Suspension Design

[vside]

Hi everyone.

I'm from the UK and am doing the suspension and steering on our first FSAE car build.

If I'm honest I'm a little stuck on where to even start.
We have some tire data from AVON and i know what wheels we are using. The chassis design has been done therefore a weight can be assigned to that on CAD. We know what engine we are using, but where do i go from here?

The engine needs to be weighed so that we have a weight for that, the mounting points have also been put on the chassis we we know where it is sitting.

I have been looking at some calculations people have been asking about about on here and it's really baffling me.

I just need a rough guide of where i need to start and what sort of order to work in from there.

The problem is we are yet to build a car so I'm designing from the hubs through to the steering wheel and there is a lot to do.

I really hope you guys can give a little advice and shed a bit of light on my ever so dark project lol

Really appreciated.

[acedeuce802]

Check out the different books posted here: http://fsae.com/eve/forums/a/t...5607348/m/1956095883

I started with Smith's Tune to Win, then moved on to Rowley's Racecar Engineering, and then use RCVD for equations.

Also check out this thread: http://fsae.com/eve/forums/a/t...=845103284#845103284

The post by Demon of Speed gives a good very brief overview of the basic steps to take. There is definitely more to consider than just what is there, but at least he gives the main points.

[Dewi Griffiths]

Perfect advice given above! ^^

Out of interest, what University are you from? As we're a UK team, I'm sure our team would be happy to help you with any questions/queries you may have. Even come over for a spy!

[vside]

De Montfort in Leicester! we have salvaged a few parts. There is about 8 of us doing parts of the car for out final year proects and suspension is mine so I am kind of stressing as I am unsure where to begin! I'm sure there is probably a few of us that would love to come over at some point to have a look and maybe get some help on our projects!!!

I have done a real basic chassis design to start us off and allow all of the people to begin their projects. Just checking that it meets all of the specs at the moment as we are trying to enter in the 2013 comp!

[acedeuce802]

That approach may work well for a first year team, where getting the kinematics and such isn't so important, as it is to just get a car done and driving. Typically, tires are the first thing chosen, and then the suspension is designed before the chassis. You basically work from the outside in. So, choose the outboard points, to determine caster and KPI, then choose the inboard points to determine camber/caster change, scrub, etc... and your steering geometry. Then you connect the dots to create a chassis.

In your situation, if you already have a chassis designed, it should be fine to design a suspension around it, that way while you are designing the suspension, your brake designer can already be packaging pedals, while your frame designer packages the engine, diff, fueling, catch cans, and so on...

[vside]

That is what the brake designer is doing, designing the pedal box and has picked the brake calipers, so I have mounting points on the hubs,I just need the mounting points for the disk to enable them to be integrated into the hub design.

The frame has been designed to allow the engine to be mounted. It may need a bit more supporting when it is made but this will have to be looked at after but as a basic design it should be OK.

We are using solidworks for our desings but without all of the parts and weights being put into an assembly (as we don't know them all at the moment) how do I find the Center of gravity of the car. How do I know the corner weights? We have some donor shocks that we are going to use for our first car (as we don't have a lot of funding). So in some ways we are very limited. The front and back shocks are different and need refurbing but this isn't much of a problem, apart from determining what spring rates are need. We have springs on them, if I can find what they are can I design the suspension around this?

How do I find out where to mount the coilovers? and does the mounting position change the rate of the springs and how the system will work as I guess it will quiet dramatically.

[acedeuce802]

Finding the center of gravity is probably easiest using Excel. List out all major parts of the car (anything that weights over 2-3 lbs maybe), figure out a close x, y, z coordinate, figure out the mass of each part, and it's an easy math/physics problem.

To find exact corner weights, you can support the chassis, disconnect the dampers, and place the wheels on scales. This is best done while the suspesion is attached to the car, because only half of the control arms (and anything attached to the chassis) is unsprung, and measuring this way accounts for that. It may actually be better to keep the damper connected, but remove all preload from the spring, and make sure the damper is not at full droop or compression. To give you a start on calculations, ours are between 20-30 lbs, don't remember the exact values though. That's for a 13" car.

[Edit]Just realized you said corner weights, not unsprung. Take your overall weight, multiply by longitudinal weight balance, then multiply by lateral weight balance, and you have one of your corner weights...[/Edit]

Refer to my first post, and read about suspensions. You will encounter something called a motion ratio. It is a lever ratio, and there is an equation that relates spring rate to the wheel rate. Typical motion ratio's are between .7 and 1, so if you can find a motion ratio that will make your wheel rate something you're happy with (give your current springs), then yes, you can use them.

[vside]

This is one of the problems I am faced with though.We literally have nothing. We only have a CAD model of the chassis at the moment at the wheels are on order and they tyres are in the process of being ordered. The only things we have are a donor steering rack, shocks, brake calipers, and an engine. As for everything else its rather difficult to know what we will be using and how much things will weigh. Solidworks does give a weight for the chassis of 46kg (off the top of my head)

I have ordered Tune to Win and also Competition Car Suspension: Design, Construction, Tuning. The RCVD book I have manged to get from the Library there is just so much in there to look at I don't know where to start!!

[mdavis]

Make some assumptions for part weights. These things don't have to be exact, just within the ballpark. Then you can go through more of the suspension design. Once you start getting parts in, update your spreadsheet, and verify your calculations are somewhat close. Getting the car done early and tuning from there will be far better than worrying about the weight of *insert part name here* being off by .01 lbs. The thing that I learned this year is to go through the calculations and layout process once using whatever numbers you please (they don't matter really, as this is simply to get the process down) then go back through after you've added more knowledge with your final numbers for things.

As for RCVD, read what you're interested in. Don't worry about starting at the beginning and trying to read the entire thing. Use it more like an encyclopedia rather than a book.

[Owen Thomas]

Originally posted by mdavis:
Make some assumptions for part weights. These things don't have to be exact, just within the ballpark. Then you can go through more of the suspension design. Once you start getting parts in, update your spreadsheet, and verify your calculations are somewhat close. Getting the car done early and tuning from there will be far better than worrying about the weight of *insert part name here* being off by .01 lbs. The thing that I learned this year is to go through the calculations and layout process once using whatever numbers you please (they don't matter really, as this is simply to get the process down) then go back through after you've added more knowledge with your final numbers for things.

As for RCVD, read what you're interested in. Don't worry about starting at the beginning and trying to read the entire thing. Use it more like an encyclopedia rather than a book.

+1!

Your whole design process can get held up by a couple people waiting for other systems to be done because they "can't do anything without the numbers/pickup points/forces/lengths/someotherexcuse". Get the thing close with reasonable assumptions and iterate once you have the hard numbers.