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This is something that has been bothering me for quite some time, how are aftermarket recovery points rated? Not just the piece of plate, but more importantly the attachment point.
I'll start by saying that I am not anti mods, I've done plenty to my Prado, but for something as important as this there needs to be more than opinions on a blog. This is my take on aftermarket recovery points and the existing tow points on the chassis (taken in part from a previous post)
Recovery plates are only as good as what they are attached to, and importantly how they are attached. I can find no information on the design of the holes in the chassis that are used to bolt on the recovery points, and whether they were designed for this purpose
Just as important is the method of attachment. When designing plates to bolt together to take a load in shear, the friction of the joint is an important factor. The joint should be designed to create sufficient clamping force so that the bolts create a friction joint and don't just take a shear load through the bolts as seems to the case for many recovery points. And using 'high tensile' bolts doesn't do away with the need to use them correctly. They are designed to be high tensile in tension, clamping a joint together. They are not designed specifically for shear loads as they are often used when attaching a recovery point.
Some questions:
My take on the welded tow loops which come on the vehicle from the factory is that they are engineered by the manufacturer to take loads. Take a close look and you will see that they are not some flimsy bits of metal, not on the Prado anyway, they are welded directly into the chassis
I decided to do some quick calcs on the tow points. I'm a mechanical engineer but no welding expert so if there are any welding experts please let me know if I got this wrong.
Each u-shaped 'tow point' is 16mm diameter and has 4 full circumference fillet welds holding it in place. That gives 5cm for each weld x 4 is a total of 20cm = 7.9 inches of weld for the u shaped tow point. The fillet welds look to be about 8mm but lets round that down to 6mm (1/4inch) for safety. Rule of thumb says that 1/4 inch weld can hold 1t / inch (a conservative number backed by some complicated welding theory).
So that gives a theoretical static rating of 8t for each 'tow point'. And by my calcs the 16mm bar should be good for about 10t, so the welds should fail first.
Question:
I'll start by saying that I am not anti mods, I've done plenty to my Prado, but for something as important as this there needs to be more than opinions on a blog. This is my take on aftermarket recovery points and the existing tow points on the chassis (taken in part from a previous post)
Recovery plates are only as good as what they are attached to, and importantly how they are attached. I can find no information on the design of the holes in the chassis that are used to bolt on the recovery points, and whether they were designed for this purpose
Just as important is the method of attachment. When designing plates to bolt together to take a load in shear, the friction of the joint is an important factor. The joint should be designed to create sufficient clamping force so that the bolts create a friction joint and don't just take a shear load through the bolts as seems to the case for many recovery points. And using 'high tensile' bolts doesn't do away with the need to use them correctly. They are designed to be high tensile in tension, clamping a joint together. They are not designed specifically for shear loads as they are often used when attaching a recovery point.
Some questions:
- Are the attachment points on the chassis used to bolt on recovery plates actually designed for this purpose? Where are the facts on this?
- How are the calculations done for recovery points. Not just the piece of steel plate, but the complete unit including attachment points and method?
My take on the welded tow loops which come on the vehicle from the factory is that they are engineered by the manufacturer to take loads. Take a close look and you will see that they are not some flimsy bits of metal, not on the Prado anyway, they are welded directly into the chassis
I decided to do some quick calcs on the tow points. I'm a mechanical engineer but no welding expert so if there are any welding experts please let me know if I got this wrong.
Each u-shaped 'tow point' is 16mm diameter and has 4 full circumference fillet welds holding it in place. That gives 5cm for each weld x 4 is a total of 20cm = 7.9 inches of weld for the u shaped tow point. The fillet welds look to be about 8mm but lets round that down to 6mm (1/4inch) for safety. Rule of thumb says that 1/4 inch weld can hold 1t / inch (a conservative number backed by some complicated welding theory).
So that gives a theoretical static rating of 8t for each 'tow point'. And by my calcs the 16mm bar should be good for about 10t, so the welds should fail first.
Question:
- Can someone point me to information on failure of the existing tow points?
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