Thread: Tractor Carrier

Originally Posted by jatt

Yeah used to drive one as a kid. 3rd or 4th gear take offs from stand still (hey I was 10 and couldnt get crunch box shifting down pat). Jocky the hand throttle to keep the front wheels down.

I had to use one at a vineyard planted on a steep hillside a couple of years back. It was petrol, a strange experience in itself having only driven diesel tractors - it had a crank start, which got used once or twice when the battery was flat, and I couldn't believe how readily the front wheels came off the ground.

They subsequently bought an Antonio Carraro tractor, which was night and day by comparison - modern diesel, 4WD with diff locks front and back, very low centre of gravity and with its horizontal articulation, no matter how steep or what angle, you couldn't get a wheel off the ground.

That said, it cost about 10 times what the Fergie was worth

Originally Posted by Styx

I think this is a very good idea and I will give it a try.

Some surfing has brought up a ball park figure of 70% weight goes to rear wheels.

Doing a spreadsheet calc FOR MY TRACTOR ONLY assuming the above loading spread, I reckon the centre of the axles should be 1 ft (11.5") behind the centroid of the trailer deck.

However, if tractor is loaded backwards (recommended way for higher safety), the centre of the axles should then be 3.6" IN FRONT of the centroid of the deck. This is a huge range so a decision has to be made about which way to load the tractor.


Seems practical to load frontwards, as axle position for this loading would be more suitable when the trailer is loaded more uniformly, such as a load of gravel.

Styx

I think there is an easy way to do this. Start with the trailer and as mentioned earlier, set the CofG forward of the centre of the axle set. Maybe look at tandem car trailers at the servo. They lean/tilt forward, but generally not enough to lift the rear axle.

Once you know where the trailer's CofG is, the tractor's CofG probably needs to be slightly behind the trailer's CofG. That should ensure you have pretty much the same weight over the towball of your car/truck so that it tows safely loaded or unloaded. I think one way to find out where the fergie should be, is to measure the tow bar height from the ground with just the trailer attached. Then load the fergie and move it forward or back until the tow bar height is the same as it was when it was unloaded.

This document for Victoria mentions a tow ball load of about 10% of the trailer's ATM (weight of the trailer and tractor, in this case). You already have a ball park figure of the weight of the fergie, so all you then need is the weight of the trailer to then work out the tow ball load. That will determine your CofG of the trailer.

Trailer & Tow vehicle specifications explained | Motoring | RACQ

Cheers

Thanks Jemijona,

I need to find the position of the CofG of the tractor first. I think using a trolley jack is a simple way to do this.

Then I need to decide how much bias to provide to obtain a desirable towball load and safe tracking. If the rule of thumb is 1/2" for each foot of tray, then perhaps I should add this to the CofG of the tractor to position of the centre of the axles.

I can't move the tractor back and forth to get the best position as the trailer will only be just long enough to fit the tractor. There will be only two positions for the tractor: facing forward or facing rearward. Facing forward seem the better position.

The critical loading will be when the tractor is on board. When the trailer is empty, the axles might be too far back, which for rocker suspension means extra load on the towball. I would think slipper suspension would not have this problem.

I have plenty of time to decide the axles' position. In the meantime I've got to go and do some ground work for the shed.

Styx

Originally Posted by Styx

Thanks Jemijona,

I need to find the position of the CofG of the tractor first. I think using a trolley jack is a simple way to do this.

A trolley jack should work too as long as it's rated for 1200kg's or more. You will have to do it on concrete though.

Then I need to decide how much bias to provide to obtain a desirable towball load and safe tracking. If the rule of thumb is 1/2" for each foot of tray, then perhaps I should add this to the CofG of the tractor to position of the centre of the axles.

I don't think there can be a rule of thumb. It all depends on the weight and how it is spread along the trailer. You'll probably just have to use trial and error.

I can't move the tractor back and forth to get the best position as the trailer will only be just long enough to fit the tractor.

Just make up temporary deck extensions so that you can move the tractor back and forth. Once you know the spot, cut each end of the deck to your final length. The other way is to move the trailer axles back and forth. That would be more time consuming though, I would think. If you want to nail the trailer axle position on the first go, it may be best to see an engineer, preferably one that specialises in trailers.

The critical loading will be when the tractor is on board. When the trailer is empty, the axles might be too far back, which for rocker suspension means extra load on the towball. I would think slipper suspension would not have this problem.

I think you might be right there. At a guess, the weight transfer possibly only changes once the load's CofG is either forward of the front hanger, or rearward of the rear hanger. If the equalisers have swung to their limit one way or another, then that would be the CofG's limit, so to speak.

Maybe just have a chat to an engineer anyway.

And what is the rating of your vehicles tow ball load? Will it be enough?


Cheers

How often do u recon the tractor is gunna be towed around on the trailer?

Do u recon its gunna be used more for carting materials?

If it was me I would build it longer. It will soon be apparent where the tractor rides best and u can position it as such. not to mention how handy the extra length will be.

Our solution was to pick up an 8 tonne GVM truck. Currently fixing a few things on it to make it more user friendly. Why? cause towing the float heaps is killing the ute, cant put any more than 3 round bales in the trailer, the shop tractor is around 2.5 tonne.... in laymans terms things are being scaled up to accommodate present requirements.

Having some play in the position is a good idea - esp if you ever end up with a different tractor.

I have found the position of the CoG of the tractor using my heavy duty trolley jack. Surprisingly it turns out that the CoG is very close to the centre of the tractor's length.
In fact the CoG is just 1" behind the centre point (front to back distance).

This means the tractor's CoG is virtually on the same line as the CoG of the empty trailer. So there would be no difference when the trailer is loaded with a uniform load, say, evenly spread gravel.

Therefore, the centre of the axles could be placed as usual practice, 1/2" per foot length of deck. In my case 5' (125 mm), back. See drawing.

Assuming weight of tractor is 1220 kg, and sprung weight of trailer at 280 kg (i.e. neglecting axles, suspension, hubs, wheels - all un-sprung weight), I calculate the tow ball load is 41 kg. (This should be a bit higher due to weight of draw bar itself, about 50 kg.) Perhaps the axles should be placed a little further back to increase load on tow ball.

Good thing is that it won't make much difference whether tractor is loaded front-wards or rear-wards.

I'm still unsure whether to use rocker or slipper springs.

Styx

Rocker springs transfer more/less load to the towing vehicle, so are more critical of where the load is placed in the trailer. However, they will also carry their rated load for the springs and axles.

Slipper springs will be more forgiving where the load is placed, but they require the trailer to have spring and axle ratings at 120% of the ATM. So at 2000Kg ATM, you need 2 x 1200Kg axles and slipper spring sets.

Thanks Yonnee, yes the slipper type springs might be more forgiving.

I made a mistake with calcs for tow ball load but think it's OK now. Easy to find loads for rocker type suspension, but more difficult with slipper type.

Styx

Originally Posted by Styx

I'm still unsure whether to use rocker or slipper springs.

Styx

I think you will find that tow ball load will change depending on which suspension you use. So you might need to do two calculations, one for slipper and one for rocker.

Cheers

Jemijona, problem with slipper suspension is that it is very difficult to calculate loads on each axle, as the load depends on how much each spring compresses. With rocker type suspension it is assumed each axle takes an equal load.

With slipper suspension if the trailer is loaded so there is a load on the towball, then most likely the front axle will take a higher load than the rear axle. But how much? This is why the slipper axles have to be rated 120% higher, which is hopefully a conservative assumption.

I can't see an easy way to calculate loads.

Styx

Originally Posted by Styx

Jemijona, problem with slipper suspension is that it is very difficult to calculate loads on each axle, as the load depends on how much each spring compresses.

I'm not sure spring compression makes any difference, as you still have x amount of kilos forward of the trailer's CofG and y amount of kilos rearward of the trailer's CofG, giving you the load on each axle.

With rocker type suspension it is assumed each axle takes an equal load.

I believe this is true until the rocker pivot limit is reached at which point the load then starts putting more load on the axle with more weight over it.

With slipper suspension if the trailer is loaded so there is a load on the towball, then most likely the front axle will take a higher load than the rear axle. But how much? This is why the slipper axles have to be rated 120% higher, which is hopefully a conservative assumption.

I can't see an easy way to calculate loads.

Calculation will be difficult with slipper suspension. An engineer should be able to work this out for you.

Cheers

Jemijona, actually I am an engineer!

The problem is that the load is carried by the two axles and the tow bar.

With slipper suspension the load on each axle will depend on how much the axle's springs compress. Without compression there is no load transfer. Assuming the trailer chassis and tow bar don't flex, then the axles "rotate" vertically in arcs about the tow ball. That is, the front axle rotates in an arc of radius equal to the distance from the tow ball to the front axle, while the rear axle rotates in an arc of radius equal to the distance from the tow ball to the rear axle. The attached diagram might explain better.

Even for static loads on level ground, the loads can't be found without knowing the spring stiffness and the deflections. (Spring stiffness may not be constant either.) And to complicate further the tow vehicle deflects under load too. On uneven ground it is further complicated, more so with dynamic effects under motion.

So I don't believe it is possible to calculate axle loads without deflection data and spring characteristics.

Someone may have more insight.

Styx

Hi all,

This is an update on progress with the Tractor Carrier.

During a recent visit to Victoria I collected all the gear I need to build this trailer, (from the popular Bacchus Marsh supplier):

45 mm sq axles, tandem rocker springs, electric brakes (one one axle only, but brackets welded onto other axle too), controller, lights, etc.

Currently I'm building a farm trailer (not to be registered) from the remnants of a 6' x 4' trailer that rusted away leaving only the axle, springs, wheels and the DuraCal drawbar 100 x 50 x 1.6. I'm making a new frame using 50 x 50 x 2.5 DurGal, and have discovered how strong this size section is, compared to the original 40 x 40 x 1.6 (out of Keon Park).

I'm thinking now that the sizes I proposed for the Tractor Carrier might be on the heavy side, but my real question now is the size of the drawbar. The Bacchus Marsh shackles are very heavy, 110 x 80 x 9. Thinking that the drawbar should be welded to the front shackle - all the braking force goes into this front shackle and the towbar pulling force could go directly to the front spring/axle. It seems highly desirable to make the connection between the drawbar and the shackle sound and strong.

However, due to the width of the shackle (80 mm) it seems a 50 mm wide drawbar would not fit neatly, and hence I'm now thinking a 75 mm wide drawbar is the only solution. This means I need to go up in size to 125 x 75, but could perhaps go down in wall thickness to 2.5 mm.

Probably need the make the side chassis members 75 mm wide too, so the shackles can be attached soundly.

Any thoughts, thanks.

Styx