interestedparty
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| posted on 5/2/11 at 08:49 AM |
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ackermann steering question
If I wanted to use a steering system that was origianlly on a car with 2700mm wheelbase on a different car with 2400mm wheelbase, would the 11%
difference cause any noticeable problems?
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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britishtrident
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| posted on 5/2/11 at 09:52 AM |
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Classical text book Ackerman geometry can't be applied to rack and pinion , with rack and pinion steering a quasi-ackerman geometry is
created by the for-aft position of the steering relative to the outer track rod ends, this is a subtle detail missed by many kit designers.
As the tyre friction characteristics is non-linear 100% true ackerman geometry is not desirable on anything faster than a milkfloat, some ackerman
effect is required to get good turn in but what percentage compared to classical theory is a moot point.
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Angel Acevedo
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| posted on 13/2/11 at 07:14 AM |
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quote:
Originally posted by interestedparty
If I wanted to use a steering system that was origianlly on a car with 2700mm wheelbase on a different car with 2400mm wheelbase, would the 11%
difference cause any noticeable problems?
I don´t think it will.
Variable amount of ackerman can be adjusted just by moving rack forward or backwards.
You may want to design some adjustability on the mounts if you are not sure about your results.
HTH.
AA
Beware of what you wish.. for it may come true....
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cliftyhanger
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| posted on 13/2/11 at 08:52 AM |
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Maybe I am missing something, but I don't see why the wheelbase will cause a problem.
I do know moving a steering rack causes mayhem. I moved my spitfire rack forward an inch to get a zetec in, and it caused chaos, the car was hopeless.
Took a load of work to move it back to within 1/4" of the original position, not too bad, but not as sharp as it should have been.
I "thing" it just needs to stay in the same relative position to the front suspension.
Now somebody is going to confitrm I know diddly squat about it
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matt_gsxr
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| posted on 13/2/11 at 09:27 AM |
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Well done for thinking about this in advance, but surely the difference will be negligibly small.
Given that many designs don't incorporate Ackerman, and F1 run "anti-Ackerman" I wouldn't have thought it would make any
practical difference. You notice Ackerman most when pushing the car by hand, as a lack of Ackerman can make it harder.
Regarding the Spitfire comments above, moving a rack by 1 inch could be a pretty massive amount, it could also mess up bump steer, which always feels
bad. I moved my rack around 5mm to tune out bump steer.
Matt
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cliftyhanger
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| posted on 13/2/11 at 09:30 AM |
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Nope, def not bump steer. It just had a massive effect, felt very wrong, even at low speed and not anywhere near the full lock.
Felt like the tracking was maasively out, which it wasn't (when straight!)
Thought bump steer was normally sorted by changing the height of the rack?
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britishtrident
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| posted on 13/2/11 at 07:26 PM |
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Ackerman greatly helps turn-in at the expense ultimate grip. Tyre friction characteristics and type of driving/roads have a big effect on what
ackerman angle is most suitable.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Neville Jones
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| posted on 14/2/11 at 10:23 AM |
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quote:
Originally posted by britishtrident
Ackerman greatly helps turn-in at the expense ultimate grip. Tyre friction characteristics and type of driving/roads have a big effect on what
ackerman angle is most suitable.
Just where do you get the BS you put on here about Ackerman????
That statement above is wholly incorrect. In theory, and more importantly, in practice!!!!!
Just think about what Ackerman does exactly, and how it does it, and what the tyres are actually doing. Yes, life is somewhat different in the real
world of practical application. A little education and more than a few years of experience are a help as well.
Cheers,
Nev.
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interestedparty
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| posted on 14/2/11 at 12:38 PM |
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quote:
Originally posted by Neville Jones
Just where do you get the BS you put on here about Ackerman????
That statement above is wholly incorrect. In theory, and more importantly, in practice!!!!!
Just think about what Ackerman does exactly, and how it does it, and what the tyres are actually doing. Yes, life is somewhat different in the real
world of practical application. A little education and more than a few years of experience are a help as well.
Cheers,
Nev.
There's something about this posting style that is familiar. Anyone else spotted it?
Oh, and FWIW, I am in awe of the range and depth of knowledge shown on this site by BT, who I think is a huge asset to LCB.
[Edited on 14/2/11 by interestedparty]
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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hughpinder
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| posted on 14/2/11 at 01:51 PM |
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Hmm
Lets not slag each other off please - most posters on here including britishtrident have been very helpful. Sometimes someone posts something you
don't think is right, or think is dangerous. You can then put your post giving the facts you believe to show the comments to be wrong. The
others reading the post can then draw their own conclusions or take the concencus opinion.
Perhaps Neville would like to say what the actual solution is, rather than just slagging off someone else.
The Cateham CSR uses about 60% ackerman.
Was the steering actually giving ackerman steering in the first set up anyway?
A longer wheelbase steering system in a car with shorter wheelbase will have 'quicker' steering.
Check the wheel lock does not allow the tyre to strike the bodywork.
For road use, I don't think you will notice the difference.
By the way, here's a link to some info that may be useful, and a summary below:
http://www.smithees-racetech.com.au/ackerman.html
In conclusion:
Steering angles achieved are not perfect Ackerman geometry. They vary with roll/bump etc. What we want to acheve is that we have increasing dynamic
toe out, and that it is increasing with steering angle. For small steering angles, Ackerman is minimal.
Your change in wheelbase will certainly change the steering characteristics.
You cannot predict exactly what to expect unless you can do the maths to allow for the KPI/static camber etc.
Your tyres will have a slip angle that gives maximum grip for a given normal force - if you do not have normal force/coef of friction/slip angle data
you are almost guessing.
Manufacturers (who I hope are ahead of us in this area) do the calculations of ackerman angle etc to check they are in the right ball park - they
build a prototype then and run it around a track to get the final settings.
Regards
Hugh
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Neville Jones
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| posted on 14/2/11 at 04:55 PM |
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quote:
[Perhaps Neville would like to say what the actual solution is, rather than just slagging off someone else.
Regards
Hugh
Hi Hugh,
I'm not 'slagging off' the hallowed Mr.Britishtrident. He seems to be a mine of little gems relating to older cars and technology
involved in classic cars. But the man comes out with some diabolical untrue statements at times. There's just too much of it on the internet,
then people read it, see it written positively with authority and confidence, and believe it to be true, when it is nowhere close to truth.
I'd be glad to talk about Ackerman, but BT thinks that Ackerman can only be achieved on carthorse beam axle setups, and refuses to even
entertain the thought that Ackerman can and is configurable on rack and pinion/independant suspension setups.
Ackerman geometry relates to the difference in angle between the inner and outer steering wheels. Ideally, both are at rightangles to the radial line
drawn to the steering axes of each wheel, when the line is drawn from the centre of the turning circle. The centre of the circle is normally
considered to be on the line drawn through the line of the rear axle.(When the two front wheels are doing the steering.
How Ackerman geometry is achieved differs, but relates primarily to the angle between the steering arm and track rod. Steering arms can be differing
angles, meaning that track rod/rack position needs to be moved forward/rearward to achieve the needed angles.
How much Ackeman is purely up to personal thinking and how much work you think the inside wheel does in cornering.
Modern thinking is to ignore what the inside wheel is doing. Then again, some designers are of a mind to try and make the inside wheel do a bit more
of the turning work, so utilise varying amounts of the mythical Ackerman geometry.
Cheers,
Nev. 
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interestedparty
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| posted on 14/2/11 at 05:18 PM |
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quote:
Originally posted by Neville Jones
I'm not 'slagging off' the hallowed Mr.Britishtrident.
and then-
quote:
Originally posted by Neville Jones
But the man comes out with some diabolical untrue statements at times.
I think you need to make your mind up there Neville.
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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hughpinder
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| posted on 14/2/11 at 08:06 PM |
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The ackerman set up is as described - both wheels are tangential to the centre of the circle you are turning on.
It is the optimum to achieve minimum energy loss on a corner - hence ideal for muscle powered vehicles doing relatively sharp turns and was developed
by Mr Ackeman for precisely this purpose. In this situation the vehicle is being pulled rather than driven.
This is not the ideal situation for a modern car to achieve maximum grip. This occurs when you achieve the best slip angle on the tyres - typically 4
to 7 degrees at ultimate limit. For this you need to know what slip angle gives the optimum grip for a given normal force - e.g it will vary as you
turn because the weight on the wheels changes. I found this a difficult concept to visualise.
I started off wanting full ackerman in my design, but have come to the conclusion that it is not necessary, although it will give the lightest
steering.
However, none of this actually answers the original posters query!
Regards
Hugh
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smart51
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| posted on 14/2/11 at 08:15 PM |
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You could narrow the track to compensate. You'd be right back where you started then.
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britishtrident
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| posted on 15/2/11 at 08:59 AM |
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The best illustration of the bad effects of too much ackerman effect is to watch any old episode of the Rockford Files TV series, if you watch what
the inside front wheel of any of the cars in a car chase you will see the inside front wheel getting dragged squealing across the road surface
because the maximum slip angle it can tolerate at the download it is carrying has been exceeded by a very large margin and all the front end
cornering force is being generated by outside front wheel. Tyre slip angle characteristics vary greatly between tyre designs but are always
non-linear with the amount of slip angle the tyre can do use ful work at falling off steeply at light load.
As I already stated classical ackerman geometry (that is the geometry as illustrated in motor technician text books) only holds true on
vehicles without a rack and pinion steering, it only works on vehicles that employ a steering box & centre track rod on cars with independent
suspension an idler arm is also required.
With rack and pinion steering the a of conflict of arcs comes into effect ( a bit like horizontal bumpsteer equivalent of bump steer) , which has
to be taken into any calculations and is harnessed d as the major generator of quasi Ackerman effect.
An examination of the Cortina front end parts shows another difficulty with Ackerman geometry on layouts where the rack is mounted ahead of the
line joining steering axis centres. On the Cortina upright the brake disks run very close to the outer track end hence the reason why the Cortina
steering arms have very little ackerman geometry designed in.
[Edited on 15/2/11 by britishtrident]
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Neville Jones
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| posted on 15/2/11 at 09:09 AM |
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quote:
Originally posted by hughpinder
By the way, here's a link to some info that may be useful, and a summary below:
http://www.smithees-racetech.com.au/ackerman.html
Regards
Hugh
And from reading that link above, I have been exonnerated in my thoughts and comments.
I'm obviously not the only one in the world who can think and reason for themselves.
Rockford files??? Come on, watch BOTH the front wheels, for a more accurate picture of what's going on.
Cheers,
Nev.
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TheGecko
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| posted on 15/2/11 at 09:51 AM |
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Without "taking sides" (not that I'm even sure how many "sides" there are in this discussion ), here's some
more grist for the mill.
Borrowed from "Suspension Geometry and Computation" by John C. Dixon - ISBN 978-0-47051-021-6.
Dominic
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hughpinder
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| posted on 15/2/11 at 11:17 AM |
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This is my last post on this subject.
Here is Nevilles comment:
"
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
Originally posted by britishtrident
Ackerman greatly helps turn-in at the expense ultimate grip. Tyre friction characteristics and type of driving/roads have a big effect on what
ackerman angle is most suitable.
--------------------------------------------------------------------------------
Just where do you get the BS you put on here about Ackerman????
That statement above is wholly incorrect. In theory, and more importantly, in practice!!!!!
--------------------------------------------------------------------------------
"
And later:
"
--------------------------------------------------------------------------------
And from reading that link above, I have been exonnerated in my thoughts and comments.
I'm obviously not the only one in the world who can think and reason for themselves.
--------------------------------------------------------------------------------
"
So, fully exonnerated in your comments - I couldn't find anything in that artice that says you can be rude to someone.
Fully exonerated - you say the above is "wholly incorrect" this means the opposite is true, so you're saying ackerman has:
NO effect on turn in
Full ackerman always gives maximum grip.
and Tyre friction characteristics and type of driving have no effect on suitable ackerman %.
I'm sure the formula 1 teams will be please to know they have it all wrong - they just needed full ackerman to get maximum grip, and they
don't even have to worry about which tyres they're using, what circuit it is, or who is driving the car!
Hugh
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britishtrident
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| posted on 15/2/11 at 12:56 PM |
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How much current F1 practice is relevant to with the type of cars we build/assemble/drive is a moot point leaving aside that F1 cars have aero
down force that can be measured in tons and most of the suspension movement is in the tyres they don't have to be easy to drive off the
limit.
On a rwd car on the front axle static toe-out produces varying degrees of steering instability particularly on uneven roads I have always favoured a
tiny amount of toe-in as this gives a tiny degree of pre-load to the tyre wall on the outside wheel which should give a sharper response on the very
early stage of turn in.
A degree of static toe-out is favoured in some racing classes including oval racing but they don't have be driveable on public roads.
When considering ackerman and turning in to a corner weight transfer starts from zero and builds up as the slip angle of the tyres increases it
follows that in the early stages of turn in a fairly high percentage of ackerman will be benefical, but as slip angle and weight transfer increase
the loading on the inside front tyre decreases as a result the inside tyre is working at too large a slip angle for the "weight" it is
carrying.
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