Brake balance following upgrade to rear Brembo BBK

[CB72]

For many years, I used a BBK from Movi't in Germany consisting of 322x32 rotors and Brembo calipers as used on the Porsche 993 Turbo.
The setup worked very well with a home made kit with 4 piston AP Racing calipers in the rear.
Beeing a perfectionist...I bought a Brembo rear BBK with 330x28 disks.
The system works fine with a definite increase in deceleration for the same brake pressure.
The concern came when I had to do an emergency braking and a rear wheel momentarily locked up asking for quick correction!
I suppose one of the rear brake pads was not properly bedded in as a 12% braking force difference was measured between the wheels.
Another concern is that the braking force was higher in the rear by 26% on one side and 13% on the other.
This is probably due to the use of a racing pad upfront ( Pagid RS 29) vs a road pad ( Brembo OEM) in the rear.
The bite on the RS 29's is definitely on the low side.
I've ordered RS 29 pads for the rear as I'm attending a track day at Spa Francorchamps in Belgium but I wonder if there isn't a proportioning valve included in the NA2 ABS system that influences the F/R bias?
For info the front calipers carry 38 and 44 mm pistons while the rear carry 36 and 40 mm pistons that should give a 53.85 % front bias

 

[BBVNSX]

Hello John,

Your math sounds right, and bias is theoretically very close to "NA2" system, even if you have quite some increase in piston area (front and rear), in theory this should impact the pedal travel (more travel) as you are moving more braking fluid through those bigger pistons using the OEM master cylinder.


But you shouldn’t be experiencing any unbalance or rear wheels lockup, I have the same bias on my car and brake balance is great with F 205/40R17 and R 255/35R18.



2000+ ABS upgrade does come with proportioning valve, it is part #6 on the picture:

 

[CB72]

Hi Bruno,
Thanks for the confirmation on the proportionning valve!
I suppose it was added on the 2002 ABS models to reduce the pressure to the rear calipers to avoid lock up in emergency braking?
Given that I'm using larger pistons front and rear, I'm creating more stopping power at less fluid pressure I suppose?
This would put me on a "lower" part of the proportionning curve thus leaving more pressure on the rear for the same deceleration?
Worst case, I can use adjustable proportionning valves in place of the OEM setup allthough I believe that with the same pad material font and rear, the problem will disappear.
Spa Francorchamps will give me the answer!
Great to see you still active on NSX prime!

 

[BBVNSX]

Yes, you're right about the stopping power with lower pressure, so that could also impact the proportioning valve effect as you mentioned… but most probably as you mentioned before, the main reason for the rear lockup is the difference in brake pads! If your brakes were cold when you did the emergency braking, and the front pads are race oriented (need more temp to bite), probably the street pads on the rear just got to maximum bite faster and that resulted on the lockup…

Poor brake pads bedding in could in did explain the difference you felt on rear left vs right… well Spa Francorchamps will help getting conclusions for sure

I come here on daily basis (usually to read) and still learn many things about the car every time!

Have a great time on track my friend! And thanks for sharing your experience!

 

[Honcho]

I suppose it was added on the 2002 ABS models to reduce the pressure to the rear calipers to avoid lock up in emergency braking?

Nope. The early-style ABS unit has the prop valve contained inside the ABS unit. The 2000+ digital ABS unit is just the pump and has no internal prop valve. So, they had to add one externally. I believe the bias is identical to the 97-99 NSXs.

 

[CB72]

I got my RSL 29 Pagid rear brake pads today!
They are yellow and pricy ...
Here are the specs:

Product Family RSL - PAGID Racing

Racing Brake Pads - PAGID Racing RSL compounds are developed to comply with the latest requirements in endurance racing and meet or surpass all current ecological standards of the automotive industry.

www.pagidracing.com

 

[BBVNSX]

I got my RSL 29 Pagid rear brake pads today!
They are yellow and pricy ...
Here are the specs:

Product Family RSL - PAGID Racing

Racing Brake Pads - PAGID Racing RSL compounds are developed to comply with the latest requirements in endurance racing and meet or surpass all current ecological standards of the automotive industry.

www.pagidracing.com

When you try them let us know how they worked solving the brake bias issue

 

[RYU]

I ran a similar piston size mix some years ago. The big caliper pistons felt not great paired with the small factory master cylinder. The brake pedal bit quite deep into the stroke and modulation got worse (I.e. very on/off). It had no pedal pressure until deep into the stroke then it was quick to induce lockup.

I'm not sure if this helps but I'm surprised you like those setup which brings me hope that perhaps mine was wonky somehow.

 

[docjohn]

Lol Pagid is the rolex of pads...all the ballers use them...

 

[CB72]

I came back from Spa Francorchamps today and here's what I dicovered.
It's a long journey to get there from my place in France with around 1000 km of mainly highways but not only...
The pads had been bedded in before I left, at least that's what I though!
In fact when I got closer to the track the brakes created a shudder that became stronger with the miles?
As there where around 100 cars at the track day, I discussed the matter with a few Exige owners who used the Pagid "Yellows"
The answer was you must bed them in further as there is still a layer of the previous pad material on the disks.
I was I bit doubtfull but full of hope to still be able to track the car.
The initial laps were dreadfull but the situation improved as I got heat in the system after 5 or 6 laps.
I stopped 20 minutes later to cool the brakes and the next session was a dream with no shudder and very powerfull decelerations.
The back to front ratio seems perfect as the 2002 ABS locks the front wheels first with stronger braking than my previous setup.
BTW with the previous setup my front drilled disks ( 322X32 mm) would develop small cracks after one session.
Here no more, proof that they have less work to do
In fact I had to stop the second session after 20 minutes as the brakd fluid ( RBF 600) started boiling.
Totally my fault as I had omitted changing the fluid recently.
Last point: is it perfect?
Not totally and I'll explain why in my next post.

 

[stuntman]

My calculations with your 322mm front rotor is 53.1% front bias vs NA2 at 56.6% - that's significantly more rear bias than the NA2, hurting brake performance, making the ABS pump work harder, and causing rear instability.

This caliper sizing and excessive rear bias is likely your biggest problem causing the rear lockup issues. Going to the RS29 rear pad likely makes the bias worse than running a street rear pad.

You had lockup issues with the 2002 ABS system before?

Your larger rear caliper will cause more rear bias and a longer, softer brake pedal.

I'd go with Castrol SRF over the RBF600 brake fluid.

 

[CB72]

Yes, I had calculated a front bias of 53.85% assuming same disk sizes.
I believe Honda augmented the rear bias on the NA2 to improve the wet weather breaking.
The offset of that would indeed be rear wheel lock up in the dry.
Knowing that, Honda introduced the proportionning valves on the NA2 to reduce the pressure to the rear calipers when braking on dry tarmac.
With respect to the friction coefficient the RSL 29's are way lower than the Ferodo FM 1000 fitted by Brembo.
This was confirmed during the Spa trackday as I mentionned above.
What's great with the Pagid pads is their linear performance delivery from the onset of braking until you release the brakes.
The only setback with larger caliper pistons is that the pedal travel is a litle longer.
This also means lower pedal pressure that could not be to everyone's liking?
Maybe fitting an NSX-R M/C if it has a larger diameter than the OEM NSX one would be the good choice?
Thanks for the usefull comments!
BTW I found the site usefull to compare different types of pads.

Brake Pad Comparison | Race Technologies | Brembo Official Partner

www.racetechnologies.com

 

[CB72]

Coming back to the Spa Francorchamps track day here are some comments I'd like to share.
Clearly the NA NSX is no match for the current cars I met!
A majority of GT3's of course, one Z06, some KTM's and Lotus Exige's. but also McLaren's.
Even the Alpine Renault with it's 1.6 liter engine would pull away from me...
No wonder with only 1100 kg to drag, 300 HP and a 7 speed PDK gearbox.
With the NSX most of the time you are in 3rd gear and on the faster parts in 4th.
With no turbo it hurts.
I had the opportunity to go as a passenger in a Dallara Stradale; a real race car, very light with incredible grip and fantastic brakes of course.
I definitely miss my LoveFab turbo that brought me to 240 km/h at the end of the Kemmel straight while I only managed 210 yesterday.
In fact I had to drive while constantly looking into my rear view mirror to get out of the way of the faster cars.
No fun! It's a pity as it's my favorite track.

 

[Honcho]

Coming back to the Spa Francorchamps track day here are some comments I'd like to share.
Clearly the NA NSX is no match for the current cars I met!
A majority of GT3's of course, one Z06, some KTM's and Lotus Exige's. but also McLaren's.
Even the Alpine Renault with it's 1.6 liter engine would pull away from me...
No wonder with only 1100 kg to drag, 300 HP and a 7 speed PDK gearbox.
With the NSX most of the time you are in 3rd gear and on the faster parts in 4th.
With no turbo it hurts.
I had the opportunity to go as a passenger in a Dallara Stradale; a real race car, very light with incredible grip and fantastic brakes of course.
I definitely miss my LoveFab turbo that brought me to 240 km/h at the end of the Kemmel straight while I only managed 210 yesterday.
In fact I had to drive while constantly looking into my rear view mirror to get out of the way of the faster cars.
No fun! It's a pity as it's my favorite track.

I'm quite jealous that you get to do your track days at Spa! With the NSX, I try not to think about it as a pure speed endeavor anymore. The car is 30 years old and while still quite capable, you're going to be pointing by a lot of the newer stuff. I think it says something about Honda's design effort that you are comparing yourself to GT3s and Exige.

210 kph is still quite fast and you get to hear that NA VTEC screaming the whole way. You could always do ITB and cams if you want a little more power and "music".

 

[RYU]

Coming back to the Spa Francorchamps track day here are some comments I'd like to share.
Clearly the NA NSX is no match for the current cars I met!
A majority of GT3's of course, one Z06, some KTM's and Lotus Exige's. but also McLaren's.
Even the Alpine Renault with it's 1.6 liter engine would pull away from me...
No wonder with only 1100 kg to drag, 300 HP and a 7 speed PDK gearbox.
With the NSX most of the time you are in 3rd gear and on the faster parts in 4th.
With no turbo it hurts.
I had the opportunity to go as a passenger in a Dallara Stradale; a real race car, very light with incredible grip and fantastic brakes of course.
I definitely miss my LoveFab turbo that brought me to 240 km/h at the end of the Kemmel straight while I only managed 210 yesterday.
In fact I had to drive while constantly looking into my rear view mirror to get out of the way of the faster cars.
No fun! It's a pity as it's my favorite track.

it sounds like it's time for a new car

 

[Optical]

Or a turbo K swap

 

[CB72]

And a picture in the "Raidillon" at Spa.

 

[stuntman]

Yes, I had calculated a front bias of 53.85% assuming same disk sizes.
I believe Honda augmented the rear bias on the NA2 to improve the wet weather breaking.
The offset of that would indeed be rear wheel lock up in the dry.
Knowing that, Honda introduced the proportionning valves on the NA2 to reduce the pressure to the rear calipers when braking on dry tarmac.
With respect to the friction coefficient the RSL 29's are way lower than the Ferodo FM 1000 fitted by Brembo.
This was confirmed during the Spa trackday as I mentionned above.
What's great with the Pagid pads is their linear performance delivery from the onset of braking until you release the brakes.
The only setback with larger caliper pistons is that the pedal travel is a litle longer.
This also means lower pedal pressure that could not be to everyone's liking?
Maybe fitting an NSX-R M/C if it has a larger diameter than the OEM NSX one would be the good choice?
Thanks for the usefull comments!
BTW I found the site usefull to compare different types of pads.

Brake Pad Comparison | Race Technologies | Brembo Official Partner

www.racetechnologies.com

It's likely they went to the caliper parts bin and had to use a proportioning valve to get the correct bias for the caliper and rotor combination they had. Proportioning valves and front to rear mechanical brake bias remain constant constant whether the road is dry or wet. While there is a small % of brake bias shift rearward in the wet due to less weight transfer to load the front tires, making it easier to lock the front tires, the ABS module handles this by blowing off pressure to the front calipers. Likewise, if the rear brakes are causing rear lockup too easily due to poor caliper sizes, brake pad choices, etc... the ABS will blow off pressure to the rear calipers. But the ABS system can only work so well and if there is an inherent imbalance in the brake pads, caliper piston sizes, rotor sizes, etc... the entire braking ability of the car will be greatly reduced compared to having a bias similar to what the ABS pump was tuned for.

I would recommend the larger master cylinder to get the pedal travel down and effort back up.

Coming back to the Spa Francorchamps track day here are some comments I'd like to share.
Clearly the NA NSX is no match for the current cars I met!
A majority of GT3's of course, one Z06, some KTM's and Lotus Exige's. but also McLaren's.
Even the Alpine Renault with it's 1.6 liter engine would pull away from me...
No wonder with only 1100 kg to drag, 300 HP and a 7 speed PDK gearbox.
With the NSX most of the time you are in 3rd gear and on the faster parts in 4th.
With no turbo it hurts.
I had the opportunity to go as a passenger in a Dallara Stradale; a real race car, very light with incredible grip and fantastic brakes of course.
I definitely miss my LoveFab turbo that brought me to 240 km/h at the end of the Kemmel straight while I only managed 210 yesterday.
In fact I had to drive while constantly looking into my rear view mirror to get out of the way of the faster cars.
No fun! It's a pity as it's my favorite track.

What size and model tires are you running again? With a PDK I would think you could hang with some of those cars. Do you have any in-car footage?