T@B Pushes TV Around Longitudinally

sequimite

My TV is a 2015 Mercedes-Benz GLK 250 (diesel). Plenty of power and good handling towing my 2016 320 S Max. However, I get significant longitudinal push/pull action on the TV from the T@B, especially at lower speeds on uneven road surfaces. Speed bumps are really exciting! Above 50 mph it diminishes quite a bit, but is still noticeable.

My prior trailer was a LG 5-Wide Platform with a dry weight of around 1000 lbs. including my added storage boxes, I never had this issue with that trailer. Given that my T@B weighs at least 700 lbs more, it likely that explains the difference.

I have tried to mitigate this issue by using a Curt RockerBall Cushion Hitch Trailer Ball, but don’t notice much difference. I have read that the RockerBall requires break-in for optimal performance and I have only used it for about 10 miles, so it’s still fairly virgin.

Has anyone else experienced this issue? How did you address it?

MuttonChops

@sequimite;  At one time I considered getting the Curt RockerBall.  Then I tried a receiver/draw bar tightener.   The tightener fully resolved the for-aft jerks on my rig.

Do you have a tightener installed?

techietab

MuttonChops said:

@sequimite;  At one time I considered getting the Curt RockerBall. 

When I first bought my Tab, I installed a Curt RockerBall on my tow vehicle, and had a lot of noticeable forwards-backwards jerking, especially when braking. After several months, I removed it and replaced it with a normal Curt hitch mount and ball, and all of the jerking motion disappeared.

sequimite

@MuttonChops, I don't have a tightener installed, but will definitely try that. @techietab, I started out with a standard drawbar and ball. The issue was at least as bad, if not worse. Thanks to both for your help!

techietab

Out of curiosity - what brake controller are you using?

VictoriaP

Also out of curiosity, did you have any water in your tanks? They should either be empty or full. Partially filled allows for wave action while towing, and you’ll definitely feel that.

Three things made towing more comfortable for me: More weight on the tongue (12-15%). I went with a Convert a Ball hitch insert/ball mount, which smoothed things out somewhat (couldn’t get the height right with a Curt Rocker). And ultimately added a hitch tightener like MuttonChops uses.

sequimite

@techietab, I believe the brake controller is a Tekonsha Primus IQ. @VictoriaP, all the tanks were empty when I towed with a standard bar mount and ball. I did have tanks about half full when I tried out the RockerBall, so that is something to consider. Anyone have thoughts about these two hitch tighteners? My concern about the standard type is that it accommodates a 2" receiver, but mine is 1.25".

https://www.amazon.com/gp/product/B0068D3WLW?psc=1
https://www.amazon.com/gp/product/B07JDD8L3H?psc=1

I will get one of these and try it out.

kottum

If you have a 1.25" receiver do you have a 1 7/8" hitch ball, or the correct 2 5/16" hitch ball to match your trailer coupler ?

Denny16

The TaB uses a 2” hitch ball kottum, not a 2 5/16” ball.
cheers

sequimite

I have a 2" ball.

Denny16

Grand, you are all set.  Just get a hitch tightener to eliminate the sudden push/pull jerk when going slow.
cheers

kottum

Okay, backing up a bit.  Yes, the correct Tab 320 hitch ball size is 2" (my other trailers all require a 2 5/16 ball), my error.

But I don't think you are all set.  I believe your 1 1/4" receiver is a Class I hitch, rated for up to 2,000 lbs.  Would your Tab 320 weigh under 2,000 lbs when loaded for travel ?

Also, your Mercedes Diesel engine has plenty of power to pull your Tab, but is the chassis rated to pull it ?  Can you fit a larger hitch receiver if needed?

The other thing I was thinking about was the front to back level of the Tab when hitched up.  Is it level or slightly nose down, that would be good.  If it is nose high, depending on loading, it may not have enough hitch weight.  This could cause some porpoising which may be felt as longitudinal push-pull.  At higher speeds the wind will keep the trailer pushed back, mitigating the effect.

But then, as they say, never believe anything you read on the internet.  So take the time to learn good towing practice, and experience is the best teacher.

techietab

Looks like you probably aren't experiencing a brake controller quirk (as can happen with bluetooth models especially).

But as others have pointed out, towing with a 1.25" hitch is potentially an issue if it's a Class I hitch. Class II hitches still have a 1.25" inch receiver but are rated for up to 3500 lbs, which aligns to the towing rating for your GLK...so hopefully that's what you have.

sequimite

Thanks for the caveats. The vehicle is rated for a trailer weight of 3500 lbs. The hitch is a Class II with a max tongue weight of 280 lbs and max trailer weight of 3500 lbs. The level of the trailer with no load in the back of the Benz is dead level, so should be slightly down with the vehicle loaded. I figure the trailer loaded for travel will weigh near 2200 lbs. 12% on the tongue is 264 lbs, so I'm close to the max on tongue weight, but lots of headroom on max trailer weight.

Here is an article with the same vehicle towing a 5500 lb Airstream. Not recommended!
https://www.rvlifemag.com/towing-test-mercedes-glk-250-bluetec/

CharlieRN

I wonder if some of the issues that @sequimite is experiencing are related to the relatively short (108") wheelbase of the GLK-250? I'm far from an expert on the topic however, hence the question rather than comment.

kottum

Yes, Andy Thompson at Can-Am has shown the GLK 250 can tow an earlier model Airstream 25.  Even small turbo diesel engines are quite capable.  This is probably the most innovative hitch shop in North America, and that's the reason it works.  This  GLK 250 has a custom-fabricated heavy-duty hitch assembly with a stiff torsion bar linking the hitch to a mounting point near the rear axle to prevent the hitch from twisting.  That enables use of an adequate weight distribution hitch to lift and transfer hitch weight and dynamic loads to the car's front axle and trailer axles. The results are quite decent.

Don't try this at home unless you know what you are doing.

Mickerly

The airstream is also a lot less aerodynamic drag than anything NuCamp produces. 

kottum

Keeping up the guesswork here, trying to help a fellow Tab owner.  Not really sure what "longitudinal push/pull action" means.

Noting the GLK 250 is eight years old, the shock absorbers could be worn and unable to control the movement of the extra weight on the vehicle.  

I don't know what you carry in your car, but people often load the back heavy without regard for individual axle ratings.  Then drop the trailer tongue onto it as well.

And even though the car is rated to tow 3500 lbs, any weight carried inside the vehicle reduces that capability by whatever weight is loaded inside.  Sometimes this is evident in the tow vehicle's GCWR (Gross Combined Weight Rating), which includes loaded tow vehice and loaded trailer.

If the problem persists, you may want to check your shock absorbers for function under load, and do some weighing at a Cat scale.

sequimite

@kottum, all good ideas. By longitudinal, I mean fore and aft--the direction of travel and the reverse. It shakes the vehicle in that manner. The shock absorbers are original. The car rides well, and does not bounce, but they could be marginally worn out. I'm aware of the load impact on available tongue weight. I also intend to weigh at a Cat scale in the near future. I know I will be fairly close to the max tongue weight for the hitch.

kottum

If you could hitch your trailer up to a similar-sized car using your ball mount and do a towing test, and it still does the push-pull, the trailer (or somehow the ball mount) is most likely the problem.

The ball mount's tightness in the receiver is still suspect.  Maybe just force a large hardened washer in the gap until the slop is gone as a second part of the test.

Binghi

I would test a new ball mount and hitch pin in your TV’s receiver tube. The tolerances might be excessive in your existing set-up due to wear-and-tear, resulting in too much movement and resulting jerkiness. The trailer coupler may also be out of tolerance.

sequimite

@kottum, I like the idea of hitching to a similar sized car. I'll see if I can make that happen.

@Binghi, good thought. I actually have 3 ball mounts (one Reece and two Curt) of differing drops. They all seem to fit similarly loose in the receiver tube, so maybe my tube is out of spec. I've only towed a couple thousand miles with this TV, and I doubt the previous owner did much. The paint on the hitch looked untouched! I purchased a hitch tightener, but have not been able to go for a spin with the trailer yet. I'll post an update after a test drive.

Denny16

It has been my experience, that most receiver tubes and receivers are a loose fit with some play between the tube and receiver.  Very few are a tight fit.  

Cheers

sequimite

I tested towing with a hitch pin tightener. I think it reduced the push-pull action by about half, so I'm making progress.