Jerry Feather

My advise is: let it go. That is about three times what I would even consider if I wanted or even needed one. Part of my advise is also kind of subject focused.

I have now and for a long time given much thought to designing my own version of a 928 trailer hitch. I have no need for one myself, but given the need occasionally posted, I feel some desire to kind of try to help out. Most of my design thoughts have been focused on what now appears to be the very early version of the 928 hitch. However I am now very much inspired by what Kevin is posting about the 928 GTS version of the 928 hitch. Wow, is that ever a revelation for me. Basically, it converts all of the concerns from having all of the shift loads carried by frame cross members to having them all carried by the frame long members.

A 928 trailer hitch designed much like the GTS version can be made very simply. It takes the place of the rear bumper (not bumper cover) and can be made to even flex a bit with bumper impacts. When I get this all worked out I am going to make one for Jeremy designed to fit the car much better than the GTS version and designed to meet his need for 21 inches. I am working on that now.

When I get this all worked out I am going to make one for Jeremy. I need some more pictures form Jeremy and Kevin.

Petza914

Would be even better to design it with a simple 2" receiver instead of the curvy ball thingy so anyone could adjust their lift or drop just with the ball hitch they choose and also use universal accessories like bike racks, enclosed cargo boxes, a MX bike cross transporter, etc.

Gary Knox

For a light weight trailer, about 20-25 years ago a 928 owner and track enthusiast in Indiana (Randy Faunce, DMD) worked out a US Standard hitch receiver system that mounted to the rear bumper 'channel barr' behind the plastic facade. Cut a 2" square hole in the plastic bumper behind the license plate, buy a '60's GM fold down license plate holder (several GM cars had the gas filler behind the license plate). Then, when you have the insert with the ball removed, license plate covers hole, and everything looks OE. Takes about 1 minute to install the square extension with the ball into the receiver, and same time to remove it. I had one on my '89 track car, and the current owner probably has the full set up. My 2 wheel Harbor Freight trailer weighed about 1K with 4 tires and a custom made 4 ft tool box full of 'stuff', and balanced to have about 30 lbs weight on the hitch ball. I used the system for going to and from tracks for about 8-10 years with no problems. Tracks were 250 to 850 miles away from home. I have a sketch Randy provided me - and I used to make the one for my car. Having been transferred through about 5 generations of computers, it's a bit difficult to interpret, but anyone who'd like one - drop me a PM, and I'll reply with what I've got! I also have enough pictures of the items and installation to make it very easy to replicate. One 'ready to go' picture of the car/system attached.

Gary

Jerry Feather

Good Point. That is still in the design stage.

76FJ55

I agree that you will need some more pics to get the full image of how the GTS hitch is designed.

A few observations from the limited pics posted so far.
In the area identified in red, there is obviously additional structure added below the body. I can't tell the extent of this addition or where it may brace to the chassis though.


In the area identified in red. again it is a bit of a guess, but it looks to me like you can see the rear edge of a plate which I presume runs forward between the battery box and fuel tank similar to the forward torque arm on the original hitch design.


Based on the assumptions above, I don't really see any significant advantage in designing around the GTS hitch, the only thing it does is eliminate the original aluminum bumper under the bumper cover.

If I were to redesign the hitch, I think I would follow the original hitch design, but face the "C" shaped mounting ears both open to the left. this would allow you to slide the hitch over the original bumper shock mounts without having to remove the bumper itself.. It would need to slide in from the right as the battery box is on the left and is non-moveable.

I like the idea of keeping the original aluminum bumper too simply due to the fact that it follows the contour of the pumper cover better that the hitch and doesn't have the sharp plates at the end that will potentially cut into the cover it ever bumped from the rear. it also has the mounts for the rear bumperettes, which if lucky would be the point of contact on the rear, and it doesn't appear the hitch has provisions for those. .

76FJ55

Yes, I totally agree it would make much more sense to use a standard 1-14" or 2" receiver design. 1-1/4" would probably have sufficient capacity for what most need and may offer better rear ground clearance, but if ground clearance isn't a concern the 2" receiver would be more adaptable.

Jerry Feather

I disagree with Simon's conclusion about which style of hitch would be best. I think that the GTS design was created just because of the shortcomings of the earlier design.

I have gone thru several different versions of a design based on the GTS style, including a means to install the hitch and then either have it become the rear bumper or now I am working on a design to be able to install the hitch then put the bumper back on behind it.

I don't know if there is any kind of center brace with the GTS hitch but even if there is I strongly disfavor it. I think it is there only to resist the twisting moment of the hitch but to the extent that it is there to resist the pulling (or pushing) forces I disfavor it. I think all of the forces should be directed only to the rear ends of the long frame members. To the extent that some resistance to twisting might be desirable I would direct that to the rear cross member only.

My current hitch design will be made of some box tubing in about 4 or 5 inches by one inch if that can be found. It will mount to the bumper shock mounts under the bumper shocks which will space the bumper back by the thickness of the mounting plates. If that becomes a clearance problem then the rear edges of the bumper will have to be shaved. The hitch will reduce the amount of travel for the bumper in an impact, but that will be better then none and probably just a good as what there is now since I think just about all of the shocks are dry and of little impact use anyway.

I am inclined to incorporate the 2 inch hitch receiver.

hwyengr

Wow, Jerry, that’s incredibly generous.

I’m super concerned about towing liability (Thanks, GM, for putting a sentence in the owners manual for my Chevy SS that it was not designed to tow, when the Holden variant of the exact same car is rated at 3300lbs.). How close do you think your design is going to be on mounting to the factory equipment locations?

Jerry Feather

What I think is going to be my final or near final version of this new Hitch is finally coming into focus in my mind. In fact, I have kind of started a little bit of prototyping of in in my shop. I have abandoned the idea of using box tubing in the main member of it for side to side in place of using probably quarter inch steel plate there. That will restore much of the lost bumper travel in an impact situation. For the stiffness needed I will add about a 3/8 inch cross ways bar from side to side mounted horizontal and being about 3 inches front to back for most of it. The other mounting detail are just that - details. I have most of them figured out also.

Jerry Feather

I think almost none of them. That is if you are referring to the factory hitch mounting locations. It is my perception that the strongest points on the rear of a 928 are the two locations where the rear bumper mounts. That is where I intend that my design of hitch will mount, and almost only there. The "almost" is that in addition to mounting my hitch to the bumper shock locations I am going to have an additional mounting point just under each location into the bottom of the end of the frame rails. I think that will be the extent of it with just four bolts at each end of the hitch.

76FJ55

To some extent that aligns with where the factory hitch mounts. The factory attaches where the rear bumper is attached to the chassis. There is however the additional torque arm that the factory extends forward along the center line of the car between the battery box and fuel tank and attaches at the forward inboard corner of the spare tire well.

The criticality of the torque arm will depend on several factors including; how much you are wanting to tow, tongue weight, how high the ball is relative to the bumper attach line and how far the ball extends back behind the bumper attach line. the greater any of these values are, the more critical the hitch torque arm becomes. .

Gary Knox

Reference my post regarding Randy Faunce as the 'designer' of the hitch I built. He stated that the hitch Porsche sold for the 928 was not approved by the NHTSA, and he indicated that his design was approved (or approvable).. Like Sargent Schulz, 'I know nothing" about this, but perhaps our 928 fellow member Otto (known on Forum as Eplebnista) might know about NHTSA requirements).

Just a thought for anyone contemplating a new design.
Gary

76FJ55

I feel this is akin to H4 headlights not being DOT approved.

Did Randy give any idea what his design would be rated at? I can't imagine it actually being more robust than the factory hitch since his design is relying on the aluminum bumper and the bumper shocks before mounting yo the tub at the same point the factory hitch mounts. And the factory hitch adds additional structure going forward to the chassis above the trans.

I'm in no way saying you hitch is insufficient for what you do with it, but I have a hard time believing it to be structurally superior to the factory hitch.

I believe this may be the sketch for one of the other hitch threads.

Gary Knox

76FJ55,

Yes, your post is the exact sketch that Randy Faunce provided me, but much more precise than the one I currently have in my files. I too doubt that this hitch system is as robust as the Porsche design, and as I said, I have no idea why the P-car design might not have been 'legal' per NHTSA..

But then, NHTSA mandated the change in headlights for '68 and later E-type Jaguars (ruining the design effect), but currently allow headlights on large SUV's and Pickups to be at or above eye level of oncoming 'standard sedan/sport car' drivers. Nearly blinding those drivers (at least me) on many occasions. As a deceased friend used to say "there are times when I think they have their ladder up against the wrong wall".

That tow system was certainly adequate for my needs, but I would not use it for a heavy tow.

Gary

Jerry Feather

Although I think my resulting design is going to be stout enough to pull just about anything one might want to tow with their 928, I do have to wonder just what kind of trailers and loads anyone might want to pull with their 928.