
Ackermann Steering Geometry - huhtenberg
https://en.wikipedia.org/wiki/Ackermann_steering_geometry
======
erikig
The mechanics of turning are one of those things that seem simple and mundane
at first but become increasingly more interesting the more you analyze them.

For instance:

How do trains turn if the outside wheels can't spin faster than the interior
wheels? \-
[https://www.youtube.com/watch?v=Ku8BOBwD4hc](https://www.youtube.com/watch?v=Ku8BOBwD4hc)

How do you create stable train wheels? \-
[https://www.youtube.com/watch?v=agd8B-31bjE](https://www.youtube.com/watch?v=agd8B-31bjE)

How do automotive differentials work? \-
[https://www.youtube.com/watch?v=85CA4_cgZ5U](https://www.youtube.com/watch?v=85CA4_cgZ5U)

How do 6 wheel truck differentials work? \-
[https://youtu.be/LwZBnMQ40rI?t=261](https://youtu.be/LwZBnMQ40rI?t=261)

And on and on down the rabbit hole

~~~
auxym
Great video from 1937 on differentials:
[https://www.youtube.com/watch?v=yYAw79386WI](https://www.youtube.com/watch?v=yYAw79386WI)

~~~
userbinator
I think it's been discussed a few times on HN before. The fact that it's over
80 years old yet easier to understand than newer material is amazing.

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giardini
As a boy I'd always wanted a small car that I could drive. I pestered my
father for years with plans from various magazines until one came up for a
kart ("go-kart"). He got interested and bought some scrap mild steel. My job
was to cut everything to fit and assemble it. We paid a welder to weld it.

But the steering geometry was the beautiful part. We knew about Ackerman's
work and also about camber, caster and toe-in/out but had never done any
design. So I had to read up on steering design/geometry. I already had a
strong interest in auto sports. My geometry class turned out to be quite
useful. FWIW on a kart with no suspension the geometry problems are simpler.

That kart is still in the family, still running and never had a structural
failure. It's worn out a number of engines and been a blast for all involved.

[http://yospeed.com/wheel-alignment-explained-camber-
caster-t...](http://yospeed.com/wheel-alignment-explained-camber-caster-toe/)

~~~
joshu
a competition kart with a single solid rear axle has a different steering
setup from the car. the wheels also dig into the ground to lift one of the
rear tires up, which allows the cart to actually turn. otherwise they
understeer under enough throttle.

when a car goes fast enough you actually want anti-ackermann. the slip angles
and not the actual angles are what matter, and the inner wheel has a different
load which means it needs to turn less rather than more in order to still
grip.

~~~
giardini
Our kart had a live rear axle with power to both wheels and slicks on the
rear. Top speeds was < 60 mph so standard Ackerman geometry was fine.

On a track or flat parking lot you could throw the wheel hard left or right
even at max speed with no controllability problem other than the
aforementioned slight "washing out" if the surface was slick. It _would_ go
into a very nice predictable controlled slide.

I miss it! It was great training for later driving a car. I never had the urge
to do anything foolish in an automobile b/c the kart was always far, far more
exciting. Driving a car was dull in comparison.

~~~
joshu
shifter karts are faster, but the one i modified for a project only did 60mph.
you still needed to lean. you were probably still seeing a great deal of slip.

take a look at the steering on a pro kart - the wheels are deeply angled
around the kingpin and dig into the ground.

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matahwoosh
Here's a neat demo in JS
[https://www.quaxio.com/steering/](https://www.quaxio.com/steering/)

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newnewpdro
My first exposure to this term and concept was back in the early-mid 90s when
building and racing RC-10s. Ackermann was something we could adjust, and it,
combined with caster, made a big difference in how the vehicles steered
especially in the dirt.

~~~
chasd00
yep, I learned a lot about steering and suspension tuning racing rc. I raced
dirt oval and then asphalt oval back in the day.

~~~
newnewpdro
Hobby-grade RC kits have to be one of the most useful educational "toys" you
can give children, even today.

My childhood street had a circle of boys who regularly did RC stuff together,
our bedrooms overflowing with kits, parts, tools, it was borderline obsessive.
But as we became young adults, it was _very_ apparent that we had learned a
whole lot of valuable skills the kids whose parents refused to spend money on
the relatively expensive toys lacked.

I remember like it was yesterday a friend letting me take one of his
disassembled kits unfamiliar to me home without the instructions so I could
put it back together "blind" for the fun of it. It was just a box full of tiny
screws and parts, such a great puzzle, with only the box exterior photos to go
from. It was a Kyosho Lazer ZX-R, I coveted that kit.

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rocketperson7
The most bizarre application of this is drift cars

They actually have pretty aggressive negative Ackerman, which is why they can
do stuff like reverse entry drifts

~~~
mirimir
I love ice-covered parking lots in the winter!

But you gotta know that the lot has no interior curbs.

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mhb
And more generally:

Control Schemes of Steering System of a Multi-axle All-wheel-steering Robot

[https://news.ycombinator.com/item?id=21143135](https://news.ycombinator.com/item?id=21143135)

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codesushi42
This is incredibly useful for driving games. But like all physics in games, it
is better to fake it.

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zhamisen
This model always reminds me of Nelson's parking problem:

[https://rigtriv.wordpress.com/2007/10/01/parallel-
parking/](https://rigtriv.wordpress.com/2007/10/01/parallel-parking/)

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etaioinshrdlu
This would seem unnecessary if you have a differential, right? Or if your
front wheels are unpowered.

This steering configuration would seem to also require very large wheel wells.

~~~
chowells
None of the above.

The problem this addresses is facing each front wheel the correct direction
when turning.

Imagine fixing a pair of bicycles together at the bodies. When you want to go
around a corner, the inner bicycle will have to turn more sharply than the
outer one (which in turn has to go faster). That means that the front wheel
needs a higher angle of deflection on the inside than the outside.

The same dynamic exists in a true 4-wheel vehicle, except that there is a
mechanical linkage that allows you to steer both front wheels together instead
of independently. The problem this linkage geometry solves is the problem of
getting the inner wheel to deflect further than the outer wheel, by
approximately the right amount. (iirc, this geometry doesn't get that perfect,
and modern cars use a variant that's a bit more accurate. But that's a small
optimization compared to the improvement this provides over just making the
wheels both turn the same angle.)

The problem a differential solves is allowing the wheels on the outside edge
to spin faster even when sharing an axle with the wheels on the inside edge.

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IgorPartola
Is this what the Yamaha Niken uses?

~~~
salty_biscuits
Yeah, but with such a short distance between the wheelbase I'm not sure how
important this would be apart from the very tightest of corners.

~~~
IgorPartola
Well with the Niken it’s the fact that it’s got two wheels up front but can
still lean. That’s as opposed to the Can-Am Spyder which is an old school
trike.

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francislavoie
Huh, TIL. That's really cool!

