A question for you Bosch CX Gen 5 owners. After a descent at speed when I begin to pedal again there is sometimes a loud clunk that you can hear/feel as the motor engages. It often occurs with what feels like a gap in engagement. I can then feel the hub engage after the clunk and all works fine. This is hindering the bikes performance, a 2026 santa cruz bullit. eg. riding a fast descent, half way down there is a gap after a corner, you come out of the corner and go to pedal... nothing happens for a quarter of a pedal turn (approx) and CLUNK in the motor then another slight pedal turn.. hub engages then pedal to get power. Another scenario where this happens... finish a fast descent with no pedalling, coasting to rest on the raod at the bottom, go to pedal away same thing as explained above. The shop checked it out but they only took it on a flat road... they couldn't reproduce the issue. Has anyone experienced this? Is it normal?
Bosch CX Gen5 Motor Clutch clunk/engagement issue AFTER a descent?
- Thread starter han_slomo
- Start date
Maybe this is what you're experiencing. You're shredding downhill at mach chicken, bouncing off the jumps and rocks, chain flailing in all directions, the chain lands a tooth off, and it ends up riding on top of the sprocket teeth. When you begin to pedal again in earnest, the chain slides over the teeth, back to where it ought to be. You feel a clunk, and then everything is good again. The shop can't reproduce the phenomenon because the road they're on isn't a gnarly black diamond.
- Thread starter
- #5
Checked the chain length and clutch . Eagle 90 system, clutch feels ok and chain has the correct number of links.. less than 300km on the system. One up upper chain guide appears to be in the correct position. I’m going to go find a rough descent and then try to have a look before pedaling again.
ebsocalmtb
Well-known member
I've actually been noticing this more and more in my bullit. Lately, I swear that it's harsher and more pronounced and I swear that it is not something that I ever noticed in the first few months of owning the bike. I'm still sorta taking notes about all of the conditions and the extent that it's doing it to try and direct my lbs towards checking it out. To me, it honestly feels like there is a sprag clutch issue as there is a metallic slipping sound before the hard clunk. It is absolutely for sure, not chain driven drivetrain causing the issue, of that I am certain.
- Thread starter
- #8
I also feel like its something that didn't exist im my first month or so of ownership AND it seems to get worse over time. Ill keep testing and dealing with my LBS. Will post if anything gets resolved. I'm thinking of going in and test riding a bullit once I figure a consistent way to reproduce the issue and see if a new bike does it too... Also have a ride Wednesday night planned that should be good for testing.
Any updates on this?
Mine is doing exactly the same, it does it after first turning it on & riding. It takes a few pedal stroke for the assistance to kick in, it sounds like it’s trying to with a weird momentary clunk/motor noise with every stroke. It will also do it as you’ve described after periods of coasting.
Mine is doing exactly the same, it does it after first turning it on & riding. It takes a few pedal stroke for the assistance to kick in, it sounds like it’s trying to with a weird momentary clunk/motor noise with every stroke. It will also do it as you’ve described after periods of coasting.
- Thread starter
- #10
I did some testing on my rides this week and can not figure out what is causing it. The issue happened on a couple of occasions while descending then returning to pedalling. I took some time to descend then stop and look at the drive train before pedalling again to check if the chain was slightly of etc.. but all was fine making me think it is not drivetrain related. Unless it’s the derailleur clutch on eagle 90 …? I think it’s a motor sprag clutch engagement issue but I haven’t been able to source another bike with a CX gen 5 to compare.
Are you sure that it is not the rear hub? It would explain all of the symptoms.
On my mtb, I had an E13 hub with three spring-loaded pawls on a multi-tooth hub offering 6 degrees of engagement. It was supposed to give "almost instant engagement!". But the hub seals failed very quickly and first one pawl spring rusted through, then a second, until finally after only 128 miles and three months, the hub failed completely. As each hub spring failed, the pawl failed to engage and the symptoms you described emerged.
When the first hub spring failed, the hub engagement would have become 12 degrees; ie a doubling, but not all the time, just one in three! Then when the second one failed, two times out of three it would be 18 degrees of wheel turn before engagement, which was quite a clunk. Followed soon by total failure. The hub, in fact the entire wheelset came with a five-year warranty, which is one of the reasons that I bought that particular wheelset. Yet it failed in three months! It was not at the top of my list for the culprit.
On my mtb, I had an E13 hub with three spring-loaded pawls on a multi-tooth hub offering 6 degrees of engagement. It was supposed to give "almost instant engagement!". But the hub seals failed very quickly and first one pawl spring rusted through, then a second, until finally after only 128 miles and three months, the hub failed completely. As each hub spring failed, the pawl failed to engage and the symptoms you described emerged.
When the first hub spring failed, the hub engagement would have become 12 degrees; ie a doubling, but not all the time, just one in three! Then when the second one failed, two times out of three it would be 18 degrees of wheel turn before engagement, which was quite a clunk. Followed soon by total failure. The hub, in fact the entire wheelset came with a five-year warranty, which is one of the reasons that I bought that particular wheelset. Yet it failed in three months! It was not at the top of my list for the culprit.
- Thread starter
- #12
Good call! I will borrow a wheel and test .
Good idea! Much better than stripping down the hub.
Just make sure that the brake disc is in line with the pads.
- Thread starter
- #15
An update… pretty sure it is the motor clutch. Today I went to pedal after descending and there was a whirring and vibration from the motor but no acceleration then it would engage and work like normal. Apparently according to the AI gods this should throw an error code at the dealer. So I’m going to take it in.
That's not normal.
But to clarify, you're saying you have no resistance when you start pedalling (Indicating a rear wheel hub issue). Or you're saying there's resistance, but no assist for a quarter turn of the cranks (indicating a torque sensor or clutch issue) ?
Have you tried pedaling backwards then forward? That's the easiest way to hear and feel the clunk when the motor engages. Mine does it every time, very annoying.
I must add that you are right that after 1.000 kms it is more noticeable than before. All my other motors do the same (brose and giant, and ep8 when the assistance begins), but the bosch cx gen 5 is getting harder and louder.
I must add that you are right that after 1.000 kms it is more noticeable than before. All my other motors do the same (brose and giant, and ep8 when the assistance begins), but the bosch cx gen 5 is getting harder and louder.
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- Thread starter
- #18
when it happens i have resistance while pedalling... it just sounds like the motor is trying to assist but there is no support... it sounds like a spinning/whirring then it catches and will provide assistance
Good! Let us know your test results
Tuomito
New Member
Hello
I already posted this question but i thought i post it also here and hopefully someone could test if this happens also.
I have a Santa Cruz Vala and the overrun seems to behave strangely.
If i turn the cranks about 1/4 of a turn, the overrun begins and stops and then there seems the be a delayed second kick of the overrun.
You tube link
With sound on you can hear the delayed boost kick and even see it on the video when the camera shakes.
This happens with emtb and emtb+ modes.
Anyone else noticed the same behaviour?
I already posted this question but i thought i post it also here and hopefully someone could test if this happens also.
I have a Santa Cruz Vala and the overrun seems to behave strangely.
If i turn the cranks about 1/4 of a turn, the overrun begins and stops and then there seems the be a delayed second kick of the overrun.
You tube link
With sound on you can hear the delayed boost kick and even see it on the video when the camera shakes.
This happens with emtb and emtb+ modes.
Anyone else noticed the same behaviour?
I see nothing unusual.
If I accidentally use turbo mode, while I’m descending, the clunk almost always happens at the beginning, middle and end of the trail. I quickly switch to EMTB mode if I notice this happening. Most of the time the bike is coasting rapidly.
If I accidentally use turbo mode while climbing a technical trail the bike behaves like a stuttering rocket. It becomes out of control at times.
I don’t hear or experience the clunking or rocketing behaviour when I’m in MTB mode. I try to use regular cadence (80) and regular torque. Constant clunking can damage the motor or mounts or some other components.
The POE is determined by your motor, then your hub. I think the POE of my Project 321 hubs are 216 (correct me if I’m wrong). I don’t know the POE of the Bosch CX Gen4.
If I accidentally use turbo mode while climbing a technical trail the bike behaves like a stuttering rocket. It becomes out of control at times.
I don’t hear or experience the clunking or rocketing behaviour when I’m in MTB mode. I try to use regular cadence (80) and regular torque. Constant clunking can damage the motor or mounts or some other components.
The POE is determined by your motor, then your hub. I think the POE of my Project 321 hubs are 216 (correct me if I’m wrong). I don’t know the POE of the Bosch CX Gen4.
@Bearing Man will probably have a better explanation than I when it comes to the clunking in the motor. What I know is that I don’t like it…so I avoid it whenever possible.
We are monitoring this issue with great interest. However, because these motors are still in warranty, we don't get to see the issues on a daily basis yet. If we get to the bottom of this before you guys I will let you know.
Tuomito
New Member
But there is something unusual. At least I don't think the motor should behave like this. The delayed secondary motor kick is easy to replicate by first coasting with one foot down, standing up, then spinning the cranks backwards 1/4 of a turn, then letting your own weight push the cranks down again. This engages the motor, motor spins and pushes your forward, motor stops then comes the unusual bit, the motor spins again briefly and with significant force. On the trail it's unsettling when it happens.
It's a bit difficult to put in words what's happening..
- Thread starter
- #29
after two test rides on the new motor I have not had either issue again.
the problems were both random and not repeatable on demand but happened every ride at least once. It seemed like problem 1 progressed into problem 2. they were in EMTB+ mode at 90nm and 650-700 watts, assistance and dynamics at stock
1. after a rough descent section, go to pedal... 1/4 turn of the cranks or more no engagement,, then after more turning of cranks heavy clunk + clunk then power/assistance
2. after a rough descent section, go to pedal...feel engagement but no power/assistance. I would hear a whirring spinning sound from the motor area. Keep pedaling with no assistance after 1 or 2 rotations of the cranks whirring stops assistance kicks in.
the problems were both random and not repeatable on demand but happened every ride at least once. It seemed like problem 1 progressed into problem 2. they were in EMTB+ mode at 90nm and 650-700 watts, assistance and dynamics at stock
1. after a rough descent section, go to pedal... 1/4 turn of the cranks or more no engagement,, then after more turning of cranks heavy clunk + clunk then power/assistance
2. after a rough descent section, go to pedal...feel engagement but no power/assistance. I would hear a whirring spinning sound from the motor area. Keep pedaling with no assistance after 1 or 2 rotations of the cranks whirring stops assistance kicks in.
sjturner2112
Member
Oreba WIld owner here, 2025, with CX-5, about 700 miles on the bike/motor - I too am getting this clunk in my emtb+ and turbo+ modes - I can feel it through the pedals, it most often happens maybe after techy sections of trail? Or after descending for a while while not really pedaling, and after jumps. Or just riding around on level ground, I can briefly pedal, stop, and I can feel the overrun kick in, but some sort of clutch connection or something happens where I can feel motor or gears go through a revolution, I can feel it in the pedals, and If I try and pedal right after I feel the cycle/kick through the pedals, I will briefly have no assistance, and then it will sort of lurch back on and provide assistance and everything is fine.
I can mostly recreate it by pedaling hard, and then stopping suddenly and coasting, and then I can feel the overrun kick in, the weird clutch/gear thing happens with a clunk, and then if I try and pedal immediately after the clunk, no assistance for a brief second. Turbo+ and EMTB+ modes, have not tried with the others, and have not had it to the shop yet. Seems to be associated with the overrun. Is getting worse and worse over time. Did not have this behavior when new, started about 200 -300 miles in.
Very similar to Han Slomo's issue, I think.
I can mostly recreate it by pedaling hard, and then stopping suddenly and coasting, and then I can feel the overrun kick in, the weird clutch/gear thing happens with a clunk, and then if I try and pedal immediately after the clunk, no assistance for a brief second. Turbo+ and EMTB+ modes, have not tried with the others, and have not had it to the shop yet. Seems to be associated with the overrun. Is getting worse and worse over time. Did not have this behavior when new, started about 200 -300 miles in.
Very similar to Han Slomo's issue, I think.
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