Trailer Structural Steel Question

[Dave100456]

Hope someone can help please.
I'm modifying a road trailer to take a 1250kg boat which rolls on/off said trailer on a 4 wheel "yard trolley".
What size channel would be needed (min 75 mm wide to take trolley wheels) that will hold a load of 400kg on an unsupported 'overhang' of 500mm?
I am hoping to make the trailer as light as possible.
Advice re ratio of side wall height and steel thickness would be gratefully received.
Thanks
Dave

 

[Momac]

If you want a hot rolled parallel flange channel (PFC) with a 75mm wide flange I think you will be looking at a 150mm deep section which has 10mm flanges and a 5.5mm web.
Not particularly light weight - I think practical considerations govern the design.

 

[Dave100456]

Thanks
That is a heavy section. I had thought that a channel made from folded 6mm sheet with 75mm wide base and 50mm uprights might be enough.

 

[Momac]

I had in mind a trailer chassis with the channel used as a 'C' shape in cross section.
But you seem to have in mind 'U' shape in cross section - the toes of the flanges pointing up. If so you are using the section inefficiently on its weak axis.

.

 

[Dave100456]

I need the wheels to run inside the channel as ramps for loading/unloading.

 

[JSYmartini]

Maybe use a universal beam or a PFC then make something up to weld on the top as the guides for the wheels. Probably end up lighter than going OTT with the PFC just to get the width.

 

[Lakesailor]

I am probably being dim. If it's a 4 wheel trolley and the boat weighs 1250Kg (and is well balanced on the trolley) why does the rear axle only have a load of 400Kgs?
What about the weight of the trolley as well?

Why not add some extra trolley wheels 600mm in front of the rear ones so that some weight is taken by the supported part of the channel?

 

[VicS]

I am probably being dim. If it's a 4 wheel trolley and the boat weighs 1250Kg (and is well balanced on the trolley) why does the rear axle only have a load of 400Kgs?
What about the weight of the trolley as well?

Why not add some extra trolley wheels 600mm in front of the rear ones so that some weight is taken by the supported part of the channel?

400kg on each wheel I suspect ie 1250/4 plus a bit for the trolley ??????


Channels for a car transporter come to mind perhaps as suitable.

Indespension used to supply 12 ' and 14' long x 14" wide x 3/16" thick pressed steel channels for the purpose and also 5' x 13½" chanels for loading ramps.

However not qualified to say if these would be OK or not.

 

[Dave100456]

Lakesailor
Not being dim; I probably didn't explain it well.

At 1250kg the load on each wheel is 300+ and I allowed extra for any uneven loading hence 400kg per wheel. Each wheel will roll along the overhanging ramp hence the need for a channel section that will support 400kg cantilevered out 500mm.
I think PFC of 100x 50 will do it but wonder if I can go lighter?

 

[halcyon]

Lakesailor
Not being dim; I probably didn't explain it well.

At 1250kg the load on each wheel is 300+ and I allowed extra for any uneven loading hence 400kg per wheel. Each wheel will roll along the overhanging ramp hence the need for a channel section that will support 400kg cantilevered out 500mm.
I think PFC of 100x 50 will do it but wonder if I can go lighter?

The problem is the channel on i'ts back is the weakest way, 4 x 2 x 1/4 with web vertical moment of inertia is 5.06, but with web horizontal it becomes 0.70, same for sectional modulus,2.53 and 0.50.

Effectively you are holding it up with two 2" x 1/4" strips on edge, then you need to allow 2 x load to allow for pot holes, so you are 800 Kg at 500 mm.

Brian

 

[JumbleDuck]

What size channel would be needed (min 75 mm wide to take trolley wheels) that will hold a load of 400kg on an unsupported 'overhang' of 500mm?

You need to work out the required elastic section modulus (Ze) for the channel. The equation for that is

Ze = Maximum bending moment / material yield stress

400kg at 500mm is 4kN at 0.5m, so the bending moment is 2kNm

For mild steel it's reasonable to assume a yield stress of 250MPa. That gives you a required Ze = 2 x 10^3 / 250 x 10^6 = 8 x 10^-6 m^3 = 8 cm^3. You need to multiply that by a safety factor. I suggest 2.5 for that, giving you a Ze of 20 cm^3 to aim for. Any steel stockholder should be able to supply a list of moduli for the channel they sell. I think you'll find that 20 cm^3 is very low indeed, and that any channel section 75mm wide will exceed it by a long way.

Note: This advice is worth what you paid for it. I can produce three witnesses to say I was elsewhere when it was typed.

 

[halcyon]

You need to work out the required elastic section modulus (Ze) for the channel. The equation for that is

Ze = Maximum bending moment / material yield stress

400kg at 500mm is 4kN at 0.5m, so the bending moment is 2kNm

For mild steel it's reasonable to assume a yield stress of 250MPa. That gives you a required Ze = 2 x 10^3 / 250 x 10^6 = 8 x 10^-6 m^3 = 8 cm^3. You need to multiply that by a safety factor. I suggest 2.5 for that, giving you a Ze of 20 cm^3 to aim for. Any steel stockholder should be able to supply a list of moduli for the channel they sell. I think you'll find that 20 cm^3 is very low indeed, and that any channel section 75mm wide will exceed it by a long way.

Note: This advice is worth what you paid for it. I can produce three witnesses to say I was elsewhere when it was typed.

What about deflection at 1000 Kg ?

I gave up the stress lark when we got Newtons, the Germans were were sensible, they introduced a deca Newton, as close to a kilogram as makes no difference, and just carried on as before.

Brian

 

[Lakesailor]

Lakesailor
Not being dim; I probably didn't explain it well.

At 1250kg the load on each wheel is 300+ and I allowed extra for any uneven loading hence 400kg per wheel. Each wheel will roll along the overhanging ramp hence the need for a channel section that will support 400kg cantilevered out 500mm.
I think PFC of 100x 50 will do it but wonder if I can go lighter?

Ah I was being dim.

However I still think that an extra pair of trolley wheels 600mm forward of the rear wheels would take most weight during deflection as that part of the channel will seemingly be supported.

 

[Dave100456]

You need to work out the required elastic section modulus (Ze) for the channel. The equation for that is

Ze = Maximum bending moment / material yield stress

400kg at 500mm is 4kN at 0.5m, so the bending moment is 2kNm

For mild steel it's reasonable to assume a yield stress of 250MPa. That gives you a required Ze = 2 x 10^3 / 250 x 10^6 = 8 x 10^-6 m^3 = 8 cm^3. You need to multiply that by a safety factor. I suggest 2.5 for that, giving you a Ze of 20 cm^3 to aim for. Any steel stockholder should be able to supply a list of moduli for the channel they sell. I think you'll find that 20 cm^3 is very low indeed, and that any channel section 75mm wide will exceed it by a long way.

Note: This advice is worth what you paid for it. I can produce three witnesses to say I was elsewhere when it was typed.

Thank you for the calculation which is what I was after. At present, I follow it perfectly to "2kNm". However a bottle of Riocha currently fogs the pathway to me proceeding further and further complicated by finding a set of ramps which may be cannibalised.
Thanks again.

 

[JumbleDuck]

Thank you for the calculation which is what I was after. At present, I follow it perfectly to "2kNm". However a bottle of Riocha currently fogs the pathway to me proceeding further and further complicated by finding a set of ramps which may be cannibalised.

Fair enough. So here's the short version. 2kNm is not a very high bending moment (it's what you'd get at the root of a diving board with a fat man standing on the tip) and any channel section which looks reasonably beefy will do.

 

[Dave100456]

Hi JumbleDuck Thanks for the advice, much appreciated...

The ramps I've located are too wide (230mm), are 5mm thick with 50mm x 5mm flanges angled at 45 degrees. I could cut the ramp longitudinally giving the required width of 100mm and I'd need to add a second flange which could be taller (although I recognise the asymmetry). Do you think this would offer the 2KNm? My gut feeling is it probably would and and is worth trying for the cost of the flange and time taken to weld.
One last question; what size of flange would you go for, I was thinking of 60x6? I know its the "height" that gives the strength and width contributes to torsional stability; would you go bigger on one side or replace both sides with ...? I will be flanging at 90 degrees.
Thanks
Dave

 

[Lakesailor]

Erm, why does the extension have to be unsupported? Why not use the car trailer trick of putting the cantilever on top. The red bit.

 

[JumbleDuck]

The ramps I've located are too wide (230mm), are 5mm thick with 50mm x 5mm flanges angled at 45 degrees. I could cut the ramp longitudinally giving the required width of 100mm and I'd need to add a second flange which could be taller (although I recognise the asymmetry). Do you think this would offer the 2KNm? My gut feeling is it probably would and and is worth trying for the cost of the flange and time taken to weld.
One last question; what size of flange would you go for, I was thinking of 60x6? I know its the "height" that gives the strength and width contributes to torsional stability; would you go bigger on one side or replace both sides with ...? I will be flanging at 90 degrees.

Sorry, I don't have my structural tables with me and it takes ages to work this out by hand. Gut feeling: sounds OK, but make the flanges as nearly symmetrical as you can to avoid nasty torsional effects. I would also suggest having some system to spread the load when the trolley is on the trailer: don't put it all through the trolley wheels. I had vague ideas of doing something similar, albeit on a smaller scale, for my wee Hunter (~500kg) and the idea in that case was to jack each side of the trolley up once loaded and insert a long wooden baulk between trolley and trailer for towing. Since she is only ever (for now) used in freshwater I gave that up as too complicated and just dunk the trailer.

Lakesailor speaks much sense.

 

[cliff]

Lakesailor
Not being dim; I probably didn't explain it well.

At 1250kg the load on each wheel is 300+ and I allowed extra for any uneven loading hence 400kg per wheel. Each wheel will roll along the overhanging ramp hence the need for a channel section that will support 400kg cantilevered out 500mm.
I think PFC of 100x 50 will do it but wonder if I can go lighter?

That is fine IF al 4 wheels are taking their share of the load however unless teh beds are absolutely planar and the 4 wheels are planar you could find yourself in the situation where only 3 wheels are supporting the load and even worse where only 2 are carrying nearly the full load so I would start at 600kg loading then you need to consider acceleration forces induced while pulling the trailer along a bumpy road (any UK road these days). then a safety factor (2.5) - could be looking at a loading of 2500kg/wheel, not the 400kg you first come up with. I do not know what suspension you intend to have so cannot define the acceleration component of the imposed load. You really should consult some one like indespension. Better still go to an indespension stockist and pick up one of their trailer manuals - they used to have a design section in it which was pretty much dumbed down and allowed the layman to design his own trailer.
Watch out for plod if using it on the road - traler law has changed over the years and is ridiculously tight nowadays and frowns on home made jobs.

 

[Avocet]

Cars used to be stressed for the (at least at one time industry standard) "3G pothole brake". It's where you hit a pothole braking hard and instantaneously, that corner of the car (and everything attached to it) sees three times the static load. The idea would be for no plastic deformation to occur at 3x static load. I tend to try and use "yield" strength in any calculations rather than Ultimate Tensile Strength - obviously, you're looking for it not to bend rather than not to break!

Current law would require a new trailer to be taken to VOSA for an Individual Vehicle Approval inspection but of course, until there is compulsory identification AND registration of trailers, there are unlikely to be any "new" ones...

In any case, the inspection doesn't go into much detail on the structure - really focuses on brakes, tyres and lights mainly.

If a standard rolled channel section wide enough to carry the trolley wheels is too heavy, you could maybe consider four bits of channel section, facing each other, with a small gap between them? Nothing like as stiff though, and the sums would be harder).