NZ00Z3

What grade of bolts for fitting a Towbar?

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Hi

Fitting a towbar to the E46.  Its a secondhand Towbar and didn't come with any mounting bolts.  What grade of bolt is required, 8.8 or 10.9?

Regards

Murray

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Wow... I'm starting to learn how to be a copy & paste nerd

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I don't think that link really helps with the answer and you might consider using this instead for the lols: http://lmgtfy.com/

To the OP, this is better Google-foo, look at the first link: http://lmgtfy.com/?q=towbar+bolt+strength

 

Edited by M3AN
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A70 stainless bolts from Mitre10 will be just finneee :P 

Joke, I'd use 10.9 or 12.9 

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Stainless pfft! Too expensive.

Mild steel, for that rusty patina.

 

 

Edited by M3AN

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Well... its not too difficult to work out.

For your E46, assuming you have the torqiest (is there such a word?!) - you have the following:

     400 NM of engine torque, Gearbox reduction ratio of 4, Final drive ratio of 3.5, 80% drive train efficiency

You'll be putting out 4480 Nm at the wheels.

Assuming you have standard sized wheels (0.63m diameter), this equates to 14.2kN.

Your bolt will fail in shear rather than in tension.  Typically, the bolts will fail at 60% of its UTS.  

So assuming you only use 1 bolt to attached your tow bar, your bolt needs to withstand 14.2kN of shear force or 23.7kN of tensile force.

I don't know what diameter your bolt holes are... so if we go backwards and assume  you want to use the cheaper grade 8.8 bolts we can do the following calculation :

23.7 k / (800 * 0.8) = 37 mm^2

This equates to a bolt diameter of 6.9 mm

This calculation is very similar for braking... assuming you have standard brakes and your 100 kph stopping distance is 40 m.  Applying the kinematic equation of Vf = Vi + 2ad, you'll find that a = 10 ms-2.  Approx weight of an e46 is 1500kg.  F = ma, your braking force is 15kN (which is less than our approx best case acceleration force).

Apply your own safety factor... and you can decide what you need.

 

   

 

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^ did that and the answer was 42.

Where to from here? 

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i think he is saying that 4 or so 8mm dia  8.8 grade bolts will be more than sufficient?i would be more concerend what the bolts actually bolt to.

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Considering the lecture I got given when talking to The Oracle Of All Things Child Safety Related at Baby City with regards to The Sacred Bolts that secure child seat anchor points... any damn Chinese crap off the shelf will do. I'd suggest using the bolt that fits the current holes (neatly bypassing curryinahurrys fantastic physics lesson, and assuming the original fitter of the towbar hasn't cocked it up royally...) and throw 10.x or 12.x grade bolts at it

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Thank you for you replies, grade 10.9 where used and torqued to 115 Nm to provide a superior clamping connection

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