WHY DOES MY LOWERD CAR HANDLE FUNNY? (what do RCAs do?)

[Mr Revhead]

this expalins it nicely


when the wheel moves over bumps in the road, the steering arm and suspensions moves in an arc.
with a lowered car the arc causes the hub to be turned, causing bumpsteer.
which is why an incorrectly lowered car is a bad bad thing

PLEASE NOTE:
as soopachargen pointed out, the pic is a little wrong, its not the control arm angle thats a big issue (but that does affect roll centre) its the rack end/tie rod end angle..... so where the pic says control arm, replace with steering arm/rackend/tierod end etc

[KinLoud]

???
Not sure if I agree about the RCA/bumpsteer relationship you have posted Warwick. Roll centre movement with macpherson strut suspension is waaayyy different to bump steer.

Roll Centre Adjusters (RCAs) are designed to correct problems created when cars with macpherson strut suspension are lowered.
When the car is lowered the centre of gravity lowers BUT the roll centre lowers by a greater amount which in turn causes more body roll and/or weight transfer instead of less.

As macpherson strut suspension compresses due to body roll or lowering the car, the roll centre lowers. This, in turn, tends to make the car roll more. This is an inherent design problem with macpherson type suspension.

The roll centre for macpherson strut suspension largely depends on the angle of the lower control arm. When the control arm slopes down towards the hub the roll centre is high. If the arm is level or slopes up to the hub the roll centre will be much lower!
A Roll Centre Adjuster spaces the lower control arm down from the hub as per the pic in the first post. This raises the roll centre and so reduces the body roll problem that occurs when you lower the car.

NOTE - macpherson strut suspension can be made to work very well!!
BUT - it does mean that you must apply some thought to suspension modifications.


Bumpsteer/rollsteer - can occur if the suspension design is poor or has been modified without any thought. Lowering the car is unlikely to affect bumpsteer/rollsteer.
Bumpsteer/rollsteer can occur when one wheel (usually the front) goes over a bump or when the suspension moves due to bodyroll - as the suspension rises the wheel changes it's toe slightly tending to steer the car away from where it should go.

In a factory setup even when lowered you are unlikely to experience significant bumpsteer/rollsteer if any. Bumpsteer/rollsteer problems might occur when people fit different steering arms (i.e. powersteer arms on a manual rack etc.) This can also affect the ackerman steering effect.

The control arm and steering tierod should be parallel and the pivot points at either end should be aligned vertically to minimise bumpsteer. Drifters give themselves bumpsteer problems when they space out the rackends with washers to give more lock (this means that the pivot points are no longer aligned vertically - they probably don't have too much problem as the drift cars are setup with very little body roll.

If I am thinking of shagging around with the ride height and suspension of my cars I refer to several books including:
Car Suspension at Work - Jeffrey Daniels (explains how different suspension and drive systems work and how they affect handling)
Tune To Win - Carroll Smith (he is a bloke who really knows how to get the best out of what you have in handling, braking, tuning the suspension etc.)
Buy them and read them several times straight away then once every year so you don't forget why you should/should not and hot to/how to not improve your cars handling and performance.

I hope no one fell into a deep sleep trying to read this big ass post!

Ken
Ham
021 408 863

[Mr Revhead]

Bumpsteer/rollsteer - can occur if the suspension design is poor or has been modified without any thought. Lowering the car is unlikely to affect bumpsteer/rollsteer.

In a factory setup even when lowered you are unlikely to experience significant bumpsteer/rollsteer if any.

ah... have to disagree there ken! lowering alters:

steering tierod should be parallel and the pivot points at either end should be aligned vertically to minimise bumpsteer.

i assume you meant aligned horizontally?

[KinLoud]

The diagram in your post is a simplified depiction of a macpherson strut suspension setup. I don't think it does justice to your post about bump steer.
I will see if I can find or draw a diagram that will help explain bump steer in another way.

To answer your comments on my post...
NO - the lower control arm and steering tierod do not have to be horizontal to the ground to avoid bumpsteer.
HOWEVER - the lower control arm and steering tierod MUST BE PARALLEL TO EACH OTHER. This is why on a racecar with custom or non factory steering arms the steering rack or the tierod end joint may need to be spaced up or down with shims or washers to make them parallel.

I haven't mentioned another benefit of Roll Centre Adjusters.
This is in relation to the slope of the lower control arm and the change of camber as the macpherson strut suspension compresses - The lower control arm should slope downwards to the hub so that the camber becomes more negative as the strut compresses. This helps to keep more tyre tread on the road as the car rolls during cornering. THIS IS GOOD
If the lower control arm is horizontal or sloping upwards to the hub the camber will not become more negative and so with even small amounts of body roll the camber could become more positive and so reduce grip. THIS IS BAD.

Ken
Ham
021 408 863

If people are still having trouble getting to sleep after reading this I can start discussing ackerman steering in depth

[Mr Revhead]

ok, moved it into tech as it can hardly be FAQ if its disputed, and non mods cant make a comment if its in there.


i was ignoring roll centre, my aim was to point out what extreme incorrect lowering does to the steering of a car.


when i get a chance to sit down and think it through and we can tidy it up ill move it back to FAQ

[thegreatestben]

Does anyone have any info regarding having some made?

[postfach]

also are there adverse effects on the roll centers when lowering a car with double wishbone suspension?

[KinLoud]

Roll centre movement is much less with double wishbone suspension than macpherson strut...
However, your camber angles will change - so in extreme cases of lowering you may need to use adjustable arms to correct for camber. Just look at some hondas that have with huge negative camber on the rear when lowered. You could end up with less total grip and major tyre wear problems!
I guess that it can be a case of looks vs handling/show vs go.

Ken
Ham
021 408 863

[postfach]

ok no worries then as i have adjustable arms in the works

[fangsport]

ok, moved it into tech as it can hardly be FAQ if its disputed, and non mods cant make a comment if its in there.


i was ignoring roll centre, my aim was to point out what extreme incorrect lowering does to the steering of a car.


when i get a chance to sit down and think it through and we can tidy it up ill move it back to FAQ

the idea behind the thread is a good one, but i guess the information needs to be correct before it is stickied.

when her indoors gets home, i will scan a few pages fron Tune To Win and upload them. the comment Ken made regarding reading and re-reading is right on the money. i have a grasp for what is required, but still have to read the book several times to refresh the memory (or lack thereof).

[soopachargen]

the latest autosalon magazine has a great article on suspension which includes roll centres etc. i've also been reading a book called competition car suspension which has good information.
it also helps that my tutor on my auto course is an ex ralli art suspension mechanic and has interesting info and ideas

[THA SHZ]

Before u go into any of this warrick wot wheels are u running and wot car are we talking about , and wot is the major problem / symptoms that ur experiencing ?

[Mr Revhead]

Before u go into any of this warrick wot wheels are u running and wot car are we talking about , and wot is the major problem / symptoms that ur experiencing ?

no no you miss understand....

this was solely to explain why when you drasticly lower a car it can adversly affect the handling if not done correctly.

[THA SHZ]

ah my bad Basically wen the car is drastically lowered the shocks stop doin their job , as the pivot points like ball joints and bushes start taking all of the absorbtion because of how the arms sit , like you have shown in your picture at the top , hence the reason why lowered cars flog out the bushes and ball joints quite regularly

[soopachargen]

its more than the effect on balljoints, its the relationship between the different angle of the arms and the arcs that they move through changing the alignment of the wheel as it rides over bumps and also the lowered roll centres increasing body roll.

[ze-fighter]

so when you say the struts stop working properly when you lower a car, wouldnt shortening your struts help?

[postfach]

so when you say the struts stop working properly when you lower a car, wouldnt shortening your struts help?

I think it has something to do with the angle the shock ends up sitting on, as it tends more towards horizontal it becomes less effective, so shortening the shock wouldn't make a difference because it will still have the same angle, if you wanted to do a decent job of it you would either need to change the position the shock mounts in or use onboard dampers with pushrods like the ariel atom.

[soopachargen]

The need for shortened shocks

The reason for needing special shortened shocks is that when you lower a car you alter the position of the control mechanisms inside shock absorbers, they operate much closer to each other. For road use the standard unit is fine but in the event of going over a kerb or full bottom out of the suspension it would be possible to damage the valve block unit, so for those who want piece of mind or using to the ultimate we suggest the shortened shock units.

[deaf_rattle]

so whats the conclusion on the rcas?
should i buy some or not?

I thought the car handled very well before, but that was 3 years ago so cant really remember.

going to have the front suspension apart soon to install some more goodies, and if it would be a wise decision to run the rcas ill buy some.

Cheers

[RedMist]

In short, suspension is designed to sit at a certain height. By lowering the car not only do you alter arm geometry but you alter track, ackerman, spring rate, need for additional compression and rebound damping. RCA's are nothing but a stopgap measure to bring one issue back into alignment.

If you've ever attempted to design a suspension system you'll know what a massive difference one inch in ride height can do to absolutely every suspension component, chassis mounts and steering box location.

And in the end, even if you do alter all of the above, it just makes it useless for New Zealand roads. Track use certainly, roads, far too little travel to cope with bumps.