Hacky Racers – Lessons From a Scratch-Build

What would we change?

After a few races, we can point out a few lessons from our design…

Have Plenty of Load on the Front Wheels

The primary issue we’ve faced is mild understeer caused by the rigid back axle. Fortunately our design has the seat quite well forward so the front wheels carry about 50% of the load and this gives them just enough grip to get the car around corners.

Easy Understeer Solution: Duct Tape

On very smooth grippy surfaces we wrap one of the rear wheels with shiny duct-tape, to reduce its grip and allow better cornering. (The RHS wheel on RH circuits, the LHS wheel on LH circuits.)

Alternate Understeer Solution 1: Stiffen the Chassis

With reasonable caster on the front king-pins, any steering action tends to unload the inside rear wheel allowing it to slip and greatly improving turning ability. This effect is widely used by petrol go-karts. However, the effect can only work if the chassis is stiff enough – a bendy chassis just plants all four wheels and the benefits are lost. So, build a chassis that doesn’t easily twist. Ours is, perhaps, a bit too bendy.

In any case this solution will not work if most of the load is on the rear wheels because the outside front wheel will lift instead, until cornering forces cause the car to lean out of the corner at which point the inside rear wheel will lift and the steering will suddenly ‘bite’. The result is likely to be an unpredictable feel to the steering.

Alternate Understeer Solution 2: Fit a Differential

This is what a diff was designed for. However, diffs are heavy, complex, bulky, expensive, and just plain tricky to engineer into a Hacky.

Having a diff also means that rear-wheel braking becomes more complex: each wheel needs to be braked separately.

Furthermore, if one wheel loses grip and spins then both wheels lose traction equally. So the vehicle should have a deliberately flexible chassis (or suspension) to keep both rear wheels planted when steering.

Alternate Understeer Solution 3: Use Two Motors

If each rear wheel is driven separately, then the rear axle offers minimal resistance to steering action, and if one wheel loses grip the other will continue to provide traction. Potentially some electronic wizardry could be used to further enhance steering (and traction) by diverting drive power to the appropriate rear wheel.

Again, two brakes would be required.

Alternate Understeer Solution 4: Shorten the Rear Axle

If the rear axle is shorter – so the rear wheels are close together – then the rear wheels’ influence on steering is reduced in proportion. The ultimate short rear axle is, of course, a single wheel to build a ‘proper’ trike (i.e. one which has two wheels at the front.)

Use a Separate Brake Disk

The brakes-on-sprocket systems works fine. But it does mean that the chain can’t be oiled at will. Pretty soon, the chain becomes stiff and creaky. Iron filings (from chain/sprocket wear) visibly build up around the magentised parts of the motor and chassis indicating that the chain really doesn’t have enough oil.

A separate brake disk – even if it’s just a laser-cut steel disk welded elsewhere on the axle – makes more sense than trying to double-purpose the driven sprocket.

Don’t Crave a Front Brake

Front brakes are in theory better than rear brakes because they allow fiercer and later braking into a corner. However, this wisdom comes from modern motor vehicle design. And a hacky is nothing like a Mondeo or a Goldwing.

A typical hacky has far more weight on the rear axle than on the front axle – unlike a modern car design which has around 60% of the weight on the front axle. And a typical hacky has a fairly low centre-of-gravity compared to its wheelbase, so the weight is not significantly ‘thrown forward’ onto the front axle during braking – unlike a modern bike or motorbike. So, by-and-large, there is always more downforce on the rear axle than on the front axle. The maximum braking effect which can be achieved by a wheel is proportional to the amount of downforce on that wheel. So if the driver’s seat is positioned near the rear axle then braking the rear axle is the logical choice. Unless the driver is wearing lead diving boots.

A hacky doesn’t usually have ABS. Locking the front wheels would cause a front-end skid combined with the complete loss of steering control. But a rear brake can be recklessly applied to lock the wheels without losing the ability to steer: a useful feature in an emergency stop.

And finally… even if your hacky is well-balanced with 50% or more of the weight on the front wheels, there’s still the benefit of rear brake-oversteer to consider. This can be used to flick the car into a sharp turn, after which power-oversteer helps to keep it turning through the corner.

Tyres

Our sack-truck tyres don’t offer much grip on smooth, hard surfaces. We can only assume that high-speed cornering – mysteriously – isn’t a priority for sack-truck manufacturers. Lack of grip is good (drifting around corners) and bad (drifting off the track).

Sack-truck tyres are 2-ply rated and are made from a substance that looks and feels like burnt rhubarb-crumble topping. They wear away extremely rapidly on all surfaces except grass.

Fortunately, mobility scooter tyres (260×85 4-ply – usually grey-coloured) are widely available new and used. These fit onto standard sack-truck rims. They improve handing enormously. And they last many times longer.

Alternator Motors Really do Work

The typical total Wh consumption of our alternator at the end of each race was comparable to the Wh used by other competitors with ‘real’ brushless motors. There was plenty of power, too.