Steering mechanism kinematics 1

[TeoMV]

I am currently designing a trailer for large boats (up to 200t) and i am in the process of steering mechanism design. I have made a basic and starting layout for evaluation. I attach 2 pics to let you underastand how this mechanism works. What you see is the left-hand wheel set.
Problem is that for any angle of steering the wheel at the back steers less than the front wheel. For example for 39.54 degrees of steering bar angle (steering bar is the long blue bar with the grey plate underneath) the front wheel steers by 38.78 degrees and the rear wheel steers 4.42 degrees less than the front wheel. I think this configuration will pose some problems to the large and heavy trailer steering. What do you think and which are the suggested solutions? Thanks in advance!

http://img704.imageshack.us/img704/1258/steeringmechanism.jpg

http://img192.imageshack.us/img192/9194/steeringmechanism2.jpg

 

[BrianPetersen]

The wheel at the back must steer less than the wheel at the front, otherwise the trailer will never actually turn, it'll just crab sideways. Think about it; the rear wheels on your car steer by less (zero) than the front wheels. Or am I missing something here?

 

[TeoMV]

I think you didnt understand fully the wheel configuration. Probably it was my fault not to explain it clearly. Here is a drawing of the whole trailer to let you understand the wheel layout. The set of wheel you have seen in the pictures of my first post are actually the front wheel sets. At the rear of the traile are another set of non-steering wheels.

 

[cntw1953]

I think Brian is correct.

The greater the difference of the steering angle, the sharper the trailer may turn. If both wheels have same steering angle, the trailer will follow a slant line, and needs a huge turning radius to make a turn.

You can plot the position of the wheels (two curves) on a grid and visualize the movement.

 

[dvd]

If two steerable axles are good, would three be better? How about five steerable axles? I think I would take a cue from the majority of steerable vehicles in existence and have a single steerable axle set. If you have to have more axles to handle the load, then perhaps they should be attached to a single truck that steers from a central pivot-point (think railway). I would also think about moving the fixed rear axles toward the front to support more of the load at that location.

 

[TeoMV]

The load needed to carry is 200t. The total trailer length is fixed at 12m. Fixed also is the position of the rear wheels as you dont want to leave much unsupported part of the trailer at the back, where the majority of the load exists. Given this, you need to have 2 axes at the front to carry the whole load. And 2 axes at the front means that you have to have 2 steerable axes at the front.

 

[dvd]

some concepts to think about:

http://rackleybiltcustomtrailers.com/9axle/index.html

 

[MintJulep]

I think bigger problems will be that you appear to have no caster angle or ackerman.

 

[BrianPetersen]

Okay, now I get it.

On a vehicle with two steerable front axles, one behind the other, the wheels in front must turn at a greater angle than the wheels in rear to minimize side-scrubbing of the tires when turning.

Want to see why ... Draw a plan-view diagram showing the four wheels. Pick an imaginary point that you want the trailer to circle around. It doesn't matter what the turn radius is for purpose of illustration. What DOES matter, is that this center of turning be in line with the non-steered axle. It has to be this way. If you have two non-steered axles, it must be in line with an imaginary axle centered between the to (and more on this in a moment).

Now, draw lines between that turning-center and the center of the contact point of each steering tire with the ground. That line is perpendicular to the direction of the direction of travel of that particular tire. So, draw the wheels, centered on the steering axis and perpendicular to that line.

Note that the required steering angle is different for the front and rear steering axle and different for the left and right sides (look up the term "Ackermann" for more on this).

With two non-steered rear axles, you will also note that in theory, those axles should also steer slightly. Those wheels will scrub sideways a little if you don't.

In practice, if you make both steered front axles steer the same angle, it'll still work, but with more tire scrubbing. Since it is possible using the linkage to achieve better than that, why not do it?

 

[GregLocock]

There have been many trucks with 4 wheel at the front steering, the 4 axle Octopus springs to mind. You need a different ackerman setup on the two axles, if you have a real steering linkage, or else you need to turn the rear one a bit less than the front if you have soapbox derby technology.



Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[TeoMV]

Greg,
I have in mind that right now i have anti-ackermann which is bad to the steering design. I am in the process of fixing the ackermann issue. But even if i fix ackermann does the rear wheel (of the front axle) needs to steer less than the front one and why?

 

[cntw1953]

TeoMV:

Please have a look on the PDF if it makes sense. You can do better to have a better feel by tracing wheel positions at specified rate of change of angles.

 

[Compositepro]

During a turn the axle of every wheel should be pointing at a single imaginary point, which is the center of radius of the turn. If they do not there will be some scuffing between the tire and the ground. So having two fixed axles will always result in some scuffing.

The steering wheels should also turn the same, or more than the trailer tongue so that they follow in the direction of the tongue. If the turn angle is smaller the following error increases with time.

 

[GregLocock]

BrianP and CompositePro seem to have nailed it. If all of your front wheel axes point at a common point on the rear axle axis, that is the best you can do. It won't be perfect, as you have twin rear axles.

The more subtle problem is designing your steering link from the towing beam to the front axle.

If I were you I'd start with the sort of diagram Brian suggests, for the towing vehicle AND trailer, and then see what your wheel angles need to be. Only then can you design your linkage.





Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[GregLocock]

Of course given the size of the thing you are only going to be moving at slow speed, the other option is to go to an independently steered trailer, using hydraulic or pneumatic rams.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[zekeman]

quote
"BrianP and CompositePro seem to have nailed it. If all of your front wheel axes point at a common point on the rear axle axis, that is the best you can do. It won't be perfect, as you have twin rear axles.

The more subtle problem is designing your steering link from the towing beam to the front axle.

If I were you I'd start with the sort of diagram Brian suggests, for the towing vehicle AND trailer, and then see what your wheel angles need to be. Only then can you design your linkage."

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How is that even remotely possible, given you have a 2 degree of freedom system since the trailer can be disposed at an arbitrary angle with respect to the truck at the pivot?
So unless I don't get it, the best you an do is have 2 different instantaneous centers of rotation, one for the truck and one for the trailer.
I'm also not seeing how the trailer knows how much to rotate its steering angle when the truck turns. Is this done by some magic? Please show me.I would think some kind of parallelogram mechanism could steer it but that may be too naive.
I think the Ackerman is the LEAST of the problem and only second order for this application.
I also have a problem with the OP trying to design this without an in-depth knowledge of kinematics, trailer dynamics and braking, none of which he can get here at the level he needs.
My advice is to get this done professionally and use this forum for ideas, not for design purposes.

 

[GregLocock]

Design for full lock. All practical steering systems have geometry errors, if it works at full lock and straight ahead then the rest will be OK at low speed.

He designed a steering system that picks up off the tow beam, which articulates at each end. I don't think that is necessarily the right approach, but until he knows the wheel angles that is just a detail.

I doubt he cares tuppence about kinematics, a 200 ton boat is moved very carefully around the yard, in my experience with 20 ton boats.




Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[zekeman]

Greg,
Well,if that's the case, you still need some basic tracking of the trailer steering. Maybe a parallelogram to the rear axle of the truck could work since it would at least make the trailer steering wheels parallel to the rear truck wheels.
I'm having a lot of trouble with that connecting beam.

 

[GregLocock]

Agreed, and I think a patent search will be useful.


Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight