suspenson geometry

[johnnymolloy]

hi

im not sure if this is in the right section, but what im wondering is im currently building a car for time attack and have free reign over the suspension and i have asked a company to build me a full setup including as much adjustability as possible (CAD and machining). now i have looked at some of the threads on here about some of the things that have to be taken into account to build something like this, my problem is im not sure that ill be able to look at what im getting and say yes this will do what i need it to, can anyone advise me on what i should be looking for when having something like this built?

or is there anyone on here that could put me in contact with someone who designs these sort of setups?

thanks

[Tim.Wright]

You should start by telling us what you mean by "full setup"

[johnnymolloy]

no problem

mounting points to the chassis, double wishbones for front and rear and the uprights

basically anything that attaches the wheel to the body they are going to make

[Greg Locock]

You'd probably be better off defining the sorts of effects you'd like to be able to tune, for instance, and I strongly suggest you DON'T try all of these, and many of these are interrelated:

KPI
Scrub
static Castor
castor trail
static camber
static toe
bump steer
roll centre height
contact patch lateral displacement in bounce
ackerman
roll steer
then you need to consider how each change sin bump and roll (if different) and how each affects the others
etc etc

If you do try and build a fully adjustable suspension you'll end up with a flexible, unreliable, heavy, mess and I'm pretty sure you'll never win an event with it.

[johnnymolloy]

thanks for the reply

so this is where i start to need help i understand pretty much all of them are but wouldnt be able to tell which ones are most important to my setup

the car is gonna weigh about 900kg have close to 800hp ,rwd and generate a lot of downforce and the dampers are going to be more than likely have a 50mm stroke

[mep]

You'd probably be better off defining the sorts of effects you'd like to be able to tune, for instance, and I strongly suggest you DON'T try all of these, and many of these are interrelated:

KPI
Scrub
static Castor
castor trail
static camber
static toe
bump steer
roll centre height
contact patch lateral displacement in bounce
ackerman
roll steer

Why not? Could you explain why?

[Jersey Tom]

You'd probably be better off defining the sorts of effects you'd like to be able to tune, for instance, and I strongly suggest you DON'T try all of these, and many of these are interrelated:

KPI
Scrub
static Castor
castor trail
static camber
static toe
bump steer
roll centre height
contact patch lateral displacement in bounce
ackerman
roll steer

Why not? Could you explain why?

Adds complexity, adds weight, can be easy to get lost when you have too many knobs to tweak.

[Tim.Wright]

2 things that should always be tuneable especially on a self built car are bumpsteer and static toe. Unless your manufacturing tolerances are spot on - your bumpsteer is not going to be what you plan it to be and you only need to be off by a couple of tenths of a degree for the driver to feel it -especially on the rear axle.

Toe compliance is something to keep in mind too - and the wider and stickier the tyres, the more it needs to be kept under control. Analysing toe compliance on a race suspension (using spherical joints) is practically impossible to simulate but I use a ghetto-imaginary-FEA method of locating the links to (try to) guarentee a toe-in compliance on the rear and a toe-out on the front. If you set your "caster trail" and "scrub radius" to zero - or as small as possible - then you know that the toe link will see a very low load from the cornering forces. With this knowledge - you know that the wishbones will see a higher load so they will deflect more than the toe link (which can be considered to be rigid since its practically unloaded) and from this you can see that putting the toe link behind the wheel centre on the rear axle and in front of the wheel centre on the front axle will get you the desired response. To get the correct toe response from longitudinal loads - apply the same logic (assume the toe link is rigid and the wishbones deflect) and you will see that the plan view inclination of the toe links will influence the toe compliance.

Obviously this logic only works with low values of scrub and caster trail. And it also neglects the chassis compliance - but its really the best you can do in my opinion. Obviously you are somewhat limited in the degree to which you apply this method on the front axle since you shouldn't make the trails so small there because it will nullify the driver's feedback torque.

I agree that making lots of things adjustable will only add compliance and weight to the car. I would almost guarantee that anything that breaks will do so right at some badly designed adjustment mechanism. Not to mention, most of the things on gregs list are practically impossible to make independently adjustable. They will always change something else - one of which is the bumpsteer (see first point).

I personally would facilitate the following adjustments:
Static toe
Statc camber (using shims on the upright)
Bumpsteer (using the height of the steering rack/toe link)
Anti-roll bar (using different bars and attachment points on the lever arm)

Anyway, leaving aside the subject of adjustability, what are the basline kinematic parameters? If these are reasonable - you wont need many adjustments. What tyres are you using?

[johnnymolloy]

thanks for all the input

kinematic parameters is where im afraid my knowledge of this field ends or atleast im not sure what answer your looking for

as for the tires im using a 12.5x18 on the front and 13x18 on the back more than likely going to be using something from avon
that fits as for the compound of the tire and the sidewall stiffness im not sure

[mep]

At least static camber and static toe should be easily adjustable without adding to much complexibility, weight or reduce stiffness. I can't see why those are included in Gregs list.

[Greg Locock]

MEP-They're on the list because you might want to change them. As I said before "If you do try and build a fully adjustable suspension you'll end up with a flexible, unreliable, heavy, mess and I'm pretty sure you'll never win an event with it."

The way I approach it is to decide how much camber gain in roll I want. With wide tires I might say 100%, that is, keep the tire square to the road as the car rolls, or even a fair bit more than that to account for sidewall deflection. That defines my front view swing arm length. Now decide on a roll centre height. That defines my FVIC. Suddenly a whole bunch of other decisions have been made, because they are all related to FVIC.

Anyway the whole process is too long to describe in a post. Also you need to design both ends of the car at once, in fact I start with a set of targets for the car, and work down to a suspension spec and then eventually to hardpoints. Depending on how serious you are then Carroll Smith's books are a good place to start, and if you are actually spending money you'll end up with Race Car Vehicle Dynamics as well.

[mep]

You'd probably be better off defining the sorts of effects you'd like to be able to tune, for instance, and I strongly suggest you DON'T try all of these, and many of these are interrelated:

KPI
Scrub
static Castor
castor trail
static camber
static toe
bump steer
roll centre height
contact patch lateral displacement in bounce
ackerman
roll steer
then you need to consider how each change sin bump and roll (if different) and how each affects the others
etc etc

Somehow there must be a misunderstanding. Greg, you clearly state not to make static camber and toe adjustable! I think you should at least be able to change camber, toe and track width. That’s simply done by putting shims between the upright and suspension mounting bracket. That does not make anything flexible or unreliable. In fact I think that’s pretty basic to most race cars and required to stay within the legal track width.

[Tim.Wright]

Maybe he meant don't try all of them at once.

[Jersey Tom]

You'd probably be better off defining the sorts of effects you'd like to be able to tune, for instance, and I strongly suggest you DON'T try all of these, and many of these are interrelated:

KPI
Scrub
static Castor
castor trail
static camber
static toe
bump steer
roll centre height
contact patch lateral displacement in bounce
ackerman
roll steer
then you need to consider how each change sin bump and roll (if different) and how each affects the others
etc etc

Somehow there must be a misunderstanding. Greg, you clearly state not to make static camber and toe adjustable!

He says not to have ALL of them adjustable, didn't say to not have ANY of them adjustable.

[johnnymolloy]

thanks for the input guys

i am spending money, quite a bit so thats why im on here making sure im getting what i pay for i think ill have to have a look a them books