Front Rotor Shudder And Discolouration

[MrStabby]

On my second outing to Wakefield (this time with DS2500s on the front and Motul 600) the brakes stopped ok, but developed a shudder. I've just bled all round and noticed the front rotors are discoloured. Does this mean they went beyond their usable temperature range? If so, i'll probably try some 324mm front rotors with caliper adapters from unique autos. The kit comes with either DBA4000 or 5000 rotors - do the 5000s handle extra heat much better? They're a lot more expensive...

tnx,

Kot

PS - Its an R32 GTR (non-brembo)

[djr81]

What colour do you mean by discoloured? A blue tinge on the rotors doesn't mean much.

Check the thermo paint - you will probably find that all three have gone off which is indicative of fairly high temps.

What I found happened to my front rotors was they developed fine cracks which looked to me to be around the grain boundaries of the metal. These aren't pretty, but keep an eye on them.

Also, if you are running 4000 series rotors, it pays to inspect the small return grooves from the outer circumference of the rotor as these tend to be the first to instigate cracks. Check the back as well.

5000 series will save you some unsprung weight & lessen heat transfer to the hub, but are little different in terms of the rotor itself.

Edited October 6, 2006 by djr81

[SLY33]

i dont believe the stock brakes are really up for a "tough" track day, especially if your running sticky tyres and run extended laps in succession!

on my gtst which run pretty similar brake setup to your non brembo 32gtr, i had the calipers fully rebuilt with new seals, the works, and fairly new dba4000 rotors. After a few hard laps really getting on the brakes, massive shudder started when i applied the brake (which mostly went away after they cooled down) and after the track day we pulled the brakes off to find that the new rubber seals had all but melted away! Discs had a mega blue hue to the centre of the pad area, and i had pushed them to absolute fade, yet not one of the colours on the "thermocolour paint" had changed at all.

i was running semi slicks, which no doubt really worked the brakes hard!

Im sure going to a bigger rotor would help, but ultimately a bigger rotor and some better calipers would be ideal.. Are the 33GTR brembo calipers much better than 32gtr non brembo's?

[djr81]

The three thermo paint colours change colour at 460, 550 & 630 degrees. So if you are getting fade at less than 460 degrees & can't get above that temp I would suggest there is something fundamentally wrong.

Also, running semi slicks doesn't necessarilly work the brakes harder than running roadies on the track.

[Duncan]

totally agree....

Just machine the rotors if they are still above the minimum thickness and go out and do it all again.

If you do want to upgrade grab the just jap 6 piston kit it is dirt cheap.

But with good sticky tyres and decent suspension you don't have to brake as much, which means quicker lap times and easier on your brakes.

my old 33 gtst got down to 1.10 at wakefield with the standard brakes....no fade problems

[dontfeelcold]

what could be causing the shudder?

[Duncan]

the shudder is caused by the disc warping slightly from the heat, machining it takes it back to a flat surface. Even a .1mm warp will feel terrible through the brake pedal...its nothign to worry about unless it keeps happening, if so you need to upgrade to larger calipers and/or discs.

or just keep machining them flat again lol

[MrStabby]

> What colour do you mean by discoloured? A blue tinge on the rotors doesn't mean much.

Blue ring toward the outer edge, and some silver spots about 1 cm in diameter.

> Check the thermo paint - you will probably find that all three have gone off which is indicative of fairly high temps.

AFAIK it doesnt have any. They're stock looking but slotted. No idea what brand, but the slots are conventional looking, so they're not 4000s

>What I found happened to my front rotors was they developed fine cracks which looked to me to be around the grain boundaries of the metal. These aren't pretty, but keep an eye on them.

Have you tried to locally sand or machine them out? Cracks concentrate stress which makes the crack spread. The rotors may last longer if you can spread the stress over a larger area by machining the crack out. Its sort of counter intuitive in that a thinner section can be stronger.

[MrStabby]

Im sure going to a bigger rotor would help, but ultimately a bigger rotor and some better calipers would be ideal.. Are the 33GTR brembo calipers much better than 32gtr non brembo's?

Force = pressure x area, so the force exterted by the caliper would be proportional to the area of the pistons. If the total area of the pistons on both calipers are the same, i dont think there would be that much difference. I think the nissan calipers have equal sized pistons whereas the brembos have uneven sized pistons, so there's some optimisation there, but i have no idea how much effect that would have.

[MrStabby]

totally agree....

Just machine the rotors if they are still above the minimum thickness and go out and do it all again.

Both rotors are 32mm, so they're at new spec. I could machine them again, but i would expect them to warp again - dont you think?

If you do want to upgrade grab the just jap 6 piston kit it is dirt cheap.

I can see an 8 piston kit for $2299 - is that the one you mean?

But with good sticky tyres and decent suspension you don't have to brake as much, which means quicker lap times and easier on your brakes.

That's one way of looking at it, another way could be that you're not accellerating enough between corners ;-)

my old 33 gtst got down to 1.10 at wakefield with the standard brakes....no fade problems

Well i only managed 1:17 in a GTR, so i guess that means i suck.... Were you using street tyres? I'm running cheapo Federal 595SS. What brake pads?

[Duncan]

hahah no mate it was with excellent semi slick tyres - bridgestone re55s, and Bendix ultimate pads (about $100 a set lol). but if it makes you feel worse it was pretty stock as well, 160rwkw

yeah i did mean the 8 piston ones, nice big discs and calipers, about the best you can fit under 17s, they are what I'll run for targa next year at that price. Another bloke on here recently paid $8k for a smaller brembo setup by comparison.....

anyway, my theory on the rotors is spend $20 machining them and see if it happens again.....you could do a lot of $20 machinings for the price of a new brake set

[MrStabby]

hahah no mate it was with excellent semi slick tyres - bridgestone re55s, and Bendix ultimate pads (about $100 a set lol). but if it makes you feel worse it was pretty stock as well, 160rwkw

Yep that does make me feel worse - ultimates with 160kw and 1:10 - DAMN! I'm heading back in a few weeks to practice...

yeah i did mean the 8 piston ones, nice big discs and calipers, about the best you can fit under 17s, they are what I'll run for targa next year at that price. Another bloke on here recently paid $8k for a smaller brembo setup by comparison.....

anyway, my theory on the rotors is spend $20 machining them and see if it happens again.....you could do a lot of $20 machinings for the price of a new brake set

Yeah i see your point but i didnt have any problems with my Porsche 928S - it only had single piston calipers and was running EBC greens. I want a permanent fix.

[Duncan]

right....but was the 928 just plain slow? the quicker you are going, you need exponentially more braking.

Most people's problem with skylines brakes is fade or mushy pedal, neither of which were your problem. In those cases bleeding, better pads or better fluid, or even new brake lines or new master cylinder are the place to start.

In your case it stopped fine so your fluid and pads are up to the job, its just that the discs got to hot. And of course with more aggressive pads that will only get worse not better - a lot of cars these days replace the rotors with every set up pads.

But, if you want to address the discs are too hot problem just start with cooling the discs - remove the splash guards in the wheel wells, and add ducting to the centre of the rotor. And if that doesn't work upgrade to a larger rotor and caliper which have more mass = better heat dissappation.

If you decide to upgrade the brakes there are 3 main ways to go:

Brembos off a 33 or 34, about $1500 + discs + pads - and only a little better than standard. Very popular upgrade but very poor value in my opinion. I have to run these on the race car according to the regs and I still get fade at wakefield.

Mega $$ 6 or 8 pot kits from Brembo, AP racing etc....top shelf gear but way too expensive.....have a quick search for that thread i mentioned. Or something even more serious like the f40 brembos or carbon discs like giant runs

The Just Jap kit, 8 pot calipers, big rotors, pads, lines everything for $2200.....its a no brainer to me.

[Roy]

The different sized pistons on th Brembos are there to aid pad wear and attempt to more evenly force the pad onto the rotor.

And i have found that whislt you can break way harder with semi slicks, and you are probably getting higher momentary brake temps...i think its a good trade off as you are on them for a shorter period of time.

This helps two fold. If your not heating them up by stopping, you are colling them down with airflow. So less time on the brakes means shorter heat cycles, and more time for cooling.

To boot your corner speed goes up so you dont have to slow down as much. So then you have to cosnider you are probably arriving at the next corner quicker. But here again there is a trade off that works for you. The speed that you approach the corner naturally impacts greatly on your brakign distance. It can be 30mtrs etc etc.

But the time increase is far less. With it likely to be measured in tenths of a second. Braking distances is not a linear relationship with braking time.

So if your Duncan and can drive, my thoughts are semis actually help mask smaller brakes. If your still coming up to speed (Dont worry, i 3 seconds slower then Duncan in a tweaked R32 GTSt) your probably still trying to come to terms with the grip the semi has to offer...it is quite daunting. I didnt realise how much grip they had unti i went for a ride with someone, then i had to egg myself on to push a bit harder.

As for the shudder. Are they std 296mm rotors? Im guessing they may be. The rotors probably have hot spots and are shagged from way too many heat cycles...throw high temp cycles like you get at the track and they will be worse again. Temps can get that high that you can actually change the metallurgy of the rotor around the grain boundaries and you can find the austenitic structure of the rotor changing to martensitic...which gives the rotor an inherent imbalance/vibration.

Ppl hang shit on DBA4000, but they are probably a better quality casting/heat treatment as this problem is not so common with them

[Scooby]

The Just Jap kit, 8 pot calipers, big rotors, pads, lines everything for $2200.....its a no brainer to me.

it certainly seems that way, but has anyone tried them extensively on the track over time?

Cheers

[MrStabby]

right....but was the 928 just plain slow? the quicker you are going, you need exponentially more braking.

I did 1:18 in the porsche, so not that much slower, but it felt a lot slower with its doughy suspension and auto gearbox...I think i must have been driving better back then.

Most people's problem with skylines brakes is fade or mushy pedal, neither of which were your problem. In those cases bleeding, better pads or better fluid, or even new brake lines or new master cylinder are the place to start.

In your case it stopped fine so your fluid and pads are up to the job, its just that the discs got to hot. And of course with more aggressive pads that will only get worse not better - a lot of cars these days replace the rotors with every set up pads.

But, if you want to address the discs are too hot problem just start with cooling the discs - remove the splash guards in the wheel wells, and add ducting to the centre of the rotor. And if that doesn't work upgrade to a larger rotor and caliper which have more mass = better heat dissappation.

I had already removed the spash guards as i heard that alone should help a bit. I also looked at trying to run cooling ducts - it wont be easy on the driver side tho with all the BOV and intercooler piping - its very tight in there.

If you decide to upgrade the brakes there are 3 main ways to go:

Brembos off a 33 or 34, about $1500 + discs + pads - and only a little better than standard. Very popular upgrade but very poor value in my opinion. I have to run these on the race car according to the regs and I still get fade at wakefield.

Mega $$ 6 or 8 pot kits from Brembo, AP racing etc....top shelf gear but way too expensive.....have a quick search for that thread i mentioned. Or something even more serious like the f40 brembos or carbon discs like giant runs

The Just Jap kit, 8 pot calipers, big rotors, pads, lines everything for $2200.....its a no brainer to me.

You dont like the $800 unique autos 324mm front rotor with adapters for the existing calipers? That's the one I think i'll try first, as its more stock like with regular R32 pads and R33 GTR rotors. With the extra $1400 i can get powerfc and dyno tune muhahaha....

[djr81]

>What I found happened to my front rotors was they developed fine cracks which looked to me to be around the grain boundaries of the metal. These aren't pretty, but keep an eye on them.

Have you tried to locally sand or machine them out? Cracks concentrate stress which makes the crack spread. The rotors may last longer if you can spread the stress over a larger area by machining the crack out. Its sort of counter intuitive in that a thinner section can be stronger.

A couple of points:

1. What forced me to bin the rotors was not the fine surfaces crazing, but the cracks forming in & propogating from the small return slots that DBA put in their 4000 series rotors. Have a look at the photo up the top post, you can see the large crack in the rear rotor & the smaller one in the fron rotor.

2. The surface crazing can be removed by machining the rotors, but I have found that inspecting them is a cheaper option. The crazing on the rotor surface does not seem to cause crack propogation.

I hang shit ont he DBA4000's simply because of these small return slots. I would much rather they not be there as I think they cause problems, ie cracks. They certainly have for me.

In terms of caliper design there are a few things to note:

Different piston sizes (as used on the Brembo's) allow for better (more uniform) pad wear.

Larger pads will reduce the peak temperature as seen by the pad, but will not in itself contribute much to the braking torque over a smaller pad (Assuming a uniform mu value vs temperature, which you can't really assume)

[Duncan]

it certainly seems that way, but has anyone tried them extensively on the track over time?

Cheers

Well I am pretty sure these are what Steve ran at Targa, you won't get a better test of brakes than a 49klm tarmac rally stage.

[Scooby]

Well I am pretty sure these are what Steve ran at Targa, you won't get a better test of brakes than a 49klm tarmac rally stage.

agreed, i'd be interested to know how they went.

Edited October 9, 2006 by Scooby

[Duncan]

nope but since I've just put in my entry for targa next year i need to research this before the end of the year I'll report back.....