Ping rubber specialists.

[T i m]

Hi all,

I have recently acquired a galvanised dingy trailer that is 1) fairly old and 2) has probably sat with way over the design weight on it for many years [1], and so the 4 (x2) rubber rods that make up the 'independent suspension' have been over-compressed / distorted and so

3) the ride height has been compromised (tyres very close to the inside of the mudguards) and 4) the 'suspension' (ride / travel) has also been compromised.

I don't think it's likely to catastrophically fail / collapse as it looks like the rubbers just get squashed flat(er), however, it's not ideal. This sort of thing:

Now, in many cases these suspension units are independent (of the trailer chassis) and so can be replaced easily. In this case they are part of the actual chassis and so not so easy to replace.

(If I was to replace them I would have to first remove the old units and then grind back the galv, then weld plates to the underside of the box section to allow fitment of standard independent units and re protect with cold galv etc).

Now, assuming I could get the old arms out of the box section (drill, Porta Press) and could find the right rubber (section, material, elasticity) I could (hypothetically) press the arms with new rubbers back in.

However, I'm not sure that is how they make them, or how some make them new and I think I read somewhere that they might first stretch the rubber rods (reducing the section), then they are frozen (temporarily 'locking' the reduced section) and then the rods and arm inserted into the tube where the rods then defrost, regain their full section and hold the arm in place. ;-)

An alternative thought was that years ago a mate had a large tub of some (nasty) liquid that he dropped some of my hardened rubber bits into (4 motorcycle carb inlet rubbers) for a few seconds, pulled them out and rinsed them off and they were all rubbery again? ;-) Anyone know what that liquid might have been?

A final / compromise solution would be to fabricate some sort of adapter that would both bolt onto the original trailer axle and then allow the new suspension units to bolt onto that, saving the issue of having to extract the old suspension stuff (I can just cut the old arm off) and remove the galv to weld etc.

As an aside to the main question I would be looking for what they refer to as '200kg units' (so 100kg each) because the trailer gross weight is 180kg and as a typical 14' (or less) boat that I might have on their weighs between 50-100kg, I don't want the suspension to stiff (been there before). You can easily buy 200kg units bit not with such a good range of designs (and so 'rides') as the heavier units.

Cheers, T i m

[1] There has been a 14' dinghy sitting on the trailer for ~11 years that I know of and the boat itself weighs about 75kg. However, with all the other bits that were left in there it could have been nearer 100. The real issue though is for some of that time the boat also had quite a bit of water in it, possibly at least 1/2 a cubic metre and so that was an *additional* ~500kg sitting on the suspension units! ;-(

p.s. I wonder now the trailer is unloaded the suspension rubbers might recover to some degree (over the next 11+ years), and if that could be improved if I was to apply some negative pressure to the units?

[Andy Burns]

Youtube implies soaking for a couple of days in isopropyl alcohol 3:1 with wintergreen oil

[T i m]

Thanks for that Andy, I did check Youtube to see if anyone had replaced such rubbers but couldn't see anything ... but didn't look for any 'revival' tips.

Do you still have the link to the so I can see the context please? Obviously there are safety implications if I was to do something and it made matters worse etc.

Cheers, T i m

p.s. Your reply does remind me I once bought some stuff that was supposed to be for conditioning catapult rubbers but I wasn't sure if it also 'revived' tired rubbers (and they would be cheap and easy to replace in any case etc).

[newshound]

The exact material is probably not all that critical, as long as you have about the right Shore hardness. Even natural rubber or SBR would probably be OK as it won't see sunlight, or too much air or oil. Nitrile or EPDM might be a bit better.

That seems plausible to me.

Mineral oil will cause most rubbers (apart from Nitrile) to swell and soften. It won't actually "rot" them but in the softened form they will be weaker than in the hard form.

That sounds to me like it might be the simplest solution.

However, one thing that occurred to me is that you don't necessarily need to retain that geometry. Traditional "silentbloc" bushes just have solid rubber bonded between the inner and outer parts (although they might not have quite the "range" of these units). I could envisage doing a repair involving removing the existing rubber and casting two-part silicone (RTV or room temperature vulcanisation) between the spindle and the housing. However I think I would worry whether a square spindle would "key" well enough, I can see it tearing the rubber in shear. Flexible couplings with this sort of geometry usually have rubber between a male and a female "star" so that the rubber is predominantly in compression rather than shear.

There might be a bit of recovery, but I would not expect a lot.

[T i m]

I've since found this which seems reasonably 'scientific' (considering etc).

The bits of specific interest to me being how long any softening lasted and how 'easy' and cost effective it might be to apply any such process to my trailer parts.

Cheers, T i m

[T i m]

Ok. I'm not sure how I could measure that (is there a tool / test as I might be able to nick a bit from a new unit as a sample).

Air yes as it's open to the elements. The others no.

I have seen poly mentioned on some recent adverts?

Ok thanks. Would could a DIY'er use to 'freeze' the parts as I'm not sure std domestic freezer temperatures (-20 DegC) would be sufficient?

I guess that's a compromise between giving some of the desired 'suspension' whilst keeping everything in line as much as possible. I think I have noticed so increased camber on the 'lowest' arm, suggesting the rubber may not have deformed equally along it's length (as predicted probably).

I think I agree, however I felt I should at least explore the alternatives if it could turn out easier / cheaper / better (as 'original). ;-)

Ok.

I have those on another trailer and it uses them on a pair of full trailing arms (like the back of some cars). I pressed the old bushes out and the new ones into the arms on my 10 tonne press (and suitably turned tools) and you then set the ride height before tightening them fully to the trailer chassis.

Ok. This looks like something along those basic lines:

Seen / agreed.

Our old Bramber dinghy trailer had 'Flexitor' suspension units that were as you say. The rubber was used in torsion rather than compression and made for some very compliant suspension units. I think some of these use nylon bearing blocks on the inside and outside of the mount because the rubber is so soft it might not manage the other geometries (tracking / camber) accurately without.

Me neither but if they did (now they unloaded) it would save a load of hassle (and cost). I wonder if I could unload them to the original condition before treating them chemically and if that would help both their 'set' and (therefore) their ride?

On the luggage trailer I tow behind the motorcycles they use a pair of 'leading' wishbones with a soft rubber bush in compression on each to provide the suspension (and real dampers from memory). This means it can easily be tailored to a very light action and even when towing it across a camping field it seems to react very well (nothing arrives broken). ;-)

Cheers, T i m

[Andy Burns]

This one was at least methodical about trying various liquids

Many others if you search youtube for "soften rubber" but the various brake/transmission fluids seem to risk swelling it.

[T i m]

Ah, yes, as you may see sometime I found that one after you mentioned Youtube and agree it was go re how the actual tests were applied and how he even left them to see how they felt a good time after.

It looks like the Isopropyl Alcohol / Wintergreen combo seems to be used fairly commonly and with good result, including any swellings going down (oooh, missus). ;-)

Cheers, T i m

[Brian Reay]

I've seen rubber expand, as it happens when I didn't want it to, when exposed to oil- specifically the oil used to fill a compass. I assume a cheaper oil with the same 'base' would work.

My concern in your application would be controlling the result- you don't want a trailer that is going to 'bunny hop' as you tow it and there is the question of load bearing.

While it seems like more hassle, I think total replacement is the safe option.

[newshound]

There's an instrument to measure it, you poke a spike into it at a certain load and see how far it goes in. I was assuming you would have got the old bits out (to confirm dimensions when going shopping) although of course it might have hardened a bit with time. If you put the old bit side by side with the candidate, you should get some idea of the match just by poking (if the supplier doesn't have a tester, but some might well do).

I think that might be marginal too. You would have to make a jig to tension the specimens before freezing, and you would want it to be "quick release". I'd be more confident using liquid nitrogen, which is not in fact very expensive. But you probably need a friend in an industrial lab, hospital, or university science department. I once sneaked a couple of pints home in a briefcase, in a wide necked thermos, in order to freeze a rising main incomer to fit a replacement stop tap. It worked very well. I only had a couple of inches of copper above the porch floor level to work with, but I excavated a cone a few inches deep in the floor, and froze the ground around the pipe.

I see from the web that dairy farmers sometimes have a bulk tank which is topped up periodically by the supplier. Someone else pointed out that local "industrial gas suppliers" who do gases for welding and pubs might also stock it. It's safe to transport in a glass "thermos flask", but a modern stainless steel one might be less stressful. TIP: don't screw the top down.

I've never used polyurethane that way. Probably OK for suspension mounts with not too much movement, but I don't think it stretches as far as silicone before tearing.

[newshound]

Perhaps in this case the swelling caused by mineral oil would not matter so much (as there is plenty of space).

Agreed, both good points

Me too.

[T i m]

Same here, or some chemical cleaners etc.

Agreed (if by work that's what you wanted but in my case it isn't really).

I can't see it doing that though as I doubt I'd ever get the rubber as 'elastic' as it was from new?

Unless whatever I do reduces the overall strength of the rubber or dissolves it away completely I think the worst case situation here is that it just has no suspension. Now *that* when you will see a trailer bouncing all over the place, especially when unloaded.

And that's the very thing I'm trying to avoid by *not* fitting some aftermarket suspension unit that was actually too stiff.

I agree (re safety and assuming I remove enough of the galv to get a good weld (and not poison myself at the same time)) but ironically, any 'replacement' will also 'modify' the trailer, opening up other issues etc.

The 'best' solution (to / for me anyway) is to come up with a 'bolt up' solution, potentially including some fabrication etc but even little things like keeping enough room around the suspension plate mounting holes to get a spanner round the nut can be quite tricky.

eg. Image a 50mm box section axle tube, currently filled at the ends with this original suspension solution. Remove the old stuff and get a

200kg replacement that generally comes with a 4 hole mounting plate.

You can also buy matching plates you would typically weld under the chassis / axle but that would mean removing the galv (and partly why I considered the trailer in the first place).

So, if I was to make up a heavier duty plate to take the new suspension (say 6 rather than 3mm) and bond [1] and bolt it up from underneath using countersunk machine screws into a plate that can be slipped into the tube with welded on nuts, I wouldn't have to do any welding.

Ideally I would also use some angle between the plate and the front and back of the axle tube but I don't think there is the room for that and the nuts that secure the suspension unit to the plate? ;-(

I'll have to do some measurements ... ;-)

Cheers, T i m

[1] My mechanic mate has a 'gun' of some very strong (nozzle mixed, two part) glue that you use for bonding car / body parts together. Apparently such is acceptable now in place of weld (on the body particularly) for MOT's et (and many vehicles (inc boats and planes) are bonded together)?

I would be using it as 'belt and braces' over my machine screws. ;-)

[T i m]

I also agree with having concern re the last point but I'm not sure about the first?

I bought a nearly new second hand light general goods trailer (the sort of thing you take your rubbish to the dump with) and empty, unless you took every speed hump at 2 mph, all you could see in the rear view mirror was the trailer in the air?

I also fitted the lightest suspension units I could find at the time to my previous boat trailer and that too was 'very lively', even when we loaded the boat down with general luggage / camping gear etc. So, my point (from experience) is that it's more likely to get a /bunny hopping' trailer by using the wrong suspension units (and not just the nominal capacity) than I might from just freshening up the old rubbers?

Also agreed, assuming I can achieve that without compromising the longevity of the trailer (how easy would it be to re hot galv the affected parts after welding the new parts on) and me finding the right spec units etc?

And remember, not all of us have unlimited budgets for such things? ;-)

Cheers, T i m

[T i m]

OK, thanks.

I was hoping to be able to get to some of the 'excess' that often sticks out the arm end of a new unit.

See above. This would be measured on a new and suitable rated unit.

Understood.

Ok.

Crikey, just that was enough?

Tip noted. Do you think it would be sufficient to tip the LN over the stretched rubber strips, release and fit (I could also freeze the other components to help buy some time), before they started defrosting? Or would they be better off being 'dipped' (with the jig etc)?

Agreed. I think the issue with my scenario is that nothing would be in compression from the get-go, unlike the original rods etc?

Cheers, T i m

[Thomas Prufer]

What also works, is only marginally nasty, and leaves pleasantly scented result: fabric softener. Takes days to weeks, though.

I think it contains silicone oils that permeate and soften...

Thomas prufer

[newshound]

The difference between entry level trailers and car suspension is that the former don't have any proper damping, or even much friction.

With a heavy and attached load like a boat, you may not need much damping. On a tip trip, light and unsecured items may well levitate all the time.

[tabbypurr]

Rubber provides damping, as it does in rubber car suspension such as the mi ni. That's one of the major reasons why it's used on trailers, though not t he only one.

There is one key difference between car & trailer: cars are heavy unloaded, loading full only adds somewhere around 50% total weight. Trailers OTOH ha ve no engine, drive train, top or crash safety concerns, so are very much l ighter, giving a much larger ratio of full to empty weight. That's not so e asy for a crude suspension system to deal well with.

NT

[newshound]

LN provides a *lot* of cooling, with the combination of the large delta T and the latent heat.

Hard to say without trying it. My guess is that "pouring over" might need a lot of LN, but that a bath giving 100% immersion for a few minutes would be more than enough. If you can manage partial submersion with the rest of it sitting in the vapour immediately above the liquid for a minute or so I think this might be long enough for the "cold" to diffuse into the centre of the rubber; which is what you are trying to achieve.

Remember that if you freeze your bit of rubber into a banana shape, you will need extra force to insert it. I'd be trying to freeze it as straight as possible. I would also think about applying some lubrication to get the bits inserted as quickly as possible. Silicone spray or silicone fluid would be good, mineral oil spray should be OK; washing up liquid contains a fair amount of water, and might tend to freeze.

[newshound]

True, rubber does provide *some* damping, but original Minis had traditional hydraulic dampers. Later BL stuff had hydrolastic suspension with a lot of rubber in them instead of metal springs, but the damping was mainly provided by the flow of "water" through the connecting pipes. That said, what you say about the difficulty of optimising for trailers is exactly right. Although hydraulic dampers would make them perform better.

[Fredxx]

Are you suggesting Minis didn't need dampers?