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Interesting write up.


To me all of this makes sense.


Typical fastening equipment is over rated by quite a bit as no one in the real world carries load cells to measure the load in the tow line.


Perhaps I missed the detail above but, I believe VW talks about not pulling at an angle. I venture a guess that if the recovery eyelet is torqued to the proper value there is a certain amount of side load carried and some side load is to be engineered for. Has anyone ever done a destructive test to the front tray that the recovery eyelets fasten to?
I'd be curious to see how much side load they can handle.


Also it seems that a beam mounted to the tow hook holes could probably carry the load of a recovery operation without a lot of concern... if it is rigid enough and mounted with the correct geometry.


I'm not curious to pull the bumper and see what will fit under there....
 

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Discussion Starter #42
Thanks Singh!
I'll try loading the photo again.
Pretty self explanatory, sample numbers are as in the report.
We never did find the missing piece of sample B, sorry....
Thanks for the testing results. These results would be significant to a straight line winch recovery, in vertical and horizontal planes per the owners manual. For real world testing to recovery in winch or strap, shear loads are the reality in 95% of all recoveries. The failures I observed firsthand of multiple eyelets were never failures of the loop. IME, the failures were shear force failures at a very low snap strap load, and they were at the neck of the tow eye. As I recall, in all instances threads stayed in the bumper, and the loop/eye was always intact, and still attached to the hardware on my strap.

When the bumper came through the cover (cayenne and panamera) the failure was observed corrosion on one bumper (colorado infamous mag cloride) and a bend to failure on bumper mount on the other.

Your results do indicate to me that if we assume a straight line vplane/hplane steady state (winch) recovery, the most likely weak link will be the hardware attached to the eyelet. Or in the case of the strap, the hardware attaching the strap to the eyelet.

Ideally, in a winch recovery in v/h plane would indicate your conclusions are true, all appears pretty beefy and overengineered. It's the other 95% of the time that is my primary concern. Drawing conclusions to that would require shear load testing at minimum.

Cheers and thanks for the data.

Scott J
07 v10tdi Xena
03 suburban 2500 Thor
 

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A bridle is a line used to connect two points on a common structure. I use a 20 foot core dependant line (non-stretch) with an eye at either end to connect the two tow eyes on a single bumper. That makes a nice 10 foot triangle with whatever you're pulling with attached at the apex, tow strap or recovery strap. Just feed the bridle line through the eye of the inline strap before shackling it to the tow eye. My tow eyes are fitted to be horizontal when secured, so the side thrust from the bridle is in an acceptable direction. Clear as mud?
And yes, all use of a dynamic strap should start out soft, preferably with a one meter "S" of slack. If you can't get it out in a couple of tries, you need to rethink your recovery plan, or risk serious damage!
Dynamic recovery straps are a useful tool, but a very dangerous one in unskilled hands.
Ok, here's some actual data for those of you who may be interested. I apologize for no pictures, can't seem to upload them right now.....
The following are the results of my personal research and investigation into the structural strength of using the "tow eyes" fitted to the VW Touaregs, both the 7P version (VW stated GVWR of 6305 lb) and the previous 7L version (VW stated GVWR of 6493 lb). (This also may apply to the Porsche Cayenne, as there is much component sharing between the vehicles.)
(All figures given in pounds force (lbf) and refer to relevant engineering standards describing minimum ultimate tensile strength (breaking strength))
-The detachable tow eyes have four threaded receptacles permanently mounted on each vehicle; two on the front bumper impact beam, and two on the rear bumper impact beam. The bumper impact beams are bolted to reinforced welded end-plates of the front and rear vehicle sub-frames longitudinal members.
( Note that in this application, all fasteners are subject to tensile force loading, and not shear force loading, which, although values are not normally provided, is considered to be only 60% of the fasteners rated ultimate tensile strength. )

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Detachable tow eye information:

-VW Touareg T3+ (7P series) part number 7P0805783
(M18-2.5 left hand) Forged pad eye (31mm dia.) flanged and machined base, slightly longer than 7L eye but with heavier stock (24mm dia.) to compensate, 18mm neck at the eye. Two tow eye of this style were tested.
-VW Touareg T1, T2 (7L series) part number 7L0805783A
(M18-2.5 left hand) Forged pad eye (31mm dia.) flanged and machined base, 18mm neck. Two tow eyes of this style were tested.

-Porsche Cayenne part number 95572115100 (however, it does appear identical to VW 7L units) was not tested.
-Audi Q7 part number 1T0805615A (longer bodied tow eye, no flanged base, seems to be used on most Audi and VW cars) was not tested.


Tow eye attachment thread strength:

(Steel grade is unknown, however, consider the fact that all other tensile fasteners used for Touareg bumper element attachment are grade 10.9)
For an M18-2.5 thread (minimum ultimate tensile load)
Grade 10.9 steel (possible) -44,962 lbf
Grade 12.9 steel (likely). - 52,605 lbf

Oval eye section strength:

Forging of eye side legs taper down to the eye tip semicircle, cross sectional diameter at tip is approx. 12.5 mm.
(Typical Breaking strength rating for a forged steel ring of that size is 11,450 lbf)

Tow eye testing:

All four tow eyes were tensile tested to destruction by a professional rigging company, on their large hydraulic rigging test bed. The results of each test were monitored by computer and a test result sheet and graph were produced for each. All four tow eyes failed in a virtually identical manner. After some elongation of the eye portion only, they fractured at the transition of the eye tip semicircle to either eye leg, completely separating the load bearing semicircle portion in each case.
7P type- Two new, unused eyes of this type were pulled to maximum load destruction, sample one failed at 39,300 lbf, sample two failed at 41,200 lbf.
7L type- Two new, unused eyes of this type were pulled to maximum load destruction, sample A failed at 39,100 lbf, sample B at 39,300 lbf.
In no case was there evidence of either attachment thread, flange, body or neck distortion, when inspected after destruction.

Due to the very close range of failure load values in the four samples, we can reasonably assume for our purposes that the OEM VW Touareg tow eyes are of very high quality and consistent strength, with an individual component minimum ultimate tensile (breaking) strength of 39,100 lbf.

Vehicle structure information:

Front Touareg bumper (impact bar crossmember) as fitted originally by VW, is attached to engine main subframe by eight M10 grade 10.9 bolts, four on each side (surrounding the tow hook attachment points).
Minimum Bolt strength, each - 13,556 lbf
Per side (4 bolts) -54,224 lbf
Total (8 bolts). - 108,448 lbf


Rear bumper (impact bar crossmember) as fitted originally by VW, is attached to rear main subframe by eight M12 grade 10.9 bolts, four on each side (surrounding the tow hook attachment points).
Minimum Bolt strength; Each - 19,716 lbf
Per side (4 bolts) -78,864 lbf
Total (8 bolts) - 157,728 lbf

(Front and rear bumper impact bar crossmembers are of varying designs, but are observed to be highly engineered items, comprised of both pressed and flat steel elements formed and welded together, with the full thread depth tow eye receptacles of machined steel stock, both flanged (against pull through) and fully welded into the built up structure, at the tip of a tapered steel cylinder, with its base surrounded by the four (per side) side frame attachment bolts. Both in compression, and tension, these structures appear extremely robust and capable of complementing the strength of the related fasteners.)
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My Conclusion:
To me, this information does indicate, that on a VW Touareg of any series, the factory towing eyes, and related factory original substructure, are designed and constructed to be of more than sufficient strength in order to allow safe recovery of the vehicle using either single or dual tow eyes at either front or rear locations. (It makes good sense to have the weakest link being the tip of the eyebolt, so as to mitigate structural damage to the vehicle in case of overload.) As to why VW does not indicate this ability specifically, or have a load rating indication on the tow eyes, is most probably due to liability concerns.

Notes, and things to bear in mind;
-Tow eyes must be installed as per the VW operators manual, ie, fully threaded in to seat the flange, and properly tightened.
-If a bridle is used, the tow eyes should be secured in the correct orientation for side loading (this can be done by inserting a shim washer, of no more than one thread pitch thickness).
-Please note that this information is only applicable to a factory equipped VW Touareg. If any of the substructure (bumper impact beams, aftermarket tow bars and/or fasteners) has been replaced or modified, weaker components may have been substituted. This especially holds true for the use of aftermarket tow hitches, aftermarket fasteners and aftermarket tow eyes.
-Retail vehicle recovery gear (tow straps, dynamic recovery straps, winch lines, etc) has no mandated safety factor, and as such, strengths specified are often average (not minimum) breaking strengths (regardless, most professional tow and recovery operations try to maintain a minimum 2:1 safety factor). This is not to be confused with professional rigging gear (for overhead lifting) which is normally rated with a working load limit (WLL), being the minimum ultimate tensile strength (breaking strength) divided by 5, to give a material strength safety factor of 5:1.
-Dynamic recovery, when properly done (ie, with a rated, rested line capable of proper stretch, and used as intended to free from a soft substrate such as sand, snow or mud, in a direct line and using the accepted one meter "S" of line slack) develops a surprisingly manageable force level. This article has the results of using a load cell to determine freeing forces,
https://www.outbacktravelaustralia.com.au/driving-towing-recovery-techniques/snatch-strap-loading-on-test
-The ISO Standard, 5422 -1982, which has been referred to here recently, should be recognized for what it is, a MINIMUM performance standard of a vehicle component performance.

Cheers,
R
Amazing reply thanks very much. Feel much more comfortable heading out on the sand this weekend. Spare tow eyes look to be about US$14. Just checking with an AU supplier and will post findings. Thanks again 🙏
 

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A bridle without a spreader bar would insure that there is an off axis load applied to one both eyes. Just an observation
 

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Yes, and the reduction of safe load for two eyes is an easy calculation knowing the fetch of the sling. All basic rigging practice.
 

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Using my 20’ bridle, the side angle on each eye is just less than 10 degrees, so the reduced load factor would be 80%. With two eyes, that would give a strength of 160% of the straight load on one eye.
Yes, a spreader bar would be ideal, but one may not be available, or required for the forces encountered when recovering correctly.
 
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Had a great weekend on the beach feeling comfortable about the tow points! First time on the beach with the 180tdi with air suspension. Was a breeze with staun deflators to 18psi and air set to highest mode. Although special 4wd mode (the highest) was speed limited to 20kmph. Wonder if obd eleven or vcds could shift the limit to 50/60 kmph.. Car felt like it came of age this weekend :)
 

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Had a great weekend on the beach feeling comfortable about the tow points! First time on the beach with the 180tdi with air suspension. Was a breeze with staun deflators to 18psi and air set to highest mode. Although special 4wd mode (the highest) was speed limited to 20kmph. Wonder if obd eleven or vcds could shift the limit to 50/60 kmph.. Car felt like it came of age this weekend :)
You don't want to go faster in Xttra mode because it will be real bumpy ride since you are on the bump stops at the high side of the suspension. That mode is to just get you over tall obstacles, not drive a long distance in.
 
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