Which brake upgrade 322 mm or 340 mm?

[BlackShadow]

I was just having a word with a tuner about upgrades and with regard to brakes, I've decided on the Porsche/Movit front brakes. The extra £300 for the 340mm disks in comparison with the 322mm disks, is it worth it? For track day use are the 340mm better - i.e. larger, disipate heat better? Or is this perhaps overkill? Are the 322 mm adequare even on a hot summer track day (they come with steel hoses apparently)? Anybody, any views on this or, better still, any experience of this?

All comments greatly appreciated

[img]images/graemlins/audiS4black.gif[/img]

[sitas3]

go for the larger disc if you can as this will help brake-performance wise.

where are you getting your brakes from?

I want a new set as well and I'm considering buying the Porsche 993TT kit with 322 discs from the States as the exchange rate is good.

I also have my eye on the Cayenne 6 pot ones as well.

[jeffw]

There was/is a possible brake group buy going on .......BiTurbo was organising it

[DavidR]

go for the larger disc if you can as this will help brake-performance wise.

This is not necessarily true. It can front bias the car deterimentally and increase stopping distances. You should research which setup is the most compatible for the rear brakes you have (or intend to fit).

[jeffw]

I think it highly unlikely that fitting very large brakes would actually increasing stopping distances. It is possible that with some more thought with the rears in conjunction with the large front you will see and improvement overall. There is next to no weight over the rears under heavy braking and there is next to no braking effort directed to the rear of the car (less than 20%) so any improvement will be minor before the ABS kicks in.

[DavidR]

Well, there seems to be a body of evidence to suggest that there is a fairly narrow window of brake balance where optimal stopping distances are acheived. If you stray outwith this window, then you get premature front or rear brake lockup, activating ABS which (by definition) will increase stopping distances on a dry road. Even with ABS disarmed, the front premature front lockup means that the brake force that the driver applies has to be reduced and therefore the optimal rear braking effeciency is never acheived. Granted that proportionally the effect of rear braking is substantially less than the front, but if its effect was not significant then there would be little point having rear brakes at all. The ideal setup is to have a system that locks the front brakes shortly before the rears to maximise the grip available for decelleration. The net result of excessive front bias may be small (e.g. 5-10m from 80mph), but surely that defeats the point of upgraded brakes.

Essentially, an ideal upgraded front brake system will confer greater feel, modulation and better resistance to the effects of heat primarily rather than providing dramatically more braking "force" or "efficiency". A 4 wheel system will still operate within the limits of physical grip, but may, overall, increase braking force so long as balance is maintained.

[sitas3]

well i've done a little reasearch into this and the conclusion i've reached (admittedly after spending an hour on Audiworld) is that erm there isn't one and I'm more confused than when I started.. I can't believe the amount of statments made on that forum by brake distributors/manufacturers, then when challenged, they don't post the facts..

There was one fact posted however by Stoptech that stood out in that the Porsche 993TT caliper front set-up was considered to be a balanced upgrade approach (ie closer to stock brake bias performance than the 4 wheel upgrade!)

anyway doesn't having a bigger disc reduce the chances of warping when subject to repetitive hard braking (ie like track environments)?

[jeffw]

I think you only have to look rear pad wear to see how little they are actually doing.....I don't believe you can create excessive front bias on a front engined road car using 322 or 340 front discs. There are people running around with 380-390 mm discs and they seem to be able to stop effectively. There is a body of evidence ( a very large body of evidence) to suggest that Audi fit crap brakes and any 3rd party set-up is going to cut your braking distances.....

[sitas3]

David, have you decided which kit you are going for yet? 2 wheel or 4? Porsche/Movit type or Stoptech?

[DavidR]

There is a body of evidence ( a very large body of evidence) to suggest that Audi fit crap brakes and any 3rd party set-up is going to cut your braking distances.....

I haven't seen objective evidence that the audi braking system is crap, although we all know that subjectively they succumb to the effects of heat prematurely and there is a scope for improvement. The 993TT front upgrade is a fairly balanced upgrade, but the 4 wheel upgrade on the S4 is not. If a 50:50 weight distribution car static is decellerated at 1.0g then the approximate (accounting for suspension and similar variables) then the front to rear weight bias will become something of the order of 150% front of static weight vs 50% of static rear. So, agreed, the rear bias is considerably smaller than the front, but if it is still applying (for arguments sake) 10-20% of the overall braking force upon maximum decelleration (and again for arguments sake it could br greater at less than optimal decelleration) then you could add somewhere in the region of at least 5% to stopping distances with no rear brakes (whereas more realistically this would reflect the actial bias of 10-20%)

Add these variables to the fact that most people regard audi OEM brakes as "crap" then you have the potential for an even greater front bias with a front end only kit (as you fit "excellent new brakes" and use the "crap audi OEM rear") so by our subjective definitions front bias increases and overall braking performance suffers.

From the research I have done into this issue the fact that Audi OEM brakes wear the rears slower than the front is to some extent inevitable given the overall excessive front bias of the OEM setup and the relatively poor performance of the OEM front brakes means that they are working closer to their design capabilities than the rear. The way to optimise this defecit is to go for a system that redresses the balance issue and doesn't get so hot - enter the so called "balanced" brake upgrade, rather than the undoubtably effective "big brake kits" that provide front brake power in abundance with the capability to resist heat effects, but potentially upset balance and therefore affect overall stopping distances (potentially detrimentally) in, for example, a one off emergency stop from high speed.

[sitas3]

very informative response David. so, are you suggesting a front setup upgrade which isn't as powerful so that the rears are also used more?

[jeffw]

Going back to DavidR's point regarding all of this ......

I have never seen any evidence at all that you are correct....I've seen a lot of mutterings but never seen any evidence i.e. a test between different brake set-ups on the same car. I would not take the word of a company who takes a product which was made for another vehicle and fits it to an S4 as gospel especially as the opinion given to me by HiSpec (a manufacture of brake systems for 25+ years) goes against what you are suggesting.

[TarmacTerrorist]

Thought our cars had a gadgy called EBD (Electronic Brake-force Distribution), Im no techie but the name suggests that some sort of cpu controls the amount of braking force designated to each wheel before having to use the ABS system?

Anyone care to comment?

[sitas3]

JeffW, can you tell us what HiSpec's opinion is on what is the best braking upgrade for the S4.

Thanks

[DavidR]

Jeff, Evidence on the topic of car tuning and modification is always very hard to come by (unfortunately). There are data on a comparative test using the S4 with different brakes (admittedly performed by a manufacturer and therefore potentially biased) which demostrates the effect of balance on stopping distances and car stability.

I have read some information provided to me by a fairly large motorsport outfit based in the UK regarding this topic, and they too have demonstrated the effects of brake bias vs stopping distance, and we all know from watching the GP and rallying, that the brake bias (as altered in car by the driver) can alter the attitide of the car under braking.

So, whilst the evidence that exists may be biased, it does make sense on a logical level. Even if a vehicle is only provided with 20% of grip to the rear under decelleration, that is still 20% extra grip afforded to allow cornering or braking.

I think that this area does need to be looked into more thouroughly - I am not taking any one piece of information as gospel but it certainly doesn't make brake choice easy, but I think that stoptechs superficially more scientific and logical approach to braking (rather than the somewhat boy racer attitude of getting the largest, shiniest most cosmetically impressive brakes) makes sense, and I am tempted to gamble some hard earned to try their products. Of all the manufacturers approached, they have been most obliging in providing information, and again, having spoken to independant contacts, they would tend to agree.