Detachable Towbar - What stops it falling off?
Discussion
The top one is quite a clever bit of design, see here;
The other has a sizeable groove in the back which is engaged by a spring loaded pin; it's a system called Stealth Hitch.
Both are more than robust to deal with the <100kg nose weight and dynamic loading the hitches of <3.5t trailers deal with.
The other has a sizeable groove in the back which is engaged by a spring loaded pin; it's a system called Stealth Hitch.
Both are more than robust to deal with the <100kg nose weight and dynamic loading the hitches of <3.5t trailers deal with.
They usually hinge on two things -
1 - the snug fit of the removable bar into the receiver, which means the major forces of the trailer movement itself are transmitted through the bar and receiver, not through the retaining mechanism
2 - positive locking which is strong enough to resist any disassembling forces. The Westfalia one above is a good example, where a solid pin is holding ball bearings into a groove - that's more than strong enough to deal with any forces.
I have one on my Range Rover which uses a sprung, sliding stainless pin. The geometry of the hitch means that the trailer nose weight is holding the bar in place as long as there's weight on it, and downward forces are reduced by the angle of the receiver.
Very clever.
1 - the snug fit of the removable bar into the receiver, which means the major forces of the trailer movement itself are transmitted through the bar and receiver, not through the retaining mechanism
2 - positive locking which is strong enough to resist any disassembling forces. The Westfalia one above is a good example, where a solid pin is holding ball bearings into a groove - that's more than strong enough to deal with any forces.
I have one on my Range Rover which uses a sprung, sliding stainless pin. The geometry of the hitch means that the trailer nose weight is holding the bar in place as long as there's weight on it, and downward forces are reduced by the angle of the receiver.
Very clever.
I have the westfalia one (the top image).
The way it is designed the balls take the vertical load and they're at best trying to force themselves into a solid bar. The side to side load is then via the V shaped protrusions either side which "lock" into the socket.
It is very secure when fitted. Although I will mention that if you don't remove it for a long time (I often leave it on over summer as I use it for the bike carrier pretty much every weekend) it's even more secure as it doesn't want to drop out even when unlocked!
Take into consideration that technically a *single* M5 bolt (8.8) can support an 820Kg proof load, the balls are something like 10mm and it's trying to effectively cut them in half. A mere 100Kg of nose weight isn't going to cut two 10mm ball bearings in half!
The way it is designed the balls take the vertical load and they're at best trying to force themselves into a solid bar. The side to side load is then via the V shaped protrusions either side which "lock" into the socket.
It is very secure when fitted. Although I will mention that if you don't remove it for a long time (I often leave it on over summer as I use it for the bike carrier pretty much every weekend) it's even more secure as it doesn't want to drop out even when unlocked!
Take into consideration that technically a *single* M5 bolt (8.8) can support an 820Kg proof load, the balls are something like 10mm and it's trying to effectively cut them in half. A mere 100Kg of nose weight isn't going to cut two 10mm ball bearings in half!
Fastdruid said:
Take into consideration that technically a *single* M5 bolt (8.8) can support an 820Kg proof load, the balls are something like 10mm and it's trying to effectively cut them in half. A mere 100Kg of nose weight isn't going to cut two 10mm ball bearings in half!
True although a 100kg static nose-weight could in result in far higher dynamic loads. I'd imagine something like a car transporter trailer could easily put several tonnes of instantaneous load through the tow bar if someone, for example, takes a speed hump inadvisably fast and whilst that would obviously be a stupid thing to do while towing, you can bet it happens occasionally so the tow bars have to be designed not to fall apart when it does. I'd be surprised if these tow bars aren't designed to take five+ tonnes of load downwards and at least a couple of tonnes in every other direction.
Edited by kambites on Tuesday 4th July 10:42
Follow the manufacturer's instructions - as in the vehicle's owner's handbook if it's a factory fit, or the towbar manufacturer's if it's an aftermarket.
Some have a key operated mechanism & if the instructions say 'Remove the key' then do that. If you don't then it may not stay attached to the car!
Some have a key operated mechanism & if the instructions say 'Remove the key' then do that. If you don't then it may not stay attached to the car!
Ideally the breakaway cable will stop the trailer but only if the cable was attached to a part of the vehicle that hasn't fallen off & not looped over the ball.
Possible the trailer may roll or crash & if it's your caravan or boat it might be an expensive issue. That's even if nobody else is involved.
I've seen a fair number of 'guess what the make was' caravans at the side of motorways when they've gone over.
Usually a result of losing control as a result of snaking & not unusual to take the car with it.
Possible the trailer may roll or crash & if it's your caravan or boat it might be an expensive issue. That's even if nobody else is involved.
I've seen a fair number of 'guess what the make was' caravans at the side of motorways when they've gone over.
Usually a result of losing control as a result of snaking & not unusual to take the car with it.
I fitted a Witter detachable swan neck towbar to our XC90. It was chosen mostly for aesthetics as the car only tows a couple of times a month, and is used as a daily otherwise. I was a little concerned at the idea initially, but Witter state that these bars are tested and rated to the same towing capacity as the tow vehicle they are fitted to.
It's a simple, but very clever bit of engineering. The locking mechanism itself isn't subjected to the any of the towing load or the forces. The neck will only enter or exit the receiver if it is held at a raised angle relative to the car. Once it's fully home and pushed down, the neck is held by means of parts that don't move at all. The neck has two stout circular pieces machined into it (a bit like an axle) that engage with two cut-outs in the receiver, and an additional grooved section that engages with a matching lip at the back of the receiver.
The locking mechanism is a stout, spring loaded bar that automatically engages when the neck is pushed into the correct position, and prevents any vertical movement of the neck. This means it physically cannot be raised to the angle needed to detach it from the receiver. As an added safety feature, the nose weight of the trailer is pushing the neck downwards, when it needs to be raised to disengage and remove it. Both the neck and the receiver are very stout, very heavy pieces of steelwork, and fit together very precisely.
The large thumbwheel on the neck that operates the locking mechanism has a cam action which you have to compress a very strong spring to overcome. It also locks for added security, although this is for anti theft purposes and isn't necessary for daily operation.
As long as you use a reputable manufacturer, I think these things work really well. Ours pulls a 950kg horsebox with a couple of ponies (probably 350kg each) in it quite regularly and hasn't given us any cause for concern. With the neck removed, the receiver unit is invisible without getting on your hands and knees.
It's a simple, but very clever bit of engineering. The locking mechanism itself isn't subjected to the any of the towing load or the forces. The neck will only enter or exit the receiver if it is held at a raised angle relative to the car. Once it's fully home and pushed down, the neck is held by means of parts that don't move at all. The neck has two stout circular pieces machined into it (a bit like an axle) that engage with two cut-outs in the receiver, and an additional grooved section that engages with a matching lip at the back of the receiver.
The locking mechanism is a stout, spring loaded bar that automatically engages when the neck is pushed into the correct position, and prevents any vertical movement of the neck. This means it physically cannot be raised to the angle needed to detach it from the receiver. As an added safety feature, the nose weight of the trailer is pushing the neck downwards, when it needs to be raised to disengage and remove it. Both the neck and the receiver are very stout, very heavy pieces of steelwork, and fit together very precisely.
The large thumbwheel on the neck that operates the locking mechanism has a cam action which you have to compress a very strong spring to overcome. It also locks for added security, although this is for anti theft purposes and isn't necessary for daily operation.
As long as you use a reputable manufacturer, I think these things work really well. Ours pulls a 950kg horsebox with a couple of ponies (probably 350kg each) in it quite regularly and hasn't given us any cause for concern. With the neck removed, the receiver unit is invisible without getting on your hands and knees.
normalbloke said:
Nothing that I’ve seen beats the Porsche Cayenne fold away tow bar.
You don't need a Cayenne for one of those. My wife's humble 1.0 Leon has one too. I do find it a bit disconcerting that pulling on a small flimsy switch in the boot is enough to make it fold away though. Go over a big bump, some luggage moves about operating the switch, and the next thing you know the towbar could fold away with the trailer still attached.....It never has though, so probably better designed than it appears.
The other thing I find really unconvincing are those towball mounted bike carriers, both in how they attach to the towball and also the bike being held only by the bottom of the wheels and a small strap fairly low down on the frame. They're no where near as bad though as the bike carriers which sit on the back window and held in place by a couple of hooks under the boot lid, which do fall off quite frequently.
SkodaIan said:
You don't need a Cayenne for one of those. My wife's humble 1.0 Leon has one too. I do find it a bit disconcerting that pulling on a small flimsy switch in the boot is enough to make it fold away though. Go over a big bump, some luggage moves about operating the switch, and the next thing you know the towbar could fold away with the trailer still attached.....
It never has though, so probably better designed than it appears.
I'd love to know the full logic behind the button but it probably goes something like this:It never has though, so probably better designed than it appears.
For starters I guess they check if there's anything connected to the electric wiring for the tow hitch? The same resistance measurement used to check for blown lightbulbs.
Secondly there's probably something mechanical that prevents it from unlocking if there's any significant load on the hitch. Maybe stand with your foot on it while pushing the button? My guess is it needs to be lifted up a little before it can unlock.
Thirdly it probably won't respond if the engine is running.
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