In that their first kits DIDN'T do well at all, and screwed the pooch, so to speak.

Really, the math on the bias calculation is easy. The hard part is that the car is a very short wheel base, with a relativlly high Cg for a track car, and this leads to large amounts of weight transfer, requiring a lot of reduction in line pressure in the rear under severe braking to prevent lock up.

Unless you're going full race and pulling out the DSC/ABS pump system, we get what we get for proportioning.

For those that are interested in the static bias calculation. At low line pressures, front and rear line pressure is the same. A zeroth order calculation has the bias being determined by the ratio of the piston sizes (area, not diameter) and the working radius (use the center of the piston to start). IF the pad materials are the same, thier co-efficient of friction drops out. Pad area does as well. If you want high accuracy calculation, a bit more goes into it, but this will get you close.

I just measured a R53 front caliper. It has a diameter of 1.887" The Outlaw 2000 (Wilwood Dynalite clone) 4 pot caliper has 4 1.38" diameter pots. That's 213% of the stock caliper piston area. Start moving that out with larger rotors, and the net effect is even more pronounced. These are the basic numbers that got me thinking and why I started the thread.

Matt

I did some digging on the Stop Tech site, and it took a while to figure out, but they call out something called the ST-40 caliper for the mini. It's available with pistons as small as 28mm and 30mm (two of each) for a total piston area of 4.1 in^2 of piston area up to a huge 8.6 sqin of piston area using 40 and 44 mm pots (there are three combos of piston available from StopTech). FWIW, this is 146% to 308% of stock piston area. I"m sure hoping they use the small ones!

 

Actually....in fairness to all the suppliers mentioned there are some errors in these statements.

Piston area is based on the diameter of the single slider as mentioned. However opposed piston area is calculated only using 1/2 the body. Single sliders "pull" the outer pads into place and thus are doing the same work.

Meaning a 4 x 1.375 caliper is but only slightly larger: 2.9 vs 2.8sq" area.
This is for all practical purposes; the same area.

I cannot speak for ST on what they use but the bores listed are paired in the caliper body such as 40/36 or 38/32 and not produced only in square bore designs such as 40/40. Of course nobody matches a 48/30 when you'd achieve the same with more closer numbers. I have the notes on what they use somewhere but I can tell you they are (like the TCE 13s) not this large. With the additional rotor it's not needed.

 

I just did the math, and you're right!

Matt

 

Not always, but it IS what I do....lol

 

they offer three piston combos for the ST-40 caliper. With Todd's new math , they come out to

73% of stock
147% of stock
153% of stock

Matt

 

todd: by unsettled, i mean that the rear end feels light and the car doesn't want to continue in a straight line. i'm sure that the rear end IS light w/ 13" rotors, dyna-lite calipers and poly h's. the situation does not change w/ temperature. it is the same from post warm up lap to final lap.

short wheelbase may be the problem.....or some other vehicle dynamic that i have yet to comprehend. i think i have about the right bias w/ the R4 rear pads. the car feels better w/ these vs. the stock rears, but still uneasy at the limits. i'm not getting premature lock up at either end.

i absolutely love going deep into braking zones. my low hp car makes up a lot of ground here. i have no problem w/ the performance of my brake/tire combo. i just wish it felt more solid.

the last time i had an instructor ride w/ me, he described my car as "a set of brakes w/ a car attached". i forewarned him about the "unsettled" feeling. he later thanked me for the warning.

maybe i have it right. i still feel that i'm missing something.

 

spring rates and alignment have as much to do with threshold braking as brake bias

 

i haven't addressed spring rates, yet. maybe something there. my alignment specs aren't anything out of the ordinary for a track-prepped car. i'm not the only one who has this problem.....

 

Tighten up the rear shock rebound? Install a beefier front anti-roll bar (trail braking) or some slightly heavier front springs?

 

I recall reading something about rear toe settings and stiff rear spring rate having tendencies to cause the light rear end (the weight shift + stiff rear spring forcing full extension)

 

blast from the past bump.

dr. obnxs, or anyone, do you have the piston size for the jcw 1st gen brakes?

to me and i'm sure many others, the stock r53 feels like it has a lot of rear bias. And as toddtce said, a little more up front wouldn't be the end of the world.

compared to my old car (subaru), the pedal feel and bias are actually really freaking nice. Meaning you push the pedal and the car slows down and stays pretty flat, as opposed to the subaru where you push the pedal, check your watch, call home, sip your coffee, and then feel the calipers start to squeeze, and the car dives like crazy. so it is tempting to leave it alone, but that's no fun. i am thinking about boxster calipers.

unfortunately i just bought new stoptech pads for the stock calipers. so maybe a summer project.

- andrew

 

The first gen JCW stuff was the same piston size caliper only fit to a larger diameter rotor was all.

Correct me if I'm wrong.

 

thanks todd. between here and nasioc i've learned a lot about brakes over the years from you.

edit: saw somewhere it was 54mm for JCW and 48mm for standard MCS

- andrew

 

Todd: Great information to all. Beautiful write-up and explanations.

Just my $0.02:
This talk of adjusting bias is sort of relavent, but the MINI uses EBC. The ABS unit does all of the proportioning. WSS (wheel speed sensors) tell the ECU what each corner is doing and it adjusts the pressure to those corners that are not performing as the algorithm has told it the car should perform. I am assuming that the car also has a YRS (yaw-rate sensor) and an SAS (steering angle sensor) since it has ATC/DSC. All this electronic feldercarb(hah that dates me as either a new syfy geek or a scifi geek from the late 70's-I'm both) gets in the way of mechanical adjustments to performance. The ECU just compensates more for the changes you make with improvements. The BEST way to improve stopping distance is to get REALLLLLLLLLY STICKY tires.
The main benefit to a BBK is fade reduction and consistent stopping distances over time; like Todd already stated.

PS. havent been on the boards in a while, saw this at the top and had to comment. Love a good brake discussion.

 

,said Inspector Columbo

The absolute worst thing you can do during braking is lock the rears, especially during cornering.

 

Great info being shared.
Thanks

 

The only thing I'll add is that while the car has these features (and exactly how they influence every aspect of braking I'll never be certain) that adding too much rear brake will remain a problem.

The reason being; premature lock up or ABS moment. Supposing one were to double the piston area in the back; you would have an huge increase in brake pressure. My reasoning has it you'll engage the abs system sooner. Reducing pressure to the rears would come in part from abs and 'maybe' the EBC can handle it also but there may be limitations to how much pressure reduction is possible. And with twice the piston area you'd have to drop pressure a lot I think to counter that.

All interesting thoughts and I'm too lazy to really explore it all in depth. But I'll cover my butt and simply work within my comfort zone of design and play it safe!

 

Speaking of too much rear brake bias.....

Anyone ever run the new R56 JCW rear 280mm brakes?

Is it a full new caliper, or just the new rotor and caliper carrier/bracket to space out the caliper?

Also, is the width the same on that new 280mm rear rotor as stock R53 or R56?

If it's the same caliper, then a nice rear big brake upgrade would just be the rotor and the bracket.

Only a good idea with a front BBK and even then maybe not.

VERY much a bad idea with stock fronts. Can't stress that enough. Don't do it.


- Andrew

 

Check with Jan from RMW. He has the new new frt Brembo and new bigger rear JCW caliper and rotors on his car.

 

 

Be interesting to know the OD of that rotor and the piston size of the caliper.

A vented rear would be nice perhaps but for the masses it's gross overkill.

Properly paired with a larger front set up it's a fine deal. Like any other re-fit it's not going to have a huge impact on distance but the added mass makes them a more efficient package.

 

OD is 280mm, no clue on piston size. Not even sure what standard MCS rear piston size is.

- Andrew

 

If you turn off your DSC does all you mention turn off also ?

 

No...

 

No, it is a function of the ABS. Pull the ABS fuse on the other hand...though not recommended.