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Shimano Rear Derailleur RD-6400 Total CapacityHelp

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Shimano Rear Derailleur RD-6400 Total CapacityHelp

Old 03-01-19, 04:29 PM
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bikeaddiction1
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Shimano Rear Derailleur RD-6400 Total CapacityHelp

I am rebuilding a pretty sweet Miyata T6000 Titanium bike. The bike is outfitted with a full Shimano 600 Tri-color group set. The crank is 53-39 teeth and the existing (suspect original) cassette is a 7 speed 13-21 corncob. I would like to give the bike higher gearing and don't want to change the chain rings as it is less expensive to change the cassette and it is showing a bit of wear while the chain rings are fine.

I have found the specs for the RD-6400 rear derailleur on VeloBase and I have also found an original Shimano sales brochure. Both say the max cassette cog is 28 teeth. Velobase states the max capacity for the derailleur is 26 teeth while the Shimano brochure says 28 teeth. Note that the Shimano brochure also says max chain ring difference is 13 but the bike came with a difference of 14 (53-39=14).

Even if I use the max capacity of 28 I am left with a cassette range of 14 (28-14=14). I would like to go up to an 11-28 (difference of 17), or a 12-28 (difference of 16) cassette as they are quite readily available, but I am concerned that the derailleur won't handle going to this big of a change. I have searched for information and am aware that many have used a 28 tooth cassette with a RD-6400 but I have not been able to find the range of cassette that anyone who has a 14 tooth difference on the chain rings has had success with.

My question is, does anyone have experience with running a RD-6400 past the max difference, and if so buy how much?

I may just go buy a new cassette and try it as I already have to get a new chain and if it does not work I can still shorten the chain for a smaller range cassette but I was hoping someone could help me out in advance. I do not want to change the derailleur as the bike will be all original except for wear items (tires, chain, cables) and pedals.

Sorry for all the long details but I know there is a lot of experience on this forum and I am trying to avoid questions on things I have already investigated or considered.
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Old 03-01-19, 04:53 PM
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Not sure I can help other than to say that in my experience you can usually push the published limits by a few teeth by playing with chain length, position of the rear axle in the dropouts and b screw adjustment.

I've got to wonder why you want a 53-11 top gear. That's 129 gear inches, good for 35 mph at 90 rpm. That's really only good for downhills and you can probably go faster by coasting and tucking.
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Old 03-01-19, 05:15 PM
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Following as I have the same situation, updating a Miyata 512 to 914 specs. My large ring is a 52 I believe. Bike currently has 12?/28 6sp freewheel.
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Old 03-01-19, 05:18 PM
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I've got that same derailleur on two Centurion Ironman bikes. Both 53/42 in front and 13x28 7 speed in back. Never had a problem.
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Old 03-01-19, 05:30 PM
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Just to make sure we are reading this right, you are wanting more top end to go faster. You are not wanting more bottom end to go up steeper hills. Is that correct?
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Old 03-01-19, 05:43 PM
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not exactly what you are doing, I just set up an dura-ace ex (max cog 26) up for 13/28 6 speed and a 39/53 crank and it works well on the stand. I fiddle with the chain until it just barely clears the rear derailler cage in small/small. Did this by leaving chain long on first shot and then taking more links off (hoping I didn't go to far) at this it feels like it is at it's limits..
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Old 03-01-19, 05:45 PM
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Originally Posted by seypat View Post
Just to make sure we are reading this right, you are wanting more top end to go faster. You are not wanting more bottom end to go up steeper hills. Is that correct?
Actually, the opposite. I want to put the largest cassette that I can. Howerer when I get a larger cassette I increase the cassette range and I am looking for real word examples of this working.
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Old 03-01-19, 05:51 PM
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Originally Posted by davester View Post
Not sure I can help other than to say that in my experience you can usually push the published limits by a few teeth by playing with chain length, position of the rear axle in the dropouts and b screw adjustment.

I've got to wonder why you want a 53-11 top gear. That's 129 gear inches, good for 35 mph at 90 rpm. That's really only good for downhills and you can probably go faster by coasting and tucking.
I don't want a higher gear, however any new 7 speed cassettes I find start at 11 or 12. It would be awesome if I could find a 14-26 or 14-28, but they don't seam to exist. That is why I am wondering if anyone has used this derailleur with a cassette with this wide of a range. It is funny that if I was looking for the older style freewheel there are lots, if fact I have a bunch in my garage, but not the newer cassette to install on a freehub.
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Old 03-01-19, 05:53 PM
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Originally Posted by Markoshawn View Post
Following as I have the same situation, updating a Miyata 512 to 914 specs. My large ring is a 52 I believe. Bike currently has 12?/28 6sp freewheel.
Just to be clear, you have the RD-6400 derailleur. If so this sounds interesting as the dropout/hanger geometry should be similar from the same manufacturer.
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Old 03-01-19, 05:56 PM
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Originally Posted by Vintage_Cyclist View Post
I've got that same derailleur on two Centurion Ironman bikes. Both 53/42 in front and 13x28 7 speed in back. Never had a problem.
That is the kind of information I am looking for. I wonder if the extra 3 teeth on the small chain ring would make much difference.
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Old 03-01-19, 05:58 PM
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If you exceed chain wrap capacity and size big-big, all that happens is the chain doesn't tension properly in small-small and some things start rubbing. I've found with older drivetrains, things rub in small-small anyways, so not a big deal. Just don't size your chain small-small as that will cause your drivetrain to implode if you ever accidentally shift to big-big.
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Old 03-01-19, 06:09 PM
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If you haven't done it already, you need to see which 6400 hub is on the back. Velobase says the 6400 came in both Uniglide and Hyperglide. Knowing which one you have will prevent buying the wrong cassette.(that is, if the hub is a 6400 series)
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Old 03-01-19, 06:20 PM
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According to Velobase, the 6400 series has the same capacity specs as the 1050 series. I have a 1051 running a triple and a 12-28 cassette. When I tried a 30t, the upper jockey wheel makes contact with the 30 cog.
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Old 03-01-19, 06:57 PM
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Unlike seypat I have had good luck with RD-1051 and a 30t big cog., along with several other Shimano short cage RDs.

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Old 03-01-19, 07:02 PM
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The difference in radius between 28t and 30t is ~4mm, so what work well with one hanger might not work well with another.
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Old 03-01-19, 07:58 PM
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Originally Posted by top506 View Post
Unlike seypat I have had good luck with RD-1051 and a 30t big cog., along with several other Shimano short cage RDs.

Top
It could be because there is a 52/42/30 on front. I am impressed it will do that. The FD-1050 will not swing in enough for a triple, but the 6200 series will.
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Old 03-01-19, 08:14 PM
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Thanks for all the input. As I have said my big concern is ending up with to much chain in small/small. I have a number of 8 speed cassettes in good shape so I think I am going to pull the 12 and 13 rings off of a 12-28 8 speed cassette, and put the 13 ring off of the original 7 speed cassette on to make a 13-28 7 speed cassette and try it. If the chain is too loose in small/small then I will just remove the large ring off of the 12-28 8 speed and make it into a 12-24. 7 speed. I've got the parts so might as well try.
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Old 03-02-19, 12:31 AM
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Ok, so it's lower gearing that you want, not higher. Shimano offered a 13-28t 7s cassette, so the derailer should be nearly rated for 28t max cog size.

I worked on a road bike yesterday that used a 12-32t cassette with a rear derailer rated for only 28t, and it worked ok, but there can be luck involved in terms of the bike having just the right chainstay length, or having horizontal dropout slots for the axle position to be adjusted. The right choice of chain length will be necessary in any case.

And the "capacity" Shimano refers to actually has a completely different meaning. It is the combined difference in front and rear tooth counts, as in (28-13) + (53-39).
Also known as the "chain wrap capacity" of the derailer.
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Old 03-02-19, 10:37 AM
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Originally Posted by dddd View Post
Ok, so it's lower gearing that you want, not higher. Shimano offered a 13-28t 7s cassette, so the derailleur should be nearly rated for 28t max cog size.

I worked on a road bike yesterday that used a 12-32t cassette with a rear derailleur rated for only 28t, and it worked OK, but there can be luck involved in terms of the bike having just the right chainstay length, or having horizontal dropout slots for the axle position to be adjusted. The right choice of chain length will be necessary in any case.

And the "capacity" Shimano refers to actually has a completely different meaning. It is the combined difference in front and rear tooth counts, as in (28-13) + (53-39).
Also known as the "chain wrap capacity" of the derailer.
Yes, I understand derailleur capacity, As I stated in the OP it is 26 or 28 depending if you believe VeloBase or a Shimano sales brochure. Assuming 28 is the correct capacity the rear deraileur can only have a range of 28-(53-39)=14. so I would need a 14-18 cassette which doesn't exist. I plan to push the capacity and was only wondering who has done this and hiow slack the chain gets at small/small.
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Old 03-02-19, 11:28 AM
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Originally Posted by bikeaddiction1 View Post
Yes, I understand derailleur capacity, As I stated in the OP it is 26 or 28 depending if you believe VeloBase or a Shimano sales brochure. Assuming 28 is the correct capacity the rear deraileur can only have a range of 28-(53-39)=14. so I would need a 14-18 cassette which doesn't exist. I plan to push the capacity and was only wondering who has done this and hiow slack the chain gets at small/small.
you are calculating chain wrap incorrectly
the calculation is (difference between small and large on the freewheel/cassette) + (difference between small and large on crank)
so for a 13-28 freewheel and a 39-53 crank it is (28-13 = 15) + (53-39 = 14) = 29
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Old 03-02-19, 11:41 AM
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Originally Posted by squirtdad View Post
you are calculating chain wrap incorrectly
the calculation is (difference between small and large on the freewheel/cassette) + (difference between small and large on crank)
so for a 13-28 freewheel and a 39-53 crank it is (28-13 = 15) + (53-39 = 14) = 29
I think we are saying the same thing, just starting at opposite ends. Either way I am over the 28 by one, and if I use a 12-28 cassette I am over by 2.
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Old 03-02-19, 12:20 PM
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If you are going to exceed the capacity slightly, it is usually no problem at all.

The only possible problem has to do with un-lucky combinations of vertical dropouts and just the wrong chainstay length, since the chain tension cannot be finely adjusted with vertical dropouts.

But if the chain begins to lose tension on the small-smallest sprockets, what consequence is that? The small-smallest sprocket combination is already the worst possible combination and is easy to avoid. And if there is a little slack, at most you will hear some rattling and feel the chain snagging the large chainring (which it would be doing anyway, regardless of chain length and gear ratios).

Again though, the capacity rating is conservative to deal with combined worst-case possible scenarios and toavoid even a hint of chain slack under any condition.

Where you don't want to compromise is with the new, un-worn chain being much shorter than a full two links longer than is needed to wrap over the biggest sprockets while bypassing the rear derailer. I wouldn't fret over 1-1/2 links with the un-worn chain, but would never go tighter than that as a combined front/rear shift might damage parts not including the derailer itself (which is happy to move out of the way of a full-tight run of chain).

What style of dropouts does your bike have? If you have adjustment there you are completely good to go with the 13-28t cassette.
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Old 03-02-19, 12:32 PM
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Originally Posted by bikeaddiction1 View Post
I am rebuilding a pretty sweet Miyata T6000 Titanium bike. The bike is outfitted with a full Shimano 600 Tri-color group set. The crank is 53-39 teeth and the existing (suspect original) cassette is a 7 speed 13-21 corncob. I would like to give the bike higher gearing and don't want to change the chain rings...

I would like to go up to an 11-28 (difference of 17), or a 12-28 (difference of 16) cassette as they are quite readily available, but I am concerned that the derailleur won't handle going to this big of a change.
Is it a cassette or a freewheel?

You really need a 53x12 high gear? That sounds really, really big.

I will guess that rear derailleur will shift onto a 28t cog just fine, given enough chain, but it will have a problem wrapping the chain when on the small chainring and the smaller cogs. A 14-28 should work better. 53x14 is still a really big gear.
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Old 03-02-19, 12:57 PM
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Originally Posted by SurferRosa View Post
...I will guess that rear derailleur will shift onto a 28t cog just fine, given enough chain, but it will have a problem wrapping the chain when on the small chainring and the smaller cogs. A 14-28 should work better. 53x14 is still a really big gear.
They just don't make a 14-28t 7s cassette, and it would be rare for any real problem to occur WRT to chain tension if the chain is cut to the right length for the 53-28t.

Where there are hills that require a 39-28t low gear, the other side of the hill should really benefit from a 13t (vs. a 14t) cog in my experience. I actually prefer a 12t for even the gradual descents in many cases, since I ride rolling terrain, and I am more comfortable pedaling than coasting.
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Old 03-02-19, 01:04 PM
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Originally Posted by bikeaddiction1 View Post
I think we are saying the same thing, just starting at opposite ends. Either way I am over the 28 by one, and if I use a 12-28 cassette I am over by 2.
My early gen dura ace is similar and as noted earlier i have 13-28 and 39/53 working well on the stand..... I am 2 to 4 links longer than big/big in chain sizing. I focused on a) no less than big/big length, and then added lenght to make it so that small/small did not rub on derailler

Shimano is known to be conservative.....I think 1 or 2 over will be ok

Also my derrailler listed max cog size as 26 and is handling the 28 nicely


the 6400 is listed on velobase as 28 cog and chain wrap of 26, you seem to be using 28 for chain wrap, not 26. so I would go 13/28 and max 39/52 if you can

edit 6401 is listed as 28 cog and 28 chain wrap so it does make difference as to which one you have

whatever you do, don't go so big/big does not work......I did this once in error and nearly fell off my bike.....and that was low speed......don't want to think high speed
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