Mazda MX5 Conversion Blog
6 January 2008: Subframe prototyping - how to fit the motor and batteries!?
It's been a few months since my last blog entry - partly I've had other distractions stopping me from spending as much time as I should on the conversion, and partly much of the time has just been spend prototyping and discovering what WON'T work! But I think I have the mechanical design sorted now.
I built several prototype revisions of the main subframe(s) holding the front battery pack and motor(s) out of MDF. I will get the real thing fabricated in aluminium - unfortunately it's something I'll have to outsource because I don't have the equipment (waterjet cut the pieces and get a professional TIG welder to assemble).
The original plans of using two Advanced DC L91s were scrapped due to the added difficulty of coupling two motors, and the fact that it turned out they weren't going to fit very far down the transmission tunnel as I had originally hoped. (Plus, as a NetGain distributor, it only seemed right that I should use a NetGain motor!)
The first alternative I considered was one NetGain Warp9 motor, which would be easy to fit and most would consider to be plenty of motor for a 1000kg vehicle, but the 9" motor is a bit borderline in terms of torque for direct drive applications - at least if performance is a priority.
Since I am determined to make this MX5 the fastest electric car in southern hemisphere, I figured I might as well go "all out" and put a Warp11 in there - located mostly in the gearbox tunnel with a little extending into the old engine bay. This motor is certainly not going to be the weak link in terms of power!
The arrangement of the batteries also changed a few times, as I was trying to avoid cutting the existing body/chassis at all and trying to keep the weight closer to the middle of the vehicle (for lower polar moment of inertia = better handling). Unfortunately neither of those quite worked out as I'd hoped. I got a set of foam blocks cut up to the same dimensions as the ThunderSky TS-LFP160AHA cells around November so to test battery fitment etc, which was really valuable - much easier to test fit than the real cells!
I've known for a while that I'd have to split the pack (some cells up front, some in the rear). I was originally hoping to squeeze 8 cells into the old fuel tank cavity and 32 up front, but I asked the experts on EVDL and apparently the ThunderSky cells don't last very long when installed horizontally (i.e lying down), and it's just too shallow a cavity to have them vertically. Also, fitting 32 cells up front was a little too tight.
So after much head-scratching and playing with foam blocks, I'll be going with 24 cells up front in a 3x8 arrangement and 16 cells in the back arranged 2x8, for which I will have to cut a hole in the boot and sink in a battery box. If all goes well it should fit flush with the existing boot floor and extend down to around where the bottom of the muffler was. For the sake of cornering performance it's kind of a shame to have such a lot of weight behind the rear axles, but it won't hurt quarter mile times.
It's a really awkward shaped cavity where the fuel tank used to be, but I'll probably be able to use the space for a few things like the charger, DC-DC converter, circuit breaker, etc.
(1) Park brake mechanism prevents me from putting the 6.7
(2) Would be plenty of space to fit a Warp 9 where the gearbox used to be
(3) An early prototype of the subframe, with 28 cells up front and the Warp 9 motor
(4) Fitment of the above assembly into the engine bay
(5) Test fitting of a Warp 11 mockup, on a stand for correct height
(6) View from the front, star towers used to support the battery box in the correct position for measuring up motor frame
(7) Assembly of the new prototype, holding 24 cells plus Warp 11 motor
(8) New prototype installed in engine bay, fitted to existing engine mounts. One real LFP160AHA cell shown
(9) Eight cells in the old fuel tank cavity, before I found out they shouldn't be installed horizontally - darn.
(10) 16 cells to be installed into a battery box in the rear - will be sunken so as not to consume boot space!
(11) The most expensive part of the project: 40x 160Ah Lithium Iron Phosphate cells from ThunderSky
(12) The prototype frame holding (dummy) motor - looks wonky but in fact it's the engine bay which is assymetical. The battery box attaches on top of front of this frame