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Someone recently went into the back of my Disco (a shunt).  Guess what!  No damage to Disco but made a bit of a mess of her brand new shiny car.  Disco checked out by garage, to be on the safe side.  Now I need to replace the tow ball, curtesy of their insurance.  (Always a good thing to do - tow balls being cast iron and can have 'hidden cracks' etc).  I need a A50-X tow ball to fit the Al-Ko AKS 3004 stabiliser/hitch on the caravan. 

When I started looking at these there are different makes.  I know I need one with an extended neck, but what are the meaning of the figures that some brands quote?  e.g.

D value = 17kN,
S value = 150kg

Most have the same D value, but some that I have seen have S value as 150kg, some have 120kg.  I just wondered if the S value has anything to do with the noseweight that the tow ball can take? (I am aware that the nose weight that the car can take is different to the stress on the towball - I may have answered my own question here by using the word 'stress'?)  Or if not, what this might be?  Might as well go for the best I can if I'm not paying (sorry, that sounds a bit cheap).

Any advice?

sorry - forgot the link

http://www.western-towing.co.uk/acatalog/Towballs.html

"(Always a good thing to do - tow balls being cast iron and can have 'hidden cracks' etc)."

I am surprised to hear that tow-balls are cast iron and not forged steel.  . Cast iron is brittle and I would have thought not the best material for a safety critical part. 

Brian  

Quote: Originally posted by G7ACU on 11/3/2010

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The 'D' rating is generally attributed to the towing bracket more than the ball
A towbar manufacturer is obliged to include a 'D' value in kn on the towbar identification plate.
The "D" value is calculated as follows:
(GVW x GTW ) / (GVW + GTW) x 9.81/1000 (gravity)= kn
GVW is Gross Vehicle Weight, and GTW Gross Trailer Weight.

It's recommended the towball ratings are higher than those of the bracket itself, so the 'D' rating should be equal or exceed that of the towbar and the 'S' value is the maximum nose-load of the ball.

So yes, you have answered your own question; aim for the higher 'S' rating. The Al-ko original tow-ball would be the best option I'd have thought as at least you know it'll be fully ok with your hitch etc. I'd be dubious of cheaper 'pattern' parts, it's just not worth it especially as the third party will be footing your costs.

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Quote: Originally posted by brianconwy on 12/3/2010

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"(Always a good thing to do - tow balls being cast iron and can have 'hidden cracks' etc)."

I am surprised to hear that tow-balls are cast iron and not forged steel.  . Cast iron is brittle and I would have thought not the best material for a safety critical part. 

Brian  

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I many have got that bit wrong.  Personally, I haven't a clue - just quoting what the garage man said, and the insurance co. swallowed it, so I assumed it was right.

Thank you all for your replies.

Quote: Originally posted by G7ACU on 11/3/2010

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The 'D' rating is generally attributed to the towing bracket more than the ball
A towbar manufacturer is obliged to include a 'D' value in kn on the towbar identification plate.
The "D" value is calculated as follows:
(GVW x GTW ) / (GVW + GTW) x 9.81/1000 (gravity)= kn
GVW is Gross Vehicle Weight, and GTW Gross Trailer Weight.

It's recommended the towball ratings are higher than those of the bracket itself, so the 'D' rating should be equal or exceed that of the towbar and the 'S' value is the maximum nose-load of the ball.

So yes, you have answered your own question; aim for the higher 'S' rating. The Al-ko original tow-ball would be the best option I'd have thought as at least you know it'll be fully ok with your hitch etc. I'd be dubious of cheaper 'pattern' parts, it's just not worth it especially as the third party will be footing your costs.

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Given that (GVW+GTW) = (GVW+GTW) then (GVW+GTW)/(GVW+GTW) = 1 and therefore (GVW x GTW ) / (GVW + GTW) x 9.81/1000 (gravity)= 0.00981kn. I suspect you must have misquoted this equation somewhere as the answer will come out the same, regardless of GVW and GTW.