andrew,
Thanks for taking time to answer my question. For the moment, I have 5 rear cogs. But I want to change the freewheel and install one with 6 cogs. I was also going to change the chain. I was planning to buy a KMC Z51 chain. The specs for the chain are ½”x3/32”. The problem is that I can't find equivalent measurements of width in the specs of cranks. Therefore, I have nothing to compare the width of the chain with when it coomes to the crank.

Ironfish653

As far as the chain goes, 5, 6, 7, and 8-speed all can use the same chain. KMC Z51 is my go-to when i'm putting together a mix-n-match driveline as well.
Once you hit 9 speeds in the rear, the chain gets thinner. 10-speed is likewise thinner than 9-speed, and 11-speed is it's own thing as well.

Modern bike nomenclature refers the 'Speeds' to how many cogs are on the back, the fronts are Single, Double or Triple. 1x, 2x, or 3x.
WRT the cranks, some of the vintage cranks from the 5-6 speed era (1980s and earlier) have very wide spacing between the rings. By the 7-/8-speed 1990's, the spacing was pretty much standaridized, thanks to the market domination of Shimano. 9-speed brought the chainrings a little closer together, to deal with the narrower chains needed by a 9-speed cassette. 10-speed also made the rings themselves thinner, as the chains got slimmer.
You can usually go 'backwards' with a crank, so a '9-speed' crank will work on a 7/8-speed driveline without much drama.

Once you start getting down in to 5- and 6-speed, you are either talking about vintage or very cheap bikes, in which case there's a lot of variation, so the component manufacturers dont' necessarily spell out compatibility.

It seems like you're just adding a new crank, using your old friction shifters? In that case you can use pretty much anything, 7/8-speed, 8/9-speed. Shouldn't make much difference. a double front crank with friction shift is about the easiest thing to set up. Since you're using a modern chain, and modern crank, it should be no problem.

Gael

Thank you for your help. I find it very surprising (and disappointing) that the industry hasn't decided to settle on a good metric to define the spacing between rings. If chains are defined by their width, why can't the spacing between the rings be given? It would make pairing with the chain so much easier.

Ironfish653

They did. It's just that in the early '90s Shimano became THE dominant supplier for bike drivetrain components, in part due to their size and their SIS indexed shifting system.
In any indexed system, the shifters, deraileur and cassette are designed to work together as a system; The shifter pulls a certain amount of cable per 'click' the derailleur moves the jockey wheel a certain distance for that cable pull, and the cassette cogs are that same distance apart, so that the chain moves smoothly up and down the cassette.
For 6, 7, and 8-speed, these spaces were all the same, you just had to make sure the number of cogs was the same (or less) than the number of 'clicks' on the shifter.
For each extra gear, though, the rear wheel gets wider, 5-6 speed was 126mm, 7-8 speed used 130mm. When 9-speed was developed, they chose to make the gears closer together, rather than make the wheel wider. 10 and 11-speed and the adoption of disk brakes have seen the axle width go up again, to 135mm.

So, to sum up, there IS a standard, It's just that the standard applies to a certain family/generation of components, not all of them at the same time

Kimmo

10 & 11 speed are not responsible for 135mm OLD. Rim brake 11s road bikes are still 130.