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Old 24-05-2019, 12:58   #1
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How far do brakes pads need to move for full braking performance to be achieved?

A bit of a geeky question perhaps, but can anyone tell me just how far brake pads (ie caliper pistons) typically need to move in order for full braking effect to be achieved? At a guess I thought it might be around 1mm, but I now think it must be a lot less than that. Does anyone have good data on this?
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Old 24-05-2019, 13:22   #2
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That depends on the brake setup:
Floating calibers need far more travel then fixed ones.

Also kickback is an issue ( wobbeling Disc i.E. ) , loose Wheel Bearings, type of Bearing i.E. Taper or Ball, Floating Disc mounting.

Thats the reason that in real race cars they use anti kickback springs behind the Pistons, so they minimize the travel needed. ( and have an far better "Point" on the brake any time with consistant Heigh of the Pedal )

I did an static measurement with an rotating HUB / DISC assembly ( Evo 8 front bearing / mount with an Stoptech ST60 Brake / Disk setup ) with brand new disk.
The "travel" including caliber deflection was around 0.2 mm total ( Max Pressure hydraulic the stock Brake system is able to Apply incl. Evo stock booster , vacuum by an extarnal vacuum pump ). With an used disk, High bearing load ( Car weight ) you can expect 2-3 times that ( not including kickback from High Cornering forces from Wheel to bearing....)

So, its hard to tell for any possible setup....
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Old 24-05-2019, 15:38   #3
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Originally Posted by evo7_GERMANY View Post
That depends on the brake setup:
Floating calibers need far more travel then fixed ones.

Also kickback is an issue ( wobbeling Disc i.E. ) , loose Wheel Bearings, type of Bearing i.E. Taper or Ball, Floating Disc mounting.

Thats the reason that in real race cars they use anti kickback springs behind the Pistons, so they minimize the travel needed. ( and have an far better "Point" on the brake any time with consistant Heigh of the Pedal )

I did an static measurement with an rotating HUB / DISC assembly ( Evo 8 front bearing / mount with an Stoptech ST60 Brake / Disk setup ) with brand new disk.
The "travel" including caliber deflection was around 0.2 mm total ( Max Pressure hydraulic the stock Brake system is able to Apply incl. Evo stock booster , vacuum by an extarnal vacuum pump ). With an used disk, High bearing load ( Car weight ) you can expect 2-3 times that ( not including kickback from High Cornering forces from Wheel to bearing....)

So, its hard to tell for any possible setup....
Thanks for the response although I can't see why a floating disc should indicate the need for any additional pad travel. Isn't the "floating" aspect purely to allow for heat expansion rather than disk runout?

The point about caliper flex is a good one.

For a pedal box arrangement (currently my main area of interest) the "normal" sizing for the fronts master cylinder is reckoned to be 0.625", and by my calculation that would equate to master cylinder piston to caliper piston ratio of something like 1:50. And that being so, it means that even for a 0.5mm pad movement the throw of the MC piston would need to be 25mm. That seems a lot to me, and in any case would be pretty much the total throw of the MC piston! So I was rather expecting/hoping that something more like a max caliper piston travel of 1-2mm is all that would needed.
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Old 24-05-2019, 16:19   #4
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The theory behind a floating disc is that it minimises or completely prevents pad knock-back. The disc is "fixed" in the rotational sense, but has limited freedom of movement in a lateral sense so that side loads don't push the pad and piston combination back into the caliper. So from a movement perspective, floating discs *ought to* mean less pedal travel compared to a fixed disc.
In terms of total movement, there ought to be far less than 1mm if travel. When the brake is released, the pads still "skim" the disc since there's no motive force acting to press the pistons back into the caliper.
As described above, however, the total brake and hub assembly is not completely rigid, so side forces acting on the hub with a fixed disc do push the pads back a little and that is the source of dreaded pad knock-back. So a tiny bit of extra movement is needed to move the pads back into direct contact with the disc is required before the (zero movement) pressure applied can create braking forces.

I'm also drinking gin in the sun right now. So if that doesn't make entirely good English, my apologies in advance...

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Old 24-05-2019, 17:28   #5
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Originally Posted by Rampant View Post
The theory behind a floating disc is that it minimises or completely prevents pad knock-back. The disc is "fixed" in the rotational sense, but has limited freedom of movement in a lateral sense so that side loads don't push the pad and piston combination back into the caliper. So from a movement perspective, floating discs *ought to* mean less pedal travel compared to a fixed disc.
In terms of total movement, there ought to be far less than 1mm if travel. When the brake is released, the pads still "skim" the disc since there's no motive force acting to press the pistons back into the caliper.
As described above, however, the total brake and hub assembly is not completely rigid, so side forces acting on the hub with a fixed disc do push the pads back a little and that is the source of dreaded pad knock-back. So a tiny bit of extra movement is needed to move the pads back into direct contact with the disc is required before the (zero movement) pressure applied can create braking forces.

I'm also drinking gin in the sun right now. So if that doesn't make entirely good English, my apologies in advance...

Slŗinte Mhath

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As far as I know, most, if not all, really decent makes of calipers incorporate knock back springs. Also, I always checkout total disc/hub runout (as learned from NR days) and will rarely be satisfied if the value is greater than 0.05mm. Of course that is assuming rigid of semi-floating discs only. Can you sensibly check runout on fully floating discs?


I agree that total pad movement should be less than 1mm, but what I'm really looking for is confirmation that it's substantially less than that (ie more like 0.2mm max) otherwise a 0.625" diameter MC with 25mm of throw simply won't be up to the job!


Enjoy the gin :-)
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Old 25-05-2019, 08:19   #6
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MC size is a drivers choise ,some prefer short pedals others longer ,its entirely up to the driver

pad to disc distance should be as little as possable ,in the case of knock off springs fitted to the pistons then its basicly nothing ,but its not down to the MC to dictate this ,a pad will only come off as far as it needs to once pressure has been released of the piston
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Old 25-05-2019, 09:17   #7
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MC size is a drivers choise ,some prefer short pedals others longer ,its entirely up to the driver

pad to disc distance should be as little as possable ,in the case of knock off springs fitted to the pistons then its basicly nothing ,but its not down to the MC to dictate this ,a pad will only come off as far as it needs to once pressure has been released of the piston
I fully accept that MCs can be selected to suit specific driver preference on pedal travel, but altering the MC diameter sizing will necessarily alter the amount of required pedal pressure too ie two two go very much in hand.

What I trying to achieve here is a servoless twin MC system that retains the existing OE brake pedal (which supposedly has a mechanical advantage ratio of only 2.9:1 - whereas with a pedal box arrangements I believe it's generally 5 or 6:1), but avoids pedal driver input forces needing to be in excess of around the 50kgs. We have chosen to use fairly large aftermarket 4 pot calipers (with a combined piston area of 10000sq/mm) and so when combined with a 5/8" MC (piston area just 200sq/mm) the ratio of movement between one and the other is 50:1 - so even a movement of the caliper pistons by 0.1mm will require MC piston movement of 5mm. That of course is not a problem because the MC piston can travel up to 25mm (the full stroke of the particular make of MC we have chosen), but that means the caliper piston travel can never be greater 0.5mm (unless you routinely pump the brakes, which clearly is undesirable). So I'm just hoping that 0.5mm (from either side of the disc ie 1.0mm in total) will be enough - which it sounds like it should be.
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Old 25-05-2019, 19:33   #8
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id be suprised if there any thing like 1/2 mm id be inclined to think its 1 or 2 tenths at most ,i know when ive allways put new pads in my evo theres been VERY little room for error often resulting in me taking the flat pad and sandpaper to the pads to clean off the slightest amount
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Old 29-05-2019, 09:10   #9
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I have a further geeky question on a closely related point - when the brake pedal is pressed the applied force (foot force multiplied by the pedal ratio) is transmitted through to the master cylinder(s). In the case of a pedal box type of arrangement that force is split between the pair of master cylinders such that they each receive around half of the total applied force (assuming broadly 50:50 bias has been set). But in the case of a tandem master cylinder (as in most OE situations) I can't decide whether all of that force acts both on the fronts and the rears or whether in fact they are also subjected to half each?
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Old 29-05-2019, 21:34   #10
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I have a further geeky question on a closely related point - when the brake pedal is pressed the applied force (foot force multiplied by the pedal ratio) is transmitted through to the master cylinder(s). In the case of a pedal box type of arrangement that force is split between the pair of master cylinders such that they each receive around half of the total applied force (assuming broadly 50:50 bias has been set). But in the case of a tandem master cylinder (as in most OE situations) I can't decide whether all of that force acts both on the fronts and the rears or whether in fact they are also subjected to half each?
you will often find theres reducers or restrictors in the rear lines of most standard systems
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Old 30-05-2019, 07:29   #11
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you will often find theres reducers or restrictors in the rear lines of most standard systems
On this particular car the abs has been removed and the only kind of restrictor is a Tilton proportioning valve in the rear circuit.
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Old 30-05-2019, 08:28   #12
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but avoids pedal driver input forces needing to be in excess of around the 50kgs.
Actually, 90 kg for full brake force is quite acceptable, and I would even say normal for unassisted race car

Quote:
Originally Posted by plip1953 View Post
I have a further geeky question on a closely related point - when the brake pedal is pressed the applied force (foot force multiplied by the pedal ratio) is transmitted through to the master cylinder(s). In the case of a pedal box type of arrangement that force is split between the pair of master cylinders such that they each receive around half of the total applied force (assuming broadly 50:50 bias has been set). But in the case of a tandem master cylinder (as in most OE situations) I can't decide whether all of that force acts both on the fronts and the rears or whether in fact they are also subjected to half each?
Tandem brake masters are made for diagonal brake circuits.. i.e. connecting left front and right rear brake caliper on the same master cylinder, and vice versa.. In such a circuit you do need a brake force adjuster to regulate rear brake force, and it has to be a "twin" regulator. This is indeed required by some regulation, so do check. (for example grN mitsubishis run just such a system,.. servoless, twin diagonal circuit with such a brake force regulator)

You could connect the front brake calipers to one half of the master cylinder, and rears to other, and in such a case you can use a simpler and cheaper brake force regulator. In any case, tandem master cylinders have two pistons of same area, hence same pressure in both outlets..
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Old 30-05-2019, 20:36   #13
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Actually, 90 kg for full brake force is quite acceptable, and I would even say normal for unassisted race car
Interesting, although we Iíve read in quite a few places that around 120lbs, or 55kgs, is about the max that a normal human being can normally manage?

The application Iím talking about here is a Mk3 Mazda MX5 and it doesnít need to confirm to any particular usage regulations nor does it need to pass an MOT.

Assuming the brake pedal is pushed with 120lbsf and the pedal ratio is around 3 (OE MX5 brake pedal ratio), that will provide 360lbf to act on the master cylinder(s). But where, as on this particular car, the pedal is operating twin pistons (ie pedal box style) is the force going to each MC just half the total being applied (ie 180lbf each)? I think it must be the case, but I canít quite get my head around why! Is it different with a tandem master cylinder ie where the full force is acting on just one physical MC unit, albeit that it contains two in-line pistons.
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Old 31-05-2019, 06:54   #14
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a bias pedal box often runs smaller clyincers .625 and .7 most of the time, a standard servo system runs 1 , 1.25 or 1.5

nearly all bias pedal boxes ive seen being used still run the standard pedal system so all OE pivits are used ,and only testing provides the ideal pedal travel and feel for what ever needs the systems being used for
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Old 31-05-2019, 08:11   #15
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a bias pedal box often runs smaller clyincers .625 and .7 most of the time, a standard servo system runs 1 , 1.25 or 1.5

nearly all bias pedal boxes ive seen being used still run the standard pedal system so all OE pivits are used ,and only testing provides the ideal pedal travel and feel for what ever needs the systems being used for
Yes, I'm well aware of that, and in my case the kit I'm using is supplied with .625" and .7" MCs.

I can't remember what the OE MX5 MC spec is (and don't have the car to go and look at right now), but I think it's a 7/8" or 15/16". A 7/8" piston has a surface area of 0.603sq ins and that of 5/8" piston is 0.3068sq ins, so roughly half the former, and therefore the fluid pressure being generated in the smaller 5/8" MC will be double. But I'll be losing servo assistance (roughly 4 times force multiplier?) so was rather hoping that use of the smaller diameter MC in the new setup would compensate at least to some extent for the servo loss. Right now with the servo disconnected the required brake pedal force is huge!!


And as previously mentioned, by retaining the OE brake pedal arrangement we are pretty much stuck with the OE pedal ratio which is a very lowly 2.9:1. In a pedal box arrangement it would normally be double that, so I was rather hoping that the "half size" MC of the new arrangement would compensate to at least some extent.


Once installed, we will of course test everything before using in anger, but I'd rather get things as near possible right at the design stage rather than have to go through too much of a trial and error process.
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