Capacity of rd 7800 dura ace

[jpgibson]

Thanks in advance Im buying(well,bought) some bits from wiggle which included a 11-28 cassette and the above rd..now the max capacity is ok but shimano states max rear as 27t.will a 28 work or should I get 11-25?
Cheers

[Uncle Just]

No you'll need a 7900 RD to accept a 28T rear. 7800 RD will take up to a 27T rear cog.

[jpgibson]

Thanks very much ,hopefully wiggle can change my order.

[Myddraal]

To be honest, it might just fit but if the order hasn't been shipped yet you're definitely better off getting a 27 tooth cassette.

[Robdog]

No you'll need a 7900 RD to accept a 28T rear. 7800 RD will take up to a 27T rear cog.

Wrong. I'm running a 28 no worries
While I was doing some looking to see if it would work I came across some posts by MTB'ers who claimed to be using 7800 RD's on their MTB's with anything up to 32t with no problems (not that I'm advocating that, but personal experience tells me 28 will work)

[RonK]

shimano states max rear as 27t.will a 28 work or should I get 11-25?

According to Sheldon Brown this arrangement will work fine. Read the section on capacity.

This cassette will give you a low gear, but with uncomfortably wide ratios. A compact chain set with an 11-25 cassette would be a better, if more expensive solution.

[toolonglegs]

I am also using one with a 11-28T and compact cranks...no problems,although the small chain ring used with the smaller rings on the rear doesn't have a lot of chain tension...never been a problem thou for me and you are not really going to cruise around in the 34 11/12/13 very often.
..and yes I have also heard that they run happily with XT 9 speed 11-32 cassettes but also I have never tried it.

[Aushiker]

No you'll need a 7900 RD to accept a 28T rear. 7800 RD will take up to a 27T rear cog.

Wrong. I'm running a 28 no worries

Hi

+ 1 and I went from 23T to 28T with no change in the chain length either.

Regards
Andrew

[jpgibson]

they couldnt change the rd order ( to 7900) so I kept the 7800 and the 11-25 cassette.just awaiting delivery.thanks again.

[Uncle Just]

Oh well, anyone want a low mileage 7800 RD? On second thought I may use it on my fast tourer with an XT 11-32.

[thomas_cho]

The 32T will not clear the jockey wheels.

[Uncle Just]

T'was a joke! I wouldn't dream of going past a 28T.

[queequeg]

shimano states max rear as 27t.will a 28 work or should I get 11-25?

According to Sheldon Brown this arrangement will work fine. Read the section on capacity.

This cassette will give you a low gear, but with uncomfortably wide ratios. A compact chain set with an 11-25 cassette would be a better, if more expensive solution.

For both the Dura-Ace 7900 and Ultegra 6700, the sprocket configurations for 11-25 and 11-28 are:

11 - 12 - 13 - 14 - 15 - 17 - 19 - 21 - 23 - 25
11 - 12 - 13 - 14 - 15 - 17 - 19 - 21 - 24 - 28

So, they are identical for all but the last two sprockets.

I run with an 53/39 + 11-28 and find it is just right. I'll usually run it up to the 24 and keep the 28 in reserve for the steep bits. I toyed with the idea of a compact crankset but my bike frame would not accommodate one due to the braze-on front derailleur. In the end this was a good thing!

The issue with the RD capacity for the large sprocket refers to the clearance between the jockey wheel and the large sprocket. I think you can safely get away with using an 11-28 on a RD that can handle up to a 27, but I would not be trying anything more than a 28.

[Robdog]

For both the Dura-Ace 7900 and Ultegra 6700, the sprocket configurations for 11-25 and 11-28 are:

11 - 12 - 13 - 14 - 15 - 17 - 19 - 21 - 23 - 25
11 - 12 - 13 - 14 - 15 - 17 - 19 - 21 - 24 - 28

I'd give up the 12 in a heartbeat for a 21-23-26-28 I think - only complaint I could think of with the 6700

[thomas_cho]

I toyed with the idea of a compact crankset but my bike frame would not accommodate one due to the braze-on front derailleur. In the end this was a good thing!

I have been successfully running a compact crankset on my Colnago with a brazed on FD tab.

I am the same with Robdog, I'd give up the 11T for some closer lower gearing.

[queequeg]

I toyed with the idea of a compact crankset but my bike frame would not accommodate one due to the braze-on front derailleur. In the end this was a good thing!

I have been successfully running a compact crankset on my Colnago with a brazed on FD tab.

I am the same with Robdog, I'd give up the 11T for some closer lower gearing.

In my case, even with the FD as low as possible on the Braze-On, I still had about a 1cm gap to the large chain ring. It probably would have worked, but was well outside the suggested gap of 1-3mm

[RonK]

I still had about a 1cm gap to the large chain ring. It probably would have worked

Jeez - you think so? With a 1cm gap?

I'm surprised to learn a 50T chainring is 2 cm smaller in diameter than a 53T.

[toolonglegs]

It would work absolutely fine with a 1cm gap....I can go from a 55 to 50 if I want to hit the hills and have no issues.Yes if I had the 50 on all the time I would drop the derailleur down.
And obviously when you have no choice...

[queequeg]

I still had about a 1cm gap to the large chain ring. It probably would have worked

Jeez - you think so? With a 1cm gap?

I'm surprised to learn a 50T chainring is 2 cm smaller in diameter than a 53T.

The only real concern was the chain dropping off the outside of the big ring, which is what a friend says happens to him on his compact crankset with a 2cm gap. I wouldn't have really bothered me, but in the end the 53/39 crankset works better for me. I ride my hybrid to and from work every day, and that is 48/36/22 (the 22 never gets used anymore). I was initially worried the 53/39 was going to be too hard, but the first time I jumped on the road bike I found that it was perfect as I don't carry all that extra weight on the the road bike!