My general take on fixed or equalizing bridles. If I know I am going to be pulling off line, rigging the bridle to have 'proper' leg length to have roughly equal forces is worth it. This does a few things. 1- This does provide true redundancy if one point was to fail, however, we should never be rigging or operating an attachment point for vehicle recovery under a 2:1 Factor of safety knowingly. 2- A fixed bridle will help prevent the vehicle from spinning out of your control AND force the vehicle to spin in the direction you want it to quicker.I don't understand the advantage of a fixed leg double bridle. For an even slightly off center pull, the vehicle will be rotating during the recovery, so only one leg will have all of the force at a given time. The extra leg would serve only as a backup to keep the vehicle from rolling away if the tensioned leg failed... but it's hard to say if that would be the failure point vs elsewhere in the system. A sliding bridle doesn't offer the redundancy, but also puts half the load on each recovery point. It would seem to me that preventing failure would be more valuable than having an unlikely to be useful partial redundancy.
I have been thinking that in the age of synthetic lines and wireless remotes, in many situations it may be safest for vehicle recovery to be totally 'unmanned,' with everybody far away from the vehicle, and no driver. In this case, the vehicle itself could have a second safety line always in the uphill direction that is belayed out manually, like a rock climber. This would be a 100% redundant system sharing no points in common with the main recovery line.
Metcalf - I've been reading through offroad recovery techniques on here and have learned a lot reading things you've written in the past. Thanks!
Commonly, a lot of people think of bridles as only being used on bumpers for something like a kinetic rope recovery. That is a great use for them, one of the reasons is that works so well is because it gets the rope to the middle of the vehicle. As far as strength....see #1 above. With questionable recovery points by all means use a bridle, but if they are questionable, wouldn't having the redundancy of a fixed bridle be better? Personally, I don't think rigging around the idea of doing a max effort offline pull, that is constantly changing directions, is THAT likely. I find it much more likely do be doing high force pulls in a known direction where I really don't want the vehicle to be able to equalize itself.....think pulling the vehicle mostly sideways from the frame and/or bumpers and you absolutely don't want the vehicle to be able to pivot one way or the other over the shoulder of a road.
There are some very real issues with equalizing bridles that are getting glossed over a bit I want to touch on. 1b- The movement between the two 'lines', be it strap on strap, strap on rope, rope on rope, etc....all cause high levels of wear at low loads. If the bridle IS actually trying to equalize itself consistently, it will wear itself out over time even under very low loads. rope on rope connections that move are generally regarded as a no-no across all industries and sports. We know that causes issues over time even at low loads and shortens the service life of the item. To solve this problem, you have to add extra items ( typically something like a pulley ) into the rigging which then adds mass which is not only a safety issue, but also another item of gear you have to have wear on, wear out, and make sure will reset itself under this constant changing force direction and loads. 2b - At high load levels, the friction between the rope/strap connection will lock up, causing uneven loading anyways, then suddenly release causing rapid movements. As mentioned in 1b, you can add something into the system to help with this, but then you have more mass ( probably metal moving parts) and connection points in the system. You also have to worry about that device skipping down the road if you are doing something like a tow/kinetic recovery.
I hope that helps explain my position on things a bit more. So far, I have never had a situation where I have broken a mounting point with either style of bridle rigging. I HAVE had connection points come loose ( hooks and hard shackles) more than once though. In those cases, the equalizing bridle caused a total disconnect as it pulled through, while the fixed bridle remained connected on one side and the connection to the vehicle.
Cheers. Thanks for the interesting topic to babble about.