Thoughts on a BBK?

BBK meaning big brake kit. I can upgrade the front brakes on the FiST and stay in my current class. Why would I want to do this? Well, for autocross I am less worried about heat dissipation and more interested in increasing clamping force. I feel like whenever I have a long section where I am hitting 55 (top of second) and I have to stand on the brakes for a sharp turn, I either have to brake early and overslow, or I overshoot while waiting for the brakes to do something. Nothing in between, I want to be able to dive a bit deeper into a turn and still slow down.

I have good pads, SS lines, high-temp fluid, so I think it comes down to being able to clamp harder on the rotors at speed. Am I crazy? I am not a brake engineer, in fact, basic math escapes me. Help me out here. 

Are you locking up the tires/engaging abs when you stand on the brakes?

Define "good" pads. You probably have pads that are poorly matched for the usage. Look at some temp/friction charts (or ask for some recommendations) and get a pad with higher friction at lower temperatures. That will probably solve your problem. 

It sounds to me that rather than "clamp harder", you're describing a braking system that is hard to modulate to get exactly the desired amount of braking.  Throwing a BBK at it is probably overkill, at least as a first step.  Different pad compounds are a much cheaper/easier way of experimenting, and pedal technique (the loose nut behind the wheel) is also very important in getting the brakes to do what you want.

For driving on the big track I like AiM data systems for this kind of thing, not sure if people use them for autox or not.  Does the Fiesta ST have. a CAN-bus accessible brake pressure channel?

Also, don't forget that your tires stop the car. What tires are you running? (And what class are you running in, too?)

We'll get you all sorted out.

The main benefit of BBKs on a modern car is increasing thermal capacity for the track, with the secondary benefit of increasing braking force, which may improve performance if the stock brakes aren't enough for big, sticky tires.

For AutoX, I would think experimenting with pad compounds would be a better course, because you usually end up adding a lot of weight with a BBK, and the extra thermal capacity isn't usually a benefit.

Rodan said:

because you usually end up adding a lot of weight with a BBK.

BBKs often replace cast iron calipers with aluminum ones which can actually result in a reduction in weight despite the larger rotors (and rotor-on-hat designs can help here too).

Tom Suddard said:

Define "good" pads. You probably have pads that are poorly matched for the usage. Look at some temp/friction charts (or ask for some recommendations) and get a pad with higher friction at lower temperatures. That will probably solve your problem. 

This is the question. This sounds like track-based pads when autox needs something closer to street.

The stock "S" type "summer" OE pads are some of the best bite pads in an autocross scenario. They can tolerate a good bit of heat during autocrossing also. Are you using something different?

Some related reading that might help:

Do Performance Brake Pads Really Matter at Autocross Speeds?

How to Go About Upgrading Your Brakes

Project Z06: Exploring Brakes and Their Technology

Rodan said:

The main benefit of BBKs on a modern car is increasing thermal capacity for the track, with the secondary benefit of increasing braking force, which may improve performance if the stock brakes aren't enough for big, sticky tires.

For AutoX, I would think experimenting with pad compounds would be a better course, because you usually end up adding a lot of weight with a BBK, and the extra thermal capacity isn't usually a benefit.

 

That's pretty atypical. Like codrus said, they are aluminum calipers and typically two-piece rotors with aluminum hats. For instance, the front AP Sprint kit for the BRZ/FRS/GR86 drops 10lbs PER CORNER. So a BBK litterally drops 20lbs off the nose of the car.

In reply to z31maniac :

Depends on the car, and the kit, but yeah... if you're going from all iron to aluminum calipers and two piece rotors you'll probably drop a little weight overall while gaining rotational inertia in the rotor.  

codrus (Forum Supporter) said:

Rodan said:

because you usually end up adding a lot of weight with a BBK.

BBKs often replace cast iron calipers with aluminum ones which can actually result in a reduction in weight despite the larger rotors (and rotor-on-hat designs can help here too).

 

I agree. I lost weight when I went to 4 piston fixed calipers.

It's been a while since I had 200TWs on my fiesta, but I don't remember ever having any braking force issues, with either the stock pads or the hps5.0s I replaced them with. Sounds more like a modulation issue, which is a pad compound issue more than anything else. Or just a physics problem...heavy tall fwd car on street tires isn't really going to be able to go deep into a corner regardless of what brakes are on it.

Thanks for the feedback and questions. 

Pads: I too like the OEM pads for the Fiesta, a 2-time HS Nats winner in a FiST suggested the same to me. I have tried others (Hawk 5.0, HP+) and am currently running Ferodo DS2500. They were suggested by another Nats winner, and I really like them for their modulation.  I believe that I am running decent pads. When I went with aggresive track pads, I would indeed lock the fronts, that is not what's happening here. 

Tires: I run a 205 or 215 series 200tw tire, RE71rs lately, but have run BFGs and Falkens as well. Same issue on all tires. 

Wheels: 16 x 17.5 Enkei RFP1s. 

Suspension: Coilovers, Eibach front bar, 2-point traction bar. 

Driver: 20+ years of autocross and a few schools. Not an expert, but not a beginner either. I think that I am goood enough to be able to progressively brake a little later each run to find the ideal braking point. The issue is that run #1 is way too early. Run #2 I get a little closer, and feel like I could hold off for a few more feet. On Run #3 I stand on the brakes hard and the car is slowing, but not enough and I understeer through the corner. No lockup, no ABS, just not enough whoa. 

I guess what I am looking for is to be able to shorten my braking distance from 55mph. I have already upgraded tires and pads, is a BBK going to make an incremental gain, or am I simply at the limit of the platform? 

pinchvalve (Forum Supporter) said:

When I went with aggresive track pads, I would indeed lock the fronts, that is not what's happening here. 

 

Just to clarify... with your current pads you are unable to lock the fronts?

I am tuned in to this discussion as I feel skeptical about the idea of adding bigger brakes in autocross. 
I know they have improved dissipation. That makes sense. But I wonder if clamping force increases at all with a bigger brake? Isn't that a function of master cylinder?
 

The weight is definitely a factor. If you can get lighter brakes that are bigger, that seems like a win. But I know of people adding heavier brakes to light cars (like my old Festiva) and I have never seen anyone do a and b testing to verify anything. They just say it feels like it stops better.

From your description; you are having trouble getting the car to turn at a given entry speed. 

You either need to slow it down more or figure out how to get it to turn in at the higher speed.

Have you checked the pads to make sure they are wearing evenly, I.E. there isn't something wonky with the calipers?

Have you tried trialing the brakes a bit longer (rather than threshold braking) to get the car to rotate more aggressively? In the Datsun rotating the car to the point of almost chucking it into a corner scrubs off that extra bit of speed..............it's good for that extra 3 feet your looking for. Trailing the brakes down into the apex of the corner is the only way to make that happen in that car. By contrast if you do this in the F500 it will lock the inside front a push like mad...........in that car you have to be off the brakes earlier.

What are the alignment specs?

What are the National champs doing that you're not would be my only other question.

CyberEric said:

I know they have improved dissipation. That makes sense. But I wonder if clamping force increases at all with a bigger brake?

Clamping force, not necessarily (thats a function of the hydraulic side of the system).

Brake torque, probably (further from rotational axis)

Most of the time, clamping force is kept similar and torque is increased.  There are some systems (WRX Stoptech comes to mind) where clamping force is reduced and torque is kept constant.  What you get is increased thermal mass and less pedal travel (wrxs have awful pedal feel)

The physics is really simple.  A great book on this stuff btw is 

https://www.pegasusautoracing.com/productdetails.asp?RecID=5963

Inconsistency in braking points to something responding to heat. Pads are the most likely. Here's what the heat vs mu (friction) chart looks like for the DS2500.

Never terribly high friction (thus needs more pedal pressure for a given amount of brake torque) and it does fall off with temps. So step 1 is to find out how hot your brakes are actually getting. "No stopping power even when I really romp on the pedal" is what overheated pads feel like, and you'll probably be able to smell them too. That's unusual in autox due to the short runs and long cooling time. Still, that chart says the DS2500 isn't a big fan of heat.

Here's what the Hawk pads look like. Some unit conversion will be necessary - but see how the mu increases until it falls off a cliff for the HP+ and 5.0? The DS2500 falls off more slowly than those two, but it falls throughout.

Single stop distance is about brake balance and modulation, assuming you're not overheating something. Going to a BBK will probably increase front brake torque which will move the balance forward. This may or may not solve the problem.

Great input, thanks, I will try to wrap my head around all this. 

According to EBC, going from a single-piston, sliding caliper to a multi-piston caliper is what increases what I am calling clamping force. It also more evenly distributes it across the pad surface and across both pads.  That seems like it would perhaps allow me to grab the rotors harder and slow the car a bit more. Its hard to explain, but imagine grabbing a spinning metal rod with a leather glove on. If you grip with slight pressure, it will eventually stop spinning. If you grip it with more pressure, it will stop sooner. That's my thought. 

Heat should not be my problem, there is plenty of cool-down time between runs, but different pad choices may be worth experimenting with. If I go with a more aggressive pad, I know they will lock and slide, but I can try a few an see if something works better than what I have tried. 

pinchvalve (Forum Supporter) said:

Great input, thanks, I will try to wrap my head around all this. 

According to EBC, going from a single-piston, sliding caliper to a multi-piston caliper is what increases what I am calling clamping force. It also more evenly distributes it across the pad surface and across both pads.  That seems like it would perhaps allow me to grab the rotors harder and slow the car a bit more. Its hard to explain, but imagine grabbing a spinning metal rod with a leather glove on. If you grip with slight pressure, it will eventually stop spinning. If you grip it with more pressure, it will stop sooner. That's my thought. 

Heat should not be my problem, there is plenty of cool-down time between runs, but different pad choices may be worth experimenting with. If I go with a more aggressive pad, I know they will lock and slide, but I can try a few an see if something works better than what I have tried.

Increasing the friction of the glove against the metal will also stop the rod sooner with the same amount of pressure squeezing it.  The key is finding a higher friction pad that doesn't need a bunch of heat to start working and still modulates well (vs being grabby and hard to control). 

FWIW, I love the Carbotech AX6s I have on the Jeep as a street pad (and that thing has tiny brakes, single piston calipers and 11" rotors to stop 4300 lbs).  They have tons of stopping power, work fine cold, are easy to modulate, etc.  But they're expensive, very dusty, and occasionally a little loud under light braking. They take heat well enough that it took some work to get them hot enough for bedding on the street, and I'm sure they'd be fine for AutoX. 

In reply to pinchvalve (Forum Supporter) :

Are you not running ABS?

Heat should not be your problem, but this

On Run #3 I stand on the brakes hard and the car is slowing, but not enough and I understeer through the corner. No lockup, no ABS, just not enough whoa. 

says overheated pads. Did you ever bed them properly? If you're street driving the car, it might be worth bedding them again. I've had glazed good pads behave like this, they came right back after a bedding session.

Yeah this sounds like the pad being out of its correct temperature range one way or another, or more likely, glazed pads to me. No remotely modern car with properly bedded pads in their correct temperature range should have any problem activating ABS.

I can't honestly say that I  have ever been able to activate ABS outside of snow and ice. In fact, I will go out today and make sure ABS is working since there is snow everywhere. : )

I assume they were bedded correctly, but that was a while ago and they are street-driven. I also got new rotors last season and may not have done anything to bed the pads. Is this something that I can see? I may just go with a new set of pads, properly bed them and see what happens before buying anything more expensive.