BIMOTAL ELEVATE DYNO V3
End of Line Efficiency Testing for Elevate E-Bike Drive Units (DU)
What Does It Do?
- Measures torque via a torque sensor which is compared with electrical power input to determine DU efficiency
- Catches defects such as bearing slip or excessive NVH before the customer receives their unit
- Establishes quality control for DU efficiency
- Creates an efficiency map for various scenarios (i.e. climbing: low rpm/high torque, or descending: high rpm/low torque)
How Does It Work?
- Power input is provided to the test DU and measured
- The test DU drives a gear which is coupled to a torque sensor
- The torque sensor measures RPM and torque
- A fixed, non-testing DU translates the load into 3 hysteresis brakes which provide braking force.
- Efficiency is calculated from electrical power input and torque sensor data.
Objectives:
- Reduce NVH over previous V2 rig by increasing adjustability of gear mesh
- Add mounting points for 160/180/200/220mm disc brake rotors
- Add compatibility for 2 different torque sensor brands
- Add additional hysteresis brake mounts for additional braking force (3 vs. 1 in V2 rig)
- Keep same reliability parameters as V2 rig (110Nm/306RPM maximum power threshold, survive 250k cycles)
BACKGROUND
Bimotal Elevate System
Elevate is a removable, lightweight, and powerful ebike motor that mounts to a bike's disc brake mounts. Power is driven through the disc rotor, which has a gear bonded to it (called Gear 6 for ease of labeling).
Diagram of Generic Mountain Bike Rear Wheel Assembly
There are various sized disc brake rotors for different bikes and riding styles. Common sizes include 140mm diameter, 160mm, 180mm, 200mm, 203mm, 220mm, and 223mm.
Each bike has a different “native” spacing, which is the size of rotor it is designed for. So bikes that require more braking power, such as downhill bikes, will use larger rotors and their frames will be designed to accommodate this while others may use smaller rotors and those frames will be designed for smaller rotors.
Bikes with larger rotors will have the mounting points for brake calipers farther away from the wheel axle, so there is no collision between the rotor and caliper
Disc Brake Standard: 6-Bolt
- 6 x M5x0.8 bolts screw directly into the hub and secure the rotor in place (typically T25x10mm)
Disc Brake Standard: Centerlock
- One large lockring secures the rotor to the hub
Proprietary M31x1 threading
Driving Side Tower
Driving Side Tower (Full Exploded View, Final Assembly)
Function:
- Hold and position one Elevate DU (Drive Unit) at the correct spacing while power is driven through it.
- Hold adapter hub (pink) and bearing (blue) to simulate wheel/DU interface
Design Consideration: Determining Mount DU Points
Measure within full DU assembly from inside of Gear 6 and Elevate Mount Bracket to determine the offset between the disc brake rotor mount and DU mounting surface. (dZ = 10mm)
Design Consideration: Disc Brake Rotor Spacing
Dashed circles show all possible mounting points for all 4 rotor sizes (smallest is 140mm, largest is 200mm), these are overlaid on the V2 tower. Dimensions for these circles are set by brake caliper spacing standards. Recall that since the brake rotor is circular, the caliper can be mounted at any point as long as the spacing is maintained (hence the circles)
I ultimately decided to design compatibility for 2 sizes on each side of the tower (red lines) due to size constraints. Rotating mounts around the circle did not provide enough clearance to mount in other locations.
Initial Design
As seen, driving tower has compatibility with 140mm and 160mm rotors. 180mm and 200mm rotors compatible with shims (see below picture).
Initial Design Assembly with 160-200mm Shim
Blue shim piece moves mounting surface outwards to accommodate 200mm disc rotor. Through holes ensure secure attachment and quick removal for changing rotor sizes.
Final Objective: Set Screws
Implementation of Set Screws
Implementation of Set Screws
After a design review, it was decided to add set screws to precisely locate the Elevate Mount Bracket for repeatable testing. Red boxes indicate the position of set screws
Implementation of Set Screws
Implementation of Set Screws
Implementation of Set Screws
Final Driving Tower Design
First number denotes native spacing (e.g. 140mm), second number denotes Bimotal Gear 6/rotor size (e.g. 160mm). Elevate requires a rotor one size (20mm) larger than the native spacing.
This design has compatibility for 4 different rotor sizes with only two bolts needed swap. Two DUs can be mounted simultaneously for even faster swap time between tests.
Hub Adapter
Hub Adapter (Final Assembly)
Function:
- Simulate wheel and mount Gear 6/rotor
- Couple to torque sensor shaft to obtain accurate torque readings
Design Consideration: Torque Sensor Compatibility
The torque sensor used on the V2 rig had shorter shafts to couple to while the torque sensor intended to be used on the V3 rig had longer shafts. The V2 design with set screws was incompatible with both torque sensors in the same design.
Solution: Clamp-style Coupler
A clamp-style coupler allows for compatibility with both torque sensors and their respective shaft lengths (and even more torque sensors, provided their keyway dimensions are the same).
It improves concentricity between torque sensor shaft and hub adapter, provides a stronger clamping force, as well as reduces damage to the torque sensor shaft by removing the set screws.
Design Consideration: Gear Swap Speed
With quickly changeable DU mounts, the limiting factor for test set up speed was changing Gear 6. Removing and retorquing 6 individual bolts between each test was unfeasible. However, the Bimotal Gear 6 is only compatible with a 6-bolt mount standard (see background section for more info)
Solution: Centerlock Adapter Hub
By designing holes to press-fit dowel pins into, the Bimotal Gear 6 can still be secured in place then torqued down with one large centerlock (see background section) lock ring, reducing time between gear changes by 600%.
Driven Side Tower
Driven Side Tower (Full Exploded View, Final Assembly)
Function:
- Hold and position one Elevate DU Gearbox to absorb high torque load and translate it to hysteresis brakes at a lower torque.
- Hold Elevate mounting points perpendicular to the base plate for adjustability ONLY in one axis.
Design for this tower was not as complex as the gearbox does not need to be removable. Its only function is to translate the load received from the driving side Elevate.
1-Axis Adjustability
The mounting surface on the tower was designed such that the mounting surface for Elevate is perpendicular to the base plate. Shims may be installed under the mounting studs (small light green parts) to adjust gear mesh, and this mounting angle ensures that the DU's position is only changed in the lateral axis. The ability to shim and micro-adjust the DU improves gear mesh greatly and reduces NVH.
In turn, this ensures that the position change can be accounted for when aligning the hysteresis brakes, further reducing NVH from shaft concentricity issues.
braking
Hysteresis Brake Assembly (Full Exploded View)
Function:
- Absorb load transmitted by DU
- Allow for adjustability to maintain hysteresis brake shaft concentricity
The design for braking remains largely unchanged from the V2 rig, other than the addition of 2 brake towers and 2 more hysteresis brakes, as well as slots in the brake tower bases for adjustability (see below).
Brake Adapter
The brake adapter inserts into the driven side Elevate DU and receives the load transmitted through the torque sensor. It then couples to the series of hysteresis brakes to provide braking force.
Hysteresis Brake Assembly (Top View)
With slots in the base of the brake towers and slots on the baseplate, compression is provided to the brake couplers and alignment of the brake shaft is ensured. Recall that on the driven side tower, the Elevate DU mount angle was such that adjustment only occurred in one axis. The adjustability in the brake assembly takes advantage of this to dial in a perfect alignment.
Final Elevate Dyno V3 CAD Assembly
To Calculate Efficiency:
Efficiency = Power_In/Power_Out
Power_In = W = V (voltage) x I (current)
- Recall that Power_In is electric power
Power_Out = T (torque) x w (RPM)
- Recall that Power_Out is mechanical power