Why Slippery Strana is Fast

One word - Bosch - will explain it all.

Each of Slippery Strana's two wheels is driven by a 7.2 volt Bosch Cordless Drill - but driven at 12 volts.
The 6 inch garden-lawnmower wheels are directly connected to the Bosch drill chucks, with no other gearing.

The innards

You can see the drills here, lying on their sides, back to back.

We dispensed with the Bosch 7.2 volt batteries and instead fitted a 6 ampere hour 12 volt 'gel' battery to drive both motors. The 12 volt battery is at the top, with the yellow "2" label on it.

We do use an original Bosch 7.2 volt battery to drive the receiver. You can see two here, straddling the 12 volt battery, but in the final design we used only one.

We have not had any overheating problems with any component, in spite of the higher voltage.

(Click on the picture for a bigger view)

The two Bosch drills gave us plenty of speed (400rpm) and torque. Charlie worked out that the motors pushed Slippery Strana forward with a force of 233 Newtons during acceleration. That translates to pushing the final 21.6Kg robot from 0 to 3 metres per second in about 3 seconds.

Going In Reverse
These Bosch drills were designed to run equally well in forward or reverse, so Slippery Strana originally had no 'front' and no 'back'. The direction was arbitrary. It was only when we decided that having spikes on both ends was too cumbersome that Slippery Strana got 'directional'. But the freedom to manouvre fast in both directions was a big design aim from the outset.

Speed Control
We knew that speed-control electronics were possibly the most difficult part of any design using variable speed motors. We also knew that Bosch knew this ! What better way was there to control the speed of our motors than to use Bosch's own design ?
So rather than re-invent this part, we used the original Bosch triggers, separated from the handles, and mounted so that the servo arm could press the trigger, as though it was a finger. Job done. Reliable speed control achieved.

Well - not quite that simple, because we blew two expensive triggers during testing, and aligning the servos and triggers to get full 'finger' travel took some time. But once we had replaced the original heatsinks with flat aluminium equivalents and carefully aligned the triggers, they just got on with their job as Bosch intended.

Inside corner view

(Click on the picture for a bigger view)

The only sensible wheel type in our opinion had to have rubber tyres (or spelt the American way, tires). We expected Slippery Strana to have to push itself and an opponent on a metallic surface. And, as in a tug-of-war or a Sumo wrestling match, he who has no traction probably has no hope !

So we got traction. Slippery Strana was able to push 7 year old Alex in his pedal car on grass, and could push Charlie along the road in his roller-blade boots - effortlessly. Very difficult to balance however !




Slippery Strana Home



The Name

The Speed



Amazing Shell

A Good Fight

Page updated on 2007-04-15