Dynamics of tongue weight questions

[angib]

Trailers get by without shocks at present because they have very high spring rates (ie, very stiff with very little travel) - hence my comment about Model Ts, as cars back then had nearly the sort of spring rates that trailers have today and they (just about) survived without dampers.

It's vital not to think that what's good for a car is good for a trailer, unless it is actually delivering a benefit in the trailer application. My personal opinion is that most of the advanced suspension design from cars is no use to trailers because there's only about 10 chassis engineers in the world that know enough to make good use of that technology, and none of them are forum members!

So anti-roll (US: sway) bars may sound good, but I think they have very limited benefit on a trailer. In a car, adjusting the roll rates at the front or back by fitting or changing a sway bar alters the loading on individual tires - so for example, it can reduce understeer by increasing the rear roll stiffness.

But that doesn't work at all on a trailer as 100% of the roll stiffness is at the one axle - fitting a sway bar doesn't change that. It might look more 'sporty' by rolling a bit less, but that's about it. The only benefit I can see would be to increase the tire grip on smooth roads with a torsion axle - and with a rigid axle, not even that.

And fitting a sway bar to compensate for softening the springs makes little sense - the sway bar effectively stiffens up the springs again, so the same result would be achieved by not softening the springs so much.

And, Gus, your comments about weight distribution are spot on - the 'sportiest'-handling trailer would have all its weight between the wheels, not spread out in front and behind the axle. Of course actually achieving that isn't at all simple - putting the galley in the middle of the bed is not a convenient feature....

Andrew

[eamarquardt]

Years ago I converted a boat trailer to a 4X8 utility trailer and did not move the axle. It's sprung softly (my brother has the trailer now). If not loaded properly it get really exciting. Give that the springs are soft I think shocks would help a lot as you can see it dip on each side as it sways. I've offered to move the axles but we haven't gotten to it just yet and if you pay attention when loading it, it tows just fine.

I agree if a trailer is really stiff, shocks won't help because there isn't much motion to dampen but if you are experiencing problems and your kinda softly sprung, shocks might improve the situation. Given that it's easier to install shocks while building the frame, I plan on doing it then. Overkill, maybe, but what the heck "get it your way".

Again, "the world according to Gus".

Gus

[High Desert]

Andrew, any thoughts on standard vs gas charged shocks on a trailer application? That is, if you can find them in the correct configuration.

[Scooter]

My approach would be start with a proven design, then tweak from there in small enough steps you can tell what makes a difference. Concepts, imagination, even engineering doesn't mean squat until the rubber hits the road and it actually works.

[angib]

Any thoughts on standard vs gas charged shocks on a trailer application?.

In a trailer, for example on a torsion axle, it's quite likely that you want to install the shocks nearly horizontal, in which case I think you need gas-charged to get them to work correctly. But otherwise, I'm hard pressed to believe that going for higher-spec shocks will deliver a noticeable improvement. Fitting any shocks is the big step.

Andrew

[TD Beej]

It's vital not to think that what's good for a car is good for a trailer, unless it is actually delivering a benefit in the trailer application. My personal opinion is that most of the advanced suspension design from cars is no use to trailers because there's only about 10 chassis engineers in the world that know enough to make good use of that technology, and none of them are forum members!

Certainly there are lessons there and dynamic behaviors that would make sense and could and should be applied.

The biggest problem is likely not tuning an existing automotive suspension to suit a trailer but the packaging, most TDs are just not designed around fitting a modern suspension.

So anti-roll (US: sway) bars may sound good, but I think they have very limited benefit on a trailer. In a car, adjusting the roll rates at the front or back by fitting or changing a sway bar alters the loading on individual tires - so for example, it can reduce understeer by increasing the rear roll stiffness.

Roll bars have limited usefulness on cars too so why should it be an impediment to use on trailers?

Roll bars are a kludge, but so are short springs and shock absorbers. In the days of the horse drawn carriage the springs were long and matched the natural harmonic of the carriage.

Really the springs need to be just stiff enough to keep the rubber on the road, too soft or too stiff and they wont do that well. If at that point you need to control the lean further, well you do that.

But that doesn't work at all on a trailer as 100% of the roll stiffness is at the one axle - fitting a sway bar doesn't change that. It might look more 'sporty' by rolling a bit less, but that's about it. The only benefit I can see would be to increase the tire grip on smooth roads with a torsion axle - and with a rigid axle, not even that.

Benefits would also include limiting the shifting of the center of mass.

And fitting a sway bar to compensate for softening the springs makes little sense - the sway bar effectively stiffens up the springs again, so the same result would be achieved by not softening the springs so much.

It makes sense if you need the softer springs to keep the tires on the road. The sway bars will primarily stiffen the spring on the outside wheel in a turn by transferring the now unloading inside springs force. In typical driving the roll bars impact on normal spring operations is fairly limited.

Beej

[eamarquardt]

I'm not familiar with how to quote in this format but I can copy:

And, Gus, your comments about weight distribution are spot on - the 'sportiest'-handling trailer would have all its weight between the wheels, not spread out in front and behind the axle. Of course actually achieving that isn't at all simple - putting the galley in the middle of the bed is not a convenient feature....

Andrew

Thanks for agreeing! I don't get that too much at home from my wife and two boyz!!! I appreciate your "keen insight" as to the placement of the galley. I have purchased a regular rv range/oven and 3 way reefer and had planned on putting them right over the axle. But as you so astutely pointed out, being located in the midst of the bed would be inconvenient, so I am rethinking my design (just teasing about the placement but I do have the oven/range and reefer).

But I have been thinking "outside the box" once again and have had a "eureka" moment. Most people seem to put the head of the bed at the front of the trailer. As most trailers have a curve there this may affect one's ability to have nice headroom when sitting up. If you have the head against the rear bulkhead (assuming your galley doesn't intrude into the sleeping area) you have nearly the full height of the tear to work with. In addition, baring suicide doors, It seems to me that the opening of the door will be a more natural motion (pushing the door open rather than trying to fold the door behind you) making egress a bit easier. In additon, putting the usual shelves over the area of the feet will now be more forward in the trailer and perhaps provide a bit better weight distribution. As I intend to only store clothes and sleeping items there the weight won't be significant it will be more "balanced". Also if I mount an air conditioner in the "foretriange" it will be easy to plumb the air intake/exhaust over the feet which I think is more desireable than having it right near your head. Any thoughts?

[Frog]

This has all been very interesting reading.

Common sense is also a prime factor. Locating the axle properly to maintain a loaded 10 - 155 tongue weight, matching the TV to the TD, driving at a safe and reasonable speed, not overloading the trailer, keeping both tires at the same pressure, having adequately sized tires, keep the weight as low as possible and as centered as possible and using your head should pretty much eliminate most problems. The 100 mph illustration might seem to be over the top, but driving at 60 mph into a 40 mph headwind would be the same thing. Reducing speed will always increase stability and save fuel.

A minor point I don't believe was mentioned that a trailer with inadequate or negative tongue weight will not back worth a darn either.

Early TD's had simple suspension cobbled together from old junk automobiles and towed fine. Granted speeds were lower, but those trailers were also usually 4' wide and used some pretty well worn parts and for the most part worked. Sophisticated suspension for a non sophisticated load is probably not worth the cost and effort.

Just my humble, nonscientific opinion.

[eamarquardt]

After considerable thought, I've come to the conclusion that there is too much though and discussion over tounge weight. Tonge weight is good. More tongue weight is better except:

1. When you put too much stress on a trailer frame that cannot deal with the tongue weight you have (weak trailer design).

2. The tow vehicle (including your hitch components) cannot support the tounge weight that you are putting on it without compromising it's weight distribution and handling characteristics (small light cars towing trailers weighing a significant amount compared to the tow vehicle).

My logic: all of the "big boys" on the road (the big tractor trailer rigs) put the axle on their trailers as far back as practical because trailers track better the further the axle is from the king pin, tow ball, or pintle. End of discussion. The laws of physics make this a fact.

If I had a big heavy tow vehicle that could handle putting the axle on the trailer as far back as it could go, that's where my axle would be. The trailer would track well and swaying would not be a concern. I believe the ride on the trailer would also be smoother. Unfortunately I (and most of us) don't have vehicles that can tolerate high/unlimited tongue weights and we have to compromise and balance the load. So, my thought is to go for the maximum amount my tow vehicle can tolerate (my trailers are BUILT and can handle any tongue weight the trailer is capable of generating). The limit in my mind is set by physical limits of the strenght of the trailer and the ability of my vehicle to tolerate a given tongue weight. If my vehicle, hitch, and trailer can tolerate a 500# tongue weight for a #1000 trailer, that's where I want to be for highway travel.

Rebuttals?

Cheers, 73, K,

Gus

[TD Beej]

After considerable thought, I've come to the conclusion that there is too much though and discussion over tounge weight. Tonge weight is good. More tongue weight is better except:

1. When you put too much stress on a trailer frame that cannot deal with the tongue weight you have (weak trailer design).

2. The tow vehicle (including your hitch components) cannot support the tongue weight that you are putting on it without compromising it's weight distribution and handling characteristics (small light cars towing trailers weighing a significant amount compared to the tow vehicle).

My logic: all of the "big boys" on the road (the big tractor trailer rigs) put the axle on their trailers as far back as practical because trailers track better the further the axle is from the king pin, tow ball, or pintle. End of discussion. The laws of physics make this a fact.

If I had a big heavy tow vehicle that could handle putting the axle on the trailer as far back as it could go, that's where my axle would be. The trailer would track well and swaying would not be a concern. I believe the ride on the trailer would also be smoother. Unfortunately I (and most of us) don't have vehicles that can tolerate high/unlimited tongue weights and we have to compromise and balance the load. So, my thought is to go for the maximum amount my tow vehicle can tolerate (my trailers are BUILT and can handle any tongue weight the trailer is capable of generating). The limit in my mind is set by physical limits of the strenght of the trailer and the ability of my vehicle to tolerate a given tongue weight. If my vehicle, hitch, and trailer can tolerate a 500# tongue weight for a #1000 trailer, that's where I want to be for highway travel.

Rebuttals?

Cheers, 73, K,

Gus

3. The tongue weight needs to be generated close to the trailer axle, as the mass gets closer to the the hitch its utility diminishes. the reason is it increases stability (in the trailer) by forcing the suspension to do its job. Tongue weight alone does not do anything it is about making your three points of support on the trailer work the best and the hitch is a long lever and the axle gets the side to side stuff.

Your point about the wheels being far back on the semi trailers is correct. The further the wheels are from the hitch the more stable, this is where increased tongue weight really equates to increased stability. Longer distance from hitch to trailer axle = more stable.

Beej

[High Desert]

here's a couple of thoughts in regards to comparing big rigs to small ones from someone thats driven a lot of big rigs;
Even though a fifth wheel combo handles and carries weight different than a rear mounted hitch, balance is still important. Get a long trailer with the bulk of the weight forward and you won't like how it handles or rides. And with a rear hitch setup like a truck/trailer combo these things have even more effect. The length of the trailer is secondary to how the load is carried. Most 53' foot trailers have about 12' of adjustment on the rear axles to get the load balanced and legal. Most trucks also have 2' to 3' of adjustment on the 5th wheel fore and aft, to balance the weight between the steer and drive tires. It can be out of balance and will get the job done but the best handling and ride is still to be had with a balanced load. My 3/4 ton pickup and car trailer will also mimic this. I can move a car on the trailer less than a foot fore and aft and effect the handling considerably.

Just a couple of observations and a little info from a long time truck driver. Unless you've spent time around these things most folks have no need or interest to know this stuff. Just my $.02 worth.

[eamarquardt]

I think we're agreeing. Balancing the load over the trailers axle and not adversely impacting the balance between the front and rear wheels of the tow vehicle are more important than shooting for an "ideal" tongue weight of X% of the trailer's weight.

Not understanding this and relying on the "simple formula" that is kicked about so often without understanding the physics behind it all may just get you into a situation that you can't get out of safely.

Cheers, 73, K,

Gus (AB6KS)

[High Desert]

I agree Gus. I'm just not as good at putting things into a technical description as some here. A properly balanced load is key for the best performance. Tongue weight is part and parcel of the larger picture, but the other factors each have their own effect too. There are folks with tongue weights well on both sides of the 10-15% rule and they have no problems. I'd bet with some examinatoin it's because the overall trailer is well balanced.

Thats a nice thing about TDs & TTTs. There is always some extra gear that can be moved around to get things just the way you want them. Cast iron and coolers make for great moveable ballast lol.

[regis101]

I'm a newb to towing. I'm trying to keep things at 10-15%.

Much of the talk here has been with low tongue weight. What about high tongue weight. My understanding is that it makes the TV steering light.

Is it better to err on the side of heavy tongue weight and if the TV has issues to then move the cargo around to bring this under control?

[eamarquardt]

I'm a newb to towing. I'm trying to keep things at 10-15%.

Much of the talk here has been with low tongue weight. What about high tongue weight. My understanding is that it makes the TV steering light.

Is it better to err on the side of heavy tongue weight and if the TV has issues to then move the cargo around to bring this under control?

That's what I would do.

Cheers, 73, K,

Gus