This is a link to a very nicely illustrated website describing ackerman in glorious detail. Suprisingly, it's for RC cars, but I think some of
those people are pretty serious about their little vehicles! 
And of course those same principles apply no matter what the size of the car. 
RCTek Ackerman Link
As a bonus, there are a number of other nicely illustrated pages on the site explaining KPI, caster, camber, toe, etc... Very interesting and nicely
detailed.
Enjoy,
Graber
What a fantastic site and read. Well-done Grabber.
I had realised that the steering arm angles should be pointing towards the center of the rear axle when building go-carts as a kid, but didn’t know
that it was called the Ackerman Principle.
This is a bit of a can of worms because all of us who are using donor uprights will have this angle set wrong which for a performance car is a bit of
an annoyance. That is of course only true if the wheelbase and track width is changed from that of the donor! It would be interesting to know what the
correct angle should be for a standard “book 7” and if wrong how with minimal cost it could be adjusted?
These RC guys are nuts!
Some time ago I bookmarked this RC tech page that has a lot of great info about roll centers, weight transfer, etc,:
http://www.teamassociated.com/racerhub/techhelp/marc/car_handling.html
Got a kick out of the note on the main page that say for more information, see Milliken & Milliken!
I've got one of the little radio controlled methanol/nitro powered monsters. 
Its permanent 4wd and does 0-45mph in about 0.8seconds! Flat
out is 50mph! Its stupid. The power to weight ratio works out at about 700bhp/tonne 
which makes them nearly impossible to control, since they
weight about 1.2kg!
Anyway most of those tips are worth taking into account on the models and definately worth it for real cars! Especially the weight transfer stuff,
suprising the difference driving style makes. Just because they are small doesn't mean the stuff doesnt apply.
bit more history - king pin type steering was developed because top heavy stagecoaches had a disturbing tendency to fall over while being manouvred -
if the steering front wheels are round at 90 degrees there's NO roll stiffness at all. So some english guy patented swivelling the wheels out at
the ends of the axle.
Problem with this was that with the parallel steering arms it would break the wooden cartwheels due to the two wheels fighting each other on every
corner. So everyone remembers Mr Ackerman who worked out (and patented)how to stop that.
Or so I was told........
Bob C
quote:
Originally posted by flak monkey
Just because they are small doesn't mean the stuff doesnt apply.
Pete
Indeed they are! And no offence taken anyway..was just saying
Cheers
Does moving the rack not have an affect on bump steer?
Julian - according to my reasoning; rack up& down affects bump steer and fore & aft affects ackerman.
Bob C
quote:
Originally posted by sgraber
Suprisingly, it's for RC cars, but I think some of those people are pretty serious about their little vehicles!
Very serious.
I have digital corner weight scales and laser allignment kit for my 1/5th Viper
Tim.
quote:
Julian - according to my reasoning; rack up& down affects bump steer and fore & aft affects ackerman.
Stu,
I'm not disagreeing with you (yet anyway...) but can you explain how fore/aft movement could affect bump steer? There was a discussion on this a
little while ago and nobody disagreed with me when I said that longitudinal position wasn't important (even when I invited disagreement). I guess
you must have missed that one eh?
Anyway, I'm intrigued now so please do tell...
Cheers,
Craig.
ps. I'm assuming we are talking in the context of locost building and not some wierd suspension system. Therefore the upper and lower wishbones
mountings are each parallel to the chassis centreline.
quote:
Originally posted by Viper
Very serious.
I have digital corner weight scales and laser allignment kit for my 1/5th Viper
Tim.
quote:
Originally posted by Viper
Very serious.
I have digital corner weight scales and laser allignment kit for my 1/5th Viper
Tim.
Stu & Craig,
I only get fore/aft affecting bump steer because of castor angle, didn't want to make things too complicated - what haven't I thought of ?
?
Bob C
I would have thought if the rack isnt inline with the steerign arms, in bump and droop you'd get toe-in problems as the track rods dont follow the same arc as the wishbones??
Go,
Yes but you are talking about vertical positioning of the rack. Stu is saying (I think) that by moving the rack body fore and aft (longitudinal axis)
you will affect bump steer. This is the bit I don't get.
In my view rack height affects bump steer but longitudinal position only affects ackerman steering angles.
Cheers,
Craig.
Hi Craig,
I was actually meaning with the rack not inline fore and aft. As far as my mental modelling of the situation looks I think it would affect bump
steer.
But I am suffering from a stinker of a hangover so my mental ability probably isnt at its usual stupendously high level!
I think a lego model is called for... i will see if i can make a setup with my sons technics lego and try it
Cheers
Evening Craig. I ought to clarify my previous statement by saying fore and aft *may* affect bump steer-and for our causes, probably not by much.
IF the car has suspension which is completely rigid, i.e. there is definately going to be no flexing in suspension bushes, wishbones etc, and the
wishbones are parallel mounted top and bottom-vertically and horozontally, foe and aft movement will make no difference.
But in the real world, there is flex. Also a lot of cars (not necessarily Locosts) wishbone mounts are designed for anti dive, or offset to the centre
line of the vehicle.
Using the 'flex' as an example, when a front wheel hits a bump. or the car is subjected to heavy braking, the front wheels try to go back on
themselves, by a greater or lesser degree. If the rack was mounted closer to the centre line of the vehicle-i.e. the inner rack joints are further to
the rear of the vehicle than the track rod ends- it will encourage the front wheels to toe out, and visa-versa. Throw in the weird and wonderful arcs
the susupension geometry takes on some vehicles, and it becomes a nightmare....
Stu,
Thanks for putting my mind at rest as I was thinking that maybe I had missed something fundamental. What you are describing is what I would refer to
as "2nd order effects". (ie. those which are not fundamental but which should be given consideration once the fundamentals are fully
understood)
Yes I can see your argument about flexing movements (although with my Nylon bushes I don't expect to get much...) Mind you I'll probably
lose my fillings if I hit a bumpy road!
At the end of the day my own fore/aft choices are fairly limited as I have a crankshaft pulley at the back and the chassis at the front and not much
space in between. I think the key thing to maintain some ackerman effect is to make sure that the steering track rods point forwards slightly and that
they make an angle more than 90 degrees with the steering arms. This will ensure "some" ackerman although "how much" is anyones
guess at the moment...
Cheers,
Craig.