Roll center height/position of arms

[Aussie_Haynes_MX5]

Can anyone shed some light on what a good roll center height would be for a Haynes? I've worked out the dimensions for where the top and bottom ball joints sit in relation to the wheel and tyre combination i'm going to be using, but from here i will need to know the RC height to start drawing up the geometry. Looks like my ride height will be about 120mm or so.

On the second part of the question/topic, do any of the arms have to be horizontal when the car is sitting at it's designate ride height, or is a matter of as long as the arms are the correct length and the ball joints are in the correct position, it doesn't matter.

Reason for the question, when using the NC MX5 uprights, the distance between the upper and lower ball joints is only about 161mm. This means that when using a 205/50R15 wheel/tyre combination, if the bottom arm was to be horizontal at rest, it would need to sit at 230mm from the ground.

Any thoughts, or if someone can steer me in a good direction, i'd appreciate it. There's quite a bit of calculation and maths that i've had to call on which i haven't used since i left school.

[ozzy1]

From what i recall in a previous thread i think handyandy or spud said that when the ride height is set correct the front arms are parallel to the ground.i stand to be corrected though.

http://www.racingaspirations.com/?p=286

[flyerncle]

Ride heights 4ins front 5 1/2 ins rear under chassis rails seems to work fine after corner weighting.

[Bonzo]

Originally Posted by flyerncle

Ride heights 4ins front 5 1/2 ins rear under chassis rails seems to work fine after corner weighting.

Just written that in my book Paul

[flyerncle]

Measured from Spuds Roadster (when he was not looking ).

Did you see the housing price Ronnie ?

[3GE Components]

Suspension design is a real black art that's only understood by 3 people, and they don't agree

Seriously though, too much emphasis is placed on roll centres, they are not the be-all and end-all of suspension design, there are lots of factors to get you head around and the end result will always be a compromise. In reallity what you want to concentrate on is how the outside wheel, the one taking all the load in cornering, behaves, it's important that the wheel is upright through various angles of lean, with CAD you are able to plot all this.

Kind regards

John

That link i posted will give folk some idea what happens when you change things.

[Tilly819]

Oh boy what a question.......

Well it depends on many factors but it is something that you have to work out for yourself it is not a figure that can be quoted.

First you need to work out how high your front and rear CofG are. then this will alow you to decide how much leavarge you want the cornering forces to apply to the RC which will detirmine how far you want the roll center away from the CofG, like adjusting the length of a spanner to undo a nut, the longer it is the more leaveridge it has.

idealy you want the RC to move with the CofG by equal amounts so as to keep your leaverage the same.

once you have all this data you will probley wind that the camber curve for your wheels in bump and droop are awful and will find that you have to make a compromise beween
1) outer wheel control in roll
2) camber change in bump/droop
3) roll center location / control

you will find that the less suspension travel you have the more control you will have over these factors however another compromise then comes into the fray that you require a realistic amount of suspension travel do drive the car on the public highway due to all of our lovley pot holes etc

also consider the effects the KPI and caster have on the geomitary of the outside wheel when the car is steered more KPI will put on more positve camber when the car turns and caster will have the oposite effect.

i would highly recomend you get a copy of
"compotition car suspension" by allen staniforth
this covers all of the basics of suspension design and weight transfer

hope this helps i am awere it is not quite the answer you were looking for but the answer is not a simple of.

if you have any other question of this nature dont hesitate to ask, i will be as helpfull as i can though a little moe detail in the question may be required.

all the best of luck, it is a complicated process designing suspension

tilly

[Tilly819]

oop missed a bit

in answer to your second question

the book car with the sierra upright has the lower bone say level with the ground at ridehight

however this is irelivent for yourself
you will find that having the lower bone inclined towards the chassis will make the wheel gain positive camber in droop and having the upper bone declined towards the chassis will make the wheel gain negative camber in bump
this can be desirable in some cases at it can help the tyre contact patch stay upright in a corner under roll conditions but is detremental under acceleration and braking.

tilly