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One of the P38's useful traits that helps justify me keeping it around is its towing capacity. I have a 3T twin axle car transporter that i use infrequently, which goes nicely behind the P38.

The last few times i've had it out however, towing it has been bloody awful. When unloaded, the trailer seems to transmit some sort of vibration or oscillation into the car, which causes a vertical "jiggling" at a frequency which feels like someones shaking my internal organs to bits. Did 100miles in it on satruday, 80 of those unloaded and i was feeling verging on ill by the time i stopped after 40miles. I was having to sit forwards off the seat back to minimise the vibration. Road surface certainly played a role, with one section of road being incredibly bad, but the vibrations were there the whole time.

Now, i know the trailer hitch itself has some wear in it. The draw tube wiggles up and down inside its bore. I've been meaning to buy a new hitch for a while, but its expensive and hard to justify for something used so infrequently. The part i'm trying to figure out though is why its changed. The draw tube has always had this play in it for as long as i've had the trailer, but its never been that bad to tow with.

In the past, the hitch slack seemed to causes the trailer to sort of "kick" the back of the car when the trailer hit a bump or similar. Uncomfortable, loud and annoying, but not continuous. It would also get unsettled over uneven road and the hitch would kinda rattle and bang around, but again, it would only happen over the bit of rough road, not continuously for 40miles!

With the trailer loaded with a car it gets MUCH better, though its still not the most pleasant thing to tow, and it seems to like "snaking", not violently, but enough to mean that your sat there on red alert the whole time.

Anyway, enough waffling, i'm trying to figure out if ALL of this issue is with the trailer itself, or if the vehicle could be playing a part? Perhaps tired shocks or wear in bushes or something? Anyone experienced anything similar?

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If it's used infrequently, could it be a tyre that's gone out of round?

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I bought a P38 for that exact purpose, towing car transporter trailers, and there's all sorts of variables that make a difference. Twin axle trailers are worse for it than single axle too. Empty they can be uncomfortable and that varies with the make and model of trailer, Indespension ones are horrible to tow when empty but locking the EAS at motorway height helps (even though the owners manual says to lock into standard height). Trailer tyre pressures make a difference and so do the state of the wheel bearings. Snaking is usually caused by too little noseweight, shifting the load when loaded or dumping your spare wheel on the A frame can help with that. I've found that the best ones to tow are Bryan James trailers, as said, Indespension are horrible with Ifor Williams somewhere in between. If it has got progressively worse, I'd say a bit of maintenance on the trailer might be a good idea. I did own a Bryan James tilting transporter until it got stolen so since then have hired one whenever I need it. The hire place I use regularly replace their hire fleet so I usually get a fairly new trailer with little slop in anything so do tow much better.

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I am not quite in the same league, but I have a heavy'ish caravan @ about 2 tonnes and have done quite a few thousand miles throughout europe over the years. Not even remotely near Richard's mileage though.
I would also say check the tyres. Jack one side up and spin each wheel. See if anything is amiss. Wheel nuts loose? Any play in the wheel bearings? Older tyres with steel reinforcing are more prone to flats if it has been layed up. Check tyre pressures and examine the treads. It is also worth checking the date letter on each tyre. Bin them if over 7 years old.
Is the trailer hitch worn? It may come off the ball if it is. Is there a stabiliser? Friction pads worn?

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Thanks for the pointers. The tyres arent ancient, i replaced them all when i bought the trailer, but yeah they could have deformed or something. I'll check the date codes.

The trailer itself doesnt have any branding on it, it appears to have been built probably 25-30 years ago. It all looks fairly DIY, but done properly with solid steel sections, proper 5 stud trailer wheels with load rated tyres and a nice bradley hitch. The original wheel rims were dated 1989. The hitch is dated 1997. I've replaced a few of the steel crossmembers over the years, mostly because it was C channel and had been torn by the previous owner ratchet strapping things to the leading edges.

I wouldnt even say its a progressive change. It was fine, then it wasnt, and its been equally bad the last few times i've had it out. Like i say, its never been nice to tow empty, but this vertical jiggling motion is very much new and moves it from a bit "uncomfortable" to basically being unusuable for anything more than a short local trip.

I guess i'll start with the wheels/tyres and check all the bearings etc. Costs nothing.

Tyre pressures are something that hasnt changed much, they're at about 60psi, sidewall recommends 90 for the full 3 tonnes, but Most inflators wont go that high though. I think i had them around 75 at one point but they've slowly crept downwards over the years and are now around 60, which feels like "enough" for a typical saloon car on the back.

I'm just a bit loathed to spend £300 on a new Bradley HU12 to find its still just as bad afterwards!

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It might be an idea to get someone else to drive it and have a look at it from a second vehicle if you can do so. May be nothing to see, but its surprising to see how many vehicles you see going round with shocks that have clearly completely failed and the wheel bouncing all over the place as it goes along a dual carriageway or motorway with very little roughness on the road.

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Trailers also have got to be type approved now i.e. from a reputable manufacturer. Home built trailers are illegal.
There is however a grandfather clause that trailers built before a certain date are excempt. Can't remember off hand what the date is, but It said you had to have the date of manufacture on the A frame. I have got a small trailer that I made back in the year dot and I just painted a date on in case I am stopped by someone just out of Hendon.

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My bet would be tyres, especially if they are 13" or even 12", ... (or just possibly a wheel bearing) I have actually altered my Ifor W. to take 16" wheels because the small tyres kept going egg shaped. Jacking up each side (as above) is good advice and check the nose weight as well, even if there is a little play in the draw tube it shouldn't be a big problem by itself on main roads.

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Thanks, i think they are 13". Hopefully at some point this week i'll get it jacked up and check the tyres and bearings.

Is it worth trying to have the wheels balanced?

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Unlikely to be brakes if it's a braked trailer but worth a mention if they haven't been mentioned already.

The thing about wheel bearings on trailers is you don't hear them when they've gone wrong like you do on a car.

Which reminds me, I need to change wheel bearings on a trailer (not a car trailer) before I can use it and I need to use it at the end of next week.

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had a look over it this evening, and certainly to the eye, i cant see anything wrong with the wheels.

Bearings all feel fine, no play, no roughness or odd noises. Spun the wheels by hand and theres no obvious flat spots or egg shaped running.

They are dated 2014 though. I bought new wheels and tyres as a package back then, as the original wheels were needing refurbed and it was cheaper to just replace the whole lot.

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Check the wheel nuts as well. I remember a few years back I had severe vibration with my small trailer.
I found my son had put the wheel nuts on the wrong way round. The conical bit of the nut should fit into the recess on the wheel to centralise it.

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how do the brakes work , are they the over ride system , if so put the reverse plate over and go for a drive see if its any different . remove the wheels and visually check the brakes . are they discs or drums ? hydraulic ,cable or electric.

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Not sure if theres an override actually, i'll have a look.

Its fairly standard cable operated drum brakes.

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There may be nothing wrong with the car or the trailer, it might be down to the council. Yesterday I towed a loaded Brian James flatbed trailer over to France. The load was more bulk than weight so probably weighed no more than 500kg. Got it to the destination where it was unloaded and I set off back. The trailer had towed perfectly on the way down and was just the same on the way back, until I got off the ferry at Dover. For the rest of the journey home, except for stretch of A14 that was only opened a year ago, the trailer was causing the car to 'jiggle' as you put it, pretty violently. Leaning forward so I was not leaning against the backrest at least meant my eyeballs weren't being shaken around to the point where I couldn't see where I was going! It's not the trailer, it's the shit quality of the roads in this country......

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I had a similar issue on an Audi that had wear on the tube that "holds" the (detachable) tow ball. That wear is noticeable is you grab the ball and shake it, even uncoupled. Might be a similar issue?

Is been a while I have not towed with mine (sigh), but I do recall a smooth and proper 'towing ride'. Miss some HP on inclines, but that is understandable ...
One thing I never liked is when loading/unloading, the rear of the car lifts alarmingly, to the point of almost lifting the wheels ftom the floor. I never liked it since I was worried on the capability of the EAS height sensors to resist the extension without issues ... I will need to think of something when I get restarted with this, to "lock" the trailer preventing the movement...

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It won't strain the height sensors as the shocks will limit how much the suspension can extend. Most car transporter trailers have legs at the rear so the back doesn't drop right down. I use the extension on the suspension as a guide when loading a car. Watch how high the back of the car is then drive onto the trailer and when the back of the car has dropped to just below normal height, the balance is about right.

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The extension limit only with OE type shocks ... I am still not sure whether to mount them, but I have a set of TF (std) which nevertheless seem to be longer than the originals ....
My trailer unfortunately does not have the stands, I am thinking (if I keep it) to add them.
In 'extreme' cases I have been using couple of bottle jacks, cheap and simple, but not always have to time to set them up ...

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Yeah the roads certainly play a part, theres a stretch of newish road right after the queensferry crossing which was REALLY bad, and the amplitude massively reduced as soon as i was off that new section and back onto the original tar. However its also doing it, albeit to a lesser extent on all roads. I suspect its perhaps a few factors working together, play in some components and vibrations from the road surface setting up a bit of resonance or oscillation, and perhaps the rear shocks are getting a little tired and are struggling to damp the faster movements.

I've loaded a car at the weekend that i'll be taking to the scrappy at some point this week, so i'm going to try the return leg with the brake override enabled and see what happens.

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okay so as one might have expected in hindsight, the brake override doesnt actually stay enabled once you start driving. Its spring loaded in such a way that it pops out again.

I did notice very visible pitching of the trailer on the motorway, the nose of the trailer bouncing up and down with a very close correlation to the jiggling the car is experiencing.

So i guess we're back to the original question, is it simply too much play in the hitch/coupling allowing the trailer to bounce around at a high frequency. A new hitch is £320 which feels like a lot of money if it doesnt fix it, although to be fair, it probably needs it regardless. Its more that it might be better to sell this trailer and buy something better instead of spending the money on this one.