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consumeratlarge's avatar
consumeratlarge
Explorer
Mar 24, 2015

Help with physics of dolly turning

I just finished an uneventful first trip (1200 miles) with our new Stehl dolly. I went to get the Master Tow, but couldn't get the electric brake model, so I found this one, which looks like a virtual copy of it.

My motorhome is a 31ft, and has a nice tight turning radius, and I knew I would have to be moderate on the turns. After I was putting the dolly away, I noticed the metal stops had been obviously hit, the stops that limit the amount the pan turns. So, after looking closely, I'm thinking there is less than 10 degrees of turning available at the pan.

Simply doing a right turn at a light seems like it would take more than 10 degrees of turntable movement. I strapped the steering wheel down to the seat frame, and it wasn't going anywhere. Now I'm wondering if the turning forces in anything but a lane change are bending and twisting the front steering assembly, as something has to give, right?

I know from pulling tandem trailers that you can scrub tires sideways to some extent with totally parallel axles, but this is my wife's car I was pulling around, and I don't want to bend anything.

I also know that there has to be some limit on the turning, or the car can hit the fenders (steel in this case!). I have read that there are other models that may turn tighter, such as the steerable axle style. However, people have used these swivel pan models for decades with seemingly no problems. Or, have there been extra loads put on the front end that it wasn't designed for?

Can somebody definitively tell me what is 'giving' when I turn a corner? (greater than 10 degrees). If it's just tires, no problem, if it's something that's going to break, I need to rethink this. I can't imagine looking for a gas pump without having to turn the wheel every now and then to get into position, (V10 gas model motorhome).

My first topic post, so go easy. I have read lots of searched archives, scoured the internet, and nothing I've found has enlightened me on how to figure my realistic, practical turning radius, or angle. If it's truly 9 degrees of wheel turn, I'm going to be limited to highways only, because my front wheels of the motorhome seem to turn double that with normal maneuvering, such as turning from a 2 lane road.

I like the dolly in actual towing, no problems tracking, decent brakes, stable, good over bumps, and easy to hook up. Thanks in advance.

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  • consumeratlarge's avatar
    consumeratlarge
    Explorer
    Mar 25, 2015
    Thanks for those thoughtful replies. I will try some tests next time. I like the analysis of the many points of stress, and I like the idea of marking how many degrees of pan movement results from a steady turn. Maybe I will count how many degrees I can move the RV steering wheel in a constant turn before it hits the stops. Then, at least I would know what my point of stress-free turning is.

    I always try to round every corner with the largest radius that will fit, but I live on one of those 2 lane roads that isn't very wide, and we travel on small roads often. Any turn with traffic gets tight, especially right turns. I guess I have sort of driven it like a delivery truck, because I have taken it out on many errands, but do try to park out of the way. The dolly makes us more mobile, of course, but I do need to reset my mind and really think before turning into a small gas station while towing!
  • navegator's avatar
    navegator
    Explorer
    Mar 24, 2015
    You obviously have big rig experience, the question is, are you trying to drive the RV as a car rather than a bus, taking corners to close to the curb instead of moving to the edge of the lane you are in and taking a wide turn into the lane that you are going into, taking tight corners will require the dolly a d the toad to take a sharper turn, thus having the pan swing to the stops.

    May I suggest that you take the RV dolly and toad to an unused parking lot, take some masking tape, paper and pen and a protractor.

    Drive straight and mark where the pan sits on the dolly that is what the masking tape is for, now make a shallow turn and again mark the angle of the dolly.

    Continue until the pan is at the stop, and look at how the toad is in relation to the whole set up, maybe the steering needs go be completely locked so that the toad does not wander of track and makes the toad push the pan against the stop.

    navegator
  • boogie_4wheel's avatar
    boogie_4wheel
    Explorer
    Mar 24, 2015
    consumeratlarge wrote:
    Since I've hit the stops for sure, what is 'giving' that allowed those turns to be made? I'd like to understand the physics to see where the stress goes.


    The stress is everywhere...
    • The dolly is pushing sideways on the receiver hitch in an attempt to not be turned any sharper
    • The front tires are being pulled in opposite directions on the car. On a right turn, the driver's tire is being pulled forward, and the passenger tire is being pushed backward
    • The dolly tires experience a side load force that would push them to the inside of the turn (a right turn would cause the dolly to want to slide to the right)
    • And the car's rear tires will want to slide to the outside of the turn (slide to the left)


    Since the lightest axle between the rear of the MH, the dolly, and the car's rear axle, is the car's rear axle. You can expect that on a tight turn the backend of the car will slide to the side, just as a dual axle trailer will scrub one of the axles on a tight turn.
  • consumeratlarge's avatar
    consumeratlarge
    Explorer
    Mar 24, 2015
    Thanks for those replies. I can visualize the pan turning the opposite direction at first, because of the tail swing. Then, in a constant radius turn, it seems like the pan's angle would have to turn the same angle as a car's wheels in the same radius turn. I understand the pivot at the hitch, but if you pulled the loaded dolly around a turn (if somebody was strong enough) doesn't it seem like the swiveling angle would limit the amount of turn? Since I've hit the stops for sure, what is 'giving' that allowed those turns to be made? I'd like to understand the physics to see where the stress goes.
  • Bobbo's avatar
    Bobbo
    Explorer II
    Mar 24, 2015
    I towed with a Stehl dolly for 3 years before selling that car and buying one I can tow 4 down. Never had a problem with any damage to the car.

    Definitely do NOT unlock the car's steering. That is only for a tow dolly that doesn't have any swivel of its own. The Stehl has a swivel pan.

    I can't answer about YOUR car, but mine was fine on the Stehl.

    I agree with the post that says the dolly will pivot a lot behind the MH, and the pan only has to make up the difference.
  • Ivylog's avatar
    Ivylog
    Explorer III
    Mar 24, 2015
    If you add in the angle of the dolly to the motorhome In a turn I think you're find you have plenty of turning angles for the turn. It's the sum of the two angles, not just the 10°.
  • diazr2's avatar
    diazr2
    Explorer
    Mar 24, 2015
    Yep Acme is the kind my buddy has. I know his turning radius
    is not effected with the dolly back there.
  • consumeratlarge's avatar
    consumeratlarge
    Explorer
    Mar 24, 2015
    the model, I think ACME, that has a fixed pan, says you're not supposed to tie the steering. The ones with the swiveling pan say to tie it, though. I could see a case for tying it with stretchable bungees, maybe, so that it could 'give' a little in a tighter turn, but then if the steering wheel went over 180 degrees, the bungees would be messed up.
  • diazr2's avatar
    diazr2
    Explorer
    Mar 24, 2015
    I am certainly no expert on towing dolly's but my friend just bought one and he says the steering wheel cannot be tied down it must be able to move freely. Maybe yours is the same and you shouldn't have tied the wheel down.

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