Beware: Trail Turn Assist causes axle weld failure that dealer won't warranty!!!

[Merc4x4]

Why do you use a long handle breaker bar, or slip a pipe over the handle to lengthen it even more to loosen stubborn bolts?

Well, that handle is the tire radius and the stubborn nut is the axle tube when using TTA.

Bigger tire diameter, means bigger radius, means more force applied to the center for the same amount of friction/traction on the tire tread (outside).

As explained in this thread, I read the break is fixed to the axle tube. Now, instead of breaking an axle (common when going to large tires), if the break has enough clamping friction to keep the tire from rotating, all the force is directed to the axle tube.

I don't know exactly how all that pans out, but this is my explanation of the simplified physics model people are referring to. It has nothing to do with engine torque. You can do the same by towing a dead vehicle with 1 rear break locked. And I think the Bronco has open diffs, so all engine torque would be directed to the wheel with least resistance; the outside tire in the case of TTA.

I would think you'd need an awful lot of friction between the tire and ground to break the plug welds, but that is a complete guess on my part. I do know as you move up tire diameter you need less traction to apply the same breaking force to the axle or, the point of this thread, the axle housing.

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[ByrdDogX]

So if I have low profile mud terrain 40's with 26 inch rims and a 12 inch lift you are saying I should not use trail turn assist to get out of the starbucks drive thru?

Only if you do it under 5mph and someone has spilled a Venti Frappuccino in the drive thru lane to wet the surface.

 

[flip]

Great analysis! However the axle TUBES are well supported against rotational forces on both sides on top and bottom. The top “trailing arm” support has a 7” offset from the center of the axle. The bottom trailing arm support has a 4” offset from the center of the axle. The 4 of these links are able to carry a HUGE torque load (moment) that is placed on the axle tube by TTA, braking (@ 100 MPH) wheel torque or whatever.

The only ”un-restrained“ torque in the system is as you indicated, the pinion climbing up (or down?) the ring gear resulting in the opposite torque of the differential housing. The only restraint to this force is the trailing arm axle tube supports, which will not allow the axle tubes to twist. This places all of the torque stress on the 6 plug welds.

I’m one of those cursed mechanical engineers….

The the driveshaft is transferring all of the force through the carrier side and pinion gears. There should be no stress on the housing unless the outside wheel is inducing it. Should be no to almost no power transfer to the pivot wheel/axle. Now, if the outside wheel gets traction and grabs hard, yes, there could be power transferred to the other one. The assumption is the rear carrier is truly unlocked when the turn happens and it's operating like the standard "open" differential. If it's somehow staying locked, then you will have torque transfer to both axles but the pivot wheel is being restrained by the brake caliper via the housing.

Has anyone with a popped weld indicated what direction they were turning when this happened? Theoretically the pivot side should be the one that breaks, if not something else is going on.

 

[flip]

As a side note, the few times I've seen TT used, it seems like the vehicle is under load. Yes, I get we're trying to push the front around one of the rear wheels but to me, it give me the feeling the rear end isn't disengaged. I'm probably wrong about this but the pivot should just be a dead drag, no power applied both due to the open diff and brake that is supposed to insure it doesn't spin if it gets more traction than the outer wheel.

 

[Coloradonatives]

I am getting sick of how often Ford blames the driver for using the vehicle for what it was built for. I would never use the feature on anything other than slick surfaces though. Should have just had a true dig then it would not happen.

You don't read and understand before spewing your venom, do you. His vehicle is NOT stock, certainly NOT "for what it was built for". He is whining for a freebie and looking for support. You give him that.

 

[Tricky Dick]

The top “trailing arm” support has a 7” offset from the center of the axle. The bottom trailing arm support has a 4” offset from the center of the axle.

The rule of thumb for link separation is 1" for every 4" of tire so we're certainly solid in that aspect.

 

[Bronkers]

Great analysis! However the axle TUBES are well supported against rotational forces on both sides on top and bottom. The top “trailing arm” support has a 7” offset from the center of the axle. The bottom trailing arm support has a 4” offset from the center of the axle. The 4 of these links are able to carry a HUGE torque load (moment) that is placed on the axle tube by TTA, braking (@ 100 MPH) wheel torque or whatever.

The only ”un-restrained“ torque in the system is as you indicated, the pinion climbing up (or down?) the ring gear resulting in the opposite torque of the differential housing. The only restraint to this force is the trailing arm axle tube supports, which will not allow the axle tubes to twist. This places all of the torque stress on the 6 plug welds.

I’m one of those cursed mechanical engineers….

This, coupled with @BigMeatsBronco intro seems like it was getting close

Taller tires definitely seem like they would 'push back' against the (locked) brake-and-axle-tube, harder-than-stock. Not the weight of the tire, but the distance from center-hub to ground contact patch, goosing the horizontal axis thru the axle centerline, and a larger contact patch, goosing the vertical axis thru the wheel/tire centerline.

I'm no expert on involved formulae but any perceptively 'minor' differences in these dimensions may be inducing major multiplied stress. And yet THEN, we have the link mounts obviously fighting all of that, to a point, BUT could there be enough slop in the link bushings to pop these welds?

Wonder what OP's link bushings look like. The lack of additional pics is unhelpful.

And, exactly how the diff is reacting is also a great question per @flip but I thought these were 'standard Dana-spicers' that the collective brainpool here would have fully explained by now...where's my FEA images and animations?

 

[RHeinz]

The the driveshaft is transferring all of the force through the carrier side and pinion gears. There should be no stress on the housing unless the outside wheel is inducing it. Should be no to almost no power transfer to the pivot wheel/axle. Now, if the outside wheel gets traction and grabs hard, yes, there could be power transferred to the other one. The assumption is the rear carrier is truly unlocked when the turn happens and it's operating like the standard "open" differential. If it's somehow staying locked, then you will have torque transfer to both axles but the pivot wheel is being restrained by the brake caliper via the housing.

Has anyone with a popped weld indicated what direction they were turning when this happened? Theoretically the pivot side should be the one that breaks, if not something else is going on.

This thread has required me to crawl under my Bronce rear drive train numerous times…..COME-ON…. I’m 72 YO….

Because of this thread, I have observed a lot about the rear suspension. A good thing! I also want to get my TIG machine under there and redo one of the plug welds!

These are the 2 “struts” that are attached to the rear axle tubes on either side of the Bronco rear suspension. To me, they look pretty stout, especially compared to the rear leaf spring on my past CJ’s. These 2 “struts” will prevent any torque (bending moment) from being passed down the axle tube from the tires or mostly brakes. The torque from the engine/tire traction is going to pass down the (interior) axles to the differential, ring gear, pinion and housing.

I’m a mild Off-Roader. However, I am going to keep an eye on the condition of the 6 plug welds.

 

[indio22]

This, coupled with @BigMeatsBronco intro seems like it was getting close

Taller tires definitely seem like they would 'push back' against the (locked) brake-and-axle-tube, harder-than-stock. Not the weight of the tire, but the distance from center-hub to ground contact patch, goosing the horizontal axis thru the axle centerline, and a larger contact patch, goosing the vertical axis thru the wheel/tire centerline.

I'm no expert on involved formulae but any perceptively 'minor' differences in these dimensions may be inducing major multiplied stress. And yet THEN, we have the link mounts obviously fighting all of that, to a point, BUT could there be enough slop in the link bushings to pop these welds?

Wonder what OP's link bushings look like. The lack of additional pics is unhelpful.

And, exactly how the diff is reacting is also a great question per @flip but I thought these were 'standard Dana-spicers' that the collective brainpool here would have fully explained by now...where's my FEA images and animations?

Friction of tire to surface is key though. Regardless of tire diameter (lever length analogy), friction is going to be the same (generally speaking), and friction will be overcome same. So are we really going to see greater leverage?

 

[BigMeatsBronco]

This thread has required me to crawl under my Bronce rear drive train numerous times…..COME-ON…. I’m 72 YO….

Because of this thread, I have observed a lot about the rear suspension. A good thing! I also want to get my TIG machine under there and redo one of the plug welds!

These are the 2 “struts” that are attached to the rear axle tubes on either side of the Bronco rear suspension. To me, they look pretty stout, especially compared to the rear leaf spring on my past CJ’s. These 2 “struts” will prevent any torque (bending moment) from being passed down the axle tube from the tires or mostly brakes. The torque from the engine/tire traction is going to pass down the (interior) axles to the differential, ring gear, pinion and housing.

I’m a mild Off-Roader. However, I am going to keep an eye on the condition of the 6 plug welds.

in 4lo you can apply the brakes and the throttle and literally watch the bushings compress and the axle begins to twist. the deflection of the rubber is possibly enough to create a twist on the metal tube during TTA event. the rubber flexes but the plug welds do not.

additionally TTA is not a smooth hydraulic type operation it is electrical and the brake mechanism bis electrical not hydraulic...
this could explain the un smooth jerkyness that the braked wheel seems to exhibit. mine seems like it's constantly unlocking and relocating or trying to match the inner tires speed to some algorithm? definitely could put short but heavy shock loads on those plug welds.

it's likely a combination of several things together, and could also be related for what @flip mentioned about the locker not being fully released when TTA event is happening....this could definitely cause damage!

 

[BigMeatsBronco]

Friction of tire to surface is key though. Regardless of tire diameter (lever length analogy), friction is going to be the same (generally speaking), and friction will be overcome same. So are we really going to see greater leverage?

whatever friction or traction force is available is magnified by a bigger arm (tire size) so yes the bigger tire is like a longer lever with the same preasure (traction) on the end....the longer the lever the more the force is amplified.

 

[Aaonter]

You don't read and understand before spewing your venom, do you. His vehicle is NOT stock, certainly NOT "for what it was built for". He is whining for a freebie and looking for support. You give him that.

Thing about a solid rear axle is for the most part is the geometry gearing wise does not change with a lift. The front cv I would agree. “If you actually read his post”. He has the sas bronco that comes with 35s. He only went up 2” which is a difference of 5%. Engineers should know anyone buying a vehicle to go against a Jeep is going to lift but that is very mild. Get out of your blue oval fanboy and help a fellow wheeler out. Should not have happened if the feature was properly tested.

 

[Tricky Dick]

additionally TTA is not a smooth hydraulic type operation it is electrical and the brake mechanism bis electrical not hydraulic...

I always assumed it was using a type of ABS pulse with the hydraulics. Is it the parking brake that does the work? That seems less than optimal.

 

[BigMeatsBronco]

I always assumed it was using a type of ABS pulse with the hydraulics. Is it the parking brake that does the work? That seems less than optimal.

uncertain, however I thought I read the models with TTA have a totaly different electronic brake booster, mine sounds like a spring stretching or vibrating when activated and with the window rolled down I can hear the rear brake applying and releasing and its a NOT smooth linear action...

I've used it alot, mostly in gravel or mud. does not work well in deep snow.

 

[flip]

Anti-Lock Brake System (ABS) and Stability Control - Overview
Overview

The ABS and stability control systems are comprised of the following subsystems which assist the driver in maintaining control of the vehicle:

• ABS
• Curve control
• EBB
• Drive away release (EPB )
• EBD
• EPB control
• ESC
• Hill start assist
• RSC
• Selectable drive modes
• Supports adaptive cruise control
• Supports collision avoidance
• Torque vector control
• Traction control
• Trail control
• Trailer sway control
• Trail one pedal drive
• Trail turn assist

The ABS helps maintain steering control during hard braking by preventing wheel lock up. The ABS also includes a brake assist function which provides maximum brake system pressure during a severe braking situation.

Curve control enhances then vehicle’s ability to follow the road when cornering severely or avoiding objects in the roadway.

Some vehicles are equipped with an EBB unit. The EBB replaces the vacuum brake booster and master cylinder with an electric motor and linear actuators to assist with vehicle brake applications.

The drive away release function automatically releases the electric parking brake once certain conditions have been met.

The EBD system helps maintain vehicle control by keeping a balanced braking condition between the front and rear wheels.

The ABS module is the ECU for the electric parking brake system. For information on the electric parking brake system,
Refer to: Parking Brake - System Operation and Component Description (206-05 Parking Brake and Actuation, Description and Operation).

The ESC system helps prevent skids or lateral slides by modulating brake fluid pressure to individual brake calipers and reducing engine torque.

The hill start assist feature is designed to assist the driver during hill-starts. Using the ABS , the hill start assist system holds the vehicle on an incline for a short time, allowing the driver to release the brake pedal and press the accelerator pedal without needing to use the parking brake.

The RSC system helps prevent excessive vehicle roll by modulating brake fluid pressure to individual brake calipers and reducing engine torque.

The selectable drive mode system helps maintain vehicle traction by adapting the responses of the engine, transmission, AWD system, the EPAS system, the ABS and the stability control system to the demands of the terrain.

The ABS supports the adaptive cruise control system by applying the brakes as necessary to maintain the distance gap set by the driver. For information on the adaptive cruise control system,
Refer to: Cruise Control - System Operation and Component Description (419-03B Cruise Control - Vehicles With: Adaptive Cruise Control, Description and Operation).

The ABS supports the collision avoidance system by monitoring information and precharging the brake system allowing the vehicle to stop in the shortest distance possible. For information on the collision avoidance system,

Torque vectoring control applies brake torque on the inner driven wheel in a curve for better traction to avoid an understeer, or oversteer situation.

The traction control system helps prevent loss of traction by reducing drive-wheel spin during acceleration.

The trail control feature maintains a set vehicle speed below 31 km/h (20 mph) in high range and below 15 km/h (10 mph) in low range.

The trailer sway control system helps maintain vehicle stability while towing a trailer by detecting and aiding in the reduction of conditions causing trailer sway.

The trail one pedal drive system assists when driving through difficult off-road terrain by allowing the driver to accelerate and brake using only the accelerator pedal.

Trail turn assist helps to reduce the turning radius of the vehicle by applying the brakes to the inside rear wheel in low-speed, high steering-angle maneuvers.

Some noise from the system and pulsations in the brake pedal are normal conditions during most ABS and stability control system activations. Longer than normal brake pedal travel may also be experienced immediately following an ABS or stability control system activation.

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