Vince, you woke me up..................

Location: Near junc 8. M25

We've been here before haven't we but last time it was about not being able to track in sufficiently rather than too much toe in. I've just had a thought that it may be that when the wheel base changed from 6'3" to 6'9" the factory had to alter the angle of the track rod ends as according to Ackerman theory the angle of the the two track rod ends must intersect at the centre point of the back axle. Using track rod ends from an early chassis on a late set up would result in the centre line being in front of the centre line of the axle and vice versa. The difference in angle of early and late type track rod ends would not be discernable by eye. Is this a possible explanation?
Steve

Location: somerset

Not sure why Mr Leek is so bored- I found this discussion rather more interesting than -

"My Austin Seven is making a funny noise- what is causing this?"

type of question.

But that is just me

Tony.

Location: Malvern, Melbourne, Victoria, Australia.

Steve,

Would they have different part numbers if this was the case?

The Cross Tube Levers were BM 91 and BM 92 from 1926 to 1936 at least!

I don't think Mr Ackerman was quite that fussy

Tony.

Location: Malvern, Melbourne, Victoria, Australia.

Not for Vince or Jon (unless it was the wood stuff that bored)

The axis of the turned wheels projected meet at a point on a line extended from the back axle. But a line from the kp thru the steering joint meets the centre line far forward of the back axle. Apparent from the chassis greasing diagrams for old cars.

I particularly know this because an owner of a Jowett weirdly modified for rack steering asked me the question recently and I initially gave the answer as the earlier post above!

Bob Culver

Location: Auckland

Location: Devon

Location: somerset

All is well and good provided you believe the Ackermann theory. I don't.
The theory is that the front wheels need to turn in at different angles, relative to the arcs they describe when being steered around a corner. Unfortunately, that's not what happens. The cornering force is provided by the reaction to the slip generated when the wheels are turned into the instantaneous direction of travel. They don't roll around, they get pushed. It's just Newtons laws of motion, if you think about it. It follows that for good cornering you need the heavier loaded wheel to get more of a push than the lighter one. That's the outside wheel in a corner. So, you need to turn the outside wheel in further than the inside one. That's the opposite if Ackerman.
I've explained that very badly. Am I starting to write in a New Zealand accent. If so tell me to shut up, someone.

I don't know either Steve but the thought occurred to me that it might provide an answer to your theory :)

Location: Devon

And of course lots of A7 racers fit the track rod out front, which must give an anti ackermann effect?
Steve. Edit.... although if they swap arms side to side maybe not?

Location: South East Cornwall

"String computer"?
Have you been reading my old mate Allan Staniforth's book?
If you haven't, you should.

Location: Devon

Location: Devon

As Mrs Google advises on Wikipedia (not always right I know, but some here are not always right anyway so what the ****)

"A simple approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle. The steering pivot points are joined by a rigid bar called the tie rod which can also be part of the steering mechanism, in the form of a rack and pinion for instance. With perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point. Note that this may be difficult to arrange in practice with simple linkages, and designers are advised to draw or analyze their steering systems over the full range of steering angles.

Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low-speed manoeuvres. Some race cars use reverse Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tyres while cornering at high speed. The use of such geometry helps reduce tyre temperatures during high-speed cornering but compromises performance in low-speed manouvers."

As suggested, your basic Austin Seven at ordinary road speeds does gain some benefit from a reasonably correct Ackerman geometry but a racing version can be and often is quite different. Sorry if this is all too boring for some sensitive souls

Tony

Location: Malvern, Melbourne, Victoria, Australia.

For a boring topic seems to have attracted a lot of curiosity and comment. Ranks right up there with the oft mentioned relish!

For what it is worth, I have a copy of Austin Cars 1936-39 Running and Maintenance Instructions publication 843/c 1957 with chassis diagrams. For all, the projected line thru kp or rim centre meets driveshaft about half way between axles. I presume the point where kp projected meets the road, usually just inside middle of rim, is the key datum.
Maintenance manuals make much of the importance of the relative wheel turn angles, but probably more as a check against damage. Unlike racing, a lot of cornering is done with similar weight on both front wheels so simple Ackerman is not entirely inappropriate.

There must be a dimensioned drawing of the track rod somewhere. (A sketch in the Edge notebook) Has anyone had one break? Many other cars often remain controllable at least until braked! One virtue of the early Seven, can stop with the rear brakes alonebut it does require non instinctive use of the pedal!

Bob Culver

Location: Auckland

Fol de rol

'Fol de rol'


Boring and obscure!

Tony.

Location: Malvern, Melbourne, Victoria, Australia.

To take the boredom to a new level, I placed a length of straight steel tube across the rough center lines of king pin and track rod end on my Ruby chassis earlier today. The hub was as straight as I could judge with no wheel on. Anyway, the projected line would have crossed the center line of the car several FEET behind the rear axle. So any talk of akerman in relation to an Austin Seven is probably entirely pointless!

Location: Devon

Oh Dear

Definitely not for Vince or Jon.

Please note it was not me that first mentioned Ackerman!

The whole notion that the projected kingpin should meet on the back axle for true Ackerman may be a red herring. One engineering textbook I have does not mention it as a criterion, but it does not say it isn’t, and I am too old to wade thru the geometry.

On reflection, the deciding axis is the kingpin at steer arm level, not axis at road level (both the same on the Seven). Going thru the 1930s Austins again this places most of the convergent points nearer to back axle, or beyond. I ignored those with track rod in front as cannot determine but seems likely some may give parallel lines ie no Ackerman. Kingpins often inclined and difficult to guess where axis is at the level of the arm, and many dwgs may be diagrammatic.

They say stimulation is good for aged brains.

The various Seven chassis diagrams for greasing certainly seem to converge in front of back axle, and from what I can see of it in its storage corner I am not sure my car differs from the several drawings. For the curious it is easy to verify with pieces of straight wire held above or below the two pivot points and sighted toward the diff.

For those not building a special special and so not intending to change anything , it is all a mite academic.

PS. For those avidly following, the slip angle is the angle between the direction of patch of tyre on the road and the rim when cornering. It is apparently a major part of the secret to success of the pneumatic tyre, although modern low profiles strive to eliminate it! (Steel tyred wagons have none so do benefit from dinkum Ackerman.)

Perhaps if new side steer arms are ever made these could be less curved...would give more modern reduced Ackerman for those with rigid chassis and suit all the stretched trackrods!

The main problem with Sevens is the huge adjustment steps for toe in. At worst may have a choice of 1/3 inch or none. Maybe mechanics have deliberately sprung the track rods, much as they used to routinely ruin headlight shells and mudguards by trying to realign without loosening.

The end?

Bob Culver

Location: Auckland

Or even read my Wikipedia extract from three days ago which I thought explained the whole idea very neatly.

Tony.

Location: Malvern, Melbourne, Victoria, Australia.

For those avidly following, BC is wrong in his definition of slip angle. The easiest way to visualise slip angle is to think if it as the angle between where the wheels are pointing and where that axle is actually going. The difference being caused by slip between the tyre and the ground. Actually there is always slip between the tyre and the ground, not only in cornering, but also accelerating and braking. If there wasn't, the tyre wouldn't generate the forces necessary to provide drive/ cornering force etc. so it doesn't matter if the wheel has a crossply, radial or iron tyre there will always be a slip angle. The only question is, how much.

I risk inducing ennui (Noun: A feeling of listlessness and dissatisfaction arising from a lack of occupation or excitement).
It is very easy to work out the point on the chassis where the Ackerman steering geometry acts.
1) Measure:
K, The distance between the kingpin centres
T, The distance between the track rod joint centres.
S, The distance between the track joint centre on the steering arms and the axle centre line.
2) Divide the kingpin spacing by the difference between the kingpin spacing and track rod joint centres and multiply by the steering arm length.
As a formula this is:
K * (K - T) / S
Result:
On my car (short wheelbase) this works out at 6ft 8ins. However the measurements I took are not particularly accurate as it is a bit awkward to do with the parts on the car.
For homework, and to avoid the said ennui, go out and measure your axles!
Note that this works whether the track rod is in front of or behind the axle. When it's in front, the track rod is longer than the axle, when behind it's shorter.

It also indicates that the point changes if that axle is shortened by being bowed and also varies a bit with track adjustment.

It is only a matter of geometry to work out the angle of each wheel on lock and hence work out variations in Ackerman Geometry taking into account slip angle etc,

However, I don't think I'll bother!

Location: Melrose, Scottish Borders

Strewth Alan!

The patch of rubber on the road moves in the same direction as the axle.

Some may be interested in more elaboration...
Most instinctively realise that some degree of slip against the road exists when cornering, but in moderate driving this is small. After all, the rear wheels drive the car without scores of extra revs per mile due traction slip. The sliding part of slip angle can be large. ..some manage 90 deg in the wet! Not so generally realised is that in normal cornering a significant angle to the rim exists due to distortion of the tyre carcass. It is the different angles associated fat crossplies, thin crossplies, fabric radials, steel radials, low profile, wide tyres, stiff old tyres etc which largely determines the different handling. All pointedly illustrated by flat tyres which at slow speed don’t much slip on the road but produce whopping carcass angles with exciting effects on steering and handling.
Generally the greater the load on a tyre, the greater the slip angle.

I suppose I should have expanded that with a steel tyre wagon wheel it is tyre carcass slip angle which is absent.

I hope that did not rate too badly on the Boredom Index, Tony?

Location: Auckland

A little knowledge is a dangerous thing. Don't listen to him he doesn't understand what he's on about.

I suspect that some forum contributers are unaware of what Alan did as a career!
Compared with my career (estate agent) I find this thread most interesting.
It really makes you wonder why the little Austin has such delightful steering!

I am not sure that esoteric discussions on slip angles etcetera are very applicable to our often very original ordinary road going Austin Sevens- especially as many seem to use ten year plus old tyres with Bakelite characteristics.
As mentioned recently the suspension and steering is fairly basic and often the car goes where it wants regardless.
'High speed' cornering in my 1929 fabric saloon is not something I do regularly

Tony.

Location: Malvern, Melbourne, Victoria, Australia.

Hi Tony

Slip angles not entirely irrelevant. Long before I had read about such matters I had figured that my RP handled better with the front tyre pressures low, increasing the slip angles and reducing the inherent severe oversteer.
In fact I was amused to later read books about handling of GP race cars and realise that I had noted many of the effects under discussion driving my 13 hp RP.(ie improved wet adhesion with weak s.a.s)

I suspect crossplies with soft walls are, like radials, less prone to road camber induced wander, and so feel better even if the slip angles are greater. Most of my driving was on stiff, probably cotton fabric tyres, also old. Comparison with new modern soft walls would be interesting. Others in old car circles have noted huge improvement of old cars with new tyres (crossplies).

Bob Culver

Location: Auckland

Only for the seriously curious.

I recently stumbled upon my copy of “The Sports Car” by Colin Campbell, an introductory text into vehicle performance etc, popular 50 years ago and sometimes appearing at Swap meets. The book is unnecessarily obtuse in places, has errors, and his conventional concept of slip angles would presumably be dismissed out of hand on this Forum. However, for what it may be worth, he states;
”On the older beam front axle the modified Ackerman geometry was given by inclining the links which connect the wheel steering pivots and the track rod ends so that, if extended, they would intersect on the car centre-line about two-thirds of the wheel-base from the front.”

Bob Culver

Location: Auckland

Interesting point, Steve, and worth checking by someone with a good stock of track arms.
Bob, I'd put that book away and read one by Donald Bastow or even better David Crolla.
The point of having the intersection point in front of the rear axle is to promote the effect I described earlier, and it doesn't just apply to beam axles.
What happens, and I'm probably not describing it well is the wheels no longer turn in to Ackermann geometry and by adjusting toe angle you can make the outer wheel turn in more or less than the inner. Toe in makes the outer wheel turn in more in relation to the inner, toe out, the opposite. If the intersection is behind the rear axle the effect if toe is reversed. Only at the Ackermann point toe has little effect. Car designers don't like that because you can't tune for steering sharpness using toe. The downside is tyre wear is probably marginally increased.

Frank, designing suspension/steering is all about compromises. So, it some respects all this isn't relevant to road cars, but in others it very definitely is. So where on a racing car you might use Anti-Ack to get a sharper turn in to corners,on a road car you would maybe use it to tune the degree of understeer. We always try to make cars understeer, its safer that way. The point is true Ackermann is not very tuneable, so its better not to use it.
Negative toe(ie "in")will certainly help with braking stability, but what you really don't want is for the toe to swap from negative to positive under braking. Not everyone agrees but I think its a big contributor to instability.

"Interesting point, Steve, and worth checking by someone with a good stock of track arms."

I know no-one reads my comments - possibly because I don't speak from great knowledge or experience in high speed road transport but I did point out-

"The Cross Tube Levers were BM 91 and BM 92 from 1926 to 1936 at least!"

I also quoted -

"A simple approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle."

Note this states 'a simple approximation'

Experts can now attack.

Tony.

Location: Malvern, Melbourne, Victoria, Australia.

Apologies to long suffering Vince and Jon

I have long known that full Ackerman does not suit modern cars ie those made since 1900. I have not been advocating for full Ackerman, simply responding to the initial post which suggested that for full Ackerman the axis meet at the diff. (A notion I also had loosely in my head years ago. I guess it arises from confusion with the turning centres.)

I did slog through Bastow years ago. Unless higher maths is a hobby, I do not recommend. I suspect most using this Forum will get a better understanding of slip angle and the effects of from the flat tyre analogy. Books such as that by Carrol Smith are more understandable.

(Incidentally, Bastow worked for Jowetts in their dying days. Their designs were then old so I do not know what he contributed. Perhaps the very satisfying handling of the Javelin prompted him to ponder the subject of handling. Like the Seven the Javelin has an element of roll induced oversteer which gives it a very responsive and lively feel. But the steering geometry is fully adjustable and can be and normally is set up for no front bump steer and negligible free travel!)

Ackerman effect applied to typical 1960 cars gets more involved as there is often three track rods moving in small arcs and with slave levers, and often with one rod behind the pivots.

And Tony, the fact that contributions are not all acknowledged does not mean these are not appreciated. If everything was acknowledged would get very cluttered. If not quite to the point and comment is made with a reservation it may offend so many don’t, but it does not mean the contribution was not useful. All Ackerman with pivoted instead of sliding links is an approximation, except at one angle.
I suspect you reflect the thoughts of many on occasions. Without your comment this post would probably have died!! Vince and Jon clearly wish it had, but others less experienced seem to have found some interest. The Forum is not only for the old hands.

A problem is that most understandably do not always read every word of every post that has gone before, but respond to a quick general impression. Some repetition helps ideas to sink in!

Bob Culver

Location: Auckland

Bob,

I didn't need acknowledgement - just making the point that sometime ago I mentioned that the 'track arms' which were in fact called 'Cross Tube Levers' by The Austin didn't change angle from the original pieces (going by Part Numbers) even though the chassis lengthened and the Ackermann steering was an regarded as an approximation when defined by a line drawn between the steering kingpins and the centre of the rear axle anyway.

Tony.

Location: Malvern, Melbourne, Victoria, Australia.