Blackbird - Land speed record breaking car

December 2008

I have been contacted by Jeremy at Greenpower with an intriguing idea; try to break the UK land speed record for electric cars. What a brilliant idea!. So this is henceforth my hobby... Initial thoughts are;

1) there's nowhere long & straight in UK to go for the record

2) use BLDC aero engines for RC planes, light cheap & powerful

3) use lithium batteries - light & "high-C" discharge rates possible

I bought a couple of motors from www.hobbycity.com - only £30 each with 2.5kW output. I did a simulation with 4 wheel drive & 200kW, geared for 230mph. With 0.6m2 cross section and Cd = 0.15 we ought to be able to exceed 300mph on an 8 mile track(!) - or reach nearly 170mph over 1km at elvington (3km track suitable for kilometer record), and well over 200mph at Pendine sands (5 mile straight beach suitable for 1 mile record).

I started thinking about drives for the motors. I can't afford a drive each (there are 80 of them....) and there are question marks about starting the motors with back emf sensorless control, so I reckon I'll make my own controllers. Making a big PWM controller like this also looks a bit daunting so I plan to have an input synchronous buck converter feeding the bridge as a current source, with the bridge switched at fundamental frequency from shaft sensor feedback. So far, I've made prototype PCBs for the sensor and for a single motor drive: I'll make these work on a 2 seat training car for the CAUC team (2.5kW should suit them just fine) & then move on up to a 10kW version for Blackbird. My plan is to have each wheel driven independently, and each wheel to have its own battery pack and 5 independent 10kW drive spindles. This will give modularity and redundancy in the drive system. The BLDC motors will be geared for about 200mph in UK from about 36V  (note we are talking BIG currents here)

Looking at other stuff I'll need, I was encouraged to find "off the shelf" dragster tyres rated at 300mph(!), heard encouraging noises from "lifebatt" - a supplier of lithium batteries, and "ondrives" gearboxes, and have determined that vauxhall corsa uprights look like the thing to use on all 4 corners. Next weekend will see me down at the scrapyard to furnish myself with 4 of these. I will start talking to APracing to try and get some carbon/ceramic brake discs.

I have also spoken to Dr. Kenji Takana at Southampton University aerodynamics department and we might be able to get some help from that quarter (note the Cd = 0.15 above.........)

I have started a 3D design project with a "safety cell" for the driver & battery pack outlines & motor mockups to start working towards target weights & packaging.

It all looks very encouraging, with all-up weight around 700kg. Physically we should be able to get it much smaller than "Dieselmax", and a world record might also be on the cards (note that UK record is presently 137mph, world is ~315, an altogether different kettle of fish). If everything goes to plan in the UK, it's a logical next step & would be much more interesting for any sponsors!

January 2009

I bought a couple of Corsa front uprights from the scrapyard for £60 - I'll get down there again when the weather is nice for a couple more. The bottom balljoints look usable. I also took away the driveshafts & will use part of them. I spoke to a local propshaft company, Congleton Propshafts, on the industrial estate where I work & they put together a suitable package of hooke joints & splined shaft for the driveshafts. These are quite small because there are no shock loads with the electric drive. The hooke joints are light and efficient when compared to CV joints & my steering angles will be infinitesimal. So I have to engineer the Vauxhall hub spline onto the UJ - piece of cake - drill & weld! I'll leave a bit of the CV joint on there so the wheel will stay on if the weld breaks.......

The uprights cleaned up quite well, I've measured them up carefully & used a geometric suspension program to determine wishbone lengths & chassis pickup locations to give me a stable geometric roll centre over 3 inches of bump and 3 degrees of roll. The roll centre is about 30mm higher at the back than the front.

All upper wishbones are 190mm so these can all be made on the same jig. Similarly all the lower wishbones are 300mm, Chassis pickup points are:

front: bottom pivots 183mm from ground and 320mm apart. Uppers 200mm higher and 84mm further apart.

rear: bottom pivots 223mm from ground and 320mm apart. Uppers 195mm higher and 54mm further apart.

At the rear, a turnbuckle from the rear upper wishbone pivot on the chassis to the steering arm will be used to set the tracking. Springs and dampers will be acuated via pullrods from the upper balljoint mounting brackets on the uprights.

March 2009

I bought another pair of corsa uprights, these were SO much easier to remove now I've done it before! The idea of using the old corsa driveshafts was a complete non- starter (they're made of the hardest metal known to man.....), so I've had some machined, splines, threads and all, out of billet EN8 steel by a very nice man in Rotherham. He's done a lovely job, very prompt & very reasonable; they're being welded to the hooke joint carriers I got from Congleton Propshafts (I turned them up in myself). They said they'd preheat prior to welding. I should be able to put that lot together fairly soon. I have solid models of the uprights & the required extra brackets to make the uprights work with double wishbones;  all looking good. I need to source some billet steel to mill out these brackets; 8 to make, I have the material for 2....

I've added a couple of pictures showing the wishbone geometry for the car. This will allow the use of the same wishbones front & rear & the geometric roll centres stay fixed w.r.t. the chassis, within a centimetre or so, for 2" bump, 1" droop and a few degrees of roll. The overall width is 1.2m

July 2009

It's been a while since i added to this diary. I've done most of the milling on the suspension brackets (dug out of billet EN8 steel) and started making the wishbones and wishbone jigs. I've designed the composite motors and gearboxes in detail and begun the manufacturing. This has mandated a reduction in the wishbone lengths.

The motors have been partially dismantled and the stock connections separated (the motors were hard wired in delta connection) So I now have 6 wires coming out of each motor, meaning that I can wire the 4 motors on a shaft in series (for perfect load sharing). In conjunction with switching to star connection, this will lead to the drives being much higher voltage and lower current. This will allow a big reduction in the amount of copper (wiring) and silicon (power switching). Previously we required 36V; this has now gone up by 4*sqrt(3) to 250V. Note this is now a potentially dangerous voltage. Furthermore the slotted opto disc for position feedback has been abandoned in favour of hall effect sensors mounted between stator poles. This gives guaranteed fixed indexing and continues working in sunshine!! The inverter part of the drive has been run up to 10A (the limit of my power supply), but the current source front end has not been run yet.

The gearboxes: I have been in contact with Glebe engineering in Stoke & they say they will manufacture gears and spurs at cost for the 1st stage of the car (UK record attempt). They were talking about £60 per gear. Rob in Rotherham (who made the driveshafts) says he can do the gearbox casings for £125 each (I'll need 8); so that's looking like about £2500 for all four gearboxes complete. I have a source of the PTFE lined steel rod end bearings for the wishbones, whom I now owe about £400.

Still no word on wheels, though to be honest I don't see why I can't use stock corsa steel wheels, at least for the UK runs!