Question regarding design

[prateek11]

Which is better to follow negative or positive ackerman for fsae cars and why?? And how to decide the rack position ??

[Goost]

Using ackermann can certainly lead to desirable pole placement...

Though we don't have qualifying in FSAE it may not help you much :)

~~~

More seriously:

how about if you don't understand tires; just use something close to true ackermann steering, maybe a bit more than that to get a yawing moment on corner entry.
It is a complex subject I think and certainly worth your time to consider.

Rack position, on the other hand, is not a complex subject; you should be able to handle that one with a little effort.

[The_Man]

Hi Prateek,

There are multiple posts about Ackermann on this forum. Some of them asking the same question as you. In fact, there are a few with you asking the same question, you have replies on some of them so I am not sure that you are looking for by this same post again. Having said that -

Which is the right ackermann to have?
Well if 'negative' was better than 'positive' or vice-versa there would not be so much discussion about it. Clearly there are some situations where 'positive' ackermann is the way to go and in some cases 'negative' ackermann. Which one is the right one for you to pick at the design stage (which I assume you are at) depends on the data you have about you car, and your design goals. The question you need to answer for yourself (and may be explain this along with the post) is how does the ackermann affect your design goals. For example, the maximum lateral acceleration you want to attain in skidpad can be your design goal and then you have to figure out how the ackermann is going to affect the lateral acceleration in stead state. A basic vehicle dynamics textbook can help you out understanding this. If you have confusion over understanding this effect feel free to write about it here.

The other situation I imagine you could be in (based on the very limited information you have provided in your question) is that you might not have access to some of the data that is required for making elementary simulations that help make this decisions. In which case write about what you have tried so far, I am sure you will be helped with troubles you have been facing. There are some discussions on this forum about rough estimates of some of these parameters.

If you do not have a great understanding of tyres. In that case Goost's advice of starting with a true ackermann steering is not a bad one. You can start with that geometry and play around with your simple model to try and see how ackerman is affecting your design goal. There does not exist a formula that is going to give you the correct ackermann value.

I really wished that I could give you a true/false answer but unfortunately (or fortunately) that's not how engineering works.

[Jonny Rochester]

Another way to look at it: If you are designing for a low speed hairpin corner, then you may want zero ackerman (parallel steering) to get a tighter turning circle.

(At low speed, little tyre slip), with zero ackerman, when you turn your inside wheel to 45 deg, the outside wheel is also at 45deg, which is a sharp angle. The outside wheel does the work and you turn a very tight corner. (Fun doing a U-turn on a narrow street).

With the same car setup with ackerman, when you turn the inside wheel to 45deg, the outside wheel is only turned about 37deg. As the outside wheel is doing the work, you can not turn as sharp a corner. So, expect a 3 point turn on a narrow road (requres a whole team for fsae!)

Another thing people forget, for everything else being equal, parallel steering gives a slower steering ratio and more turn of the wheel lock to lock... think about that... makes your brain hurt.

[Claude Rouelle]

Jonny,

This approach is a bit too simplistic.

You need to understand your 4 tires Fy, Fx Mz Vs Load, slip angle, camber etc... and then decide what is the best compromise of grip (lateral acceleration) and balance (yaw moment) you want / can achieve depending your corner radius, your chassis "attitude" (or chassis slip angle) and... your steering input to decide, at least theoretically, the ratio you want between inside and outside wheel steering angle.

Then more practically you will want to make several tests with different Ackemann configurations and see with a good driver what he likes the most, and also look at the tire graining as well as the tire temperature (ideally with IR sensors).

The practical phase is the most important. The theory "just" helps you to decide what to modify in your steering geometry and how much to ho in a given direction. In amateur racing the second phase (again I insist with a good, experienced, "sensitive" driver) is the most important. In FSAE / FS you ideally need both as it is a design competition, not racing.

[prateek11]

thnkuu every1!!! yeah actually it will depend on my chassis!!!!

[Pat Clarke]

Prateek,

Use the search function and you will find out lots about Ackermann.

Pat