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Prodrive P1 Lowering Springs - Rare Purchase opportunity.

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Old 03 June 2017, 04:48 PM   #571
TurboMaup
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Thanks for the reply.

About body roll:
I also have the OEM 19mm and 20mm bars.
Here are 2 photos to illustrate, first one with Pro Kit springs (yes that's a teddy bear), second one with WR15 springs.

The bodyroll was indeed reduced, but not drastically.

My front settings (in degrees and minutes):
430' caster (previously a bit less than 3 before using the whiteline top mounts and ALK)
-140' camber (previously -126' before using the whiteline top mounts, shy result)
008' front toe in total (004' per side)

The rear is a bit more messy:
-126' at the left, -049' at the right (no camber bolts at the rear, planning to buy some maybe)
007' toe in total (002' at the left, 005' at the right)

Any suggestions welcome.
I'm thinking of buying extra camber bolts for both front and rear (Whiteline KCA414 if I'm not mistaken). That would allow me to add more negative camber at the front (using the second strut hole that does not allow camber setting from factory, the bottom one I believe), hopefully to reach -2. And I could at last tune the rear to an homogeneous value, -130' for example.
I believe my toe settings are the ones suggested in the so called Prodrive setup (although not perfectly done at the rear).

My struts looked in good condition when I swapped to the WR15 springs. I installed these AGX brand new in october 2014, they are not very old and I ran less than 20,000km with them yet.

I got the clips today in the mailbox, I will try them next time on the track.

What would be the benefit of a softer front bump stop in my case? Reduce understeer? For that matter, I have other options to try:
- the rear clips as already mentionned
- camber bolts and alignment settings
- play with tire pressure (I'm using 0.2bar more at the front, I could try the same pressure all around)
- rely on the suretrac LSD of the 6-speed gearbox I'm going to install
- improve rear braking strength, to free up some front grip for turn in

Bigger rear bar is indeed another option, that I'm not willing to try yet with the WR15 as I'm already not super happy with the ride quality on the road.

New struts are tempting, but I'm so afraid of being disappointed and not feeling any real improvement in ride quality. Maybe I'm just looking for something impossible and I'm being too demanding on ride quality with decent performance on the track. A friend who owns an STI X is about to install B16 PSS10 coilovers in the next weeks/months. Looking forward to test it and see how it compares on the road, with stiffer springs than the WR15.

Do you know how these Subaru inverted struts you have in stock compare to some other famous struts (Bilstein, Koni)? Are they much better than AGX?

Last edited by TurboMaup; 03 June 2017 at 04:55 PM.
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Old 03 June 2017, 09:53 PM   #572
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Is there something bent/misaligned in the rear suspension?
What is the rear thrust angle?
OEM rear camber is neg 1deg 10mins each side.

The following are set with a full fuel tank:

Set the rear camber to neg 1deg, each side - you can release the strut mounting bolts and there is some movement in the mounting holes. You shouldn't need camber bolts.

Leave the front camber at neg 1deg 40mins, each side.

Increase the rear toe-in to 0.15mins each side = 0.30min total, as used on the Prodrive P1. You are intentionally increasing the slip angle.


A softer front bump stop will understeer less and ride better on poor roads. But, the front will pitch more under hard braking.

Agree - Don't bother with a 22mm rear bar, yet.

The wr15 do work with the agx's, I can't see any need to change the struts, as yet.

Road tyre pressures:
Maximum: 2.48 front, 2.34 rear.
Minimum: 2.27 front 2.14 rear
Keep the front pressure 0.14, 0.21 or 0.27 higher than the rear

Last edited by 2pot; 03 June 2017 at 10:30 PM.
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Old 03 June 2017, 11:11 PM   #573
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As far as I remember my rear camber has been like that, with more neg to the left. I'm not very surprised on such an old rally car. Maybe something is ever slightly bent, but it's minor proportions.

That's a lot of rear toe-in you suggest, x4 to what I currently have (and I used to have half less - only 0.04min total - before my current alignment, which felt fine with Pro Kit).
I'm not a pro... isn't a lot of rear toe-in going to make the rear less stable under hard braking?

According that much importance to bump stops is new to me. Is the bump stop so easily triggered by light bumps, that a softer one would give ride quality improvement at moderate speed (kind of "daily" usage)? My concern is for example when doing 50km/h on a crappy street full of patches and stuff. It used to be totally okay with PK. Now it feels like it wouldn't be much worse with chinese coilovers. Same problem on backroads at 90km/h to a lesser extent, when I'm just trying to drive normaly and not go Colin McRae.

I guess I'm going to experiment, and try different things. Maybe put back the OEM front bump stops (if softer than the ones you provided me), to look for ride quality improvement. Uninstall ALK to reduce diving when braking. Retry the Pro Kit springs to check if I'm being biased and too nice to them, or if they really were as creamy as I remember. All of this will mess up my alignment, but I won't care and focus on the ride quality first. Alignment will be done later if I find a satisfying setup.

Also, I should switch back to OEM 16x7 ET53 wheels with 205 tyres when doing this. I currently have 16x7.5 ET45 wheels with 225 tyres installed, and the wider track makes the car follow every level change on the road, like it's steering by itself... It's nice on a track though.

Last edited by TurboMaup; 03 June 2017 at 11:14 PM.
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Old 04 June 2017, 10:36 AM   #574
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Increasing the rear toe in increases stability under braking.
The Impreza toes out under braking, especially with oem rear bushes.
Toe out destabilizes the rear under braking.

The Impreza's high speed, straight line, stability is increased with rear toe in.

Increasing the slip angle, by more rear toe in, gives instantaneous response to steering movements.

I wouldn't use that much rear toe in if the rear bar is larger than 22mm.

I'd use zero front toe to begin with.
If you find the steering to responsive or twitchy use toe out 0.04mins each side.
To improve corner entry - use 0.04mins toe in each side.

Do you want me to send the softer front stops? They will fit the dust covers you are currently using. You can use them with clips, if necessary.
You could use your 60mm tall oem front stops, but cut off the smallest diameter section, leaving a 40mm tall stop. You'll have to use the oem covers with the oem stops. Again, you can use them with clips.

Find a way to equalise the rear camber at neg 1deg-1neg10mins. What was the thrust angle? It should be on the alignment print out.

Bump stops are important:
The Bump Stops Here - Tech Talk - Modified Magazine

Last edited by 2pot; 04 June 2017 at 08:32 PM.
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Old 04 June 2017, 11:41 PM   #575
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Quote:
Originally Posted by 2pot View Post
Increasing the rear toe in increases stability under braking.
The Impreza toes out under braking, especially with oem rear bushes.
Toe out destabilizes the rear under braking.

The Impreza's high speed, straight line, stability is increased with rear toe in.

Increasing the slip angle, by more rear toe in, gives instantaneous response to steering movements.
Good to read

Quote:
Originally Posted by 2pot View Post
I'd use zero front toe to begin with.
If you find the steering to responsive or twitchy use toe out 0.04mins each side.
To improve corner entry - use 0.04mins toe in each side.
I'm looking for good corner entry yes, so I think I will stick to my current front toe in setting of 0.04mins each side.

Quote:
Originally Posted by 2pot View Post
Do you want me to send the softer front stops?
Thanks for the proposal but I just bought a 6-speed, I'm broke.
I might experiment with the OEM bump stops though.

Quote:
Originally Posted by 2pot View Post
What was the thrust angle?
left: 1341'
right: 1358'
I honestly don't know what that means. I will google about this.


I remember this article about bump stops, I kept a PDF link of it:
http://www.pcadynamics.com/app/downl...10400-TECH.pdf
I will read it again thoroughly.


You should hear again from me later in the summer, when I have new concrete things to share. Thank you very much for your time, Mark
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Old 07 June 2017, 11:50 AM   #576
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Quote:
Originally Posted by TurboMaup View Post
Good to read


I'm looking for good corner entry yes, so I think I will stick to my current front toe in setting of 0.04mins each side.


Thanks for the proposal but I just bought a 6-speed, I'm broke.
I might experiment with the OEM bump stops though.


left: 1341'
right: 1358'
I honestly don't know what that means. I will google about this.


I remember this article about bump stops, I kept a PDF link of it:
http://www.pcadynamics.com/app/downl...10400-TECH.pdf
I will read it again thoroughly.


You should hear again from me later in the summer, when I have new concrete things to share. Thank you very much for your time, Mark
Softer front stops - sent

Toe in at the front will give good turn in. But, on track, you might not be so happy mid-corner or corner exit - hence starting with zero front toe.

Bars sizes:
19mm front, 20mm rear - uneven/undulating roads.
19mm front, 21 or 22mm adjustable rear - flat, smooth, roads/track days.
22mm adjustable front, 22mm adjustable rear - tracks days with sticky tyres.
Increasing the bar size will increase your wheel rate - you'll need to adjust your struts, to match.

A 27mm front is going to give you understeer. The end with the stiffest spring + bar combination will slide first.

Last edited by 2pot; 07 June 2017 at 12:39 PM.
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Old 07 June 2017, 02:39 PM   #577
TurboMaup
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Thanks for the stops, I will tell you when they arrive.

My pick in bar sizes for track days would be 22mm adjustable front and 24mm adjustable rear. It gives more room to correct understeer than a 22/22 setup, while still capable of a neutral 23/23 setting if needed.

Of course I'm not planning to use the front 27mm bar alone on the long term. It's indeed going to understeer much more, but I'm just curious to try and feel how the front will react to bumps with it, as I got it for free during a limited time.
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Old 07 June 2017, 05:00 PM   #578
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The benefit of awd is traction - why destabilize the tyre contact patch, by massively increasing the wheel rate with oversize bars, which also limit droop travel. You want to de-couple the suspension, side-to-side, not equate the behavior of a solid axle.

Bars reduce grip - although, a bigger rear bar transfers grip diagonally, from the the rear, to the front.
So, although you've lost more grip at the rear, than you've gained at the front, the overall balance is more neutral.

The P11L's and wr15r/t's, like the P1 and RB320, have a, relatively, higher rear ride height - this moves the roll centre rearward, increasing rear roll resistance.
That, in turn, makes the front end roll more - helping turn-in response and reducing understeer, without the use of stiffer bars.
You want the smallest bars possible, on a road car, to avoid 'roll-rock'.
Roll-rock:
If the spring rate is relatively low and the bar is too stiff, a suspension movement, initially, occurring on only one side of the vehicle, will be transmitted to the other side, inducing an unsettling 'roll-rock' motion and destablizing the tyre contact patch.
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