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chain wrap + cassette choice confusion

Old 08-01-20, 06:36 PM
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specialmonkey
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chain wrap + cassette choice confusion

My bike came with 53/40 chain rings up front and a 13/23 7-speed cassette in the back. The rear derailleur is a Shimano RD-6401 with SL-BS50 bar end shift levers.

The chain wrap calculation (53-40 = 13) + (23-13 = 10) -- or 23t (if I have that right), is under the 28t max capacity according to the sheet. The front 13t difference is max but doesn't exceed it.

I'm looking to get some mainly lower gears in the back but wouldn't mind a higher one than the current small 13t cog (and recently purchased a 12/28 cassette) but am confused by the following ...

If the max capacity is 28t and the max difference up front is 13t, how can a 12/28 cassette be in the 28t capacity? My calculation indicates that would need to be a 29t max capacity? I understand these are conservative estimates and that with prudent shifting technique, 1 to 2+ teeth over capacity should work ... but since this is in the Shimano literature, I'm scratching my head. I also noticed my bar end shift levers are not mentioned, yet they're doing fine shifting SIS or friction.

I also found this page, which looks like my derailleur (but doesn't mention the model number), it says the smallest cog compatible is 13t and largest 28t with 13t max difference up front (which does seem to add up to the 28t capacity).

SHIMANO 600 ULTEGRA REAR DERAILLEUR - BikePro.com / Buyer's Guide / Shimano Road Rear Derailleurs / bicycle parts

I'm assuming the 12/28 cassette I just bought should be OK, except possibly in the low front high rear position?



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Old 08-01-20, 06:53 PM
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Originally Posted by specialmonkey View Post
If the max capacity is 28t and the max difference up front is 13t, how can a 12/28 cassette be in the 28t capacity?
It cannot, the numbers do not add up. You would need less than 13 teeth difference up front not to exceed the 28t wrap capacity. You cannot necessarily have the max front difference AND the smallest rear cog AND the largest rear cog all at once and still not exceed the max capacity. Since you have bought the cassette, why not install it and see how it works? You will need to ensure that your chain is long enough to go into the 53-28 combination, if the chain is too short you can damage the derailleur or frame. The worst that could happen is that you will have a slack chain in the 40-12 combination but that will not break anything.
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Old 08-01-20, 07:07 PM
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Put it on there and carefully try shifting on the stand before you test ride it. Like Mr Brant Jr stated, if the chain is too short you run the risk of pulling the rear dr into the cogs, which is never good. If that happens and you can safely lengthen the chain so that it doesn't fall off the gears when you're in the lowest combination, you're good to go and I envy you. I can only wish that I had that range of gears on any of my bikes because if I did, it would quickly become my favorite goto bike. Good luck,
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Old 08-01-20, 08:15 PM
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If this combination does not work you can replace the derailleur with a Shimano mountain derailleur up to 9-speed (or road up to 10-speed) which has sufficient chain wrap and large cog capacities.
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Old 08-01-20, 09:30 PM
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Thanks for the insights. I'm hoping one tooth doesn't make a difference. I'll report back in a couple of weeks when I try installing the new cassette and new chain. I plan to use a KMC X8. I'm pretty sure that will be OK for 7 speed?

I plan to keep my current chain and cassette in case the above doesn't work out (each are still good) ... but I expect to be able to make the 12/28 work ... what's 1t on the rear small/high side? ... (I don't know what I'm talking about) ...

Not to mention 7 speed cassettes in this configuration are not that easy to come by and are expensive. I believe Shimano still make one in a not so quality, but probably just fine model, the MF-TZ500.

The 12/28 I found is an HG70 that looks contemporary with my bike.

I always use the big / big plus 1.5 link (or something like that) method when fitting a new chain, but will have my mechanic friend do it since I don't have a chain whip or much experience with cassettes.

Originally Posted by dsbrantjr View Post
You cannot necessarily have the max front difference AND the smallest rear cog AND the largest rear cog all at once and still not exceed the max capacity.
Why not!

I found the quote interesting ... it seems to align with what the seller said when I asked a similar question:

I believe I have come across some instances where the max difference on the front and rear adds up to more than the max total difference per shop catalogs. I've always taken that to mean the manufacturer is telling us you cannot have the max difference on the front and back at the same time. So for example, if running the max difference on the rear sprocket, then the front chainring difference has to be something less. Then there is your comment that probably makes all this moot...and that is manufacturers have a tendency to be overly conservative with their capacity limits...and you can usually squeeze out a few more teeth with sensible shifting habits.
Does this mean that if my front was under max by let's say 4, or a 53/44, I could have a smaller high gear (like 11t or 12t) in the rear with a 28t for low? Does that extend to being able to have a larger lower gear (like 30t) in the back as well, assuming the capacity wasn't exceeded? For some reason I thought max rear cog was a hard limit for the RD. This is kind of confusing.

Are you also saying the Shimano document is not wrong, because it's expected that not all variables can be at max?

Originally Posted by dsbrantjr View Post
If this combination does not work you can replace the derailleur with a Shimano mountain derailleur up to 9-speed (or road up to 10-speed) which has sufficient chain wrap and large cog capacities.
Noted.

I'd like to keep the bike as original as possible, since as far as I can tell it's pretty much entirely original ('93 Bridgestone RB-1).

I'm familiar with the Deore RD-MT60 and use them on 2 other road bikes, it's one of my favorite derailleurs, and I have at least one spare.

I'm willing however to sacrifice some originality for more gear choices (I do not want to be seen walking my bike ). 13/23 seemed a bit high but then again I'm getting more comfortable using the 40 up front. I'd been in the large-front-(at almost all times)-is-better camp for a while. I will also appreciate a higher 12t in back for downhill speed.

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Old 08-01-20, 10:05 PM
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I'd be very surprised if your current rear derailleur doesn't work just fine BUT:

Before you start, buy a chain with a removable link and a chain tool. (Almost every 7/8-speed chain out there has a removable link in the box. Common ones are SRAM and KMC. Swap the cassette. Shift onto big sprockets front and rear (on the stand! Or leaning against a wall - not riding the bike.) Do this shift and the pedaling (with your hand) gently. Does the chain and cage of the rear derailleur go tight? Probably does, That chain will not work. Take that chain off (either with the chain tool or by the removable link). Lay the new chain on both big sprockets and feed the rear end through the derailleur pulleys. Now hold the links together, male to female, going from one extreme end to a corresponding middle link of the opposite "sex" so that the derailleur cage is pulled forward but you can still pull it another inch or so. Break the chain 1 link back so you have two smaller links at the ends. (Your removable link will be the missing large link.) Install the missing link.

Now (take a deep breath, that was a lot) shift into the small-small combo. Look at the rear derailleur cage. It will be folded all the way back..The chain may be loose. If it looks OK and the chain runs smoothly and quietly, you are good to go. If it sags excessively or contacts itself more than just barely there are two options. You can tweak the rear derailleur back with the "B" screw, a small screw at the top back of the derailleur body (close to the rear axle. Usually a Phillips or Japanese Phillips-looking JIS screw. Screw it in a little. This will pull the whole derailleur back, tightening all shift combinations Go back and check the big-big to make sure that is still not at or near the limit. Using the "B" screw may also degrade shifting a little.

The other approach is to try taking a pair of links out. Pull the chain together an extra pair and look before you do this. Is the big-big combo going to be OK? (I keep hammering on the bib-big combo because - you will shift onto it someday. If it is too tight, things break. You are going to make the "call of shame" or walk. You may even crash. Damage to the bike might be $10s, $100s, even fatal. You may be able to ease the derailleur forward with the "B" screw but make sure before you commit yourself.

I am near certain that if you do all this with your eyes open, keep the chain as long as possible (noise and rubbing rarely cause issues and small-small is a combo you aren't supposed to use) that you will find the 12-28 with your existing chainrings works just fine. I've been setting up my bikes with triple so they rub a lot in that combo and have been doing this for 40 years. I have no fear of running in small-small as far as I need to on epic climbs when the road levels out and I don't want to do the big double shift.

So go for it. Get that new chain and tool first. as long as bib-big works well (it doesn't have to sound good - you just want to be good and sure that nothing bad is going to happen when you climb that last hill of that epic day and completely space, hauling that front lever instead of the rear first. We've all done it. And small-small needs to work but rub and noise are rarely issues anywhere besides between the ears. Manufacturers spec their derailleurs so big-big works and small-small is pretty.. One tooth more just means you have to sacrifice a touch of pretty.

Edit: Oh, I forgot the other constraint - the big cog on the cassette. Every rear derailleur has a limit to how big a cog it can clear. But it would suprise the heck out of me if any Shimano of the past 45 years cannot clear a 28.

Ben

Last edited by 79pmooney; 08-01-20 at 10:10 PM.
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Old 08-02-20, 06:19 AM
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Originally Posted by specialmonkey View Post
Does this mean that if my front was under max by let's say 4, or a 53/44, I could have a smaller high gear (like 11t or 12t) in the rear with a 28t for low? Does that extend to being able to have a larger lower gear (like 30t) in the back as well, assuming the capacity wasn't exceeded? For some reason I thought max rear cog was a hard limit for the RD. This is kind of confusing.
You cannot (officially at least) use a 30T in back because that violates the max large cog spec. You can possibly violate any of the limit but results are not guaranteed, you are now in test pilot mode. The straightforward thing would be to install your RD-MT60 and pretty much be free to go crazy with gearing but you have the self-imposed originality constraint....
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Old 08-02-20, 06:22 PM
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I find I like small/small sizing as it will give me the most possible length of chain. the critical is to make sure big/big works, because is it doesn't karma will dicate that you shift into that combo, no matter how many times you told youself not to, and depending on your speed you will have something from emabarassing to very bad happen

also to note I am running 12/28 with 53/39 and using dura ace 7200 which is supposed to be 26 max cog front will handled 14 teeth difference....and I didn't get chain wrap. Shimano is known to be conservative with it's numbers


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