More on the 4×4 fiddle blocks – forces and minimum rope lengths
The fiddle block I published a video about, and wrote about on Motofomo has generated a fair bit of interest. Here’s a couple of extra answers to questions:
What’s the minimum rope for a given distance?
Multiply the distance you intend to pull by 6, so for a 10m pull, allow 60m of rope.
Now it’s a 5:1 mechanical advantage so in theory it should be 5:1, but that doesn’t allow for curvature of the pulleys or a bit extra in hand. I think a factor of 6 would be safe.
I would also suggest using an extension line for the fiddle block as per the diagram below – the reason is that even for a pull of 5m you’ll need 25m of rope, and that’s quite a lot for the average winch drum. By using a synthetic rope extension you can eliminate that problem, and possibly even leave the fiddle block rigged at all times.
What are the loads on the block and what size shackles do I need?
Here’s a diagram:
USA people, see I converted to imperial for you, can you do the same for us? Thanks 🙂
OK, so if you’re using the 24t version you will have a large truck, Unimog, Isuzu or similar. This means you’ll have a mighty big winch too. The maximum diameter for the fiddle block is 14mm, and your winch rope might be greater diameter that that. But, that’s ok. You just connect your winch rope to an extension rope as I suggested above, and run that through the block – in the diagram above the winch rope is green, and the extension rope is black. The extension rope only has about 1/5 of the total force on it so it doesn’t need to be 24t rated.
The two areas of concern are the anchor point rating, which could go up to 24t and requires a soft shackle. You’ll need a very highly rated shackle there, but the maximum is 14mm. Some of the 14mm shackles will go to 24t – look for RED Winches, Factor 55 and SABER for examples.
Then, connecting the other fiddle block back to the vehicle requires about 4/5 of the load – again, a 14mm shackle should do the job there.
Not all 14mm shackles are created equal, and they don’t have working load limits like metal shackles (watch for a video on that soon!)
Fiddle block line lengths
Something else to consider is line length – at 5:1, you will pull in 5m of line for every 1 m you move forwards. This means you have two options:
- run the winch rope through the fiddle block
- use an extension rope
For option 1 your winch rope needs to be 14mm or smaller diameter, which may not be the case for the big trucks. And, you’ll need at least 25m, preferably 30-40m because of the 5:1 MA. That’s why a 50-70m rope extension make sense. However, if you do use a rope extension then the shackle you use to connect your winch rope to the extension rope cannot go through the shackle. This means the fiddle block needs to be located 5 x the distance away relative to the distance you need to pull, like this:
The red line is the winch line, green is a static line (doesn’t get shorter) and black is the extension rope.
Of course, you may not be able to find a suitable anchor point that far away. In which case, you’ll need to do some re-rigging at very regular intervals. The ability to shorten the winch extension line (black) would become very useful.
This problem is no different to using a normal snatch block – when you have a 2:1 MA you take twice the line in for the distance you move, so re-rigging becomes more frequent. And the greater the MA, the greater the line taken in, and the more frequent the re-rigging. So, only use as much MA as you need.
I can’t really see many situations where you’d need 5:1, given you can get 4:1 with two blocks as explained here.
I haven’t tested the fiddle blocks in the field, but I do plan to and will be reporting back when I have. I will be testing how close the 5:1 theoretical MA is to reality.
Here’s the video explaining the concepts:
RED Winches have now released the price for the fiddle blocks, which is 495UKP for a pair – need a pair to be useful. That’s around $900AUD, so maybe we’ll pay around the AUD$1000 mark.