edge

Hi.

I see it mentioned over here all the time, and know that Powerstation does it, but what exactly is it?
I live in Portugal, so a trip to Powerstation is not an option Would like to know if it's something I can get done at the shop I normally use to do the alignments on my car (tyre / suspension alignment shop).

Looking for some more technical info, like if it's some specific geometry setting or similiar.

Cheers,
Paulo

Chelspeed

They fit spacers under the rack to raise or lower it a little bit relative to the rest of the chassis.

This changes the relationship between the geometry of the steering arms and the bottom wishbone. The benefit is that when you hit a bump with one wheel both the steering arm and wishbone move together and the steering stays pretty much straight ahead.

Before the mod the steering arm moves through one angle and the wishbone through another. Net effect the wheel thinks it's getting a input from the steering rack and turns a bit, hence "bump" causes "steer".

It's nothing too clever, every race car has an adjustment, usually where the track rod fixes to the upright, so that you can minimise bumpsteer. Powerstation have just found a simple way to adjust it on Imprezas, worked out how much adjustment is needed and made up the spacers. Any competent race car engineering type place should be able to do it. They'll just need to work it out from scratch which will cost more than Powerstation just repeating a tried and tested solution.

vindaloo

What he said ^^^

The main point of it IMO is to reduce steering lock required to maintain cornering. As the car rolls, the outside wheel tends to understeer as the suspension compresses. The bumpsteer mod. makes the car track truer on roll.

In theory the mod. may make the back end liable to be a bit more active, though also IMO, that's more dependent on your driving style.

J.

StickyMicky

4mm was what i adjusted mine by

garethbrunt

Does this mean raising the rack 4mm then?

Does anyone know how high powerstation go?

911

I had my car there 2 weeks ago. we checked for the bump steer correction and needed NONE.
Some car can have the ideal set-up and the mod is not required.

Trick:

I have Whiteline steering rack bushes which are different thickness to the stock bushes, this moves the rack up a bit so correcting the bumpsteer mis-alignment, but you may not need it, and others do.
Any alignment shop can do it if they understand the term.

Graham.

garethbrunt

So what is it that is to be looked for?

Is it that the tie rods should be straight, I.E. Parallel to the ground?

Thanks

911

Not really.
It is the relative movement of the toe in/out as the suspension goes from full droop to full compression, but really you are only interested in the movement about 50mm +/- of the normal running height.
Powerstation do the check at the end of a full alignment job. Two sturdy characters pull down on the front end and the Computor notes the change in toe. They also push up and do the same.
If there is no appreciable change in toe (mine was almost no change at all) then the rack stays put.
If there is a change then the rack is raised to a level to negate the toe change.
There is a real limit to how far you can go and I think 5mm is about maximum. You have to slacken-off the UJ to the steering column too to allow the rack to come up. It is the length of the spline that controls the rack height change available.

All quite simple but you need a good alignment to get the best of it.
As an alternative 'guide' my old 911 Porsche was lowered by 40mm and needed a 10mm spacer to raise the rack to remove bump steer; most Imprezas are dropped by 20 mm and a rack change of 4 or 5 mm sounds about right. The geometry on both cars is surprisingly similar (!)

Graham.

Chelspeed

If the bottom wishbone was exactly the same length as the steering arm, the end of the rack was exactly above the wishbone to chassis bush and the track rod end bolt was exactly above the bottom of the strut then it would be easy. The track rod end would need to be parallel with the wishbone for zero bumpsteer. Wouldn't matter what ride height you chose (which changes the angle of the wishbone relaitive to the ground) you'd just raise the rack to restore the steering arm to parallel.

But none of the above is true so it's not as easy.

As stated above in practice it's a case of setting the geometry, measuring steering change with bump and compression and raising the rack to minimise this. Either raise it 5mm and see the effect or use your years of experience (for Powerstation) to know what height spacer has what effect.

911

It is all a compromise in the end.
The stiffer the springing the less vertical deflection for a given load so you can adjust to a better level, but I'm not too sure if chasing the rack position to the perfect dimension is worth a lot of effort, too many variables involved.

Graham.

Powerstation

You lower the rack not raise it.

Maximum lowering we do for safety reasons is 5mm.


Andy

StickyMicky

4mm did the job on mine

ricardo

Does it work, or is it even feasible, on a Forester ?
And do the AST's fit a Forester Andy ? (You've got an email from me about that, but seems easier to get hold of you on here )

jbl

Powerstation did my Forester T/tb about 3 years ago-no problems.

JBL

Fuzz

Yes Ricardo, Both feasible and worthwhile .

We tried a set on Litchos demo just before Japfest but they didn't fit.
OEM shocks are much longer due to the extra suspension travel by the looks of it.
I'm sure if the interest was great enough we could get sets made.



Andy

ricardo

Sounds good to me - it definitely feels like it needs it, maybe the higher centre of gravity and the suspension travel makes it worse...

Didn't fit just due to ride height, or didn't fit due to other reasons ? I don't want or need the extra suspension travel, so maybe I need the Impreza-based AST's plus lifting blocks to stop the exhaust from scraping

My interest is great, is that enough ? Looks like Mrs Mog's Forester is back there now, so maybe I'm the only one at the moment