The following is the text of a response posted to the AusHarness list in October 1999, to a query by Dave Powell on the influence of driver weight on performance.
Let's start with the basics (because so very many people who really need to know this stuff DON'T):
1. A horse's energetic cost of locomotion (how much energy the horse must expend to move) is proportional to THE WEIGHT ON HIS HOOVES. Increase that weight (add a jockey; fat on the horse) and you slow the horse, REDUCE that weight (no jockey; lean animal) and you allow the horse to go faster.
2. By altering the location of the driver's centre of gravity in relation to the fulcrum point of a sulky (the axles) it is possible to either INCREASE the weight on the horse's hooves (by putting weight DOWN on the shafts) or REDUCE the weight on the horse's hooves (by putting the driver's CG well behind the axles so that the sulky tends to flip over backwards if not attached to the horse - known as 'negative balance').
3. NEGATIVE BALANCE reduces the weight on a horse's hooves, neutral balance has no effect, and positive balance increases the weight on the horse's hooves. Thus NEGATIVE BALANCE increases speed, neutral or positive balance reduces speed by comparison.
4. NEGATIVE BALANCE and its influence on speed was first discovered about 4,000 years ago, forgotten, re-discovered, forgotten again, and so on. In recent times, it was introduced to harness racing by a fellow called Weber in the 1960s, used to enormous effect by the American Joe King with the single shaft sulky and later "modified" two-shaft sulky, and introduced in Australia in the 'Time Trial Specials' which won an unequalled six straight Inter Dominions - including the richest ever - the 2000 Inters.
5. In practical terms, the upper limit of negative balance is inversely proportional to the length of the sulky. In other words, if the manufacturer knows what he/she is doing, it is possible to achieve more negative balance on a short bike than a long one.
6. When the maximum width of a sulky is regulated (as it is in the southern hemisphere to 1.3 metres overall width) BY FAR the most effective way of shortening the sulky is to centre the track (lateral or sideways distance between the wheels) on the mean (average) lateral (sideways) movement of the horse's hind legs, while centering the upper frame of the sulky on the horse's longitudinal axis. If you make a sulky like that it MUST BE an OFFSET SULKY. Thus for any given horse/track/speed combination, an offset sulky can be made shorter, and with greater negative balance, than a symmetrical sulky.
So let's look at the historical record. What have been the greatest marginal improvements on an existing world mile record (the most internationally competitive distance for harness racing) in Australia or New Zealand in a symmetrical sulky of any type? Answer: ONE FIFTH of a second (Popular Alm and Scotch Notch in the 1980s).
What has been the greatest marginal improvement achieved over the same distance in an offset sulky? Answer 3.4 seconds; SEVENTEEN TIMES (1700%) better than the best achieved with symmetrical bikes!
How many world records over the mile have been achieved in all symmetrical southern hemisphere sulkies in the last 200 years? Answer, two.
How many world records over the mile have been achieved in ONE PERCENT of that time period in offset sulkies? Answer, at least FIVE!
I could quote many more examples, but I believe the above is sufficient.
Now, to get back to your specific question: 'why the weight of the driver doesn't really matter in harness racing' we need to go to point 7:
7. All other things being equal, and the bike producing negative balance, the greater the driver's weight, the greater will be the weight reduction on the horse's hooves and the LESS will be the horse's energetic cost of locomotion. Of course all other things are NOT equal, the most important of which is the rolling friction of the sulky, which is proportional to the weight on the wheels of the sulky, and therefore increases with driver weight and with the degree of negative balance. But when you do the maths it turns out that offseting the REDUCED horse energy requirement resulting from negative balance against INCREASED rolling resistance from driver weight, results in about a one percent penalty (in terms of the horse's energetic cost of locomotion) for DOUBLING of driver weight.
Now in practical terms, a variation in driver weight of (say) 40 kilograms, will be heavily disguised by greater variations in the horse's performance from other factors, so that, on a negative balance bike, such a difference in driver weight will NOT show up in race statistics over any reasonable number of samples.
In fact, in ordinary racing, variations induced by non-bike factors tend to swamp the real difference between sulkies in terms of efficiency. This has always been the case. To get away from the possible bias of defending my own products; when the long Australian bike was introduced in 1910, it was easily two seconds slower over the mile than existing American short bikes then in use. But by cleverly stage-managing its introduction (by only using it on the best horses of that era), its inventor convinced a gullible industry that his long bikes were competitive.
In the same way, today it will not be obvious to the casual observer that there are any differences in speed between sulky types used over all classes of horses.
But to those who do their homework, there is only one bike to use (on underbanked half and five eigth mile tracks) if outright speed is your aim, and that is an offset bike.
I might just put in here that if we had mile tracks designed for a 1:50 speed, or straight tracks, there would be NO PURPOSE in offset sulkies, simply because the horse would, on such tracks, throw his hind legs equal lateral (sideways) distances. On such tracks, with a 1.3 metre sulky width limit, a symmetrical sulky could be made shorter than an offset and I would NOT recommend an offset sulky for that very reason.
'Offset' is simply a means to an end. The end is greater negative balance and reduced vehicle weight - both have been shown to have a significant influence on horse speed on the tracks we now have.
Copyright James S. Walsh