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High speed braking issue

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Old 30 December 2017, 08:49 PM   #1
SoNiCa
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Default High speed braking issue

I don't know if the video tells enough but the car feels jerky during high speed braking. Especially over 200 kph.
It is the first time I've been on the track so I have no such experiences before.
First question? Is that normal at that speed? If not, what could be the problem?
It is MY98, k-sport 356/330 mm brake kit and sti master brake cylinder

Happy new years!

ht tps :// youtu.be/fszhqGv6Uec?t=124

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Old 31 December 2017, 02:38 PM   #2
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The brakes shouldn't feel jerky at any speed. How does the pedal feel?

First thought would be a warped disc, it would be more pronounced at higher speeds and would feel like you were pressing on and off the pedal even though you were holding a consistant pressure, which might be the jerky sensation you describe.
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Old 31 December 2017, 02:58 PM   #3
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Pedal feels okay. No shuddering at all.

I just edit video link timescale.

It feels like the back off the car wants to move forward. I'm trying to stabilize the car with some minor steering movements.
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Old 31 December 2017, 03:15 PM   #4
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Is the back end getting light I.e a brake balance issue. K sports front and rear?
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Old 31 December 2017, 03:29 PM   #5
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are the bushes all original on the front wishbones and rear trailing arms ?
you could be finding the rear arms moving under braking, making it feel bad,
just an idea.
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Old 31 December 2017, 03:46 PM   #6
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Thanks..
Ksport rear and front...

Yes all bushes, wishbones and rear trailing arms are stock 20 year old

98-2018
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Old 31 December 2017, 05:17 PM   #7
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What condition are the front wheel bearings in?
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Old 08 January 2018, 05:00 PM   #8
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Default Worried about brakes ?

Even a minnor amount of disk warp will cause an awful Vibration Feeling,
Its downright dangerous if you cannot press hard enough, most of the UK Folks won't know this unless they track as they don't , should not be up and over 200kmh on public roads.


The answers / causes are many, A few that I have experienced are ;

Warped disks / machine the existing ones flat or replace new.
Seized sticking Piston accompanied by disk overheat, clean up-replace seal. WBearings as suggested above.
Bent/mis aligned Suspension Geometry.

I found I could the warp Standard disks on one single 250-80 Kmh decelaration , and after warping the 3rd Std set ( best Std pattern were from TRW ) I swapped to Godspeed 330mm fronts , and changed pads to
Syntered Ferrodo 2500 Front Carbon lorain LC5 rears. ( Standard rear disk) The Balance is good and I can pull it down from 285 Kmh to Zero full application if I have to without damage. The other thing you must do is get some race brake fluid. Motul do a good one DOT 4.
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Old 08 January 2018, 05:08 PM   #9
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Should add to this , if you are running too much braking effect on rear , the car will attempt to rotate , you can add a brake Balance valve or just swap the pads at rear to some with less bite. The Feeling will tend to be worse if the front coil spring rate is too low , as the car will dive under braking as lifting pressure from rear wheels makes it rather more unstable.
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Old 12 January 2018, 06:07 PM   #10
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Quote:
Originally Posted by joe v3sti View Post
What condition are the front wheel bearings in?
Custom 5x114 bearings running over 5 years.


Quote:
Originally Posted by Linksfahrer View Post
Should add to this , if you are running too much braking effect on rear , the car will attempt to rotate , you can add a brake Balance valve or just swap the pads at rear to some with less bite. The Feeling will tend to be worse if the front coil spring rate is too low , as the car will dive under braking as lifting pressure from rear wheels makes it rather more unstable.
The brake fluid was Motul RBF660. Disks were machined for a smooth flat surface recently. I actually suspect 20 year old bushings that could make the car jelly on a g force like that.
I'm planning to buy a Super Pro full bushing kit.
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Old 14 January 2018, 01:01 PM   #11
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Default Swerves under braking

I took another look at the video of this on a PC ( not my Iphone ) , this is an awful amount of swerving around is probably caused by the suspension, That amount of steering wheel sawing only happens otherwise when your rubber is going off / too hot or you are driving winter tyres with 8mm profile in 30 degress , must be a horrible feel for you.

Given that the rubber is 20 year old , the more you throw out for new the better. The bushing kit is most definitely the way forward here.

I'd normaly start with the front roll bar, but this is so bad I'd also expect all the rear trailing bars.

For road / track mix Id try something like Powerflex bushings, or even new standard rubber, I wouldnt do the hard polypropelene unless you only track , as it tends to stress and flex everything else much more , so for a 20 year chassis its questionable and hard polyprop can be harder to fit too.

Once you are done , get the geometry checked / adjusted. You will find you will enjoy the car so much more. Then the lap times tumble
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Old 16 January 2018, 11:48 PM   #12
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Yes, not comfortable!
Suspension is new upgraded KW Variant 1 and I'm very happy with them.(after JIC FLT-TARs!)

It is the first time I have been on this track and 2.08 with Porsche 918 Spyder is the record.
I hope to get 10 seconds for the next year with the same 16G setup.

I need to sort out fuel surge problems first.
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