Ackermann steering when three axles?

[morson1]

Hi,

Trying to build a truck here, and have a question about Ackermann steering geometry. My truck will have two driven rear axles, fixed axles, non of them steerable. If you look at the attached picture, which alternative is the correct one? Should I line up my steering according to the blue lines, pointing at the center of the first rear axle? Or should I aim for the red lines, which points at center between axles? Or are both alternatives wrong, and in that case, what is the correct solution?

Thanks!

[deehtha]

I found this image on Jennifer Clark's website. This might help you figure it out. Looking at the picture, you would want to use the midpoint of the rear axles.

http://www.genuinemodels.com/skip_lrg/steering_trig_lrg.htm

[1974]

With the slack in LEGO parts I highy doubt either your or Jennifer's drawing is right or wrong. Just build what turns out the best with LEGO parts. Afterall, this is LEGO, not actual vehicles you're designing

[Jeroen Ottens]

The red line is the right one for non-steered rear axles

If you choose the blue one you have to make the last rearaxle steerable as well, but in the opposite direction and with a much smaller turningangle then the font axle

[morson1]

With the slack in LEGO parts I highy doubt either your or Jennifer's drawing is right or wrong. Just build what turns out the best with LEGO parts. Afterall, this is LEGO, not actual vehicles you're designing

I have a hard time doing things "halfway"

Maybe between axles is correct then. I´m just a little worried about the fact that both axles will then need to slide a little when steering, and maybe that will cause that sometimes axle no1 has best traction, next second axle 2 and so on, and that it will make the truck do strange movements when turning. Well well, in time we will see how this turns out...

[1974]

I have a hard time doing things "halfway"

You'll have a harder time when I move those front wheels with my hands into any of those "proper" positions that you (and Jennifer) have drawn and you'll not see your HOG knob move - get my drift?

It's LEGO, you gotta work on LEGO terms

Cheers,

Ole

[morson1]

You'll have a harder time when I move those front wheels with my hands into any of those "proper" positions that you (and Jennifer) have drawn and you'll not see your HOG knob move - get my drift?

It's LEGO, you gotta work on LEGO terms

Cheers,

Ole

Yeah I know... But I like the idea of at least doing things right in theory... But I´m well aware about the play there will be in all three axles of my truck

[Saberwing40k]

With any truck like this, there is a little bit of tire scrub in the rear axles, and this goes for real ones, too. The correct solution is to have the Ackerman correction pointing to the point between the rear axles. Although with Lego, having Ackerman or not does not really matter, at least for me.

[Lipko]

That's interesting. How do you implement the difference between the red and blue version with Lego? I mean at the front axle the difference is a few percent of a stud.

[dhc6twinotter]

And to throw a wrench into things, if one rear axle has dual tires and the other axle singles, the pivot point is not centered between the axles. The pivot point would be roughly 1/3 the distance from the axle with dual tires.

Years ago I found a website that had diagrams for a bunch of different wheel and axle configurations. It was pretty neat, but I can't seem to find it now.

[Didumos69]

If you have a steering setup that is not wobbly, then Ackermann steering certainly makes sense (with wobbly I mean a setup that suffers from substantial toe-in and toe-out depending on the driving direction).

You have to find out in practice what will work out the best. It could be that one of the rear axles carries more load than the other, which will affect the ideal pivot offset. My experience is: When your pivot offset is too small (insufficient Ackermann), then your front wheels will effectively suffer from toe-in in turns. Due to slack in the wheel hubs, this will visibly add unintended positive camber to your front wheels when driving forward and negative camber when driving backwards. When your pivot offset is too big (too much Ackermann), then your front axles will effectively suffer from toe-out in turns. The effect will be the opposite; it will visibly add unintended negative camber to your front wheels when driving forward and positive camber when driving backwards. I personally refuse to settle for this kind of behaviour.

As for the theoratical part: If you would decide to make one of the rear axles steered, then it needs a smaller steering angle (as Jeroen Ottens already pointed out), but also a bigger pivot offset.

Edited May 13, 2016 by Didumos69

[morson1]

That's interesting. How do you implement the difference between the red and blue version with Lego? I mean at the front axle the difference is a few percent of a stud.

The truck is quite large, so it´s easy to laborate with length and shape of steering arms and also lenghten or shorten the frame a stud or two, to achieve perfect Ackermann.

If you have a steering setup that is not wobbly, then Ackermann steering certainly makes sense (with wobbly I mean a setup that suffers from substantial toe-in and toe-out depending on the driving direction).

You have to find out in practice what will work out the best. It could be that one of the rear axles carries more load than the other, which will affect the ideal pivot offset. My experience is: When your pivot offset is too small (insufficient Ackermann), then your front wheels will effectively suffer from toe-in in turns. Due to slack in the wheel hubs, this will visibly add unintended positive camber to your front wheels when driving forward and negative camber when driving backwards. When your pivot offset is too big (too much Ackermann), then your front axles will effectively suffer from toe-out in turns. The effect will be the opposite; it will visibly add unintended negative camber to your front wheels when driving forward and positive camber when driving backwards. I personally refuse to settle for this kind of behaviour.

As for the theoratical part: If you would decide to make one of the rear axles steered, then it needs a smaller steering angle (as Jeroen Ottens already pointed out), but also a bigger pivot offset.

The rear axles should have same load as I aim for pendular suspension (load from frame travels to axles though only two points in center between axles). I have spent alot of time on the front axle, making it as unwobbly as possible in theory, not jet tested IRL. A steering rear axle is no option for me, the aim is to make the truck look and fucntion as a heavy duty from the 80's era, not many steerable axles back then. I also want it to carry plenty of load, future dream is hooking up a lowbed which I can transport my Forwarder on. But that forwarder weights in at 7 kilos....

[thomol]

But that forwarder weights in at 7 kilos....

Any info on this beast?

[morson1]

The Forwarder:

PowerPuller wheels, 8 wheel drive, one XL on each wheel, straight into hub reduction. Forced steering. Four LA:s for lifting boom, driven together by one XL. Claw rotates with micro motor, claw open/close with pneumatics. That's about it :)

Edited May 14, 2016 by morson1

[andythenorth]

Real trucks with tandem rear axles have substantial tyre scrub when making sharp turns. There's also tyre lean (tendency to walk out of the vertical) and tyre deformation (tyre bulges to one side). With walking beam suspension there's also a tendency for the rear bogie to walk out from under the centre line of the chassis.

With power puller wheels, they are ridged along the centre, this tends to accentuate tyre lean when turning. Depending on the construction of the axles and suspension this leads to axle deformation (bending in the horizontal plane), and/or body roll.

Larks eh?

If you're most interested in realism, have a dig around the websites for suspension vendors, and find the technical data (usually at the end of product PDF datasheets). For example, Hendrickson's USA site has some interesting stuff on it. http://www.hendrickson-intl.com/Truck

Edited May 14, 2016 by andythenorth

[morson1]

Real trucks with tandem rear axles have substantial tyre scrub when making sharp turns. There's also tyre lean (tendency to walk out of the vertical) and tyre deformation (tyre bulges to one side). With walking beam suspension there's also a tendency for the rear bogie to walk out from under the centre line of the chassis.

My main interest from the beginning was really if tyre scrubs occur on both axles on a real truck, or if real truck manufacturers goes for the blue line in my drawing and put all the scrubbing in the rear of the two axles, meaning that the truck acctually turns around the first of the rear axles.

With power puller wheels, they are ridged along the centre, this tends to accentuate tyre lean when turning. Depending on the construction of the axles and suspension this leads to axle deformation (bending in the horizontal plane), and/or body roll.

The note about Power Pullers was about my forwarder. On the truck I will use ordinary truck wheels.

Edited May 14, 2016 by morson1

[efferman]

with two unsteered axles in the back the red lines are correct to calculate the steering links and get a proper turning circle. the same method worked superb on my etf testchassis

sure, i have only one unsteered axle but the concept works

[PunkTacoNYC]

with two unsteered axles in the back the red lines are correct to calculate the steering links and get a proper turning circle. the same method worked superb on my etf testchassis

Awesome test and test chassis

And wow

That is a lot of unimog tires