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Old 10-19-2007, 09:27 PM   #1
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Root causes and cures

Roger made some comments in another thread that really got me thinking (no mean feat).
Quote:
........ if their problems can be resolved by increasing the tongue weight, making sure the tires are aired up, or as was in my case, modifying the suspension of the tow vehicle to cause it to act as it should have from the factory?

........I just think that folks need to understand what is happening that causes them to experience what they're experiencing, and then be able to work through those issues until they're satisfied that sway isn't a concern. .......

What I can't abide is someone spending the $3k for the panacea without knowing what the alternatives really are, and without fixing the actual problem that plagues them to begin with, but pronouncing it "cured".

Roger
---edited heavily

His comments relative to root causes of towing problems made me realize that I don't have as good as a grasp on the topic as I would like to. So here is my question: What are the factors contributing to an unstable towing configuration above and beyond hitch choices? In effect, what are the factors we try to mask with our hitches and how can they be corrected rather than compensated for with hitches? While these discussions are free for all in nature, I respectfully request that contributers address this central theme and not discuss hitch-based solutions.
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Old 10-19-2007, 09:47 PM   #2
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Of course the first thing always mentioned is the tire cornering stiffness of the tow vehicle. Primarily the rear tires, but the front tires need to be stiff also. Passenger tires are the worst.
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Old 10-19-2007, 09:56 PM   #3
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Rodney,
EXCELLENT and, well stated.
I will just touch on one aspect. "Wheel base of the TV mis-matched and, thus not up to the towing requirement. That, which being said, is quite simply: Getting from point A to point B safely.
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Old 10-19-2007, 10:17 PM   #4
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good topic rodney,

certainly an overloaded trailer and/or tow vehicle will contribute to an unsafe towing configuration. i think people are mindful of the numbers to an extent, but when reality sets in and they need to get out of town and take one more thing or cousin joey is now coming along in the TV, the extra weight can add up quick. all that info is important for determining tire pressure in both the tow vehicle and trailer.

during my private pilot training, we had to always calculate the load of the plane accounting for full fuel and number of passengers and check that against the airport we're taking off from and landing at to make sure the runway was long enough (not a major thing with the small planes, but necessary calculations nonetheless). those little cessna 172's can't take off with full fuel and 4 people in the plane.

it'd be an interesting poll to see who really knows what there weight is of their tow combination when headed out on a trip. i'd bet most of those who "have a pretty good idea" are heavier than they think.

one cure would be to keep some sort of log of items you take with and their weight, almost like a weight and balance log. or you could pay a trip to the local scales and get your tow combination measured. it comes down to just paying attention and taking the weight issue seriously.

once i get my shell back on, i'll take the trailer to get weighed because i'm curious to know how much just the shell and frame weigh.
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Old 10-19-2007, 11:26 PM   #5
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Quote:
Originally Posted by urbanfood
<bhig snip> once i get my shell back on, i'll take the trailer to get weighed because i'm curious to know how much just the shell and frame weigh.
Hmmm... Have you considered starting a pool? Five bucks a shot, closest without going over takes 50% of the pot, the other 50% goes to the restoration fund...
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Old 10-19-2007, 11:55 PM   #6
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Quote:
Originally Posted by Gen Disarray
Roger made some comments in another thread that really got me thinking (no mean feat). ---edited heavily

His comments relative to root causes of towing problems made me realize that I don't have as good as a grasp on the topic as I would like to. So here is my question: What are the factors contributing to an unstable towing configuration above and beyond hitch choices? In effect, what are the factors we try to mask with our hitches and how can they be corrected rather than compensated for with hitches? While these discussions are free for all in nature, I respectfully request that contributers address this central theme and not discuss hitch-based solutions.
Rodney, this one's sorta difficult, as they are too many variables. However, I am sure that perhaps some of the root causes will be revealed in your thread.
Here's my scenario as of 8 weeks ago:
Suburban 1500 2WD, stock LRC tires, rear anti-sway bar, full tow package, excellent shocks and tight suspension ( according to my alignment shop)
Air pressure in tires when towing is 50R 45F
Towing a 1963 Overlander, dual axle, weighing in at give or take 5300lbs travel ready, with a tongue weight of about 12%. Air pressure 50lbs
Hitched with a Reese Dual cam HP, adjusted as good as possible as per Reese's instructions.
It sways. It does so whenever trucks or fast SUV's pass me. It does not seem to be wind sensitive. But it's a bear with the trucks and buses.
I don't know why. I tried numerous settings, more or less chain links, adjusted the cams for each time I went out, according to load. Simply put, the combination was not anywhere near what I would call satisfactory.
My observations on the tow vehicle are that I can move the rear of the truck left and right substantially without too much strain, simply by pushing on the bumper sideways. I can see the tires wobble slightly when I do this, but it seems to be the leafsprings that give way also.
Maybe one of the root causes are poorly controlled tow vehicle rear axles.
Another possibiity is that my tongue weight is too low, relative to trailer weight. My plan is to relocate the batteries to the tongue, increasing the weight by 130lbs, getting closer to the 15% number.
Unfortunately, I totalled the Suburban recently, and have not towed with the new vehicle yet. I am deliberately not changing anything in my tow setup yet, other than the truck. Hopefully this will give me personally a better idea of what doesn't work well for me. I will then go one step at a time, to find out what made the biggest difference. My new tow vehicle is a 2007 Suburban, half ton.
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Old 10-19-2007, 11:56 PM   #7
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Ok, I'll jump in! One of the most important considerations is the set up of the brake controller. At a young age, I was taught you want the trailer brakes to always engage a bit earlier and a bit more than those on the tow vehicle. The tow vehicle and trailer should stop as a unit, with the trailer doing more of the stopping than the truck. Properly adjusted, this will stop sway. Improperly adjusted, it can contribute to sway especially if the trailer brakes are set very light and the tow vehicle brakes are very aggressive. Throw in poor load layout in either and overloading and you have a recipe for an accident.

One more consideration! Sometimes, I think we work too hard trying to get the tow vehicle to be perfectly level, by transferring too much of the load away from the rear tires of the tow vehicle. By experimenting, I found the most stable towing to be when the rear of the tow vehicle is just squatting down a little. For me, when I set up the trailer and truck to be perfectly level, it was a bit twitchy and nervous, especailly when being passed by trucks and when going over rough portions of road, so I let it squat down a bit and the twitchness went completely away.
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Old 10-20-2007, 12:41 AM   #8
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Quote:
Originally Posted by Gen Disarray
... So here is my question: What are the factors contributing to an unstable towing configuration above and beyond hitch choices? In effect, what are the factors we try to mask with our hitches and how can they be corrected rather than compensated for with hitches? ...
i don't think many (any?) folks actually try to 'mask' issues with a hitch...

i don't think hitches "compensate" for poorly prepared rig combinations...

and

i absolutely don't think most modern hitches, properly adjusted "contribute" to unstable towing configurations...

these seem to be flawed premises, but still the question is "factors that affect towing dynamics" more or less...

regardless the issues that contribute to stable or unstable towing have been covered many times in many threads...

bryan (leipper) has written MANY great posts on the variety of issues and offered reasonable corrections as well.

the sierra nevada unit site has some of his well composed and sensible thoughts...

so check it out.

others have gone to great lengths to recount/tabulate the issues and corrections...

that's the magic of the search feature that works so well here.

typically 'we' think of trailer...

-tires (rating, wear, inflation, size, pairings)
-axles (number, condition, positiion, symmetry)
-length (especially as related to tv wheelbase)
-weight (absolute and relative to capacity)
-tongue mass (as a % of total weight, and in absolute numbers)
-aerodynamics (box vs twinkie)
-attitude (level ft/rr, side/side, ball height)
-loading (water, gear, center, low)


and tow vehicle...
-tires (rating, wear, inflation, size, configuration)
-frame design and stiffness (rail vs unibody)
-wheel base (especially as related to trailer)
-weight (absolute and relative to the trailer wt, total and % on each axle)
-suspension (spring rates, shocks, anti-roll bars, height)
-receiver (type, rating, wear)
-break controller (response time, proportionality, gain, design type)
-visibility (forward, reward, and in traffic)


the hitch design and characteristics are PRIMARILY focused on...

'controlling' unexpected, excess movements OR sudden events which driver response might make worse...

most of the obvious, user apparent issues (listed above) are important but secondary factors...

in the stable/unstable towing concepts....

while the fundamental issues take more digging to grasp, modify and 'design away' in the trailer OR tow vehicle...

things like center of gravity, trailer suspension response (roll, stiffness, dampening), brake designs, response to braking, yaw dynamics and other 'handling' parameters inherit to both vehicles...

on the tow vehicle side again ALL of the dynamic stability issues and the differences when paired with a trailer....

many of the things that make for a great handling auto do NOT make for the ideal tow mule...

apart from the vehicles are weather, road conditions/design, gradients, traffic conditions, day/night, rate of travel and so on....

also the BIG issue at the steering wheel...

- the driver's training, awareness, preparedness,
-response to sudden events,
-vision, motor skills, physical conditioning, reflexes, fatigue, medical conditions
-and experience towing and experience/practice with 'oops' moments...


at the connection between tv/trailer is the 'design' issue (ball, 5th wheel, goose neck...)

there are some easy reading papers on the fundamental issues of towing stability/dynamics and here are just 2...

A Design Analysis Approach for Improving the Stability of Dynamic Systems with Application to the Design of Car-Trailer Systems -- He et al. 11 (12): 1487 -- Journal of Vibration and Control

http://people.bath.ac.uk/en8cjk/Caravan.pdf

the 2nd paper in particular applies well.

there are also MANY published studies on EACH issue, like brakes or wheel base or frontal mass or yaw control or suspension and so on...

lastly we NOW have auto makers building IN 'sway control' properties just like abs or traction control...

as in this....

Boasting unsurpassed capability, Chrysler Aspen will give customers more peace of mind in towing conditions with segment exclusive Trailer Sway Control, ” said Mike Donoughe, Vice President – Body-on-frame Product Team, Chrysler Group. “Chrysler Aspen is designed for safety and stability with standard Electronic Stability Program, Electronic Roll Mitigation and Trailer Sway Control.”

Trailer Sway Control
Trailer Sway Control reduces trailer sway (an alternating yaw of the vehicle) and improves handling in adverse towing conditions caused by crosswinds and traffic, provides trailer stability and increases towing safety.

Software monitors the vehicle’s movement relative to the driver’s intended path. The vehicle yaw sensor recognizes sway. Once the system determines the sway is increasing not as a result of the driver’s steering input, Trailer Sway Control applies brake pressure on one front wheel to counteract the yaw induced by the trailer and applies brake pressure to all four wheels to slow the vehicle. As the trailer sways to the other side of the vehicle, Trailer Sway Control will increase pressure to the opposite side of the vehicle. The technology will continue to apply alternating brake pressure and reduce engine torque until the trailer is under control.


so it's a big world out there in trailer towing dynamics and the vehicle manufactures are even looking at these issues...

relaxed, safe and expedient towing is what we all (mostly) desire...

and lastly i think the conclusion that 'if properly set up' the top hitches all perform similarly, is ALSO flawed

because MANY of us have done that and

STILL we like the fact that bending (angulating) at the hitch pivot point is IMPOSSIBLE with a haha...

unless WE initiate the bend at the steering wheel..

no one seems to have addressed this issue in the tv or trailer yet or other linkage systems...

although a/s has/did briefly consider incorporating the haha (or the concept) into it's tongue area...

with integrated brake controllers here now, and 'sway control' as noted above, it may not be long before some DOES look at the linkage issues...

cheers
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Old 10-20-2007, 11:12 AM   #9
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A running list

Thought I would start compliling a list of the factors.

Here are the factors as I read it so far:
  • Stiffness of tires.- Presumably the rating of the tire.
  • Mismatched wheel base.- Is there a wheelbase to trailer length ratio that applies here?
  • Overloaded vehicles. -I am assuming the point here is exceeding capacities.
  • Wrong tire pressure. Ė Normally when I see this addressed it is referring to under inflation but is there suggestion that over inflation can also be a culprit?
  • Poorly controlled vehicle rear axles.- Uwe, Iím going to need you to explain this one to me.
  • Inadequate brake controller.- not sure if this one belongs on the list or not since Iím trying to get at factors contributing to instability rather than fixes. On the list for now.
I think this is a great start, I know it is giving me food for thought. If any of you who contributed to the list so far feel I have got your comments wrong by all means let me know. I wonder how many factors we will come up with in the end?
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Old 10-20-2007, 12:55 PM   #10
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the 2 widely quoted calculations for wheel base match are here in post 15 and other threads too...

http://www.airforums.com/forums/f463...tml#post219479

but neither account for vehicle height, center of rotation, center of gravity and so on...

and only one considers tv rear overhang, which is a big factor in yaw forces...

for example a 2 identical tvs or identical trailers will 'tow' differently just based on the vertical relation of mass to suspension...

also neither account for single versus multiple axle trailer configurations....

and almost none of us have control over axle placement relative to trailer length...

so view these as extremely gross guidelines, like a cubit or hand or biblical yard of measure...

'overloaded vehicles'....

simply exceeding the carry capacity isn't the issue.

many 'over gross' rigs are very stable at a constant speed and direction and even with wind or bow waves...

BUT as the conditions vary control becomes problematic....

so loading as more to do with 'placement' of the load.

fore/aft, high/low, related to the axles/tongue and relative to the tv AND trailer...

tire pressures...

over or under inflated can be problematic.

under inflation issues are well covered elsewhere.

over inflation can alter vehicle suspension characteristics and contact patch and grip and so on...

IF the tire is airborn it can't provide much grip/control, right?

many folks have mismatched tires (brands, load ratings, age and even SIZE)....

they are either unaware or believe it's not really that important, since the rig seems to perform fine in the driveway.

'poorly controlled rear axles'

many of the leviathan sized suvs have inadequate rear shocks, springs, no anti-roll bars and other issues...

resulting in softer rides while unleashed,

but excessive floating, wiggle, roll, squat or axle wrap and wheel hop, under heavy loads (like towing)

cheers
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Old 10-20-2007, 04:24 PM   #11
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Quote:
Originally Posted by Gen Disarray
  • Poorly controlled vehicle rear axles.- Uwe, I’m going to need you to explain this one to me.
I consider the leaf spring mounted solid rear axle more of an insult than automotive genius. It's cheap, easy, always been like that...etc. It sucks in terms of roadholding, wheel control, and handling. It's a small step up from the covered wagon, the small step being the rubber tires. The spring must perform double duty, both as the suspending element, and the part that fastens and controls the position of the rear axle. The unsprung weight is huge, making true control of this axle system very difficult, if not impossible. It's the quintessential reason for the term "rides like a truck". With added sway bars and the best shocks money can buy, to me this entire system is just barely acceptable. There is an inherent tendency of the axle to not only slightly steer when going over one-sided bumps, but also yaw and float. Much of which 2air has already posted. Combined with a rear overhang of 3ft or more, this could easily be one of your root causes.
My hopes are that my 07 Suburban behaves nicer in that regard. It still has a solid axle, but it is now controlled with control arms and finely tuneable coil springs. The spring and shock location seems more sensible to crisper handling than the leaf sprung variety. I hope this maes sense to you.

Quote:
Originally Posted by Gen Disarray
  • Inadequate brake controller.- not sure if this one belongs on the list or not since I’m trying to get at factors contributing to instability rather than fixes. On the list for now.
Not sure on the brake controller either. Perhaps we should focus on the rear root causes, the brake controlle rcould only mess up things while braking, which to my understanding was not included in your original question.
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Old 10-20-2007, 07:13 PM   #12
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As for brake controller, it's not about the adequacy, it's about the driver selected sensitivity! If the driver doesn't know he needs to dial in the sensitivity, it contributes to a condition which can turn a small problem, a little sway, into sway so severe it can cause a wreck.

The original question was: What are the factors contributing to an unstable towing configuration , and poorly adjusted brake controller would be high on my list of things to avoid because it contributes to an unstable towing configuration. I have a friend who thought he had big time brake problems on his trailer. Every time he applied the brakes, the wheels would lock up and leave long black marks on the highway. Of course, he had the controller set on the most aggressive setting. He didn't know it was something he needed to "dial in". He wanted good brakes so he set it on the highest setting!
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Old 10-21-2007, 06:48 AM   #13
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All good stuff, I disagree with driving with the TV rear squat, that takes the weight off the front end and must affect steering - not a good thing.

Using ST tires IMO is critical, towing needs those stiff sidewalls.

The other point, is the distance from the rear axle to the ball.

Anything you can do to decrease this distance is golden.

But it's not easy in many cases, the drop bar should be inserted as far as possible to shorten the distance.

The other option if there is still room to shorten the drop bar but you're on the last hole is to have a shop drill another hole for the pin to allow the bar to insert into the receiver as far as possible.

I learned this at a towing seminar, that even if you shorten this distance by an inch it can have a dramatic increase in towing performance.

My rig with the Reese dual cams worked great for us driving 13,000+ miles this year. I drive on highways at 70 most of the time and often at 75.

When I see I a semi coming up along side I have no concern for their wind buffeting.

Still I'm having a shop drill my bar to gain another 1 3/4" of distance.



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Old 10-29-2007, 11:51 AM   #14
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Quote:
Originally Posted by Gen Disarray
Thought I would start compliling a list of the factors.

Here are the factors as I read it so far:
  • Stiffness of tires.- Presumably the rating of the tire.
  • Mismatched wheel base.- Is there a wheelbase to trailer length ratio that applies here?
  • Overloaded vehicles. -I am assuming the point here is exceeding capacities.
  • Wrong tire pressure. Ė Normally when I see this addressed it is referring to under inflation but is there suggestion that over inflation can also be a culprit?
  • Poorly controlled vehicle rear axles.- Uwe, Iím going to need you to explain this one to me.
  • Inadequate brake controller.- not sure if this one belongs on the list or not since Iím trying to get at factors contributing to instability rather than fixes. On the list for now.
I think this is a great start, I know it is giving me food for thought. If any of you who contributed to the list so far feel I have got your comments wrong by all means let me know. I wonder how many factors we will come up with in the end?
Adding to the list:
*distance from the rear axle to the ball.
*Tounge weight. I think I'm going to do some more reading on this one.
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