I asked the question of a very experienced and respected suspension engineer: what do we (amateur builders) need to consider when designing a vehicle from scratch?
The answer is as below, take from it what you will -

Here's a list we compiled of the suspension-related kinematic characteristics that need to be considered when designing a vehicle from scratch. By its nature this is a top down approach. I've tried to edit out the redundant entries, but can't claim it is 100% sorted. It obviously is not complete.

1 WHOLE VEHICLE:

Sprung mass
Sprung CG X and Z
Moments of inertia
Maximum steady state lateral acceleration
Ground clearance
roll stiffness (degrees per g of lat acc)
ride frequency, including front to rear percentages
lateral weight transfer percentage distribution front to rear
Torsional stiffness of the body structure
Roll axis gradient and any gradient change relative to roll
roll couple distribution

2 AXLE and WHEEL:

2.1 Packaging and general
spindle length
position
track
tyre size, hence rolling radius
Jounce travel
rebound travel
wheel offset
unsprung mass

2.2 Toe
Static toe setting
Ride Steer
Roll Steer
Tractive Force Steer
Brake Force Steer

2.3 Camber
static camber
Ride Camber
Roll Camber

2.4 Castor
static castor setting
castor gain
castor trail

2.5 Roll centres
roll center height at design load
roll centre height gain
TLLTD (tire lateral load tranfer distribution)

2.6 Track
track gain in roll
semitrack gain, ie lateral displacement of contact patch per unit of jounce travel

2.7 King Pin Geometry and Steering
KPI
scrub radius
steering axis angle
hub trail
pin lead/trail
Ackerman percentage/change
steering ratios
Lateral force steer (deg/g)
Aligning torque steer (Nm/deg)

2.8 Compliances
Stiffness of the major suspension mounting points, ie on a McPherson Strut, do the strut towers move around
Aligning torque Camber compliance (small effect)
Lateral Stiffness (at ground w/o tire)
Fore-Aft Stiffness (at wheel center w/o tire)
Lateral Force Camber Compliance

2.9 Other
Anti-x
Side View Swing Arm Angle (SVSAA)

[Edited on 14/7/08 by rpmagazine]

Yup...pretty certain Ron Champion did all of that...

I'm not surprised, it is all basic stuff. The complex parts are for those that can be bothered...no doubt some will decry this as all too complex, but then I guess it is dependant on how what you want to achieve and what you can be bothered doing.

That's a good list, thanks for posting. It includes some concepts that are mentioned occasionally and I keep meaning to learn more about, like trail in its various flavors.

Sometimes this stuff just scares the hell out of beginners before they start so that is why they don't!
Whatever you do to a car suspension design, it will be a compromise. One small change to one component can upset the whole setup.
There is just not one right way to do it.
Practical experience coupled with a bit of book reading will set most people on the right track.
People who bang on about one aspect of the suspension without considering other factors, will leave you confused.
My advice is Read, Read, Read first, then ask,ask, ask, until you have the basics in your head.
Limiting factors for your suspension will be the layout of your engine, passenger compartment and where you want your wheels in relation to the chassis.
Example: Sometimes your wishbones (if you go this route) maynot be the length you want due to engine width etc.
Work stuff out with cheap paper models like making 2D wishbones with pins at the fulcrums to check camber gain/loss etc.

hi all,

on the whole I don't post in threads like this because i'm not even an armchair expert. However I have seen it posted that as ammeter builder many things are all ready set for you in the upright you choose. assuming you don't fabricate your self.

Blake

It's not just beginners it scares the hell out of it frightens me!

Very comprehensive but could it be prioritised?

For example as already pointed out most people will use a standard upright and buy available hubs brakes etc so most of the decisions re the wheels are made for them and we have to work round it.

In my case I had to work round the tyres on the market and others will have different priorities.

Just thinking aloud I suppose but my idea of prioritising the list might involve different lists for different purposes.

Hell of a work though! Probably impractical.

Martin, perhaps it is simply think of this list as a reading list - things you need to be aware of. This is the way I took it and in truth, although it was a bit daunting it was also a relief as I had a list of areas to research - of course other areas/things have come up since.
Because we really are not designing from scratch, but are in effect recycling parts which will likely define some of these criteria, we should know a bit about them so perhaps we can use them ourselves e.g. the castor wind off from bush compliance in braking of modern cars assists braking performance and ability to turn under braking. If we use the bushes we can alter the compliance by changing the bush characteristics and also in using compliant bushes we get away from the bugbear of stiction in bearings.
V8kid, as I said take from it what you will. This is not a comprehensive list from an OEM perspective. Some of it can clearly be eliminated by choosing certain paths e.g. solid bearings (Spherical bearings) will get rid of the majority of the compliance section, but even minimal knowledge of the area will allow greater insight into the OEM practice and might inform us as to the choices we make. Impractical...in a totality - yes probably, though perhaps we should have some awareness of these things?

I am not disagreeing on the awareness, just that there are so many things that builders will be frightened off.
As mentioned, the use of existing parts will dictate to the builder the way things are connected.
Sometimes it is easier to build your own uprights and start from scratch.
This is what I opted for because I didn't want the hassle of locating my rack for bump steer etc.
It wasn't your post that made me mention about "banging" on about one aspect of suspension design, I was just pre-empting those comments (you know who you are!)

I would love to be able to make my own uprights. Unfortunately local rules preclude the option, in that the level of costs due to the need to formally engineer such things makes it pretty much impossible in the budget I have.

Seems a little OTT to me, im sure some of those variables arn't really needed in the design of a suspension setup.

I don't really have anything constructive to say, but i'll scratch my head about it and just wait to see how my car handles... then do something about it if it dosen't

I don't think you're alone CaptainJosh

OTT...gods I haven't heard that phrase since I worked in Kent years ago!
Yep as said before parts of it we cannot alter and some of it will be less important than others...after all how many of us actually weigh components before choosing one item...including wheels?

Posted this in the "other" thread, but it applies here as well:

Our cars exclusive strong point is their light weight. This one factor is responsible for making more of a difference than anything else, including the RC. Face it, we aren't building F1 cars, we don't have a national runoff at stake, and we don't have sponsers to please. Plus, our driving skills are so inconsistant, any small improvement in RC placement will be buried in the noise of our scattered lap times. (I'm sure there are exceptions here, but you know what I mean.)

Summed up, as long as the tires are wearing evenly, that's all that counts.

quote:

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Originally posted by rpmagazine
OTT...gods I haven't heard that phrase since I worked in Kent years ago!
Yep as said before parts of it we cannot alter and some of it will be less important than others...after all how many of us actually weigh components before choosing one item...including wheels?

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I often use 'awareness checklists' in my work as an accountant, and they are helpful for bubbling up issues that might be easily overlooked, such as tax and regulatory requirements. The idea is to not spend a lot of time on them.

Take 'scrub radius', for example. Usually there's not much you can do about it, but knowing what you have might save some trial and error in wheel alignment or deciding whether to buy that quick rack.

If you make your own uprights then you should be able to get the scrub right!

quote:

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Originally posted by nitram38
If you make your own uprights then you should be able to get the scrub right!

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quote:

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Originally posted by CaptainJosh

quote:

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Originally posted by nitram38
If you make your own uprights then you should be able to get the scrub right!

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I second that! But one can only dream

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But how much scrub is the correct amount?

I haven’t posted on the threads re geometry design. Problem is it seems there is a lot of noise around the theory but a distinct lack of real practical useful info from the experts. See upright thread here for example

upright question

So we go back to making educated guesses based on what we can read / learn from those who do share. If it wasn’t for Staniforth I would not have had the confidence to at least attempt to design and build a car.

Edit - add fabricated upright thread

update here a while ago

Cheers

Fred W B


[Edited on 15/7/08 by Fred W B]

thing with practical and useful information is it might well be wrong for you but right for me. Apart from that I am obviously not an expert so fear to provide advice on specifics given all of the variables.
Re uprights can I suggest a couple of things:
Have a look at the bolt on FWD bearing/hub sets. The bearings are much larger than the RWD cortina style and have far less compliance...which is a good thing. They are also compact and do away with the need for an axle.
Also try and get the steering arm and top ball joint mounted on the same arm that is attached to the upright via shims ala RALT cars. This allows you to alter camber without changing toe!

Its all very well saying you need to consider things like this when even the experts cant give any kind of indication of what combination of the factors will work.

All they do do is allow you to work out when you have something wrong... which is no doubt imortant

I ended up paying an expert to design my geometry simply because after reading the books and doing calculations they were a complete waste of my time as they gave no indication of what I needed to aim for... not even a baseline.

quote:

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Originally posted by rpmagazine
But how much scrub is the correct amount?

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All you "arm-chair" experts spouting off like you actually know something. Like headless chickens you are...

Sorry, could resist. I really miss Syd... just kidding.

[Edited on 2/2/09 by kb58]

There will be arguements forever on this one!
Suspension is a compromise because in the real world, limiting factors like space and available materials are imposed.
Also, when you improve one area of design it normally causes a detrimental change in another area.
Whatever you build, I can guarantee that you will find the limit.
Instead of arguing who is the better designer, try actually doing something and get out there and drive the dam thing!

quote:

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Originally posted by nitram38
Suspension is a compromise because in the real world, limiting factors like space and available materials are imposed.
Also, when you improve one area of design it normally causes a detrimental change in another area.

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It's all a compromise. If you want a good starting point, work on your roll line and trail, they are the biggies and both often overlooked and the roll line is often not even mentioned.

A roll line is the line drawn between the 2 RC's - thinking along these lines (pun intended) will help you to see things clearer.

Scrub radius is one of the most talked about load of crap out there probably through the wheel industry out of the mouths of mag wheel countersalesmen. It does very little in the real world and if I used wheel spacers, I will take a bit of steering feedback with the better handling from the wider track any day of the week.

Scrub radius is easy to adjust through wheel offset or wheel spacers and with McPherson struts. changing the top strut mount in or out.

One note is that zero scrub can be bad if you have compliance in the suspension because you can suffer from sign change and that can upset a car. (positive scrub on one side and neg srub on the other at the same time).

Note that not all that many years ago, cars had up to 4" of scrub radius (and no one was dying).

Kurt said it well, if your getting good even tyre wear your probably close to ideal all round geometry.

Just a note, go to a historic race meeting and watch the 60's sedans, terrible suspension, bad geometry, live rear axles etc. and yet see how they go around corners with MODERN TYRES and SHOCKS - thats the secret.

For that matter, look how the new 4000 pound(!) Nissan GTR goes around corners. It's amazing what good tires and suspension can do in spite of all the weight. (and 4WD and 480hp, and...)

I approached it slightly differently.

My reasoning is that the tyre contact patch is the most important thing so the tyre choice should be first. Suprisingly there is not such a big choice of tyres out there! and I ended up with - yes you guessed it 13" rims. At this point the use of the car is cast in stone

Next by my logic was to get the most out of the tyre contact patch and that means loading them as evenly as possible - basic physics tells us that this means the Cofg must be as low as possible. In turn this determined my engine and gearbox choice (dry sump and gearbox input shaft higher than driveshaft outputs)

I wanted to get the best braking I could and reckoned that to be 50/50 front/rear at 1.5g (might as well be ambitous!) and coupled with the expected Cofg height that gave my target weight distribution.

Wheel rate choice is a fairly standard procedure see Stanniforth et al.

Roll stiffness distribution was a puzzler untill a kind soul on Engtips pointed me in the right direction and after 6 months I managed to produce a spreadsheet that should work

Tyre width choice is a minefield I dont know how to solve it. My first stab at it was way too conservative and resulted in clouds of smoke at every standing start. Tried and trusted method is to copy others and watch tyre temps - must be a better way!

I totally ignored roll centers and just concentrated on keeping the camber right under all variations of roll/braking/acceleration using SLARK and my spreadsheet above. Mark Ortiz was very helpfull and I even understood what he was saying this time! Guess what? Yup the geometric roll centeris static. Spookey or what!

Of cource practicality jumps up and smacks you in the nose now and again I'm now on my third gearbox rebuild due to some extremely bad advice but on the bright side I know a lot more about 'boxes now!

Sounds easy don't it but it has taken me 3 years to get this far and at times it drags.

Cheers All

David

so, time flies...how is the project v8kid?

Slower than I hoped RPM I'm sprinting my wife's shopping car this season so I guess that says it all.

I got well and truly ripped off with the gearbox by both the UN1 "specialists" in the UK and it has been a constant weakness despite splashing out on quaiffe internals and "new" parts.

Hopefully it will be finally fixed this month. Curently I'm learning about the mysteries of tyres and am finding it most instructive.

The only high spot is the engine which is absolutely superb I highly reccommend Ian Richardson at Wildcat Engineering

How goes the roadcar project?

[Edited on 9-4-09 by v8kid]

David,

Spotted you latest entry and it sounds like the UN1 'box has been keeping you entertained. You may recall I contacted you a while back because I too want to run the UN1 inverted.

Is running the thing upside down what has caused your problems, or the amount of torque you are putting through it?

My own build progresses slowly, but I was still banking on the unturned UN1 idea. Any information would be gratefully received right now!

My project?...well slower than expected would be an understatement. Similar to you I have just started using my Rochdale Olympic, that and a job and running Race Magazine means little time. So two months off work coming up just so I can get the chassis done and the engine in.
This is a project that has side lined me for a bit: carbon wings -
http://i129.photobucket.com/albums/p234/NRoshier/wingAS.jpg

[Edited on 11/5/09 by rpmagazine]

Hi I have just found this forum and just been reading this very interesting thread on suspension, the only reliable way to evaluate your suspension geometry is to look at any Track type car that has to race against the clock and see he difference the tinyest of adjustments make to lap times,so all I will add is if anyone is interested there is a very nice and cheap bit of Suspension Design Software for these wishing to know or learn developed by Bob Small www.susprog.com check it out you can even download a trial version which allows you to get a feel for it efore you buy, Regards Dinoman

That looks like a good program Dinoman, but I wouldn't call it Locost at US $215 I suppose if you were build ing from scratch, it could save you a lot of headaches. I can't imagine anything worse than spending part of your life building a car to find out that it's an evil handling bitch of a thing!

I own a copy of Susprog, its probably the most expensive software I've ever personally paid for!

I've used it on the design of the suspension for my car, I'd like to point out its not a panacea for suspension design you need to know what your aims and goals are when using the thing.

So there's no guarantee you won't have designed a pile of poo even if you've invested a lot of hours in developing the suspension model in the software.

Where it is very good though is sorting through the gazillion (technical term) iterations of design to get to the goals you seek.

The real brains though is working out what those goals should be in the first place, which as I've discovered in the process of designing the car is by far the hardest part of the exercise because as you look further and further into it, it just gets more complex.

Where you draw the line in how much complexity you are prepared to deal with when you are designing suspension is entirely up to you.

Here's a picture of my front suspension in Susprog...
Rescued attachment Capture.JPG

I paid for Susprog too. I do not know where I would be without it. It is not cheap but its designer is a really good bloke - I have lost my password about 6 times and he patiently sends me a new one.

It has a really useful function for locating chassis mounts in a way to limit roll centre movement - for those who believe in such things.......

It would be quite fun to run the standard "book" dimensions and see where things like roll centres bump steer etc came out.

more grist for the mill.
http://zzyzxmotorsports.com/library/roll-center-myths-and-reality.pdf

My epiphany came after I realized that the traditional roll-center math ignores wheel load. In a turn, when RC location really matters, the inside tire hardly has any load on it, right? Than why does that lightly loaded tire still have equal say in determining RC location? It can't...

The alternate methods by Mitchell and Ortiz, while they differ slightly, do take into account wheel loading.

[Edited on 7/7/09 by kb58]

your view of the tyre load is absolutely true for a car at max grip say on a dry skid pan at a constant speed or say at that section of a corner where the same loads occur, which as you know will depend on the corner. Where this view falls down is of course at the section of the corner where the car leads into and comes out of this dynamic situation, which is where the inner tyre is indeed important to consider...particularly if you consider such things as swerve and recover situations.
Having said that we have to use the tools we have at hand, whatever this is. I'm tending to look at the geometric, then checking the FAP and then doing a 'Beckman'...then relying on the advice of a pro!

[Edited on 9/7/09 by rpmagazine]

quote:

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Originally posted by rpmagazine
...Where this view falls down is of course at the section of the corner where the car leads into and comes out of this dynamic situation, which is where the inner tyre is indeed important to consider...particularly if you consider such things as swerve and recover situations...

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Oh sure, if you focus on one aspect thinking it the key to performance then you overlook how complex it all can be e.g. only looking at max cornering can mean you design in 3 deg neg camber plus camber gain in bump/roll. Cool, you get good max G cornering, but then you loose out in less aggressive cornering situations and your braking performance is compromised...flat spotted tyres get expensive and lose much performance.