[Race_Prep] Batfinks Garage - The Projects

[allanallen 528 2 Cars]

Nothing wrong with wanting the engine bay to look tidy whether its a track car or not.

Tidy looms and 'tucked' looms are completely different animals!

[Batfink 201]

Not a lot to report at the moment. I wanted to buy a set of custom split rim wheels but the company wont make them until I give them dimensions of the brake system .

Slight spanner in the works as I have no front brakes for the car.

Today I had delivered a set of Alcon 4 pot calipers of the rear of a Subaru WRC car.

These only weigh 1.9kg each (205 1.9 GTI front caliper is 3.7kg)

I'm in talks with Alcon about what thickness discs I can use but the initial plan is to go for 17mm x 280mm bell discs. These only weigh 1.5kg per side plus the weight of the bells.

[welshpug 1,622]

nice, pistons look a bit small? not going vented

[Batfink 201]

AP racing do 17mm vented disc.

The brakes have 4 x 30mm pistons, the standard gti is one 48mm piston (18 sq cm). Overall piston area is significantly greater 28 sq cm) Not sure what that will mean lol

Edited April 4, 2013 by Batfink

[Vili 17]

Actually the clamping force will be less with 4 x 30 mm pistons than with 1 x 48 mm piston. That is because in single piston caliper the second pad half is solid. The pad carrier "pushes against the piston". When in the four piston caliper the "reactive" force is coming from the other half's pistons. Remember the Newtons law "for every force there is an equal and opposite force". Effectively the clamping force would be same with 2 x 30 mm and solid pad carrier.

So actually if you are compering the clamping force in terms of piston area you should use 14 sq cm for the Subaru caliper and 18 sq cm for the OE caliper.

[Batfink 201]

Ok! I'll see how the clamping force affects the pedal feel. Master cylinder size will have a big effect. The slightly smaller pistons will possible require a slightly larger master cylinder or pedal ratio

[Vili 17]

Well actually to get the same clamping force with the same foot pressure you would need smaller master cylinder which will result in longer pedal travel.

Edited April 5, 2013 by Vili

[welshpug 1,622]

smaller pistons will need smaller MC to retain the same travel.

[Batfink 201]

my mistake. thought it was the other way round..

[Cameron 16]

Pressure = Force / Area

Force = Pressure * Area

 

At the master cylinder, smaller Area = larger line pressure

At the caliper, smaller Area = lower clamping force

 

[brumster 135 1 Cars]

FWIW, 17mm sounds very narrow and 30mm pistons sound very small to me. I've just gone up to 41.3+44.5 (x2) piston calipers, the braking force of which is wonderful although the pedal travel has lengthened ever so slightly - but I intended this, because I felt I wasn't getting enough leverage on the old master cylinder/caliper setup. I'm not sure what size MC you're going to need with 30mm pistons - these are for on the front right? Wondering whether this setup is going to work for you, to be honest ? Please prove me wrong though

[welshpug 1,622]

rears are 30 or 32mm, that'll need a fair difference in MC sizes if running dual to prevent rear locking

[buchanan84 1]

Actually the clamping force will be less with 4 x 30 mm pistons than with 1 x 48 mm piston. That is because in single piston caliper the second pad half is solid. The pad carrier "pushes against the piston". When in the four piston caliper the "reactive" force is coming from the other half's pistons. Remember the Newtons law "for every force there is an equal and opposite force". Effectively the clamping force would be same with 2 x 30 mm and solid pad carrier.

So actually if you are compering the clamping force in terms of piston area you should use 14 sq cm for the Subaru caliper and 18 sq cm for the OE caliper.

 

I'm not 100% but I think you've got this wrong

 

with the 2 pots your force is being applied from 1 side and reacted against by the other side (therefore F = P x area of 1 x pistons)

with 4 pots the force is being applied from both sides as the caliper and disc are fixed (therefore force for 1 side = p x area of 2 pistons + Force for the other side)

 

also with your hydraulic side

currently your master cyl is pushing 2 x 48mm pistons (ignoring rear brakes)

with the new calipers your master cyl needs to push 8 x 30mm pistons

 

say 25mm master cyl - area = 490mm2

 

current 48mm pistons - area = 3617mm2

therefore a hydraulic advantage of 7.38

 

new 30mm pistons - area = 5652mm2

therefore a hydraulic advantage of 11.53

this would give you a lot more braking force for touching the pedal but will also give a larger pedal travel

 

 

as i say i'm not 100% so... am i talking crap?

Edited April 5, 2013 by buchanan84

[Batfink 201]

Only one side is considered on multi-pot. These may not work and I may have to sell on. Seems I cannot use 17 mm discs say alcon but they have not yet told me what will work. Not many calipers will it seems! Nothing from the original braking system remains so I'll see if I can still use them. Seemed a good idea at the time. I never thought to ask about piston size.

Edited April 5, 2013 by Batfink

[stu8v 68 1 Cars]

Only one side is considered on multi-pot. These may not work and I may have to sell on. Seems I cannot use 17 mm discs say alcon but they have not yet told me what will work. Not many calipers will it seems! Nothing from the original braking system remains so I'll see if I can still use them. Seemed a good idea at the time. I never thought to ask about piston size.

 

I'm not so sure, remember fluid is moving all the pistons.

[welshpug 1,622]

stick them on the back

 

if the pistons were large enough it would be worth making the caliper wider, quite straightforward to do that given its two halves.

[brumster 135 1 Cars]

I'm not so sure, remember fluid is moving all the pistons.

 

"Only consider half" seems to make sense to me - firstly, just from the numbers, but also if you think about it, if one side of the caliper was extering more pressure than the other side it would only result in the imbalance pushing the disc over and the pots on the other side back in... surely they just find their natural balance when opposing each other, and thus only consider half of the pistons (ie. one side; or 50% of both sides) as effectively putting pressure on the disc?

 

Dunno, that's how my minds eye sees it

Edited April 5, 2013 by brumster

[Vili 17]

I'm not 100% but I think you've got this wrong

 

with the 2 pots your force is being applied from 1 side and reacted against by the other side (therefore F = P x area of 1 x pistons)

with 4 pots the force is being applied from both sides as the caliper and disc are fixed (therefore force for 1 side = p x area of 2 pistons + Force for the other side)

 

also with your hydraulic side

currently your master cyl is pushing 2 x 48mm pistons (ignoring rear brakes)

with the new calipers your master cyl needs to push 8 x 30mm pistons

 

say 25mm master cyl - area = 490mm2

 

current 48mm pistons - area = 3617mm2

therefore a hydraulic advantage of 7.38

 

new 30mm pistons - area = 5652mm2

therefore a hydraulic advantage of 11.53

this would give you a lot more braking force for touching the pedal but will also give a larger pedal travel

 

 

as i say i'm not 100% so... am i talking crap?

 

You're not totally wrong. I was right on the clamping force but wrong on the MC size and the pedal travel. Of course you have to move all four pistons when braking so you would have longer pedal travel with four pistons.

 

Lets say that we keep pedal pressure, MC size and pedal leverage constant and name their "product" ie. brakefluid pressure as P1.

 

The piston(s) pushing force on side of the caliper will be brakefluid pressure*piston area. F=PA

 

Now remember the Newton's third law. It doesn't matter if you have pistons or solid pad carrier pushing against the piston(s) on the other side. The force will be equal to the piston(s) pushing against. This is because on the single piston caliper is mounted floating so the only reactive force against the single piston is the solid pad carrier. If single piston caliper would be mounted solid it would bend the disk. The four piston caliper can be mounted solid because both side pistons are moving and there is exactly the same pressure affecting on both side pistons so they centralize them selves according to the brake disk.

 

So the clamping force with single 48 mm piston will be F1=P1*((0,048/2)´2*pi= 18,1*P1*10`-4

 

And 4x30 caliper F2=14,1*P1*10´-4

 

So we're having (18,1-14,1)/18,1= 22 % less clamping force with the same pedal pressure with the 4 x 30 mm calipers.

 

 

But you have to remember that clamping force is only one part of the braking force equation. Disk size and pad to disk friction coefficient are equally important. And when you're driving on track heat dissipation plays a big role.

Edited April 5, 2013 by Vili

[buchanan84 1]

the way i see it...

both caliper and disc are fixed, if 1 side was siezed then you would wear only 1 of the pads (the same with a sliding caliper if the sliders are siezed)

the fluid pressure is in all the fluid so provides the same pressure to both sides which is then turned into force by the piston...

 

17mm thick discs are another problem, not sure how you get round that... you could widen the caliper, it uses an external pipe so only the forces to consider...

[buchanan84 1]

You're not totally wrong. I was right on the clamping force but wrong on the MC size and the pedal travel. Of course you have to move all four pistons when braking so you would have longer pedal travel with four pistons.

 

Lets say that we keep pedal pressure, MC size and pedal leverage constant and name their "product" ie. brakefluid pressure as P1.

 

The piston(s) pushing force on side of the caliper will be brakefluid pressure*piston area. F=PA

 

Now remember the Newton's third law. It doesn't matter if you have pistons or solid pad carrier pushing against the piston(s) on the other side. The force will be equal to the piston(s) pushing against.

 

So the clamping force with single 48 mm piston will be F1=P1*((0,048/2)´2*pi= 18,1*P1*10`-4

 

And 4x30 caliper F2=14,1*P1*10´-4

 

So we're having (18,1-14,1)/18,1= 22 % less clamping force with the same pedal pressure with the 4 x 30 mm calipers.

 

 

But you have to remember that clamping force is only one part of the braking force equation. Disk size and pad to disk friction coefficient are equally important. And when you're driving on track heat dissipation plays a big role.

 

Think I'm with you now, would love to do a school experiment type thing to prove it so that my head would believe me...

its amazing how a simple thing can confuse so much

and yeh its definately only 1 part of it

[Batfink 201]

To use a thin disc I'd have to make the caliper thinner rather than wider. These will take up to 28mm disc I think. Certainly do able. I'd not want vented on the rear though so a little overkill if I stuck them there

Edited April 5, 2013 by Batfink

[welshpug 1,622]

ohhh, I thought being rears you said they'd only take 17mm, crack on with 28mm then!

[Cameron 16]

I would go for 25.4mm discs tbh.. no need for 28mm on a light car as you'll just be carrying around 20-30% more weight!

[Batfink 201]

It seems that most companies only make calipers for thick discs so my idea is trashed lol. Seeing if I can get some different calipers but new this time.

[Batfink 201]

Ordered some brand new Alcon CRH304 - these have a 38 and a 41mm piston so far more meaty. Was looking at the AP Racing CP3228 but this is a blank lug caliper which needs drilling to fit. Fook me these are expensive though so glad I get a bit off for trade and can claim the VAT back! Subaru ones can go on Ebay. Going to look into some cheap discs that have the same bell pattern as AP or Alcon so If they are sitting on the car a while its no biggie if they get a bit rusty