Empty single axle trailer fishtail - tire pressure related?

[lhfarm]

Have a 5x10 single axle tilt trailer that I purchased in the early 90s for hauling mulch and other light duty hauling. Use it around the farm too for cleanup work. Yesterday, I loaded my ZTR mower and drove 20 miles on the interstate to the dealer. Before I left home, I checked the tire pressure and they were at 24lb. These are big 235/70 R15 tires that are only a couple of years old. I have no idea of the correct PSI, so pumped them up to 29lb. On the way to the dealer the trailer pulled great @ 70mph. On the return trip, I felt the trailer at above 65mph and saw it was fishtailing. Slowed down for the rest of the trip home.

As stated, this is an old trailer and I've had it on the interstate many times, but never noticed the fishtail issue. I was towing it with my '07 F150 and there hasn't been any issues in the past. So a couple of questions. First, would improper inflation cause the empty trailer fishtailing? Second, how do I determine the proper pressure?

Thanks,

 

[Egon]

Look at the tire sidewalls and you should find the max pressure and load rating.

Think about trailer wheel alignment??

 

[bobmisi]

More likely cause was not enough weight on the front of the trailer and too much on the rear half. That is the only time I have ever had a trailer fish tail. When I tow my tires are at 35 psi.

 

[ray66v]

More likely cause was not enough weight on the front of the trailer and too much on the rear half. That is the only time I have ever had a trailer fish tail.

Yep.

You either have to load it so there is more weight on the tongue, or in some cases, move the axle more to the rear, to stop fishtailing.

The tires should have the maximum inflation printed on them, for a given weight.

Your load weight, divided by 2 should be below the maximum the tires are rated for.

On a trailer, usually you run the at, or near the max, depending on the weight of the load.

 

[tractchores]

Almost definitely had to do with load VS tire pressure (never heard of tire pressure causing a fishtail). With small trailers a lot of time they're low to the ground and are angled up towards the tow vehicle taking weight off the tongue. The solution is to have the trailer tilted slightly down toward the tongue when empty to keep weight on it. It seems like little / light trailers are more prone to this than others. My heavy trailers are balanced enough that empty even a little tile up towards the front is fine...

 

[the old grind]

More likely cause was not enough weight on the front of the trailer and too much on the rear half. That is the only time I have ever had a trailer fish tail. When I tow my tires are at 35 psi.

+1! With your car, truck, or motorbike you can have handling or braking issues nearer the upper pressure limits when loaded lightly, but with trailers you usually just get more bounce when empty. 10% minimum weight on the ball is critical for the load to caster (trail), and IMO on the light side of ideal. Otherwise, the trailer tries to steer itself and even the tiniest bit of clearance in hitch or ball gives it room to wander. IMO, tires whose pressure is so low as to cause wander have a life expectancy in mere minutes or miles.

Good point about tongue angle and ball height, too! What also seems to matter most is ball height to center of gravity of the load. Don't see much going on when loaded, but wow ... when empty....? :irked: That's when it can be tougher to get the 10% min weight bias to the front. btw, sometimes a spare tire on the tongue can make a reasonable diff when unloaded.

btw: When I stripped & repainted my bike trailer I moved the spring mounts back one set of holes, ~4" IIRC. (a lucky option) It not only reduced the empty wagging but made backing it muuuch, easier. Of course, lengthening the tongue would have done the same. My heart goes out to those who build their own trailers & miss a sweet spot by just a few inches. Got options?

 

[alchemysa]

Severe fishtailing of an empty trailer is somewhat unusual. But thats quite a long trailer for a single axle. Theres a lot of weight hanging out the back there so, even when its empty, it should be pretty heavy on the ball if its weighted properly. I would not have thought tyre pressure had much effect unless they were exceptionally low, or one was significantly softer than the other.

As you said, you have towed this trailer many times without fishtailing but have you towed it empty over that distance and at that speed before? Or perhaps your truck bed was loaded heavier previously. Might also be worth checking that the axle, springs and hangers on the trailer are all straight. Also check the towhitch on the truck.

Another thought. Perhaps one trailer spring has lost its stiffness. Differing spring rates could result in fishtailing I imagine.

 

[lhfarm]

Thanks for all the replies. Had to pick up the mower today, so before I left, I threw (OK put) a 50lb weight in the front of the trailer. It towed perfectly at 70mph. I think that will be an OK solution for the small number of times I'll have this trailer on the interstate. I think the geometry of the hitch on the F150 is just enough different from the other vehicles I've towed with to cause an issue.

Again, thanks for the help!

 

[jejeosborne]

Thanks for all the replies. Had to pick up the mower today, so before I left, I threw (OK put) a 50lb weight in the front of the trailer. It towed perfectly at 70mph. I think that will be an OK solution for the small number of times I'll have this trailer on the interstate. I think the geometry of the hitch on the F150 is just enough different from the other vehicles I've towed with to cause an issue.

Again, thanks for the help!

For this reason I had to mount a spare tire to my tongue. Tilt trailers are the worse because the fulcrum point is the axle thus no tongue weight. Tongue weight is crucial. I once stopped along the highway and stole a couple pieces of rip rap from the state to throw on the front of my empty trailer for an on the fly fix.

 

[ray66v]

Thanks for all the replies. Had to pick up the mower today, so before I left, I threw (OK put) a 50lb weight in the front of the trailer. It towed perfectly at 70mph. I think that will be an OK solution for the small number of times I'll have this trailer on the interstate. I think the geometry of the hitch on the F150 is just enough different from the other vehicles I've towed with to cause an issue.

Again, thanks for the help!

Sometimes, a little more drop on the draw bar of the hitch, can be enough to stabilize it too.