I know some manufacturers specify two different maximum tongue weights for their vehicles - one for using a regular hitch, and a second (typically higher) value if using a weight-distributing hitch. So near as I can tell from digging into the T4R's owner's manual and Toyota's website, there is only one value provided: 500# (with no mention of hitch type).

I came across some info on Car and Driver's website stating that the max tongue weight when using a dead weight hitch is 470# (or 10% of the truck's 2014#max towing capacity). Interestingly, it also gives a value for max tongue weight with a WDH: 705#. Thing is, I have no idea where they got these numbers. Does anyone maybe have a newer revision of the 2014 OM that has this info?

Otherwise, can anyone speak in general terms about how a WDH affects max tongue weight, if at all?

Maybe this link will help: RV.Net Open Roads Forum: Towing: Weight Distribution (WD) Hitch --- How it Works

Just remember though you still ultimately have "X" weight back there that you have to accelerate and stop. With a WDH it has better weight distribution over both your axles and not just the rear.

As far as I've seen, Toyota has never published a tongue/trailer weight for the 5th Gen 4Runner when using a weight distributing hitch. So, you might assume it is the same as a weight carrying hitch. I call the 705# number bunk. WDHs are only mentioned in the 2012/2013 owner's manual. There is no WDH reference in the 2010, 2011, or 2014 manuals.
1engineer's link gives a good (probably the best on the internet) explanation of WDHs and their operation. If you want to burrow deep into the rabbit of of towing physics/dynamics, search for Ron Gratz on the rv.net forums.

The WDH transfers weight from the rear axle up to the front, lessening sag and increasing the understeer gradient. I think the SAE towing standard requires a positive undsteer gradient at 0.3g steady cornering.
Adding airbags to the rear actually decreases the understeer gradient (drives the system towards oversteer). Not necessarily a bad thing if the system (tow vehicle + trailer) is still understeering after adding the bags.

Regarding system stability, you have two main components.
Is the trailer stable?
Is the system (tow vehicle + trailer) stable? [understeers at 0.3g]

Trailer stability is related to tongue weight and the trailer's polar moment of inertia [Ron Gratz has a nice thread explaining this]. Once you pass a minimum tongue weight % (SAE uses 10% in their towing standard, but it varies), the trailer is stable. However, adding more tongue weight decreases the understeer gradient of the tow vehicle + trailer system. So there you have to walk the line between having enough tongue weight for the trailer to be stable, but not too much to make the system unstable. This is really only an issue when you get near the tow limit. A 1500 lb trailer with a 25% tongue weight (375 lbs tongue weight) isn't going to make the 4Runner an oversteering death machine.

Yeah I call BS on the 705 lb number too. Sure, it's 15% of the GVW but it's only a rule of thumb. I am a firm believer in the 10% rule even with a WDH. I have only used them a few times when driving for others but as near as I can tell it's breaking-the-laws-of-physics beating magic I say. They work.

Thanks for the insight, guys. I'll stick with 10% of loaded trailer weight, not to exceed 500#.

Got the following response from Toyota some time back. Haven't been able to post because I've been engaged with a cross-country move, but I figured I should share this to close the loop. Interesting that they're quoting a total towing capacity of 5000# and a max tongue weight of 550#.

Dear Mr. Xxxxxxx:

Thank you for contacting Toyota Motor Sales, U.S.A., Inc.

We appreciate the opportunity to address your inquiry.

Toyota's official specification for maximum tongue weight for your 2014 Toyota 4Runner SR5 is no more than 11% of the vehicle's maximum towing capacity, or 550 pounds. The use of a weight-distributing tow hitch will not increase this recommendation, and exceeding maximum trailer weight specifications may cause deterioration of steering or braking control.

Please thoroughly review the towing recommendations given in section 4-1 of the 2014 4Runner Owner's manual prior to using the vehicle to tow a trailer. An electronic copy of this document is available for review at this address.

If you would like to discuss your intended towing setup in greater detail, please feel free to speak with your local Toyota dealer. If we can be of further assistance, please contact us.

Sincerely,

Alex Xxxxxx
Toyota Customer Experience

glwt!

So since TS and I had traded numerous PMs on this and he's made numerous threads ...

@Bimmer-Bob - what setup did you settle on for your cross country trip?

Getting ready to do a full write up. Short version is that, properly set up, this truck has no problem towing at it's limit.

I expect charts and graphs containing engine/tranny temps. TIA

This is interesting as I've been looking at a trailer with a 550lb tongue weight and my 2017 T4R shows max 500lb. Not sure how much difference 50lb would make though.

I can tell you claimed dry tongue weight and the actual tongue weight are two separate things. Usually the tongue weight is more than advertised. Sometimes way more. Always weigh before you buy.

Good point, I will have to certify that on site when I look at the trailer. As for tongue weight of the 4Runner, I looked in the manual and it gives a range 9-11% of the towing capacity so that puts it 450-550 lbs.