300SD Trailer Hitch Receiver - Update - PeachParts Mercedes-Benz Forum

Gene Horr · #1 05-07-2009, 09:35 PM

About a month ago I posted that I was in the process of designing a proper receiver for the W126 model as there were none on the market. Since then I've been receiving messages asking for updates and so I decided to post a status update and list a couple of problems that I've found.

Tax season threw a, well, a hitch in the process and delayed things. Then trying to get the bumper off delayed things further. I am supposed to meet with the fab shop tomorrow with the mockups and after that I should know when the first model should be ready. My goal is to have it in time for the Texas run in June.

The biggest problem I had was getting the bumper off. The sort of good news is that apparently the vehicle has never been in a wreck and all of the bolts are original. The bad news is that all of the bolts are original. Some of the nuts came off fairly easily, some required a day of Kroil soaking, and one of the bolts wound up shearing from the force trying to remove it. This sheared bolt is one that I recommend everyone to inspect as there is a slight design, er, oversight in the vehicle which can cause a long term problem.

There are six bolts holding the two bumber shocks to the body. five of these bolts are in cavities with water drains. One is not. That cavity had filled with water over the years and the bolt was rusted solid. The bolt is on the left hand (driver's side in the US) shock and is the single bolt on the interior side of the shock. To reach the nut there is an access flap in the triangular area between the spare tire well and the side storage wells. Lift the flap then remove the foam rubber. If you've had leaks in the trunk in the past you'll probably find a mess. As a safey issue I would recommend everyone to inspect this bolt and replace if damaged.

When it came to design I was considering two options. The simplest and cheapest method would be to use the European hitch as the inspiration. The second is to use the 300D receiver made by Curt. After looking at the bumper it became obvious why the European design was not used in the US and so why the US version was not rated for bumper towing. The US version has the bumper shocks, the European does not. Since the bumper receives the main force from towing and you can't use the shocks for towing this eliminated this design.

The next design is to mount to the body at the bumper points similar to how the 300D Curt receiver is made. This is a far superior design as this point of the body was designed to handle high force and in the end this is where the force in the European design is ultimately applied.

There are a couple of problems that I had with the 300D design shown in this thread . The first is that the main force is not fully carried by the bumper mounting points but by the storage wells. These wells do not appear designed to be a high stress point. Admittedly the loads that these engines can tow is fairly light anyway and the design would probably be fine in most cases. However I prefer a stronger attachment. The second issue is that by placing the plate behind the shock it moves the bumper back and so the side trim no longer matches the wheel wells. I had hoped that it would not be noticeable but unfortunately for me even the 1/4" steel produced an objectionable gap.

The solution I came up with is to have angle channels for the vertical portions with spacers designed to fit within the ribs of the shock. The longer replacement bolts will be either stainless steel or galvanized to help prevent corrosion in the future. I will also look at a possible drain for the one bolt mentioned above. Even stainless will corrode if kept underwater and away from oxygen.

The angle channels (two on each side) will then be welded to a square tube for the horizontal member. The receiver will be welded to the bottom of the horizontal member. The only major part visible will be the receiver itself. Most of everything else will be behind the bumper. The receiver will be 2".

mobetta · #2 05-07-2009, 09:47 PM

do you have forward bracing planned?

the TD hitch(which is the one your somewhat copying= not the D) uses the box frame, not the storage wells- it is quite strong. and 4/6 of the bumper mount bolts are replaced w/ 3/8 grade 8 hardware- with antisieze, these high grade bolts should be fine.

good luck.

Gene Horr · #3 05-08-2009, 10:10 AM

Corrections noted. Thanks. I now see the frame you are talking about. The 126 appears to have a similar design.

pawoSD · #4 05-08-2009, 11:14 AM

DaLan used to (or may still) offer a receiver that bolts to the bumper and underside of the back of the car. I have such a setup and have used it many times with no issues. Here it is:

In fact I just used it this morning to pull a trailer of junk to the junk yard about 25 miles round trip.

oldsinner111 · #5 05-08-2009, 11:21 AM

How did you wire for trailer? Will stock flasher work?

al b · #6 05-10-2009, 10:01 PM

Most people don't understand that pulling a load down the road is the easy part . I would not doubt that these little cars would do a worthy job .What most people don't think about is stopping !!! My 300SD has new brakes and there no way I would pull any kind of a load across the street much less down the road . Not trying to upset anyone just my two cents.

cmbdiesel · #7 05-10-2009, 10:16 PM

Quote:

Originally Posted by al b

Most people don't understand that pulling a load down the road is the easy part . I would not doubt that these little cars would do a worthy job .What most people don't think about is stopping !!! My 300SD has new brakes and there no way I would pull any kind of a load across the street much less down the road . Not trying to upset anyone just my two cents.

Indeed, braking is the most important issue when towing. I would probably feel comfortable towing under 1000 lbs with a rig like pawoSD's, but I would give myself plenty of stopping distance.

pawoSD · #8 05-10-2009, 10:16 PM

Quote:

Originally Posted by al b

Most people don't understand that pulling a load down the road is the easy part . I would not doubt that these little cars would do a worthy job .What most people don't think about is stopping !!! My 300SD has new brakes and there no way I would pull any kind of a load across the street much less down the road . Not trying to upset anyone just my two cents.

I can lock up my tires pretty easily with my cross drilled rotors and ceramic PBR pads.....I can barely even tell my trailer is attached even with over 1000lbs loaded on it. Stopping is not an issue.

300SD's have awesome brakes. As good or better than a lot of pickups on the road....

pawoSD · #9 05-10-2009, 10:17 PM

Quote:

Originally Posted by oldsinner111

How did you wire for trailer? Will stock flasher work?

You need one of those converter boxes that combines the separate brake/blinker circuits to operate a single light on the trailer. About $10-15 at most auto stores....and easy to splice into the wiring.

300SD81 · #10 12-28-2009, 11:20 PM

Old thread, but have you finished this? I'm trying to build a hitch for my SD and some pictures would be awesome.

seanarcher · #11 12-28-2009, 11:36 PM

i had a hitch on a w123 that attachted to the three bolts on each side of the two bumper posts.It looks like there is the same thing on a w126.anybody know if this is true?if so i could just buy the same hitch for the w123 i got on that body.

4x4_Welder · #12 12-29-2009, 12:34 AM

I've never been fond of towing with unibody cars. I had an old Galaxie that had this massive 2" receiver hitch setup, but it bolted to the rear frame rails and was quite stout. That bumper hitch scares me-

Quote:

Originally Posted by pawoSD

300SD's have awesome brakes. As good or better than a lot of pickups on the road...

I'm actually using mostly 300sd brakes on my Courier. Front calipers with Nissan Pathfinder rotors, and the rear brakes complete. It won't tow much, it's more a trail toy and awesome economy commuter, so those brakes will be quite adequate.

layback40 · #13 12-29-2009, 05:23 AM

I build a tow hitch for my old 300d about 20 years ago, it is mounted between the bumper and the body using the existing bumper mounts and has a plate that bolts to the underside of the spare well. The biggest thing I have towed was a New Holland round bale hay baler. It weighs about a ton and a half, not braked and is unsprung on balloon tires. I had to move it about 150 miles. It pulled it well, got up to 45 mph a few times. Head winds were a pain as the baler is aero-dynamically like a bucket being dragged through water.
I need to make a hitch for my 124 wagon soon.

layback40 · #14 12-29-2009, 05:24 AM

I build a tow hitch for my old 300d about 20 years ago, it is mounted between the bumper and the body using the existing bumper mounts and has a plate that bolts to the underside of the spare well. The biggest thing I have towed was a New Holland round bale hay baler. It weighs about a ton and a half, not braked and is unsprung on balloon tires. I had to move it about 150 miles. It pulled it well, got up to 45 mph a few times. Head winds were a pain as the baler is aero-dynamically like a bucket being dragged through water.
I need to make a hitch for my 124 wagon soon.

MaydayMike · #15 12-29-2009, 09:07 AM

http://www.etrailer.com/pc-H~11711.htm?vehicleid=19836171

Looks like you can still buy this one. It looks to be the same one in pawoSD's post.