Trek Rail 9.8 Chain Slip, why?

[seabeast]

Bought my Trek Rail 9.8 XT in July, and have put about 160 miles on it so far. During this time I've had two problems: persistent creaking noises from the motor area, and now some slipping in the transmission. After a couple weeks at the bike shop, the creaking noise seems resolved (LBS said he removed and reinstalled the motor and noise went away), so this post is focused on the slip.

Symptom: on my last two rides, 30+ minutes into the ride the transmission would occasionally skip and make a loud clunk I could feel and hear. I haven't been able to see it in action, but I think the most likely explanation is chain slippage. Here's what I know:

1. I've confirmed it happens in the top three gears (smallest sprockets). Not sure if it happens in any lower gears, but don't think so.
2. It seems to happen most often in the smallest sprocket.
3. The sprockets themselves all look fine, but Shimano sprockets have sorta funky tooth shapes, so it's possible I'm missing something.
4. I generally can't reproduce it when I try. For example when riding around the street in front of my house.
5. Shimano XT 8100, less than 175 miles on everything.
6. I haven't checked the chain for wear, but that seems unlikely given mileage. My chain wear tool should arrive soon and I'll double-check.
7. The bike shifts fine. In fact, better than any prior bike I've owned. No phantom shifts, no clicking as it hunts for the next gear, nothing.
8. The slippage/skipping occurs when I'm pedaling relatively hard and not shifting.
9. I do almost all my riding in Tour (eMTB-lite) and eMTB modes. Pretty much never use Turbo.
10. I'm a careful shifter, and try to never shift under load.

Any suggestions on causes and ways to debug? Are there potential explanations other than chain slip? Something in the hub?

Another thread suggested the B adjustment screw as a potential cause of chain slip, but that screw is already backed out all the way, and appears to be adjusted correctly according to Shimano instructions. I also noticed the clutch switch was "off"; I don't know if that could causes slippage but based on descriptions of this switch, it seems kind of unlikely.

I might add that I reported this problem to my bike shop when I took it in for the motor creaking, and their tech said he "adjusted the derailleur", but on yesterday's ride the problem occurred again. In addition to the clutch switch being left off, I also noticed today that the derailleur cable attachment wasn't clamped correctly (the cable was not routed through the U-shaped guide), so I'm not super confident with my LBS.

 

[slomtbr]

Hanger alignment
B-tension screw

 

[seabeast]

Hanger alignment
B-tension screw

As mentioned above, I did check the B-tension screw, and it is adjusted correctly according to Shimano docs: the largest sprocket teeth are lined up with the derailleur markers.

I've only checked the hanger alignment visually, but it sure looks perfectly aligned to me. And since the bike is just a couple months old with no crashes or spills, and it shifts great with no noises or hunting, a misaligned hanger seems like a long-shot. But I'll try to double-check, anyway, next time I have a chance.

Reading more about the Shimano Shadow PLUS clutch mechanism, I'm hopeful that maybe part of my problem was having that switched off. I mean, the chain shouldn't be slipping even without it activated, but maybe having it on will keep the chain tensioned enough to avoid the problem.

 

[Slapbassmunky]

Make sure the cable is clamped in the correct position, it can effect the pull ratio. Make sure your chain doesn't have and stiff/twisted links. Make sure the cable is routed properly through the back or the motor assy and into the swing arm (it should be crossed over with the brake hose), if its too tight or incorrectly routed it can pull as the swingarm goes through its arc, this usually manifests in the upper and lower ranges of the cassette.

160 miles is nothing. My last XT chain did 1000 miles before i swapped it for a new one. If anything it will shift better and behave itself better with the clutch off.

Creaking noises from the motor are usually between the motor housing and the small alloy interface plates that hold it into the frame. The small (M5 IIRC?) Torx screws that hold these plates to the motor housing have a relatively low torque value. I have no idea why Bosch specify this value, but I cranked mine up to something more respectable and all the creaking went away immediately.

 

[Asnowmobiler]

I had a similar issue, It only did it on the smallest gear in turbo mode. It ended being my fault, I had the quick link upside down and it would cause the chain to pull away from the gear enough to cause a nasty clunk.
This was on my old Scram NX.

 

[folmonty]

Looking at that cable / derailleur photo I'd say your last comment is correct.

 

[seabeast]

Looking at that cable / derailleur photo I'd say your last comment is correct.

You mean my comment about lacking confidence in my LBS? You may be shocked to hear that this wasn't the only time they've disappointed during the months I've known them.

 

[seabeast]

I had a similar issue, It only did it on the smallest gear in turbo mode. It ended being my fault, I had the quick link upside down and it would cause the chain to pull away from the gear enough to cause a nasty clunk.
This was on my old Scram NX.

This is interesting. I didn't realize quick links were directional, so I'll double-check this! Between my bike shop's inattention to detail, and me cleaning things, it seems entirely possible the link is on backwards.

And after more research, it seems the quicklink for my chain (Shimano SM-CN910-12) is directional, single-use, and out-of-stock everywhere. Great.

 

[folmonty]

You mean my comment about lacking confidence in my LBS? You may be shocked to hear that this wasn't the only time they've disappointed during the months I've known them.

Exactly! Do have some sympathy for the LBS these days with all the challenges of being in business. The eBike thing is pretty new for many and takes a little different mindset. I've seen lots of growing pains in this regards. But cable routing shouldn't be one of them. Not when you're spending north of 10K on an eMTB!

Manufactures would like for you to buy a new master link each time but don't think many people do. You'll be fine reusing it IMHO. One of the chain pliers tool really help make it easier. When reconnecting get the quick link on the top chain line and just press on the pedals and it will click in place.

 

[seabeast]

This is interesting. I didn't realize quick links were directional, so I'll double-check this! Between my bike shop's inattention to detail, and me cleaning things, it seems entirely possible the link is on backwards.

Well, checked my quicklink and it's on correctly. So I can't blame my skips on that. Still, thanks for pointing out that this is something to check.

 

[Alexbn921]

Derailleur hanger needs to be straight. They never are. New hanger, new bike doesn't matter. It needs checked.
edit. You can't look at a hanger and "see" if it's strait.

New chains need to be stripped of grease and then lubed with your favorite lube.

 

[Aikone]

+1 for a proper hanger check, I bought the Park hanger alignment tool a few years back as it’s almost always not straight if there’s a gear issue.

 

[seabeast]

In the name of science, I took a long lunch and went on a quick trail ride this afternoon to see if having the clutch in the ON position helps.

No slips or skips at all. Now, this was only a 7.5 mile ride, and mostly in lower gears, but still, I'll take it as a good sign.

I still intend to do a proper hanger alignment check, and fix that cable clamp, and give the chain a good cleaning and check for any stiff links, but for now I'm hopeful that keeping the clutch ON will avoid whatever was causing the skips. It seems possible the root cause is imperfect derailleur adjustment, due to hanger alignment or cable stretch or who knows what, and the clutch masks it by keeping the chain taught enough to avoid skips.

 

[DrStupid]

Is the chain length correct? A chain that was too long, caused me similar trouble to what you describe. What is the chain tension like on the small cog?

 

[gfunkmc]

Bought my Trek Rail 9.8 XT in July, and have put about 160 miles on it so far. During this time I've had two problems: persistent creaking noises from the motor area, and now some slipping in the transmission. After a couple weeks at the bike shop, the creaking noise seems resolved (LBS said he removed and reinstalled the motor and noise went away), so this post is focused on the slip.

Symptom: on my last two rides, 30+ minutes into the ride the transmission would occasionally skip and make a loud clunk I could feel and hear. I haven't been able to see it in action, but I think the most likely explanation is chain slippage. Here's what I know:

1. I've confirmed it happens in the top three gears (smallest sprockets). Not sure if it happens in any lower gears, but don't think so.
2. It seems to happen most often in the smallest sprocket.
3. The sprockets themselves all look fine, but Shimano sprockets have sorta funky tooth shapes, so it's possible I'm missing something.
4. I generally can't reproduce it when I try. For example when riding around the street in front of my house.
5. Shimano XT 8100, less than 175 miles on everything.
6. I haven't checked the chain for wear, but that seems unlikely given mileage. My chain wear tool should arrive soon and I'll double-check.
7. The bike shifts fine. In fact, better than any prior bike I've owned. No phantom shifts, no clicking as it hunts for the next gear, nothing.
8. The slippage/skipping occurs when I'm pedaling relatively hard and not shifting.
9. I do almost all my riding in Tour (eMTB-lite) and eMTB modes. Pretty much never use Turbo.
10. I'm a careful shifter, and try to never shift under load.

Any suggestions on causes and ways to debug? Are there potential explanations other than chain slip? Something in the hub?

Another thread suggested the B adjustment screw as a potential cause of chain slip, but that screw is already backed out all the way, and appears to be adjusted correctly according to Shimano instructions. I also noticed the clutch switch was "off"; I don't know if that could causes slippage but based on descriptions of this switch, it seems kind of unlikely.

I might add that I reported this problem to my bike shop when I took it in for the motor creaking, and their tech said he "adjusted the derailleur", but on yesterday's ride the problem occurred again. In addition to the clutch switch being left off, I also noticed today that the derailleur cable attachment wasn't clamped correctly (the cable was not routed through the U-shaped guide), so I'm not super confident with my LBS.

View attachment 74917

I have this problem quite often when I catch the mech on stuff... It rotates backwards on the hanger and I have to push it forward by hand

 

[Geedog]

Hello, sometimes there's a build up of mud or maybe a little twig or grass in between the gap on the smallest gears and that can cause the chain to slip over the teeth on the cog, it might not be that but it's always good to check now n again anyway.

 

[seabeast]

Hello, sometimes there's a build up of mud or maybe a little twig or grass in between the gap on the smallest gears and that can cause the chain to slip over the teeth on the cog, it might not be that but it's always good to check now n again anyway.

Full disclosure, prior to the above-mentioned no-slip ride with the clutch enabled, I did find and remove some plant matter from between the two smallest sprockets.

However, I've experience slipping in all three of the top gears, plus both my LBS and I checked the rear sprocket after the first time I had this problem, so presumably at least one of us would have noticed stuff between the sprockets. That said, I suppose it's possible some junk got stuck in the sprocket, the chain slipped a couple times, then the twig or whatever worked its way out, only for similar events to occur again on my next ride.

And regarding the chain length, this is the original chain, plus it has very little slack when the bike is in the lowest gear: with that giant sprocket in use, the derailleur is just about straight. So I don't think a shorter chain would even work.

Anyway, I fixed the cable clamp, then wiped down and lubricated the chain, so I'm going to see if the problem recurs before any more steps. I also "special ordered" a replacement chain at another local bike shop, so when Shimano eventually ships them some stock (December?) I should have one reserved for me.

Thanks everyone for the suggestions. This has been helpful.

-Brett

 

[seabeast]

Full disclosure, prior to the above-mentioned no-slip ride with the clutch enabled, I did find and remove some plant matter from between the two smallest sprockets.

Went on a big ride today: 23 miles, mix of street, fire roads, single track, rock gardens, climbs, downhill. Not a single chain skip.

So I believe the problem is fixed. My money is on the clutch; I think enabling it "fixed" the problem. My guess is that with the clutch off, the chain can flap around, and minor derailleur misalignment or junk in the sprocket can potentially cause a skip. Turning the clutch "on" makes it much less sensitive to those things.