Talk About Tongue Weight

[dznf0g]

You're right Bill, On many of the newer units, which are not pendulum type, there may not be any adjustment for "leading" the trailer brakes. On the old pendulum type you could lie to the controller about the levelness of the unit and adjust in an early application.

[Ron Gratz]

Originally Posted by Bill M.

"With a 4-bar hitch it's always a good idea to have a brake controller which can cause the TT brakes to "lead" the TV brakes"

Somebody please explain how is this even possible??

Some brake controllers use the brake light circuit to activate a feature which sends current to the trailer brakes before TV braking action takes place.

Examples are the acceleration-based Prodigy's "boost" feature and the pressure-based BrakeSmart's "Initial Brake Constant" feature.

The small delay time between brake light circuit activation and application of TV brake force is sufficient to cause the trailer brakes to "lead" the TV brakes.

Ron

[ROBERT CROSS]

Originally Posted by Bill M.

"With a 4-bar hitch it's always a good idea to have a brake controller which can cause the TT brakes to "lead" the TV brakes"

Somebody please explain how is this even possible??

The only way I can see to assure that the trailer brakes come on first is to delay the TV brakes slightly from the initial push on the pedal. I do not think I want a setup like that. You are sure not going to get the trailer brakes on first with one of the very common inertia based controllers.

Determine how much pedal movement you have before the brake lights come on.


With the Tekonsha's I previously used..

The best way I found was to wire to the brake pedal and not the light circuit.

The first 1/2" or so of pedal movement usually won't apply very much TV braking....hence you see brake lights on while following a car with a headless driver.
A switch taking advantage of that 1/2" took care of the problem on our 95 Burb.

POI..I've had no 'bumps' since using the TruControl Gold with the haha.

Bob

[slowmover]

BenK over on WOODALLS did a very nice job of showing how he modded a brake light switch to get "TT brake first".

[mstephens]

Originally Posted by slowmover

Where the rubber meets the road with an articulated vehicle, (as SAE likes to call it, I prefer "combination vehicle") is the TV rear axle tire contact patch.

What are your thoughts about that?

[slowmover]

If the contact patch is lost, then real trouble ensues.

The gold standard in all this, IMO, is in braking to a full stop. All else depends from this.

Remaining upright and lane-centered is the ongoing, lesser, goal which feeds that.

[mstephens]

Originally Posted by slowmover

If the contact patch is lost, then real trouble ensues.

The gold standard in all this, IMO, is in braking to a full stop. All else depends from this.

Remaining upright and lane-centered is the ongoing, lesser, goal which feeds that.

Right. In spite of the confusion that ensues over the second law of friction, it certainly appears true that large patches with sticky rubber definitely shorten stopping distances. This is another example of why there is more to tires than load capacity.

I think the idea of being able to come to a panic stop in your own lane in a short distance should be the goal of any tow vehicle design. A good standard for a TV with large TT might be 225' from 60MPH, as an example. We rarely see such data reported for TVs. Wouldn't it be fascinating to take a particular TV, and then test stopping distance from 60MPH with say a 22' a 25' and a 30' Airstream? To me, that would be far more enlightening than simply reading "9500# towing capacity."

[Jim Flower]

I wonder if the manufacturers have this data and it helps to explain their reluctance to adopt J2807.

[Jim Flower]

Adopt was incorrect. Implement is more like it. Jim

[mstephens]

The SAE J2807 Braking Test is a bit limited, I think.

QUOTE
Braking Requirements

The test vehicle and trailer must stay within an 11.5 foot wide traffic lane during stopping tests. Stopping requirements from 20 mph without use of trailer brakes are;
1. in 35 feet or less with a tow rating of 3,000 lbs or less and no trailer brake requirement
2. in 45 feet or less with a tow rating of 3,000 lbs or less and a requirement for trailer brakes
3. in 80 feet or less for tow ratings above 3,000 lbs
The parking brake must be capable of holding the rig on 12 percent up and down grades
END QUOTE (from a blog, not from an SAE document)

There apparently is no test for 55MPH or 60MPH braking. I can see why the 20MPH test is useful, but it seems incomplete.

[Ron Gratz]

SAE J134, J134: Brake System Road Test Code - Passenger Car and Light-Duty Truck-Trailer Combinations - SAE International

and SAE J135, J135: Service Brake System Performance Requirements - Passenger Car-Trailer Combinations - SAE International

might be of interest.

Ron

[switz]

I just paid SAE $61 each for those two documents plus a membership fee. Appears these will put you right to sleep, not thrilling reading.

[mstephens]

Originally Posted by switz

I just paid SAE $61 each for those two documents plus a membership fee. Appears these will put you right to sleep, not thrilling reading.

I was briefly tempted to buy them also, and I thank Ron for pointing to them. But I decided I could do some homework, googling, and probably find someone posting ABOUT them instead.

[JeremyB]

Originally Posted by Ron Gratz

Without providing the derivation, the relationship among these four variables is given by:
a*b = I/M
From this equation, if you knew the value for I, you could calculate an "optimum" value for a/(a+b) which is numerically equal to TW%.

From the book Vehicle Stability, by Dean Karnopp: he gives mab>Ic as a stability criterion for trailers. If that's the case, then ab = Ic/m is a minimal condition for stability, and more tongue weight adds stability, at the cost of more lateral hitch load. Thoughts?

As CAD practice a few years ago, I modeled my 5x8 utility trailer. While empty, ab=Ic/m at 8.4% TW. Actual TW=10.7%.
Actual Ic was 72% of Ic when using the formula for a rectangle: Ic=m*(h^2+d^2)/12

[Ron Gratz]

Originally Posted by JeremyB

From the book Vehicle Stability, by Dean Karnopp: he gives mab>Ic as a stability criterion for trailers. If that's the case, then ab = Ic/m is a minimal condition for stability, and more tongue weight adds stability, at the cost of more lateral hitch load. Thoughts?

I agree that, for this single criterion, ab = Ic/m is a minimal condition for stability, and greater TW% (greater distance of CG forward of axles) adds stability.

However, the vertical force due to greater TW also will tend to cause less yaw stability.
On one hand, greater TW% is good for yaw stability. OTOH, greater TW is bad for yaw stability.

The challenge is to keep TW low while keeping TW% high enough for minimal Routh Criterion stability.

Ron

[ROBERT CROSS]

Now these guys must have have it figured out...

Bob

[JeremyB]

Originally Posted by Ron Gratz

I agree that, for this single criterion, ab = Ic/m is a minimal condition for stability, and greater TW% (greater distance of CG forward of axles) adds stability.

However, the vertical force due to greater TW also will tend to cause less yaw stability.
On one hand, greater TW% is good for yaw stability. OTOH, greater TW is bad for yaw stability.

The challenge is to keep TW low while keeping TW% high enough for minimal Routh Criterion stability.

Ron

I'm missing why increased greater TW is bad for the trailer yaw stability. (Not being facetious ) I can see where it would cause worse combination vehicle yaw stability.
A later derivation in Vehicle Stability adds rotational damping and comes up with stability criterion that includes speed:

c(a+b)/U + mab > Ic

c= rotational damping
U= forward speed


Although if it stable by the previous criterion (mab > Ic), it will always be stable with the criterion with rotational damping.

[Ron Gratz]

Originally Posted by JeremyB

I'm missing why increased greater TW is bad for the trailer yaw stability. (Not being facetious ) I can see where it would cause worse combination vehicle yaw stability.

Sorry, I should have been more explicit. When I introduced "vertical force due to greater TW", I was alluding to effect of TW on the TV.

A later derivation in Vehicle Stability adds rotational damping and comes up with stability criterion that includes speed:

c(a+b)/U + mab > Ic

c= rotational damping
U= forward speed

Although if it stable by the previous criterion (mab > Ic), it will always be stable with the criterion with rotational damping.

I agree. However, with the introduction of rotational damping, stability no longer requires mab>Ic. With rotational damping, you can have stability with Ic>mab as long as U does not exceed a critical speed, Ucrit.

Ucrit can be defined by replacing U in the above inequality with Ucrit and changing the inequality to an equality.

Then, Ucrit = c(a+b)/(Ic-mab)

This equation says the critical speed decreases as the polar inertia increases and/or the product, mab, decreases.
This could explain why some improperly loaded trailers suddenly develop oscillations at a certain speed.

Ron

[SteveSueMac]

What do you think about c(am)/p-ing ?


:-)

Sorry - just being a wise guy.....back to the adult table :-)

[mefly2]

Good to know about the smart car ...guess that we can ditch the 3/4 diesel and just use our chev spark to tow now ... lmao!