trailer fitting

[motthediesel]

So next it's time to grit your teeth and cut metal. The 45 degree bends at both ends were done by sawing the plates and then bending the webs. The top and bottom plates were then welded with a MIG unit equipped with a spool gun. I'm an OK steel welder, but I have little confidence in my abilities with aluminum. So, these welds will be used to hold the parts in alignment, but I will add gusset plates top and bottom as well.



The bridging between the rails will be 5" aluminum I beams, bolted in, as you see here.



You also see there the gusset plates used to connect the "outrigger" beams to the original trailer rails. That is another place where I would not trust my welding. Cutting those 5" beams was done with my old Delta bandsaw, that old machine continues to punch above it's weight every time I ask it to.



(to be continued)

[real550A]

Following this thread closely, as I'm on the hunt for a trailer, and will be retro-fitting to fit my 27FC in December. Lots of great ideas here!
motthediesel- you have great design skills, and your welds look pretty good to me! Keep up the good work.

[motthediesel]

Thanks! I'm still banging away on the trailer project, in between other work demands. We're having a nice early spring here, so that's a help. (As an aside, and as a Porsche 356 owner, I have to ask about your name -- do you have a "real 550A"?)

Here's the outrigger unit bolted onto the starboard side of the existing trailer.



It's plenty strong, and well attached once all the bolts are made up, but there is a potential problem with this structurally. The springs will be mounted underneath these, and the weight of the load would put a very large torque vector on the original frame rails. I thought the best way to handle that would be to add another set of bunks on top of the new outriggers to send that load right up to the hull. In this way, the hull would actually strengthen the trailer while at the same time the trailer would support the boat.

I used those same simple screw jacks again to force the PVC faced 2X6 into conforming to the shape of the hull. Then more 2" conduit was cut to make the uprights, as you see here.



Shapely! It's not hard to see now why these boats will never really plane, no matter how much power is bolted into them -- there's just not enough straight lines in the aft underbody for that.

(to be continued)

[motthediesel]

So now to the axles. I had planned on using three 3500# torsion units, but after I did a little research on trailer design, I changed my mind. As great as the rubber-spring torsion axles are, they have a real disadvantage when used on multi-axle trailers.

The problem is that, in trailers that spread the load over two or three axles, there is no equalization between the wheels with the torsion units. Conventional leaf spring axles can be mounted with equalization rockers that transfer the load from one axle to another, the typical torsion axle setup is entirely independent. Why is that bad? Let me try to illustrate.

Say you have a boat trailer supporting a total load of 3 tons (6k#) on two 3500# axles. Also say your tires are rated for 2200# -- sounds fine right? You've got 1000# extra capacity on your axles, and a full 2800# over on your tires. But -- now say you hit a big pothole, or go over a speed bump. With an un-equalized setup, all of the load could (in the case of the speed bump - would) be supported by one axle alone. So now you have 6000# of load supported by one 3500# axle, with 3000# on each tire.

This is not good with dual axles, but maybe you would get away with it -- with three axles though the math gets even worse. So, I decided to go with conventional leaf springs with the equalization mounts that will "share the load" over all three axles. Here's a picture of my setup with the axles not yet clamped to the springs.



For anyone interested, here is a good article on the subject of multi-axle suspensions. That website is a great source for anyone interested in trailer design.

https://mechanicalelements.com/torsion- ... or-triple/

[tribologist]

Very cool! That is a serious project! I would love to have a spool gun! I have a Miller Diversion Tig but that's of no use on heavy aluminum.

I'm a little bit worried on how the load from the axles are going to generate moments on your 5" bridging cross members and the full weight of the trailer is carried by the relatively small bolts connecting the flanges. It looks like the distance from the bolts to the center of the outer beam is about 3X longer than the height of the beam so the shear force on the two fasteners will be 3X that of the load on the axle. You quickly end up with pretty high stresses on those fasteners if you bottom out the springs. With the spacers in there between the flanges you also add bending on those bolts.

You probably need pretty beefy tie plates on top and bottom to keep it together. As it looks, you have very stiff members between the beams that is held with relatively small bolts. Nearly all the load will go through there and not to the ends since they are so much stiffer but maybe I am missing something. I am assuming the weight is on the keel and not the bunks. If most of the load is carried by the outer uprights/bunks it would be less of an issue. If you could have the cross members all across the trailer you would not have any issue. Maybe you could add sister beams that you tie to the current cross members? The could be galvanized steel. You gain 3X stiffness with steel over aluminum

DA had a similar issue with flexing frames and stiff cross braces that generated tremendous forces at the cross member. He solved it by making a massive cross brace that was capable of taking the forces. (I was trying to convince him to go the other way and make the cross brace soft and compliant. )

[motthediesel]

It's difficult to say with certainty, but I believe that most of the weight aft is supported by the bilge bunks, not the keel. Those cross members are rather light 2"x4" channel, and function more to prevent frame spreading rather than to support the keel weight vertically. They show little to no deflection when loaded, and if they were supporting a significant proportion of the weight, I think that it would show there.

An unbalanced torsional force as shown in your diagram would require unequal loading on the bunks, and I don't think that is likely with the boat on the trailer. Without the boat, it would be a different matter, but in that case the empty trailer would only be supporting it's own weight, and I don't think it will be much of a problem, we'll see.

You are right though that the very best situation would be full width cross members, and I'll certainly consider that if it does prove necessary. Thanks for your input!

[tribologist]

Sounds good. Yes. Try to carry the load on the outer bunks. Thats how my trailer is set up. The keel rest on a rather springy set of cross beams and then i have solid bunks resting on the I-beams. I think all dynamic loads are on them. The keel probably carry a lot of the static weight. You should end up in the same situation since your bunks are stiffer than the cross members.

[real550A]

motthediesel- Sorry for the delay.
I wish I still had the 550A, have had several 356's, and currently have a '72 911E coupe. The 550A was a '57 model with racing heritage, and a unique car. Love the Porsches!

[motthediesel]

Wow! That’s awesome — I’ve never even seen a real Spyder! I’ve only owned one 356, a signal red SC coupe. I bought it in March of 1975, and I still have it.

(Sorry all for the off-topic — now back to boats)

[Coonneck]

Motthediesel can you please tell me the width of your donor trailer. I’m getting ready to fo the same as you did. By the way your info is priceless and your fab work great.

[motthediesel]

Sure, the 4"x6" aluminum I beams of the original trailer are on 57-1/2" centers. It was pretty narrow, I think it was intended for a "Cigarette" type performance boat, as the axles were mounted way back as well. That is common for those type of boats as they have all their weight in the stern.

Sorry I haven't posted any updates on this thread lately, but this is the busy season for my work. I'm mounting the hubs and brakes now - I post something soon.

Tom

[Coonneck]

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Tom thanks for the reply. The trailer I’m getting is 20 inches wider so that makes me feel better. Still trying to figure out about the rear and how much if any it will hang over. Still waiting on closing on the purchase of the boat with a clean survey. Trailer is mine when I want it so is the boat which I will need to get rid of. Figure I can something for the boat and the tee tower and other parts. I made a deal with the owner for 2K for the trailer and half of what I get for the boat. I’ll try but won’t put much effort into selling it for much. Any suggestions please pass them on . Take care.

[motthediesel]

I’m sure you can make that work. How long is that dirty old CC?
Raising the bunks will be your first job. My way worked well, but you may have other good ideas, I’m looking forward to seeing how you go about it.

Tom

[Coonneck]

Tom it’s 26 feet long and 8 foot wide. I’m thinking I’m going to do it just like you did but with the wider trailer I don’t think I’ll have to go with two sets of bunks. Where did you get the PVC for the bunks?

[motthediesel]

Wow, that’s a big one — it was hard to tell, with no motor to scale it from.

I got that 3/8” PVC in a 4x8 sheet at my local Lowes. I think it goes for about $70/sheet — not much different than sheathing plywood now