Cannot shift to largest cog in rear
 

I am helping someone in my RV park with their very old bike. The shifters disintegrated. He has Sram 5.0 components, eight speed on the rear, three on the front. I put on a Sram 3.0 3x8 speed shifters. The front shifts fine. No matter what I do I cannot get it to shift to the largest cog on the rear. I can physically push the deraiileur and it will shift to the largest cog but it won't by pulling the cable. I went through the usual routine of checking the limit screws but when it won't shift to the largest cog I am not so sure how to check it. Letting it out more did not help. Cable tension makes no difference. Bent derailleur hanger maybe? I can't see anything obvious by looking at it and other than the big cog it shifts perfectly.

I remember your screen name so I'm going to assume that you have done all of the typical things. Bent derailleur hanger is a good suspect. In fact, on an old bike, they have very frequently gotten bent inward at soma time.

Shift into a gear combination that makes the hanger point downward. Then prop the bike up against something and look at the derailleur from the back. If the derailleur arm seems to point toward the rear wheel, that's it.

Park tool makes a device to measure and straighten it but, on a 7 or 8-speed, I have been successful just eyeballing the hanger into line. My technique is to leave the derailleur in place so the mounting bolt hole doesn't get ovalized and clamping a big Crescent wrench over the whole thing. Eyeball the derailleur arm until it is parallel with the cassette cogs and you should be good-to-go.

Checking the hanger is good.
You should if you haven't check the upper jockey wheel for play. It should not move more than mm or so.
It could affect the shifting if it has excessive play.

Is the RDER an "X" series? I think they take a different pull ratio than the n.0 series.

Jockey wheels seem fine. Eyeballing the derailleur hanger isn't telling me anything obvious but maybe just maybe a slight inward bend. I have a local bike co-op here in Tucson and tonight is women's night. I'll take it by there and can use their derailleur hanger tool to check and straighten it. I don't have a big crescent wrench at my winter home to try to fix it myself.

I made sure I got the ones with the correct pull, the .0 series.

Check the wheel dish?

Are you sure you had all the cable fed out from the shifter before you hooked up the derailer?

Will do so tonight.

Yup.

And the shifter gives you 7 clicks?

How would dish affect the cogs?

I'm thinking maybe somebody got a spacer on the wrong side and now the hub's not centered and too far from the derailleur. Not exactly a dish problem, but a check of the dish would reveal it.

Long shot since the other cogs all shift well.

While I'm thinking of it - maybe there should be a spacer between the hub and cassette that's not there?

Yup. The last click (to the big cog) doesn't move it far enough to shift.

I loaded the bike in my truck to go to the co-op. Otherwise, I would check with a different wheel. I likely can do that there if all else fails. It seems unlikely, there is no play in the cassette to speak of and just eyeballing it, nothing looks off.

Just sounds like an indexing miss-match? Perhaps a 3.0 isn't compatable with 5.0, perhaps it's designed for a narrower chain. How does the chain sit on each cog on the way up?

Are lower-end SRAM derailleurs/shifters compatible now?
http://www.bentrideronline.com/messa...t=61767&page=2

Both the 3.0 and 5.0 have 1 to 1 actuation ratios so they should work. And it shifts fine until it gets to the biggest cog.

Limits adjusted properly?

Sorry, I should've read that better.

Hmmm....

Once you've eliminated a bent RD hanger, just for fun I'd try increasing the upper limit screw to just beyond where you think it should stop, and see if it doesn't allow the RD to go a bit further than it's going now.

Unrelated. Rim position has nothing to do with derailleurs.

My initial thought of a bent derailleur hanger was correct. The mechanic at the co-op and I both looked and thought it wasn't bent or was only very very slightly bent. We were wrong as it was bent enough that there was a half an inch difference between the top and the bottom of the wheel. Now oddly it looks bent but is straight. It shifts fine. Six bucks of shop time well worth getting it properly straightened.

Well done goldfinch, I'll put this one in the back of the memory and recall it if I ever get a similar problem.

Nice job.

Keep an eye on eBay for a used Park DAG-2, you can get one relatively cheap and it's nice to have in the toolbox.