70 replies to this topic

[TheRealVXed]

 

Incase anybody searches this thread, I found what I think the issue is with the pedal travel.... The piston area of the original rears are 2036mmsq, however of the original front APs is 6194mmsq... so roughly I now need to move twice as much fluid to squeeze the disc, and yet the area of the new 4 pots on the front is 9099mmsq, which isn't the same increase ratio (approx 1.46:1 on the front and 3:1 on the rear).  I think this is causing the major issue, so, I am going to try reducing the fronts to the 34/36mm EPs rather than the 36/40 I have on there now, and revert back to the standard brembos on the rear.

 

Alternatively I guess I could put a brake bias valve in to send more fluid to the rear.... 

 

What's the best option here?

 

??
Original AP front pistons are 44mm => 1521mmsq      (60%)

Original Brembo rears 36mm pistons => 1018mmsq   (40%)

 

You changed to:

Front 36mm +40mm = 2275mmsq  (60%)

Rears 44mm => 1521mmsq  (40%)

 

So you have the exact same brake bias as before (The VX can use a bit more rear bias on track), but your main issue is that you have changed from total 2539mmsq piston area to a total of 3796mmsq, which is 50% more!

 

 

I think you are missing the multiplication of the number of pistons... there is 1 in the brembo and 2 in the AP, above are the calculations for one half of each AP caliper...

 

Should be "# of pistons x piston area" so for the AP, it's 2 x 1521 = 3042 per caliper.  For both calipers on the axle it would be 6084 (my original calculation was based on 44.4mm pistons as this was the measurement I found online).

 

I verified my maths with the below

https://brakepower.c...bc_27_PAC_t.htm

 

Pretty much identical results per axle.

 

Original AP (44mm) = 6079mmsq

Brembo (36mm) = 2034mmsq

New large 4 Pots (40/36mm) = 9093mmsq

New small 4 pots (36/34mm) = 7699mmsq

 

So by moving the AP to the rear in place of the Brembos it's increased the surface area of the pistons by almost a factor of 3 and the front with the large by a factor of ~1.5.  With the small 4 pots this increase is reduced to a factor of ~1.25 and the original rears would be 1:1 so the bias would move rearward slightly on this set up.

 

Without changing the ratio between the master cylinder and the rear APs by a factor of ~2.5:1 you wouldn't maintain the same balance as standard

 

Hope this makes sense...

 

ETA, missed the floating caliper issue;  need to multiply the Brembos by 2

 

Original AP (44mm) = 6079mmsq

Brembo (36mm) = 4068mmsq

New large 4 Pots (40/36mm) = 9093mmsq

New small 4 pots (36/34mm) = 7699mmsq

 

So to correct my earlier calculations;

 

6079:4068 = 1.5:1

9093:6079 = 1.5:1

 

So now I am really confused as to what the problem is

 

Thought I had solved it there...

 

VXed

Edited by TheRealVXed, 21 May 2019 - 10:50 AM.

[Bargi]

Wheres Arno when you need him 🙂

Sent from my SM-G950F using Tapatalk

[TheRealVXed]

Wheres Arno when you need him 🙂

Sent from my SM-G950F using Tapatalk

 

Isn't it!

[Exmantaa]

So you calculated per axle, but I think you're missing the point that the floating Brembo rear caliper must be multiplied by 2, as it acts the same as a fixed dual caliper of the same piston size.

 

Original front axle: AP 44mm dual => ~1521mmsq x2 pistons x2 calipers = ~6084mmsq (60%)

Original rear axle: Brembo 36mm floating => ~1018mmsq x2 (floating) x2 calipers = ~4072mmsq (40%)

 

Total 10156mmsq

 

You changed to:

 

Front axle: 36mm +40mm dual => ~2275mmsq x2 x2 calipers = ~9100mmsq (60%)

Rear axle: 44mm dual => ~1521mmsq x2 pistons x2 calipers = ~6084mmsq  (40%)

 

Total 15184mmsq

 

So you have the same front -rear brake balance as original, but with 50% more surface than original... (So 50% more volume to pump with your original master cylinder.)

 

 

Edited by Exmantaa, 21 May 2019 - 11:09 AM.

[FLD]

I don't think you need to multiply up the brembo. A sliding caliper exerts half the force of the piston per side.

[Exmantaa]

I don't think you need to multiply up the brembo. A sliding caliper exerts half the force of the piston per side.

 

Well, you suggested to use this calculator:

 

Total area of caliper pistons per axle in case of FIXED MOUNT calipers.

This is the area of one piston, multiplied by the number of pistons in one caliper, multiplied by the number of calipers on that axle.

In case of different piston diameters per caliper, calculate the area of every different piston, multiplied by the number in which every one of those different pistons occur in one caliper, add those numbers and multiply the result by the number of calipers on that axle.

 

Total area of caliper pistons per axle in case of FLOATING calipers: Do the calculation as described above, and multiply the result by two.

The area of one piston = Ï€ • r² = ¼ • Ï€ • d²       (Ï€ = 3.14)
So: the area of one piston = 0.785 • d • d        (d = piston diameter in inches or millimeters)

If you prefer to have the math done by your PC, than just enter the required numbers in one of the calculators below. You can overwrite/delete the current numbers. These are just some popular values entered as an example.

REMINDER: For floating calipers, multiply the result of the calculator by 2.

 

[FLD]

My bad! They were just two I found by the power of google.

[Exmantaa]

......

ETA, missed the floating caliper issue;  need to multiply the Brembos by 2

 

Original AP (44mm) = 6079mmsq

Brembo (36mm) = 4068mmsq

New large 4 Pots (40/36mm) = 9093mmsq

New small 4 pots (36/34mm) = 7699mmsq

 

So to correct my earlier calculations;

 

6079:4068 = 1.5:1

9093:6079 = 1.5:1

 

So now I am really confused as to what the problem is

 

Thought I had solved it there...

VXed

 

Your issue is that you massively changed the total area/volume of your brake calipers and still use the original master cylinder.

So if you apply the same pedal force (=brake fluid pressure) as you normally would, you now get 50% more braking force.

And due to the increased caliper surface/volumes, the master must travel 50% more to pump the required fluid volumes...

=> Long pedal travel, No feel => fit a bigger master cylinder.

 

 

 

 

Edited by Exmantaa, 21 May 2019 - 11:25 AM.

[Exmantaa]

From the EP site =>

The OEM master cylinder is only 19.05mm which gives a long brake pedal with 4 pot Brake calipers.
Our EP Tuning Master Cylinder has a 22.22mm bore and will flow 36% more brake fluid.

 

That will be a nice match to your 50% increased system. And it fits... 

[TheRealVXed]

From the EP site =>

The OEM master cylinder is only 19.05mm which gives a long brake pedal with 4 pot Brake calipers.
Our EP Tuning Master Cylinder has a 22.22mm bore and will flow 36% more brake fluid.

 

That will be a nice match to your 50% increased system. And it fits... 

 

I asked them about that, they said it fits the non-servo Elise, not the VX.  Apparently the VX master is 23mm already (http://www.vx220.org...-cylinder-bore/), so 1.5x this would be 34mm ish, could probably reduce this a little as 34mm seems quite large....

 

Been talking to Arno direct and he has cleared my mind on this, it also tallies with what you and FLD have been saying, so thanks : :

 

Going to try and find a servo assisted replacement MC to (roughly) match the increase in piston area factor.  Any suggestions on what size/where to get one that will fit the servo too?  This one in 1 1/8 inch is approx 28mm https://www.demon-tw...der-kit-243384/

 

Alternatively removal of the servo and reduction in MC size could also work?

Edited by TheRealVXed, 21 May 2019 - 02:32 PM.

[Exmantaa]

From the EP site =>

The OEM master cylinder is only 19.05mm which gives a long brake pedal with 4 pot Brake calipers.
Our EP Tuning Master Cylinder has a 22.22mm bore and will flow 36% more brake fluid.

 

That will be a nice match to your 50% increased system. And it fits... 

 

Scrap that; apparently the VX has a 23mm master bore, so I assume the above original 19.05mm is for a non-assisted S1 system.

 

There is a topic somewhere where someone changed his VX to a Subaru (29mm?) master cylinder to compensate for bigger calipers.

[Exmantaa]

Digging a bit more (the power of Google...)  

There is 29mm quoted on Joshua's modified master, but Subaru seem to have only 15/16", 1" and 1-1/16" size master cylinders? So that biggest one is 27mm, which would give you +38% over a stock VX220 master. Not bad.


And a quote from JohnTurbo I found somewhere:

... its worth reading a little about hydraulics:
Firstly nothing will make the car stop quicker without you increasing grip or improving balance. If you want to stop faster, push the pedal harder. ..this increases system pressure!
If you aren't strong enough to lock the wheels id be surprised! If joshua hadnt increased his master size his pedal would have felt like a soft sponge whilst insta-locking the wheels.
Broadly you should keep master to slave area ratio the same to give decent feel...small adjustments to change feel.

If you want the car to slow more for less pedal force the nest bet is higher cof pads.
What most people achieve by cocking around with brakes is a car that stops slower than standard. My first couple of efforts are included...

[TheRealVXed]

https://www.demon-tw...der-kit-243384/ this one is 1 1/8 inch (28.5mm)

 

Will it fit though with the servo too.... 

[Exmantaa]

Not sure if that Tilton is a dual master cylinder for ABS use? (2 outlets)

 

The Subaru master apparently fitted with only some minor mods to the mounting holes. Do a search which models have this 1- 1/16th size. But for sure others can be made to fit. just calculate with the piston area. ;-)

 

[TheRealVXed]

Not sure if that Tilton is a dual master cylinder for ABS use? (2 outlets)

 

The Subaru master apparently fitted with only some minor mods to the mounting holes. Do a search which models have this 1- 1/16th size. But for sure others can be made to fit. just calculate with the piston area. ;-)

 

Checked with Demon Tweeks and wouldn't fit apparently.....

 

Looking at this thread http://www.vx220.org...t-the-vx/page-2 it appears the Cayenne MC would work and has a 27mm bore...

 

I found one on ebay (RHD versions are like hens teeth apparently!) and for £20 it's probably worth taking the punt....

[Exmantaa]

So the same as the Subaru STI/WRX master. Not sure if the Cayenne fits well in the VX.

Like on the Subaru, you probably must modify the length and/or diameter of the internal actuator pin between the servo and master cylinder...

[Exmantaa]

Edit; the Cayenne MC mounting flange looks angled. Subaru layout is similar to the straight up VX220 MC.

 

Edit; Think our VX MC mounting plate is also a bit angled, right?

Edited by Exmantaa, 21 May 2019 - 08:35 PM.

[TheRealVXed]

The VX ones bolt holes are offset. So also angled

[FLD]

Thinking about this a bit more.....
The brembo doubling issue applies when considering MC bore BUT not for when comparing brake caliper cylinder areas front to back (at least that's how it works in my head.

WRT the cylinder; have you tried taking the numbers off the side of the OEM cylinder into a generic shop to look in their catalogues at options? The other thing you could do is get a second one and get it over bored and a new piston made by an engineering co.

[FLD]

Have you considered over boring the rear calipers? I've done 40mm rears but not found a way of getting the pistons in without nipping the boot yet. That should balance things up a bit.