carcentric
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| posted on 28/1/06 at 12:52 AM |
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Ackerman angle - what if
Let's say your donor's front suspension was designed well with the Perfect Ackerman Angle (see diagram).
Then let's say you put the front suspension in a car (your Locost) with a different wheelbase than the donor car's (and change nothing
else).
1. What would be the effect of the Locost's wheelbase being GREATER than the donor's wheelbase?
2. What would be the effect of the Locost's wheelbase being LESS than the donor's wheelbase?
M D "Doc" Nugent
http://www.carcentric.com
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smart51
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| posted on 28/1/06 at 09:46 AM |
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Ackerman steering is designed such that when turning, the axis of all the wheels intersects at the same point. The car turns about this point and
none of the tyres scrub or try to pull the car in a direction other than the simple radius of turn. It works as slow speeds too.
If you move the front axle backwards, the point at which the front wheel axies meat the rear axis moves inwards towards the car. The front outer
wheel moves in more than the front inner wheel. The effect is that the outer front wheel tries to turn the car on a sharper radius than the front
inner wheel.
Due to body roll, the front outer wheel dominates and so the front inner wheel gets draged away from it's preferred course. That is until the
front outer wheel starts to slip, and I don't mean understeer just yet. The more cornering force a tyre has, the more slip angle it has. That
is the angle bewteen the direction it points and the direction it follows. If the outer front wheel has enough slip angle (more than the inner front
wheel) then normal ackerman steering gometry is resumed.
the result is that the geometry isn't quite right at parking speeds but gets a bit better as cornering speeds increase. If you go too fast then
the slip angles will get worse until you reach severe understeer.
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Syd Bridge
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| posted on 28/1/06 at 11:38 AM |
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Simple answer to the questions put....
If you lengthen the wheelbase and don't change the settings, Ackerman will be less than needed.
If you shorten wheelbase, reverse the situation above.
Syd.
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carcentric
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| posted on 28/1/06 at 05:43 PM |
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Thanks for both responses. I've read about inside tire scrub before, but have never heard if the direction of the error matters or not
(wheelbase too long versus wheelbase too short, for a particular front end).
A wheelbase too short example (one of my projects) is using a Mustang II front end (designed for a car with 96.2" wheelbase) in a Morris
Minor (86" wheelbase). I know I'm mixing inches with angles, but how much scrub would you expect in street driving (negligible,
noticable, or obnoxious)?
A wheelbase too long example (another of my projects) is using a Triumph Spitfire front end (designed for a car with a 83" wheelbase) in
VW volksrod (95.2" wheelbase stock, but this one will be extended about 6" more than that). Again, how much scrub would you expect in
street driving (negligible, noticable, or obnoxious)?
Both projects are described at www.carcentric.com if you're confused or curious.
[Edited on 28/1/06 by carcentric]
M D "Doc" Nugent
http://www.carcentric.com
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smart51
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| posted on 28/1/06 at 06:58 PM |
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Some people talke about having a percentage of ackerman. I guess that 50% ackerman means half way between having the correct ackeman angle on your
steering arms and having non at all. This is the same as having too short a wheel base. In arguements about ackerman, many people put this forward
as being the best choice. I've heard few if any people who want it the other way.
You can get over having the wrong wheel base by widening or narrowing the track to restor the ackerman angle. To widen, simply put a threaded spacer
between the rack and the track rod. shortenening the track is altoghether more difficult as many seven builders will tell you.
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Liam
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| posted on 28/1/06 at 10:42 PM |
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Ackerman geometry came about in the 18 hundreds and 100% ackerman (as in the diagram) only really applies to very slow moving stiff-wheeled vehicles.
Modern, high speed vehicle/tyre dynamics, with constantly changing contact patches and significant slip angles have made analysis of Ackerman geometry
as much a black art as most other suspension geometry choices 
If you want to avoid disturbing your gravel when turning sharply in your drive, or to drive silently over polished car park floors, then 100% Ackerman
is useful. Nowadays something like a fork-lift truck, for example, will have 100% Ackerman - your donor car certainly doesn't.
If you want to achieve maximum cornering performance, then you will want anything from over 100% Ackerman, through 0 Ackerman, to anti-Ackerman,
depending on your vehicle, your tyres, what you use it for, who you listen to, the weather...
There's reams to read about it. Prepare for lots of cans of worms, swings and roundabouts. A quick skim through something like
This will reveal how complex and subjective the subject is!
In short, for a locost just stick with whatever Ackerman your donor uprights give you and don't give your Ackerman/wheelbase/track relationship
more than 10 seconds of your time .
Liam
[Edited on 28/1/06 by Liam]
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carcentric
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| posted on 29/1/06 at 03:36 AM |
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quote:
Originally posted by Liam
. . . If you want to achieve maximum cornering performance, . . .Liam
I appreciate your input and the perspective it adds, but I had trouble with your phrase above.
Did you forget that I'm starting with Morris Minor, Triumph Spitfire, Mustang II, and VW bug parts?  
M D "Doc" Nugent
http://www.carcentric.com
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