How CVT makes EVs more efficient
CVT is a type of transmission that can convert the rotational power from the motor steplessly. The most distinctive feature of the CVT is transmitting steplessly, not like a bicycle’s gear move step by step.
The CVT makes it possible to continuously change the variable speed ratio, so the motor's rpm can always be kept at the optimum level. This has many advantages, including higher EV efficiency, acceleration performance, hill climbing performance, and load capacity.
Electric mileage improved by 5%
We conducted a comparison of the driving force using commercial CVT electric bikes on the market, with one bike remaining CVT and the other remodeling CVT to a fixed speed ratio transmission. If you want detailed comparison data, please click on the link below to download the document.
In addition to driving force, we also measured electric mileage. The results of the measurement showed that electric mileage was improved by approximately 5% when CVT was installed. This measurement was conducted with WMTC mode.
WMTC mode: A driving mode (vehicle speed command) for evaluating fuel economy or electric mileage. WMTC mode is a global standard developed by the United Nations World Forum for Harmonization of Vehicle Standards.
Structure of CVT
The CVT consists of a pulley with a roller called a "weight roller" and a power transmission belt. The following is an explanation of how stepless speed change is operated.
Figure 1, the top half is a view from the side, and the bottom half is the cross sections of driving side and driven side. The driving side consists of weight rollers, a movable sheave and a fixed sheave, and the driven side consists of a movable sheave, a fixed sheave and a compression spring. Figure 1 shows a low speed, in which the diameter of the belt wrapped around the driven side is larger than that of the driving side. At this time, the weight rollers are on the center axis side. As a result, rpm decreases and the torque increases on the driven side compared to the driving side.
Figure 2 shows a medium speed. As the rpm increases from low to medium, the centrifugal force of the weight rollers on the driving side overcomes the force of the compression spring on the driven side, causing the weight rollers to move radially outward. This causes the movable sheave on the driving side to move downward, increasing the diameter of the belt wrapped around the driving side. The driven side wrapping diameter is reduced accordingly. Compared to low rpm, rpm increases and the torque decreases on the driven side.
Figure 3 shows high speed. At high rpm, the centrifugal force of the weight rollers increases more than at medium speeds, so the diameter of the belt on the driving side becomes even larger. Therefore, the driven side’s rpm increases and torque decreases than at medium speed.
In this way, the CVT enables smooth gear shifting by varying the belt winding diameter with the weight rollers.
CVTs can be divided into two types: belt-type CVTs used in motorcycles and passenger cars, and toroidal CVTs used in large passenger cars. Belt-type CVTs are further divided into two types: metal belts and rubber belts, with rubber belts used for motorcycles and tricycles and metal belts used for passenger cars.
BANDO’s rubber belt for CVT
Extensive experience in belt for CVT used in ICE
There are two types of rubber belts: friction belts, which are also used for fan belts in automobiles, and synchronous belts, which are used for OHC and precision components in automobiles.
Friction belts ・・・ Automotive fan belts, etc.
Synchronous belt ・・・ OHC, precision machinery, etc.
Rubber belts are used in belt-type CVTs for motorcycles and tricycles. Our friction belts are widely used in CVTs for motorcycles equipped with internal combustion engines.
Friction belts used in CVTs include single cog, double cog, and mini-double cog. The belts used differ according to output (displacement), and examples of use are shown below.
Single cog ・・・ 50cc to 150cc scooters
Double cog ・・・ ATV/UTV, snowmobile and big scooter over 250cc
Mini Double COG ・・・ 125cc scooters
With such extensive experience in CVT belts for internal combustion engines, we are able to optimize belt design for CVTs for EVs as well.
World's first! High-load belts with cellulose nanofiber composite rubber
We have developed and sold the world's first double cog belt using cellulose nanofiber (CNF) composite rubber. The belt using this CNF composite rubber meets the high elasticity requirements of next-generation CVT friction belts, and has excellent transmission capacity, separation resistance, abrasion resistance, and crack resistance, which are the basic performance requirements of belts.
The graph below compares the current belt and the belt containing cellulose nanofibers. Compared to the current belt, the belt containing cellulose nanofibers has a transmission capacity of approximately 1.7 times that of the belt containing cellulose nanofibers.
The higher the elasticity of the CVT belt, the better the belt durability and transmission capacity. This has the great advantage that the size of the belt can be made smaller and the CVT unit can be made more compact.
