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MG MGA - Beef up rear springs?

Hi Guys
When I have my boot full, the spare wheel on the boot lid and the trailer secured (73kg on the tow bar), the rear suspension is under real strain! I measured the difference in ride hight between "normal" and loaded conditions (35mm) and asked a local spring manufacturer to quote to "refresh" an old set of springs I have to offer the "normal" hight when loaded.
Does this make sense? We are doing South Africa to Kenya return next year and this was the only problem we encountered with the syspension during our 21500km trip around Australia earlier this year.
Regards, Dave
David Godwin

Dave, I reckon that ideally the rear springs when operating under normal working load should be in a flat, zero camber, condition and oscillate around this position.

To get this flat condition for your abnormal loading you could increase the free camber on your springs by your 35mm but then you may not be able to connect your rebound straps. I suppose you could always connect them up for your journey when loaded. Also the unloaded rear height would be rather high.

What you really need is springs with the same free camber, 3.6" as normal, but with a lower deflection rate. The deflection rate of a normal spring would be around 125 lbs. per inch and this would mean that you have an extra load of 175 lbs per spring (350lbs on the backend) for the extra deflection of 35mm. So you would need springs with a deflection rate of around 175 lbs per inch.

I would guess that the easiest way to achieve this would be to replace the top leaf (the one with the eyes) with one having a greater steel thickness. Your spring man should be able to calculate this extra thickness to give your required deflection rate.

Hope this helps - Mike

PS I made the assumption that all your extra rear loading is taken up by the springs but some of it will be taken up by the tyres in giving your 35mm reduced ride height. Therefore 175 lbs per inch might be an over estimate. I suppose you should anyway increase the rear tyre pressure significantly for the extra weight on the journey.

















m.j. moore

I have to ask, how heavy is the trailer you are towing? That 73kg hitch load seems extremely high, unless the trailer gross weight is more than 1/2 of the car loaded weight. I'm wondering if you have something like a huge ice chest or propane bottles sitting on the trailer tongue ahead of the load box. I seem to recall pictures of your trailer (the traveling coffin?), and it struck me as overkill at the time (rather heavier than necessary for the job at hand).

It sounds like you need to redistribute the load in the trailer to reduce the hitch load. The ideal hitch load should be 10% to 15% of the gross trailer weight for good trailer tracking. For a heavier trailer, 10% on the hitch is enough. Additionally, if you can put the spare tire in, on, or under the trailer (with the proper trailer balance), you would remove 90% of the spare tire weight from the rear of the car.

My luggage trailer with 19 cubic feet of volume (150 pounds empty) has over 100,000 miles towing behind my MGA. See here: http://mgaguru.com/mg11.htm
This was one of the heavier travel loads. Hard to see in that picture because of shadows, but there are two spare tires for the car attached underneath the trailer. Inside is a full size ice chest, some water and oil and car parts, two large suit cases, a couple of duffel bags, food, cooking utensils, a tent and sleeping bags, and other camping gear for an eight week trip. Every morning there was a 10 minute packing job and sit on the lid to get the cover latches fastened. When I'm not touring the trailer commonly carries a set of race tires, tools, helmet, air tank, fuel can, camping gear, and the ice chest doesn't fit inside.

I commonly have it loaded to about 600 pounds (272 kg) gross weight and sometimes more (maybe 800#/364kg on occasion). That would be about 1/4 the weight of the loaded car (sometimes up to 1/3). When the trailer is more than 1/4 the weight of the tow vehicle you need to be very careful with run out space and stopping distance, or the trailer needs its own brakes on all wheels.

The only trick with my trailer is to have the load properly distributed so I can still pick the hitch up with one hand and also push it around by hand (at least on a hard surface). With the moderate tongue weight I hardly know it's there at road speed, the tail does not wag the dog, and the rear of the car does not sag appreciably.

If you must tow a trailer weighing more then 1/4 to 1/3 the weight of the tow vehicle, the trailer should have brakes as a matter of personal safety (simple preservation of life). If you have a hitch load greater than 90-100 pounds (40-45 kg), then you might consider supplementary helper springs. These are generally inexpensive, relatively easy to install, and can be adjusted for variable load conditions. See here for an example (not MGA specific): http://www.jcwhitney.com/autoparts/Search?catalogId=10101&sku=ML15983G

For the MGA these would fit on top of the original leaf spring, below the axle, using longer U-bolts if necessary. For initial adjustment raise the car and let the axle go to full drop. Pull the end U-bolts up just enough so they are not loose to rattle, giving minimum tension on the helper springs. Back on the ground the rear of the car may sit 1/2 to 1-inch higher, and lower is better for good handling. When the trailer is hooked up, if the ride height is more than 1/2 inch lower than normal you can pull up the end U-bolts to increase helper spring tension, and you can have any ride height and load capacity you want.

Suggest you look for the lightest helper springs available. These are sometimes used on trailers, so they may be available in lighter duty than you find for P/U trucks and heavy cars.

Some years ago I installed these single leaf helper springs on a Ford Ranger P/U truck to tow a small stock car on a tandem axle flat bed trailer. In that case the loaded trailer was heavier than the tow vehicle, had it's own brakes, and the greater problem was engine power, but load capacity was no problem.
Barney Gaylord

Mike and Barney

The spring man said he would put a known weight on the spring and measure the deflection and calculate the thickness of spring required to replace the top leaf to give the desired loading. Sounds straight forward to me, but is it that easy? I have my doubts.....
Barney, the tow ball maunfacturer (Plans from your site - thanks) said I should have max 100kg on the ball. I loaded and reloded the trailer and 73kg kept coming up. Sounded well within the 100kg limit so went with that. On flat roads, everything was fine, but once it got a little rough, it bottomed out too soon and caused a few hairy moments!
Mike, I will only fit the straps when loaded - thanks for that.
I will call the chap who manufactured the tow ball - maybe he said 100lbs! I'll let you all know tomorrow.
Thanks, Dave
David Godwin

David, -- I designed the hitch to be a full class-I hitch with 2000 pound towing capacity. That is mostly a function of the strength of the cross tube. That means you could tow another MG with it in a pinch, and if you hit the brakes hard it would just skid the tires with no other mechanical problem. The hitch can support hundreds of pounds, but the MGA cannot take that much weight added at the rear bumper. I recommend not more than 100 pounds hitch load for good handling of the car. Heavy hitch load makes the tail wag the dog in an emergency handling situation, leading to nasty over steer and possible jack knife situation.

There is an easy way to determine and adjust the hitch load for your trailer. Get a stick about a foot long, a wooden rule perhaps. Stand it on the ground next to your unladen hitch, and mark the height of the hitch on the stick. Then put a known load on the hitch, preferably about 100 pounds (a small person perhaps), and mark the new height on the stick. Load divided deflection gives the spring rate at the hitch. It is then easy to determine the tongue load of the trailer by noting the amount of hitch deflection.

If it looks like too much hitch load, then move some heavy things in the trailer from front to back to balance the load. You can do this any time if you keep the measuring stick in the car or in the trailer. If this seems to be impractical or nearly impossible, you may need to move the trailer axle farther forward.
Barney Gaylord

Great Idea, Barney! Will save a lot of work - I have been resting the hitch on a axle stand on a bathroom scale and weighing the load as I pack the trailer - a laborious process, but accurate!
Thanks, Dave
David Godwin

Hi All, Saw the chaps at King Springs today and they suggested 1) Retention the existing springs, and 2) adding an 8th (top) spring - to give an extra 31mm, which with the shackle swing they estimate will rise 35mm.
Discussed adding a single, lower, "load" spring, but apparantly requires a redesign if a longer leaf is added below a shorter leaf - something about noise and rubbing - couldn't work it all out, but went with their recommendation. Quoted $193 - not bad for 2 springs, I thought!
Dave
David Godwin

Sherline.com has an excellent guide to trailer towing. They also manufacture a relatively low-cost trailer tongue scale. I bought one recently when I towed my "A" for 3000 miles on a trailer. The trailer plus MG equaled the Jeep in weight. Although the Jeep squatted a bit from a load of spare parts and a lack of overload springs, it handled the trailer very well thanks to trailer brakes and proper tongue weight.

Ken


k v morton

Thanks, Ken. Certainly very thorough article!
Dave
David Godwin

Dave

Check out the helper springs that Barney references. Back in the late 60s I was towing a sailing outrigger with an overall trailer + boat weight of less than 600# with my 49 YT. Towed it many miles with no problem using a similar set of bolt-on helper springs. When I was not towing it was a relatively easy job to remove them and return the car to its original condition. This way you have your original ride characteristics when not towing, and adequate spring rate when towing. Best of both worlds.

FWIW

Larry
Larry Hallanger

This thread was discussed between 15/12/2007 and 18/12/2007

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