Thanks for the responses.

Chris - "it is well accepted that it takes far more energy to get something rolling than it does to keep it rolling... ". Doesn't this mean that driver weight is an important factor for 'standing start' harness races?
I like your idea about driver weight being a factor in 'ability'.

Chrome Prince - "You could walk a far longer distance with a brick, than you could run." Wouldn't the speed you can walk be influenced if the brick weighed 20lbs. rather than 7?
Your comment "The weight he is pulling is only fractional to what he is capable of.." is probably the key to all this. A few kg.s either way would probably not be noticed by an animal capable of pulling far greater loads.

Makes me wonder if we over emphasise the importance of weight variation when assessing gallops form. Can a couple of kg.s really make a difference to a horse? Maybe variations of less than 5kg.s are totally irrelevant.

Does anyone know if there has been a scientific study of weight vs performance in horse racing?

bomber