If you really want to measure it, measure the toe out on turn angles, and bisect it back from the wheelbase, this will give you the turn raduis of each front wheel

Ackerman sterring geometry isn't a good idea, racing cars tend to used either parrallel or some toe-in in turns (anti ackerman) all to do with tyre loading and the relationship between tyre loading and maxium slip angle.

[Edited on 7/5/04 by britishtrident]

I didn't know about the tyre loading aspect, but anything with a true ackerman, is VERY unstable at even moderate speeds in a straight line - still not sure why F1mini thought I might be on drugs!

Sorry didn't mean to offened thought it was a funny... So ackerman is not worth worrying about, or are you saying not true ackerman but more ackerman or less ?

I do need help to see weather it is worth me changing what I have at present.

With a standard Mini wheels base of 80" I presently have a 71" converging point (that is if it is the top ball joint you use).

On a more serious note, the GTD40 owners club have pin pointed that there Ackerman lenght was at the engine bulk-head. A bit to short so they altered there steering arms and managed 2 Seconds faster around Goodwood race track. So it make me think what do I need ?

Confused..... I am alittle...

I suppose being technical, the point to use should be where the kingpin line is bisected along the spindle height.

I would have thought the convergance point should be behind the rear axle, parallel would be at infinity

Sorry again, my sense of Humor. Life's normaly to short not to fool around, and have a laugh.

Yes I am new to this forum, but am realy glad to have found it... Must remember to attach () when making a Joke....

Point taken, we do sometimes get over engrosed in the technicallities!

http://www.locostbuilders.co.uk/viewthread.php?tid=7012

[Edited on 7/5/04 by Mark Allanson]

I was told about the high speed stability thing on my C&G course, I just took it as gospel, but thinking about it, if there is no movement in the suspension, how would it know!

For ackerman 101 just watch an episode of the Rockford files --- most Yank tanks of this period had (almost) true ackerman geometry, watch the inside front tyres squealing uselessly in the car chase scenes.

To generate cornering force tyres must run at an angle to the true tangent the the circle vehicle is following this difference is called the slip angle. Because of the nature of a tyres friction characteristic the more highly loaded outside tyre generates its maximum cornering power at a greater slip angle than the lightly loaded inside tyre.

For this reason over about 10 mph true ackerman steering geometry would over load the inside fron tyre causing it to generate little or no cornering force -- the Rockford files syndrome.

Cars with rack and pinnions cannot have true ackerman geometry anyway -- it is impossible the geometry is dictated by the position of the rack. It can does however have a quasi ackerman effect, on the book Locost setup move the rack backwards you get more ackerman backwards less, on a Cortina or Sierra front end the steering arms have little or nothing to do with it.

At the end 1950s after studying tyre data Colin Chapman adopted a lot of anti-ackerman on the Seven S1 and other Loti racers of the period such as the 18. It did give some increase in cornering power but drivers found made the car twitchy and unstable at high speed.

With the Seven S2 Chapman went to rack and pinnion steering and at the same time the anti-ackerman was removed replaced by a small degree of quasi-ackerman as used on the other Lotus roads cars of the period.
This now standard practice on rwd cars. Shopping cars like the Cortina and RWD Escort used quite at lot of this quasi-ackerman the steering rack being mounted a few inches behind the track rod end centrelines. While in the race car world on a typical Formula Ford the steering rack would be postioned nearly in line with the track rod ends to give some quasi-ackerman at small angles of steering changing to anti-ackerman as you approach full lock.

With the book Locost I suspect any ackerman angle would be swamped by the large ammounts of bump/roll steer.

[Edited on 8/5/04 by britishtrident]

[Edited on 8/5/04 by britishtrident]

Hmmm, I am no expert here but would agree with Syd and his points.

On my grasser which runs a very short wheelbase, I altered the cortina uprights to gain more steering lock per mm movement of the rack (now have 0.9 turns lock to lock) but also to move the trackrod to steering arm attachment points outwards (as you mentioned F1 Mini) to give more ackerman angle (well, to get the inside wheel to turn tighter than the outside one, I don't really give a crap what this is called but it has to happen for the car to steer properly!!!!)

I have read several articles in CCC magazine where the tech ed Dave Walker (of Emerald fame not Newark Engines!) has talked about needing to move the trackrod to steering arm attachment points outwards to give a better approximation to ackerman geometry in order to REDUCE understeer (totally the opposite to what BritishTrident says).

All I know is I have a lot of ackerman (Mike R saw it in the car!) and it turns in on a dirt track really well (probably better than any of the other cars in my class) but before I changed the ackerman it was a real understeerer! (and this is noticeable on a dirt surface!)

Back to the original point of measuring it, I don't know a particularly sciantific way. I just make things adjustable, then adjust them and see what goes quickest! Not terribly scientific I know but it works (and the sciance behind the adjustments usually becomes obvious afterwards!)

Study this image carefully it is of a Van Diemen FF chassis -- Van Diemen were the builder with the longest run of success in this class spanning decades.
Formula Ford is the class with suspension demands quite close to a the demands of a Locost style car.

Can anyone one spot any ackerman or quasi ackerman ? infact it has a tiny ammount of quasi ackerman.


http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=9883&item=2477033370&rd=1&ssPageName=WDVW

[Edited on 8/5/04 by britishtrident]
Rescued attachment 49_1_s.JPG

Another couple of Formula Ford picture

This time showing car with a bit of anti-ackerman --- toe-in in turns.

[Edited on 8/5/04 by britishtrident]
Rescued attachment foxfrontcornerbarelrg.jpg

Plan view QED

[Edited on 8/5/04 by britishtrident]
Rescued attachment foxtopfrontshockslrg.jpg

Yes Syd, if I read you correctly, in saying that the trackrods mount outboard of the top balljoint (and one assumes, unless the geometry is very strange, the lower balljoint too) you are quite right.

I modified my cortina uprights to suit a short wheelbase by moving the connection on the steering arm out my around 8mm, this gave increased ackerman which seemed to work well.

Certainly the steering arm connection points on the last photo above would give some degree of ackerman steering angle.

A series of articles written by Staniforth appeared in Race Tech magazine #31-#35 which covers the subject in detail ,including rack mounting , very interesting read , but in short it came down very much in favour of Ackermann and plenty of it, to the point where the converging lines through TRE's and KPI line meet between the drives legs

to my thinking, ackerman related specifically to the line between the hub pivot and the steering rod end. if a pair of hydralic pistons were used (as opposed to a rack where to knuckles move) then ackerman geometry would be simple. moving the rack forward and back undoubtedly has a significant effect on the steering geometry, but i previously wouldnt have counted this as ackerman effect/geometry. would i be wrong? did ackermans thinking go beyond just the hub design?

personally i think it would be good to use a pair of hydralic pistons to move the wheels, then the geometry wouldnt change so much from lock to lock. it seems to me that when you take into account the steering rack knuckles moving it becomes a case of 'best fit' rather than complete control. unless it is advantageous to have a nonlinear relationship between steering wheel movement and degrees of either wheel movement.

quote:

---

Originally posted by WIMMERA
A series of articles written by Staniforth appeared in Race Tech magazine #31-#35 which covers the subject in detail ,including rack mounting , very interesting read , but in short it came down very much in favour of Ackermann and plenty of it, to the point where the converging lines through TRE's and KPI line meet between the drives legs

---

quote:

---

Originally posted by Mark Allanson
http://www.locostbuilders.co.uk/viewthread.php?tid=7012

[Edited on 7/5/04 by Mark Allanson]

---

Typo, should be drivers legs, same as using herd instead of heard

quote:

---

Originally posted by Syd Bridge
In my experience(for what that's worth to some!), the critical angle is that between the steering arm and track rod, then the angle between the steering arm and longitudinal centreline comes into action.

Full ackerman geometry, and more, can be attained on cars with the steering arms parallel to the centreline.

Naturally, with rack and pinion setups, the angle between track rod and steering arm is dictated by the fore/aft position of the rack.

Cheers,
Syd.

---

Rescued attachment Ackerman.jpg

am I seeing that pic wrong mark, or does your car have massive ackerman the wrong direction?


atb

steve

When you all are talking about ackerman, is this for parking lot speeds, road handling or race handling?

Assuming race (from the pictures), what's the desired outcome? Better alignment of the inside tire during the turn, right? Anything else? Turn in response?

I thought the inside, unweighted tire will want at a lower slip angle than the outside tire. This seems to argue for neutral or even anti ackerman - what am I missing here?

It strikes me that there must be a very complex relationship to tire type, tire traction and road traction on weight transfer and slip angles. These will affect steering angle, changing effective bump steer since the chassis has rolled (by how much?). Is it possible to come up with a rule of thumb setting? For all cars in all conditions?

I wonder whether the toe setting has more overall effect than ackerman,- does it?

The rack position determines the ammount of quasi-ackerman because the effective length of the track rods change as the streering arms move through an arc. Large ammounts Kpi and castor also have a small effect because they have effect on the height of the TRE as it swings the steering angle.

Unlike true ackerman geometry perfect theorhetical alignment should it be desired can't be maintained through the whole arc of movement, for that should it be desired you need a steering system with a centre track rod and idler arms.


Just to clarify on a question asked early in the thread with classical ackerman the construction lines are drawn from the outer TRE centres through the steering axis (virtual kin pin) at track rod end centre height --- not the top or bottom ball joint.

Mark your car has a very small ammount of ackerman built into the Cortina uprights but quite a lot from the position of the rack, If you turn the wheel to full lock you will see that a line drawn between both the track ends rods is in alignment with the rack --- the track rods have a longer effective length than in the straight ahead position --- giving toe-out in turns --- adds up to a good bit of ackerman.

With full Ackerman the difference in angles can be quite large, leaving aside for now the effect the difference between inside and outside tyre loading has on ideal slip angles, the effect of even equal slip angle throws full ackerman out the window. To have lateral grip must have a slip angle.

Personally I am going for a little ackerman all gained from having the rack positioned 25mm to 40 mm behind the TRE centres in the straight ahead position. If building a car for hillcimbs I would use more (to warn the front tires), also lots of ackerman for autotests and PCT, for circuit racing i would use a bit less..

Some of the formula 1 cars in the 70s had "anti ackerman" ie negative ackerman effect so that the loaded tyre had greater slip angles. On a road car I'd go for full ackerman so that you don't sound like a prat squealing round every bend in the car park! My old talbot sunbeam was a bugger for doing that...
Cheers
Bob C

and I should read the rest of the thread before repeating part of it wrongly.....
spanner!
Bob C