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SJ23 Tandem Axle Conversion - Betsy and Dave

It's unfortunate that professional people build trailers that are under rated.  We wont even discuss some of the home made disasters we've seen at launching ramps.  We also picked up some very good ideas by looking at other trailers and incorporated them into ours.  The final straw was a broken front spring hanger that Dave found on the driveway late one night after a 100-mile tow home from the San Juan Islands.  The previous owner, an engineer for Boeing, installed a heavier axle without upgrading the hanger.  The problem was that the new spring assembly was too long for the frame attachment points.  The stress eventually broke the weld at the frame, resulting in the broken hanger.

BEFORE YOU START - Before you tackle a tandem axle installation, be absolutely sure of what you are doing.  Dave has been a professional heavy-duty diesel mechanic for the past 20 years, most of the time as an engine specialist.  He didn't feel qualified to tackle this modification.  He is very meticulous about the quality of work and is generally dissatisfied with 89% of what he sees in this world.  He is a competent welder and has had lots of advice.  He is also very resourceful, able to differentiate between good and inaccurate advise.  We mention all this to caution against diving into a project that may be beyond your abilities.  Be very sure and comfortable with what you are doing or pay someone who is.  Certified mobile welders are abundant and usually very good.  Mistakes in design, or workmanship could easily result in an accident or loss of life!

The second question you should ask yourself, "Is your current trailer worth upgrading and is it structurally sound?"  Our project involved at least a 100 hours of labor and about $1000.00 US in material.  We replaced a single 5200 lb. axle with two 3500 lb.  Dexter axles equipped with marine drum brakes, four springs, and mounting hardware (4" spindle rise, 84.5" rim mounting surface).

SALT WATER CORROSION - Another problem Dave discovered was a build up of rust on the inside of the axle tube.  This was due to salt water, trapped in the tube, which entered through the poorly sealed brake wire holes.  All Dexter axles come with these predrilled holes for the electric brake wiring.  Our open channel trailer is galvanized and we wanted the axle protected as well.  We convinced the galvanizer to do the inside of the axle but the responsibility of protecting the spindles during the process was ours.  They were very hesitant.  Dave wrapped them well and it was fine.  After the axles came back from the galvanizing shop Dave tapped those holes and installed pipe plugs.  He managed to do this work with the boat still on the trailer.  That, coupled with the fact that we wanted everything galvanized, lead Dave to weld the spring hangers to a long piece of flat stock and then U bolt this to the existing trailer.  This is a common practice, which enables you to move the axles to correct the tongue weight.  Our bow support is also U bolt attached so we have the option of moving the boat on the trailer if need be.

TONGUE WEIGHT - Tongue weight is a function of axle location under the load.  It should be about 10% to 15% of the trailer weight.  Our center spring hanger is located approximately 1/3 back on the window.  The center spring hanger is in the same location as the center of the old single axle.  We looked at Caulkins and other power boat trailers for ideas on attaching the spring/axle assembly with U-bolts.  San Juan's seem to be stern heavy, with outboard, fuel, and water commonly stored in the aft.  Sailing and trailer performance can be improved by moving weight forward; moving our water tank forward is on our project list.

BRAKES - Brakes are likely law on trailers in this weight class.  We would not even considering towing this kind of weight without a functioning brake system.  We have a Surge-O-Matic hitch activated brake system with the master cylinder just behind the coupler.  I heard a good idea the other day, disc brakes.  They are more open and less likely to corrode, and are easier to clean.  I would look into this option.

CLOSING - Since we modified our trailer from a single axle to a tandem axle, the trailer tracks much, much better and we have not experienced any trouble turning it.  When Dave put the new axles on, he also moved them back to increase the tongue weight.  I think that is why it no longer swings when we change lanes.  It just pops right in behind us.  Turning the trailer also doesn't seem to be problem.  We use our tractor, with a hitch attachment on the forklift, to park our boat.  Dave turns the trailer very sharply to get it into the barn.  So hard in fact, that the inside tires pivot without rolling.  I doubt it is good for them but the trailer moves over nicely.  I have hauled horse trailers and I believe that the proper tongue weight is the critical element to ease in hauling.  Too little weight is dangerous.   Betsy & Dave Schultz

Editor's Note - Betsy and Dave sold their SJ23.
 

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