FD track car brake bias issue
 

Hi guys,

I'm dealing with a weird brake bias issue in my FD track car (93). When I go threshold braking, I lose the front steering, like the car wants to go straight, it's almost as if the brake bias goes all the way forward under hard braking.

On track this makes it impossible to trail brake into turns, it just won't turn under braking. Have to cruise in off the brakes in order to turn in.

For ex., if i'm coming up to a corner, that require a slight steering input, while braking at the same time, the car just wants to go straight as soon as I hit the brakes hard.

Here's my setup
Stoptech Front Brake, 929 MC, 18x 10.5 wheels all around, 285/30/18 rcompound tire (hankook td)

at this point i'm thinking it must be the stock abs unit/proportion valve, so I ordered a abs delete kit from Alex Rodriguez.

Any one have any ideas what could be causing this? or have had similar issues?

What pads are you running

carbotech xp10 front, xp8 rear

I had the same issue with stock rear brakes. I went to a 13 inch rear rotor and mounted a corvette c5 rear caliper which was 50%more piston size, from 30 to 45 I think. You could try a more aggressive rear pad as well.

The abs is a big advantage. With the 929 mc and a booster the pedal effort is a bit light and I found I was locking wheels all the time and killing tires as it didn't take a hard push to lock them. Keep the abs ! I ditched the rear prop valve but it didn't make a big difference.

You have too much front brake. You could have some wonky ABS issue too. The system was not designed for large race tires, but this is likely just a bias issue.

I run a wilwood prop valve in place of the OE prop valve with no ABS. It is an FC but I have larger front brakes and the 929 MC and booster.

I would start with adjusting your pads before ditching your ABS. What adjustment did you make prior to your bias getting out of whack? That should be your starting point

Probably the pad difference and the front BBK.

You have plenty of front spring?

Back when my FD was stock class (stock springs) it was like that on DOT-R slicks- wouldn't turn in when sitting on the bumpstops under braking. Had to get off the brakes long enough or on the gas momentarily for the front end to pop back up and then turn in.

Stock RX-8 was the same story (on the FD wheels/tires)- could only turn in on the throttle.

Good call here.

All good suggestions.

I'd have to dig a little however I recall the stoptech kit pushing the brake bias forward a little. Factor in the more aggressive pads front vs rear and the more touchy 929 mc, this could all contribute to what you are experiencing.

Before doing anything extreme:

-You could try running the same compound front/rear
-Put the stock 15/16" MC back on.
-Would cost some money, however you could upgrade the the larger RZ size rear discs to gain a little more rear bias.

'

I wish it was that easy =) 16kgmm front spring rate, it doesn't appear o be bottoming out, as it's very front biased braking even at 50% moderate braking pressure.

The rear xp8 pad is the suggest setup from Carbotech, it works in a temp range lower than the xp10, this commonly used in the rear in conjunction with xp10.

I should mention that I had a Brembo GT BBK in the front before (with stock mc), and it was the same front bias behavior (perhaps even more extreme due to the sloppy pedal with stock mc, and the huge rotor of the brembo).

The stoptech brake is alittle more front biased, but it should not be this extreme. My buddy also has the stoptech front on his FD, and he loves his setup.

It sounds like you already know the answer to your problem.. put a more aggressive pad out back to start. If that is not enough you need an adjustable prop valve on the rear brake circuit. After you put in the prop valve you may have to go back to your XP8 rear pads. After I put the adjustable prop valve on my car I had to run a less aggressive rear pad. I run Hawk DTC 60 in front and DTC 30 in back. I got too much rear brake lock up with DTC 60s in the rear.

I have a question about the prop valve, so I plan to bypass the abs (just to eliminate the variable), and use a wildwood prop valve to control the rear brake pressure.

by using the prop valve, it can only reduce the rear brake pressure? that means if the car was too front biased to begin with, can prop valve help?
or does installing the prop valve changes the starting rear brake pressure?

The prop valve will not give more brake pressure, it will only take it away. Fully open it is like not having a valve in the line at all, and you turn the rear brakes down until you get the bias where you want it.

If you have too much front brake bias now with the stock bias setup, you'll be able to get good brake bias with an adjustable prop valve, unless your Stoptech calipers have the wrong size pistons in them.

thanks guys, i think i know what to do.

btw, would a kinked rear brake hard line cause this front bias issue?

Well that's certainly relevant information

A kinked line will hamper brake pressure, depending on how kinked it is

haven't checked it yet, but that's another potential issue that I will check

you are either a really good driver or very lucky that your FD hasn't had an intimate encounter w the Armco.

let's generalize and assume your FD weighs 2950. that would mean your corners weigh in around 737.

16 kilogram per second springs translate to 896 pounds per inch.

enter a road racing braking zone and a minimum of around 20% of rear weight shifts forward.

737 X 1.2 = 885 weight on each of the front corners assuming you are going in a straight line which you should be initially.

you just added 147 pounds and your 896 pound spring deflected about .16 of an inch. flow it thru the front motion ratio and you have a quarter inch of wheel travel.

you have effectively no suspension. kind of like driving on ice. just like F1, Indy Cars as to suspension travel, but they have the benefit of between one and 3 Gs of downforce. you have none.

the transition between static and fully loaded is the most important factor in driveability as it is in this period of time and dynamism that the driver is able to feel for the limit of adhesion. you have effectively none. in addition, there should be a transition of force fed into the tire rather than an abrupt shock.

further exacerbating this are your very low profile tires, i run 18s also but they do give up a bit of feel w their very low profile. take a look, again, at F1, Indy cars, TransAm etc. it is not an accident you don't see minimal aspect ratio. again, not saying you need to go to 17s, just saying that 18s make driving at the limit a touch more difficult.

as to your brake situation... i ran my FD for four years w the AP package on the front end and stock rear brakes and the ABS. this setup totally screws up the proper longitudinal bias and essentially turns the car into a 2 wheel brake setup.

like so many things on our cars, the stock brake system is perfect for what it was intended which was spirited street driving. the system was not designed for aggressive tracking. simply put the front rotor does not have enough mass. put any pads/ducting on the stock brake system and i will be out of brakes in less than 3 laps.

it is also possible to have an enjoyable experience on track w the stock system and better pads at 7 tenths.

anyway, just FYI, w the 13 inch AP rotors and AP4200 calipers, proper pads and the ABS working i just modified my driving to just barely stay out of the ABS and had no problem beating every car i ran against at the Ferrari Club of America's events at Brainerd.

i was running RSR coil overs which are 8/6, 18 by 10 wheels and Toyo Proxes. air pressure was no more than 30 fr and 27 rear.

what is your tire pressure?

i did race SCCA GT3 Nats for 22 seasons and held the track record at Brainerd at one time.

years later, running my street FD i was using my pyrometer to set camber etc etc but my point is that you can actually do pretty well w a large front brake kit without removing your ABS. you just sort of drive around it. it isn't optimum but it will work.

i currently have totally removed the power brakes on my dual purpose FD and have twin master cylinders w a cockpit adj bias bar. generally in a 30 minute session you burn off 60 pounds of fuel so you need to do an adj near the end of the session to stay in balance. brake bias is super critical as you look for more time on the track.

two essentials for lowering times are a good tire pressure gauge and a pyrometer so you can set camber.

if i have your starting the session tire pressure and your ending pressure and tire temps i can help you set your suspension from my laptop.

(anyone on the board is invited to post their starting and ending tire temps and pressures and i will be happy to make setup suggestions)

you will end up w different tire pressures at all 4 corners as well as differing cambers.

what tire pressure are you currently running?

a totally optimised setup which will generate the fastest lap times is also the easiest setup to drive. if you are not comfortable in your car it is the setup.



Howard

Is this a new experience or has the car always done this?

I don't understand threshold braking or braking hard into any corner. For example if I'm going 100 plus MPH and just before I arrive at a slight turn I hit the brakes hard I'm not going to turn no matter what I'm driving because a car can't turn and stop at the same time. Trail braking is a really subtle (pedal should be rising along with the nose of the car not falling) form of braking or the car is pretty well balanced.

Do you have any braking issues or lock up when braking in a straight line/threshold braking?

"a car can't turn and stop at the same time"

:)

The car came to me with a front Brembo GT brake kit installed, and it was like this when I first got the car. I thought it was just the enormous brembo front brake (really large diameter).
But even after changing to Stoptech the problem still persists.

Yes I'm reducing pedal pressure as I start the turn in, though I still have pressure on the pedal. I like to think of trail braking as being able to brake later by extending the braking zone into the corner entry phase.

I'm not a pro race driver, but I like to think I can feel when something is wrong with the brake =) Just to eliminate driver as a variable, I drove 2 other cars recently, a mid engine rwd car with excellent OEM brakes, and a 85 Toyota ae86 stripped out track car, both I can brake just fine, they behave as I expect them to.

The diameter of the GT kit is about the same. In fact you can use the stop tech rotors and hats on the GT kit.

If you have any video of the lock up that would really help us figure out what's happening.

Do your front brakes lock up in a straight line or does your ABS prevent serious lock up. Fast forward to approx 6:30 of this video and at about the 6:38 mark the fronts lock up a little. My front brakes do this all the time (I'm currently using the GT kit) Sure I'd be faster with more rear bias but the last thing you want is the rear locking when say trail braking. I've driven these cars without ABS and lets just say I'd never choose to drive any car without ABS UNLESS the ABS was interfering. I think the ABS on the FD is very good compared to the GT3s I've driven. My favorite overall rear pad is Hawk blue (cheap and work) and I like PFC 08s in the front but 01s will shave a little time at the expense of rotor life. You can't use cheap pads in the front but you can get away with it out back.


I can't recall ever locking up the front when trail braking so I suspect you have some major ABS issue.

Typically if you have a brake line that's kinked it will cause the pad to stick or the piston to not release so I doubt that's your issue. It's likely a bad relay in the pump, bad ABS ecu or some other issue effecting the function of the ABS.

I'd save myself some reading/confusion and deal with the prop valve and the kinked hard line, then start from there, swapping out pads, etc.

In my experience, with stock brakes and Porterfield R4S pads, RX-7s trail brake just fine.

What Natey said. IF the line is kinked, replace it. And put same compound pads all around. I run XP10s all around and trail-braking is not an issue at all, on softer springs and running 24.75" front ride height.

I know running staggered pads is a thing, but honestly why would you want to overburden the fronts when the rears can still contribute? I've never had rear lockup issues running same compound front/rear on the Z, S2k, or FD.