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High speed braking issue

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Old 30 December 2017, 07:49 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

Last edited by SoNiCa; 31 December 2017 at 01:50 PM.
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Old 31 December 2017, 01:38 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, 01:58 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, 02:15 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, 02:29 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, 02:46 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, 04:17 PM   #7  
joe v3sti
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What condition are the front wheel bearings in?
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Old 08 January 2018, 04:00 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, 04:08 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, 05:07 PM   #10  
SoNiCa
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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, 12:01 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, 10:48 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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Old 30 April 2018, 10:53 PM   #13  
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A photographer catch the moment ! 220 kph to 60 kph


I'll change the bias valve to oem sti and see how it feels.
I did change some bushings as well.
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Old 03 May 2018, 10:46 AM   #14  
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Super photo ,

Somehow you have to get the rear down some more.
Despite the full braking it really should not be up in the air like that.


If you do not want to or cannot physically drop ride height then your next alternative is increase front coil rate. However upping spring rates is not always a good thing for compliance over undulating circuits. If you go to far with spring rates you end up with a twitching feel that can be very uncompromising and uncomfortable. If you touch the curbs the whole car skitters laterally or spins out.
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Old 04 May 2018, 11:34 AM   #15  
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Are you running an anti-lift kit?

If you are, you removed the anti-dive component of the suspension. The longitudinal load transfer, under dive, is now all going through the front springs - it was previously shared (20%) with the front control arms.

You're already running 340lb/in front springs - that is unhelpful for road use.

Are you running any rear toe-in? Try positive 0deg 15minutes (0.25 deg) rear toe-in per side - to control the rear under heavy braking. This will also mean you're running a rear slip-angle, which will give instantaneous response to steering inputs.

Stiffer front bump stops, as a form of pitch control under braking. But again, you'll lose compliance on the road.

Clip-on spacers, on the damper rods, would work - they can be added/subtracted front and rear when at the track.

What ride heights are you running?

Under heavy braking, which end of the car is locking up first? Or no locking?

Last edited by 2pot; 04 May 2018 at 04:38 PM.
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Old 05 May 2018, 10:40 PM   #16  
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I can adjust the ride height of KW V1 suspensions. It is a bit high just because 235/40/18 tyres rubbing issues. I should stay on 225/40/18..

Yes I'm running a GTSpec ALK.
Fender to floor 66 cm for rear/65 cm for the front.
AFAIK all toes' were 0.04 front camber was -3.0 rear -2.2.
Thanks to 20 year old steering bushes the response was very lazy.

The problem continued last month session. As you can see from the yaw rate sensor, it shows up to 0.26G lateral forces during the brake.
It feels like that I put the handbrake first so it should be the rears locking up but actually I really don't feel any locking. Moreover, I'm in doubt if the abs triggered.


This is a huge braking for me really. I try use the brake like on/off. Since I'm noob I can't concentrate on downshifts which makes the oil pressure down to 1.0 bar and Syvecs limpmode.

What I like on EJ207 is the temperatures. Stock tmic(max 30c), stock radiator(max 83c), no oil cooler.(max 105c)
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