a frame? how do the car be hind steer?

[stuboy]

And steering not flop wrong way and wreck the tyres???

[dugong]

The rods on the A frame act on the bottom arms. They have a hook which is pushed through the gap and chained round, so as the towing car steers, the towed car follows (provided you've taken the steering lock off). Guess what I didn't do the first time I tried to A frame something?

[nigel bickle]

Ackerman principle.

 

Unless its a beam axle -in which case strap the steering & corner slowly!

[barefoot]

...or unless its a Trabant which had such infinitesimally feeble self centring ability that after the first corner it just remained with the wheels turned to a jaunty angle until I unhitched the ungrateful little bastard and drove it home again. 

[Ratdat]

The castor that naturally self centres the steering on cars is what this works on. Same as a shopping trolley. The wheels will point in the direction of travel. Anything that upsets the stock geometry usually makes a car hard to A-frame such as wider wheels or different offsets.

[overrun]

Clearly this is car pixies at work.

 

The same ones who plod on with the projects that you cba with.

They like to leave the workshop whenever possible, just like we do.

[dollywobbler]

The rods on the A frame act on the bottom arms. They have a hook which is pushed through the gap and chained round, so as the towing car steers, the towed car follows (provided you've taken the steering lock off). Guess what I didn't do the first time I tried to A frame something?

 

This is incorrect, at least it certainly isn't always the case. A 2CV doesn't have bottom arms but they still self-steer when towed with a set-up that either attaches to the suspension arms or even just the front of the chassis. The first time I saw a 2CV being A-framed, I wondered why it was still making a noise. Yup, it had been left in gear...

[dugong]

This is incorrect, at least it certainly isn't always the case. A 2CV doesn't have bottom arms but they still self-steer when towed with a set-up that either attaches to the suspension arms or even just the front of the chassis. The first time I saw a 2CV being A-framed, I wondered why it was still making a noise. Yup, it had been left in gear...

 

It's correct about 90% of the time, although it works on weird arsed stuff like 2CVs and sort-of gets by with beam axled cars as Nigel mentions for different reasons. You make me out to be completely wrong, which I'm not. The majority of A-frames built for the majority of cars work on the principle I mentioned above. 

 

The stuff you'd struggle to drag would be a hydropneumatic Citroen. I had to explain repeatedly to a bloke who wanted my GS (time waster # that he couldn't A frame it because the engine needed to run to power the suspension pump. Left at rest it would be too low and hitting bumps would be perilous because the front end would try to self-level even if the engine didn't stall. 

 

Anyway, he wouldn't have it and didn't buy the car, like most of the other idiots I had to tolerate. 

[dollywobbler]

I'm pretty sure I've seen other cars with A frame attachment points physically bolted through the front bumper though, so I don't think connecting to the suspension is actually, definitely required. 

 

It's the same physics that makes a car want to steer down hill. It wants to turn in the manner that's easiest. For a car being A-framed, it naturally wants to follow the car in front as long as there are two attachment points rather than one. Well, that's my understanding of the black magic of a-framing anyway!

[Ratdat]

The A-frame just has to be rigidly attached... it doesn't matter where, but the majority of commercially available set-ups are designed to fit onto the lower arms, simply because that's the most common type of suspension (usually struts). So long as the car being towed has inbuilt castor in the front suspension geometry and there's a rigidly attached towing frame then it'll work.

 

To be honest, I am surprised they don't actually build car transporter trailer this way. Fixed axle at the rear end and steering axle at the front using proper Ackerman geometry with a sufficient amount of castor. I bet it'd be really stable.

[forddeliveryboy]

Like continental lorry trailers used to be - wonder why they've died out? Always thought they made a lot of sense.

[Geep]

Never A framed but wouldn't mind having a go. As other say, caster helps point wheels in right direction, a very under checked but important angle. Most tracking places never check/adjust secondary angles, simply toe and record camber. In fairness, a lot of cars are time consuming to adjust caster/camber or can't be adjusted, but it's always worth asking for the readings when being tracked. Modern wheel alignment machines mean it's no more complicated than following on screen directions and turning steering one way then the other. Caster can cause cars to be 'nervous' in straight line, cause a steering pull, heavy steering or poor self centring. Not really a tyre wear issue so that's probably why tyre shops are not bothered and customers don't realise/ask.

[Ratdat]

The first time I took my MX5 to get alignment after I fitted urethane bushes throughout I gave them all the figures for the geometry that I wanted.... and they didn't even know what castor was! I had to explain it to them! Needless to say they made an utter cock up of it and left handling worse than when it went in, un-aligned. I only took it there as they had an all new super modern laser alignment setup.

 

So, I took it to my usual tyre fitting place who sorted it no probs. The first place had managed to set one side of the car with negative camber and the other with positive. Well done chaps!

[dollywobbler]

That first 2CV A-frame I saw did get alarming at one point. For some reason, the 2CV started 'convoy wobbling' trying to turn one way then the other very rapidly. It may have been because the suspension height wasn't quite right. 

[KruJoe]

To be honest, I am surprised they don't actually build car transporter trailer this way. Fixed axle at the rear end and steering axle at the front using proper Ackerman geometry with a sufficient amount of castor. I bet it'd be really stable.

 

Because it'd be impossible to reverse, wouldn't it?

It would just get 'crossed up' very quickly. That's why trying to A-frame backwards rarely ends well...

[Ratdat]

Because it'd be impossible to reverse, wouldn't it?

It would just get 'crossed up' very quickly. That's why trying to A-frame backwards rarely ends well...

 

That's true. Reversing an A frame in anything other than a straight line is a chore and often not possible. The front wheels would need to be like giant castors then! Mind you it would still be a bugger to reverse round a corner with the fixed axle so far back. Maybe a fixed central axle and castors on all for corners!

[dugong]

Hooray for trailers. 

[Lankytim]

I've reversed A frames loads and never had an issue, the wheels stay straight. Its not like the front wheels suddenly lock over when you try to reverse a car its it?

 

To be fair I've never tried any sharp corners in reverse.

[KruJoe]

I've heard you say that before Tim, but it just won't work for me.

 

Its not like the front wheels suddenly lock over when you try to reverse a car its it?

In my limited experience they do. Maybe I R DOIN IT RONG.

 

With the rig in a perfectly straight line I've managed a few feet, then it all goes Pete Tong. It's like pushing string.

[Lankytim]

Maybe i'm doing it wrong, lol.

 

Maybe certain cars are more susceptible to the steering locking over and having a spak attak when pushed backwards on an A.

[nigel bickle]

You can  generally reverse, VERY slowly -if its straight /flat/ level. But even a pebble will upset it.

 

A spare driver sitting in the towed vehicle may help a bit, failing that, set it up straight and strap the steering wheel tight.

 

Its a pain, but it will enable you to back it up -gently.....