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Old 10-07-2016, 09:33 PM   #29
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Church Point , NSW
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Have now read Andrew's refutation (and his associated writings) but he does not address the question that I was asking: what is the reason - in J2807 FALR -for reducing the WDH's effect on front-end mass (that thus reduces understeer.

I suspect it is to increase the ability to withstand higher g forces by ensuring the rig can follow a tighter radius (without incurring oversteer). If not, then what and why.

I have been reading Ron's postings in this area and they do reflect that of most (of the few!) people working in this field worldwide.

Collyn Rivers

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Old 10-07-2016, 09:39 PM   #30
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Would you be kind enough to have a look at my postings re FARL - I would very much appreciate your comments. (Can be found by entering Collyn into the Search engine.

Kind regards
Collyn Rivers

(in Australia)

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Old 10-08-2016, 01:36 AM   #31
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The simple answer is that -- after more than fifty years of success -- that "restricting" tow ratings to pickups enhances their desirability. Sales driven. (No different from other industries, alas).

Leave the tongue weight on the rear axle and it cancels out vehicles actually better suited to the job. Not to mention worse dynamics, overall.

One may as well examine the state of trailer suspensions. I believe you've compared them unfavorably to late 19th Century horse-drawn buggies which also used leaf springs. Shock absorbers are rarely even optional.

Not even a whiff of aerodynamic design (well understood before the 1930s), and hobbled farther by raised suspensions to accommodate the latest fad of "slide-outs". Worse in winds than their 1960s forebears.

Vehicles that once had adequate tow ratings (and still may in foreign countries) have had those dropped to token assignments. "Not tested".

The real world proves otherwise.

One could drive a pickup thru the omissions and obfuscations.

1990 35' Silver Streak
2004 555 Cummins
1990 35' Silver Streak Sterling; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 10-cpm solo, 18-cpm towing
Sold: Silver Streak Model 3411
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Old 10-08-2016, 07:30 AM   #32
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Originally Posted by slowmover View Post
The only expert around here (or on other RV forums) is Andrew Thomson of Can Am RV Centre, London, ON. Is a consultant to both Airstream and to the SAE committee on the above.

I'd recommend a thorough reading of his commentary at his website, on the Canadian magazine, RVLife, and his many posts at this site.

His refutation of "other than FALR" is solid. 31/1/2016 date at his website; Setting Torsion Bars.

Specifically, (and IMO) J2807 purposefully ignores whole classes of vehicles, both as potential TVs and wind loads on TTs versus the unrepresentative trailer employed.

Now, the differences between travel trailers here, in England or in Australia make for other problems in analysis.

I've enjoyed reading your contributions elsewhere and wish you well in your endeavor.

A few points:

SlowMover is obviously a big fan of Andrew T. To state that he is the ONLY expert here or else where is obviously SlowMover's personal opinion and not necessarily accurate. I would surely read Andrew T's articles as they are very informative. Just don't take them as ground truth. There are many that disagree with his opinions based on their own many years of experience.

J2807 is SAE's standard for determining vehicle tow ratings. Everybody who is anybody has had input in setting up the standard. Its a set of well defined repeatable tests that allows (to some extent) apples to apples comparison of vehicles from different manufacturers. It tests pulling/stopping/cooling capacity of tow vehicles, their hitch strength, and their stability.

Testing is a means to verify the design. If the design says that a vehicle can tow 10,000#, we need an objective test to verify that. There is no point in testing a vehicle that is not designed for towing. Thats why auto manufacturers do not test the Camry's, Miata's, etc. but tests the pickups, SUVs, etc that have a substantial tow rating.
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Old 10-08-2016, 11:51 PM   #33
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Thank you

I have studied J2807 at length since it was first announced and understand that its intent is often misunderstood (and in some places possibly misrepresented).

The query I have is very specific.

It is why (by reducing the amount of understeer as a result of 100% FALR) is this seen as enhancing safety.

As I understand this part of J2807 full FALR results in the degree of understeer preventing a lateral acceleration of 0.4 g (it suggests 0.3 g).

Is it thus seen that it is better to limit understeer such that a higher g level is feasible?

This appears to be the only explanation. I was hoping that Ron might respond. I have read a fair number of his responses and understand his point of view.

If interested I have a piece in the new Australian in late October re AL-KO ESC, Dexter DSC and the lesser known IDC systems.

I do I fear share many of Slowmover's reservations re the state of trailer design. The EU is way ahead but does not build trailers of the size used in the USA.

Ours are somewhere inbetween - and with far too low nose mass (as low as <5%) and towed by vehicles often two thirds their weight. The physics relating to their behaviour is however just the same.

I have a large number of articles on my website re caravan & tow vehicle stability (but need to check this forum's rules re Links etc before mentioning that again).

Collyn Rivers
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Old 10-09-2016, 05:34 AM   #34
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Hi Collyn

Below is a column I wrote about the SAE weight transfer recommendation. Certainly there may be something here that I don't understand but try as I might I cannot get the SAE recommendation to be stable.

If I can help feel free to call sometime and if you get to NA sometime feel free to come for a visit and we can do some test drives.

Andrew T

SAE J2807 Tow Rating Standard.
Setting The Torsion Bars:

In previous articles we discussed how to set your torsion bars properly. If you need a refresher here is a quick summary.

1) To set the torsion bars you need a level cement pad. To start, you want the trailer and tow vehicle in a straight line, disconnected, with the coupler ready to drop on the ball.

2) With the tow vehicle in position but disconnected use some masking tape and mark a height on all four corners of the tow vehicle’s bumpers, for example 22 inches, or whatever is appropriate for your tow vehicle.

3) Now that we know how the tow vehicle sits without the trailer, we want to determine the change in position when we connect. Generally the goal is to have the tow vehicle pushed straight down, so the front is pushed down the same amount as the rear.

4) Connect the trailer, do up the torsion bars and measure the change on the masking tape.

5) You will find that the tow vehicle will have been pushed down by the weight of the trailer. For example, the rear measurement may now be 20” instead of 22” but the front may have come up to 22.5”. In this case, you need to go up to the next link (adding tension to the torsion bars) and measure again.

6) However, you may find that the next link puts you into the opposite position where the front is pushed down 1” and the back stays even at the 22” mark. This means that the torsion bars are transferring too much weight forward.

7) If this is the case then you need a partial link. To do that, overlap two chain links and slide a ” bolt through them. A ” bolt is 1/3 of a link of adjustment so in some cases you will need 2 bolts to achieve the correct transfer.

8) Most tow vehicles will end between 21 ” & 21 ” front and rear if you started with marks at 22”.

9) If you are setting up a new hitch the bars will wear in quite quickly. You’ll likely need to add a third of a link in 2-500 miles. You will need another third after another 500-1000 miles. You’ll probablyy feel the difference in the steering as this happens. After a while it just won’t feel as planted as it was after your initial set up. It’s fine to experiment 1/3 of a link at a time you should notice the difference in steering feel right away if it was the right or wrong change to make.

10) If you can’t get the weight to transfer to the front wheels without your torsion bars meeting the trailer frame then you don’t have enough reward angle on your ball mount, torsion bars too light, a hitch receiver that is too weak, or all of the above.

This is how we have configured hitches for the past 45 years. Because of these articles and internet searches, people with towing stability problems regularly seek us out. This is how we wind up helping them most of the time. A couple of hundred times every year we reconfigure a problem hitch that was installed elsewhere to these new settings. The difference is usually quite dramatic and customers are quite pleased. Now they don’t have to sell their trailer or buy a bigger truck.

The SAE has been working on a tow rating standard that they can use to better determine a vehicle’s tow rating. The standard will be better than nothing but it will still have many issues, the main one being that it is dependent on weight alone. The issue that concerns me most though is that they have come out with a recommendation on how to adjust torsion bars that in my experience is quite flawed.

What most vehicle manufacturers’ use for tow testing are enclosed cargo trailers with test weights inside. This gives the cargo trailers a low center of gravity, and most of them use torsion axle suspensions. Connect a tall RV trailer with a slide-out, leaf springs and no shocks and those handling tests are no longer meaningful. In a cross wind a tall RV trailer will behave much worse, however they did no testing on cross wind effects and really how could they? However most loss-of-control trailer accidents are the result of sudden wind changes.
The next barrier to effective testing is another SAE standard that we discussed in previous issues. Many of the vehicles being tested have hitch receivers that are too weak to transfer weight properly. If you cannot set up the hitch optimally how can you perform a good handling test? And most of the testing appears to be done with little or no rearward angle on the ball mount so the weight transfer on the tow vehicles is wrong. Without angle on the ball mount when turning, weight is taken off the front wheels and inside rear wheel and all dumped on the outside rear wheel. Last but not least the testing was done with the ball positioned well behind the bumper, and no effort was made to reduce overhang. The bottom line is that they never tested a truly dialed-in hitch system.
The next concern with J2807 is the handling tests themselves. I’m not certain how useful they are. One involves a test where traveling at highway speed, the steering wheel is snapped 180 degrees and immediately snapped back to the straight ahead position. The problem with this test is no correction for steering gear ratio or wheelbase. We talked about this in the article on steering. In a vehicle with a short wheelbase and a quick steering ratio this is a very violent maneuver, whereas in a long wheelbase vehicle with a slow steering gear it’s relatively relaxed.
J2807 is not the only time this method has been used. A few years ago Transport Canada set out to test the handling safety of 15 passenger vans. In this maneuver they used a steering robot to snap the steering wheel. This procedure removes the human equation but again they did not correct for steering ratio or wheelbase. The robot would seem like the ideal way to test; but is it? When in an emergency situation, as important as your steering input is the steering’s communication back to you. You can feel if the front tires are starting to plow or the back end is coming around. Take the human equation out of testing if you want, but in the real world people do the driving. A better test might be to flag down the first 20 everyday drivers and see how they perform.
The second SAE handling test is a steady state circle around the skid pad. As the G force increases the tow vehicle will start to go outside the circle, either the back end will break loose (oversteer) or the front tires will plow (understeer). Car companies like understeer. Now my own testing of this is on a delightful exit ramp with a downhill decreasing radius turn. What I find with my combinations is that the front end will almost always start to plow first but if you overcorrect which is the tendency then the back end will come around. If you just let it drift a little it pretty smoothly continues around the ramp drifting a slight bit off line. In the SAE test they kept breaking the back end loose. Which is to be expected since their hitch set up was unloading the inside rear tire and overloading the outside rear.
This combination of events led them to recommend a strange way of adjusting a weight distribution hitch. Instead of setting the hitch to push the tow vehicle down the same amount front and rear they use another method. Measure the height of the front of the tow vehicle. Drop the trailer on the ball without torsion bars. Measure again. Let’s say the front raises 2” then they want you to set the torsion bars so the front raises 50% from its solo position or in this case 1” higher than its solo position. This results in a considerable unloading of the steering axle. If you set your combination up this way it will be quite unstable at highway speeds especially when there is truck turbulence or cross winds.
In theory it would have a little more traction decelerating into low speed sharp turns. I have never seen an accident that was at all serious on a low speed sharp turn but there have been plenty due to loss of control at highway speed. After the SAE came out with this I thought possibly we’ve been wrong for 40 years. I tried setting up some combinations with their method and none of them were what I would consider stable at highway speeds. In fact most of the 200 trailers a year that we fix for people come in close to the SAE spec, usually due to a combination of weak receivers and improperly set ball mounts.
Before writing this column I wanted some more track testing just to check again that there was not something I was missing. I felt I should use a pick up as that is what the SAE used mostly in their testing. We used a ton GM truck with a 34’ Airstream for testing. We tested it to the industry spec where the truck is pushed down evenly and to the suggested SAE spec. Two differences from the SAE testing were that our ball was close to the bumper as possible and we strengthened the stock hitch receiver. In their test the back axle lost traction with the industry specification. In the 100’ slalom we achieved 81 KPH with the hitch set to our spec, 81 KPH is really moving in the slalom. At the 6th cone the Airstream drifts sideways on all six tires which is no easy feat. In theory if the back tires were to break loose on the truck this is where it should happen when the trailer is sliding sideways but the truck stayed planted and slid very little if at all. When we tried the same speed with the SAE spec we made it to the second cone and had to give up the run due to lack of control. On the track there is a nice declining radius turn where we expected the SAE spec truck to do better but there was no speed difference to speak of, on both settings the front tires started to plow before the back wheels broke loose. Next we tried a steady state turn that took us down and then up a lip of asphalt as well as over several bumps with less sticky pavement. The SAE setting bounced more and felt less planted but again we could not measure any appreciable difference in speed. I think though most drivers would find it much easier with the industry spec.
If I were you at this moment I would be saying “who is this RV dealer schmuck who thinks he knows more than the Society of Automotive Engineers?” That’s a valid question. I have some advantages that the SAE does not have. I’m sure I’ve driven more combinations of trailers and tow vehicles with more hitch systems, tire changes etc. etc. than anyone you’ll ever meet. I don’t just wander down the road them either. I do quick lane changes, panic stops, and all manner of emergency maneuvers. I am also close to the real world customer. If a customer says they are having an issue, I’ll say let’s go for a ride and you can show me. We then get a good feel for what is really happening out there. We take most of our own vacations with trailers, so I always have something to experiment with. Additionally, we’ll have 7 to 10 different tow vehicles of our own that we use for pickups and deliveries. I can focus on RV trailers, while the SAE must worry about cargo, boat and horse trailers as well. The test track is a great tool and we spend a lot of time there but nothing is quite like real world testing, and that we have in spades. We have the opportunity to pay a lot of attention to something that no one else wants to be bothered with.
The other parts of SAEJ2807 are acceleration and cooling system tests. The combination at its rated weight needs to be able to accelerate from 0-60 in 30 seconds, unless it is a dual wheel truck, and then it can take 35. The cooling system test is done on a 10-mile long 6% grade in Arizona in 100+ degree heat at a minimum speed of 40 MPH, unless it is a dually. Then 35 MPH is magically ok.
One thing I asked for in the SAE standard was publication of the individual test results. This would allow people to find a vehicle that suits their individual needs better. For example, a Ford Edge crossover SUV may not have the acceleration of V8 F-150 with a V8, and it also might not do as well on the cooling system test but it would almost certainly be better in a handling test and be comparable stopping. If you are concerned about fuel economy and safety but not in a big hurry, and willing to back off a little in extreme heat, the Edge performs very well without compromising safety. For many it suits their lifestyle better. As it is, you only know the rating is lower but you don’t know why. Even then they would have to test the Edge to find its limits but since they only plan to rate it for 3500 they test to that and leave it there. Coincidently the Explorer which is built on a similar platform with the same drivetrain is rated to tow 5000 pounds but it is not nearly as good a tow vehicle. It sits higher, has a softer suspension, and a longer overhang on almost the same wheelbase.

I have the greatest respect for the SAE. What the auto industry has accomplished is nothing short of amazing, and the technologies reach far beyond the car industry. I think in this case the SAE has tried to take on something that should not be their job. Then whose job is it? I believe it should be the RV dealer. The dealer is the only one who sees the trailer, vehicle and hitch system and who also meets the driver and understands how he’s going to use the combination. But since many RV dealers are afraid to get involved over fear of lawsuits etc. the towing equation tends to be neglected. In fairness, if we didn’t have a 45-year track record I doubt we would do what we do either. The good news is you don’t have to take my word for any of this. It’s easy to experiment with your own combination. Set it up to both specs and try it each way and you can decide for yourself if the industry spec of the last 50 years is right or the SAE spec.
In the end, getting the right combination and making sure it is configured properly is often up to you. That’s why you need to look beyond tow ratings and solely weight, and focus on the core principles behind the design of the tow vehicle and the trailer. And of course never compromise your hitch set up!

Andrew Thomson
London, Ontario

"One test is worth a thousand expert opinions."
Tex Johnston, Boeing 707 test pilot
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