StickyMicky

i have been having a few thoughts,
ive read a lot about lowering a impreza and have heard a lot about "only doing it 25mm" to keep the handleing right etc etc

now, this 25mm thing, is this because genraly if you go lower then you need to go far stiffer to compensate and therefore your car will not track the roads properly?

asume that the geometrys are correct
for maximum grip i belive that the car should be as low as it can go while still running full suspension travel

so as long as my car has full travel and the spring poundage rates ate "good" i should be able to lower the car as far as possible and still have the car preform properlly?

or is the whole thing more like "lower it more then 25mm and you will destroy you draveshats and stuff?

got a few track days lined up for next year and i like to plan theses things in advance

thank you for your time

superstring

This is the way I understand it: Because of the geometry of the Impreza's MacPherson strut suspension, if the car is too low, when the car rolls during cornering and the suspension compresses, you actually start to LOSE camber (and hence grip).

To compensate for this, you need to dial in a large amount of static negative camber (and/or use BIG anti-roll bars!) and, of course, this adversely affects tyre wear etc.

There may, of course, be other factors I'm not aware of!

John

StickyMicky

ahh but if you have a harder set up the car wont roll much, if at all (mine didnt budge 1mm on the old springs )

intresting note tho, i had not thought of that

ex-webby

Hi Guys

This could turn in to a good one.. great question.

On a perfectly smooth surface, *theoretically* it is true that "lower = more grip". But there are two major things to start the ball rolling..

1) lowering means less suspension travel in bump, which (as suggested) means increasing roll and bump resistence, which on their own can reduce grip (possibly negating the benefit of being lower - unless done well).

2) When you lower a McPherson strut car, you lower the roll centre further than the c/g so you effectively make the car want to roll MORE, not less. This also means you need more roll resistance to keep the cambers useable which again can reduce grip.

Cheers

Simon

diogenese

If you make the suspension too stiff in roll with a monster front anti roll bar you get less overall grip due to weight transfer from one side to the other, the outer tyre gets too much weight to cope with and the inner one just lifts off! hello ditch, goodbye front end

vindaloo

Simon,

If I understand you properly, lowering a McPherson strutted car will aggravate roll and increase camber (or reduce negative). I can visualise the geometry of the situation. ISTM that the geometry can be improved if the lower arm/balljoint can be lowered as well. Try to keep the angle of the lower arms to the balljoint, in relation to the bodywork (whilst avoiding wheels, brakes, the ground etc.)


Vindaloo.

vindaloo

Diogenese,

I agree that can happen at the limit of suspension movement and/or if there's too much G in a corner.

Aren't ARBs supposed to do the opposite though? - Prevent the outside wheel from becoming overloaded by tranferring load to the inside wheel?

Vindaloo.

diogenese

No vindaloo, they do the opposite, they transfer load to the outer tyre. when cornering the car rolls,the ARB tries to stop this. think on this sat in your chair now, with your hands on the desk. Imagine you are turning right, you lean to the left, to pull yourself back upright you can push down with your left hand and pull up with your right, transfering load to the left, the outer side Some anti roll is good, to keep the geometry of the suspension in it's optimum range, too much will reduce your ultimate cornering force.

ex-webby

Hi Vindaloo

Agreed re lowerlink, if you were able to mount it lower again, you would effectively raise the roll centre, but that's a much bigger project, and not necessarily possible.

On the camber thing, I was talking about dynamic camber, and simple roll. When the body rolls, the wheels camber / roll also (turn left, body rolls right, wheels camber / roll right). This is the only really major reason for roll bars, to keep the contact patches as flat on the ground as possible by not allowing the wheels to camber as much during cornering.

Also, diogenese is absolutely right, and that's the very best layman's terms way of explaining it I've ever seen.

All the best

Simon

[Edited by webmaster - 12/31/2003 3:55:43 AM]

vindaloo

Diogenese, Simon,

Thanks, looks like I flunked the ARB section and will have to re-take

I never did understand how fitting stiffer bars would 'reduce' grip at that end.....maybe I have an inkling how it works now.

Cheers,

Vindaloo.

ex-webby

It is without question the most common assumption that I've noticed, as it seems so obvious that it would do the reverse (I can still remember the stunned realisation when I discovered ths).

When discussing the "in relation to the other end" part, it is also even more powerful in it's effects. It is also a little easier to understand / picture.

Imagine absolutely huge ARB on the front and next to zero roll resistance on the back. Going round a bend, the rear would happily roll, and front wouldn't. This would eventually force the front inside wheel to lift off the ground as the whole body of the car pivots over the outside front and the inside rear.

Cheers

Simon

StickyMicky

so would a softer sprung car slammed right down handle better then a stiffer one sitting higher up?

or is this a "how long" is a piece of string type question to ask

ex-webby

It's a little bit of a peice of string question, but..

A soft car at really low static ride height would be dreadful as there would not be enough suspension travel for the soft springs, etc. You've be running on the bump stops continuously.

Cheers

Simon

diogenese

That would be a stiff, lowered car then running on rubber springs!


I worked out the arb thing the hard way. I built my first car with socking great 'bar at the front and DeDion axle at the rear with no roll stiffness at all
then I wondered why I kept understeering into the ditch!

[Edited by diogenese - 1/7/2004 7:57:49 AM]