# Calculating the appropriate cable strength.

1. May 18, 2013

### The450Man

5 years ago I was at penn state, for engineering of all things. I did things similar to this but everything is eluding me at the moment.

My situation:

I have a car that Ive attached a class 1 trailer hitch. Its rated at 2000lbs and i don't intend on towing more than 1000 lbs.

For a peace of mind, id like to have an extra safety barier.

Id like to run some steel cable from the hitch itself to the frame of the car (the hitch is bolted to the frame).

The cable should prevent the trailer from taking off in an even the trailer hitch fails.

Consider the maximum speed of 75.

I know deceleration/acceleration is important here, numbers im not sure if I can reliably provide.

2. May 18, 2013

### Simon Bridge

Welcome to PF;

You draw a free body diagram for the trailer and run $\sum F = ma$ - remember how to do that?
You will have to consider friction dragging on the trailer - which would depend on speed - which you have to measure.

So: $T-f(v)=ma$ where T is the rated break-strain of your system - then you know everything except "a" which will be the maximum acceleration you can expect to achieve.
Since you want the cable to take up should the main (2000lb rated) system is accidentally overloaded - say you accelerated too sharply - then you'll have to guess at how much over acceleration you want to protect against.

(Units are important too - 75kmph is quite different from 75mph and this is an international forum.)

Personally, I'd try a heavy-duty tow-rope looped from the tow-point to the trailer and back... this is usually good enough to tow quite big cars.

In NZ the main failure mode of a tow-hitch is a coupling failure - it's just not attached properly like if the operator is in a hurry - and there is a chain that must also be attached to prevent the trailer running off.

Last edited: May 19, 2013
3. May 19, 2013

### davenn

if he is in New Zealand it will be 75km/hr which is about the max legal speed to tow a trailer

towing trailers at 75mph (~ 125km/hr) would be pretty scarey haha

are you in NZ, Simon ?

Dave

4. May 19, 2013

### 256bits

I got the impression he is worried the hitch is going to fall off the car if the bolts fail or come loose.

5. May 19, 2013

### Simon Bridge

I am in NZ - land of the long white cloud, no-helmet football, and personal jet packs :)

In NZ, the open road speed limit for a light vehicle towing a trailer is 90kmph.

I have been overtaken by vehicles towing trailers doing 130kmph+ ... and, yes, it's pretty scary.
I'd rather have them in front than behind though - people forget the extra mass when they have to brake.

@256bits - well yes, that crossed my mind too ... makes the safety chain a bit redundant.
I've yet to see tow bars that were not welded on... bolted? If OP is worried about the bolts, or the bit it's bolted to, giving way, then that may explain the cautious loading. Don't think it affects the answer though ...

6. May 19, 2013

### The450Man

Correct, im worried about the bolts breaking loose and the hitch + trailer going loose.

Im a little obsessive about these kind of things, a safety barrier will make me much more confident on the road.

7. May 21, 2013

### davenn

thanks for the reminder .... been a bit over 13 yrs since I moved to Oz .. couldnt remember the exact speed limit for a trailer .... not that its obeyed by many anyway as you have seen haha

Im ex Dunedin -- Edinburgh of the south, now am in Sydney ;)

Dave

8. May 22, 2013

### Simon Bridge

I'm on Waiheke Island (NZ) which is "somewhere near Taupo".
Note - about 20 years ago the top open road limit was 80 and the trailer limit was <drum-roll> 75.
This sort of stuff hangs around long after it stops being useful ... :D

9. May 22, 2013

### davenn

I know this is off topic but .....

gosh havent been to Waiheke Is since ~ 1982, got a cousin and family that I think still lives there, he's an electrician by trade.

Do they still fly the ol' Grumman (sp?) Goose to and from the Island ?

Dave

10. May 22, 2013

### Simon Bridge

No - that got retired ages ago - the old slipway is used exclusively by the vehicle ferry now.
Perhaps I should put social media contacts in my profile?

Doesn't appear we can do any more for OP.