Posted in r/triathlon about changing my rear derailleur so I can put a larger rear cassette on for a hilly course. Someone suggested that although not specified by the manufacturer I might get away with putting a larger cassette on anyway. So i tried putting my road bike wheel on and it seems to shift fine on the stand.
For reference derailleur is a RD-9070 di2, max specified cassette is 11-28 the pictures are of a 11-32 cassette.
Buying the medium cage derailleur is going to cost ~£200.
Picture 1 is derailleur on small chainring, large cassette. I haven’t touched the B-screw, there’s a couple of turns left in that. So could get a little extra gap if required.
Picture 2 is also small chainring, large cassette
Picture 3 is large large, i appreciate this looks horrendous, clearly i need to lengthen my chain, but would avoid this gear.
Picture 4 is small small, again would avoid this gear, but there is a tiny bit of capacity there.
If I lengthen my chain, will this work for 4 weeks of training and a race, or should I bite the bullet and pay the £200.
by boredcourgette
16 Comments
If you have a dura ace sworks you have a responsibility to your bike to use the proper equipment. Get the right derailleur, don’t do some xbike bs. If you’re going to ride this, put another link in the chain, it’s better to have too long than too short.
I used a derailleur extender to get a larger cassette on (36T, will probably try a 38T in future), they don’t cost much, 5-10 bucks.
Can’t you just buy the longer cage, rather than the entire derailleur?
Wolf tooth Roadlink
Derailleur is fine, u need a longer chain
Check the B screw, there needs to be a bigger gap there. Otherwise the chain length looks normal, except the large chain ring and large cassette which is normal, never cross chain like that.
Look up the Shimano chain sizing guide. It usually involves wrapping the chain in the big/big combo (bypassing the derailleur) and adding a set number of additional links. There’s usually leeway for 1-2 links so I bet your chain is now short by at least a couple links.
Also check the pulley wheel clearance spec for that derailleur. It’ll tell you how much clearance you need between the big cog and the upper pulley wheel. You should be able to adjust this with the b screw if it’s not already all the way in.
After making those two adjustments if your chain stays tensioned the full range of the drive train and it shifts ok, then you should be good.
The main problem is clearly the derailleur’s capacity, so this will never work. You’ll need a new derailleur.
Also cmon man, clean your chainset…
Break? No. Prematurely wear out? Yes.
One thing is sure: you’re overstretching everything when you chose extreme gearing, like big chainring/ biggest cog or small chaining/ smallest cog. Those two gear ratios are not recommended.
You can usually get away with going a couple of teeth over, eg a 30, but 4 teeth is pushing your luck
Yes you will absolutely break your derailleur and no there isn’t another cage that will fit on this one. [This guy](https://www.bikeforums.net/road-cycling/1289507-dura-ace-9070-di2-gs-mid-long-cage-conversion.html) goes into why that is. Best bet would be to post it up on eBay before you break it and buy the medium cage Ultegra.
Lots of people will show you hacks to make this sorta thing work but as a shop mechanic I’ve seen SOOOO many of them fail because the b spring can’t handle the stretch and they feel like garbage until they die as quickly as the wear parts.
So this looks terrifying in big/big and I’m shocked you had the guts to try and make that shift. It looks like it wants to break.
Small/small looks fine. So does small/big.
With the E-Tube Project app, you can lock out certain gear combos by using synchro shift.
>Di2 Synchronized Shift is Shimano’s name for the Di2 automatic shift modes. Yep.. modes. There isn’t just one Synchro Shift, but actually two distinct modes, *Full Synchronized Shift* and *Semi Synchronized Shift*.
>According to Shimano; “*(full) Synchronized Shift is a function that automatically shifts the front derailleur when the rear derailleur is shifted, allowing the derailleurs to work together efficiently.*”
>When the other mode – *Semi Synchro Shift* – is selected, the bike will shift the rear derailleur automatically when you shift the front derailleur. In this mode, the bike compensates for the front shift by moving the rear derailleur one or two gears.
So, what you *could* do is run the bike with synchronized shift enabled, and that will keep you out of this big/big combo. You could actually have a set up that will completely brake the bike in big/big, but if you are using full synchro, the bike will never actually shift into that gear combo that will brake the bike.
I think you can run this, as is, just fine, but I would set up synchro shift, to be certain you never shift into that terrifying gear combo. Also, if I were you, I would remap the buttons on your shifts to remove your ability to shift the front derailleur manually at all, so that you don’t accidently make a FD shift and break your bike.
To clarify how this works….
Map your Di2 buttons so that you can only shift the rear derailleur. Remove the FD shifting buttons completely. Then, turn on full synchroshift.
Then, lets say you are riding on the flats and in your big ring and say your 22t cog. Now, lets say you approach a hill and want to make the gears lower. You shift down, to an easier gear, and the bike will shift to the 25t cog. No problem, but now lets say you still need an easier gear and you keep shifting lower.
At this point, instead of the bike shifting the rear derailleur to the 28t cog, it will shift the bike back down to the 22t cog, and shift your *front derailleur* to the small ring at the same time. All with one click of a button. You’ve shifted your FD and stayed out of this danger zone of bib/big, without ever actuating a FD shift manually.
You see whats going on here? You’ve built a safety mechanism into the bike. Lastly, I say remove your ability to make a FD shift so that you don’t fat finger a button a brake your bike.
I would do this if I were you. I’m actually doing this right now on a SRAM AXS setup and its running fine, however, my reasons for doing so are way more lazy than the problem you’ve got.
You can’t really push cassette size on a 2x system due to capacity issues. Guys saying they run a 34 or whatever are thinking about 1x, where it is ok to do this.
Sorry but that short cage derailleur just isn’t going to work. It might feel ok in some gears until it breaks.
Do you really really need 32 on the back? I’m guessing the chain rings are big 53-39. Could you fit a smaller low ring?
Get a long cage 105 derailleur. Just as good shifting performance, and maybe only costs like 60usd or something. Seriously. This current setup could end up costing you a lot more than that.