chain slipping in high gear with new 12-42 cassette

[JBeasty]

A little background: My bike is a 2020 Giant Sedona DX (comfort bike), and I recently converted it to an ebike. It's now got a BaFang BBS02B 750w. The front has a Lekke 42t Bling Ring - which gives a good chain line. I changed out the rear 11-34 cassette (Shimano) for a 12-42 (Box 4). I also installed a Road Link to drop my Shimano Altus deraileur down, and the cage wheel clears the largest cog. The problem is now the chain slips in 7th and 8th gear. I think there's too much slack in the chain now? Many hours spent watching youtube videos on deraileur adjustment, but its just not working with my current setup. I believe my Altus is a medium cage (63mm center to center). Would a Shimano Deore RD-M591 (long cage) solve my problem?
Thanks, John B.

 

[theemartymac]

34 to 42 is a wee bit of a range change. How does the B tension (Jockey to cassette spacing) look in 7/8 vs 1? Is there too much space down there?

 

[JBeasty]

Thanks for the quick reply! If I understand you correctly (and I measured right); 20-25mm in 7th & 8th, and ~5mm in 1st (with the B tension screw about 1/2 way in. I'll try to post some pics so you can see what it looks like in high & low gear.

 

[theemartymac]

5mm is good. 25 is too slack on the small cogs. You're probably losing an entire tooth worth of engagement, and with a motor adding to your own power, that is a lot. An 11 tooth cog only engages 50% (at best adjustment), and you might only be grabbing 4 teeth.

To test that, you could adjust the b tension to 5-8mm in 7/8 and go for a ride. Don't even try to shift lower as it will hit the cassette. If it runs smoothly in that case, you need to either replace the derailleur with a properly matched one, or perhaps someone else on here can give you some ideas to adjust it for the new range?

 

[JBeasty]

First pic is a close-up in 8th, 2nd pic is in 1st, 3rd pic is in 8th. I'll test it out tomorrow with the B tension set closer to 5mm on the smallest cogs. Might have to find a longer screw...

 

[JBeasty]

So I tried it out with the B tension screw all the way in, all the way backed out, and removed. Still no luck. To be honest, the b tension screw doesn't really do much of anything, at least with the road link on there. I then looked at converting my derailleur and shifter to Box components, that way the system would be all one mfg. They have a derailleur that is specific to the 12-42 cassette. But I think my bike is set up for a direct mount derailleur, and their stuff is all traditional mount. Starting to get frustrating. Maybe I should just go back to my Shimano 11-34 that worked fine? Is it that important to have a 1:1 gear ratio in low for an ebike? I do live in the hills.

 

[theemartymac]

Can you get a B-link to use with the direct mount? Shimano typically makes one.

 

[JBeasty]

After a bit more searching, it appears Microshift advent derailleurs are direct mount. The long cage is compatible with 11-46. From what little I've read, Microshift is comparable to Box 4 components in terms of durability and function. And it's a tiny bit cheaper..