Suspension Design Step by Step Process

[Benn Hulbert]

Hello forum.
I'm fairly new to FSAE and suspension design. One of the overwhelming problems that I'm having is figure out the step by step process in designing the suspension system. I basically don't know where to start there is so much information. If you could give me a list from start to finish (1,2,3,etc)in terms of calculations and parameters to address, that would be great.
Benn Hulbert

[Lemon Lime]

Hello Benn,



First warm greetings and afterward;



I'm a beginner too so don't take this as an expert talk ... I mean I'm just sharing what I know .



First; I want to tell you a tiny little thing ... Introduce yourself properly; name , country, team ... etc.



Secondly; There is a "Find" button at the utility bar up at the right just beside the "New" button, So try to search for your topic if it had been discused before so as to save time and effort.



About your topic it had been discussed quite alot, But I'm going to share what I got anyway.



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First; If you haven't already read RCVD "Race Car Vehicle Dynamics" then you must.



Secondly; Start by tires ... Study the pneumatic tires very well ... Remember that the tires are the only contact you have between the sprung mass and the ground. I remember watching a video where Mr.Claude Rouelle who is a famous FSAE judge and president of one of the leading companies in vehicle dynamics softwares "OptimumG" saying that if you know the forces acting on your tire contact patch then you will be able to choose a suspension template.



Thirldy; If you want to start your design process, Then start by visualizing a 3D model of any type of suspension you want ... Start by 2D visualizing .. Start drawing on one of the CAD softwares "I recommend SolidWorks .. Not for a particular reason except that there is quite a lot of tutorials on youtube " Start by darwing two planes; Front view and a side view .. And start sketching on the front view geometry by drawing lines to express your wishbones and your unsprung mass and then move to the side view ... After which; You should be able to transform the whole 2D drawing into 3D you have 2 planes and the third one is the top plane.



Then; Put the whole geometry on a kinematics analysis software ... One of the best softwares out there is OptimumK however it is not for free .. Another free one and quite successful is Wishbone.exe it is an old program with old interface but it will help much trust me.



Then start analyzing the results from the softwares such as the camber change due to body roll, bump, droop ... etc.



You won't be able to be sure about the results unless you have tire data !



Then go back to your model and edit it again and analyze once more and start iteration.



Work well on the compromises ... You'll have to loose something in order to win the other.



Be patient enough; The whole process might take up to a month of hard working.



Don't stop studying; Once you start design; Study more not less !



Know why before knowing how !! <--- VERY IMPORTANT NOTE HERE .



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That's it; I shared all what I have of knowledge ... I really like one quote or an OptimumG motto that says "Knowledge shared; Is knowledge squared".



God be with you and Good luck with your design. http://fsae.com/groupee_common/emoticons/icon_smile.gif



P.S : Sorry for any typing mistakes .

[Benn Hulbert]

Thanks for the information!
University of St. Thomas FSAE Team, St. Paul, Minnesota, USA

[Lemon Lime]

Benn,



Sorry, My previous post was all about kinematics ... No forces.



if you need some info on rates and calculations Check a discussion by the name "Damping Rates" and another one called "Rates Again" both posted by my team mate Karam Atteia ... It should be very helpful too.



Good luck

[Francis Gagné]

As in any design problem there are some step you should take before jumping into the details. Get the basics right first, the details only after.

I have written down one approach, there is no "best" one, but this is systematic, and "relatively" simple since it seems to be your first go at it :

0. Prior art, read on suspension design (in RCVD for example). Understand the basic physics first! Read and understand about the bicycle model, the understand the behaviour of the model and the tire.

1. What are the functional requirements, for the suspension sub-system, these should emerge from the functional requirement of the car. Example : The suspension must permit vertical movement between the suspended mass relative to the tires. The suspension permits control of the vehicle direction.

2. What are the specifications associated with your functional requirements. You will have to make basic assumptions to estimate the values. You only need to get yourself in the ballpark (The suspension must move ±25mm, the front outer wheels must turn at least X degrees, The suspension must resist a vertical load of X newtons, steering torque should be X Nm etc.)

*The car layout design and team goals should drive your suspension goals. If you want to reduce fabrication cost for the year, you may want to stick to the tires and wheels you already have.

These specifications should be written in a table for fast reference.
(Some function and associated specifications can be derived from the RuleBook)

3. Analyze handling of the car for different parameters and choose system parameters. Use the Bicycle model first for practice, then build to a Four Wheel 2D model.

-You will first need some sort of tire model that fits Mz, Fx and Fy, it need to accounts for the shape of the curve, and load sensitivity. You might not need combined loads at the beginning, but it could be useful for looking at trail-braking and power corner exit (and differential selection) (You might want to use the Stackpole Engineering Pacejka model from TTC Round 3, it might not be your tire, but it will give you a good indication of the common behavior of a fsae tire. All the fitting is already done and all the equations of the model written down)

-Write a simple code for the Four Wheel model that is able to calculate the slip angle for a given radius and steering angle, you will have to make assumptions about the slip angle at the rear. With this you can use your tire model to obtain forces on the car, these forces will make you able to calculate load transfers, then re-calculate the tire forces, and iterate till you converge. (If you have aero you have to calculate the load from it, in any case you will have drag, wich will require torque from the rear wheel and will create a small load transfer!)

-With this code you will be able to input steering angles (or front slip angle), rear slip angles, slip ratios (if you have combined load code), and obtain Lateral acceleration, Longitudinal Acceleration, cornering speed, etc. But most importantly you will also have a resultant moment on the car, this car will be the indication of the instantaneous oversteer/understeer of the car. With this tool you will be able to make graphics, table or whatever you like by example, by iteration you can find the maximum lateral G that gives a zero yaw moment for different radius. Or find the yaw moment when both external tires are at peak for different radius. Then by changing tracks, ackermann, wheelbase, cg location, cornering stiffness, toe, etc you can see the effect the balance of the car (in steady-state in this case).

-You will also be able to include scrub radius and mechanical trail, and put your tie-rod location and calculate the forces in the steering.

*REMEMBER, if you make such a program, you will need to validate it's output, Change inputs and verify if the output changes in the appropriate direction (from logic or experience), validate with test results from a known car if you can.

-When you are satisfied with the analysis, set these global parameters goals.

4. Only then will you have to worry with roll centers, roll stiffness, kinematics, motion ratio, dampers and such things. There is not point in finnicking a camber gain that will "optimize" the tire output of about 1% if you understeer at 80% of the peak G of the tire in the rear...

I hope I haven't said something stupid, if so I duly apologize.

*A note on tire choice, the best tire is not necessarily the one with the most peak grip, it's the one, that onto your car, makes the most grip in the direction wanted (In fact it's the one your driver makes the best lap time with (and possibly burn less fuel for enduro while so)). So to choose a tire you need a car, and to make a car you need to choose a tire.

[Edward M. Kasprzak]

I'd start by analyzing and understanding a few existing suspensions. You can't truly design something if you can't perform an analysis on it. Jumping straight to "design" without sufficient analysis capability/knowledge removes the engineering component--at that point you're just drawing stuff and rationalizing.

That said, the need to design must drive the analysis/learning and the analysis must drive the design. Time is short. You can't wait to perfect your knowledge and analysis--you'd never design anything! There is a knowledge-analysis-design-(evaluate) cycle...or perhaps it's a helix. The more times you go around the cycle the more you learn, the better your analysis gets and the more engineering you have in your design. How far up the helix can you get in your FSAE tenure?

[Z]

Originally posted by Benn Hulbert:
I basically don't know where to start there is so much information.
Benn,

My views on this subject appear at length on this "Suspension Design" (http://fsae.com/eve/forums/a/tpc/f/125607348/m/73320357151/p/1) thread. A summary of that thread is that I reckon the problem is a simple one, but, unfortunately, most FSAE students make it far more complicated than necessary.

In brief, you are required by the Rules to build a "suspension" that could potentially have +/- 1" of up-down movement. Importantly, it doesn't actually have to move that much on the track. In fact, many FSAE events have been won by cars with effectively NO SUSPENSION AT ALL (ie. springs so stiff that the suspension never moves).

So, IMO, the big question is;

How much time and effort should you invest in "suspension design" in order to get a car capable of winning the competition?
~~~o0o~~~

Briefly, here are some issues RELATED to suspension design that you have to consider;

1. Car mass distribution? Includes mainly Total-mass, F:R%, CG height, Yaw-MoI.

2. Tyres? Consider what brand, size, ply-type, rim-widths, inflation pressures, camber-angles, and toe-angles, they should run at.

3. Structural implications? No point designing the best ever suspension if it is a structural nightmare (eg. too heavy, or too flexible, or gets in the way of more important stuff...).

4. Steering geometry? This is a big issue from a performance point of view (ie. how fast the car can go around corners), and also for driver comfort (ie. how long before driver gets too tired to steer...).

5. Differential type? Again, a big influence on how fast you get around corners...

6. Aero? How much ...???

IMPORTANTLY, these above issues are much more important than most of the suspension details. And, in fact, they don't require a suspension at all.
~~~o0o~~~

So that leaves the details of the suspension that you have to consider.

1. Lateral Load Transfer Distribution (aka Roll Moment Distribution)? This is really only a MINOR tuning tool. It involves either working the front tyres harder by lifting the inner-front when cornering, or working the rear tyres harder by lifting the inner-rear. Look up "TLS" (yes, you have to learn lots of acronyns... http://fsae.com/groupee_common/emoticons/icon_smile.gif).

2. How to stop the car "bouncing"? This is the original purpose of a suspension. But since (current) FSAE tracks are so smooth it is of less importance than on "real roads". However, even without bumps on the track a car can "bounce on its tyres" due to a stick-slip-stick effect of the tyres during cornering. This is easily suppressed in FSAE conditions by having a small amount (say, +/- 5 mm) of SOFT suspension movement, together with a small amount of damping (exotic dampers are NOT required).
~~~o0o~~~

Summing up, most FSAE teams I have seen seem to spend far too much time designing and building suspensions that are far too complicated. As a result, they overlook a lot of the more important issues, such as structural stiffness, and end up with wheels that point NOWHERE near their intended directions when on track. (Hint: Toe-angles of the wheels are very important, and can be (often are) more affected by "structural compliance", than by "suspension kinematics".)

In short, DO NOT OVERCOMPLICATE a simple (mainly structural) problem.

Z

[MCoach]

Formula Student Germany 2013 Endurance (http://www.youtube.com/watch?v=7q6kturw6CA&feature=youtu.be&t=1h37m13s)

Here is the entire endurance video from FSG 2013 last week. It's skipped forward to the final 5. One thing that can be noticed is actually how bumpy the track is. The next thing that becomes obvious is the teams that have a good car design, and the cars that dialed in ( one very in particular if you watch it). Vehicles without suspension have won, and they can be fast, as long as the proper adjustments are left open for balance.


Brain storm. What do you want to achieve? Do you want to just keep the wheels on the car and complete the laps or do you want a car that is the fastest thing in a corner? Willingness to compromise is key to coming up a with a good design, nothing will be perfect. Is there something that can be innovative to the design? Anything novel?

Research. Who is using what, what has been used before? Why were certain concepts used and others abandoned? Is there a reason why many have converged on a certain concept? If tires change, what becomes different.

Books. Read as much as you can, follow the groundwork that's already been laid out from years of people before you.

New Brainstorm. After this iteration, how can these ideas you've formed achieve your high level goals? Now slim the ideas down to a core goal.

Design. Simulate. Iterate. Repeat. Then, build, race, win.

[EPMPaul]

Hi Benn,

Another thing I'd look into is setting some goals for your suspension by identifying what parameters will really affect your laptime. OptimumLap would be a good place to start as it will give you a decent idea of what really affects laptime on one of these cars.( They have Formula SAE cars and course already modeled into the software).This will also have to integrate with the kind of car you want to build overall (tradeoffs aren't the same with a 4cylinder than with a single, aero or non aero)

[mdavis]

Going along with what MCoach was saying about FSG being quite bumpy, I think the TUFast onboard video shows the bumps quite well:

https://www.youtube.com/watch?v=wEdMO7GfkbA&hd=1

-Matt

[Kevin Hayward]

Benn,

It is very frustrating to dive in and start designing suspension for a vehicle. There are no definitive written guides, and many of the books straight out contradict each other.

When you extend suspension into what you are really working with, which is non-linear vehicle dynamics with huge unknowns the problem gets very big very soon.

I started trying to get a list of steps starting from 1 and increasing in a rational order, only to find that you often start on number 4 then end on number 2, while passing a good 20 or so steps along the way.

I won't offer a list of steps, but more like a couple of ideas that might help you eliminate some of the things not to worry too much about early on.

- Tyres are super important ... but within reason you will be running the same ones as everyone else. Going through piles of data before you have a starting point is almost completely useless. You will learn more by the first few days of testing than the first year of study. Watch the car, look at the tyres after a run. Listen to people at your local kart and racing tracks look at their cars. Touch the tyres if you can. Dig your nails in. Look at old vs new, notice how the old tyres aren't as hot after a couple of laps, feel how they aren't as sticky. Look at tyres that have been running big camber vs small, notice what the wear pattern looks like after a few laps, and then again what it looks like when they are shagged. Talk to the competitors. Nurses may not be doctors, but they often know through experience how to treat a patient. Remember that rubber is just another type of material. Did you know that the friction of rubber varies as a function of it's sliding speed? Did you know that it contains long polymer chains? Did you know that it is cured through a process of heat with the addition of sulfur (or other additives) both or which are present when racing? Also note that a tyre is pretty much just a cloth and rubber balloon. This sort of insight into the basics demystifies what some people would like to think of magic. Want to understand how camber produces camber thrust ... then imagine how the contact patch of this balloon would look like if it pressed into the ground at different angles and then rolled. Would it be any different if the sidewalls of the tyre were stiffer or softer? How would it look if the pressure was increased or decreased?

- Good sturdy, stiff mechanical design is very important. Understand statics, load paths, materials, and general mechanical design principles. If you want your suspension to match your calculations in any way it needs to be clear of slop and stiff. If you want it to keep working it needs to be strong. Nearly all forces applied to the vehicle (i.e. lateral load transfer) are able to be solved using simple engineering mechanics principles. The equations for lateral load transfer (fancy name, much simpler concept) are simply derived from mechanics.

- Develop an understanding of dynamics. What you need that you can't derive from statics fundamentals, you can from dynamics. The car is a series of connected spring-mass-damper systems with the main forces applied at the tyres. For most initial calculations the car can be treated as a point mass. Start with simple models that you completely understand and then move on. Try to avoid using simulations that you do not understand the inner workings of. That means knowing what each input is, what each output is, how the model is likely to be constructed, and the type and rough magnitude of errors.

- Understand the driver. The car is only as good as how well a person can drive it. It is important that controls are manageable (i.e. not too heavy) and give decent feedback. This is very important for steering. The car needs to be predictable and controllable, and adjustable car with testing time really helps make this happen. Go to your local racetrack and work with some drivers. There will be a 99.9% chance that there will be somebody there that will want help. Watch different drivers. See how a car can be driven almost completely differently and achieve similar speeds. Two cars with similar lap times are likely to have different suspension settings, with drivers who control the vehicle differently. You will need to deal with it when your car is built. You may find a couple of tenths in exhaustive tuning, but seconds by working with the pilot. Account for this in your design. It also means that we design cars as windows, rather than trying to hit a bulls-eye. We want to be in the right region with the design and then fine tune to win.

- Understand economics. What you are after is the best return on time and money spent, not a complete knowledge, or the best vehicle. Focus on your biggest, most easily fixed weaknesses first. Sure understanding the effect of camber on the vertical tyre stiffness will help you to improve accuracy of tyre loading. However if you have a poor understanding of mechanisms you are pretty much stuffed. Likewise if your team is seconds off the pace you wont be gaining much by having $2000 dampers instead of $200 ones. You might be much better off by having another week of testing.


...

Read a lot, but think a lot more. Vehicle dynamics is not a separate field to the rest of engineering, it is a subset. All the black magic hand wavy stuff, and the claims of expert knowledge is just the application of empirical knowledge to attempt to simplify an otherwise very difficult engineering problem.

Be patient. It is a big field, and no matter where you start you will not get too far in a year, but it is one of the most interesting fields of engineering to sink your teeth into, and you will never run out of things to learn.

Kev