Re: Width of strings for hay wagon

3" x 8" solid oak is what is used on out 16 ft long wagons that carry 130 small squares (plus the steel cage). With your proposed length of 26 to 30 ft, there is no way that you will have sufficient support with any of these stringer sizes you have described. You might consider an old straight truck frame for your planned length. Maybe you can find a frame from a mobile home. It is a thin I beam that I would cut and double, welding 2 I beams side by side for each stringer. Where you locate the rear bolster, and how much rear overhang you have will affect the design. A 30 ft long wagon is huge!!

Does your 12 ton wagon gear have a tandem rear axle, making it a 6 wheel wagon? If so, I think you could get by with about 8' of rear overhang, but with a single axle rear bolster, I would guess that 5 to 6 ft would be your max overhang. The overhang length and future load on it will counter balance a similar length of weight/load in the critical span length between the front and rear bolsters. In engineering terms, deflection is the beam being bent /flexed down (the span between the bolsters). But that can be reduced by the inflection (push up) from the load behind the rear bolster (like the balancing of a teeter tooter). With a 6 ft rear o'hang, and a 3 ft front o'hang, you will have a 21 ft span in the middle....and that is huge.

Paul in MN

If you look at this website http://www.farmcomanufacturing.com/roundbale.php you see they use 3x8's on their 24' wagons... so I could step up to 4x8 and be perfectly fine I think.

An 8' overhang seems to be way too much, I'm going based on the existing designs I see. A 3-5' overhang from the rear axle seems to be about the most.

And yes the rear is a tandem axle.

Yes sir, I did look up this link.

It brings up further questions about your proposed design. They show 3X8 rectangular steel tubing for their frame on the 24 ft model. But they do not give any spec on wall thickness, and no spec about the tensile strength of the steel. It is likely they are using a high tensile steel for the stringer members (as would be common in truck frames and much farm equipment).

Next up, a design which is good for 24 ft is likely to fail when stretched to 30 ft. The main span is just too long for the weight it needs to support.

As to the o'hang distance, it is measured from the center hub of the wheel if it is a single wheel, and from the center pivot point if it is a tandem. From the photos, the front o'hang exceeds 3 ft if you measure to the front of the extension which keeps the bales from rolling forward. With a tamdem axle, the rear axles should be carrying a minimum of 2/3 of the total weight, and probably 70% is more realistic. Thus the need for more rear o'hang (and consequently a better turning radius).

But you are going to design and build this wagon, and I have only offered advice as you requested. So in the final analysis, do what you want to do. I provide advice based on my experience in building 4 of our 10 hay wagons, and the experience of many years of teaching physics (thus the design theory), and the lessons learned from my son who has a Masters in Ag Engineering. By no means do I think I have all or the only correct answers, and I am certainly open to other's ideas and possible solutions. But I do think my advice was on track.

Have a good day!

Paul in MN