The effect of weight in horseracing – a definitive answer

Weight – so much of our racing revolves around it. Every day we hear phrases like “5lbs better off”, but can anyone really say they know what that effect weight has on a racehorse?

Does it give horses equal chances in handicaps or does it, as a friend of mine once put it, “have about as much effect as sellotaping an empty crisp packet to the back of an Olympic 100m runner”?

I've been thinking about how to get a definitive answer to this question.

The obvious races to look at are handicaps. After all, assuming the handicapper has it right, all the horses should finish in a line.

If he’s got it wrong, there should be a way of working out the correct values from the average beaten lengths for each extra pound carried.

The idea I came up with is to compare the performance of horses carrying less than top weight against the performance of top-weights.

Using Raceform Interactive, I looked at all the UK handicap hurdles races from 1998-2004 except for amateur races (which I feel are a law unto themselves).

The first thing I did was to throw out all the horses that failed to complete and all those that carried overweight. I also left out any horse carrying less than 10st6 to avoid any horses running out of the handicap.

From those left, I chose a horse to be the “top weight” according to the following rules:

(1) Pick the horse which carried the most weight (AFTER deduction of jockeys allowances and including any penalties)

(2) If there is more than one horse on top weight, choose the first alphabetically

This gave a horse that could be used as a benchmark for all the other horses.

For each horse, I assigned a weight value and beaten lengths value as

Weight = weight of topweight – weight of horse

Beaten Lengths = total beaten lengths of horse - total beaten lengths of topweight

(i.e. positive if behind top-weight)

This gave me a list of 5352 races and over 31,000 runners.

Here are the results:

Lbs less than topweight	Runners	total lengths beaten	runners*lbs	Average lths behind TW	avg/lb
0	5839	1152.5
1	966	1270.7	966	1.315	1.315
2	1148	1352.3	2296	1.178	0.589
3	1141	-1006.85	3423	-0.882	-0.294
4	1183	46.7	4732	0.039	0.010
5	1345	345.25	6725	0.257	0.051
6	1348	-184.3	8088	-0.137	-0.023
7	1477	3295.2	10339	2.231	0.319
8	1488	3480.4	11904	2.339	0.292
9	1450	472.2	13050	0.326	0.036
10	1568	1280.9	15680	0.817	0.082
11	1429	2415.6	15719	1.690	0.154
12	1453	2789.75	17436	1.920	0.160
13	1488	2301.8	19344	1.547	0.119
14	1350	4209.7	18900	3.118	0.223
15	1455	3561.05	21825	2.447	0.163

(results for horses with greater weight differences have been left out)

To explain what these numbers mean, let’s look at those horses that were carrying 5lbs less than the topweight.

There were 1,345 of these horses, giving a total of 6,725lbs (1345*5lbs) difference. These 1,345 horses finished a total of 345.25 lengths behind the topweights, an average of 0.257 lengths per horse, or 0.051 (0.257/5lbs) per pound.

The table shows 25,630.4 lengths beaten (ignoring equal top-weights) for a total of 170,427lbs less carried, which suggests that each extra pound slows a horse down by 0.150 lengths less than the scale used by the handicapper.

I believe that these results show that weight does matter. I think it also shows that the scale used by the official handicapper is pretty accurate and that the advantage towards high weights is far smaller than many punters imagine.