|The MGA With An Attitude
MGA TRAILER HITCH, Light Duty - TH-102
At 08:26 AM 6/20/2007 -0400, Vaughn X wrote:
>>"I bought the hitch new in about 1972 when the Twin Cam was my daily driver. It was made by Valley and was their part number EMG8. .... I used to flat-tow a Morris Minor racing car behind the MGA Twin Cam, using that hitch and the hitch was very strong and up to the job, in spite of it's simple design. The hitch uses the stock bumper bolts to hold it in the vertical plane, but the load is pulled in the horizontal from the round cross tube in the back of the MGA."
First order of business here is,
WARNING: This is a light duty hitch not intended to tow another car!
When I first saw this picture I had an immediate knee jerk reaction to the mounting configuration. The rear mounting point attaches to the bumper which is mounted on spring bars. Those bars are in a neutral position when at rest and will flex easily a fraction of an inch in any direction, so afford zero load capacity in the pulling direction. That leaves the entire tow load on the U-bolt around the center of the round frame member cross tube, which is not particularly strong. This is the same size tube used in the front frame extension. If you wrap a web strap or chain around that front tube to tow the car you can easily bend the tube. If there was any tongue load on this hitch it would cause some vertical motion of the ball with any bumps or undulations in the road. That motion would in turn cause a slight rotation and chafing where the U-bolt attaches to the frame tube.
Before doing a lot of stress calculations, to be sure I got it right, I contacted Carl Hiedeman at Eclectic Motorworks to verify the exact diameter and wall thickness of the tube. His response was: "It is 14 gauge 1-3/4" OD. I agree that it's not strong enough. I've seen frames with things welded to them (including trailer hitches) and they bent/cracked."
I was skeptical about this exercise but would do it anyway, expecting to come up with a fairly low safe weight for the trailer. As an example, the car weighs 2000 pounds with two gallons of fuel, and a light load of driver and luggage may be another 200 pounds (2200 total). Suppose a trailer would weigh 550 pounds (1/4 the weight of the car). Assume the trailer has no brakes, and the car has good sticky tires so it might have a coefficient of friction of 1.00 with the road. You have a total weight of 2750 pounds and breaking force of 2200 pounds, which will yield a deceleration of 0.8g. That would result in a compression load on the hitch ball of 440 pounds.
For a one time gently applied load you need a strength safety factor of 1.5. For a load to be applied regularly you need a safety factor of 3.0 to avoid long term fatigue cracks. That means the frame tube would have to be able to take a peak load of 1320 pounds before it bends. I don't think the 14 gauge tube will calculate out to take that kind of load in the center of that rear span. If I'm right, the trailer weight allowance will have to be smaller. Now I was cranked up about this, so I crawled under the car and measure the length of the tube between the frame rails.
At 09:24 AM 6/27/2007 -0400, Vaughn wrote:
>>" I hear you on that.. but I did flat tow both a Morris Minor race car and a SAAB 96 street car behind the Twin Cam using that hitch for many miles and for a number of years with no problems.. haha.. maybe I got lucky, but I never saw any movement in the hitch."
Yeah, I think you were lucky. 100 HP doesn't give much acceleration for two cars hooked together, so the more important point for normal operation is what happens when you hit the brakes and lock up the wheels of the tow car. It would be much worse if the tow car also bumped a parking curb at the same time.
Here's a very conservative estimate in round numbers. MGA with one person on board =2200 pounds. Towed vehicle =2000 pounds. Coefficient of friction between tires and pavement =0.8. Lock up all four wheels and you get 2200 x .8 =1760 pounds of deceleration force. Divide by combined weight and you get 1760/4200 =0.42g deceleration. Multiply that by weight of second car and you get .42x2000 =840 pounds compression load on the hitch. That is equivalent to hanging two engines plus from the center of that round tube with a cable loop.
So I measured it, and the tube is 34-inches long between the side frame rails. 14 gauge is 0.075" wall thickness, so it is 1.750 OD and 1.600 ID.
Section modulus is Z= Pi/32 x (D^4 - d^4) /D = 0.185
Cross sectional area = Pi/4 x (D^2 - d^2) =0.395 sq-in.
If you assume free pivoted ends on the tube (because it is long),
Bending stress is WxL/4Z = 840x34/(4x0.185) = 38,595psi
Sheer stress is W/A = 0.84/0.395 =2127psi
Combined stress S= 40,722 psi.
Cheap mild steel has a tensile strength around 60,000 psi, so you were running with a safety factor less than 1.5, which is a little scary. If you have sticky tires with COF=1.0, then deceleration and hitch load and stress increase 25%, making S=50,902 and safety factor =1.18. All you need is to hit a small pot hole when braking to bend the tube. Even if you never hit a pot hole, repeated application of load that near to the yield point will eventually result in stress cracks in the tube. Safety factor for repetitive loaded structural parts (not involving human passengers) should be 3.0. That means the tubing wall thickness would have to be close to 0.220".
And Vaughn continued:
>>"I figured as long as the axle on the MGA didn't bottom out on the frame or unless you hit an immovable object with the front of the MGA while at speed, any excess force on the hitch would cause the MGA to skid, thus relieving the excess stress. In other words, a heavier hitch would provide no more safety or towing capacity than the Valley hitch. Valley was pretty hooked up in the engineering department then and they still are."
You would be wrong. Locking up the wheels on the tow vehicle will generate a hitch load of 40% to 50% of the weight of the towed vehicle (if the two vehicles are near the same weight). That is, 800 to 1000 pounds of hitch compression load for a 2000 pound towed vehicle. That round tube in the rear of the MGA frame is designed to be a frame rail spacer (with very little load), and to carry the weight of the fuel tank (about 100 pounds full). Even the weight of the fuel tank is distributed on two points closer to the ends of the tube.
I would be surprised if Valley still sells the same hitch today without a significant disclaimer on towing capacity. There is no way that thing should ever be used to tow another car. Do you recall ever seeing a class 1 "Hidden Hitch" brand (or equal) towing hitch on a modern car? It is commonly attached to the vehicle frame or uni-body about in line with the bumper mounting points, two points about 3-feet apart. The structural cross bar is often a square pipe with wall thickness of 1/4 to 5/16-inch, depending on the required span.
You might look here: http://mgaguru.com/mgtech/trailer/th201.htm.
This is the full class 1 hitch for my MGA, designed to pull (or resist) up to 2000 lbs hitch load along with 200 pounds tongue load. The structural cross tube is 2" square with 1/4" wall thickness. This does indeed have the capacity to tow anything with the MGA, as long as it doesn't exceed a reasonable tongue load. If you had a Mack truck hitched on there traveling at speed, and you locked up the wheels with sticky race tires, it could generate a hitch compression load of about 2000 pounds to make the tires slide, and it does have a 3 to 1 safety factor on top of that. This isn't overkill, it's just safe and correct.
At 09:44 AM 6/27/2007 -0400, Vaughn wrote:
>>"The u-bolt is 3/8" diameter stock and the vertical bolt that goes through all three pieces is 7/16" stock. Also, the photo doesn't show it, but the lower piece has a 1" diameter hole in it for attaching the safety chain(s)."
All pretty irrelevant. The round frame tube is thin, almost like exhaust pipe tubing. If you tighten the U-bolt it will crush the tube before it gets anywhere near the yield point of the U-bolt. The heavy appearance of the structural parts of the hitch is about 500% overkill compared to what you have it attached to in the car frame.
I found the Valley Industries web site here: www.valleyindustries.com. After poking around there quite a bit, it appears to list mostly more modern models under fixed standard part numbers. There is a listing for what appears to be a generic part number for a custom order Class I hitch. Look under:
Product Lookup - All Valley Items - Hitches and Receivers - Class I - Page 5 - See last item on the list:
85710 Hitch-Class I Receiver, 2000 lbs Rating, Special Order Only
This may be one of those "Call us to ask and we will look up your application". Through life of the company they may have accumulated hitch designs for almost any car ever built. If they ever did have a hitch for some car model they would likely retain the records for the design and construction. They may or may not sell you one for an MG, depending on existing inventory or possible order quantity. They may in some cases discontinue a certain item if it subsequently fails to meet some changing design parameters (which might be the case with your MGA hitch). Would have to call them to ask, but it might be worth a try to see what they have to say about it.
>>"With today's litigious conditions, I expect they are a lot more conservative now than they were in the 1950s and 1960s."
No kidding. That doesn't mean they can't still sell the same hitch. It just means they now have to clearly label the hitch for the appropriate towing load limit, and also give you a book along with it that can educate you in all the nuances of trailer towing, and provide a raft of caution and warning statements to cover their ass, and then rely on the "legal" assumption that you will read the owners manual and adhere by all of the recommendations. That gets them off the hook legally (usually).
>>"I would add though, that under deceleration, the width of the flat bar may spread the load slightly more than the width of a cable loop,"
Yes, but not much difference. Spreading the load along 2" in the center of the tube doesn't shorten the length of the cross tube significantly. It is significant though that the flat bar is less likely to smash the thin frame tube under compression than the little U-bolt under tension.
>>"but that probably doesn't make much difference to your 50/500 recommendations. I bet Valley had the same recommendation.. 50 & 500 seems reasonable."
I don't know if they can legally apply lower than 100 lb tongue load limit. Tongue load needs to be a minimum of 10% of gross trailer weight, and there has to be some allowable range for tongue load, so a 50 to 100 lb range of tongue load would be reasonable for a 500 lb trailer. This might be a good option for someone who only wants to tow a small luggage trailer, especially if this particular hitch is still available for a moderate price (in bolt on and run form). Time for a phone call to Valley Industries to see what's up these days.