Andy_Mac

Issue out on the track today

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Anyone care to take a stab at what this noise might be. The repeated clunking when on the brakes is what concerns me.

It's done it at both track days now, this one worse than the last, but never when on the street no matter how hard it's pushed. Not sure if it's two separate issues or whether they’re related. Brakes got super juddery today with light or firm application.

 

My initial thoughts are rear diff mounts, driveshaft centre bearing or passenger rear shock as the clunk seems to come from that area. 

I'm slightly stumped as these are the only two times it’s done it so can't really replace parts blindly and then test out changes.

 

 

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Juddering will be warped disks... happens to me every time I go to the track... need bigger disks.

 

As for the noise... does it happen on the straights or only the corners?

 

Can you feel the knock through the brake pedal? Does it only happen when the brakes are hot?

 

Do you have DCCD and is it locked up pretty tight when on the brakes? Do you know the condition

of the rear diff? 

 

Could be something loose or worn... no idea what. Sounds bad enough that it might be doing damage though.

 

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@Loren 

 

The noise happens when turning or going straight when on the brakes.

 

Hard to say if I feel it through the brake pedal or whether its just the regular brake shuddering feel something through the steering wheel too.  Almost feels similar the hoping you get when the rebound on coilovers is set too high. 

Was still happening when colder on starting laps but not as bad.

 

No DCCD on this. No idea on the rear diff condition other than that it’s a viscous r180. Have replaced the housing fluid but not the actual diff fluid. 

 

Also both times it’s done a single clunk that sounds the same when coming off the throttle when doing long pulls on the straights. 

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Can't be the rear diff or front diff  if it happens on the straights...  you can get hoping under very heavy brakes when the centre diff is locked up very tight...

but if you don't have DCCD, then probably not that as you'd get chronic binding on low speed corners if the centre diff was that tight.

 

I think I can see the knock through the gear stick... correct? So the drive train is bouncing around quite a bit... could be what you are hearing.

So maybe the bushes on the gearbox are a bit soft for track work... or maybe something else causing it to bounce excessively.

 

Edited by Loren

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17 minutes ago, Loren said:

Can't be the rear diff or front diff  if it happens on the straights...  you can get hoping under very heavy brakes when the centre diff is locked up very tight...

but if you don't have DCCD, then probably not that as you'd get chronic binding on low speed corners if the centre diff was that tight.

 

I think I can see the knock through the gear stick... correct? So the drive train is bouncing around quite a bit... could be what you are hearing.

So maybe the bushes on the gearbox are a bit soft for track work... or maybe something else causing it to bounce excessively.

 

 

Think the knob moving about is mainly because the shifter bushings are pretty rooted so it has quite a lot of play and everything was shaking around so that was too. Good place to start though. The stock gearbox ones are pretty floppy.

 

6 minutes ago, sobanoodle said:

 

+1

 

Yea agree without a doubt on that being the cause of the shudder. Wouldnt think the shudder could be causing the knock though

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A cracked disk would knock and shudder... but the braking performance would be S*** as well.

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21 minutes ago, Andy_Mac said:

Yea agree without a doubt on that being the cause of the shudder. Wouldnt think the shudder could be causing the knock though

 

I'd back up Loren and say that some tired bushings or something loose will cause the knock.

 

My BR9 came with warped stock discs and was shaking madly when braking from around 110/115 down to a hundred. Passenger seat (when vacant) would be jumping all over the place, rather embarrassing :D 

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1 hour ago, sobanoodle said:

My BR9 came with warped stock discs and was shaking madly when braking from around 110/115 down to a hundred. Passenger seat (when vacant) would be jumping all over the place, rather embarrassing :D 

 

I've worked on my fair share of cars with stuffed rotors/drums. I've had it so bad once in a customers car it was shaking the whole car! Nissan Patrol - have to pull the front bearings apart and all that to replace the rotors :/. I was paid to do it though! Plus Ford Falcon's and Holden Commodore's are known for warped front rotors. 1.8T(ish) of car, with inadequate brakes.

What you've got there is warped rotors - you said you feel it in the pedal, and you can see it in the steering wheel. If you feel it in the pedal still after doing the front rotors, the back ones will be warped too most likely.

Best get some slotted ones (if you haven't already)! If you plan on doing more track work, probably worth getting some track pads for the front to swap in at the track too. My Dad did this back when he had his Impreza STI. No point in doing the rears, just some good fast road pads/street pads will be fine.

 

Andy, you're missing out on the shifter bushings! They're AMAZING! From the slight sloppiness I had, to tight AF, it was one of the best things I've done to my car!

Can understand though why you haven't done them yet though. The horizontal bolt can be a c*** to get out, depending on which way the head is (I got lucky, I think).

If your trans/engine mounts are on the tired side too, look at the Hardrace set.

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16 hours ago, sobanoodle said:

 

I'd back up Loren and say that some tired bushings or something loose will cause the knock.

 

My BR9 came with warped stock discs and was shaking madly when braking from around 110/115 down to a hundred. Passenger seat (when vacant) would be jumping all over the place, rather embarrassing :D 

 

No cracks on any of them on the outwards facing face but not sure on the inwards.

They’re DBA T3 front’s T2 rear’s with bendix ultimate’s all round so should be more than adequate on the track. Besides the shuddering the performance was still mint. Could stand on them super late and besides shaking around they pulled up fine.

Got them earlier in the year just before the last track day. They were getting bloody hot but not quite smoking like last time.

There’s a very mild shudder at 100+ when braking on the street.

 

@Individualities Yea I know I need to get them done. Almost did the rear the other day but ran out of time and motivation. Need to grab the large lower ones to do at the same time still.

 

Since the clunk happens on liftoff after long pulls as well the trans and engine mounts will probably be the first things I replace, have been torn between group N and those Hardrace ones.

 

Super keen to buy a semi decent press so I can do most bushes myself and not have to go for poly ones.

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31 minutes ago, Andy_Mac said:

 

@Individualitiessince the clunk happens on liftoff after long pulls as well the trans and engine mounts will probably be the first things I replace, have been torn between group N and those Hardrace ones.

 

Super keen to buy a semi decent press so I can do most bushes myself and not have to go for poly ones.

 

Hardrace also make race versions of the same mounts. Not sure how they would compare to the Group N ones.

 

A vice, hammer, and some cheap sockets = cheap press :P. If you know someone who has a press, or have a mate who works at an automotive shop, just go round after work hours and use it then.

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My old be5 use to make a clunk on lift off but was after every shift under power, was rear diff bushes (pretty common on the be/bh I believe, and pretty sure its the same design as bp/bl).

Other thing I was thinking was binding center diff or cv's but you'd think that would only happen around corners.

 

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As per the above, your shimmy or shake etc is because of the disks. We all agree there. 

 

I disagree on getting vented, however, until you make a small but VERY significant change in your braking habit. 

Late and hard braking are your friend in racing scenarios. Taking it 'easy' on the brakes, aka braking early and lightly is what will root your pads and rotors.

 

I learnt my lesson the hard way when I took a very expensive set of Znoelli S12 rotors on track and cracked them because of this, and ruined a set of pads while I was at it. 

 

Now. The knocking. 

 

I'm willing to bet it's your transmission mount. Reason being, as your cars weight shifts forward during braking, this unloads the weight of the transmission from weighing down, to neutral to ever so slightly lifting. it'll be rattling on the top (where it keeps the mount captive) because the mount is a bit worn. 

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My brakes did that also last track day I raced would shudder quite badly feel it in the pedal & steering and made a knocking noise when getting hot which I presume was something else shaking due to the warped discs. 

 

This track day I had no issues with the Znoelli discs and pads now I just got blow by from faster cornering 9_9

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35 minutes ago, Dairusire said:

 

Late and hard braking are your friend in racing scenarios. Taking it 'easy' on the brakes, aka braking early and lightly is what will root your pads and rotors.

 

 

I have to disagree here... it's usually an insufficient cool down that warps or even cracks the disks. Parking up when there is still a lot of heat in

the calliper causes a hot spot... which leads to localised expansion... and a warped or cracked disk. 

 

Personally I think the subaru 4 pot is too small for the track and so much heat in generated that even a nice slow warm up and a good cool down will not save the disk. I have to machine or replace the front disks after every track session... one of the reasons I don't bother with the track much.

Edited by Loren
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Yea was never disagreeing with warped rotors causing the shuddering. May have found the issue just now. All the shaking around managed to loosen off one of the adjustable camber arm nuts which has quite a bit of movement when loose. I know it was tight-ish before the day as it was one of the checks I did prior to driving down.

That particular one makes sense as it felt like it was coming from that corner and with the exhaust where it is it can't be tightened properly. Have taken the Y section out today and given the bolt a decent crank down this time which should prevent it coming loose again. Think there may still be something else causing issues when lifting off which probably is the gearbox mount.

 

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Interesting point @Loren. Had a chat to the local Subaru rally/race expert this morning and he reckons the Subaru 4 pots are way too small to dissipate heat effectively and that even the Brembo’s don't do that well. Making up a duct for the fronts along with 2 piece rotors was the way he reckons I should go. I think some of the warping was already done prior as there’s been a very mild shudder at road speeds for a few months now.

 

Had been rolling the car half a turn at a time for a few mins after every session but guess that wasn't enough.

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what loren said above.

will take more than just rolling the car to cool the brakes.

usually a full cool down lap and a circuit of the carpark.

;)

 

as for late and hard braking to conserve the brakes... no instructor ever advised that !

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Yea rolling was just to not let the remaining heat get stuck on one spot wasn't expecting much from it.

Might bring my infrared thermometer next time. Would be interested to see what the temps are at straight off the track vs after a cool down lap of the carpark.

 

Also were you there on the weekend @newsuba? Did see a unmistakable bright yellow hatch parked up at one point

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Removing the metal shielding when I used to go to the track that was on the back of the discs made a massive difference to heat. I damaged one and removed it and the next time I went on the track was a a massive difference in temp and wear on the brake set up each side

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28 minutes ago, Kiwiflyboy said:

Removing the metal shielding when I used to go to the track that was on the back of the discs made a massive difference to heat. I damaged one and removed it and the next time I went on the track was a a massive difference in temp and wear on the brake set up each side

 

Some cars don't even have these. I've always wondered why. Now that's possibly one reason (saw this on a C63 Merc I worked on).

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Or you can cut a hole in the shield, tack on a round sleeve, and run ducting from the front bumper to the back of the discs. Tony did something like that; I believe Viv has that setup now.

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1 hour ago, GC8E2DD said:

Or you can cut a hole in the shield, tack on a round sleeve, and run ducting from the front bumper to the back of the discs. Tony did something like that; I believe Viv has that setup now.

 

Might have to give that a crack. Shouldn’t be too hard.

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