Rear-wheel drive versus mid-engine

The Swiss manufacturer Stromer uses rear wheel motors (hub motors) for its S-Pedelecs. This is in contrast to most e-bikes on the market, which are equipped with mid-mounted motors. The following advantages are in the foreground:

The rear wheel motor transfers its power directly to the hub without going through the chain and pinion. This ensures:

Comparison of engine performance of STROMER models

* Evaluation of torque specifications for electric bikes

The torque specifications are often misinterpreted when compared to the mid-engine (bottom bracket motor).

For example, the Bosch “Performance Line Speed” mid-engine is specified as 85 Nm. Bosch S-Pedelecs are used by Riese and Müller, for example. At first glance, 85 Nm sounds significantly stronger than 48 Nm for the STROMER ST5.

The torque is usually measured directly at the motor output. This means that with a mid-engine, the entire drive between the front chainring and the rear chainring/gearbox must be taken into account. With a mid-engine, only around 18 Nm**) is effectively delivered to the rear wheel in the highest gear. Depending on the maintenance status of the chain and the type of gear box, this can be significantly less.

This is also the reason why a speed pedelec with a mid-engine is just about sporty up to around 38 km/h, after which the performance drops massively. 45 km/h can only be reached with great effort. With a rear-wheel motor, the full torque is transferred to the road and 45 km/h can easily be reached and maintained.

We receive the same feedback from customers who have test-driven both STROMER and a mid-engine.

** Additional calculation - Highest gear ratio of 52 teeth (front chainring) to 11 teeth (rear pinion) = 1:0.21. This means that only a maximum of 21% of 85 Nm reaches the rear wheel. That is a maximum of 18 Nm minus losses due to the chain drive.

Because of these different views on torque specification, it is worth looking into this topic.