Offtrack trailer turn calculator

 

Introduction

I recently moved house and the access to the new place is down a very narrow road with a right hand dog leg turn in it.  I have a caravan and a trailer which I want to store in our back garden but the big question is “will it go around the corner?”

Now you might think it would be a simple thing to just tow the caravan and trailer in and give it a go.  After all reversing out if it doesn't go around is no big deal.  Well in this case it isn’t that simple.  We are on the side of a hill and the approach to this corner is down a 1 in 3 hill.   If the trailer or caravan won’t go around I can’t unhitch and manhandle it around...   So I really do need to know “will it go around the corner?” before I get it there.

A quick bit of web searching and I discovered that this towing characteristic is called outtracking.  A trailer following on behind a tow vehicle in a curve or turn will take a path that is closer to the corner than the tow vehicle.  This trailer offset is called an outtrack.  At first sight it seems to be quite a simple thing, but calculating it is not so straight forward.  You need to know how the car turns, it’s turning circle from curb to curb or turn radius.  The specific dimensions of the turn or curve that you need to maneuver around and the dimensions of the trailer or caravan.

I searched the web and found many people asking the same question but no one providing the answer.  There are a few expensive programs that calculate trailer sweep paths for commercial design of road layouts and parking lots but nothing simple and cheap, or better yet free.  If you are looking for a commercial solution then I recommend you take a look at Autotrack

I also couldn’t find anything showing how outtracking is calculated, so I dusted off my circle theory and trigonometry and several sheets of paper later finished up with this spreadsheet.  I make this outtrack calculator available as is.  There are absolutely no guarantees that I have got the calculations right or that it will work under all situations.  Feel free to make use of the outtrack turn calculator if you like.  If you do and like it you might like to send me a $1 through PayPal to say thanks.  If you don’t want to, thats fine, but drop me an email to say it was useful.  I am paulrbarnard@mac.com