DA 7800------>7900 Compatibility?
 

I have DA 7800 standard cranks and a 12-25 cassette with DA 7800 deraileurs and bar end shifters on my tri bike. I am switching to the new DA 7950 50/34 compact cranks and an 11-28 cassette in the interests of increasing my hill climbing ability. Are the 7800 deraileurs compatible with the compact crank and 11-28 cassette or do I (unfortunately) need to spend more money and get 7900 deraileurs, or can I keep the 7800s and use them?

I believe that the cranks and cassette will be fine. I believe that compatibility becomes and issue when you're talking derailleurs and shifters.

So, if I understand what you are saying, I can put the new crank and cassette on with the 7800 derailleurs and shifters, but if I got new derailleurs as well, I won't need to get new shifters?

You don't need an RD, and probably not a FD. Same number of speeds, so same shifters.

All you need is the crank and cassette. If you want to switch to 7900 shifters, then you also need the FD. The new shifters have no trim from the big ring, so you need the correct FD to match the shifters and their new trimming setup.

The new shifters can also only shift two cogs larger with one sweep of the brake lever, which sucks with a compact. A 3-cog shift is quite common.

Thanks

Quick report back: The DA 7950 Compact crank and DA 7900 11-28 cassette work great with the DA 7800 rear and front derailleurs and DA 7800 bar end shifters.

The 50-34 crank and 11-28 cassette also make for a great hill climbing combo, and I still have good top end with the 50 ring and 11 cog combo.

Has anyone used this setup with a 7800 brake/shifter? I'm looking to do this same setup.

Bar end shifters, just like I said in my post.

This subject is particularly interesting to me too. I have a stock 2008 Madone 5.5 which came standard with a Bontrager 50/34 crank and a SRAM 11/26 Openglide cassette (and SRAM 1090 chain). I think the DA group (shifters, deraillers, and brakes) for 2008 Madones is the DA 7800 series.

I've been wanting to put an 11-28 cassette on my bike to assist climbing but my local shop tells me it will not be a good fit. I currently have issues with the jockey wheel lightly contacting the 26 tooth cog even with the B-tension screw adjusted to the limit.

I'd love to consider the DA 7950 50/34 crank and DA 7900 11/28 cassette, but am still concerned about the fitment and clearing the DA 7800 RD jockey wheel.

What do you think? Will this work well in my mixed DA 7800 setup?

I wish I could tell you, but all I can say is that my exact setup worked, yours mayor may not.

You can get it to work. I'm surprised to hear that your jockey wheel is rubbing on the 26 tooth cog. Is your chain sized correctly? Is the b-tension screw bent or your drop out damaged? You can always use a longer screw to get more clearance.

Worst case, you could easily pull off this gearing change by using a medium cage MTB derailler. You'll save some weight over the long cage MTB derailler which has way more wrap capacity then you need.

It should work without any issue. Worst case, like the other guy in this thread, you'll have to either switch rear deraillers or modify the b-tension screw to get some more clearance out back for the 28 tooth cog. People on this forum have reported getting Ultegra and Dura Ace deraillers (pre 6700 and 7900) to work with 32 tooth cogs.

As best I can determine, Shimano lists the total capacity of the DA 7800 SS RD as 29t. The DA 7900 SS RD is listed as 33t capacity.

That being the case, 50/34 and 11/28 gives 33t... which would just make the specs of the DA7900 SS RD. Is that important at all in the above considerations?

Edit: maybe that's why I'm getting some contact between my RD jockey wheel and the 26t cog... I have 31t and the DA7800 is rated at 29t capacity.

I have to confess that I have no idea what will happen if you exceed the rated capacity of the RD.

Yes, the chain is sized correctly (is the factory chain, but length looks perfect when I check it in the little/little combo). B-tension screw looks good and a check of the drop-out is also good... all is in proper alignment.

I priced the DA7950 crank, DA7900 cassette and DA7900 chain and it is over $800 at Probikekit. Yikes. A bit more than I thought it would be. That crank is expensive.

Somewhat. It all depends on how you size your chain. If you know you are at or above the limit of the wrap capacity of your RD, size the chain using the big/big combo. Worst case, if you ever happen to shift into the small/small by accident (who uses that combo anyway?) you'll just have a rattling chain as opposed to a broken RD or hanger if you used small/small then accidentally went into the big/big combo. Shimano's specs are typically conservative though so you'll likely be ok.

You can see for yourself by checking your current set up. First, shift into small/small combo and see how close the chain comes to the jockey pulley. Estimate how many links could be added and still retain clearance. Next, shift into the big/big combo and see how many extra links you could remove before the derailler would be pulled completely straight. Add those two numbers to your existing wrap requirements and you'll have a good idea of whether or not 33 teeth is too much.

FWIW, I recently set up a 50/34 compact crank on an old Cannondale with an 11/28 7 speed cassette. The 105 (7 speed era) short cage derailler handled all gear combinations just fine.

Well, there's no reason you need to use DA 7900 parts. You could save a ton of money just by using an SRAM PG-1070 11/28 instead of the DA cassette. It'll last longer too (no titanium cogs).

Thanks for that info. While it sounds very promising... I hate to gamble $800+ at this point.

Arghh. Sometimes I'm tempted to yank it all and replace with an Ultegra SL triple groupo. But that would be even more $$.

Other than the cassette, I'd have to use the DA7950 crank, wouldn't I? I don't believe there is a DA7850 compact crank. Sometimes I get lost in all the detail.

I thought Ti would last longer than steel. Not so?

And would the chain matter? I thought there was a special DA7900 chain for the 7900 series.

Well, no there isn't a Dura Ace 7800 series compact but there's no reason why you couldn't use an Ultegra/105/Tiagra level compact to save some (or a lot) of money. There is a special chain for 7900 but I'm 99.9% certain that it will shift just fine using a normal 10 speed chain. There's a lot more fudge room than Shimano would like you to believe.

Ti will certainly not last as long as steel. Think of Ti's durability/strength somewhere between aluminum and steel. Given the same design of a component (in the case of a cassette, the same cog thickness), steel will last longer than titanium which will last longer than aluminum. Ultegra cassettes (or the SRAM PG-1070) are all steel and are the wise choice for the cost conscious consumer.

Good advice there. Thank you.

Am running a 7800 triple crank with the new 7900 11-28 cassette.....had to change out the derailleur though to the new series, am running a normal 10 spd chain.