Does changing tire size / circumference effect Bosch motor output??

[Zrt1200]

Does changing the tire circumference in the Bosch software effect the torque output??

 

[Greg Watts]

Does changing tire circumference in the Bosch software effect the torque output

Good question, @Zrt1200, and the short answer is: not in the way you might hope. The circumference setting in the Bosch software (adjustable via the Flow app or display) is there for speedometer and odometer accuracy, not motor output.

All the system knows is rear wheel revolutions per minute - it multiplies that by the wheel circumference to calculate your speed and distance. Change that number and your display reads differently, but the motor's torque output is unchanged.

The more interesting bit: the motor's assistance cutoff is based on a wheel RPM threshold hardcoded into the motor itself, not what you've entered into the display. That rotational speed at cutoff is not user-adjustable.

So if you run a physically larger tyre and set the circumference correctly, you actually get a marginally higher real-world top speed before assistance cuts out, because the same wheel RPM carries you further per revolution.

But to directly answer your question - no, the circumference setting doesn't affect torque output. The Bosch CX Gen 5 puts out 100Nm regardless of what tyre size you've entered. The motor responds to your pedal input, cadence, and ride mode. Tyres don't change that equation.

 

[Zrt1200]

Good question, @Zrt1200, and the short answer is: not in the way you might hope. The circumference setting in the Bosch software (adjustable via the Flow app or display) is there for speedometer and odometer accuracy, not motor output.

All the system knows is rear wheel revolutions per minute - it multiplies that by the wheel circumference to calculate your speed and distance. Change that number and your display reads differently, but the motor's torque output is unchanged.

The more interesting bit: the motor's assistance cutoff is based on a wheel RPM threshold hardcoded into the motor itself, not what you've entered into the display. That rotational speed at cutoff is not user-adjustable.

So if you run a physically larger tyre and set the circumference correctly, you actually get a marginally higher real-world top speed before assistance cuts out, because the same wheel RPM carries you further per revolution.

But to directly answer your question - no, the circumference setting doesn't affect torque output. The Bosch CX Gen 5 puts out 100Nm regardless of what tyre size you've entered. The motor responds to your pedal input, cadence, and ride mode. Tyres don't change that equation.

I should have clarified this is the Bosch Gen 4 Non smart system with Purion display. I am also more curios if it produces more start up / take off torque VS top speed

 

[Greg Watts]

I should have clarified this is the Bosch Gen 4 Non smart system with Purion display. I am also more curios if it produces more start up / take off torque VS top speed

Right, @Zrt1200, important clarification and the answer is essentially the same but with a notable caveat for your specific setup.

Your Gen 4 CX with Purion is pre-Smart System, which means you don't have the Flow app or any user-accessible way to adjust the circumference setting.

With the Purion display, the wheel circumference can unfortunately only be changed by the workshop using Bosch diagnostic software. So even if you wanted to fiddle with it, you'd need a dealer visit.

But to your actual question about startup torque versus top speed: no, it doesn't change either. The modification has no effect on the motor support. Only the display values for distance travelled and speed will change.

The Bosch system stores a second, factory-hardcoded wheel circumference value inside the motor controller that you (and the dealer) cannot touch. That hidden value is what determines the actual cutoff RPM. The user-adjustable one just tells the display what to show you.

Your Gen 4 CX with Purion has fixed power modes: the tour setting gives you 140% of assist and a whopping 340% on the turbo mode.