FatManAndRobin
| FatManAndRobin | |
|---|---|
 | |
| Built By | Ian Beaver |
| Theme | The three-wheeled van from Only Fools And Horses |
| First Race | Makers Central 2019 |
| Current Status | Active |
| Motor | Vevor 2000W brushless |
| ESC | Generic 2000W eBike controller |
| Battery | 48V Li-Ion (2x 6S) |
| Transmission | Chain 9T to 74T |
| Contact | |
| View all Racer Profiles | |
Construction
Chassis uses the front crossmember from a severely crash-damaged '8mph' 4-wheeled mobility scooter which was fished entire from a skip. The front crossmember includes steering and suspension components. Bespoke mild steel sections are welded to make the front and rear of the chassis.
Front track has been widened up to the maximum allowed, to enhance stability. The front axle is positioned well rearwards - under the driver's knees - to further improve stability. The scooter's original front springs and dampers are fitted but, since the total laden weight of this hacky is perhaps only 1/3 the gross weight of the source vehicle, they don't move much in use so are unlikely to have much effect on handling.
Steering handlebars and head-tube are from a 'PUKY' child's cycle.
The single rear wheel is from the crash-damaged scooter but is mounted on a bespoke axle.
Braking is rear-wheel only by a standard cycle disk with cable caliper.
Bumpers are 12mm HDPE, removable for transport.
Theme is constructed from 6mm builder's ply, with LDPE foamboard decorations.
Powertrain
Current: Vevor 2000W brushless motor with a 9T sprocket driving the rear axle via T8F chain and a 74T chainwheel. Rear wheel is 10" rim with 3" tyre giving an approximate dia of 16". Gearing comes to approximately 10,400 motor revs per mile. Motor's no-load speed is 4300rpm, giving a theoretical speed of 25mph - which is perhaps 20% higher than would be ideal but the limited choices of T8F gears means it is difficult to gear down further.
Previously: The original powertrain used the original transaxle. The crash damage included the destruction of one of the half-shafts on the transaxle, which is how this hacky was originally forced to be a three-wheeler. So the original transaxle, complete with mahoosive 24V brushed motor, was mounted offset on the chassis so that the remaining wheel was centred. Initially the remains of the broken half-shaft were locked which, via the diff, doubles the output speed on the remaining half-shaft giving a theoretical no-load speed of 16mph at 24V and 32mph at 48V. This was found to be vastly over-geared so instead the diff was locked, giving in theory about 16mph at 48V, with subsequent timed runs suggesting about 18mph. The brushed motor takes well over 100A from 48V so a current-limiting motor controller (based on an Arduino Nano and a Porter 10 from 4QD) was employed. The Porter usefully also incorporates regen. Mechanical braking was via the emergency stop-brake which is mounted on the motor itself - this wears quickly in use but was never required because of the powerful regen braking from the controller which allowed the kart to be driven entirely on the throttle - including emergency braking. The kart was extremely reliable and the motor survived many races despite being run at twice its rated voltage and rpm, often almost continuously flat-out for 10 minute races. Even with this "mere" brushed motor and ~100kg of driver, the kart was very competitive, demonstrating that brushed motors shouldn't be dismissed. However, when a reverse switch became mandatory a c/o relay was fitted (with a lock-out to prevent it switching when drive power was on). This became a frequent source of failure. The unreliability of reverse eventually forced a changeover to brushless power. With hindsight, a reliable solid-state c/o relay wouldn't have been too difficult to engineer from FETs.
Electrics
Generic 2000W ESC. 12S of 10Ah hobby-grade LiPo, housed in a padded and vented ammunition box. 100A emergency switch with removable knob. Thumb-lever throttle and momentary reverse-switch mounted on the handlebars.
Performance of a Trike
The single rear wheel has some advantages and disadvantages. The obvious question is whether stability is significantly reduced when cornering, but for the most part this trike corners remarkably similarly to a four-wheeler. The worst-case instability scenario is when accelerating out of a corner, occasionally causing a wheel to lift briefly. A more significant issue is that the single rear wheel has less area on the ground and so less grip for forwards drive than two wheels would have, which can be a problem on very muddy circuits. On the other hand several successful four-wheelers also send power to only one wheel - which will have even less weight on it than a trike's single wheel does. The advantages of a trike are that no mechanism is required to prevent the scrubbing losses and understeer associated with having a pair of wheels on a solid axle, and three wheels offer less loss through rolling resistance than four.
Driving
FatManAndRobin is quite an energetic drive. The rider's weight is best moved substantially forwards while cornering to keep the centre of mass within the widest part of the base triangle, increasing stability and maximising the grip of the steered wheels. Weight should be moved rearwards when accelerating to maximise grip of the driven rear wheel. Just as with most four-wheelers, weight should also be moved to the inside when cornering. The current driver (the eponymous 'Fat Man') is therefore grateful that races are now only five minutes duration.
Theme
All good trikes have two at the front, one at the back. Clearly, then, Del Boy's SuperVan required some minor modifications resulting in the rear window becoming the windscreen and the bonnet becoming the boot. You know it makes sense.
BOM
| Fat Man and Robin | ||||||
|---|---|---|---|---|---|---|
| GRAND TOTAL (ACTUAL) | £200.95 | |||||
| GRAND TOTAL (FMV) | £352.63 | |||||
| PART | ACTUAL COST |
FMV | QTY | TOTAL | SOURCE | |
| Accident-damaged scooter | Rascal 8mph 24V moby scooter. Wrecked. Chassis bent. Steering crushed. Transaxle snapped. Missing its electronics. And a wheel. And both brakes. And the seat. |
£3.99 | £0.00 | 1 | £0.00 | Centre Mobility, Aberaeron. Fished from skip. Paid with a box of chocs. FMVs for parts listed separately |
| Motor | Vevor | £98.57 | £98.57 | 1 | £98.57 | AliExpress |
| Controller | eBike 2kW cheapie | £27.51 | £27.51 | 1 | £27.51 | AliExpress |
| Throttle | E-bike hall-effect trigger | £8.00 | £8.00 | 1 | £8.00 | ebay: electricbikecomponents |
| Front suspension arms | Came with accident-damaged scooter. Bent but with usable parts. | £50.00 | 1 | £50.00 | Estimate | |
| Chassis | Bits and pieces of steel | £10.00 | £10.00 | 1 | £10.00 | Estimate |
| Wheels | Came with accident-damaged scooter | £0.00 | £33.00 | 3 | £99.00 | Estimate |
| Knobbly rear tyre | Generic cheapie | £12.99 | £12.99 | 1 | £12.99 | ebay: petrolscooter |
| Rear bearings | Pillow bearing blocks | £6.67 | £6.67 | 2 | £13.34 | ebay: hendersonbearings |
| Chainwheel | 74T / 8mm Mini Moto | £13.22 | £13.22 | 1 | £13.22 | ebay: carhouseuk |
| Ply for body | 6mm builder's ply 3/4 sheet | £10.00 | £10.00 | 1 | £10.00 | Davies Builders Merchants |
| Ply for seat | 10mm builder's ply 1/4 sheet | £5.00 | £5.00 | 1 | £5.00 | Davies Builders Merchants |
| Steering head | Puky child's bike. | £5.00 | £5.00 | 1 | £5.00 | Garage sale, June 2008. At the big white house, the one behind Moylgrove garden centre. |