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Old 05-30-09, 06:01 AM   #1
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dura ace 7400 crank with 9spd chainrings

Just curious whether something like a dura ace FC-7400 crank -- presumably designed for 7spd -- can easily take 9spd chainrings? Are there any issues with 9spd chainrings being "thinner" than rings designed for 7-spd and therefore leaving a gap when mounting to the older crank?

thx
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Old 05-30-09, 05:08 PM   #2
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The tab thicknesses on the crankset should be the same although my knowledge of older Dura Ace is very limited and it could well stray from typical standards. Chanring thicknesses have pretty much stayed the same throughout all of the speed changes regardless of manufacturer. The only changes have been the ramps and pins (until you get into the 10/11 speed stuff).

The only issue you are likely to encounter is if you trying using a 6/7/8 speed chain with the 9 speed rings. I tried this and the chain kept getting caught on the ramps and half shifting any time I strayed from a perfect chainline. Apparently not all chainring configurations have this problem though.
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Old 05-30-09, 07:14 PM   #3
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Chanring thicknesses have pretty much stayed the same throughout all of the speed changes regardless of manufacturer.
That is NOT true. As I've shown elsewhere, ring thicknesses vary all over the place, even between models of the same manufacturer. However, thickness is not the only variable in determining tooth centerline, which is what you're really interested it.

Bottom line is, it's a crapshoot. The result might be off, but not off so much that it annoys. Or it might be unshiftable. Only experiment can tell for sure.

If you're really concerned about getting the tooth centerline as-designed, buy the direct replacement part.
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Old 05-30-09, 07:58 PM   #4
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That is NOT true. As I've shown elsewhere, ring thicknesses vary all over the place, even between models of the same manufacturer. However, thickness is not the only variable in determining tooth centerline, which is what you're really interested it.

Bottom line is, it's a crapshoot. The result might be off, but not off so much that it annoys. Or it might be unshiftable. Only experiment can tell for sure.

If you're really concerned about getting the tooth centerline as-designed, buy the direct replacement part.
Where are you measuring the ring thickness at? I've measured probably 20 chainrings at the teeth and they all measure right around 2mm (with some small manufacturing tolerance). Have you measured tooth centerlines? How much have differed from one ring to the next?
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Old 05-30-09, 08:44 PM   #5
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The thickness is not as important as the lateral center to center distance. From my experience I would expect the 9-speed chain and front derailleur to work will with a good 7-speed crankset. However, a 7-speed Dura-Ace rear derailleur is not compatible with 9-speed shifters due to the difference in accuation ratios. I know that may not be an issue with what you are trying to do, but just in case.....

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Old 06-01-09, 01:20 PM   #6
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Where are you measuring the ring thickness at? I've measured probably 20 chainrings at the teeth and they all measure right around 2mm (with some small manufacturing tolerance). Have you measured tooth centerlines? How much have differed from one ring to the next?
Measured at the bolt holes. One expects that thickness measured at the teeth would be pretty uniform since it is dictated by chain dynamics.

If one assumes that the teeth are centered on the ring plate, then the thickness directly implies the plane of the spider face. Change the thickness and the tooth centerline will be off by 1/2 the difference in thickness.

Complicating matters, naturally, is that some rings appear to have offset cut teeth. I haven't tried measuring tooth centerline directly since the teeth are asymmetrical and one would need an explicit understanding of cut patterns and machining reference points to do a comparison. For the same reason offset would also be difficult to measure.
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Old 06-01-09, 01:26 PM   #7
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in short, yes it will work.
does it work perfectly? no.

I've used 5500 chainrings on a 6400 crank with 9sp chain for a 9sp setup and this seems to work well enough that I don't notice anything outstandingly wrong with it.
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Old 06-01-09, 04:52 PM   #8
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We already know all road chains 7,8,9,10 have an i.d of 3/32 so the question about chainring thickness is moot.
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Old 06-01-09, 04:58 PM   #9
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I running a 7400 crankset with a 9 speed cassette and chain and it runs fine. No noise or shifting problems.
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Old 06-01-09, 05:05 PM   #10
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Measured at the bolt holes. One expects that thickness measured at the teeth would be pretty uniform since it is dictated by chain dynamics.

If one assumes that the teeth are centered on the ring plate, then the thickness directly implies the plane of the spider face. Change the thickness and the tooth centerline will be off by 1/2 the difference in thickness.

Complicating matters, naturally, is that some rings appear to have offset cut teeth. I haven't tried measuring tooth centerline directly since the teeth are asymmetrical and one would need an explicit understanding of cut patterns and machining reference points to do a comparison. For the same reason offset would also be difficult to measure.
Changes in thickness, assuming the thickness is all removed from one side as it is on every chain ring I've ever seen, will not impact the tooth centerline. The mounting surface stays the same regardless of the thickness of the tab so the only dimension that can affect the tooth centerline is the offset of the teeth from that mounting surface.

I hadn't considered that offset before and upon inspecting a few different Shimano chainrings last night I did note that the tooth centerline does appear to be at different distances from the chain ring mounting surface (different only by about 0.5mm though). With that said, I would expect to only find significant variation for this dimension on stamped rings as creating that offset is much easier with a stamping than a CNC ring where considerable extra material would need to be removed to create a big offset.

Measuring offset would as easy as placing the ring on it's mounting surface, indicating off that surface, and then touching off on either side of the teeth (where the thickness is uniform) and taking the average (or just subtracting half the tooth thickness).

Does anyone know if all/most Shimano chainrings are stamped (and possibly secondary CNC'd) as opposed to solely CNC'd? All of the rings that I have are stamped (new Shimano Tiagra, 9 speed 105, Exage, 7 speed 105).
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Old 06-01-09, 05:56 PM   #11
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I use an FC-7400 with 9-speed rings on my bike and it works fine. The 9-speed rings aren't any thinner than the original 6-speed rings but the teeth of the 9-speed inner ring are offset closer to the big ring.
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Old 06-02-09, 10:34 AM   #12
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We already know all road chains 7,8,9,10 have an i.d of 3/32 so the question about chainring thickness is moot.
You usually have some method behind your terseness, operator. But this one makes no sense at all.
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Old 06-02-09, 10:53 AM   #13
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Originally Posted by joejack951 View Post
Changes in thickness, assuming the thickness is all removed from one side as it is on every chain ring I've ever seen, will not impact the tooth centerline. The mounting surface stays the same regardless of the thickness of the tab so the only dimension that can affect the tooth centerline is the offset of the teeth from that mounting surface.
No. Incorrect conclusion as I understand your argument. (What "thickness [that] is .. removed" are you talking about, the machining for the teeth?)

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The mounting surface stays the same regardless of the thickness of the tab..
Possible, but there's no evidence (yet) to support that statement. Therefore we can't assume that it's true.

If you have a variety of rings (most of mine are Ultegra - which appears to be all CNC, btw), we might be able to get enough data to establish the truth (or falsity) of the statement.

A better method for measuring tooth offset would seem to be as follows: Place the ring on a flat surface and mike the height of the tip of a tooth (mark the tooth for later). Repeat for several differently-shaped teeth. Then flip the thing over and re-measure the same teeth. No need to take tooth thickness into account provided that you measure from the corresponding point on each side. The difference between each of the two measurements is the datum of interest. It may vary by type of tooth, but if there is an overall offset the difference should be roughly the same for each tooth.

Then we can correlate any discovered offset to ring thickness to see if it tracks.

I'll post a link to my thickness data as soon as I find it. Please contribute.
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Old 06-02-09, 11:01 AM   #14
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Here is the thread.

Again, please measure your rings as per the protocol described therein.





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Old 06-02-09, 12:09 PM   #15
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No. Incorrect conclusion as I understand your argument. (What "thickness [that] is .. removed" are you talking about, the machining for the teeth?)
Let me try to explain again as I think I know where the confusion is coming from. The machining I'm referring to is the counterbore for the chainring nut that is present on each of the 5 tabs on an inner chainring. Changing this counterbore alone will not change the tooth offset. Whether the counterbore is 1mm deep or 2.9mm deep won't change the chainring mounting surface location. I can take pictures if necessary.

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A better method for measuring tooth offset would seem to be as follows: Place the ring on a flat surface and mike the height of the tip of a tooth (mark the tooth for later). Repeat for several differently-shaped teeth. Then flip the thing over and re-measure the same teeth. No need to take tooth thickness into account provided that you measure from the corresponding point on each side. The difference between each of the two measurements is the datum of interest. It may vary by type of tooth, but if there is an overall offset the difference should be roughly the same for each tooth.
On the chainrings I've seen, the tips of all of the teeth, regardless of how the individual tooth is manipulated, are in the same plane. You will not see any wobble when spinning the crankset. Further, all of the teeth start from the same planar surface on either side of the chainring, with the thickness between those surfaces being 2mm. These are the surfaces at which I would take my measurements to find the tooth centerline. Note that all of the Shimano chainrings that I have are stamped and 105 or below.
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Old 06-02-09, 01:10 PM   #16
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Let me try to explain again as I think I know where the confusion is coming from. The machining I'm referring to is the counterbore for the chainring nut that is present on each of the 5 tabs on an inner chainring.
Okay, now your first paragraph makes sense. I agree, the counterbore is irrelevant to the tooth centerline. (And not all rings have counterbores anyway.) I wonder how it entered the conversation? Oh, when I say that I measure at the bolt holes, I mean I measure the main slab in the area of the bolt holes rather than at the outer edge (just in case the thing tapers). Using calipers I straddle the hole (and counterbore) because that's where the ring mates to the spider.


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On the chainrings I've seen, the tips of all of the teeth, regardless of how the individual tooth is manipulated, are in the same plane.
That's the way it generally appears to me too. But I've seen cases where there appears to be an offset. They are noted in the table in the thread I linked to.

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Note that all of the Shimano chainrings that I have are stamped and 105 or below.
I've been anxious to get some data on 105. Please check out the linked thread.
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Old 06-02-09, 01:37 PM   #17
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Okay, now your first paragraph makes sense. I agree, the counterbore is irrelevant to the tooth centerline. (And not all rings have counterbores anyway.) I wonder how it entered the conversation? Oh, when I say that I measure at the bolt holes, I mean I measure the main slab in the area of the bolt holes rather than at the outer edge (just in case the thing tapers). Using calipers I straddle the hole (and counterbore) because that's where the ring mates to the spider.
Glad we settled that.

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That's the way it generally appears to me too. But I've seen cases where there appears to be an offset. They are noted in the table in the thread I linked to.
Interesting. I'll check it out.

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I've been anxious to get some data on 105. Please check out the linked thread.
I'll work on it for you.
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Old 06-02-09, 01:57 PM   #18
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I believe the 7400 crank was designed for 8-speed. But my comments will apply even if it was originally marketed with 7-speed gruppo.

operator is right about the internal diamter of chains being all the same. and his conclusion, that the thickness of chainrings is therefore moot, follows from the initial statement.

As I understand it, the difference in chainring spacing comes from the rings themselves, not from the crankset. Therefore, you'll be fine using newer chainrings on an older crankset.
Or, you could just stick with the original chainrings. If you happen to get the chain stuck between the 8-speed chainrings, then switch to newer chainrings. Otherwise, run the old ones happily.

I'm running 9-speed drivetrain and chain on examples of both options that I listed above:
- Sugino XD2 crank with chainrings optimized for 8-speed - shift perfectly with the 9-speed chain
- Shimano 600 crank with Truvativ chainrings designed for 10-speed. The crank originally shipped with an 8-speed gruppo. Using a 9-speed Dura-Ace front derailer and shifts well with the 9-speed chain.
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Old 06-02-09, 04:14 PM   #19
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I think all this is just a conspiracy by the manufacturers to confuse the heck out of the users and make them buy new stuff.

I say f'em'

Slap on those so-called "9 speed" rings onto that Dura Ace crank and forget about it. The labeling of chainrings is so confusing not even Einstein could figure it out.
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