Steering/Suspension Related Questions

[ccsharry]

Hi everyone,

I'm current designing the front suspension geometry as well as the steering for our car. Due to the limited resource I have, I hope someone can give me some directions to solve the questions below.

1. Steering Force Calculation
I found an equation to calculate the static steering force and it was suggesting the driving one will be about 1/3 of the static one. But I want to have a more accurate one which takes the trail and scrub radius into account. Can anyone tell me how I calculate/find this?

2. Ackermann Calculation
What is the actual way to calculate the Ackermann Angle? I have read several books but none talk about the equation. I know it depends on the position where the imagine lines, which connect the Steering axle and Tie rod on upright position on the same plane and extend it until they meet together, are joined together. If it is on the rear axle, it is 100% Ackermann. For 90% Ackermann, is it at "Wheelbase/Percentage of Ackermann"? i.e. 90% ackermann for 1600 wheelbase = 1600/0.9 = 1778?

3. Reason for Ackermann
Can anyone confirm this with me? Is it becaure there is load transfer and because the lower load on tyre requires less slip angle, so a reduction of Ackermann is needed to achieve the optimum Lateral Force (High Lat Acc >> Less Ackermann)?

4. Castor Angle Negative Effect
I understand that Castor Angle is desirable for the outer wheel. But at the same time, it produces positive camber on the inner wheel. As the inside wheel has less load, the positive camber is always higher than it is needed. Is there any way to achieve the required negative camber on the outside, while the positive camber on the inside is less? (Or is this too insignificant that I can just ignore?)

Thank you,

Harry

[ccsharry]

Hi everyone,

I'm current designing the front suspension geometry as well as the steering for our car. Due to the limited resource I have, I hope someone can give me some directions to solve the questions below.

1. Steering Force Calculation
I found an equation to calculate the static steering force and it was suggesting the driving one will be about 1/3 of the static one. But I want to have a more accurate one which takes the trail and scrub radius into account. Can anyone tell me how I calculate/find this?

2. Ackermann Calculation
What is the actual way to calculate the Ackermann Angle? I have read several books but none talk about the equation. I know it depends on the position where the imagine lines, which connect the Steering axle and Tie rod on upright position on the same plane and extend it until they meet together, are joined together. If it is on the rear axle, it is 100% Ackermann. For 90% Ackermann, is it at "Wheelbase/Percentage of Ackermann"? i.e. 90% ackermann for 1600 wheelbase = 1600/0.9 = 1778?

3. Reason for Ackermann
Can anyone confirm this with me? Is it becaure there is load transfer and because the lower load on tyre requires less slip angle, so a reduction of Ackermann is needed to achieve the optimum Lateral Force (High Lat Acc >> Less Ackermann)?

4. Castor Angle Negative Effect
I understand that Castor Angle is desirable for the outer wheel. But at the same time, it produces positive camber on the inner wheel. As the inside wheel has less load, the positive camber is always higher than it is needed. Is there any way to achieve the required negative camber on the outside, while the positive camber on the inside is less? (Or is this too insignificant that I can just ignore?)

Thank you,

Harry

[Bete Noir]

Steered wheel forces are a function of tire self aligning values (available from tire mfg) and mechanical leverage of steering wheel, steering box and steer arm lengths.

There are two Ackerman angles of interest. One is the degrees necessary to negotiate a corner of a given radius. It is determined by the corner radius and vehicle wheelbase.
Another is the change in front toe angle with a change in steer angle. Possibilities are positive, negative and neutral Ackerman. The actual Ackerman steer angle will change with steering angle, so a toe change curve must be calculated.

Ackerman is used because the inner wheel traverses a tighter radius than does the outer wheel. And...

...since kingpin angle produces a positive change in camber with steering lock, castor can be used to offset this effect. Ackerman settings will determine the steer angle for each wheel at any total steer angle.

[JHarshbarger]

3. Both of you are correct in your posts. Ackerman is used because the inner tire sees a tighter radius, but less ackerman is used because the tires will create more lateral acceleration at higher slip angles when at higher loads. An example of this is F1 cars that run anti-ackerman. The best thing to do is make your ackerman adjustable.

4. While castor can be used to offset KPI, it is often used in excess for camber gain in steering. In order to get less positive camber on the inside tire, run some static negative camber. Castor adds to steering force and FSAE size tires don't see that much difference in lateral acceleration with different camber angles, so some teams avoid using castor excessively. Do a search for "TTC."

[ccsharry]

Thanks for the replies!

Now I have better understanding of Ackermann. However, it is impossible for me to decide the desirable Ackermann angle on our car since we cannot afford to acquire the tyre data.

How significant the time is improved with a correct Ackermann angle used for a particular event? Do you think it worths paying £500 to get the tyre data?

I have a question from my curiosity. Do you notice that on the VW Golf/Passat, there is a huge +ve camber on the inside wheel at steering lock, but the outside wheel only gain comparatively small -ve camber. Do you know the reason behind and how to achieve this?

Thanks,

Harry

[Peter7307]

Harry,
To add a little to your original Q2 in the first post: Yes the Ackermann is 100% if the intersection in plan is at the rear wheel centre line. If it is behind the RW C/L it is over 100% and ahead of the RW C/L is corresponding less then 100%
In your example with a wheelbase of 1600 ; 90% would equal 1540 and 110% would equal 1760.

In your post of April 27 (sorry but the posts on this board are not numbered) the time improvement difference really depends on a huge range of variables of which this is one. In my opinion it is not a significant factor.

The strut type suspension on the VW's accounts for the attitude of the of the front wheels duing the change in steering lock.

This is well explained in the firt mentioned book in the list below:

The Automotive Chassis Engineering Principles
J. Reimpell H. Stoll J. W. Betzler.
ISBN 978-0-7680-0657-5
Description: Vehicle dynamics and chassis design from a production car perspective. Very useful, if you are designing a production car.

Race Car Vehicle Dynamics
Milliken and Milliken.
Description: Vehicle dynamics and chassis design from a race car perspective. Probably the most useful generally of the big books, it is a good read as well. A fair amount of aero.

Fundamentals of Vehicle Dynamics
Thomas Gillespie.
Description: Mathematically oriented derivation of standard vehicle dynamics equations, and definitions of standard terms. A bit dry.

Chassis Design - Principles and Analysis
William F. Milliken and Doug Milliken.
Description: Vehicle dynamics as developed by Maurice Olley from the 1930s onwards. First comprehensive analytical synthesis of vehicle dynamics. Great historical stuff, and some nice explanations. Not as generally useful as RCVD.

Tires Suspension and Handling
John Dixon
Slightly more analytical than RCVD, covers much the same ground, but no aero.

Car Suspension and Handling
Donald Bastow
I only read a good discussion of steering effects due to camber gain, but it seemed useful and analytical.

Cheaper books

Carroll Smith - Tune to win etc. Generally a good introduction, but his explanation of roll axis in one book is confusing at best. I also disagree with him about single sided mounting for Heim's, but agree it is nicer to do it his way. Good on practical details, and fundamentals. Probably good enough for FSAE, for example.

By the way the poster is on the www.eng-tips.com site which is well worth a look. There is a section on suspension systems there.

Good luck , Pete.

[PatClarke]

Harry,
Your questions are kind of generic and resourcing the answers is part of your FS challenge. I am a little surprised you have escaped without being flamed!

Suggestions have been made and book lists given, but the very best resource on the planet when researching this subject is right here!

The FSAE.Com archives and the links therein are a priceless resource, they are free and you have access. Start researching!

Some advice.
The most important decision on the entire car is the tyre choice! You cant afford 250 quid for the TTC data? So you will just guess what the best tyre and it's characteristics are? How much is just one set of tyres for research?
What price the frustration of realising you have made a dumb choice and not only you, but everyone else, cansee that?
250 quid for the TTC tyre data? Cheap as chips!

Cheers
Pat

[ccsharry]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Peter7307:
Harry,
To add a little to your original Q2 in the first post: Yes the Ackermann is 100% if the intersection in plan is at the rear wheel centre line. If it is behind the RW C/L it is over 100% and ahead of the RW C/L is corresponding less then 100%
In your example with a wheelbase of 1600 ; 90% would equal 1540 and 110% would equal 1760.

In your post of April 27 (sorry but the posts on this board are not numbered) the time improvement difference really depends on a huge range of variables of which this is one. In my opinion it is not a significant factor.

The strut type suspension on the VW's accounts for the attitude of the of the front wheels duing the change in steering lock.

This is well explained in the firt mentioned book in the list below:

The Automotive Chassis Engineering Principles
J. Reimpell H. Stoll J. W. Betzler.
ISBN 978-0-7680-0657-5
Description: Vehicle dynamics and chassis design from a production car perspective. Very useful, if you are designing a production car.

Race Car Vehicle Dynamics
Milliken and Milliken.
Description: Vehicle dynamics and chassis design from a race car perspective. Probably the most useful generally of the big books, it is a good read as well. A fair amount of aero.

Fundamentals of Vehicle Dynamics
Thomas Gillespie.
Description: Mathematically oriented derivation of standard vehicle dynamics equations, and definitions of standard terms. A bit dry.

Chassis Design - Principles and Analysis
William F. Milliken and Doug Milliken.
Description: Vehicle dynamics as developed by Maurice Olley from the 1930s onwards. First comprehensive analytical synthesis of vehicle dynamics. Great historical stuff, and some nice explanations. Not as generally useful as RCVD.

Tires Suspension and Handling
John Dixon
Slightly more analytical than RCVD, covers much the same ground, but no aero.

Car Suspension and Handling
Donald Bastow
I only read a good discussion of steering effects due to camber gain, but it seemed useful and analytical.

Cheaper books

Carroll Smith - Tune to win etc. Generally a good introduction, but his explanation of roll axis in one book is confusing at best. I also disagree with him about single sided mounting for Heim's, but agree it is nicer to do it his way. Good on practical details, and fundamentals. Probably good enough for FSAE, for example.

By the way the poster is on the www.eng-tips.com site which is well worth a look. There is a section on suspension systems there.

Good luck , Pete. </div></BLOCKQUOTE>

Pete,

Thanks for pointing out the issue and this is the actual bit I want a confirmation. As far as I understand, from the method you suggested, over 100% ackermann will turn the inner wheel at less than 100% ackermann angle. I got this idea from you saying the intersection in plan behind the rear axle. However, how about 0% ackermann? Isn't it a parallel ackermann?

For my method, I have assumed that the percentage of ackermann is the severity of the ackermann angle on the inner wheel. For instance, over 100% ackermann means the inner wheel will turn more than the one at 100% ackermann angle. So my assumption is that by decreasing the percentage of ackermann, the point of intersection in plan will goes further behind the car and at 0% ackermann there will be no intersection since the distance will be at infinity (which is parallel ackermann). Can you confirm if i'm heading a totally wrong direction?

Thanks also for the list of books and the link to http://www.eng-tips.com/. I will try looking for the rest of them since I have read half of them already! And the eng-tips forum is very interesting!!

Regards,

Harry

[ccsharry]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PatClarke:
Harry,
Your questions are kind of generic and resourcing the answers is part of your FS challenge. I am a little surprised you have escaped without being flamed!

Suggestions have been made and book lists given, but the very best resource on the planet when researching this subject is right here!

The FSAE.Com archives and the links therein are a priceless resource, they are free and you have access. Start researching!

Some advice.
The most important decision on the entire car is the tyre choice! You cant afford 250 quid for the TTC data? So you will just guess what the best tyre and it's characteristics are? How much is just one set of tyres for research?
What price the frustration of realising you have made a dumb choice and not only you, but everyone else, cansee that?
250 quid for the TTC tyre data? Cheap as chips!

Cheers
Pat </div></BLOCKQUOTE>

Hi Pat,

Thank you for your reply. I apologise for my questions being generic in my previous post. I absolutely agree that it's a very interesting, challenging and exciting process to search for each answer of the question and definitely this forum is a well of useful knowledges.

I started to use the search function in this forum and to read posts relating to steering a couple of months ago. I have also been reading books relating to car suspension/handling/dynamics. However I am not too confident with the answers I found myself so I just want to raise the questions here and start a discussion with the experienced people.

For the advice, thank you for pointing out the importance of choosing the right tyre. I think we are heading the correct direction so far by choosing a light weight, very low profile and soft compound tyre but since we have no previous experience on this tyre company I am sure tyre data is very important to us. We managed to buy a set of them and ran them on our past year car. The performance is promising.

Thanks again,

Harry

[Schumi_Jr]

Ackerman has been discussed a fair bit on these forms so you might want to try searching the forum. Here's my opinion on steering for those that have a background in vehicle dynamics.

Your dynamic toe is one of the variables that affects your front slip angles. Tire data (which you can get from the TTC) can give you an indication of the "ideal" dynamic toe split (difference between left and right dynamic toe). The usefulness of the tire data comes from the quality of your analysis and understanding of the limitations of the data. In general you are looking to maximize the lateral cornering force of the front tires while they are at or near their slip limit. This means getting both tires to peak at the same time (its more complicated but i'll get to that)

To properly analyze tire data you need to measure or predict the operating conditions of your tire. Normal load, camber, air pressure and slip ratio are some variables that affect the slip peak of a tire. If you know these variables you can plug them into a tire model and solve for your slip peak. You will obviously need to analyze different cornering conditions- knowing what places to analyze comes down to experience.

The next step is understanding what determines your dynamic toe. There are 4 factors that affect dynamic toe: ackerman (and amount of steer input), static toe, bump steer and toe compliance. As an engineer you should know their magnitudes and understand their affect on dynamic toe at every point around the racetrack. For example toe-in and ackerman will give less dynamic toe split during the entry and exit of a corner and more in the middle. Don't neglect the effect of lateral force toe compliance- consider that you are generating significantly more lateral force on your outside front tire.

There are a few limitations to this that make this analysis ineffective. First your driver might have preferences about the transient handling of the car that require a theoretically less-than ideal set-up. For example your driver might like a high control yaw moment coefficient (gain in yaw moment per degree of steering input). At the tires limit this can go to zero unless you run lots of ackerman and drag the inside tire. There tons of other limitations- is it a combined slip tire model? have you considered normal-load variation affects? is your tire model transient? does it account for thermal affects?
Because of these limitations you need to test any theories on track. This will validate your analysis and help improve your understanding of any unknown parameters. In racing, the final judge is always the drivers feedback and the stopwatch. Engineering is just a tool to be smarter about improving your car.