Should have something to start assessing, by the end of this month.

 

A quick theory reminder, not for 2Pot, as my man know's his onions, but some other readers might benefit from first principles.

You car rotates around a mythical thing known as the "roll centre". Actually, it does this around an axis passing through the front and rear roll centres. Visualise your car on a spit roast. Then again that might be too painful.

So, the Roll Centre. What is it?

Well, any suspension system has one. It's the effective and instantaneous pivot point about which the car will rotate on the wheels as a side force is applied. On a wishbone system, it's defined by drawing a line from outer to inner pivot bolt on each arm and continuing the line in through the centreline of the car and beyond until the lines intersect. Then from there draw bac kto the centre of the tyre's contact patch and where both sides cross each other more or less on the centreline is the Roll Centre.

Roll torque is the action of the centre of gravity under centripetal force acting on this point.

With a little thinking you can deduce a parallel double wishbone would have an infinite intersection, and acts like a live rear axle. In both the roll centre is ALWAYs at ground level.

On a McPherson Strut, our cars, the upper wishbone is defined as a 90 degree line from the top mount inwards, again with a little thought you can figure out it's a very sensitive system to ride heights and angle of the strut.

Older Mc Pherson Strut cars had short lower arms and angled struts and drove like....well. Prehistoric.

From the Mondeo circa 1990, (and most cars shortly after their designers had driven the Mondeo), you can see a sea change is design, where everyone ( Except the idiots at Vauxhall/Opel) went for more vertical Struts and larger longer arms. Thank goodness for that.

What it practically means for us is this. As you lower your car, the lower arm angle changes. The outer pivot (your lower balljoint) moves out increasing Camber, caster, track and toe out. If you shorten your struts 1", you'll lower your Centre of Gravity 1". Your Roll centre geometry will drop about 3-4".

This increases the lever arm moment between CoG and Roll Centre (RC) so the car will roll more. Yuk.

Here is the thing. The Roll axis on a standard New Age Impreza is slightly tail down. The Spit Roast pole is not level but lower at the back. On rolling, the car sinks into the rear corner and un weights the inside front wheel. This causes predictable understeer. Safe.

The general first fix people try is a stronger anti-roll bar. And yes, it helps. It will support the car's outer rear by trying to lift the inner. Geometry doesn't change so it's marginal, but it also causes more transfer side to side on single bumps and makes you bang your head on the window on speed bumps. I hate too strong ARBs for this reason. Very agricultural choice.

By lifting the rear slightly, 10mm or dropping the front the same, you change the angle of the roasting pole, and the car will now corner better. By not dipping at either end, balance is better and it's win win. Too far though, and the front will dip loading the outer front, wearing the tyre and causing sudden understeer, or oversteer as the inside rear is now lifted up. A Hot Hatch is OK, on three wheels , but if you want to put power down on a AWD supercar, this is not a good idea.....

Hope that helped.

 

Thanks Jamie
Front 363mm
Rear 369mm 3/4 tank (6mm difference full to empty).
KYB ultra sr inverted front
KYB ultra sr non-inverted rear - that'll stop any subsequent knocking The KYB inverted rears only used 40mm of bearings.

 

ahh spot on 2 pot and answers my email question about seeing pics of a finished cars ride height-- embarassed to say i had,nt seen these posts at all until tonight lol- ride height and balance looks good

 

I have a set of these from 2pot non inverted all round as soon as they are fitted I will be emaling him some pics and measurements of my car with them fitted 2003 blob sti

 

Piccies?
Latest measurements?

 

^ With Feal adjustable struts, awaiting heights after 300 miles running - time for the bushes/new struts to settle into their ride heights - well made springs don't settle.
Excessively high gas pressure, in some Impreza struts, does affect ride height, ride and grip.
The inserts I'm working on will have low gas pressures; but high enough to avoid cavitation and nitrogen column collapse.

 

Hi 2pot I've sent you a Pm about some springs. Thanks

 

Hopefully, collecting this week, then for some thorough testing!
Increased bearing surfaces
Less rebound damping
More compression damping
Reduced gas pressures
Digressive pistons (unlike the rb320)
Thoughtful bump stops - shorter, softer, front/stiffer rear - mitigating against understeer, without automatic use of a bigger rear bar

 

In my case I've swapped the BC 200mm length springs for 260mm length ones removing the flat wound assister springs and enabling me to use all the extra 40mm of travel I ordered with my inverted fully adjustable BC ZR struts. 5mm preload all round and now waiting for the car to settle over the next couple of weeks. I have them now in the bottom mounts by 100mm in the rear and 90mm in the front and it is quite dippy in the back and although pretty close to O.E. not pleasant. Once settled I will be able to set the car to any ride height independant of spring setting. Lot of fun this winter.




Springs are 8kg f 6kg r. Quite stiff but these dampers contorl it and make the car feel quite a bit softer and supple if I want. Really nice adjustment from the clickers. Then again they should be for the money!!!

2Pot's efforts are very well appreciated. I maybe playing with high end solutions, but we all have aging cars and different budgets! Something really decent for sensible money is emerging from Mark's efforts and it's all good stuff.

 

I am going at it differently.

My criteria were:
Ride frequencies - 1.5Hz - 2Hz for a sports/occasional track day car.
Use flat ride - how quickly the chassis settles itself, after each bump/manouver (requires less damping to be used).
Ride height - don't restrict bump travel: form to follow function - 360-365mm front (this will lose sales ).

My assessment was that, for the road, coilover springs are only that stiff to avoid wheel to arch contact, when slammed. They're also so stiff, you don't have to do any interaction calculations, with regard to bump stop design, durometer and length.
I like the use of stiffer rear stops, as opposed to an over-large rear bar. On a single wheel bump/movement, you're not transmitting the majority of the suspension movement to the other side. A bigger bar will transmit the movement, to the opposite side of the chassis, destabilising the tyre contact patches on both sides.

Calculate the flat ride frequencies - I use 60mph for a road car - 4.4kg/mm front, 4kg/mm rear 1.6Hz front 1.8Hz rear.

8kg/mm front, 6kg/mm 2.1Hz/2.2Hz - flat ride would be at 220mph - might be an issue with pitch at lower speeds? You can artificially restrict the problem, through the damping. But, I'd rather use the low, medium and high speed damping to control the spring oscillations, rather than constrain track spring rates on the road. I do like more low/medium compression damping, than rebound, at the front.

8kg/mm front, 8kg/mm rear, would have flat ride at 60mph. But, the ride frequencies would be 2.1Hz front 2.6hz, too high for a road car.

 

Hi 2pot!What is your opinion on arai springs?They are stiff for road?I think they are 60N front and 50N rear and are the ones that did the lap record on sti spec c type ra on nordschleife.I am debating between a coilover setup or strut spring combo strictly for fast road use,currently on kyb sr on stock black springs.Sorry for the hijack but you prefer a 20mm arb(again the one that set the record on nordschleife on type ra)or the sti 21mm arb that fitted on type ra r and s203 s204 models,i have the oem 19mm front and rear arbs because the early 2004 Sti's came with those instead of 20mm.Thanks!

 

The spec c springs are equivalent to 6.1kg/mm front, 5.1kg/mm rear, flat ride at 100mph, 1.85Hz front, 2Hz rear - spot on for a priority track day car, driven to the circuit. Way too stiff for a road car.

 

Hi 2pot ! I just have a few questions regarding KYB ultra SR dampers.to use with your kit. I found several references and I'm a bit lost ...

For front dampers I have following references:

Inverted bug STI
Left 321007 + Right 321006

Inverted blob STI
Left 321003 - Right 321002

Non inverted Bug & blob front WRX saloon
Left 324033 - Right 324032

Why different references for bug and blob front inverted ? Damping rate is different ? Do I just take the reference corresponding to the car ?

With the WA/A2 you explained we have to use the bug Rear top mount and non inverted Ultra SR due to higher gas pressure in inverted ones. There are different references for bug or blob non inverted rears, which should be used ? We have to use reference corresponding to the car ?

Non inverted rear bug WRX saloon
Left 324031 - Right 324030

Non inverted rear blob WRX saloon
Left 324029 - Right 324028

Thanks for your help

 

Bonjour

Bug and blob damping rates are different. Use the struts that best match the WA/A2 spring rates.

For SALOON bug/blob/hawk - not widetrack sti:
On smoother roads use
KYB Ultra SR
321002 inverted - must change internal bump stop
321003 inverted - must change internal bump stop
324030 non-inverted
324031 non-inverted

On poorer roads, use all non-inverted:
KYB Ultra SR
324032
324033
324030
324031

WA/A2 springs were designed to fit both inverted and non-inverted struts.
Bug rear top mounts fit bug/blob/hawk struts.
Blob/Hawk rear top mounts do not fit bug struts - must use spacer.
The high gas pressure in the inverted, rear, Ultra SR acts as an additional dynamic spring I would also like to see an extra bearing in the strut housing.
I have matching bump stops, for each variation.

 

Prototype insert for inverted strut, with widebody front strut housing.

 

Good work!

Any pricing yet?

 

Thanks
Haven't dared ask yet! Still throwing money at prototypes,
The R&D is just relentless and time consuming.
Still working on gas pressures and damping.

 

So, if I'm right, the bilstein cartridge will work with both widetrack and non widetrack inverted struts housing?

 

Correct. OEM Subaru inverted and the inverted KYB Ultra SR.

 

Thanks for the piccies, measurements and feedback.

WA/A2 + KYB Ultra SR non-inverted.

 

@2pot been sharing this thread on facebook to try get more interest.

 

That's really thoughtful of you

 

I will add some pics too soon, just waiting for lockdown to end so Ican get my car back from having them fitted

Ian

 

I've got a wagon in lockdown as well - frustrating isn't it.
Can't wait to test the Bilstein inserts and WA/A2 springs on that.

 

Rather than derail the whiteline top mount thread, what sort of timeline are you likely looking at with these inserts? And, most critically, how will they compare on the road with the Koni's I've currently got with my setup?

My setup is a bit of a mix (but works well); S05 front springs, RB320 rears with spacers to give the same rake difference of the full RB320 setup, Whiteline ALK/offset top mounts, flipped pins and a 22mm Whiteline RARB currently set on softest. And of course Koni inserts with stock WRX bump stops (P.C. recommended to stick with these vs any of the custom ones).

 

Got the old girl back this morning with the springs fitted. Nice drive home, much more refined and positive than with the old Teins. Only about 50 miles on it at the moment, and the pic is with 1/3 of a tank but it does have the old Teins in the boot
Ian

 

Really helpful if you could give the the ride heights after 300miles, with the fuel level.
How close did they get on the initial alignment?

 

Hi
I don't have the printout to hand. I forgot to ask for it when I picked it up. Will email Dan and see if he has it. They must have got close or he would have said.

Ian

 

I can't give a timescale at this point. Still waiting for the wagon test mule back from lock down: it needs revised bushings, brakes, gearbox and front and rear diffs - I have a test route inc speed bumps. Then, I don't know how far behind Bilstein will be.

I'm happy with the matching bump stops, I now have, for inverted and non-inverted.

The Bilstein inserts might need:

The rear damping to be more digressive.

Maybe another bar, front and rear, off the fill pressures.

Koni inserts and RB320 struts were my baseline, being very picky:
The Koni's may jack down on to the bump stops - need more compression damping/less rebound damping.
The RB320 need to be more digressive - tendency to launch, over some road imperfections.