Technical Fitting Witter towbar to '04 Multijet

[ferrymanr]

I have purchased a Witter FT58A towbar kit (with electrics and ball) to fit to my Punto. All looks a doddle as most fitting points are present in the main structural box section member bthat runs either side of the spare wheel housing - just remove plastic bungs.. However Witter instructions require a hole (about 20mm diameter in 3mm steel) to be drilled into this 'chassis' member to get a strap with captive nut/bolt inside the box sections and locate over existing holes.. I am not keen on drilling holes in chassis members. Has anyone else done this or is there an alternative?
The destructions are at:-
http://www.witter-towbars.co.uk/uMedia/FittingInstructions/FT58A.pdf
Richard

 

[MikeSalt]

I am not a structural engineer, so please speak to someone more suitably qualified before attempting this. That seems like a big hole just to get the captive nut strap in, would it be more preferable to slightly over-drill the holes that already exist in the chassis and to use M8 rivnuts for bolts H to thread into instead?

 

[s and b]

you need to drill this so that the nut furthest from bumper can be dropped in so make tow bar safe and not able to pull through lower chassis rail
its just a matter of putting a little sound deadening pad over the holes once done if you took it to a professional (the type of sticky pads you wil see in there now from factory
you must do this

 

[ferrymanr]

I'm not so sure it would be legal to fit riv-nuts as this would invalidate the Witter specs. Also riv-nuts won't have the same holding strength as the nut/bolt welded to a 20mm wide by 3mm thick 'strap' which Witter supply. The instrucions say to drill a suitable size hole 35 mm in front of the frontmost fixing 'H'. There is however a bunged hole 16mm in diameter about 100mm in front of 'H' which I could open up to 20mm to get the strap through but I will have to fnd a way to pull the strap into place. At least that would save drilling yet another hole in a structural member.
Can't go further now - pouring with rain and I don't have a garage. Anyway my 70 year old bones seize up when I get wet.
Richard

 

[ferrymanr]

Just adding a photo of the RH member showing the two mounting holes where the towbar fits and to the front is another blanked hole which I could enlarge to get the strap assembly into the member. The strap assemble is shown not quite correct scale in the inset.
Richard

 

[fingers99]

S & B is right. Rivenuts have their uses, but in a load bearing situation like this, do not use them.

If in doubt, talk to Witter.

The danger is that, incorrectly fitted, the towbar will either compress chassis members on fitting or rip through them after fitting, or simply fatigue.

 

[MikeSalt]

As I said, I know rivnuts have their limitations, usually low-stress jobs, which is why I added my caveat. As far as I can see, you need the 20mm hole 35mm in front of the holes for the bent section of the captive nut strip to stick through anyway. If you were to modify this, you would be invalidating the the Witter specifications anyway, so you may as well have the hole. Enlarging holes elsewhere would probably be out-of-spec too.

 

[s and b]

Just adding a photo of the RH member showing the two mounting holes where the towbar fits and to the front is another blanked hole which I could enlarge to get the strap assembly into the member. The strap assemble is shown not quite correct scale in the inset.
Richard

thats not the way to do it
you have a box section and ive explained as has manufacturer what to do

f= is a load spreader even if it has captive nuts welded on

the strength is in this box section not boot floorpan

 

[ferrymanr]

Thanks for all the comments. I am going 'by the book' and drilling the recommended hole. Got as far as pilot holes and metal seems to be about 2mm thick. Is it easier to cut a 20mm hole with a tank cutter (circular saw) or large twist drill? This steel is fairly thin and I'm guessing a twist drill will tear at it and lock up towards the end. [Excuse for not knowing is that I am a retired electronics engineer not a mechanical engineer].
Richard

 

[fingers99]

I'd use a bimetalic (Bosch white one, etc. -- prices vary enormously, so shop around) holesaw, the lowest speed you can get on the drill, the side handle, and lots of lubricant.

But it will depend on the size of the hole -- up to 15mm or so a twist or blacksmith's drill will be fine. You could always drill slightly undersize and use a round file to take it to the required size.