Designing fully functional rear towing hook for MX5/Miata MK

The finished projects

Designing fully functional rear towing hook for MX5/Miata MK

Postby jass » Sat Feb 11, 2012 10:52 pm

The original NA MX5 towing bracket is ... well mine at least, already rusty. It is heavy and it is not alowing me to use any aftermarket rear spoiler, as it is not adjustable. As I do racing with the MX5, nice to have option is to comply with the FIA requirements for tow hooks.

On the other side, the original towing bracket is quite well engineered and dimensioned piece of mild steel. It is designed to withstand very high loads. And it is the guiding light in terms of strenght, dimensioning the new one.
Defining towing strenght, even for a small and light car like the MX5 is impossible. Towing it on a tarmac road doesn't require much strenght, but pulling the car from mud could be even impossible.
I desided to dimension my tow hook to withstand the strenght of a standard 1500kg rope. The rope has to tear before the hook breakes or bends, basicaly asuring the towing hook is reusable.
Broken hook can be a deadly weapon, as the rope accumulates very high energy, that accelerates the hook, when torn.

Lets collect the requirements so far:

Strenght: it has to at withstand more than 15000N (1500kg) pull without any plastic deformation
Weight: lighter than original
Height adjustable

This below are animatated GIFs you can see by on clicking on them. Many other market designs were calculated to come to the optimal design in terms of strenght and weight.

1. I-shaped 4mm steel. Almost afordable stress levels for highest position.

2. As we want adjustable bracket, we test this also in the lowest position. This is already too much stress for the thin steel.

3. Next is to stiffen the steel design, by bending a wall and screwing the hook one more time at the back. Stress levels are affordable. The stress levels even in the lowest position are acceptable.
Downside on this design is the weight which is 522 grams. Still it is a good option, offering 3 point connection.

4. Next step is to change material - we took strongest airspace 2024 T3 (approx. 1,4 stronger than 6061-T6) duraluminum instead of steel. The calculation is with 10mm thickness and in already lowest position.
With a weight of only 300 grams, fully adjustable and covering the requirements this is another option to concentrate on.

The 10mm billet 2024-T3 duraluminium prototypes ready for real life test:

Both tow hook designs passed the real life tested with two tones with no deformations.

Here are the ready products:





The corresponding front stainless steel design was taken further to the over-extremes as it is designed for track use.


The car was
[*] towed for 10 miles
[*] with 4 tones bus
[*] with 2 10mm steel ropes
[*] in a stop & go traffic
[*] with the tow hook being fixed only with two points
[*] and with final pull up on a towing truck.

Please note, this is extreme test, that will (hopefully!) never happen in real life. The MX5 was pulled left or right of 4 times heavier bus in stop & go manner many times & with a rope that wont stretch or tear.
The result was slightly twisted tow hook, as it was held with only two of the side bolts and slightly plastic deformation of the eye with no sign of problems with towing further.
Basically the extreme test harmed the design, but not the function. And most important the car body - there were no paint scratches to the bumper at all!
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