Hi

Are there any steering geometry experts out there? I need help! My current project involves fitting a 3.2 omega engine mated to a getrag gearbox into a viva E, I can fit the engine in with no modifications to the crossmember but I would like to fit the engine further back in the car, to do this would involve modifying the crossmember( not a problem) and to move the steering rack(problem?). What I would like to do is move the rack forward of the engine installation ( approx 300mm of its current position) and moumt it on its own cross member, but keep it at the same height from the road as it is now, keep the same cast links that are currently bolted to the hubs, and increase the lengths of the tie rods on the steering rack. I have been trying to get my head around ackerman angles and bump steer, the way I understand it, by relocating the rack in one dimension only, ie keeping it central across the width of the car as it is now, keeping its height the same, and keeping the cast links the same I should not alter the designed ackerman angle, and by lengthening the tie rods should reduce any chance of bump steer. Am I way off with this? I hope this is written in a understandable way.
Regards
Alan

Alan, it is impossible for anyone to say if your setup will work or not.
There are some basic rules regarding steering racks, but the main one is bump steer.
When your rack is centred in the straight ahead position, the swivel joint of the rack track rod arms, must be in line with an imaginary line between the top and bottom wishbone's inboard mounting points.
If you haven't got top bones, this makes the set up more difficult.
You are looking for the track rod arms to follow the same arc as your wheel.
If it doesn't, then as you go over bumps the uneven set up will cause your wheels to toe in or out, making the car change direction or give you poor steering feedback.
You should keep the rack in line with the upright control arms as much as possible, so if you move the rack forward or back, it is advisable to extend/reduce these as well.

If you move the rack forward you will knock the steering geometry completely out.

Forget most of the stuff in motor technician text books as it is only valid for steering designs that use an old fashioned steering box and centre track rod.

Rack & Pinion steering cannot have true ackerman geometry, toe-out in turns is largely achieved by mounting the steering rack further aft than an imaginary line joining the centres of the (outer) track rod ends.


If you move the rack even 25mm forwards you get less toe-out in turns. Move it 300mm ie 1 foot forwards and you will a lot of toe-in in turns ie anti-ackerman.

On the bright side if the rack is at the same height bump steer will be unchanged.

[Edited on 28/3/08 by britishtrident]

[Edited on 28/3/08 by britishtrident]

My rack is in front and not exactly in line (frame is in the way )
IMO you must not worry too much.
My old Kadett (chevette) scrubs the whole street in tight corners.
In most off the driving you steering just a little. The tyre will also provide some compensation if there is bad ackermann.

Wow... 300 mm is altought a lot!!!
It's too much IMO....
(next time better reading first......)

[Edited on 28/3/08 by t.j.]

Britishtrident,, I am interested in your coment that true ackermann is not achieveable with rack and pinion would you like to expand....Ians

quote:

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Originally posted by Ians
Britishtrident,, I am interested in your coment that true ackermann is not achieveable with rack and pinion would you like to expand....Ians

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Lots of production cars contain less than perfect geometry mainly to deal with packaging issues. My query was the statement that pefect ackermann could not be obtained using a rack and pinion setup. Ians

quote:

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Originally posted by Ians
Lots of production cars contain less than perfect geometry mainly to deal with packaging issues. My query was the statement that pefect ackermann could not be obtained using a rack and pinion setup. Ians

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t.j. is partly right in at least with respect to the Mk3-5 Cortina and rwd Escort and many other rwd cars --- on these cars the space where the track rod ends would have to be to give ackerman geometry is occupied by the brake discs, however this only part of the story.

With a Rack & Pinion looking down from above when the road wheels are turned from the straight ahead position a conflict of arcs will always exist that will shorten or lengthen the equivalent effective length of the track rod. This conflict of arc between the steering arms and the track rods has a major effect on the steering angle (" toe " ) of each wheel.


On vehicles with steering box and centre track rod -- such as a beam axle as on a truck, no conflict of arcs exists and the road wheel angles (ie "toe" ) are exactly as ackerman theory would predict.


[Edited on 29/3/08 by britishtrident]

[Edited on 29/3/08 by britishtrident]

And learned again....

thnx

All this talk of rack and pinion steering not being able to achieve Ackerman geometry is total and uninformed bullshite.

The critical angle to achieve Ackerman geometry is that between the steering arm and track rod. Ignore what the 'experts' and books say, regarding rear axle midpoints and all that rubbish. It 'might' work on trucks, and that's a fairly sizeable 'might', but with a rack and pinion setup you need to do some complex geometry to get what you need.

With rack and pinion you design in excess, full, or partial Ackerman movement, depending on what you do with the rest of the suspension. In my everyday work, where I design this sort of stuff regularly, I usually aim for a bit of excess.

And contrary to all common thought and 'rules of thumb', the vertical position of the rack ends is totally dependant on suspension geometry,(wishbone lengths and angles), and more often than not falls well outside of the legendary 'line between the wishbone pivots'.

In summary; the wishbone lengths and position of the outer steering joint dictate the position of the inner end of the track rod/outer end of the rack joint.

Macpherson struts can be utilised using the same basic principles I've outlined above. Knowing the arcs of critical point movements, then allows precise placement of the rack, and lengths of steering components.

Most motor manufacurers ignore all of the above, in general, and design firstly for cost. It must be cheap to build, and install; and hopefully will work in some sort of acceptable manner.

Cheers,
Syd.

Syd

Thanks for agreeing with what have always been my basic design philosophies of included ackermann. There is no reason apart from designed phyiscal constraints that ackermann cannot be included with correct positioning of rack and pinion. The angle of the steering arm is the primary factor, but the tierod angle and length play a significant role . However, the arm-to-tierod angle is a better predictor of ackermann than the angle of the steering arm. Classic ackermann dictates an intersection point at the centre of the rear axle (an accurate assumption when used on wooden wheeled vehicles). Modern tyres operate at significant silp angles so the mathematical relationship changes. The actual process may not follow the traditional pattern. Reference to this can be seen in an SAE White Paper (Society of Automobile Engineers) 2006-01-3638, the paper being presented by myself and Wm C Mitchell (William C Mitchell Software, Racing By Numbers). A further reference can be sought in Competition Car Suspension 4th Edition Allan Staniforth who kindly included within chapter 4 all the results of my 1998/1999 published work. Ians.

A long time ago I gave up trying to explain this to Syd Bridge.

(1) A Rack and Pinnion Steering can not have true ackerman geometry --- end of story.

(2) On a rack & pinnion steering with there track rods mounted ahead of the front wheel centre lines (such as Cortina Mks 3 to 5, Escort Mk1 & 2, Hillman Avenger ) its is just about impossible to have ANY conventional ackerman the track rod ends would have to bein the space occupied by brake disk.

(3) The fore-aft track rod aligment geometry on for example a Cortina or Mk2 Escort gives a quasi-ackerman effect that is much bigger than the effect possible from angling/bending the steering arms on these cars. as t.j. pointed out the convergence lines of the virtual steer arms on a cortina suspension is way beyond the ackerman point (the rear axle centre line).

(4) Information on the geometry of rack and pinion steering isn't widely published in the text books which stick to simple ackerman. In fact by mounting the steering rack aft of the virtual line joining the track rod end centres about 20 mm change in toe out (at the tyre tread) is generated.

END OF STORY

I try to help those who need more knowledge, I can put up with fools but not those who don't consider anyone but themselves might just have something to contribute.

A few years back Syd Bridge with constant back bighting & p2p messages seeking support drove out the most knowledgeable and respected contributer to this forum.

British Trident

I am very sorry to have to disagree with your statement that perfect ackermann cannot be achieved when using rack and pinion steering. In 2001 a PhD thesis titled 'The Calculus of Ackermann' requied I build in my workshops a half scale chassis. This was used for emperical testing to qualify a computer programme designed by Mike McDermott (technical contributor to Race Tech magazine). The half scale model enabled calculations to be verified working on wheel base lengths from 80 inches to 104 inches, varying track widths and the ability to move the steering rack both for an aft a total of 10 inches from the front axle centre line. And whilst I totally agree with your comment on how this has a dramatic negative effect on ackermann, it also became apparent by modifying the steering arm on the upright ackermann could be restored. In some cases this required an intersection point 50% forward of the standarised rear axle intersection point. The next step was to apply this to a real life situation, this being because of certain design contraints Allan Staniforth's hillclimb Megapin had been built with the rack in a less than desireable position of 6.5 inches forward of the front axle centre line. A modification to the steering arm provided a intersection point 54% forward of the rear axle. This changed the vehicle from formerly running full anti-ackermann to a vehicle with near perfect ackermann. We were able to verify this by dialing in known factors of steering input - corner radii and data login proved the differential in angles between inner and outer wheel. I agree also with your comments that most ackermann information resorts to basic lined drawings and do not take into consideration the rising/falling angles of the steering rack outer pivot. The earliest reference to this is a book called "Tyres, Suspension and Handling", John C Dixon, Cambridge University Press (1991). "Moving the rack forwards or backwards to change the tierod angles can be a useful way to adjust the ackermann factor, the most important angle being the angle between the tie-rod and the steering arm in plan view, the ackermann factor being proportional to the deviation of this angle from 90 degrees" This sentence was published in 1966 in his first edition. If you would care to continue this discussion off forum (to save filling the ether with script) I would be welcome the discussion.

Ian Scott (Ians)

Your work was somewhat predated by a guy called Pythagoras of Samos as far as I know his work has never been over turned.

I have never said that cars with rack & pinnion steering cannot have ackerman effects. A car with rack & pinnion steering can have 100% of the toe-out on turns demanded by Ackerman theory BUT I repeat they cannot use classical Ackerman geometry.

Perhaps if you were to examine the steering rack location on a real world production car you might understand.

[Edited on 30/3/08 by britishtrident]

'A few years back Syd Bridge with constant back bighting & p2p messages seeking support drove out the most knowledgeable and respected contributer to this forum.'

Dear Mr Trident, please explain the above statement fully, as I am at a complete loss as to what you are talking about. Well, no changes there anyway. If anything, you are consistent. No, I am doing you a disservice, as you are very knowledgeable about old cars, which leads me to the conclusion that you are a much older person than myself, and probably a retired motor mechanic, with too much spare time on your hands.

Regarding Ackerman...

As far as I am aware, Ackerman geometry is all about the change in steering angle, ie angle of the wheel to c/l, of each steerable wheel, and the need for the inner wheel to turn more than the outer.

Ians seems to have an education that goes past Wikipedia, as do I. He has reached the same conclusion as I did more than 30 years ago, and that is that the angle changes dictated by Ackerman theory and geometry, are a reality and achievable with rack and pinion steering, whether the rack be ahead of or behind the axle line, as with the steering arms as well.

To me, and the rest of the engineering world, this is called 'Ackerman Geometry', and is arrived at from 'Ackerman Theory'.

That you choose to call it something else is your problem, but you may wish to educate yourself properly and fully, before putting yourself forward as the font of all definitive knowledge on this subject, when others here are obviously far more educated in the subject than yourself.

As for Staniforth, I gave up on his books in the early 80's, when I could make no connection in Maths or Physics of his 'Roll Centres', to that which is the reality of the situation. Staniforth's books are very helpful to the amateur, but can be very misleading and even confusing, to someone with more than 'A' level Maths and Physics.

Have a nice day,
Cheers,
Syd.

[Edited on 30/3/08 by Syd Bridge]

come syd, im sure we all know which much loved ozzy ex poster he refers to

I dont think theres much point in hammering the ackermann discussion to death, since we all agree that cars dont need precise ackermann anyway. It would merely be a theoretical debate about how exactly ackerman applies to rack and pinion steering, which would take much longer than it is worth. A definitive waste of paper and time, though it would undoubtedly help us understand ackermann to a much greater degree than we need.

Id rather use my spare time catching up on paperwork, but if anyone fancies accurate worked examples for different steering rack positions, im all ears.