STi M'8

Hi there,

After about a week off spent for mecanics, I must admit I reached my limits... (mainly nervously )

Lets go for an accurate description of the car and condition, then my problem...

STi8 20000km
H&R short springs (if any influence ?)

Front stk brembo 4p calipers
Brembo disks (new from last week)
CL brake pads (mu=0.52) new also
Inner thermal screens removed
Motul RFB600 Dot 5.1
Goodridge lines w/stk routing
Both front bearings replaced and torqued according mfr specs
(initially suspected)

Rear stk brembo 2p calipers
Stk pads
Stk disks (like new)
Motul RFB600 Dot 5.1
Goodridge lines w/stk routing
stock bearings (looking good with no play).

Always using standard tyres for track days (never any racing grip).

My problem :
Driving from cold or hot, every first press on the brake pedal after hard cornering (left or right) requires about 30% more travel than the second.
This never occurs in straight line, so I don't expect any bleeding pb...
No flutter, no noise neither vibration... just excessive travel at first shot

Everything leads to an excessive bearing play... I know
But both front have been replaced and torqued by my favorite Subaru agent... and the pb is still there.

I must also tell I feel some inacuracy in the wheel when playing from L to R in straight lines, but my inspections didn't revealed anything special except a slight play in the direction stk bushes, as the aluminium cast was looking "polished" for 2mm around, seen from under the car (wich I guess is not abnormal under hard cornering).

One of my mates (rallying a GrN STi 2002) told me this is well known pb for all STi 7 & 8... related to Brembo front calipers and inner piston springs...
He even mentionned Prodrive as released stiffer springs for those ?

Any comments, I really mean any thoughts welcome !

Next track day scheduled on 26/03/04

Cheers

stockcar

depends on how "hard" you are driving .............but, yes pad 'knock-off' is known with this caliper design under competitive driving...............
alyn - asperformance.com

STi M'8

Let's say I'm pushing hard

I bought this car for -very- fast driving and I guess this is what it has been designed for

Is there any solution then ? From Brembo, Prodrive or any other ?

STi M'8

bump (steer )

StopTech

I think you are experiencing pad knockback. We have a technical white paper that explains knockback and here is an excerpt:

As the wheel, hub, and wheel bearing deflect during cornering, the rotor hat sandwiched in-between is forced to go along for the ride. Because the caliper is attached to a more rigid suspension component – the upright – the parallelism between the rotor face and the brake pads is altered. In so many words, the deflection of the rotor relative to the brake pads actually forces the brake pads away from one another. This spreading action pushes the caliper pistons back into their bores a tiny amount so that when the deflection goes away (when the cornering event is over) there is not enough springback in the piston seal to push everything back together again. The pads are now pushed off the rotor and will stay put until the brakes are next applied.

If you are interested in more detail (with graphics, which helps the above to make more sense), feel free to check it out:
http://stoptech.com/whitepapers/knockback.htm

Good luck.

STi M'8

Thanks a lot for the link StopTech... It seems clear that I'm facing knockback.

I can't belive how much deflection I have with both new stock bearings... !
Anybody here with uprated bearing experience ?
Is there any upgrade available for GrN or GrA rally cars ?

On the other hand, I'm still looking for some more infos about active knockback springs, but I didn't found anything on Prodrive commercial site.
I guess these are rallying parts... so, any Prodrive buddy here ?

Cheers

Hoppy

STiM8, sounds like pad knock-off (as we call it here) is your problem. It is most common on brakes with solid discs and bells, so two-piece discs might be the answer.

Assuming your bearings are in good condition and not already worn or drained of lubrciant, two-piece discs reduce pad knock-off as the separate aluminium bell does not conduct excessive heat into the bearings, where they would otherwise overheat, the grease would run out and the problem would get rapidly worse.

I'm sure Godspeed can supply you with some quality two-piece replacement discs. But if your bearings are not 100% first, the problem will still be there.

Richard.

STi M'8

Thanks for your input as well Richard,

I called my Subaru dealer tonight, I'll let them check again front bearings on monday... I'm afraid they're already gone with only 2 track days

Back to your suggestion, I think I'll go for 2 piece next at replacement... the stk Brembo are around 800€ pair

TonyBurns

iTrader: (3)

800 euro's a pair?!
Thats more than a set of 330mm AP discs and bells

Tony

STi M'8

Sounds unbelivable isn't it ?

Actual pricing for Brembo disk is 397€ each in France... amazing