Any experience in working with steel?

[RrogerD]

Not sure if this is the correct thread?

My son is building a car trailer, using the rolling chassis from a caravan. He's CADed it all up and tbh it's looking pretty good. I'm giving it a sense check but my knowledge is limited. I'm looking for some advice on steel strengths, max hole sizes and possibly someone to cast an eye over the design, if anyone fancies it?

Any takers?

Thanks in advance.

 

[freddiemercurystwin]

Post the drawings in the thread.

 

[RrogerD]

Post the drawings in the thread.

Thanks. I'll get him to sort something out later this evening

 

[Burnerman]

Car trailers are usually four or even six wheel, for stability and loading.
The original caravan weight and the proposed towing weight needs some thought too to make the suspension and the tow bar braking system happy.
The caravan chassis will be well strong enough ( in my experience) and will often be galvanised - so precautions needed when welding!
John

 

[cdbe]

It will (technically) need type approval (IVA for trailers) although if your caravan chassis is pre 2012 you could probably get away with calling it a modification to an existing trailer. I believe that alko (who make most caravan chassis) do not "permit" either welding or drilling of their chassis. Personally (and I have toyed with the idea myself) I think he'd be better buying and renovating an existing older car transporter or modifying a twin axle trailer.

In the end I decided not to bother, I only wanted it to occasionally drag around my classic mini so I've decided to buy this and turn it into a "trailer"/"vehicle undergoing recovery"

 

[RrogerD]

Thanks guys.
We would of course have preferred a 4 wheel but it's a case of going with what we've got. Maybe perhaps adding a second axle later could be an option? There are some examples on line, although admittedly not many, of single axle car trailers. He's done the sums and reckons that everything will be within max weight limits.

The plan is a standalone bed bolted to the existing chassis. The bed is to be made of 40d x 20w x 2mm steel square-section, bolted with plates 4mm thick plates top and bottom. Possibly welded at a later date but not initially.

With 20mm wide tubing, we're thinking m6 bolts, also plated top and bottom with 2mm plates to spread some of the forces.

Here's a pic. When I posted initially it was a lot more fancy with tubing running down the length of the existing chassis and the frame extending from it. This latter setup seems a lot simpler.

The white rectangle in the first pic shows the footprint of the car, the corners being the wheel positions.
The centre cross member is located approximately at the axle position.

 

[RrogerD]

My concern may be in the strength of the original chassis. Will it flex too much? I guess, having the 'bed' bolted at various places, ten bolts in total, then this should brace it all up? Do you think?

 

[cdbe]

My one tonne building trailer is made of 3mm box and C channel, in much larger sizes than 40x20mm, the main chassis rails are about 60x60 C channel, and it's only a 2.5m bed - but if you've done the sums it must be ok. Personally I wouldn't know where to start - calculating a structure that can resist the forces of a tonne of car bouncing up and down on it at 60mph!

A lot I've seen have two side by side chassis rails forming the sides of the track so probably a feature worth borrowing.

Cars are generally front heavy so you could do with getting one on the chassis (knock up some timber ramps) to establish the correct nose weight (I believe it should be around 7%). Hopefully it will be fairly central or you'll end up with an overlong trailer with an excessively long A frame at the front - putting the car on backwards might help.

As I said, I toyed with building something myself, but my lack of engineering design skills would have meant that for my own peace of mind I'd have to so over engineer it that it would be too heavy for my car to tow. Notwithstanding the legalities of having a serious accident with a non approved trailer. You can submit it for an IVA so it might be worth considering, I don't think it's that expensive.

When I looked on eBay, if you ignore the homemade stuff (caravan chassis with some plywood on top = car transporter, unbaked things that would be illegal to carry a mini with etc), you can pick up an older, professionally designed, built and certified transporter for around the 1K mark - and if you look after it you'll always get your money back.

 

[BenSr]

This is what I have been debating, as from the drawings and my other experience working with similar 'home built' trailers they use the same design. I thought about having a stronger beam go length ways along the frame, on the outside, forming the "track" but as the wheels of the car will be on top of the original chassis (at the rear) and only over hanging 400mm at the front I thought is deemed acceptable, not perfect, not not include one.

For the weight distribution side: I will hopefully using it for a old 3 series which have a 60% - 40% weight bias to the front so this should be fine if the car is centered over the axel.

I saw that IVA's are only £70 for an O2 trailer (trailer over 750kg and under 3.5 ton) and looked at other examples and if done correctly to my design it should be fine to pass. That's with lights, brakes etc.

*P.s. I am the person designing this trailer RrogerD is my Dad lol

 

[Burnerman]

I think you are expecting us to put our heads on the block for this one......however:
The original caravan chassis design incorporates enough strength where it is needed - i.e around the axle area but it also needs to be minimal in weight. Any bending will be around this fulcrum area.
The most important thing is the 'sleep at night factor'!
Cross members in the end areas need to be at least the same section. Most car trailers have C section steel or box section running the full length and don't taper. Cut outs are also minimal. It's the tapering steel that worries me as there will be a big dynamic load when a car is loaded on.
I don't think it would be possible to incorporate another axle, even of the Indespention type.
You would need to incorporate some sort of hinged ramping to get the vehicle on board and this needs to be considered so the car towbar isn't overloaded....any jockey wheel would become airborne. Heavy duty trailers have a ramp support as well.

I know nothing about trailer law incidentally, apart from the lighting regulations. I have repaired several heavy duty trailers that transport mini diggers etc and I source parts from my local Indespention dealer.

I don't want to appear to be a wet blanket but I think I'd be looking at an existing trailer and repairing or modifying it to suit my purpose.
John

 

[Harry Bloomfield]

My concern may be in the strength of the original chassis. Will it flex too much? I guess, having the 'bed' bolted at various places, ten bolts in total, then this should brace it all up? Do you think?

Using a modern Alko chassis, is I think a really bad idea. They are very lightweight, intended to have some support from the box of the caravan, and because of the light design - Alko prohibit any welding or drilling of the chassis at all, anywhere. To even jack a caravan up with an Alko, special brackets have to be bought and clamped to the chassis, otherwise the chassis will simply bend.

 

[BenSr]

Do you think that the length way steel's would eliminate this bending in the chassis? as you would have to bend these first.

 

[Harry Bloomfield]

I don't think it would be possible to incorporate another axle, even of the Indespention type.

You can actually - Alko chassis are designed to be modular. An extra pair of wheels could just be slipped into place and bolted together.

 

[BenSr]

I think you are expecting us to put our heads on the block for this one......however:
The original caravan chassis design incorporates enough strength where it is needed - i.e around the axle area but it also needs to be minimal in weight. Any bending will be around this fulcrum area.
The most important thing is the 'sleep at night factor'!
Cross members in the end areas need to be at least the same section. Most car trailers have C section steel or box section running the full length and don't taper. Cut outs are also minimal. It's the tapering steel that worries me as there will be a big dynamic load when a car is loaded on.
I don't think it would be possible to incorporate another axle, even of the Indespention type.
You would need to incorporate some sort of hinged ramping to get the vehicle on board and this needs to be considered so the car towbar isn't overloaded....any jockey wheel would become airborne. Heavy duty trailers have a ramp support as well.

I know nothing about trailer law incidentally, apart from the lighting regulations. I have repaired several heavy duty trailers that transport mini diggers etc and I source parts from my local Indespention dealer.

I don't want to appear to be a wet blanket but I think I'd be looking at an existing trailer and repairing or modifying it to suit my purpose.
John

My theory was to use this "bed" to take all the supporting of the car's weight and just to evenly spread the load throughout.
I was also going to use 8ft loading ramps which are self supporting and use "legs" to hold up the rear of the trailer while loading

 

[BenSr]

You can actually - Alko chassis are designed to be modular. An extra pair of wheels could just be slipped into place and bolted together.

That's what I thought, the axle which is in place is just held in with 4 bolts either side and a upside-down U cut out.
You would obvs need to make/extend the brake linkage to the new axle