Telkwa

Our usual ride is about 24 miles on a rail-to-trail. Nothing radical. The Trek odometer is at about 4100 miles.

Over the last few rides I've noticed an alarming skip at the pedals. If I stop pedaling, then start again, the pedals free-fall for a short distance before catching. Not every time, but often enough to cause concern. I assume these mid-drive motors all have some sort of pawl mechanism inside that engages when you pedal but allows free-wheeling the rest of the time?

Back home, I checked the chain. Park chain checker says less than .05. Cleaned any buildup out of the cassette (wasn't much) and cleaned out around the chainring. Because of the way Bosch built these Performance CX motors, access to the little chainring is not good but I got in there with a thin strip of cedar. Didn't see any large clots of material fall out. Pedaled up and down the road. Could not make it skip.

Will go ride again today and see what happens.

Has anyone experienced internal failure of Bosch CX Performance line motors?

cudak888 

There's no chance you're in the small cog on the cassette when this happens, by chance?

-Kurt

Telkwa

Well, I try to avoid the smallest cog because the chain has to twist quite a ways to get there, but it's often on the second to smallest cog when I get the skip. A few more details about our rail-trail ride: 12 miles out, slight uphill, at about 12-14 mph and the 3rd or 4th cog. I start out in Eco, then go to Tour.
Then 12 miles back, typically on the second or third-smallest cog, 17-19 mph in Turbo.
I might get one or two skips on the way out. Sometimes none. Mostly on the return trip. I counted 14 skips the last time on the return leg. The problem is definitely getting worse. A month ago the skip occurred when resuming pedaling. Now it's skipping with consistent pressure on the pedals. Very random. At one point it skipped 3X in less than a minute.

I called the closest Trek Store a couple of days ago. He said it could be the cassette failing. Hate to admit that hadn't occurred to me.
My wife's XS Trek Powerfly 5 has also skipped, but very infrequently. They're both still under warranty. We're gonna take both bikes to the dealer. I'm afraid to go because of the pandemic.

cudak888, do you have a moment to explain where you're going with your question?

veganbikes

Bosch motors rarely fail and if they do it is usually something pretty bad. They can certainly have issues but in terms of motors they probably have the lowest failure rate at least from what I have seen doing warranty for Bosch, Brose and Shimano motors and seeing many other motors as well.

unterhausen

I think usually they have a sprag clutch. Could need lubrication, I suppose.

Telkwa

There's some sort of clutching mechanism inside. Move the pedals forward and the sprocket turns. Move the pedals backward, the sprocket does not move.

veganbikes, I hope you're right. I paid extra money for what I thought was a quality mid-drive system. My confidence in these bikes will be utterly destroyed if the motor fails after 4000 miles. I'll always be waiting for the next shoe to drop.

We're taking the bikes to the Trek shop in a few days. I'll be back with a report.

cudak888 

I'm trying to eliminate the possibility that the chain is physically slipping/skating on the cogs from wear. Not uncommon on modern ramped cogs and what you've described sounds similar to it.

The next time it starts acting up, can you shut off the motor entirely and run unpowered to see if the problem continues to show up?

-Kurt

Telkwa

Yeah, I can do that, but I think we're taking the bikes in tomorrow morning and it's raining today.

The teeth on the cassette sure do look different than the plain old teeth on the bikes we grew up with. Back in the day, all the teeth were the same. Now, they have all sorts of different shapes. I assume that a person who knows what to look for can visually assess the cogs for wear? I looked at the teeth recently, wondering if there was wear, but I can't tell. One way or the other, I'm gonna find out something about these bikes in the next few days!

Telkwa

Someone from the shop called. He was able to get my wife's bike to skip but not mine. Odd. Mine was much worse.
Anyway, he said it's the cassettes. I jumped to the worst possible conclusion thinking the motor was failing.
We discussed how cogs were built in the ancient past. When I was a kid. He said they were thicker and simpler and longer lasting. These new ones are designed to shift more smoothly but they're not as durable.
He pointed out that a Powerfly is not really the best choice for our rails-trails ride. We're clipping along at higher speeds and higher gears almost the whole time. The Powerflys were designed for singletrack.
Unless I start over with different e-bikes I'll have to get used to the idea that our cassettes will need to be replaced at roughly 3500 miles though the larger cogs will have lots of life left in them.
I asked about the front chainring. He said it lasts longer than the thin cassette cogs. A special tool is required to remove the chainring. Sooner or later I'll have to address that but it sounds like not right now.
These e-bikes are darned expensive to keep running!

cudak888 

Ah, but you would have had the same problem on an analog bike.

I've seen premature cassette wear on 13 and 12t Shimano cogs (long before the chain wears) so many times that the aftermarket ought to sell these as replacement parts. (Shimano, of course, won't offer these bits - they prefer it when you spend way too much on a new cassette).

-Kurt

Telkwa

Yes, that would be awesome to be able to swap out the little cogs and keep the cassette.

Thanks for your replies. You were on the right track.

The guy who called from the bike shop said they could not get two replacement cassettes. None of their sources had any. I wonder if this shortage is pandemic-related? I'll have to give it a shot as soon as I verify the range of cogs. Couldn't find it in the bike manuals.

unterhausen

It sounds more like a cassette body to me. Recently had this problem on my fatbike, and fixed it with a new cassette body. Very few miles on that bike, unfortunately. But very hard miles and poor maintenance.

Telkwa

The 2018 Trek Powerfly 5's took an 11-42 rear cassette. Mine runs 29" wheels. My wife's has 27.5 wheels. I thought maybe the cassette would be different but no.
Got the bikes back from dealer yesterday. An employee brought our bikes outside. They're only letting one person at a time in the store because of virus. We talked about replacement cassettes. He said the pandemic had disrupted their supply chains. Said he'd run out of front derailleurs too.
I ordered two at Nashbar. Cassettes are confusing. There are hundreds! I think the bikes came with Deore. There's a Tiagra 11-42T cassette also available. Almost identical. The "CS-HG500-10" label was exactly the same, going by the pictures online which I realize aren't always representative.
If it was Tiagra or nothing I'd buy two, then get Deore when available. Does Shimano offer more than just those two in 11-42T?

Telkwa

Went riding a few times. No skipping. Thanks to everyone for their input.

Still wondering about cassettes and component levels if anyone has the time to reply. Shimano's packaging of the cassettes did not indicate Deore or Tiagra or etc. Just "CS-HG500-10".

I find it hard to believe that an Acera mtb would have the same cassette as a Deore XTR...