I was doing some math on brake systems, and guess what. I've come to the conclusion that most BBKs that only mod the front trade short term capacity for anything resembling a sane amount of rear brake bias.

Back of the envelope calculations show that your average 4 pot floating caliper added to the front with bigger rotors make the brake bias over 80% to the fronts.

This means that the fronts are doing much more work than they need to, and that the rears aren't doing much and could have the rear lines pretty much plugged with little differency to braking.

After I did the calcs I now understand why the prototype BBK that European Car tested on a Mini didn't shorten stopping distances.

Now, pretty much every guy I've talked to (including me) has like the improvement on the track that a front only BBK gives. But I'm wornding how much better a BBK with proper bias would do....

Any thoughts on this one?

Matt

 

It would be be better if brake bias ratio was preserved/maintained.

So add a modest brake upgrade for the rear- see TCE for rear brake upgrades to compliment a big brake kit for the fronts.

Or add some better brake pads to the rear to replace and stock pads- helps a little, better than plain stock rear brakes.

Run lighter weight rims on the rears purposely????

Try contacting Todd at TCE brakes and ask him what to do.

 

I was asking more because I haven't heard anyone ask about this. But even the larger rear rotor in the back is swamped by the increased area of multi piston front calipers.

In thinking about this, I'm wondering how the Ireland Engineering kit is that uses the Al two piston fixed PBR calipers from the new Mustang. The piston area front to rear is better matched, as are the rotor sizes. And it has a rear parking brake.

I haven't been satisfied with any answer I've gotten from any brake vendor on this one. And I've talked to a few.

Matt

But you're correct, the rear kit does make it better. To really nail the numbers I have to do some more measuring to make sure I have all the dimensions accurate.

 

I guess the better question is why aren't there options for full brake upgrade kits- not just the front but both front and rear brakes?

Is it the cost for a full set?
Is it that brake bias does suffer but not enough to make it worthwhile economically to market and sell a full four brake upgrade?

It's already pretty expensive to get one of the name brand big brake kits.

Then there is how much performance difference would it make to improve brake bias?

How much better would that help not only brake stopping distance but also resistance to brake fade.

Finally if there was a four brake upgrade how much would it affect weight, brake caliper or rotor clearance for wheel sizes, or other factors.

See
http://www.stoptech.com/tech_info/wp...formance.shtml

Factors that will increase front bias:
Increased front rotor diameter
Increased front brake pad coefficient of friction
Increased front caliper piston diameter(s)
Decreased rear rotor diameter
Decreased rear brake pad coefficient of friction
Decreased rear caliper piston diameter(s)
Lower center of gravity
More weight on rear axle
Less weight on front axle
Less sticky tires (lower deceleration limit)

Factors that will increase rear bias:
Increased rear rotor diameter
Increased rear brake pad coefficient of friction
Increased rear caliper piston diameter(s)
Decreased front rotor diameter
Decreased front brake pad coefficient of friction
Decreased front caliper piston diameter(s)
Higher center of gravity
Less weight on rear axle
More weight on front axle
More sticky tires (higher deceleration limit)

 

If the Mini translates to a Porsche 944 turbo, your reasoning works. A nice track upgrade for the p-car are big blacks from a 928 with about a 25% increase in pad area with a larger rotor and just changing out the 944 bias valve for the 928 valve(more rear bias). Def more rear braking afa pad wear and brake feel would indicate. I didn't do the math, just followed the proverbial conventional wisdom.

 

up the front rotor diameter, and the piston area significantly. The only two current solutions I know of for increasing rear bias via geometry is the TCE rear kit and the Ireland Engineering kit that uses the PBR calipers.

Matt

 

BBKs are driven by sales to poseurs, i.e., enthusiasts who will never approach the limits of the car or self. Thus, the brake bias issue is moot.
Put brake temperature paint on the calipers and rotors. Know the temperatures and match the pads accordingly. Done correctly you’ll see the same heat range front and rear. With the AP kit and my pad selection (chosen for my braking style) I see approximately 950 deg F.

 

After looking over the stoptech link I mentioned-
Trying to tune brake bias has so many variables. Realistically, an elusive moving target partly dependent on driving style and road conditions.

What might work on the street would not apply on the track. Gladly we have different brake pads to choose from in some cases.

Big Brake Kits are easily seen with larger rim diameters so it becomes eye candy as bigger is better with a measure of bling. Similar to exhausts and just about every visually appreciable mod you can do. As mentioned sales are healthy based on appearance without regard to optimal function. Heck, isn't that true of the oversized wheels we see all the time?

 

What about the Tarox setup with the rear kit? Any thoughts on performance there?

 

This is a comment from Chad regarding the DT BBK v. the OE set up:
The bias stays the same because we do not change any of the valve in the ABS or anything like that. the bias is controlled buy how small the hole is that your pushing the fluid thru. some other things may change a bit because of caliper bore and such, but never enough to notice. you still have the same pressure front to back as you did before.

 

If I may...and I will; what a bunch of questionable information. From a lot of sources here. Cone on guys we can do better than this.

Let's begin with "stopping distance". If the sole purpose of the BBK purchase is shorter distance they you may well be disappointed. If you want minimal distance you need more tire grip not necessarily more brake. The max road torque will always be limited to the adhesion of tire to surface. How you get there is what defines the BBK.

The bigger benefit of the BBK does not come from distance but repeatability over oem. More continued stopping without fade and with improved pedal feel and response. That being said, those who purchase sticky tires can easily begin to tax the oem system both in demands for torque and in thermal capacity.

Fitting alternate calipers to a car does not necessarily mean that the bias is changed for the good or the bad. All four pot (and more) for the MINI are also not floating, they are "fixed". This means the caliper is bolted solidly to the spindle. That's part of the responsiveness one gets.

Bias changes are related to three things; piston area, rotor diameter and pad coefficient of friction only. Static bias is the normal method one refers to bias. This has nothing to do with pressures, line sizes or fluid transfer. It's simply a mathematical equation based upon the parts used on each end of the car.

Dynamic bias is what happens when you brake hard (on track) and have to account for weight shift. In track use (especially on a short wheel based car) more front bias is normal- you won't get the rears to do anything anyhow. If Matt wants to change to "proper" bias and add a 12" rear rotor for example his braking distance won't change enough to measure- what's going to change is the ABS will want to kick in to prevent the rears from locking up.

What's "proper" for my car? That depends on a LOT of things: wheel base, tire rating, current static bias and how you drive. You want more on track braking? For a MINI add more front pad. And in all honesty you won't know 5% one way or the other. Under hard braking 80/20 sounds about right. Maybe even less rear. What are you going to do with a 60/40 split? Answer: skid the rear tires. If you've missed it, the MINI tends to carry the inside rear tires trail braking into a corner. A sure sign that more rear brake is all but useless.

So why then does TCE sell a rear kit? Two answers- one is matching front and rear appearance. No bull, straight up; customers want a complete look. Ok but you said there are no benefits and possible down falls, what gives? The rear kit should be fit only on those cars with 13" front kits for much the reasons explained above. But for track use the larger rear rotor also adds mass. Mass prevents pad fade and controls overheating. All good for the track user.

Ok, but if you add the bigger rear rotors you said the need is minimal due to the front kits doing much of the work on the track. Why bother? With the rear rotor being larger you can run a lower Cf pad out back. No need to buy track pads for the rear. To be honest anyone buying "performance pads" for the rear of a MINI is wasting far more money on that than they would be on a rear rotor kit. But if you're not running a rear kit it may prove the only way to control temps without having the pad fade.

I'd like to think that most kit suppliers, builders and vendors here and anywhere all understand this stuff- and like me who all but refused to sell a customer a rear kit to go with his 11.75 Wilwood kit....but many do not. That's ok, I don't understand intercoolers and strut kits either. My best advise is to research your purchase and determine what makes best sense to you and your needs.

 

You had to get me started didn't you...LOL!

Ok, you got the big-*** rear rotors in place now and all those notes on the bar napkin are making sense. (sorry, I'm having a beer) Now you want to see how all this really shakes out under real world conditions. You drive to your favorite road and come back disappointed. You still can't brake from 60 to 35 short of the cross walk into the school zone. What happened??

As you add more rear brake your gains are exploited more at lower speeds. Huh, we're talking slowing not going Todd. Yes but slower speeds necessitate lower pressures for braking and that means all four wheels are slowing the car. Ah-hah! You said pressure didn't matter! Well yes it does, but not when computing conventional bias. Dynamic bias is controlled by a proportioning valve and EBD systems on newer cars. (EBD is a bandaid life saver for a lot of poorly engineered kits I bet, but that's another story)

When you slow down from 35 to 0 you use low pressure. Low pressure means all four wheels slow the car. But when you slow from 80 to 30 on the freeway the rears don't do much at all! No way dude. Yes way dude. As front torque is increased from the resultant use of the middle pedal less pressure is delivered to the rears. A prop valve works like this: more pressure in = less pressure out. So while 60/40 looked good on the bar napkin the 80/20 is net far more realistic under hard braking.

On the other hand I know personally of a number of kits that truly have poor design and nearly 80/20 splits before anything happens. The work is nearly all being done up front with the rears along for the ride. And pedal feel is anything but inspiring. Most of this comes from way too much piston area. I'll go on record as saying that if you have more than 3.5" of piston area on a MINI up front you have a poorly built kit. No offense to anyone but the math is the math here. The reality is that the MINI is an exceptionally easy car to put brakes on and even if the package is screwed up it works out ok.

***I'm in a rambling mood for now. Waiting on the wife to go to dinner. If she shows up I'll have to bolt.

 

wow interesting post todd thanks!

 

She's home...gotta run.

 

Great post Todd. I'm in charge of the braking system of our Universities Formula SAE car and I see the same thing. Even with a CG of 8", our 40/60 weight distribution turns into 50/50 at 1.5g of braking force. I can imagine how much front bias the MINI will need during high speed braking. It's a touchy science and very tough to get right.

 

Todd and Kapps, good posts! What got me thinking about this is that the first StopTec BBK for the Mini was used on a European Car mag build up mule and increased braking distances. But since I don't have the proportioning valve response curve, I couldn't model the straight line braking response at all. And yes, braking while turning makes the math more difficult to do well, and there's no proportioning valve for side to side transfers (but the DSC system can play this game, and does!)

Any you're 100% correct in saying that a bad BBK will just rely on the ABS system to take care of the fact that the system isn't well balanced, in either the static or dynamic case.

Sorry to say but Chads answer is just wrong. What doesn't change is line pressure, but bias sure does as the piston area, and the pad radius is changed by his BBK. Like Todd says, the math is the math.

Matt

 

In the end here the MINI does not present and significant challenges to produce a brake kit that works. With weight transfer here on such a short wheel based car you can over bias the fronts and still get away with it.

On the other hand you can add more rear brake and the on board systems can counter it. *Although not an ideal situation if you don't like abs pulsing.

Now before someone claims that the BBK is just not a good idea and "all you need are ss hoses, fluid and some good brake pads for the track" remember that those elements provide the same basis as the BBK.

SS hoses to firm up the pedal.
Fluid to control boiling point.
Brake pads with a higher Cf to provide more bite.

Now...those higher Cf race pads do what? Yup, they shift more front bias into the car! Because on the track that's where the gains are going to be!

So how does the BBK differ and be better?? I does much the same work above but with less effort and taxing of the parts. Where you need to run "mondo pad" on the oem rotors to make them work you can run "street pad" on the BBK because you have other things working for you. Like leverage and mass. They pads become an equal part of what makes the kit work not the ONLY part. Add to that the improved response and release of a fixed mount caliper and you have a much enhanced braking system that's far more efficient.

...now where did I put that bar napkin?

 

Good posts, Todd. I totally agree with you on every point.

 

i've enjoyed this thread. i have had conversations w/ todd on this subject. i have tce's 13" bbk up front and tce rotors/hats w/ relocated stock calipers in the rear. i used stock rear pads while using bp20 pads up front. no problems here. the bp20's couldn't last through a track weekend for me so i switched to poly h's. i didn't like the way the car felt w/ stock rear pads. i switched to porterfield R4 in the rear. much better. temps on front and rear rotors in the 900* range. the car is still a bit unsettled during threshold braking. i'm not the only bbk user experiencing this. i make it work, but it's not a comfortable thing. i added the alta psrs in an attempt to correct the situation......better but still not fun. i've talked to meb about the problem (that he knows well). any ideas here? i thought i'd add my problem to this thread since many brake/handling guru's are participating.....

 

Unsettling under braking how? Does it change with temps?

I think a lot of it is simply the very short wheelbase. Do you want more or less rear?

 

Yes, great thread - just posting to get in on the subscription and info.

mb

 

I have been looking at the TCE 13.1" kit with the rear kit also. That gets me thinking: with all this fuss about front and rear bias, does someone out there make a kit similar to race car systems; that is, a **** on the dash so the driver can adjust brake bias as he pleases? I don't think it would be terribly difficult, just valves to control the maximum pressure of the fluid in the lines. That way the driver could fine tune the bias to prevent issues like unsettled braking instead of having the bias (however proper or incorrect it is) hard-coded into the kit. After all, braking is more about feel than math (though math is still critical).

 

Adjustable bias. A great idea but not quite as cut and dry as you might think.

A real adjustable setup has a **** to move a pedal pivot. Those come only in dual master cylinder designs. (where you can tune bias with bore sizes too)

The other "****" is the popular Proportioning valve. The Bias ****! Here's how they work: input pressure in is reduced to output pressure based on what's called a Knee point- the pressure at which the valve begins to shut down. This means two things: 1. Pressure can only be REDUCED not increased. 2. More pressure IN means less pressure OUT. (go back and re read that) You adjust WHERE it begins to reduce not truly how much.

Under normal driving you get modest pressure to the rear. Under hard braking you get less pressure. Why? Because you can no longer exploit the rear line pressure and turn it into working brake torque. The wheels are not on the ground. Put two passengers in the rear and you'll get great rear braking improvements. EBD works on some of this also but the basics remain the same.

So...let's put in a **** and just add some track pads out back and then we can 'de tune' it to our liking. Ok, that's not a whacky idea but you have to plumb it. Seeing that each rear caliper has it's own feed you now have to tee it before the valve and then de tee it post valve. Now ABS and EBD are all screwy. The computer no longer controls pressures to each rear wheel without effecting the other.

All good ideas but most lead to more complications. I say; tune with pads is the easiest method. You can move bias over 5% one way or the other quite easily.

 

Todd has provided some very insightful information on this thread about brake systems. The engineering behind braking systems and brake balance is pretty interesting.

One of the reasons that I'm a big Stoptech proponent is that they really engineer their calipers to the car to take into account overall brake balance. More importantly they actually test the resulting product of their engineering/math. Very few companies out there actually do controlled testing of their BBKs.

Tuning with brake pads front and rear is the way to go for track duty. Adjustable brake bias stuff is way too complex for street. Things change according to how much speed you are carrying into a corner, front/rear weight distribution etc etc etc. The more adjustment you have the more opportunities you have to screw something up.

 

not a MINI, but an interesting and very relevant test about how a BBK affects stopping distances - click

of note - maintaining proper F/R bias yields shortest distances (duh!), but just goes to show even with the properly engineered Stoptech kit, you're not home free