Dynamics of tongue weight questions

[bobhenry]

[quote="Nobody"]I THINK there's been waaay too much 'thinking' going on here .

YEP ! and I am one of the guilty ones.

I built my 1st trailer at 13 to haul the big heavy Sunday newspapers behind my bicycle on my route.

Didn't know much about it then......know even less now after building 9 more and they all are probably going up and down the road somewhere without incident.

[madjack]

I THINK there's been waaay too much 'thinking' going on here . The dynamics/principles of trailer towing have been 'studied' to death by various agencies, individual, corporate, & government, for longer than I've been alive (I'll be 70 in a few days). They've all ended up coming to basically the same conclusion - design the trailer essentially as Andrew shows, 'balance' the load so there's more weight in front of the axle than behind, match the trailer to the TV (weight & power wise), remain alert & aware of your 'tow' even when the TV 'doesn't even know it's there', & mainly use a little uncommon 'common' sense. Trailers have been towed behind motorized vehicles for a hundred years, & with other 'power' sources long before that; the principles were learned early on, & they STILL apply! No need to reinvent the wheel.

Thanks Harvey for summing up what we have trying to say...does anyone have a link to "Zyamfers" Mini running the "Tail Of the Dragon" while towing his torsion axled tear????
madjack

[TD Beej]

Hmm...

http://journals.pepublishing.com/content/p01556p07u30r013/

Abstract

Previous work on car–trailer stability has been largely limited to theoretical studies with some reference to practical experience or accident statistics. In this study, extensive and systematic experimental investigations were carried out on a combined car–adjustable-trailer system. The influence of different trailer parameters on the system high-speed stability was examined by changing the mass, dimensions, and inertial characteristics of a fully adjustable trailer. It was found that the dominant factors affecting stability were the trailer yaw inertia, nose mass (mass distribution), and trailer axle position. The tyre pressure also affects the stability, although this effect is less significant. It is interesting to see that the trailer mass alone does not dramatically affect the high-speed stability, as this runs contrary to current guidelines relating to limits on the relative mass of the car and trailer. Experimental tests on a friction stabilizer and on car electronic stability programs demonstrate that both of these improve the high-speed stability and help to delay the onset of ‘snaking’.

Well it appears the the relative mass of the trailer and the TV is largely a myth and that polar momentum is more significant then tongue weight and wheel placement.

I will have to check if the library has a copy of this paper.

B^)

[caseydog]

I am trying to understand the dynamics of trailer handling so I can make the best choices.

The point is to make informed choices and to do that one needs to be informed.

Now a multilink suspension is attractive for a number of reasons but it also has drawbacks and when the choice is made it will be done knowing why and not because anyone may think it is "overkill".

I'm sorry if it seems like we are not taking your ideas and concerns seriously. Trust me, we are.

But, this has come up before, and guys like me and MJ and others have been towing trailers of all sizes for many, many years.

I also have been through track driving schools, and have a lot of performance driving seat time.

It sounds like your initial issue with "sway" is perhaps more of an issue with trailer "control." The two are not really the same.

The sway problem we have been addressing is a rhythmic back and forth motion that happens without any input from the TV. The TV is going perfectly strait, and the trailer starts to sway in a rhythmic fashion.

Now, the control issues associated with a trailer reacting to a change in directions from the TV is something else. In that kind of situation, the weight of the trailer, it's center of gravity, the weight of the tow vehicle, tongue length, and other factors effect what happens.

In theory, a top-notch suspension SHOULD help keep, or return control. But, again, the value of such a suspension will vary depending on all those other factors.

I have 600-700 pound TD with a low center of gravity. My experience tells me that my TD is not going to benefit much at all from a more sophisticated suspension.

At some point, if I were to swerve back and forth hard enough, something unpleasant would happen. But I'd have to really push it hard, even with my basic leaf spring axle setup.

When I did my first drive of both of my TDs, I took them out to wide open country roads and did some weaving and lane changes to see how they would react if I had to take evasive action. Could I make the kind of aggressive maneuvers that I could with no trailer behind me? No. But that was a simple matter of having a 600-700 pound trailer behind me, which would still be true if the trailer had a top-notch multilink suspension. If I have to make a sudden lane change, I am confident I could do it, but there is no doubt it will not be as smooth and easy as the same lane change with no trailer.

As far as the wife drifting back and forth in a lane, that is not going to create a sway problem, if you get the basics right, with axle placement being the most important basic. It may be annoying, but that little bit of change in direction is not dangerous.

I'd be more concerned about knowing what to do in an emergency situation. Going to a safe place, and practicing emergency maneuvers would be a much better use of time and effort than trying to make the perfect suspension for a TD build.

[caseydog]

...does anyone have a link to "Zyamfers" Mini running the "Tail Of the Dragon" while towing his torsion axled tear????
madjack

Here it is...

http://video.google.com/videoplay?docid ... gon&hl=en#



A small car, and a 500 pound TD with a split torsion axle suspension, on one of the best sportscar roads in America.

[TD Beej]

Thanks Casey,

I agree with most of what you say, I don't know why everyone decided to latch onto the multlink suspension when I was just trying to used it as an example for the benefits of a passive steering. There are other ways to do it, even a torsion bar semi trailing arm. And I will to confess to considering suspension options on many factors other then handling but that is not what I am trying to investigate in this thread. I just want to know how the handling puzzle fits together.

The snaking is certainly a problem with control and I may be in disagreement with you on that but I do concede that it is a subset of control and not the same thing.

The reason I have begun to look at snaking particularly is that it is a worst case scenario. It is hard to think of anything that would not improve handling that would not also increase resistance to snaking but if if there is something that would, it is probably not a good idea to do.

And as to my swaying driver, the problem is that the suspension can store energies (even with correct dampening) and if the sway is at the correct rhythm the sway will amplify just like pumping your legs on a swing. Same thing can be setup up by the road, or wind, or turning inputs (Hollywood stunt drivers claim they can get any car to roll by the right turn inputs).

In hindsight I wish I had called the thread something else, like trailer handling dynamics, but it is just where things got started with all the gaps in knowledge. Still I would like to get more info yet really the actual discussion about what people have experienced has been very useful to getting a mental picture. Having a towing test bed would be really useful in figuring out the relative importance of a lot of these things but if the info is already out there, it would problably be just easier to find it.

Beej

[jackdaw]

Alot of the caravan users over here in the UK use stabiliser bars to compensate for poorly loaded caravans.
Bisicly, it like a leaf spring, fixed to a frioction plate. Its mounted under the towball on the tow vehical, and then it sits in a bracket on tthe trailer tongue. I think it works on putting more force on the tongue, and the friction plate is a bit like a brake on the sideways movement of the trailer.

here's a link to a few examples

Do you have similar devises over there.

I still use one when we're towing our caravan, but have found the teardrops tow beautifully without the need for any sort of stabilisers.

[caseydog]

And as to my swaying driver, the problem is that the suspension can store energies (even with correct dampening) and if the sway is at the correct rhythm the sway will amplify just like pumping your legs on a swing. Same thing can be setup up by the road, or wind, or turning inputs (Hollywood stunt drivers claim they can get any car to roll by the right turn inputs).

I could easily make my TD roll, and I could make just about any high profile vehicle, like a minivan, van or SUV roll. But, knowing howto do it also equals knowing how NOT to do it.

I can assure you, that the amount of back and forth movement that you spoke of with your wife at the wheel isn't going to store energy and create a rhythm. You would have to turn hard one way, and immediately hard the other way, and perhaps even hard a third time to make the trailer lose control.

Remember, the energy is only stores until it dissipates. Small back and forth inputs will not generate much energy, and it will dissipate immediately. Keep in mind, that hitting a bump with one tire will probably create more stored energy than one small change in direction.

You will get 100 times the benefit of a sophisticated suspension by learning to properly drive a TV with trailer, and practicing it, than you will ever get out of designing the perfect trailer suspension.

I'd put more of my research efforts into understanding the driving dynamic of towing a trailer, and less into the trailer technology.

[caseydog]

BTW, on a side note, I have ridden with other people who drift back and forth on the highway, and if I know them well enough, I'll ask them where they are looking as they drive. In every case, they are looking at the road, maybe 50 feet in front of the car, at best. They are adjusting constantly to stay between the lines on the road.

If they can break that habit, and start focusing 100 yards down the road, they stop drifting, and do a better job of staying between the lines.

They teach the same thing in performance driving schools with staying on "the line" of a track. Don't look AT the corner, look at where you want the car to go. Your hands will follow your eyes, and the car will go where your hands aim it.

I rode across Texas with an old girlfriend this summer, and she did the same thing. Drove me nuts. However, i knew from experience NOT to try to "help" her with her driving.

[angib]

Assorted thoughts:

I see a practical problem with using more sophisticated trailer suspension which is how to decide what characteristics you want it to produce. Just having a sophisticated specification does not guarantee that it will work any better - you have to be able to use that sophistication to deliver a result.

Designing suspension for cars is quite a different matter - you've got a staff of 50 in your chassis department (or you will be copying the designs of a car company that does), you've got both ends of the car under your design control and they interact hugely with each other.

Put a ball joint (ie, a trailer coupling) between the front and back of a car and you can throw out just about all the accumulated wisdom of chassis engineers as it will no longer apply.

So I don't see how to work out what geometry you actually want and I sure don't see who's going to fund the prototypes and track testing needed to find out.

However I can believe an improvement on typical trailer suspension wouldn't be hard to achieve. My personal favourite would be to get the spring rate down (and travel up) from its current Model-T values so that the trailer would stick to a bumpy road, rather than pogo across it.

That would certainly require proper dampers as otherwise the softer springs would probably make the stability worse. And it would only be a benefit on bumpy roads - it would deliver nothing on smooth roads.

And on the issue of trailer weight limits, I think that is probably the only measurement that you can hope to test - setting a trailer yaw inertia limit would be useless as (a) no-one knows what their trailer's yaw inertia is, (b) we don't have an easy way of measuring it in the field (ie, it would be unenforceable) and (c) it changes as soon as you alter the loading condition.

One thing to look at on this subject is what the (very thorough) German authorities require for travel trailers before they can increase their maximum speed from 80 or 90 km/hr (50 or 56 mph) to 100 km/hr (62mph). I know that hydraulic dampers is one of the requirements, but it would be good to find out all the others.

Andrew

[Trackstriper]

Another consideration with complex suspension systems is the typical width taken up by the various links and/or the typical spring/damper assembly. This is not too much of a problem with a passenger car application, but think about sleeping between the rear shock towers of your daily driver. Shocks can be added (without sacrificing usable width) to a simple, inexpensive, and proven trailing arm torsion assembly like the Torflex or Flexiride.... and as Andrew mentioned you could go with a softer "spring" rate to buy a little more compliance.

Bruce

[caseydog]

Thanks Andrew for once again showing the rest of us how little we know.

Your idea of softer springs, more travel, and good dampers is outstanding. Even if all one does is soften the spring rates and put good shocks (dampers) on the suspension, it would be a big improvement on the typical roadway. The extra suspension travel would help a lot on rougher roads.

Question, though... Would that help with stability in an obstacle avoidance situation? Is there any practical way to design a trailer suspension to maintain or regain control faster than what the typical trailer can do now?

That is the big question I have. I wonder if someone spending lots of time, effort and money to find the perfect suspension would find that it does very little to improve the actual performance of the trailer.

CD

[High Desert]

a sway bar might give you some of the faster recovery you're looking for CD. The designing of it wouldn't be over the top tough either, just have to guesstimate the bar diameter starting off. Wouldn't need to be very big considering the weight of an average TD. Combined with gas charged shocks it might make a fairly sporty package.

Another thought on spring rates would be an air suspension. It increases rate with load. There are considerations as to size, placement and air supply to name a few, but it's been done. There is a manufacturer in Cal using one an on off road model but I haven't seen the details of it.

also there's the concept of keeping the unsprung weight low as possible with a light wheel/tire combo. And sidewall aspect ratios. You know where I'm going there.

Guess it all boils down to how bad you want something huh?

[eamarquardt]

The real issues are not overloading the tv so the rear end sags which may affect your ability to steer the vehicle and placing the load and axle in the right postion to minimize self induced oscillation and try and dampen the any tendency to oscillate.

We're kinda talking a pendulum here and the longer the pendulum the longer the period of oscillation. So, the further back your axle the better as long as you don't overload the tail end of the tv.

Oscillations can be dampened by the additon of some friction to the swivel between the tv and the trailer. Oscillations can also be dampened with shock absorbers as I've mentioned in an earlier post. Once a trailer is built your options are limited unless you've planned ahead. Moving the axle can be a real pain or a ten minute job. Your choice. Mounting the axle to a piece of angle iron and then mounting that to the trailer allows you to move the axle at will. See pics in my album. Loading your trailer correctly will also help. I'm kinda amazed that no one in their right mind would drive a tv without shocks!!!!! The simple truth (IMHO) is that we "get away" without shocks on trailers but they seem like they would do a heck of a lot to minimize the oscillations that we want to avoid(I'm making an assumption here but I KNOW I want to avoid oscillations).

So it's really not that complicated (IMHO):

1-Putting the alxe in the right place is easy with a little proper prior planning. Relocating it can be as simple as r and r'ing 4 to 6 bolts which should take about 10 to 15 minutes

2-Mounting a set of shocks isn't expensive or difficult (if you have the right tools)

3-Loading the trailer takes some thought but worth the time

If you have problems with sway, I'd venture a bet that addressing the three issues above will almost always solve the problem

There you have it, the "world according to Gus".

Cheers,

Gus

[TD Beej]

Andrew,

Designing suspension for cars is quite a different matter... So I don't see how to work out what geometry you actually want.

Harvesting from cars would be easiest. A lot would depend on the characteristics you want and what qualities you would want to most closely match. Some kinds of suspensions you may want to get close on the track width or ride height but like spring and dampening many things can be adjusted with some ingenuity.

However I can believe an improvement on typical trailer suspension wouldn't be hard to achieve. My personal favourite would be to get the spring rate down (and travel up) from its current Model-T values so that the trailer would stick to a bumpy road, rather than pogo across it.

Matching spring rates and dampening to the trailer would do a lot. Anti-roll bars a low tech limit roll with softer springs too.

One thing to look at on this subject is what the (very thorough) German authorities require for travel trailers before they can increase their maximum speed from 80 or 90 km/hr (50 or 56 mph) to 100 km/hr (62mph). I know that hydraulic dampers is one of the requirements, but it would be good to find out all the others.

Yes, I am curious too.


Bruce,

Another consideration with complex suspension systems is the typical width taken up by the various links and/or the typical spring/damper assembly.

Yes it is. A lot of the tears already of dropped floors so it seems that the biggest problem really is the tall items like shock towers and spring perches. Equal length double wishbones suspensions on formula cars often have the spring/shock diagonal between the lower outside and the upper inside wishbones which would solve it for that one case other would need to be more original.

CD,

Question, though... Would that help with stability in an obstacle avoidance situation? Is there any practical way to design a trailer suspension to maintain or regain control faster than what the typical trailer can do now?

That is the big question I have. I wonder if someone spending lots of time, effort and money to find the perfect suspension would find that it does very little to improve the actual performance of the trailer.

Yes, it is the big question, but anecdotally it seems that a lot of people have troubles getting their trailers setup which would imply that a lot more people drive close to the limits of their trailers without knowing it. You simply won't know until it is tried and compared.

However if you look at how much automotive suspensions have improved over the years you find that cars ride a lot better and can travel much faster through twisties and are designed (in general) to fail in a safer ways, and warn you better when the limits are reached, this is what could be achieved.

Beej