Hi All, I have just bought a new caravan and it didn't tow very well (snaking) at times so while checking everything out I checked the Alko 3005 hitch which has pads inside for stabilising the outfit, everything was brand new so that was OK, so I then turned to the factory fitted towball which is detachable on our 2009 FL Octavia Scout, on measuring the towball size it was 49.50mm and on checking the Alko manual it says the min size for the ball is 49.61mm so it was too small for the pads to lock onto affectively ! I have a replacement set of pads for the hitch that come with some washer shims in 2 sizes 0.3mm and 0.5mm that go behind the pads so I fitted one 0.3mm behind one and it now tows OK. So the question is are ALL factory fitted towballs undersized when the standard is 50mm ? Cheers Stuart

The standard for towballs used in Europe is ISO 1103:2007.The specification for the towball is nominally 50mm, with tolerances +0 and -0.39 mm: in other words the towball can be no larger than 50 mm and no smaller than 49.61 mm. There should be an ISO marking on the flat part at the top of the towball too.

 

Out of curiosity, how did you measure the diameter of the towball?

 

I'd be surprised if a difference of 0.11 mm would make much difference in towing stability. It's more likely that the pads inside the Alko hitch might be a little bit worn, than any particular mismatch in sizes.

Might have grease on em , the pads , as well!

 

Thanks for the replys, I measured it with a digital calliper today 49.44mm, there's no grease on the ball or pads, it's a new van picked it up in Feb 2018, but it looks like the towball IS smaller than the min ISO of 49.61mm ! I have a spare 2nd hand ball of the same type and that measures 49.29mm which again is smaller, I will attach some pics, I have just measured a normal ISO towball and that's 49.81mm can anybody measure their towball size to see if it's just my type that looks smaller than standard ? I wouldn't have thought the ball itself wears that quickly or does it, or is the digital calliper not brill ? Cheers Stuart

It could certainly be the caliper - if it's a cheapie one what you've measured might be in spec.

I just went out and measured mine (OE detachable swan neck) and the ball is 49.93 mm with my Aldi digital caliper.

 

Double check you have the jaws across the widest part of the ball when measuring. If you're not sure about the caliper's accuracy check the diameter of a HSS drill bit with it.

 

If it's actually wrong I'd have a chat with whoever supplied the towbar about getting the correct size fitted.

It's recommended to use the Alko ball with the Alko hitch and make sure it's clean and dry.

alko hitch

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spare car hitch

If  0.25 mm is enough to cause the caravan to be unstable i would be highly shocked.

There must be another issue as well?

 

If you stuck a 3mm felt pad on Top of the ball and hook up, check for play in the caravan hitch then? Give a drive even? Honestly that sounds like the latch mech on the caravan isnt locking on properly, by more than the hitch ball diameter "error"...