Low-end Shimano frustration...

[dookie]

First, I've been a head mechanic at a high-end shop (many years ago), so I'm not new to this stuff...just badly out of practice. So here's my puzzle...

I'm trying to help a friend with his barely better than crappy commuter bike. The RD was misadjusted and he put it in the spokes and ripped it off. Spoke replaced, wheel tensioned/trued...shouldn't matter regarding the problem however. Hangar was replaceable and was, alignment checked...all good there. Chain is also new, because it needed it, rings and cassette not, but look OK to my eye.

Drivetrain is 3x8 Shimano. Shifters are Sora front and Claris rear (crash replacement). I have no idea how well the bike was working prior to the most recent incident, but he says "fine" and was using it daily.

He bought an 8s XT derailleur off eBay, which turned out to be junk and is being returned. In the meantime, I pulled out the only 8s RD I still have on hand and am trying to get things back together for him. It's a (known-good) DA 7402. I locked out the granny ring because the short-cage can't take up the slack, and front shifting seems fine.

Bottom line...8s Shimano is 8s Shimano is 8s Shimano, no? I CANNOT get the rear to shift properly. It will shift cleanly at one end of the range or the other, but not across the entire thing. If I set it up that way, it won't shift at all on the bad side. If I split the difference, it will move the chain every time but ends up chattering on both ends. The weird thing is that it is visibly out of alignment in both directions...ie: jockeys are slightly inboard of the largest cogs and slightly outboard of the small cogs. It's almost as if the spacing is slightly different between the cogs and the resulting movement of the Claris/DA odd-ball pairing. It seems to work slightly better in the big ring (with more chain tension), but still not well.

I have also checked the cassette and confirmed nothing weird there. Crappy 8s Shimano, mostly one-piece, so no question about spacers. Lockring torque is good. Freehub body has a little play, but not to the extent that I'd expect problems, and I checked that it was itself well-secured to the hub shell. I think I have another 8s cassette that I will try, but I haven't dug it out yet.

Any advice appreciated...I know there are some gurus here.

[Jaybee]

I want to say there was a 740x version that used slightly different spacing than other Shimano 8 groups. Could be wrong, maybe check the Sheldon Brown website?

[oldpotatoe]

First, I've been a head mechanic at a high-end shop (many years ago), so I'm not new to this stuff...just badly out of practice. So here's my puzzle...

I'm trying to help a friend with his barely better than crappy commuter bike. The RD was misadjusted and he put it in the spokes and ripped it off. Spoke replaced, wheel tensioned/trued...shouldn't matter regarding the problem however. Hangar was replaceable and was, alignment checked...all good there. Chain is also new, because it needed it, rings and cassette not, but look OK to my eye.

Drivetrain is 3x8 Shimano. Shifters are Sora front and Claris rear (crash replacement). I have no idea how well the bike was working prior to the most recent incident, but he says "fine" and was using it daily.

He bought an 8s XT derailleur off eBay, which turned out to be junk and is being returned. In the meantime, I pulled out the only 8s RD I still have on hand and am trying to get things back together for him. It's a (known-good) DA 7402. I locked out the granny ring because the short-cage can't take up the slack, and front shifting seems fine.

Bottom line...8s Shimano is 8s Shimano is 8s Shimano, no? I CANNOT get the rear to shift properly. It will shift cleanly at one end of the range or the other, but not across the entire thing. If I set it up that way, it won't shift at all on the bad side. If I split the difference, it will move the chain every time but ends up chattering on both ends. The weird thing is that it is visibly out of alignment in both directions...ie: jockeys are slightly inboard of the largest cogs and slightly outboard of the small cogs. It's almost as if the spacing is slightly different between the cogs and the resulting movement of the Claris/DA odd-ball pairing. It seems to work slightly better in the big ring (with more chain tension), but still not well.

I have also checked the cassette and confirmed nothing weird there. Crappy 8s Shimano, mostly one-piece, so no question about spacers. Lockring torque is good. Freehub body has a little play, but not to the extent that I'd expect problems, and I checked that it was itself well-secured to the hub shell. I think I have another 8s cassette that I will try, but I haven't dug it out yet.

Any advice appreciated...I know there are some gurus here.

2 things I would check
-inner wire and housing..sounds like you have some drag in the system.
-make sure the 8s cogset and chain are indeed 8s..spacers included in the cogset. Also sounds like you may have some not proper spacers in that cogset. 8s Spacer thickness should be 3mm...cog thickness is 1.8mm.

HOLD THE PHONE Matilda!!! 8s DA, 7402, is unique in the world of shimano 8s rear ders and are compatible with ONLY DA shifters...ahh..need any other shimano 8 or9s rear der...

[palincss]

I pulled out the only 8s RD I still have on hand and am trying to get things back together for him. It's a (known-good) DA 7402. I locked out the granny ring because the short-cage can't take up the slack, and front shifting seems fine.

Bottom line...8s Shimano is 8s Shimano is 8s Shimano, no? I CANNOT get the rear to shift properly.

https://www.sheldonbrown.com/dura-ace.html

8 speed Dura Ace is the exception. Per Sheldon, 8 speed Dura Ace rear derailleur is compatible with 8 speed Dura Ace shifter or any 9 speed shifter (but not any Shimano 8 shifter other than Dura Ace).

[ntb1001]

I had some problems like that a few years ago building a bike for someone...found out Dura Ace isn’t cross compatible


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[dookie]

Well, ****...there you go.

I thought the Campy 11 / 11A discrepancy was stupid. Should have known Shimano could do stupid better.

Thanks y'all. Guess we'll wait on a non-DA RD. Anyone got a serviceable long-cage?

[Coffee Rider]

2 things I would check

HOLD THE PHONE Matilda!!! 8s DA, 7402, is unique in the world of shimano 8s rear ders and are compatible with ONLY DA shifters...ahh..need any other shimano 8 or9s rear der...

I vaguely recall the 8 speed DA STI shifters I got in 1992 were only compatible the DA derailleur too. I'm glad Shimano has got more mix and match since then.

[Jaybee]

Well, ****...there you go.

I thought the Campy 11 / 11A discrepancy was stupid. Should have known Shimano could do stupid better.

Thanks y'all. Guess we'll wait on a non-DA RD. Anyone got a serviceable long-cage?

Every other Shimano 8/9/10 road or 8/9 mountain RD should work.

[dookie]

Every other Shimano 8/9/10 road or 8/9 mountain RD should work.

Figures that literally the ONLY Shimano RD I have spare is the 7402. Campy 8/9/10, SRAM 9/10, Suntour 7/8, check. No, I don't like Shimano.

[oliver1850]

I probably have a NOS Altus or something similar. PM if you want me to look.

[dddd]

I think that it's worth noting that the early Dura-Ace 6, 7 and 8-speed derailers were roughly the same as many that existed before indexed shifting arrived. That includes Shimano's non-indexed models as well as Suntour's and Simplex's rear derailers, all featuring a similar "faster" action in response to any given cable movement.

Since Shimano 600 SIS was designed during the time that the first Dura-Ace SIS system was seeing millions of miles of service per week, the newer 600 system benefitted from experience with SIS showing that smaller cable travel movements tended to exaggerate the effects of cable elasticity and cable friction.
So the newer 600 and all subsequent rear derailers received an updated "actuation" or cage-to-cable travel ratio, which allowed for longer service intervals and later the use of lower-grade cabling parts with un-sealed cable housing ferrules.

In 1997 the new 9-speed systems arrived with all the different gruppo levels finally having identical actuation between the shifter and rear derailer.

While the early Dura-Ace SIS design specification has it's detractors, this reminds me of critics of old French bicycles, which similarly feature "different" standards because they were the innovators in the first place, the ones who brought forward the first-generation design.
Things such as the aluminum square-taper crankset, now seeming to many as antiquated/inferior, actually brought standards up from the cottered days when they arrived, and lasted for nearly a century (so far).

You can tell though when technology is fundamentally sound, that the particular design in question has a good long run on the market before being retired, just like the early Trek OCLV carbon frame or even the 74XX-series rear derailers.