Bosch CX Race
, by Rosher Perera, 3 min reading time
Recently we have had a top spec CX motor in for a full strip down service. Customer states the pedals were not engaging and therefore the motor had no drive. This is a known issue in the CX gen 4 motors as water can find a route past the NDS crankshaft inner seal and down in between the 2 piece crank. As pictured by ebike motor centre in this handy diagram.
To begin with we examine the state of the motor to confirm what customer has told us which was correct, motor was not engaging at all with chainring. We can then proceed to crack the casing and examine the internals, first impressions were that both weather seals were shot and within the case there was a build up of old grease which is all standard stuff to see. The potentially worrying thing upon first examination was a build up of rusty brown grease in the bottom corner which looked like it had pooled there due to the water ingress past the crankshaft seal.
In order to get the crankshaft fully out of a gen 4 Bosch motor to inspect it fully, the rotating motor assembly and gearbox must be pulled from the stator housing. This allows us to inspect the ratcheting system on the DS of the 2 piece crankshaft system employed in these motors. It was full of rusty grease as expected, which prevents the spring from keeping one half of the crankshaft engaged with the other, creating the issues the customer described.
The DS portion of the 2 piece crankshaft is not stainless and thus is the subject of more corrosion than the other parts that make up the crankshaft and ratcheting system. We also in this instance replaced the other NDS portion of the crankshaft as the sprag had contacted the shaft to the point of wearing down below what we deem to be an acceptable level. This wear to the shaft also coincidentally makes it extremely difficult to repress the sprag back on, meaning it needed replacing either way really. It's worth noting if you are in this scenario that you cannot simply file the ridge away as sprag surfaces from the factory are machined to a very specific finish, this part in general we have found requires replacement.
Later on once the motor was back together we also had to replace the sprag clutch bearing within the pinion gear. As it exhibited symptoms of slipping upon the first test ride, (Loud clicks from the bottom end when the motor is under high load). This likely was a pre-existing condition in this instance due to the condition of the crankshaft which the customer failed to mention as it was probably masked by the bigger issue of the motor not engaging. With regard to bearings, both NDS and DS bearings were either seized or close to being so as pictured above. So those were pressed out and swapped with correctly specced replacements as well as the weather seals being replaced with direct replacements from Bosch.
As mentioned earlier while we were in the motor we observed evidence of water ingress and due to the way the motor is positioned in this frame, the rusty grease and water was left to pool at the bottom of the case. To combat this and reduce potential issues in the future we also applied conformal coating to the board which acts as an additional protective layer against water ingress.
Briefly then, both pieces of the crankshaft were replaced as well as the sprag bearing within the pinion gear, both crankshaft bearings, both weather seals and we applied some conformal coating to the board. The internal idler bearings were both in good condition so no replacements were needed this time. As with our previous motor blog on the Shimano the enemy of all sealed units such as these is water ingress and unfortunately on the gen 4 Bosch they have given the water a direct path to the central part of the motor although without pressure washing or significant water pressure pointed directly at the motor this should not present an issue.