Shimano crankset differences
I recently replaced my drivetrain. I'v always used LX cranks (FC M510) and chainrings, an XT cassette and a sram pc69 chain.
This time I used a fc M530 Deore crankset (they were on sale)
a sram pc 971 chain (the guy at bike shop said its the same as the pc69)
and an xt cassette.
The first thing I noticed was that the driveside crank (and chain rings) didn't go on as far as the old set. I had to readjust the front derailleur significantly- several mm's outward.
I also notice that my chain occasionaly skips or grinds under load like when I'm climbing. I'm not sure if its the front or the rear that causes it to grind, but I'm guessing its the crankset causing it.
Whats the difference between the 510's I used to use and the 530 crankset I put on? does the new one flex more?
This is on a Santa Cruz Superlight.
Sounds like the new crank....
pushed the chainline out. The clue is you had to adjust the front d OUT a few mm. That and the grinding in certain gear combo's when climbing. This is not that uncommon a thing even if you are using different model cranks made by the same manufacturer. You can adjust the chainline to get things back to normal, or at least pretty close, by finding a new bottom bracked with a narrower spindle length. The first thing to do is measure the distance of your current set up from the side of the seat tube to the small ring of the crank. Note: you may have to move or remove the front derailleur to get an accurate measurement. Write this down. Now install your old crank set and measure the same distance and write it down also. Subtract the smaller figure from the larger. This will give you the difference in chain line between the two set ups. If the measurement is larger (which it should be) with the new cranks, you need to get a bottom bracket with a narrower spindle by approximately the difference between the two set ups. Next you'll need to find the length of your current bb spindle. You can measure it with a caliper, or pull the bb as the spindle lenght should be printed on the bb body, or it may be stamped on the spindles. Measuring is usually the best method. Now right that down. Next you'll need to do something that sounds a bit odd, but it works out. If you need to move one side of the crank a given distance you'll have to DOUBLE the difference between the the two chainline measurements. That's because spindles are symetrical. In other words if a bb has a 113mm spindle and you go to a 110mm spindle it subtracts 1.5mm from EACH SIDE of the spindle. So if you need to bring your chainline 3mm in, going to a 110mm from a 113 would only give you HALF the correction that you need.
As an example.
Old crank from small ring to seat tube = 30mm
New crank from small ring to seat tube = 33mm
Current Bottom Bracket spindle length = 116mm
33mm - 30mm = 3mm or the difference between the chainlines with the two different cranks.
The difference between the old and new set up is 3mm. 3mm X 2 = 6mm (total correction required to move the drive side 3mm in), so, 116mm - 6mm = 110mm
So getting a new bb with a spindle length of 110mm will bring your chain line back into spec, i.e. move the dirve side cank 3mm inward.
Don't worry about getting it exact. A difference of 1mm + or - won't make that big of a difference. Also don't sweat getting an exact match to the new bb spindle length that you come up with. They don't make spindles in all lengths imagineable. But you should be able to come within 1mm or so. But several mm of difference as you describe can make a difference. Also keep in mind that the above is just an example, your figures will certainly be different.
One word of caution, If you are using a square taper bottom bracket and crank, MAKE SURE to use a torque wrench and torque BOTH cranks to spec!!! Most cranks require between 305 and 430 in lbs of torque. So pick a figure in that range and use it for both cranks when making your measurements. This ensures that both cranks are being pressed on to the spindle the same amount and negates any possible differences caused by uneven press fit. If you are using a splined system it's not as critical. As long as the cranks are bottomed out on the splines you should be good, but proper torque wouldn't hurt anything.
Note: If you don't come up with more than 1 or 2mm difference between the two cranks then it's likely that you have other issues. But from what you describe I'm betting on chainline.
Anyway, sounds more complicated than it is. It's allot easier to do than it is to tell about.
Last edited by Squash; 06-10-2007 at 07:55 AM.
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