# So how is the Brembo GT Junior Kit?



## TT Tom TT (Oct 9, 2015)

I've just bought myself a Brembo GT Junior set second hand, calipers refurbed with new dust seals, pins all nice and free, braided hoses, EBC yellow stuff new revision pads, turbosmart 323mm drilled grooved discs with carriers.

Feeling pleased with my purchase, I just need some R600 brake fluid (suggested to me by the track guys on here) to complete the front brake system overhaul.

Is this brake kit typically well received?


----------



## Jay-225 (Mar 7, 2012)

They are better than the Boxters S rears put it that way :lol:

They are very similar to the Leon Cupra R Brembos using the same size pistons, pads and discs.... I have the LCR Brembos fitted to mine but with DS2500 pads and am very happy with them , pedal feel is excellent and they have a lot more braking force compared to the standard setup  In the future i may upgrade to 330mm disc but absolutely fine as it is and am looking forward to giving them a hammering on the Nürburgring next month


----------



## jamman (May 6, 2002)

Did someone say Boxter Rears..... :lol:

Good quality cheap end brake upgrade which will be made or ruined by your pad choice.

Id make a suggestion but best wait for Max :wink:


----------



## TT Tom TT (Oct 9, 2015)

Jay-225 said:


> They are better than the Boxters S rears put it that way :lol:
> 
> They are very similar to the Leon Cupra R Brembos using the same size pistons, pads and discs.... I have the LCR Brembos fitted to mine but with DS2500 pads and am very happy with them , pedal feel is excellent and they have a lot more braking force compared to the standard setup  In the future i may upgrade to 330mm disc but absolutely fine as it is and am looking forward to giving them a hammering on the Nürburgring next month


That's good to know, haha - from what I've heard the Ferodo's that you have are the best money can buy although I've heard that EBC yellowstuff despite having awful reviews in the past have been revised to be very good pads in the last couple of years I believe. I have just the 323mm disc at the moment, I'm looking forward to the upgrade - I just really hope they're what I'm expecting.



jamman said:


> Did someone say Boxter Rears..... :lol:
> 
> Good quality cheap end brake upgrade which will be made or ruined by your pad choice.
> 
> Id make a suggestion but best wait for Max :wink:


It would have been a very cheap brake upgrade for me, calipers I could have had for 90 and carrier conversion kit for 50, sell my own calipers for probably 60 or so.

I still maintain the fact that a caliper with 4 x 28mm pots will provide improved braking force over 1 large 54mm pot equipped caliper.

I suppose I could mathematically check the clamping force

Porsche rear caliper = (3.14*(14^2))*4 --> 2461.76mmsquared of clamping force
Audi TT stock caliper = 3.14*(27^2) --> 2289.06mmsquared of clamping force

So yes the Porsche rear caliper offers around 8% better breaking. Probably not worth it but to say that they are bad because they are 'rears' seems like small-minded behaviour... Evidently it is as I have just mathematically proven them to be an improvement over stock when they are not perceived as such.

On a serious note, yes I am a big fan of Max and the wealth of information he has to offer - the guy knows a hell of a lot about these cars so I would be fool not to consider his advise - he knows a lot more than me and has busted quite a few myths, created his own ideas and backed up ideas / disproven other ideas with hard data which is exactly what knowledgeable people / experts do. Yes I would consider Max an expert in this field.


----------



## jamman (May 6, 2002)

Max is without doubt an expert in many TT related things I'm not doubting that my dig was more at your instant dismissal of Yellow's advice the other day.

Max won't recommend DS2500 (or hasn't in the past) but I run them and like them. :lol:

I'm guessing "someone" PMed you the calculations because from your normal questions you ask you are not at that level. :wink:


----------



## kane (Jul 26, 2010)

How much are these kits?
I pieced together a "kit" with used boxster fronts, new ebc pads, ss lines, grooved&drillet front and rear discs, adapters... And including shipping and tax it was almost £1000. Is that good or bad?


----------



## Beunhaas (May 14, 2014)

kane said:


> How much are these kits?
> I pieced together a "kit" with used boxster fronts, new ebc pads, ss lines, grooved&drillet front and rear discs, adapters... And including shipping and tax it was almost £1000. Is that good or bad?


Terrible.
For 1000 pounds you can fix 6 pot cayenne calipers with 2 piece 350mm rotors :lol:


----------



## TT Tom TT (Oct 9, 2015)

jamman said:


> Max is without doubt an expert in many TT related things I'm not doubting that my dig was more at your instant dismissal of Yellow's advice the other day.
> 
> Max won't recommend DS2500 (or hasn't in the past) but I run them and like them. :lol:
> 
> I'm guessing "someone" PMed you the calculations because from your normal questions you ask you are not at that level. :wink:


I have mathematical knowledge in spades probably more-so than anyone else in the forum in that aspect, mechanical knowledge not so much but is improving every day hence my constant questions, it's how we learn [smiley=book2.gif].

Yes I will always dismiss someone who says something I know to be factually incorrect which is exactly what happened, I was told that Boxster S 4-pot rears were worse than stock and they are not, fact.



Beunhaas said:


> For 1000 pounds you can fix 6 pot cayenne calipers with 2 piece 350mm rotors :lol:


I paid 450 for the whole assembly, I'm quite pleased ! Coupled with my wider than OEM tyres and much lighter than OEM wheels and 200KG lighter than OEM car weight should make for a fairly good braking car I hope but I shall find out soon enough :mrgreen:


----------



## TT Tom TT (Oct 9, 2015)

EDIT: Double post.


----------



## infidel.uk (Feb 21, 2015)

ive got he leon cupra ones with some compbrake 2 piece rotors and mintex 1144 pads, and i am very impressed, im not sure if they are the same ?

but either way the brembos i have are very good, and there is little to no fade.


----------



## NickG (Aug 15, 2013)

TT Tom TT said:


> jamman said:
> 
> 
> > Max is without doubt an expert in many TT related things I'm not doubting that my dig was more at your instant dismissal of Yellow's advice the other day.
> ...


Well that's not gone well for you Tom your maths might be good, but your theory isn't and putting it to practice isn't. The clamping force in a single pot caliper is still distributed to either side, hence you need to double your calculation for the single pot :wink: (in actual fact I have it in my mind that you only count one side of a caliper when working it out, so actually your single pot caliper would be correct and your 4-pot would need halving... Either way, makes no difference to the outcome... The single pot wins)

I would add to that therefore you still owe Yellow an apology for being an ass when he answered your question correctly. :-*

EDIT: It's Newtons law I believe that is most relevant to this, every action, equal and opposite and all that Jazz


----------



## Von Twinzig (Feb 10, 2014)

When looking at my options I had a big discussion at Autosport with the Brembo guys. 3 of them plus me to be precise. They told me, for what I was planning, (track focussed car) they couldn't/wouldn't recommend their Junior kit. Many reasons, not least of which was they thought the seals would melt. To get what I needed from them put me full square in the AP price bracket, which was not an option. I had to admire their honesty.

VT


----------



## TT Tom TT (Oct 9, 2015)

NickG said:


> TT Tom TT said:
> 
> 
> > jamman said:
> ...


If this is true why on earth do Brembo and other brake manufacturers endeavor to put pots on either side? Seems like an unnecessary expense if this offers no benefit.


----------



## NickG (Aug 15, 2013)

I didn't say it offers no benefit, I said your maths was wrong. Think about it logically, you have a pad on the other side of a sliding caliper setup and the force therefore has to be acting on it.

The benefit of having Pistons either side is having a stiffer braking setup, thereby creating less wasted energy on flex in the system and a more even distribution of force either side (think sticking slider pins and the problem associated with this of uneven wear).

So yeah, no IF required.


----------



## TT Tom TT (Oct 9, 2015)

NickG said:


> I didn't say it offers no benefit, I said your maths was wrong. Think about it logically, you have a pad on the other side of a sliding caliper setup and the force therefore has to be acting on it.
> 
> The benefit of having Pistons either side is having a stiffer braking setup, thereby creating less wasted energy on flex in the system and a more even distribution of force either side (think sticking slider pins and the problem associated with this of uneven wear).
> 
> So yeah, no IF required.


So essentially I've bought some 4 pot brakes made by Brembo which are inferior to 2-pot R32 / V6 brakes based on what you have said... In fact, based on what you've said the R32 upgrade would out perform many expensive brake kits if it holds true [smiley=bomb.gif]...

2x big pots on one side > 4x smaller pots (2 either side) [smiley=bigcry.gif] [smiley=baby.gif]

A little weird considering no one has ever recommended the V6 brakes as a HUGE improvement like a BBK would be.


----------



## NickG (Aug 15, 2013)

No, again, you have to read between the lines, you cannot just look at one figure, piston area, as a measure of the performance of a caliper setup.

As I already mentioned, the stiffness of the 4-pot fixed caliper setup is a big advantage over the 2-pot sliding setup, the pedal feel and feedback will be far greater. You've also got a MUCH lighter setup, the V6 calipers are ludicrously heavy by all accounts.

It's also worth noting Fixed calipers in general are made of aluminium which is able to dissipate heat much quicker/more efficiently, helping to keep brake temps lower.


----------



## TT Tom TT (Oct 9, 2015)

NickG said:


> No, again, you have to read between the lines, you cannot just look at one figure, piston area, as a measure of the performance of a caliper setup.
> 
> As I already mentioned, the stiffness of the 4-pot fixed caliper setup is a big advantage over the 2-pot sliding setup, the pedal feel and feedback will be far greater. You've also got a MUCH lighter setup, the V6 calipers are ludicrously heavy by all accounts.


I just want to make clear that I'm not trying to argue Nick, but I simply have a thirst for knowledge and when half-concepts are thrown at me it irritates me that I'm not getting all the facts to understand something. Now - I understand [smiley=book2.gif]... I don't know, these 323mm discs are HEAVY...


----------



## Madmax199 (Jun 14, 2015)

In advance, I apologize for the long post! Brakes are one aspect of the TT's performance community that I can't quite put my fingers on ... at least when it comes to collective logic. I feel that maybe it's a lack of understanding of how brakes work in general, or the good old monkey-see-monkey-do syndrome, or perhaps even the visual appeal of BBKs that push people to make some very interesting decisions when faced with brake "upgrades". Here is what we're dealing with:

*1) Overall system pressure* 
This is a factor of the master cylinder bore, pedal ratio, line expansion rate, fluid volume, and a few other less important variables.

*2) Caliper clamping force*
This is the maximum clamping force exerted by the caliper when overall pressure (number 1) is applied. It is a factor of overall piston area (total piston bore on one side of the caliper), minus total flex in the assembly.

*3) Pad compound coefficient of friction* 
The level of friction the pad material can achieve when X caliper clamping force is applied. Each pad compound has a different level friction, modulation characteristics, and resistance to fade/temperature. Regardless of clamping loads, if the material has low friction characteristics, it will stop poorly. This is no different then the clutch material found on various stage clutch upgrades. You can have all the clamping you want the friction material will play THE biggest role because that's what is making contact with the discs to slow them down.

*4) Brake bias*
Each axle puts out some work when it comes to braking. The factory decides (through choice of components and pressure limiting valves in the rear) how much front bias they want. It is usually conservative from the factory to promote front lock-up first (car understeers when this happens) vs rear lock-up (car oversteers and is harder to recover by a novice ... aka a liability). Must note that in the TT, it is ridiculously conservative and front biased -- probably one of the worst I've seen in a "sporty" car.

Anyone else find something wrong with increasing front bias with a car that is distributed with such ridiculous front bias? I do! 









*5) Mechanical leverage*
Mechanical leverage on the caliper is its distance from the center of the disc. The further away that the caliper placement is from the center of the disc, the more leverage it has. Therefore 'X' caliper clamping force will have 'Y' braking TQ generated at 4" away from the hub -- move that same caliper placement to 6" away from the hub (obviously larger diameter discs needed), and the braking force will be 'Y' + 2 as a result of the increased leverage.

All 5 of these factors combine to give what is referred to as the *Total Braking Torque* . The braking torque is what should concern people, not just one of the factors alone, and definitely not the "number of pots" that people tend to think makes the real difference.

You can increase your braking performance by improving each one of the factors mentioned (tyres, as a component outside of the braking system, also plays a big role). What I see a trend with, in the TT community , is that we get the priority wrong when it comes to which factor plays a more important role -- or better yet, which one will have the biggest impact in your real-life stopping distance. From experience playing with brakes and depending on them to come back home in one piece with the car, besides the tyres, pad compound makes the most difference. DO NOT, DO NOT, DO NOT, underestimate the power of a pad coefficient of friction. It is litterally just like tyre compound, you can spend thousands on fancy suspension and only gain a fraction of what a good set of rubber would do to the car's handling. Pads have the same effect on brake systems, what could seem totally anemic on a budget set of pad, could turn into "superb" with the proper pad compound selection.

You guys get way too caught up on just the caliper clamping force. More often then not, you see thousands spent in front caliper "upgrades" then it's all mixed with shitty pad selection, ridiculous brake bias, and old fluid full of water and moisture contamination (fluid should be flushed and refreshed every year ideally, or at least every two years at the most). So, let's focus on the caliper aspect that you guys love so much... it's like the gold chain that people wear to show off their fashion sense... the bigger it is, the more fashionable you are.* It's the overal piston area that counts*, ie. total piston surface area on one side of the caliper. As a result, the caliper design don't affect things much (obviously a floating design will be at a flex coefficient disadvantage over a fixed one of the same piston surface area). A caliper, regardless of design, have pads on both sides. Therefore, the clamping force of a single piston of X diameter (floating) is the same as the clamping force of a fixed caliper with one piston with the same X diameter on each side. The clamping force is not multiplied because of the number of pots, it's the total piston surface area on one side of each caliper that really counts. You increase that parameter, and the caliper clamping force will increase proportionally to the percentage increased over the baseline. It's really that simple (take notice that I said the caliper clamping force and not the Total Braking TQ).

In short, Tom should have calculated the piston area of the stock TT and find it's clamping force. Then calculate each option discussed here to find their total piston area (one side of the caliper), this would allow to gauge and compare the clamping force of each aftermarket options and find out what is what in relation to OEM. Again, caliper clamping force is only one aspect of the total braking force, so don't get too caught up on one of the lesser aspect of what will make your stopping distance improve.

For example, I haven't changed my calipers yet for a reason -- and we all know I tend to go all out with mods that increase performance. Why? Because I can lock all 4 wheels (with very large and grippy slicks) very easily if not careful. This tells you what kind of total braking TQ the stock system can generate when optimized. When I finally do upgrade, it will be first in the rear to improve the horrible factory bias and make the rear do some work too. Then I will move to the front to improve weight (downgrading to 309.6mm discs from 312mm OEM) and using a willwood 4 piston caliper. Knowing what I know, it's rotational and unsprung weight, as well as rear brake improvement that really interests me in a TT's braking system. So take this post for what it is... a reminder that there is more to it than meets the eye!

Now to put some numbers behind the talking:

The stock floating single caliper piston has a diameter of 54mm. That gives you a piston area of 2461x2 (floating design) = 4922mm square.

Your Brembo GT ave 4 baby piston of 28mm each (fixed design). That gives you a piston area of 4578mm square.

Therefore there is no point in complicating things with calculating the clamping force (which is a constant anyway). Since we know total piston area on each, the GTs will be a net loss in caliper clamping force (Nick was onto something there with his posts). I'm not sure where they will be mounted in relation to the stock calipers (brackets can alter that) and do not know your disc diameter. Therefore the mechanical leverage on the disc is unknown. However based on "pots" this swap will reduce total braking force and slightly improve pedal feel and resistance to flex. Not the best move IMO!

Piston area calculated for stock single piston floating caliper (this must be multiplied by 2 due to the floating design). 









Piston area calculated for 4 piston Brembo GT with fixed caliper design









Literature in the calculator that proves the floating design piston area must be multiplied by two (or simply calculate piston area on one side which is the old school way that I was thought).


----------



## Madmax199 (Jun 14, 2015)

jamman said:


> Max won't recommend DS2500 (or hasn't in the past) but I run them and like them. :lol:


James, I don't recommend them for a valid reason. Pads are like tyres, you don't have a full scope until you try the very best and the most crappy widow makers. When I say that DS2500, or Hawk HP+, or Mintex 1144, or EBC yellow are subpar, it's in relation to better available compounds in their segment. I have tried them all to come to the conclusion -- it's not that they can't put a smile on someone face if their perspective is a set to parts store specials, they will. However, if the entire segment of dual duty street/track compound is considered, these are the bottom of the barrel.


----------



## jamman (May 6, 2002)

I know mate I remember the post a while back my umbridge was more with Toms treatment of Yellow and a newbie posting asking for info hence the tone of my post.


----------



## NickG (Aug 15, 2013)

Totally agree Max, I've been using the following guide to all things brakes and posted this the other day to sure the knowledge, but I'll post it again...

http://www.stoptech.com/technical-support/technical-white-papers/brake-system-and-upgrade-selection

As you say, and it really highlighted it to me, the CF is essentially the easiest thing to change and makes such a huge difference to the overall torque. That's why I'm looking at Carbotechs or F2R/F3R/F4R as a next pad when the time comes.

I THINK the 28mm 4pots Tom was discussing are the boxster rear calipers, whereas I believe the Brembo GT Juniors have bigger Pistons than this (at least I hope they do!!).

I'm also glad my theory regarding the use of only one side of the calliper to calculate the clamping force appears to be correct! :lol:


----------



## jamman (May 6, 2002)

Nick this just confirms to me that the Cowbit/Spalding posse are very clever :wink:


----------



## NickG (Aug 15, 2013)

jamman said:


> Nick this just confirms to me that the Cowbit/Spalding posse are very clever :wink:


 :lol: I can't argue with that!


----------



## Von Twinzig (Feb 10, 2014)

Max, you don't mention increasing the thermal mass of the disc. Back in my aircooled Porsche days and because of the interchangeablilty of parts, the uniformed just go to the next model up, take the calipers, get an adaptor and stick the things on without upgrading the disc. Then they complain something didn't work, without realising they are just sticking more heat into the game without addressing dissipation. Boxster upgrade being a common example.

VT


----------



## KarlD (Jul 23, 2015)

I love Toms posts.


----------



## Madmax199 (Jun 14, 2015)

Von Twinzig said:


> Max, you don't mention increasing the thermal mass of the disc. Back in my aircooled Porsche days and because of the interchangeablilty of parts, the uniformed just go to the next model up, take the calipers, get an adaptor and stick the things on without upgrading the disc. Then they complain something didn't work, without realising they are just sticking more heat into the game without addressing dissipation. Boxster upgrade being a common example.
> 
> VT


Heat dissipation, thermal energy were not mentioned because it only affect things when the termal energy ceiling is reached. If you take two discs of with two different thermal mass (same caliper leverage and clamping force), and measure their braking distance from cold, it will be identical with the same pad. The difference will only be apparent in real life near the limit the disc thermal curve (bell shaped). It's the same thing as provided cooling, it is a variable that will have a big impact near the limit of your thermal curve.

Another issue that is very common is mismatching rotor/disc thickness for the chosen caliper. This was not mentioned too because it's a variable that I assume is worked out by person considering the swap. Each caliper have a specific piston stroke, therefore designed to work with a very specific disc thickness range. Fall out the workable range for the pison stroke and you will have very poor braking. So slapping random caliper size on OEM thickness disc can also lead to disastrous performance.


----------



## jamman (May 6, 2002)

Max because I'm on holiday and slightly the worse for wear after too many desporados last night and can't seem to find the post what was the name of the pads you were recommending the other day/week/month ?


----------



## Madmax199 (Jun 14, 2015)

James, the DS2500 is the compound I have issues with (only 0.42 peak coefficient friction). It is somewhat consistent with an almost flat friction curve, and that's why many people live with it. But don't expect it to give you anything more once that initial plateau is reached. If you look at the comparison chart on their website, the DS300 compound (0.51 peak CF) starts working earlier and retain a much higher coefficient of friction throughout the temperature range. So even switching to the DS3000 compound as a dual duty pad (if you want to remain brand loyal), it would be a big improvement overall. 
http://www.ferodoracing.com/products/ca ... ds/ds2500/

My favorite dual duty pad to date is the low end of the XP line of Carbotech. The XP8 or XP10 (0.56 and 0.60 peak CF respectively) will be leap years ahead of the 2500 compound (0.42 peak CF) of Ferodo and still totally drivable every day. Heck, even the AX6 or the street pad of Carbotech have better peak coefficient of friction than the 2500.


----------



## jamman (May 6, 2002)

Thank you I will have a Google and check them out. [smiley=cheers.gif]

EDIT

@Max Was going to PM but other peeps might be interested so...

1. Ok I've googled and eBay is coming up as a source are these faked a lot

2. Any recommendations on where to buy

3. If you want to save me time since I'm on holiday/drinking a Desperado (580ml very nice) and this searching interrupts my flow I need pads for Porsche 993TT or 2002 S6 I think also fits

4. Don't be telling me my Big Reds are crap because I got them because they are pretty ;-)

5. Ta muchly xx


----------



## Madmax199 (Jun 14, 2015)

I wouldn't buy off ebay (unless it's an official Carbotech dealer that advertises through the fleebay). I would contact Carbotech UK (see link to their website below) -- I also believe someone (maybe Nick) found a place that retails them at a decent price, so maybe they'll chim in. Sorry, but my parts vendor/sourcing knowledge is laughable when it comes to your side of the pond. :?
http://www.carbotech-europe.com


----------



## NickG (Aug 15, 2013)

Cambridge Motorsport!

http://www.cambridgemotorsport.com

Or Carbotech Europe direct I believe!


----------



## jamman (May 6, 2002)

Thank you Max/Nick


----------



## Beunhaas (May 14, 2014)

Madmax199 said:


> Another issue that is very common is mismatching rotor/disc thickness for the chosen caliper. This was not mentioned too because it's a variable that I assume is worked out by person considering the swap. Each caliper have a specific piston stroke, therefore designed to work with a very specific disc thickness range. Fall out the workable range for the pison stroke and you will have very poor braking. So slapping random caliper size on OEM thickness disc can also lead to disastrous performance.


Not to be the nerd but...

I ran 30 mm thick rotor calipers on the oem 25mm rotors and it doesnt matter.

The pistons are long enough to stretch the gap.

As soon as the piston is pushing the pad to the disc everything is the same as an 'matching' caliper and rotor. Releasing the pedal and the o ring pulls the piston a fraction back so the pad doesnt ride the rotor. The caliper/piston doesnt have a memorry or something to know how far to retract.

Now where in the proces do you think poor braking is caused?


----------



## Madmax199 (Jun 14, 2015)

Beunhaas said:


> Madmax199 said:
> 
> 
> > Another issue that is very common is mismatching rotor/disc thickness for the chosen caliper. This was not mentioned too because it's a variable that I assume is worked out by person considering the swap. Each caliper have a specific piston stroke, therefore designed to work with a very specific disc thickness range. Fall out the workable range for the pison stroke and you will have very poor braking. So slapping random caliper size on OEM thickness disc can also lead to disastrous performance.
> ...


So, let me get my head around the deduction first. Since you used a setup that worked (within workable range), you're concluding that in general any caliper to rotor thickness is acceptable and will work the same? I'm trying to get you to answer your own question so I don't have to. Lol


----------



## Beunhaas (May 14, 2014)

Madmax199 said:


> Beunhaas said:
> 
> 
> > Madmax199 said:
> ...


My personal experience is only to be the first part of my argument why that caliper rotor thickness is not affecting braking performance.

Now on to my theoretical part. Whats wrong with that?
A piston wil always excert the same force no matter its stroke.
A piston will always retract the same ammount due to the o ring inside the calipers.

So what part makes you think braking performance is affected ?


----------



## Madmax199 (Jun 14, 2015)

^^ The part that there could be a combo with a fully extented piston and the pad is not maintaining full contact on the disc. :lol:


----------



## NickG (Aug 15, 2013)

Okay so the problem would be with discs that are smaller than the discs originally used with the caliper?

Boxster fronts are 24mm, hence just about fit the standard TT 25mm wide disc. Shouldn't be any issues there?


----------



## jamman (May 6, 2002)

Surely we can agree that there are certain circumstances where one of you is right and the other wrong and visa versa.

Max I'm actually quite surprised you are taking this "can't break the rules" stance as it tends to go against your ethos.


----------



## Beunhaas (May 14, 2014)

Madmax199 said:


> ^^ The part that there could be a combo with a fully extented piston and the pad is not maintaining full contact on the disc. :lol:


Yes and 40mm thick rotors wont fit tt rear calipers. Captain obvious.

I think you are dodging the discussion. :lol:


----------



## Madmax199 (Jun 14, 2015)

NickG said:


> Okay so the problem would be with discs that are smaller than the discs originally used with the caliper?
> 
> Boxster fronts are 24mm, hence just about fit the standard TT 25mm wide disc. Shouldn't be any issues there?


Yes, when slapping random caliper on disc thickness they were not meant to work with, you throw the front/back stroke bias. Braking systems are designed to operate with a specific bias (not just pressure). Say the manufacturer designed it for a 1/1 stroke bias -- 1mm of front piston stroke is matched with 1mm of rear piston stroke. When messing with this relationship what you end up with is a crapshoot, it's anyone's guess what you will get as a result. ABS is usually not too happy with such combos at the threshold too.


----------



## Madmax199 (Jun 14, 2015)

jamman said:


> Surely we can agree that there are certain circumstances where one of you is right and the other wrong and visa versa.
> 
> Max I'm actually quite surprised you are taking this "can't break the rules" stance as it tends to go against your ethos.


It's not a "can't break the rule stance", it's a "breaking the rules have to come with some positives, or what's the point" stance. 8)


----------



## Madmax199 (Jun 14, 2015)

Beunhaas said:


> Madmax199 said:
> 
> 
> > ^^ The part that there could be a combo with a fully extented piston and the pad is not maintaining full contact on the disc. :lol:
> ...


I am dodging it! Way too much explaining needed for no reason. As you said "captain obvious" -- if you dig in the spec sheet of any reputable caliper manufacturer, you'll see a min-max disc thickness for that specific caliper. That obvious range is there for a reason, I don't see how that needs an essay to explain. Close to the acceptable range and you will get away with it, far outside of it and performance will suffer.


----------



## David C (Apr 15, 2013)

NickG said:


> Okay so the problem would be with discs that are smaller than the discs originally used with the caliper?
> 
> Boxster fronts are 24mm, hence just about fit the standard TT 25mm wide disc. Shouldn't be any issues there?


New pads on new discs could be a squeeze unless there are thinner versions of the pad available (some pad shapes do come in various thicknesses).


----------



## Von Twinzig (Feb 10, 2014)

Madmax199 said:


> I am dodging it! Way too much explaining needed for no reason. As you said "captain obvious" -- if you dig in the spec sheet of any reputable caliper manufacturer, you'll see a min-max disc thickness for that specific caliper. That obvious range is there for a reason, I don't see how that needs an essay to explain. Close to the acceptable range and you will get away with it, far outside of it and performance will suffer.


Porsche is 24mm start to 22mm min.

VT


----------



## Spandex (Feb 20, 2009)

Madmax199 said:


> if you dig in the spec sheet of any reputable caliper manufacturer, you'll see a min-max disc thickness for that specific caliper. That obvious range is there for a reason


I get that a piston has a fixed stroke (i.e. the range of possible motion between fully retracted and fully extended) but saying that performance would 'suffer' outside this range implies that it would work, but not as well. Surely it either works or doesn't, because it can either reach the rotor or not. I don't see that a different rotor thickness would change the bias either, because it wouldn't change the piston travel needed. Isn't the distance between 'braking' and 'not braking' the same wherever it happens within the piston stroke?


----------



## Madmax199 (Jun 14, 2015)

Spandex said:


> Madmax199 said:
> 
> 
> > if you dig in the spec sheet of any reputable caliper manufacturer, you'll see a min-max disc thickness for that specific caliper. That obvious range is there for a reason
> ...


You are correct that it 'works' or does 'not work'. 'Works' if their is pad contact on the rotor -- and 'not work' if there is no contact. However, there is also an in-between, a space between the 'work' and 'not work'. Peak TQ is not reached as soon as the pad material reaches the rotor. There is a certain amount of pad compression, line expansion, and caliper/mounting flex before maximum TQ is reached. If TQ from the brakes is graphed, it will be a curve with a peak. Are we in agreement?

If the caliper piston is operating near its travel limit before the pad can be compressed enough to reach max friction, you will have a system that 'works' because it generates some braking TQ -- but also 'not work' because it can't optimally compress the pad, and accept line expansion, mounting and caliper flex etc. to reach the peak of the TQ curve. A phenomenon called "piston knockback" is common in a setup that operates in this fashion. You get normal intial bite, but the piston reaches the limit of its stroke and is knocked back and forth in a pulsing feel -- as a result, maximum braking TQ is never reached.

There are other issues associated with this also. Constantly opreating a piston near the limit of travel can eventually cause the piston to disengage the seal. If this happens, it will cause a slight leak that can lead to total brake failure. That's one the main reason why caliper manufacturers specify an operating range for pad and rotor thickness in relation to the known piston travel. They want the piston to never be operating near the limit of its stroke. [smiley=book2.gif]

Diagram showing piston knockback at full stroke


----------



## Spandex (Feb 20, 2009)

Ok, to be honest I'd put that fully in the 'not work' camp - the piston travel limit is reached before it has moved the required amount. So I'm fully on board with the idea that a calliper piston has a finite (and documented by the manufacturer) range of motion so your rotor must fall with 'reach' throughout its useable life, but as long as you meet those specs, how else can the system be affected? Force applied by a piston is a function of the area of the cylinder, not the amount of fluid behind that area, so the point in the pistons travel at which 'braking' takes place should have no effect on the force applied. If there was some change in 'bias' caused by the braking action happening at a different point in the piston stroke, then you would also experience that change in bias as your rotors/pads wear - which doesn't happen.

So if all you meant was 'if the rotor is too thin the piston will hit its end stop before full braking force is applied', then I agree completely. If there's something else going on then I don't understand.


----------



## Beunhaas (May 14, 2014)

Spandex said:


> Ok, to be honest I'd put that fully in the 'not work' camp - the piston travel limit is reached before it has moved the required amount. So I'm fully on board with the idea that a calliper piston has a finite (and documented by the manufacturer) range of motion so your rotor must fall with 'reach' throughout its useable life, but as long as you meet those specs, how else can the system be affected? Force applied by a piston is a function of the area of the cylinder, not the amount of fluid behind that area, so the point in the pistons travel at which 'braking' takes place should have no effect on the force applied. If there was some change in 'bias' caused by the braking action happening at a different point in the piston stroke, then you would also experience that change in bias as your rotors/pads wear - which doesn't happen.
> 
> So if all you meant was 'if the rotor is too thin the piston will hit its end stop before full braking force is applied', then I agree completely. If there's something else going on then I don't understand.


You are right. It works or doesnt work. Its that simple. A piston doesnt have a stop, it just pops out, so it works perfectly until it pops out and fails. No 'poor braking' inbetween.

Im happy someone like max tries to educate people that pad friction coëfficiënt makes the difference not pad area. Total piston area makes the difference not the number of pots. And that the same caliper can brake better if radial distance is increased for more leaverage.

But this 'poor braking' due to thin discs is wrong. Nothing wrong with accepting you said something wrong. We all learn.

End of discussion!


----------



## David C (Apr 15, 2013)

Beunhaas said:


> End of discussion!


Come on now...... you know THAT won't happen..... :lol:


----------



## Madmax199 (Jun 14, 2015)

Beunhaas said:


> Spandex said:
> 
> 
> > Ok, to be honest I'd put that fully in the 'not work' camp - the piston travel limit is reached before it has moved the required amount. So I'm fully on board with the idea that a calliper piston has a finite (and documented by the manufacturer) range of motion so your rotor must fall with 'reach' throughout its useable life, but as long as you meet those specs, how else can the system be affected? Force applied by a piston is a function of the area of the cylinder, not the amount of fluid behind that area, so the point in the pistons travel at which 'braking' takes place should have no effect on the force applied. If there was some change in 'bias' caused by the braking action happening at a different point in the piston stroke, then you would also experience that change in bias as your rotors/pads wear - which doesn't happen.
> ...


Oh, I can be wrong, wife can tell you that I am all the time!  I won't the first, nor the last person to be though. Ok, I learned something, and piston knockback is a myth. Won't argue or bring anything else since you ended the discusion.



David C said:


> Beunhaas said:
> 
> 
> > End of discussion!
> ...


Look, the great contributor!


----------



## Beunhaas (May 14, 2014)

Madmax199 said:


> Oh, I can be wrong, wife can tell you that I am all the time!  I won't the first, nor the last person to be though. Ok, I learned something, and piston knockback is a myth. Won't argue or bring anything else since you ended the discusion.


Then bring an argument!
Explain where in the breaking proces it goes wrong.

You wanted to let me answer my own question but not happening this way.


----------



## Spandex (Feb 20, 2009)

Madmax199 said:


> Oh, I can be wrong, wife can tell you that I am all the time!  I won't the first, nor the last person to be though. Ok, I learned something, and piston knockback is a myth. Won't argue or bring anything else since you ended the discusion.


Just so we're clear, I thought knock back was caused by flex in the 'system', typically around the wheel side of things rather than the brakes themselves. I don't see how a piston reaching its limit of movement could create the flex required (or any flex).

http://www.stoptech.com/technical-suppo ... -knockback


----------



## Madmax199 (Jun 14, 2015)

Spandex said:


> Madmax199 said:
> 
> 
> > Oh, I can be wrong, wife can tell you that I am all the time!  I won't the first, nor the last person to be though. Ok, I learned something, and piston knockback is a myth. Won't argue or bring anything else since you ended the discusion.
> ...


Yes, piston knockback is usually a result of flex. Flex in the upright (causing rotor runout), excessive caliper flex, and excessive piston flex. The caliper has a bore that is bigger than the piston so it could move freely. The clearance varies depending on the material used (aluminium pistons require the most clearance and phenolic ones the least clearance). The farther out (outside of the bore) that a piston is operating, the more it is prone to flex and vibration when contact is made. This is common in racing calipers with aluminum pistons -- even with the proper caliper to piston spec, at full stroke they tend to flex and suffer knockback once the rotor or pad begin to wear at the track. So yes, a piston extended too far out its bore will flex (especially if you spec it to operate outside its manufactured range).


----------



## Spandex (Feb 20, 2009)

Not convinced. Knock back occurs when you're not braking (otherwise the pressure from the fluid would stop the pad being over retracted). I can see that over extended pistons could cause vibrations during braking, but knock back is more than just flex or vibration - it refers to the pad being knocked back beyond its normal resting position, requiring the pedal to be pumped before full braking force can be applied.


----------



## Matt B (Apr 8, 2007)

Just to answer the OP's question. Its a good kit 

Ive actually had a kit made from porsche rear brakes, and its well better than those.


----------



## rocker tt. (Oct 12, 2015)

As above I've run this brake set up along with a "Tarox big" brake conversion on the rear for the last 2yrs & it's possibly the best breaking system that I've used for 25yrs, yes good one .!!


----------

