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Thread: Team Bitza - Hippobotamus

  1. #1
    After a brilliant time at this years BW champs (Thanks Shane & team) Ankle Biter II required some minor upgrades, instead of doing that I've designed something completely different. Drawing from some of the superb drum spinners and beaters at the comp I've come up with my first beater, Hippobotamus.

    The Hippo theme has some what taken over this design. If I can squeeze some servos in it should be able to wiggles its ears and spins its tail! Hopefully it'll pack a punch as well, the weapon motor is rated at 755W and should drive the beater to 438mph, although physics may get in the way of that top speed.

    Specs:

    Power - 4S 1000mAh lipo
    Brushless drive - DYS BE1806 2700kv coupled to a 500RPM 22mm gearbox.
    Wheels - Banebotes T81 2-7/8" Green compound.
    Weapon motor - Propdrive V2 2836 3000KV
    Beater - 100mm x 50mm x 20mm 6061 with 4x M12 caphead teeth to take overall diameter to 74mm. This runs on a 10mm shaft, turned down to 6mm in the middle of the beater.
    Materials - 12mm UHMWPE bulkheads, 5mm UHMWPE ring, 1mm and 2mm 7075 ali for top and bottom plates, as well as front shield.

    Questions:

    1. Do you think I can get away with 1mm 7075 as a base plate? The top will be 2mm and all motors are fixed to the bulkheads.
    2. Any recommendations for suitable brushless drive ESCs, and is there a guide showing how to flash them?
    3. The Propdrive 2836 has a max current draw of 55A, how would I go about calculating if I'll come near to that? A 60A ESC seems a big overkill for a beetleweight!
    4. Just as I finished the design I had a nagging feeling it may drive poorly as all the weight is at the front. There's just enough room to swap the drive and weapon motors around and run the timing belt for the weapon around the drive motors (4mm clearance either side). Would its worth the extra headache all that would bring? The last 3 pictures are a quick mock up of what that might look like.

    Any thoughts or suggestions welcome, what do you think?
    Attached Images Attached Images

  2. #2
    Heya it was great to see ankle Biter at the eruros!
    Most beetle competitions now impose a 250mph tip speed limit (im pretty sure the euros was 250mph aswell). If the base plate is not a structural component then 1mm 7075 would be okay to hold in the electronics but dont expect it to to stop a spinner in the slightest.
    So I use that exact weapon motor in Let the good times roll on 4s. I run it on an afro 30a and only killed itself during the limpet fight when limpets tyre got wrapped round the drum and i was trying to spin up the drum to dislodge it. The afro melts its plastic coating every fight but seems to run okay although the longest it ran for was 2.30 mins not the full 3, bear in mind the drum I use is 540g and that beater looks a bit lighter aswell. I was thinking about upgrading to a 40a esc but apart from the limpet fight which was just unlucky I havnt had problems with the Afro 30a.
    I had driving problems with mine first off, shortening the entire frame by about a centimetre and a half and reducing the friction at the front worked for me but i cant say for sure how your robot would drive in either configuration.
    RANDOM ROBOT CRAP
    Fite me

  3. #3
    Hi, thanks for the input that's exactly the info I needed. Good to know about the 250mph limit, I hadn't considered that at all. How did you get the speed down on let the good times roll? 200 mph is quite a bit to loose!

    Most / all drum spinners that use belts have the drive motors behind the weapon motor so it should be ok as is, but I'm going to spend some time trying to rejig it so the drive motors sit between the beater and the weapon motor.

  4. #4
    Well its all about the gearing between the pully on the weapon motor and the pully on the weapon. For me I have a 25mmish pully going to a 40mm pully and i use this calculator to work out mph ect.: http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php
    The other thing i would say is that countersunk bolt teeth give a much better bite then caphead teeth would. The profile of the teeth really allow the drum to hook under wedges and bite into plastic much more that the caphead profile!
    RANDOM ROBOT CRAP
    Fite me

  5. #5
    After many hours tweaking the CAD file I think it's there. I've managed to squeeze the drive motors inside the drive belt for the weapon, so hopefully weight distribution shouldn't be to big an issue. Running the belt around the drive motor does mean I can't use a small pulley. I'm going to opt for a lower Kv 2836 instead as an easy fix instead.

    I may resort to countersunk bolts, they would definitely be more effective, but I think the capheads add to the hippo theme nicely.

    The protective ring wont look quite like the picture in reality, it pushed my Fusion skills to their limit and I couldn't work out how to remove the sharp points at the front.

    I'm going for Afroflight 12A ESCs for the drive, any advice on flashing them? I've done Tz85A's in the past using the Atmel Atmega flashing tool, does anyone know if that will work with the Afroflights?

    Hippo 1.png
    Hippo 3.png
    Hippo 5.png
    Hippo 2.png
    Hippo 4.png

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