Rear Brake Upgrade Hypothosis & Discussion
05-15-2007, 12:04 PM
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Rear Brake Upgrade Hypothosis & Discussion
I was reading about the rear upgrade on ECS's site and they had an interesting chart that showed the brake torque of many of the different BBK systems out there and what the bias would be with their Stage I rear upgrade. Here's the part that I'm curious about.
It highly recommends that the rear is only upgraded if the front has been, just like STaSIS, StopTech and anyone else with a rear brake upgrade. But looking at the chart I have to wonder "WHY?" The stock S4 front brakes have a torque rating of 7.281 which is higher than the 332mm kits offered by any of the aftermarket vendors. So in theory, you would still have a greater front bias thean combining with anything less than 355mm. Heck, the 4 piston Alcon 355mm setup only has a torque rating of 7.319 which is a smidge better but not much.
The Alcon rear upgrade uses the stock caliper with a smaller rotor than the ECS setup so it should be even less of an issue, so why is it considered dangerous to run them with stock brakes?
Don't get me wrong, I think there are plenly of great reasons to upgrade the front brakes, just missing the reason for concern and hoping someone can help clear things up for me. Seems like if I fella was to jack up the rear rotors it would be a great excuse to upgrade!
It highly recommends that the rear is only upgraded if the front has been, just like STaSIS, StopTech and anyone else with a rear brake upgrade. But looking at the chart I have to wonder "WHY?" The stock S4 front brakes have a torque rating of 7.281 which is higher than the 332mm kits offered by any of the aftermarket vendors. So in theory, you would still have a greater front bias thean combining with anything less than 355mm. Heck, the 4 piston Alcon 355mm setup only has a torque rating of 7.319 which is a smidge better but not much.
The Alcon rear upgrade uses the stock caliper with a smaller rotor than the ECS setup so it should be even less of an issue, so why is it considered dangerous to run them with stock brakes?
Don't get me wrong, I think there are plenly of great reasons to upgrade the front brakes, just missing the reason for concern and hoping someone can help clear things up for me. Seems like if I fella was to jack up the rear rotors it would be a great excuse to upgrade!
05-15-2007, 12:50 PM
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I recently did some thinking on this subject...
The S4 has a very front biased. Most say that it would be unsafe to upgrade rear with the HP2 but I don't buy it. Do the math...
Take the S8 rear for example...mainly because I have already done the research. From what I've read the S4 has a 72%/28% brake ratio with 2.6 bias in stock form. By adding the S8 rear adding ~10% more force that shouldn't amount to more than a 70%/30% distribution i.e. 2.3 bias which isn't much change. And still well within the balanced range:
* Lifted from Stoptech so who knows maybe its just marketing hype*
<img src="http://www.stoptech.com/tech_info/img/brakebias_7.jpg" />
Now on some BBK's (due to 4pots like 332's) actually move the bias even more towards the rear...i.e. you accomplish the same rear bias (2.3) as with HP2/S8 as with 332/stock.
Now the main reason you upgrade from the stock HP2's is you get better heat dissipation and more even/predictable pedal feel...etc which may be a fair trade off. However, when you add a 332 setup with the S8 or rear upgrade MAY (I say may because this is only theory at this point and I've never driven an S4 with 332's & S8 rotors.) upset the balance of the car as its closer to the red region on the above chart at ~2.0 rear bias.
Anyways, my main point being the S8 upgrade with HP2's in my mind is a good thing. And upgrading the rear using the same caliper is a BAD thing with upgraded front brake setup.
Also I just priced a set of rotors/carriers for $300...add a set of pads and your right at $350. Now I'm not sure what the specs on the Alcon kit is but I would suspect similar results.
Again I don't claim to be an expert...just what I've red in the archives and other sites...which is why I was intrigued by your comment.
FWIW...I will be installing S8 rotors with HP2's up front on hopefully this weekend...along with new Pagid pads, DBA4000 rotors, and fluid.
Take the S8 rear for example...mainly because I have already done the research. From what I've read the S4 has a 72%/28% brake ratio with 2.6 bias in stock form. By adding the S8 rear adding ~10% more force that shouldn't amount to more than a 70%/30% distribution i.e. 2.3 bias which isn't much change. And still well within the balanced range:
* Lifted from Stoptech so who knows maybe its just marketing hype*
<img src="http://www.stoptech.com/tech_info/img/brakebias_7.jpg" />
Now on some BBK's (due to 4pots like 332's) actually move the bias even more towards the rear...i.e. you accomplish the same rear bias (2.3) as with HP2/S8 as with 332/stock.
Now the main reason you upgrade from the stock HP2's is you get better heat dissipation and more even/predictable pedal feel...etc which may be a fair trade off. However, when you add a 332 setup with the S8 or rear upgrade MAY (I say may because this is only theory at this point and I've never driven an S4 with 332's & S8 rotors.) upset the balance of the car as its closer to the red region on the above chart at ~2.0 rear bias.
Anyways, my main point being the S8 upgrade with HP2's in my mind is a good thing. And upgrading the rear using the same caliper is a BAD thing with upgraded front brake setup.
Also I just priced a set of rotors/carriers for $300...add a set of pads and your right at $350. Now I'm not sure what the specs on the Alcon kit is but I would suspect similar results.
Again I don't claim to be an expert...just what I've red in the archives and other sites...which is why I was intrigued by your comment.
FWIW...I will be installing S8 rotors with HP2's up front on hopefully this weekend...along with new Pagid pads, DBA4000 rotors, and fluid.
05-15-2007, 12:59 PM
#4
These are calculations. Different pads, different weight cars, different drivers
will experience different things. What might be a predictable pedal for you would feel like non-responsive braking to someone else.
Also, and this is me thinking out loud, but if the rears are now less prone to fade, you'd see the bias shift a little more to the rear as the fronts continue to bear the 70+% of the bias. Probably more evident on the track, which is where you don't want issues.
Also, and this is me thinking out loud, but if the rears are now less prone to fade, you'd see the bias shift a little more to the rear as the fronts continue to bear the 70+% of the bias. Probably more evident on the track, which is where you don't want issues.
05-15-2007, 05:51 PM
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I agree.
Personally, I think the balance on the stock brakes is already pretty good.
The car will lock the rear wheels first with the ABS off under good traction conditions (so lots of weight transfer). My experience is that the fronts will go into ABS first in very slippery conditions (when there is much less forward weight transfer). So I think that the balance is pretty good. I don't think it needs any more rearward bias.
And since the rear brakes don't fade, putting on bigger rotors only serves to increase the unsprung weight at the rear. In fact, I'm considering turning my rear rotors (rather than replacing them) so as to keep the mass down since they're not overheating.
I'm going to install some Alcon 6-piston with 355mm rotors and also some sticker tyres. My guess is that the balance then will be almost perfect. But I am going to experiment a little to see how it works out.
Stephen
The car will lock the rear wheels first with the ABS off under good traction conditions (so lots of weight transfer). My experience is that the fronts will go into ABS first in very slippery conditions (when there is much less forward weight transfer). So I think that the balance is pretty good. I don't think it needs any more rearward bias.
And since the rear brakes don't fade, putting on bigger rotors only serves to increase the unsprung weight at the rear. In fact, I'm considering turning my rear rotors (rather than replacing them) so as to keep the mass down since they're not overheating.
I'm going to install some Alcon 6-piston with 355mm rotors and also some sticker tyres. My guess is that the balance then will be almost perfect. But I am going to experiment a little to see how it works out.
Stephen
05-15-2007, 06:06 PM
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It will be very interesting to hear your results!
I think the Alcon rear upgrade is probably very similar in performance to the S8 but with the added bonus of lighter weight and the venting dimples. If the Alcon front rotors with the stock calipers would perform well for an occasional track day (I'm already happy with the HP2's and Pagis Blue pads on the street) this might be a nice way to drop some weight and make the setup a little more heat friendly. Plus it wouldn't look half bad with the calipers painted.
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05-16-2007, 05:32 AM
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My stock rears do get hot but don't fade.
I often see marks on the rotors from hot HPS pads so I know that they're getting pretty hot. I do think that they get worked.
On my Porsche, I have overheated the rears with interesting consequences. It might not be a lot fo braking force but when it isn't there, you sure can miss it!
Stephen
On my Porsche, I have overheated the rears with interesting consequences. It might not be a lot fo braking force but when it isn't there, you sure can miss it!
Stephen