Introduction and bachelor thesis about initial suspension design

[MP4/4]

Hi,

I am currently in my last semester of my industrial engineering degree. I am from Germany and also study here. I've choosen to write on a thesis about the suspension for a FSAE race car.

Here's the thing:
The old team never built or constructed a FSAE car. All they did was buying random stuff like dampers, seats, a stick welder.
They did't do one thing in CAD.

I don't know the reasons for this.

All I know is that I am passionate about racing since I am 8 years old. That's when I started racing go karts. From there on I raced until I was 17 years old. Then school unfortunatly interfered.

So, Iam currently in the topic finding process. The biggest constrain I have is that I absolutly start from srcatch. There is no data, I can use.

I've looked at some white papers, but everybody was building up onto something that is already there.

So my question is:

What would you concentrate on, when writting the bachelor thesis about the development of a suspension system?
I can't make it to big, because I only have three months time. So I need to cut out some things.

From what I know, you obviously start from the tires and move towards the inside from there.

The initial things, would be the attachement points of the suspension to the upright and to the chassis right? From thereon, the chassis can be designed.

When I was speaking with my supervising professor, he also said, designing and constructing them in CAD would probably be to much.

So from your experience, for a first year team, what should I concentrate on in the thesis? Kinematics, or also the design of an upright with load cases etc.

I want to make an initial step, for further teams to come, to finally get started with the project.
Developing the suspension, I think is the right starting point for that.

P.S. They built a chassis, but heck knows, from what drawing the built it.

[Karam Atteia]

MP4/4

This year was my first year in FSAE, but for my team it is the 4th. from the time the team had started till the last year, we didn't designed the suspension accurately nor calculate any thing. It was like building it from google and pics (monkey see .. monkey do).

but when i joined the team and started working on designing and building a suspension system for a FSAE car i was alone in this group. did every thing from scratch in a country that didn't have industry (Egypt).all these difficulties made me resolute to do it from A to Z and here the journey starts !

1-TIRES.. Read about tires even you wouldn't use the TTC in your design, but it will make you see things from another point of view. Tires are the most single important aspect in this whole system, it is the only contact between your vehicle and the road without it you can't transfer forces to the road to acc. or brake or even turn, keeping it in contact to the ground along all road conditions and circumstances is the big deal. all you have to do is to keep the tire working in the best range of camber angles under different loads. Tires are load sensitive where the load transfer is a very bad effect in our trim which affects the overall grip. if it is your first year don't waste time and money on TTC and be satisfied only to know why people use TTC, that would help!

2-SUSPENSION GEOMETRY (kinematics).. Before starting your geometry, you have to know your tires and wheels dimensions (Rim diameter, width, offset)then you have to pack every thing in it (brake disc if outboard, calipers, wheel bearings). now you can start your design from the outside and then move towards the inside from there as you said. starting from your front view geometry with decisions on RC height, front view swing arm length(rate of camber change & roll center lateral migration)and the length of the upper arm relative to the lower arm(amount of camber change with bump).Regarding the KPI angle as small as possible achieving a scrub radius near zero.
then now you can move to your side view knowing the positions of your UBJ and LBJ on the upright. take the decisions on your amount of anti-dive/squat/rise and your side view swing arm length(rate of caster change).
Now you have two 2-D planes(front & side), all what you need to do is to join these points in the 3-D world on your CAD to get your wishbones planes. then get your chassis pivots. don't miss the toe link or your steering geometry.
Then use a kinematic analysis program to check what you have done and know if you need to change any thing ( i used SusProg-3D ). [Read RCVD chapter 17 sus geometry & Chassis Engineering (Herb Adams)]

3-RIDE&ROLL RATES.. they have significant effect on cornering performance as they directly affect the tires normal forces distribution, and these rates also depends on your wheel loads. before starting these iterations you have to know your springs&ARB motion ratios, vehicle weight and static weight distribution , RC height, max. lateral acceleration, front and rear track width and wheel base. then you have to start with choosing your ride spring rates depending on your decisions on your front and rear Ride frequencies, then calculate the amount of roll resistance from these springs and compare it with your target on the Roll Gradient to size an appropriate front and rear ARB. put in consideration your TLLTD (total lateral load transfer distribution) to balance your front and rear ends. start with an initial understeer and make you ARBs adjustable.(take care from your units)
[Read RCVD chapter 16 (ride and roll rates)and chapter21(suspension springs) and OPTIMUM G technical tips]

4-DAMPERS.. as a first year team you have to know dampers and their adjustability and how to use them on the track to tune up your vehicle. don't waste time on designing them, just select a damper with numbers of damping forces near what you need for your case and then you can design your rocker mechanism to use the full damper stroke(from here we get our MR). choosing your damping ratios with a compromise between response time and overshoot(compromise between vehicle control and ride comfort which we don't care about in our trim) start with ride and single wheel bump damping then try to know your Pitch and Roll frequencies and Inertia of your sprung masses, for your pitch and roll damping understand.
[Read OPTIMUM G techtips and RCVD and RCVD chapter 22 (Dampers) and Learn&Compete chapter 7 (Dampers)]

5-DESIGN YOUR SYSTEM .. before designing any part of your suspension you should know the material you are going to use on manufacturing it (never design an upright without knowing its material). you have to eliminate any friction in your system specially at your revolute joints ( the rockers), take in consideration your wheel impact as all your upright and hub and ball-joints and bearings will be packed inside it. make every thing adjustable , but as your first year be satisfied of your camber and toe adj. and your antiroll bars and your pushrods(for ride height).

Try to keep it simple and working...

Karam

[Owen Thomas]

I am not sure how your thesis structure works, but seeing as you only have 3 months I would suggest keeping it simple. Here's my advice:

Focus on developing kinematics that you think are appropriate for typical FSAE tires. If you can buy in to the TTC, I would do so, but failing that there are some broad assumptions you can make about camber variation, load sensitivity, slip angle, etc. that apply to slick tires. Since it is a theoretical exercise, you could probably find some random tire curves on the internet and base your decisions off of that. What is important is that you document your reasoning behind your decisions in your thesis. This will help any future team that picks up your work more than anything else you can do.

Note that when I say "kinematics", I mean looking at motion only and not the forces involved in your suspension system. Component design is not something you will have time to do, so I wouldn't even touch it. Keep it simple with points and lines.

[MP4/4]

document your reasoning behind your decisions
I like that.

Here's what I plan to achieve/learn with this bachelor thesis:

I want to know basic vehicle dynamics, because we never went through this in one of our courses.
We had translation of a point mass, but not more.
Cloude underlined the idea to go from a bicycle model of the car to a rigid body and apply forces there.

I probably can just do that, when I have done a basic weight calculation of the parts I want to use and the overall final weight of the car.

I have no experience in tire modeling, kinematics.
We have a budget of 2000€ right now, I may use.


Component design is not something you will have time to do, so I wouldn't even touch it. Keep it simple with points and lines.

But I do need some expected lateral and longitutional forces, to know at which point the tire is capable of supporting these forces, right?

Looks like I am not going to be able to construct something in NX, I was eager to learn while writting the thesis.

[MP4/4]

Hi,

another question:

My professor and I have talked about the three different load cases.
Do I need to think about them?

I don't think I need to do that, because I firstly want to know the kinematic of the car.

That question steers roughly in the same direction I have asked above.

So what you think?

I had the impression, he likes to see this.
He emphasized, that we can not just draw something on the CAD (where I am absolutly along with him), but it is almost impossible for a first year car to know the exact loads on each corner, without strain gauges or testing an actual vehicle, and than design a part according to these data.

[Francis Gagné]

Hi,

It seems that the «team» was pretty unorganized. If you want to make a long lasting impact on your team future and make something that can build on, may I suggest you go back further to the basics.

What will benefit your team/school the most. I don't think a suspension design is the answer because it seems you would have a suspension with no car, not very useful.

In any engineering projects (product development), there are some important steps to be followed. The first one is to set a goal, defining the project to be made, the problem to be solved. Then, you should define the functions (what does it do) and specifications of the product (how much does it have to do it). You then find system solutions that meets your specifications and choose the "best compromise" solutions based on your goals. Then you make the detailed engineering, parts, detailed cad, nuts & bolts thing. The build the thing and verify it meets the specifications that have been chosen.

In this case your goal should be something along the lines of : Design and build a car that makes a maximum of point in a Formula Student competition.

How to resolve this problem has been discussed here often, particularly look at the Reasoning your way through FSAE design process thread.

If I had to start from nothing I would do something along the lines of a) Understanding the relative influence of Power, Tires, CG height, Mass, Aero, Cost, etc. in regards of lap time and most importantly in regards of total competition point. Create a LapSim (Matlab, Excel VBA, doesn't matter) (validate with OptimumLap) that recreates these parameters. Make the program so that you can compare concepts quantitavely. You will also need to evaluate and compare other qualitative aspect/constraints of the concept such as, design time in man-hour, build time, cost, ergonomics, etc.

The more input you will want to investigate the more complex the program will have to be. If you want to look at track width, you will have to calculate weight transfer and tire load sensitivity. If you want to include mass repartition you will have to calculate the limiting lateral acceleration at both front and rear, etc. It is up to you to decide what information you want from this tool, and how far you can go in the time you have.

This tool will make the next design step of choosing a global system view of the car. You want a full aero car,mid lighweight, mid power. Or low power, very lightweight, low cg-car?

To know what input data are reasonable (mass of a 13" single cylinder car) you will have to look at the competition. Make a database of the cars, and their competition results. You can even look for trends between a parameter and the results in a particular event. (example, acceleration results and power to weight)

And if it is well documented and valid, any tool like this help team grows and refrain from doing the same calculations year after year.



If you still want to go for suspension design by personal interest, I would still encourage you to build a tool that can be reused. Example, a four wheel model with steering and weight transfer, in which you input a tire model (Pacejka or another?) and calculate the resultant lateral and longitudinal forces and most importantly yaw moment. That would be a good tool the find steering geometry, steering forces, and understand the effect of toe.

You could also make basic models/calc sheets to find other different parameters, such as spring and arb rates, or damping coefficients, these would still have to be fined tune but make a good rough starting point.

To make the tire model, the data Pacejka of the Hoosier R25a is available in an older round of the TTC. It is a good starting point to test your models.

I probably wouldn't touch kinematics at that point. Start by understanding the physics of it all, the forces and moment that act on the car and how the basic geometric parameters (wheel base, ackermann, track, mass repartition, toe) influence the max lat & long. acceleration as well as the balance of the car.


In both case, do a litterature review, take time to read and understand the basic underlying principle and physics of the problem, and document your findings for the next one (and your thesis writing!!). Good luck!

[Owen Thomas]

But I do need some expected lateral and longitutional forces, to know at which point the tire is capable of supporting these forces, right?
You have that backwards. The amount of longitudinal/lateral force the tire can support is based on the vertical load, slip angle, etc. and you can see this from the tire curves. You can solve your three load cases using weight transfer formulae to get the normal load on the tire and go from there; that might appease your professor and still give meaningful results.

The point I was making is that with the amount of time you have, keeping it "higher level" might make more sense. I would only look at the tire data to see what conditions you want the tires to operate in and design the kinematics to suit. Knowing the weight on each tire in different situations would definitely help, but I wouldn't suggest getting overly technical about it. Maybe figure out which load cases are most important (happen most often) and focus on that, while compromising for the other scenarios.

[MP4/4]

The point I was making is that with the amount of time you have, keeping it "higher level" might make more sense. I would only look at the tire data to see what conditions you want the tires to operate in and design the kinematics to suit. Knowing the weight on each tire in different situations would definitely help, but I wouldn't suggest getting overly technical about it. Maybe figure out which load cases are most important (happen most often) and focus on that, while compromising for the other scenarios.

Thanks Owen,
by "higher level" you obviously mean, keeping it rather theoretical on the tire data, than practical, right?

I like that, although I prob. will miss some CAD work.
There really is an aweful lot of equations involved in just going about a tire model and car dynamics.
But as I see it these are the fundamentals, right?
Assume this about me, I am passionate to learn everything I can about why a vehicle does this or that, but I don't know where to start. I am as confused by KPI, Roll Center Hight and Instant Center as everybody who starts with this and didn't had an automotive vehicle dynamics course I/II in uni.
For me the purpose of this thesis is to learn to fundamentals to built up on them, as I go along and pursue my masters this fall in Mechanical Engineering with a specification in automotive engineering.


If you want to make a long lasting impact on your team future and make something that can build on, may I suggest you go back further to the basics.

Hi Francis,
I still have hard feelings about what has happend last year. I was working with a very good friend of mine from brazil (a mechanical engineering exchange student) in the CAD lab for many many hours every day. We put all our effort into it.
We wanted to built a formula fsa car and said, it's stupid to use this frame from a former team.
The frame wasn't compliant to the rules anymore and would have compromised the correct design process of the suspension.
Everything we said, be it, we have to design from the outside in, has been not taken seriously from an older member who thought he knows best. He was from the team that has built this mysterious frame that is still sitting at the shop. It's a shame when you think about it, that they would built something.

He had the brilliant idea to built a hillclimb racer out of it, when we discussed next steps towards a start at this years FSG in Hockenheim. Using the formula student team for his personal interests.
Great.

I was so stuffed up with it after taken up with it for a while that I eventually,after he said that we can't afford the tires me and my buddy had choosen after some long discussions, eff it, I have put enough energy, nerves and time in it.

I had fun doing it, thinking about the design constraints with my buddy, but if you work in a team, that didn't had any interest in competing in the actual race, I can't help them anymore.
When my friend went back to brazil, I left the team and I have to say that the time he was here and we worked on the car, was the best time I had during uni.
He also had experience with their own team in brazil, who already had a working car. I think already the 3th or 4th.

The purpose of this thesis is to leave a legacy of somebody who really wanted to compete in the FS championship. Somebody who uses this stuff to become a great engineer and not just effin about it, wasting time and air.

[Jay Lawrence]

MP4,

If your purpose is to leave a legacy from which future interest can grow, listen to Francis. A very high level "what is required to make a FSAE car" will suit your stated purpose best. As mentioned, BigBird's "reasoning your way through FSAE" thread is a goldmine for this kind of thing.

For example, instead of saying "my design has -2° of static camber because graph x of tyre y shows that this will give me z force at n normal load with Q slip angle", say something like "as tyre slip angle increases, lateral force increases up to a certain point and then falls away... turning the inside wheel more than the outside can be helpful to increase yaw acceleration and therefore helping the car turn, this is based on the Ackerman steering philosophy... a carbon monocoque frame can be described as x, a steel spaceframe can be described as y... a single cylinder engine vs. a 4 cylinder engine, see following table:..." and so on. If you follow this kind of logic for as many systems as you can think of, you will fill a great number of pages with very useful information. You can then do a basic model consisting of your recommendations regarding each system. This ties it all together and let's you show something sexy to your supervisor and future interests. Combine this with a lapsim to compare some different design philosophies (I would recommend using one that's already built, given your time constraints) and voila, epic thesis!

[MP4/4]

If you still want to go for suspension design by personal interest, I would still encourage you to build a tool that can be reused. Example, a four wheel model with steering and weight transfer, in which you input a tire model (Pacejka or another?) and calculate the resultant lateral and longitudinal forces and most importantly yaw moment. That would be a good tool the find steering geometry, steering forces, and understand the effect of toe.

You could also make basic models/calc sheets to find other different parameters, such as spring and arb rates, or damping coefficients, these would still have to be fined tune but make a good rough starting point.
I really like the post of Francis.

I have been to my professor today and he made it clear that he does not want to hear things everybody already knows.
That includes vehicle dynamics.
He limited the thesis to 50 pages. I find that reasonable, given the fact I am not able to include a theoretical part about the fundamental vehicle dynamics and models which are used.

He seemed to want a factual physical work. I could see, that he never supervised a thesis about a FSAE topic before.
I think at least at the beginning it is hard to get to the little details of things, but that's what he obviously tryied to make clear, that he doesn't give a shit about the bigger picture.

With bigger picture I mean, not the future of a particular team, which could gather in a future, I mean the bigger spectrum we have to see the design task.

We are designing a race car to do a specific task for a specific racetrack, Hockenheim. With the boundaries that are given through the technical reglement and the dynamic tasks, we are creating a list of desired properties.
I think even he can see, that without the broader view, you are unable to design this car.

I have found a 3d suspension model for solid works, I will simulate the basic movements with, to show him, what this task actually means and what we are talking about. He asked for something like that today to make it more visual for him and I think this will be very helpful to see what we are actually talking about and the constrains that are underlying the tasks of designing a suspension model.

I think with the model we are beginning to understand each other a little better and what we are trying to achieve here.

I want to learn about the fundamentals of vehicle dynamics and learn Siemens NX and
he wants to teach me to think more like an engineer from a theoretical standpoint, which I like.
I come from a lot of years racing karts and can tell you practical why the back oversteers, but always wanted to know the underlying physical principles.