Play and wear on SX and CX: Bosch comments on tolerance problems

Update from 28.7.25

Having already discussed the issue in detail with the manufacturer in advance of this article, we now have an official statement from Bosch eBike Systems on the problem. We have added the detailed statement HERE.

Bushing play at Bosch: wear and tear in everyday testing

Over the last two months, a problem with Bosch's current Performance Line CX engines has crept into our test fleet. Initially, the problem was reported to the Whyte Elyte Evo RS endurance test bike with a noticeable flex and a dull clacking noise in the bottom bracket area.

The cause: massive play in the engine mounting. Specifically: the two bolts that fix the engine in the frame are visibly "wobbling". This is due to two worn plastic bushes on the left non-drive side. Important to know: A certain amount of play is not a problem and is even intentional given the "floating" suspension of the engine. However, in some cases the play gets worse or disturbing noises occur. Then there is a need for action.

When remeasuring, it turns out that a bushing with an outer diameter of 15.9 mm meets a 16.3 mm wide bushing seat. This discrepancy causes play, which becomes noticeable over time in the form of noise or significant wear on the bushings. Hoping for a quick solution, we procured a pair of new bushes as a precaution. However, even before installation, it turned out that these bushes were also undersized for the seat in the frame. According to the frame manufacturer, the dimensions of the frame are correct, so the problem could lie in the accuracy of fit of the bushes.

Not an isolated case

In the current E-Enduro test (BIKE 09), two out of four bikes also showed very noticeable play. Only a few kilometres on the speedometer, but despite the correct tightening torque, there was already noticeable wobbling of the bushings in the frame on the motor suspension - this indicates excessive tolerances in the accuracy of fit between the motor hardware and frame.

Briefly explained: Bosch engine suspension

With the introduction of the Light drive SX was presented by Bosch a new fastening system for the motor in the frame: Two bolts should be sufficient to secure the drive. Both can be accessed quickly and easily from just one side. This should make maintenance and service easier for dealers and end customers. On the drive side, the motor is screwed into the corresponding threads with two bolts and tensioned there. On the non-drive side, the fastening bolts run "floating" through plastic bushes, which are intended to compensate for tolerances between the frame and motor. And this is precisely where the problem lies: on our test bikes - and according to feedback from customers - they were unable to sufficiently compensate for the manufacturing tolerances of various frame manufacturers. The result: too much play in the motor mounts on the non-drive side. In extreme cases, this leads to rattling noises, more flex in the bottom bracket area or increased bushing wear - as on our Whyte endurance test bike.

Bosch reacts

According to Bosch, the problem has been known for some time, but is limited to a few isolated cases. The manufacturer also emphasises that minimal play due to the floating bearing is design-related and unproblematic. Even a visible wobble under load is therefore no cause for concern. Only if the play increases, noises occur, the driving experience suffers or bushings wear out is there a need for action. Nevertheless, Bosch says it has been working intensively on a revised solution in recent months due to problematic individual cases.

Result: From autumn, Bosch Service and specialist dealers will be offering New bushes and bolts available free of charge for the affected engines be. The "upgrade kit" works like an expansion plug: when tightened, a conically shaped bolt head presses on the bushing - which then clamps in the frame and is now also firmly seated in the frame on the non-bolted side. In this way, Bosch aims to better compensate for manufacturing tolerances and prevent too much play in the engine mounting. All-clear for all those affected: According to Bosch, slight bushing play is not an acute problem. There is no safety risk anyway, as the motor should sit firmly in the frame even without bushes and there is therefore no risk of falling or anything similar.

The Bosch statement in full

"On eBikes with current Bosch Drive Units (BDU31YY, BDU34YY and BDU38YY), in particular Performance Line SX and CX from 10/2024, there may be play in the motor suspension due to excessive tolerances. This is caused by the bushing, which is designed as a floating bearing on the non-drive side and ensures that the motor floats in the bike frame. This design is generally associated with certain movement tolerances.

Depending on the frame design, frame rigidity and the tolerance pairings between the frame bore and the bushing used, this has an effect on the play. Depending on the shape of the hole, the bushing inside may move more or less. This radial movement does not impair function or safety, but can be noticeable visually or when pedalling and can lead to increased wear of the bushing.

Safety, customer satisfaction and the enjoyment of eBiking are top priorities for Bosch eBike Systems. That's why we have already switched production to a new bushing variant that spreads out and better compensates for possible tolerances in the interaction between the frame and drive system. From autumn, affected customers will be offered a free replacement of the bushings via specialist dealers." - Bosch eBike Systems