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Grand National Course Guide: The fences most likely to claim your horse ... and when to lay

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Fact: horses fall in the National, but which obstacles take the most horses with them? Simon Rowlands has the (surprising) answers

The John Smith's Grand National at 4:15 at Aintree on Saturday is bound to see triumphs and disasters along the way for connections and punters alike. Thirty fences, some of them quite unlike anything you will find elsewhere, a marathon trip of four-and-a-half miles and a likely maximum field of 40 will see to that.

I remember the wins of Aldaniti, Corbiere and Miinnehoma fondly. I remember the agonising defeat of Garrison Savannah (six lengths clear at the last and not stopping, but stop he did) rather less fondly.

From an early age it was my ambition to forecast the winner in a national newspaper, and the opportunity finally came in 2006 when I was front-page tipster on the big day in The Sportsman. After much research and viewing of videos and the like I tipped up Innox ("Opportunity Nox" said the headline, clever chaps). Innox himself tipped up at the first.

In the old days, once your bet had been placed, you used to have to sit and suffer until your fate was decided. But, in the modern era there is an alternative provided by in-running betting. I would not have had much chance to "lock in a profit" in 2006, but I wouldn't have minded having one in some of the years gone by.

So, let's picture the scene: your horse is bowling along with the leaders and trading at a fraction of the odds you backed it at. The problem is that it is not the safest of jumpers and you suspect it may come a cropper at some stage. At what point do you take out a bit of "insurance" by laying some of your bet back?

Well, it depends on a lot of things, but a crucial factor will be how much of the race has passed. There are 30 fences to be jumped, but the chances of departing, through falling, unseating, refusing, being brought down or carried out, are much greater at some obstacles than at others.

To illustrate this I worked my way through the last 10 runnings of the National on good and good to soft going (ignoring Red Marauder's win in unraceable conditions in 2001 and Earth Summit's win three years earlier on soft, in other words) to find out the percentage of non-completions at each fence along the way. Similar conditions seem likely to prevail this weekend.

This is not quite so straightforward as it might seem, as you need to strip out horses which have departed or been pulled up earlier in the race in order to get meaningful figures. The following are my findings:

It is no great surprise to find that second Becher's is the fence that has taken the highest proportional toll over the years and that the first fence - when the runners are often going too quickly with little room in which to manoeuvre - is next. You can expect in the region of three falls/unseateds etc at each on Saturday. More counter-intuitive is that the otherwise innocuous fourth-last comes in at third, just ahead of first Becher's.

Granted a maximum field of 40, you can expect in the region of 18 non-completers during the course of the race, with another 7 pulled-ups on average, making for 15 finishers. I would be thinking twice about backing Cloudy Lane at as short as 7.0 on Betfair in the circumstances. A place lay at 3.1 looks more inviting.

Finally, for those looking to play in the in-running markets, it is worth noting that 3 of the last 10 winners on good/good to soft going were in the lead at second Becher's, 4 were in the lead at the fence after second Valentine's and 7 were in the lead at the last.

It all seems so clear at this stage, but I suspect that the great race will somehow contrive to produce the unexpected once again!