IDK if its just me but its seems if you can manage to make a platform solidy locked to the wheels and then make a basket which has shocks mounted to the lower solid axle it tends to move alot faster.
Like this...
And my fastest one yet which made 14secs along time ago was something like this. I still was not able to completely copy it in order to make it perform just as well as before. I also put a wheel on the top supported by shocks and a stick attached to the wheel which closes the hatch on top to somewhat help keep the coconuts in while bouncing.
