Ackermann

Has anyone got any thoughts on Ackermann re the GTD chassis?

How do you measure it & what should it be??

Presumably you take a line from the Track rod ends , thru the kingpins & somehow measure where the lines meet ?

However, the Kingpin is inclined, so do you go thru the top or bottom joint ??

Any input greatly appreciated /ubbthreads/images/graemlins/smile.gif

Ackermann is complicated, and must be refferred to as how the car is used. Yes, we could say that what is called 100% Ackermann is sort of "common used". This is when both front tires is following the turn radious. However, depending on the steering caracter of the car, diffrent Ackerman may be used.
Also, in slow turns more + Ackerman should be used and in fast turns a larger amount - Ackermann is to be preferred.
This is due to, that in faster turns the outer front wheel works under greater slip angle and therfore needs more "toe in" for the same traveling direction.
Sorry, this may make you even more confused, but there are no easy way of understanding Ackermann. Especially if you want to use it for making the car faster. But for regular street driving, 100% Ackermann is the way to gofor any car.
Goran Malmberg

Goran,

An excellent answer and welcome to the forum.

The major problem with the majority of GT40 replica's is that there is so LITTLE adjustment with the suspension that the majority of REAL performance requirements like Ackerman have been neglected or not even considered. The total balance of the car and in particular how the car "turns in" is 100% based on these adjustments.

We have a GT40 chassis at my factory which has adjustable steering (forward and backwards AND up and down), and will have adjustable steering arms also (my buggy has these already - ask Hershal about the buggy). The car has fully adjustable castor and camber and we are just finishing adjustable shock absorber mountings.

That car is adjustable and will be a true track car.

Best wishes,

Robert

[ QUOTE ]
Goran,

An excellent answer and welcome to the forum.

Robert

[/ QUOTE ]

Thanks! As you probably already noticed, I am from Sweden
and reverse grammatik now and then. I must also admit that I do not own a GT-40, but should if I had the energy to own two cars, but my Pantera takes all the time left.
Anyway, I love to share my 40 year of car experience with other serious sportscar owners.

Cheers
Goran Malmberg

Goran. Are you the guy with a 426 hemi in a pantera? If you are not then this is not a joke. I saw a article with a lot of pictures of just that a while back.

Anyway that guy had a lot to say about suspension and chassis setup. Very well put and a real nice car too.

Goran. Are you the guy with a 426 hemi in a pantera? If you are not then this is not a joke. I saw a article with a lot of pictures of just that a while back.

Anyway that guy had a lot to say about suspension and chassis setup. Very well put and a real nice car too.

Howard
Yes, thats my car, good memory... /ubbthreads/images/graemlins/grin.gif
Goran Malmberg

What website was that article published on? Could you leave a link? You guys got to see this one! What a nice job!

Goran,

Thanks for your informative reply. I've done a bit of web searching on Ackerman & there seems to be alot of confusion on the subject, but your answer puts it in laymens terms, which suits me!

Just to check I've got it right,

100% Ackerman = both wheels steer the right amount for the turn radius (is this also reffered to as 'true' Ackerman?)

- Ackerman = Inside wheel steers less than turn radius

+ Ackerman = Inside wheels steers more than turn radius

Is this correct ? /ubbthreads/images/graemlins/smile.gif

[ QUOTE ]
Goran,

Just to check I've got it right,

100% Ackerman = both wheels steer the right amount for the turn radius (is this also reffered to as 'true' Ackerman?)

- Ackerman = Inside wheel steers less than turn radius

+ Ackerman = Inside wheels steers more than turn radius

Is this correct ? /ubbthreads/images/graemlins/smile.gif

[/ QUOTE ]

That’s right. But in my example I used the term + and - for the direction of the adjustment needed. For a street car and even if driven fast, regular Ackermann appear the best.
In racing, or when driven with slip angle through all corners, special Ackermann settings should be used.
If we talk racing for a while....
Ackermann is closely related to toe settings. Different toe may be used for creating "turn in", and Ackermann was related to the slip angle of the front-outer wheel. Now, if different toe is used, this will affect the Ackermann needed.
Even the whole car could be said to have a "slip angle" which is the angle position of the car body in relation to the radius, and depend on the over-understeer of the car in question. The more oversteer, the less toe-in position is needed not to create roll-drag in the steering direction.
Whats said will not make anyone an Ackermann expert, by maybe put some light on whats going on, which hopefully is good enough.


Howard, didnt you reffer to my site, http://hem.passagen.se/hemipanter/?

Goran Malmberg

Goran,

Lots of information on your website. Sounds like you've got yourself a serious beast!!

How are you getting on with the Ohlins shocks, I have some fitted to my forth coming GT40?

Regards,

J.P

I am sorry to say that I have only driven a few miles this year. This is due to the yearly "carinspection" in Sweden.
I am not allowed to drive the car right now. I must arrange this in a special way not to get in to trouble with the authority.

I am supposed to deliver driving data to Ohlin:s, for them to create a suitable setup for different conditions. This would be quite close to a set up for the Gt-40. As soon as I get the car out I will start the testing and if you want me to, bring it up in the forum.
Cheers
Goran Malmberg

Just a thought, I understood ackerman to be that each wheel turned differently in a corner for reasons clearly explained above. Therefore if each wheel turns the same as the other then there is zero ackerman not 100% or regular ackerman. You then go to positive or negative as described. But the languages of the world do differ so I guess we all have said the same thing in many different ways!

Ackerman on the GTD is not so hot as the standard parts do not allow for adjustment. With the rack mounted in front of the axle line this is even worse for making some form of adjustment as the steering arm connection needs to be moved outwards.

This is how I understand Ray's thinking when he designed the GTD. I may or may not be right with this but from my discussions and club debates over the years ackerman is always a hot potatoe!

The generalisation is that whilst the GTD was primarily dsigned as road car, it can also go quickly and that is one of its main design goals. Therefore Ray chose his ackerman set up to suit high speed corners which means that at slow speeds on tight lock, the car makes funny tyre noises and sometimes can even jump a little. Depends on road surfaces etc. Combined with standard spec narrow front tyres ie 215 60 15, small king pin inclination angle, and spindle flex, the turn in was always not as good as people wanted it to be. His racing uprights allow for a greater king pin angle and different ackerman (on some sets with possibilities of adjustment) and zero spindle flex by use of better bearings. But still designed for high speed cornering.

When I fitted the GD uprights to the front of my car I made it so that the ackerman was much more positive. It turns on a sixpence in slow corners very smoothly but I think I am using more steering input in high speed corners. To me this is not so good.

Therefore my next effort will be to revert partially back towards Ray's set up or at least to zero ackerman and rely on slip of the tyres to help out.

As an aside, just by putting on wider front tyres ie anything up to 245 width will greatly improve your turn in. Just need to find a manufacturer that makes the exact size you want and that you have proper clearance under the wheel arch!

Yes, it looks like misunderstandings. Divide the Wb by the convergence point of the steering arm line and convert to %. If the converging point is at the rear axle this is called 100% Ackermann. If twice the wheelbase we got 50%, and if 2/3 we got 150" Ackermann.

Then at 100% Ackermann the car tracks without slip under slow rolling condition. But , even if the steering arms pointing as described, the wheel want go in the exact % angle as we have to deal with other geometry factors. But it describes what we are talking about.

Also, the direction of the front wheel in a given situation is a product of both Ackermann and static toe setting.

Malcolm, tries your theories with Zero Ackerman and different toe, get back and let us hear how it comes out.
Regards
Goran Malmberg

Goran,
I think this is probably the clearest explanation of Ackermann that I have come across.
A diagram would help immensely. . . . . . /ubbthreads/images/graemlins/cool.gif

There are plenty of very good books explaining ackerman from Demon Tweeks. Worth a look!

Here is a nice, simple explanation of what Ackermann is all about. There is even a diagram that explains the principles: Click here

Tony
This is a good diagram showing 100% Ackermann. The more complex part of it starts with racing cars, or when we like to use it in a more advanced situation.
Regards
Goran Malmberg