Starting a design

[ed_pratt]

To anyone and everyone!!!


I was wondering if you could help me in any way with regards to suspension design. I've got a few books but I'm still struggling to get everything together to come up with something solid, i.e I can't find anywhere which actually talks you through the design process.

I have;
Wheelbase: 1700mm
Track (Front/Rear): 1220mm/1200mm
Desired C of G; 250mm
Desired ground clearance: 55mm – front and back
Weight distribution roughly 45:55 front:rear
13"�x7"� wheels
20x7.5 (13"�) tyres
I have a target weight of < 250 Kg
I sort of know what I need but I'm unsure how to go about it.
Camber gain?
Roll centre displacement?
Scrub radius – <10mm?
Ball joint locations ; how do I get these? Do I just pick them out of nowhere? Or is there a method?
Inner control arm pivot points?
KPI angle?
Caster Angle?

Im sorry about the essay! But any help you could give would be fantastic.

[ed_pratt]

To anyone and everyone!!!


I was wondering if you could help me in any way with regards to suspension design. I've got a few books but I'm still struggling to get everything together to come up with something solid, i.e I can't find anywhere which actually talks you through the design process.

I have;
Wheelbase: 1700mm
Track (Front/Rear): 1220mm/1200mm
Desired C of G; 250mm
Desired ground clearance: 55mm – front and back
Weight distribution roughly 45:55 front:rear
13"�x7"� wheels
20x7.5 (13"�) tyres
I have a target weight of &lt; 250 Kg
I sort of know what I need but I'm unsure how to go about it.
Camber gain?
Roll centre displacement?
Scrub radius – &lt;10mm?
Ball joint locations ; how do I get these? Do I just pick them out of nowhere? Or is there a method?
Inner control arm pivot points?
KPI angle?
Caster Angle?

Im sorry about the essay! But any help you could give would be fantastic.

[PatClarke]

Edmond,
Its good manners to sign a request, that way people can relate to who they are talking to.

There has been an enormous amount of angst developed on this forum recently by requests like yours, so before someone flames you I'll give you a little advice.

Firstly, I would be interested in how you came up with the values you have already decided on?

Assuming you have done some research, why didn't you keep going? The information required for you to make your design decisions (I said YOUR design decisions) are only a Google search away.

There are numerous papers out there relating specifically to the design and building of a FSAE car, and probably more importantly, to running a FSAE team.

I have made this point before, Formula SAE, or in your case Formula Student, is a Design Competition, not a Back Engineering competition.

You have to make your own design decisions and you have to defend them at the competition judging.

The information for you to calculate the answers is out there, not difficult to find, and not difficult to process when you find it.

Its highly unlikely someone on here is just going to say "Hey, hang on a minute and I'll send you my design"...or even "..part of my design"

To help you get started, the Universary of Missouri at Rolla published a paper that will help. All you have to do is find it to start with. Hint: Its an SAE paper

Regards
Pat Clarke

[ed_pratt]

Hi pat,

oops thought i had signed it...
I realise that this post will probably get shot down in flames but i had just got really stuck.
thanks for the help with the paper im just finding it now.
Ive been reading the staniforth books, ive found them really helpfull but im still struggling to make sense of whats happening to the car as it moves along, obvioulsy its complicated and can be simplified in some ways, but ive just been unsure of how to proceed.

before i get "flamed"! i dont want people to think im wanting their designs or anything!
Ive read pretty much the whole forum and i know there are about 30 topics about the same as this one, but im doing this for the 1st time and on my own just now so i needed the input. anyway, thanks for your help and advice.

my preliminary numbers are from my research(staniforth books, the formula student event magazine, team websites.. etc)and come from my idea of the sort of car that we want - lightweight, reliable, probably single cylinder.

hope that doesnt sound snappy or anything, i really do appreiciate the help.

Regards

Edmund

[JerryLH3]

If you're serious about it, there's a world of books to buy that would help you understand vehicle dynamics.

Tune to Win by Carroll Smith is the book of his that focuses the most on vehicle dynamics and aspects of design. I would recommend buying all of Carroll's book as they are a tremendous resource.

Race Car Vehicle Dynamics by Milliken/Milliken. It's very in depth and sometimes referred to as the racer's Bible.

Those are both available at the SAE bookstore. I've certainly made the most of the 20% discount for members.

[ed_pratt]

thanks jerry i'll check them out, infact i think there might be a copy of tune to win floating about somewhere at uni.

i've heard nothing but good things about the milliken/milliken book, i'll try and convince someone to buy it for the team!

thanks again

ed

[CMURacing - Prometheus]

if you're doing suspension design, not only RCVD, but maurice olley's "chassis design" (also edited by milliken/milliken) is a great resource.

[Mark TMV]

If you want some 'ballpark' numbers you may wanna check this out:

http://www.brunelracing.co.uk/cars/br6.html

[ed_pratt]

thats fantastic guys.
thanks for your help, im just getting to grips with susprog and trying to get some values in and let the witchcraft happen! hopefully should get some answers.

thanks again

ed

[Ben Beacock]

as with most design processes, there are a few things I would suggest laying out before jumping into the calculations

#1. define your problem and goals for solving the problem. do you want to learn new technology? do you want something simple that will work for a long time?

#2. look at the constraints. The rules pretty much define this. minimum wheelbase, engine size, minimum suspension travel. These are the things that cannot be changed.
Project finish time is also a constraint. You cannot extend it by talking to your prof and saying you were sick. http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

#3. define your criteria. eg. low mass, low inertia, quick turn-in, low cost, high value, reliability, high torsional stiffness, innovation, ease of manufacturing, low development time

#4. since you don't have a car to test with and it's your first time, try assigning values to your criteria to match your goals. eg low value (0.1) for innovation, a high value (0.6) for reliability. The sum of all your weights should equal (1.0). As you build experience and data in the years ahead, these weights would eventually be influenced by the scoring system.

#5. keep these weights and your constraints in mind while making decisions.

This short example was for an entire car, but it is easily applicable for any part or subsystem by inheriting some of your goals, constraints, criteria from the overall project goals.

[^_^]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Race Car Vehicle Dynamics by Milliken/Milliken. It's very in depth and sometimes referred to as the racer's Bible. </div></BLOCKQUOTE>

Stop kidding yourselves. RCVD is a terrible book anyway you look at it. Milliken describes simple things in very convoluted language. If you are a beginner trying to learn, the book sucks at teaching. If you already know what you are doing, the book is way too basic to be useful. There is a whole "Emperor's New Clothes" thing going on with this book. Everybody says its so great that nobody wants to be the one to cut it down. The book is only the "Racer's Bible" to the people who don't know what they are doing and want to look smarter than they are.

[NetKev92]

Got to admit that I haven't read many good sources on the topic yet, but there are a few basics that I've been going by. For grip, I want to keep my tires roughly vertical to the track. With a bit of geometry, you can work out the camber of the wheels as a function of ride height for a given geometry. With a bit more effort, you can work out camber as a function of roll angle. Remember the law of cosines and you can figure out how to iterate for a solution. My homebrew tool rolls the body of the car, guesses the position of the uprights, then iterates if my ride height or upright positions don't work out. If you have a better budget, there are software packages that you can buy to help do the job.

From there I'd bet the rest is detail work that you're not likely to guess right without having a car to start taking data on. Being relatively new to this myself, my approach is to keep it simple. My second version will be much better probably than my first, so I just need to get my first version done.

Kevin

[Nick McNaughton]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
I have;
Wheelbase: 1700mm
Track (Front/Rear): 1220mm/1200mm
Desired C of G; 250mm
Desired ground clearance: 55mm – front and back
Weight distribution roughly 45:55 front:rear
13"�x7"� wheels
20x7.5 (13"�) tyres
I have a target weight of &lt; 250 Kg
I sort of know what I need but I'm unsure how to go about it.
Camber gain?
Roll centre displacement?
Scrub radius – &lt;10mm?
Ball joint locations ; how do I get these? Do I just pick them out of nowhere? Or is there a method?
Inner control arm pivot points?
KPI angle?
Caster Angle?
</div></BLOCKQUOTE>
Most of these you'll find out as you go, because you'll start to see the tradeoffs between them when you get to the number crunching bit. Invest a bit of time in getting the maths set up properly for any design study you do, it'll pay you back tenfold once you get into it.

Some comments:
COGH and rideheight: 250mm with driver is pretty competitive, but achieving it with 55mm ride height on a first year car is going to be difficult. The rules require 25mm of bump travel, you're unlikely to need much more than that in ground clearance.
Camber gain and rollcentre displacement: Start by deciding your global roll stiffness in deg/g, and go from there. Camber gain and rollcentre displacement play off against each other, as you will find, and you'll settle for something you're happy with.
Pivot locations and KPI satisfy all of the above, and are then largely determined by packaging.

Good luck.

[CMURacing - Prometheus]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ^_^:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Race Car Vehicle Dynamics by Milliken/Milliken. It's very in depth and sometimes referred to as the racer's Bible. </div></BLOCKQUOTE>

Stop kidding yourselves. RCVD is a terrible book anyway you look at it. Milliken describes simple things in very convoluted language. If you are a beginner trying to learn, the book sucks at teaching. If you already know what you are doing, the book is way too basic to be useful. There is a whole "Emperor's New Clothes" thing going on with this book. Everybody says its so great that nobody wants to be the one to cut it down. The book is only the "Racer's Bible" to the people who don't know what they are doing and want to look smarter than they are. </div></BLOCKQUOTE>

we've been over this before in this forum. and those who felt like posting big long dissertations liked olley's book much better. those of you not familiar with it and independent suspension design, should be. there's a lot of math and a lot of proofs in there worth looking at.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by new_to_this:
I have;
Wheelbase: 1700mm
Track (Front/Rear): 1220mm/1200mm
Desired C of G; 250mm
Desired ground clearance: 55mm – front and back
Weight distribution roughly 45:55 front:rear
13"�x7"� wheels
20x7.5 (13"�) tyres
I have a target weight of &lt; 250 Kg
I sort of know what I need but I'm unsure how to go about it.
Camber gain?
Roll centre displacement?
Scrub radius – &lt;10mm?
Ball joint locations ; how do I get these? Do I just pick them out of nowhere? Or is there a method?
Inner control arm pivot points?
KPI angle?
Caster Angle?
</div></BLOCKQUOTE>

Many of these are not simply suspension design parameters but actually whole vehicle parameters... So, are you working alone or as part of a team? How much responsibility for the whole vehicle do you have? How do you relate to the other team members and their design responsibilities? How will you manage conflicts? These questions are far more relevent to a successful design outcome than deciding on +/-5mm of scrub radius (as an example).

As Pat says this is a design competition. A good place to start is to study a systems engineering approach (I googled and found this as a start):
http://www.lboro.ac.uk/departments/mm/research/product-...es_int/ipps/sys1.htm (http://www.lboro.ac.uk/departments/mm/research/product-realisation/res_int/ipps/sys1.htm)
Has that lovely system V diagram everone likes...

Pursuade all your fellow team members to do likewise and when you reach the stage of the detailed design of the suspension
1) you will find you have a lot of your constraints already defined.
2) you will have most of your vehicle parameters (mass, wheelbase, track etc.) already defined.
3) you will find it straightforward to design the suspension with reference to any / all of the texts mentioned above.
4) your suspension design will integrate properly with the chassis, brakes etc designs.

Regards, Ian

[ed_pratt]

just reading the loughborogh link now...
i see what your saying though. At present there are only 4 or 5 people doing any work so we all have had to take quite a big share of the car each.

thanks again to everyone for helping out

ed

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by new_to_this:
At present there are only 4 or 5 people doing any work so we all have had to take quite a big share of the car each. </div></BLOCKQUOTE>

1) Generally the smaller the team, the easier the communication process (assuming you get along with each other!)...
2) However, you all have more work to do so you can't afford to waste time...

Following the basic principles of a systems engineering approach will make each sub-system's design that much more straightforward, assisting point 2). But don't overdo it, you don't need to dot every i and cross every t as long as you work with a sound methodology!

Regards, Ian