A trophy truck, features drive with A Continuously Variable Transmission (CVT), steering and suspensions.
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Preview of the instructions at the end of the article.
Building a vehicle with a constinously variable transmission was a will for a long time, since I have built the first CVT mechanical system, inspired by Zblj’s work. Here is some explanation you can find on the first link to understand how does it work :
It enables a variable ratio on the output from 1:1 to 1:5. The CVT chooses the most suitable ratio functions of the resistive torque on the output. The principle is based on two differentials which are connected side by side with a first gear : 1:1 and a second gear on the other side : 1:5. The side with the 1:5 gear has also a friction pin in order to limit the rotation.
While there is not resistive torque (or low), the rotation goes through by the 1:1 gear because the 1:5 has locking torque thanks to the friction pin. When a torque is detected, the friction pin starts to spin and the rotation goes through the 1:5 gear which reduces the speed and increases the torque, the speed of the motor remains constant.
This CVT enables to have :
- A constant speed of the motor (200 rpm)
- A infinity of variable ratio between 1:1 and 1:5
- A variable speed between 200rm (1:1) and 40 rpm (1:5)
- A variable torque between 50N.cm (1:1 = motor torque) and arround 250N.cm (1:5)
Of course, some torque are lost because of the friction pin but the torque is increased by 5 so the loss is negligible regarding the increase.
Nevertheless, including the CVT in a vehicle was very hard to manage and success, especially because of the weight. The performances of the CVT are functions of the friction pins in, it determines the amount of torque the CVT can pass before switching to lower ratio. If the required torque on the wheels of vehicle is to high, the CVT is always in lower ratio, never in higher ratio, which is of course useless. There are two basic ways to solve this problem :
- increasing the friction of the friction pin which permit to have more available torque before it starts to turn (hard to manage and not really energy saver)
- having the less amount of torque on the friction pin, like that, the CVT can use the two ratio (Then, will the CVT have enough torque to move the vehicle ?)
Therefore, the CVT must be placed nearest of the motor in the drive-train with no torque amplifier such as gears. That is why I have placed the CVT directly after the motors and the two motors drive the CVT with a 1:1 ratio. The choice of the motor has been also important : using a high torque motor will cause always slipping on the CVT and 1:5 ratio as result (no use of 1:1). That is why I have chosen a motor with high speed and lower torque as a XL : two L motor.
After the CVT in the drive-train, I have placed a gear reduction in order to have enough torque on wheel without compromise the functioning of the CVT. I placed a 12/20 reduction, then a differential on each axle (20/28t) and then a portal live axle with 8/24 reduction. This combination of gearing ensure to have good speed at 1:1 ratio (12/20, 20/28 ad 8/24) and low speed / high torque in 1:5 ratio (8/40, 12/20, 20:28, 8/24). The differential on each axle is essential for functioning of the CVT. If not, it will be always in 1:5 ratio when turning (due to speed difference between inside and outside wheel).
Concerning the steering, it is very simple, powered by a servo-motor attached directly on the axle and gear a rack pinion with Ackermann effect. The suspension are based on my previous crawler with 5 links : a ball-joint, 2 connecting rod, and 2 shock absorbers.
The last but not least : the rotating wheels of inertia. They are connected to the drive-train. Each wheel is connected to a side of the CVT. The front wheel is connected to the 1:5 ratio axle, and the back wheel to the 1:1 axle. When a wheel rotates, it indicates the chosen ratio. If the two wheels rotate, the ratio is a combination of two (proportional to each speed of wheel). Therefore it has a design and demonstrative roles. But the first aim of each wheel is to give inertia to the system of CVT. The wheels are difficult to put in rotation and when they rotate, difficult to stop, thanks to inertia. This property is used for the functioning of CVT which tends to have a functioning smooth and progressive. Without wheels, the CVT switches of the ratio very quickly, and when I don’t want (acceleration, small obstacle). Thanks to that, the CVT has a latency and changes its ratio when the conditions are required (climb, steep), and not on small or ephemeral obstacle.
Here is the video :
The performance are good, of course the vehicle is heavier with a CVT (1kg500, 2-300gr more) but the functioning of the trial truck is really good : fast on flat ground, and slower with high torque for climbing, also the fact that the CVT changes automatically is really appreciable and valuable for driving. It is handier for the remote of the truck. The clearing abilities are the same of my Trial Crawler but with a automatic adjustable speed.
The pro instructions are available, thanks to ForelockMocs, which are step by step high resolution computer generated instructions with partlist, flexible parts, highlighted parts in the same way as official Lego one. The picturing bill of material are visible in the instruction previews and includes in the instructions.
You can buy the instructions with the following button for 10€. You can also buy other building instructions and get an automated discount, 15% off for 2 PDF, and 20% off for 3 PDF on the total). The automated discount is available for all the instructions for sale on nico71.fr.
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Here is some preview of the building instructions :
with dedicated part list here.