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post #1 of 3 (permalink) Old 09-18-2017, 10:27 PM Thread Starter
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Rear differential swap?

I own a 2010 Armada SE. When I purchased it we never thought we'd be towing but has life changes so do the needs of our vehicles. I love my Mada and would not trade it for anything (maybe a newer one ?).

Long story short. We were told we had the tow package (oem hitch and tow button). After much research we have determined we don't?. Big towing difference 9100 to 6500.

I can address the suspension with after market parts but the rear differential is what has me worried. I want to tow a TT that's 7k and not have 8mpgs with a possible fried tranny/diff. If I swap my differential with a 3.357 from another Mada what are some of the issues I can encounter? Please forgive my ignorance.

Thank you all for your help.
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post #2 of 3 (permalink) Old 09-18-2017, 10:33 PM
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food for thought:

Originally Posted by scr38 View Post
There are only two options for the ratios in the Armada. The non-big tow is 2.94 and the big tow/off road is 3.36. There are no aftermarket gears available. Even if there were, changing the ratio causes problems. The Armada's computers are looking at vehicle speed from two sources; the sensor in the transmission and the wheel sensors. If these two signals don't match it causes problems, such as incorrect transmission operation and failure of the cruise control system. If you change final ratios, these two signals will not match.
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post #3 of 3 (permalink) Old 09-19-2017, 10:15 AM
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There are two major factors that go into the reduced towing capacity of the "non-big-tow" Armadas: The gear ratio and the rear suspension support.

First, let's talk about the gear ratio. The reason this is important is because of the torque that must be applied to the driveline in order to generate force at the pavement. The lower the gear ratio in the rear end, the greater the torque must be in order to generate the same force. So, if you look at the 2.94 vs. the 3.36 ratios, the former is going to require approximately 14% more torque at the transmission output, on the drive shaft and universal joints, and the input to the differential, to maintain the same pulling/driving force. These higher stresses require a reduction in the tow rating of the vehicle in order to prevent breaking things.

Second, let's talk about rear suspension. The rear springs have a particular spring rate (k) that is expressed in terms of force per unit displacement. In other words, so many pounds of force per inch of displacement. In order to tow safely, you must still have adequate suspension travel at the rated tongue weight. The addition of the airbags effectively increases (k) so that more force can be placed on the spring for the same displacement. This increases effective tongue carrying capability and maximum tongue weight.

With adequate aftermarket parts it is possible to achieve an improvement in tongue weight capability (even though doing so does not change what you can legally tow without recertification). It'll still drive a whole lot better with a higher effective spring rate even at moderate tongue weights. Similar improvements can be seen by using a weight distribution hitch, which you should always use if your trailer exceeds about half the weight of the tow vehicle or half the tow rating weight, whichever is less.

So, in short, here are the takeaways, bearing in mind that IANAL:

1) There is nothing you can do to your car with aftermarket parts to increase the legal tow rating of your Armada. If you are in an accident and are towing more than the nameplate tow rating, I believe you would be excoriated in civil court regardless of what you might have done to mitigate it. IANAL and YMMV, but my personal feeling is that I would never take that risk, even though I am sure people do it all the time.

2) That said, you can use common sense when determining the mechanical capability of your vehicle to tow a particular trailer load. The SAE J2807 tow rating test is harsh. If you are pulling on flat, level ground or mild grades, you can use that to determine a lower level of mechanical stress on your drivetrain, compared to the 5.5% average grade used in the SAE certification test. Accelerate slowly, climb gingerly, and just hope your u-joints don't break.

Last comment: I would not tow more than the nameplate tow rating, and just enjoy the better ride that aftermarket airbags and a WD hitch provide.

Whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things. -- Philippians 4:8
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