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Feb 16

eRally VXh (2014)

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Hello! I am happy to share with you the eRally VXh, the EV3 successor to the very popular eRally. With its unique hybrid drivetrain, all wheel independent suspension and all wheel drive, the eRally VXh set a time of 23.76 seconds, beating the original eRally and the ThunderSmart III, claiming second place! For more information, read on or watch the video below!

See a video of the eRally below: https://www.youtube.com/watch?v=uYygy5tTT0s

Styling

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Due to the overwhelming amount of praise generated from the last eRally’s styling, I decided that it was best not to stray too far with its successor. That’s exactly what I did; “If it ain’t broke, don’t fix it,” as people always say. The major design has carried over, but there have been many other additions. The most obvious change is that the color has changed from the white of NXT to the black and red of EV3. It may be difficult to see, but the entire body has been widened, as has been the track, giving a more aggressive stance. In addition, the back has been redesigned, including a slightly modified new spoiler. Side panels have been added to give it more substance, and are positioned to look like side air scoops just ahead of the rear wheels. Lastly, since the internals of the VXh have been completely redone, the brick is positioned lower, giving the VXh a much lower, sleeker appearance than its predecessor. Sadly, the rear diffuser has been deleted. 🙁

Steering and Suspension

SuspensionRear Suspension

The steering is a rack-and-pinion system that is powered by an EV3 Medium motor on a 1:1.67 ratio. The front suspension is an independent setup similar to the previous eRally’s, but for the VXh edition, the knuckles have been redesigned to reduce friction resulting from rubbing against the constant velocity (CV) joints. To reduce wobble, the front wheels are attached with a 3-point hub. (pic 1 is an LDD close-up of the front steering/suspension.) The rear suspension has been completely redesigned (pic 2). The geometry is all-new. Control arms have been added; they completely eliminate rear wheel wobble. (As you can see in the video, wheel wobble was a serious problem of the original eRally.)

Drivetrain

Drivetrain

As usual in my builds, the drivetrain is the most important part of the eRally VXh. It is a hybrid setup that employs an EV3 Large motor, coupled to a PF train motor through an adder mechanism. (This is something recently used by DamonMM2000, but only now in the eRally VXh has its potential been fully realized.) The adder mechanism averages the characteristics of the slow but torquey EV3 motor (140 RPM, 43 Ncm) and the fast but weak Train motor (1500-1900 RPM, 3.6? Ncm) The power is sent to the front and rear differentials, then to the tall wheels by a pair of universal and CV joints. The final drive ratio is 1:1. However, the train motor is geared down 3:1 before entering the adder. Why? Without the Train motor’s gearing, instead of the EV3 motor and the Train motor working together to drive the car, the torque of the much stronger EV3 motor takes the path of least resistance and back-drives the Train motor. With the gearing, the Train motor’s torque is multiplied, but its RPM is cut to ~500. However, this gearing has proven necessary and beneficial. Also, there is no center differential; it has been removed to compensate for the extra friction produced by the adder mech.

Program

Full ProgramSteering Center

Contrary to the rest of the car, there is nothing special about the eRally VXh’s program; it is the “simple” RC program I have been using for a while, with the steering values optimized for this particular vehicle. For those of you who are not familiar, this program (pic 1) allows you to control throttle and steering with the EV3 IR remote and receiver. The steering self-centers using the Steer_Center (pic 2), Steer_ReCenter and Steer_Right/Left MyBlocks. The RPM value is displayed on the EV3 screen through the HiTechnic Angle sensor and the Display_RPM MyBlock. No driving modes yet… unless you guys have any suggestions? I’d love to hear them!

Conclusion

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This build started with the ambition of reclaiming 2nd place and I’m happy to report this goal has been achieved. However, the building process did not go at all as planned; what was originally going to be a simple swap ‘n’ drop with parts in the existing chassis ended up being a total redesign of the vehicle. I had to overcome challenges such as friction in the driveline, the power imbalance of the EV3 and Train motors in the adder mech, improving the flawed suspension designs of yore, retaining the awesome styling and above all, meeting the speed goal. I am very pleased with the new eRally VXh; it is a faster, more advanced and better-looking vehicle than before. Tell me you thoughts on the VXh edition; I would be very interested to hear from you! Thank you for reading!

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