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Tow dolly question
3

Tow dolly question

Tow dolly question

(OP)
I have another tow dolly question. As you know, the front wheels go up on a dolly and get tied to the dolly with wheel nets. Most dollies have a pivoting wheel pan. When cornering, the pan pivots while the wheels of the towed vehicle stay straight. Another kind has a fixed wheel pan. When cornering, the front wheels of the towed vehicle pivot. I cannot comprehend this. The front wheels of the towed vehicle do not contact the road. But their turning makes the dolly+vehicle turn. Would someone explain this to me?

RE: Tow dolly question

Simple. It can't work that way you describe.

RE: Tow dolly question

Taking a look at pictures of that type of dolly, it appears to have no provisions for allowing it to rotate under the tires as the tow vehicle turns. So, it must push back on one wheel and forwards on the other wheel during a turn. This appears to result in the wheel moving on the dolly as seen in the wheel view of that video. This won't be particularly kind to the suspension of the towed vehicle as you approach the limits of the wheel restraints or the limits of easy wheel movement. To answer your question, something to do with these forces during turning must also cause the steering wheel to turn.

It doesn't appear to be a good design at all. The turning is facilitated by the dolly moving under the wheels and twisting the suspension of the towed vehicle instead of rotating on a king pin.

RE: Tow dolly question

(OP)
Actually I have a hard time understanding why the steering wheel (towed car) turns. My lame answer is this. When the tow car turns right, centrifugal pushes towed car left. Steering rack is terminated at the front wheels that are fixed, the steering column and pinion gear are pushed to the left, making the steering wheel turn right. But how does this make the tow dolly turn right?

RE: Tow dolly question

It doesn't. Each axle of the towing vehicle, and the dolly, and the rear axle of the car on the dolly, are all tracking the same radius. Draw it properly and you will see why the steering wheel turns.

Cheers

Greg Locock


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RE: Tow dolly question

The towed vehicle has caster and a lot more caster than normal because of the ramp angle. The front axle side-force acts on the total caster angle during turning and the tierods are loaded via the steer arms. The tierod forces overcome the steering rack friction and the pinion turns. Try it with the towed vehicle motor running to see the steering wheel motion smooth out.

RE: Tow dolly question

(OP)
cibachrome,
Your explanation makes sense to my dumb head. Now I see why the steering wheel lock pin failed on my car last time I pulled it on a U-haul tow dolly which had a pivoting wheel pan. The lock pin was no match to the torque from excessive caster. Had I wrapped the steering wheel with seat belt for reinforcement, I might have saved the locking pin.

Now, does steering wheel turning have anything to do with tow dolly + towed car following the tow vehicle? Does it help reduce tire dragging because of non-pivoting wheel pan? I don't see how they are linked but would like to hear from you.
Thank you.

RE: Tow dolly question

Preventing the steering from turning is the wrong approach. The right one is to have it unlocked so that it is free to turn without restriction.

RE: Tow dolly question

(OP)
Sorry for not being clear. If a tow dolly has a pivoting wheel pan, steering wheel needs to be locked. This was the case of my previous tow. The steering wheel was locked. What they say is this: if you leave steering wheel unlock on a pivoting tow dolly, the tail of the towed vehicle will fish tail. (I don't understand why but that's the common knowledge)

RE: Tow dolly question

Go back to basics here.

There are two fundamental possibilities.

1. The tow dolly is capable of "steering" on its own independently of what the wheels of the towed vehicle do, and the design is such that it does not even attempt to apply any "steering" torque to the wheels of the vehicle being towed no matter how the wheels of the dolly "steer" themselves independently of those wheels. If this is the case, the steering of the towed vehicle may be locked.

2. The tow dolly is locked to the steered front wheels of the vehicle being towed and "steers" with those wheels, thus relying on those wheels "steering" to allow the whole system to turn around corners. If this is the case, the steering of the towed vehicle must not be locked, because if you do, the dolly attempting to steer itself will either drag its wheels sideways while attempting to turn, or will break something in the towed vehicle by attempting to force the towed vehicle's wheels to steer.

Situation 2 is analogous to flat towing (steering must not be locked) except that the wheels of the car don't actually have to rotate. They have to steer, but not rotate.

RE: Tow dolly question

(OP)
If I appeared to be rambling, I apologize. I have a question because I have yet to determine the type of tow dolly to use for my upcoming 1000 mile tow. My question is about the non-pivoting wheel pan. This type weighs less and costs less.

I understand now why the steering wheel turns. Dolly manufacturer claims the front wheels do not turn though steering wheel does. The linked video in the first post also shows the front wheels are not turning. How does this happen and how does this steer the dolly?
Thank you.

RE: Tow dolly question

The really cheap ones have no provision for steering, and just drag the dolly wheels sideways while turning.

For a serious tow, if the car cannot be flat towed (lots can't but some can, sometimes with minor modifications), a proper car trailer is the way to go.

RE: Tow dolly question

The use of a rotating pan mount in combination with the unlocked steering column amounts to 2 hitch systems in series with very high gain. Keep in mind that the non-negligible rack-plus-tierods mass also adds a sizeable force to the net steering moment recipe. Since friction is probably the only major source of damping present (some power assist fluid movement may have a damping contribution), you ought to appreciate why the double hitch combo could become limit cycling (or worse). Remember the free-control problem in the FWD Omni-Horizon vehicles ? The whole apparatus also generates some sizable f/a forces on the clamped steered wheels that could cause the tire belts to flex, stretch into plastic deformation and loose their restraint capability. BTW: I've always used cotton tie down belts for this reason.

RE: Tow dolly question

For that sort of distance - is the vehicle automatic? If so, you really should consider disconnecting the tailshaft from the diff (if RWD) to avoid possible damage to the gearbox. Or, safest option would be a truck, really.
The non-pivoting dolly is going to generate more force on all the suspension components - you can get an idea of that in the video. And yes, it is going to turn the wheels. Granted, the video only shows small movements of the wheels and steering wheel; by default if the steering wheel turns, then the road wheels must. They are physically connected on every vehicle I've ever come across.
Regarding a tow of that distance - I've seen plenty of grey nomads use the setup to tow small 4x4s behind their camper vehicles, and it seems to work for them. Personally, if it were my vehicle, I'd spend the extra for a pivoting dolly and still leave the steering free, just for peace of mind. But that's just me, and doesn't take into account your situation, price/availability of replacement suspension components, etc.

RE: Tow dolly question

(OP)
Thank you for the notes. I understand tow dolly much better now. I am settled for the pivoting type. Tow vehicle is a 1/2 ton pickup. Towed vehicle is fwd. Flat tow would need aux trans oil pump. Tow dolly+ tow vehicle weight is less than max tow capability. I will drive slow @55mph. Thank you.

I have a follow-up question about this statement: [quote of Mongrel's note].....I'd spend the extra for a pivoting dolly and still leave the steering free.....[end quote] Manufacturer's recommendation (of a pivoting type tow dolly) is to lock steering wheel; otherwise, the rear of the towed vehicle would fish tail wildly. I don't quite understand this. Please comment. Thank you.

RE: Tow dolly question

I should have been more clear. By free, I meant running the seat belt through it so that it has room to turn without spinning freely. Usually gives up to a quarter turn of the wheel each way if you just pass the belt through the bottom of the steering wheel once and then buckle it.
Although, under normal circumstances it should be safe to leave completely free also - the rear wheels will follow along much like a trailer. That ignores the possibility of heavy braking, bad road surfaces, steeply cambered roads, etc....
So, in short - I meant to say I'd leave the ignition lock disengaged. Then use the seat belt to prevent wild steering events.

RE: Tow dolly question

(OP)
Thank you for the explanation. Looking back, I should have done the same, disengage steering wheel lock and wrap steering wheel with seat belt. Instead I followed U-Haul's instruction, engage steering wheel lock. Result was broken steering wheel lock.

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