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Old 01-05-2018, 03:08 PM   #16
250ptm
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Originally Posted by AGoaty View Post
I think the issue is not pressure, but flow. If you want to move the 4 pot caliper pistons, this will take more fluid movement than a single pot caliper. Having a large bore rear needs quite a bit of fluid to move it. Reducing the cylinder size will increase the amount the piston comes out? Am I wrong?

I said earlier that there was a 2 tablespoon difference in fluid displacement. Im revising that comment after more measuring and research.
• 4 pot metro caliper piston diameter 1.414” Area = pi*0.707^2 * 8 = 12.56 sq inches
• 2 pot 8.4” disc mini caliper piston 2.00 diameter. Area =-pi*1^2*4 = 12.56 sq inches
• Fluid displacements are exactly the same.
Also Minisport & KAD 4 pot calipers pistons are exactly the same dimensions as Metro calipers.
This info sort of debunks the idea that the rears cylinders move before the front calipers
If a small diameter piston is completely free to move without any restriction, it will move more for any given pedal travel than a larger diameter one. However it is restricted by the drum and because of its size produces less pressure
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Old 01-05-2018, 03:28 PM   #17
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Yeah, I'm running MGF front calipers, no bias valve, 1/2" rear cylinders.
This is what Watsons recommend.
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Old 01-05-2018, 08:49 PM   #18
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Originally Posted by fynnbar View Post
Yeah, I'm running MGF front calipers, no bias valve, 1/2" rear cylinders.
This is what Watsons recommend.
Funny you say that as i have been watching the Watsons Honda build dvd to see how he did the steering, but Geoff say's in the dvd to use the largest cylinders on the rear and this is what he told me over the phone when I rang for some advice a couple of years ago. Much to my alarm when I get a bit to forceful on the stop pedal the rears lock up and the back try's to pass the front so I'll be changing down to a smaller size
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Old 02-05-2018, 11:37 AM   #19
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Because with a dual braking system you do not have an "entire system" you have two completely separate systems which only share a brake fluid reservoir and are operated by one pedal. That's the whole point of a dual circuit brake system, if one fails the other keeps working by virtue of them being totally separate.

You cannot adjust this on a standard Mini system so if you alter one circuit (eg fit calipers with different fluid capacities to the front) it will have a knock on effect on the other circuit which, because being that it is completely separate, cannot equalise the fluid as you suggest. It is simply being operated by a pedal that is having to move that slight bit more to compensate for the extra capacity at the front calipers hence the "overthrow" at the rears, they are trying to be pushed out slightly further by the extra pedal throw but because they can't as the shoe has contacted the drum they can only increase the pressure applied by the shoes to the rear drums causing them to be more likely to lock.
Thanks for your explanation of how a dual braking system works.

Let me clarify my original statement: if pressure in any closed system is static and not increasing then that system can be considered to be in balance, or equalised (in pressure). Regardless of how many separate circuits you may have emanating from the single master cylinder, if each circuit has reached this state then the “system” in its “entirety” can be considered to be in balance or equalised (in pressure).

This point is reached when the brake pedal is moved sufficiently so that all wheels have a positive contact with their respective drum or rotor.
Any residual system pressure developed in any circuit “in this condition” will definitely NOT cause a wheel to lockup, as all the pressures are far too low !

Since I last commented on this topic I have done more research and find that the Stock 8.4 inch 2 pot calipers, have the same fluid displacement as 4 pot METRO/Minisport and KAD.

Because the front caliper pistons are much larger than the rears (stock 2 pots are 2” dia and to stock rear cylinders are 0.75 dia) it means the front pistons have a larger area by a factor of 6. (pi*1^2*4= 12.56 Sq ins /compared to (pi*0.375^2 *4= 1.76 sq ins).

Consequently the front calipers will always produce a far greater force than the rears for any given movement of the brake pedal.
Essentially, if you have 25 lbs of pedal pressure with the stock 4:1 ratio pedal, the front calipers will provide approx 600lbs of clamping force to the discs whereas the rear drums see approx. 100lbs

Which brings me back to my original point with you. The rear wheel cylinders do not cause a rear wheel lock up, because they Quote "overthrow" and lock the rears.” unquote.

The practical explanation for rear wheel lockup is a known problem for this car: Weight transfer towards the front of the car under heavy braking.
By the way, don’t assume this is compounded because of the addition of a honda lump up front. The internet abounds with data on engine weights, however one popular honda site says the B16 comes in around 405 lbs with axles, about 90lbs more than a dressed 998 A series.

As I said in Post # 4 I use a reduced rear cylinder size and fitted a bias valve. This, as has been performed by other 16valvers on this site, also works for me.
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Old 02-05-2018, 02:39 PM   #20
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WHERS HARLY? Op?
There's absolutely NO one size fits all rule hear!!
Austin/Morris/b,l rover ect ect didn't really know what was what !!! That's the reason they kept changing brake combinations ,
I don't believe it's even worth trying to understand! As such!
And if I read/understood it correctly, Mr Watson's advising
To"just fit that" is totally wrong advice!
Reason is that all our minis are one off,s
So many different brake line systems!
Some have p,w,a some may even have the factory inbuilt rear
limiter/componsator,
Some have standard rubber lines ,some have the braided ect, some will have substandard rear feed at source,
Take your mini and make it work for you!
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Old 03-05-2018, 03:21 AM   #21
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Didn't see this thread has taken off!

To answer some questions - I have standard rover brake servo / master cylinder and the proportion valve inderneath.

I did some playing around on the weekend and dropped my rear tyre pressures, re-adjusted the rear brakes, and fitted an adjustable rear bias valve in place of the rover one (that I then dismantled and determined to be partially seized so that wouldn't have helped).

End result is a much more drivable car. Rears still lock up before the fronts, but that been somewhat normal on all the minis I've had over the years.

Next mod will be heavier springs and harden the front shocks up a bit to try and reduce the front sagging under brakes and hopefully shifting slightly less weight.

Thanks for all the input / debates.
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Old 03-05-2018, 12:18 PM   #22
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Originally Posted by 250ptm View Post
Thanks for your explanation of how a dual braking system works.

Let me clarify my original statement: if pressure in any closed system is static and not increasing then that system can be considered to be in balance, or equalised (in pressure). Regardless of how many separate circuits you may have emanating from the single master cylinder, if each circuit has reached this state then the “system” in its “entirety” can be considered to be in balance or equalised (in pressure).

This point is reached when the brake pedal is moved sufficiently so that all wheels have a positive contact with their respective drum or rotor.
Any residual system pressure developed in any circuit “in this condition” will definitely NOT cause a wheel to lockup, as all the pressures are far too low !

Since I last commented on this topic I have done more research and find that the Stock 8.4 inch 2 pot calipers, have the same fluid displacement as 4 pot METRO/Minisport and KAD.

Because the front caliper pistons are much larger than the rears (stock 2 pots are 2” dia and to stock rear cylinders are 0.75 dia) it means the front pistons have a larger area by a factor of 6. (pi*1^2*4= 12.56 Sq ins /compared to (pi*0.375^2 *4= 1.76 sq ins).

Consequently the front calipers will always produce a far greater force than the rears for any given movement of the brake pedal.
Essentially, if you have 25 lbs of pedal pressure with the stock 4:1 ratio pedal, the front calipers will provide approx 600lbs of clamping force to the discs whereas the rear drums see approx. 100lbs

Which brings me back to my original point with you. The rear wheel cylinders do not cause a rear wheel lock up, because they Quote "overthrow" and lock the rears.” unquote.

The practical explanation for rear wheel lockup is a known problem for this car: Weight transfer towards the front of the car under heavy braking.
By the way, don’t assume this is compounded because of the addition of a honda lump up front. The internet abounds with data on engine weights, however one popular honda site says the B16 comes in around 405 lbs with axles, about 90lbs more than a dressed 998 A series.

As I said in Post # 4 I use a reduced rear cylinder size and fitted a bias valve. This, as has been performed by other 16valvers on this site, also works for me.
Wow, how patronising and condescending, I hope you feel better getting that off your chest. Here you might find this useful for future reference



PS you haven't actually got the point of what I was trying to say with any of this but I have no interest getting involved on a willy-waving contest on the net as these are usually only won by the biggest dick.

Have a lovely day
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Old 03-05-2018, 12:43 PM   #23
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Default Rear wheel cylinder sizes

I can’t get my head around why the weight transfer to the front causes the rear drums to lock up before the front, I can see how that causes the rear end trying to switch places with the front but I don’t really understand how it would cause the actual locking of the brakes, that surely has to do with the bias and the pressure put on the rear brakes


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Old 03-05-2018, 02:54 PM   #24
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Taken from the Mini Spares website.

When altering the front brake set-up away from the standard installation, it invariably alters the amount of rear brake bias needed to re-balance the cars brake balance.

The problems start occurring when folk graft alternative brake set-ups onto the front of their Minis. Be this 7.5" discs onto a previously drum-braked model, or going from current standard disc set-ups to one of the 4-pot varieties. The question then is 'what bore size rear wheel-cylinders do I need?' Unfortunately this isn't straightforward. Actually, that's not strictly true - more on this later. The pressure developed in the lines determines brake effectiveness - aside from disc size, pad type, co-efficient of friction, etc. that is. Understanding this may assist in choosing which way you should go for your particular set-up.

The basic facts you need to remember when dealing with brake pressures is how they are affected by the components you use. The amount of hydraulic pressure produced at the pedal is INVERSLEY proportional to the master cylinder bore, and the pressure produced at the caliper or wheel cylinder is the opposite to that. So a smaller bore master cylinder will develop a HIGHER line pressure than a larger bore one at the 'working end', and conversely a smaller bore caliper piston or wheel- cylinder will develop LOWER line pressure at the 'working end'. I know we're supposed to be looking at rear wheel-cylinders - but this is relatively important. Especially on racers where master cylinders and pedal assemblies may well be changed for something more, well, 'racy'. The former and following information, once ingested and understood, will help find a path through the jungle. However, a large proportion of the questions asked concern road use, so we'll concentrate on the rear cylinders now.

Over the years, Austin Morris, BL, Leyland, and Rover have all had a dabble at messing with the braking systems components fitted to the Mini in it's various forms. It could be said that this has left us in a fortunate position as far as choice goes, as no less than five different wheel-cylinder bore sizes were used. All of which will fit any brake back-plate with relative ease, i.e. may need to drill the locating-pin hole on the other side. GWC1101 has a 0.625"(15.88mm) bore, GWC1102 a 0.750"(19.0mm) one, GWC1126 a 0.500"(12.7mm) one, GWC1129 a 0.6875"(17.46mm) one and GWC1131 a 0.5625"(14.29mm) one. Quite a selection as stated, but confusion is added by the use of pressure regulators/distributors. Some of which don't do any regulating of the pressure at all.

There are too many variables including all-up car weight, front/rear weight split, tyre size and type, front brake choice, etc., to make an exact and correct list. As a guide to start with, working suggestions I have used with reasonable success can be generally channeled into a few groups -

Discs with 4-pots and latest built-in servo and 7.5" discs with S-type servo, use GWC1131.
8.4" discs with or without servo, use GWC1126.
7.5" discs no servo use GWC1129.
The above is presupposing NO regulator valve is used in the rear feed line as they really complicate matters, and is irrespective of master cylinder bore size - assuming a standard Mini fitment one of some type is used. I re-iterate - it's a working start.

Incidentally - that thing on the bulkhead (called a P.D.W.A. - Pressure Differential Warning Activator) on early split system 'foreign' models is there purely to maintain line pressure in one half of the system in case of failure in the other. I believe these were only used on diagonal split systems. Recognised as being a large cast-iron 4-way union with an electrical connection in it located on the front bulkhead (part number 13H5905). It DOES NOT regulate/bias the line pressure when braking with the system working normally, as many believe. Some of the regulation/biasing is done in the master cylinder, the rest with the wheel-cylinders.

The other 4-way valve on the bulkhead on later cars (part number FAM7821) is frequently wrongly identified as one of the above as they are similar. However, this unit does not have the electrical connection, and has a cylinder sticking out one end. This IS a brake pressure regulator, only fitted to front/rear split systems as far as I'm aware. The PDWA is built into the master cylinders in these applications.

The FAM7821 regulator valves are a pain. Impossible to tell if they're working properly, cause all kinds of grief when trying to bleed the complete system should the wrong method be used, and are not re-buildable. If it fails - you have to buy a new one. Consequently, I discard this at the earliest opportunity and do the following…

The 'reasonably straightforward' method mentioned earlier is basically using an in-line adjustable brake regulator valve. Simple. I now always plumb in a Mini Spares Centre MS72 type between front and rear brakes, binning the FAM7821 in the process (working or not) and use GWC1102 rear wheel cylinders (cheapest of the bunch) if not already fitted. Sometimes under the bonnet by the master cylinder (easiest to fit and plumb in) and sometimes by the driver's seat (usually race-orientated stuff). Not only does it facilitate fine-tuning of the rear brake bias/efficiency irrespective of rear wheel-cylinder size, it's generally cheaper as the correctly sized rear wheel-cylinders can be costly as can be the FAM7821 regulator valve. Make sure you get a price before ordering wheel-cylinders - some of the cost prices will make you choke!

For those who already have a regulator valve on the rear subframe, trying to establish which wheel cylinder bore sizes to use with whatever type of front brake set-up you are grafting on is a real lottery. My advice to you is the same as those with a regulator strapped to the bulkhead - bin it and use an adjustable bias valve. The simplest way to do this is to either lob the standard valve bolted onto the rear subframe, and substitute it with a simple 3-way connector, or strip the guts out of the regulator valve and re-fit it 'empty'. The only problem with this is it does weird things to the braking efficiency because of the now large volume of brake fluid it holds. Then plumb the bias valve in as detailed in the article mentioned below.

See 'Brakes - Split brake systems and bias adjustment valves' for further information.

Useful part numbers:
3H2424 3-way brake pipe connector

----------------------------------------------------------------------------------------------

Good advice as this is exactly what I have.
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Old 03-05-2018, 08:09 PM   #25
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I remember reading that - there are loads of similar advice articles on there, think Keith Calver wrote some of them. So, basically buy the biggest one you can and adjust it downwards with a bias valve as far as necessary. What valve did you use Andrew?


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Old 04-05-2018, 12:19 AM   #26
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I've done what Andrew has written above, my set up is MGF 240mm vented disc's with Mintex 1144 pads, super minifin rear drum's with 3/4 bore slave cylinders and a brand new stepped bore yellow band master cylinder. As I said above this is what Geoff @ Watsons Rally told me to fit the 3/4 bore slave's to the rear, I have also fitted a bias valve down by my hand brake, this is wound all the way in and I can still lock the rears before the fronts.
Bias valve was from Torques on Ebay; https://www.ebay.co.uk/itm/TORQUES-A...item590102e268
This is a contradiction to what Fynnbar has said he was told to do by the same company, I have a pair of other new slave cylinders somewhere so I'm going to dig these out and measure the size of them and if there smaller then I'll fit them and see how I go with the rear's locking up under heavy breaking. If they no longer lock up I'll start opening up the bias valve until they do and then close it back in a bit.

"Instant Custard; I can’t get my head around why the weight transfer to the front causes the rear drums to lock up before the front, I can see how that causes the rear end trying to switch places with the front but I don’t really understand how it would cause the actual locking of the brakes, that surely has to do with the bias and the pressure put on the rear brakes"

It is the same principal, it is weight transfer, think about it when you accelerate hard you get wheel spin in a fwd car because the weight has transferred to the rear and made the front go light, same as in a rwd car you tend to see the back end squat down and get grip because the weight has transferred to the rear when you drop the clutch.

Weight transfer
From Wikipedia, the free encyclopedia
For other uses, see Weight transfer (disambiguation).

Camaro performing a wheelie during drag racing.

A motorcyclist performing a stoppie.

A Toyota MR2 leaning to the outside of a turn.
Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects:[1] the change in load borne by different wheels of even perfectly rigid vehicles during acceleration, and the change in center of mass (CoM) location relative to the wheels because of suspension compliance or cargo shifting or sloshing. In the automobile industry, weight transfer customarily refers to the change in load borne by different wheels during acceleration.[2] This is more properly referred to as load transfer,[1][3] and that is the expression used in the motorcycle industry,[4][5] while weight transfer on motorcycles, to a lesser extent on automobiles, and cargo movement on either is due to a change in the CoM location relative to the wheels. This article uses this latter pair of definitions.

Contents
1 Load transfer
2 Cause
3 Center of mass
4 Traction
5 Rollover
6 See also
7 References
8 External links
Load transfer
In wheeled vehicles, load transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral).[3] This includes braking, and deceleration (which is an acceleration at a negative rate).[6] No motion of the center of mass relative to the wheels is necessary, and so load transfer may be experienced by vehicles with no suspension at all. Load transfer is a crucial concept in understanding vehicle dynamics. The same is true in bikes, though only longitudinally.[4]

Cause
The major forces that accelerate a vehicle occur at the tires' contact patches. Since these forces are not directed through the vehicle's CoM, one or more moments are generated whose forces are the tires' traction forces at pavement level, the other one (equal but opposed) is the mass inertia located at the CoM and the moment arm is the distance from pavement surface to CoM. It is these moments that cause variation in the load distributed between the tires. Often this is interpreted by the casual observer as a pitching or rolling motion of the vehicles body. A perfectly rigid vehicle without suspension that would not exhibit pitching or rolling of the body still undergoes load transfer. However, the pitching and rolling of the body of a non-rigid vehicle adds some (small) weight transfer due to the (small) CoM horizontal displacement with respect to the wheel's axis suspension vertical travel and also due to deformation of the tires i.e. contact patch displacement relative to wheel.

Lowering the CoM towards the ground is one method of reducing load transfer. As a result load transfer is reduced in both the longitudinal and lateral directions. Another method of reducing load transfer is by increasing the wheel spacings. Increasing the vehicle's wheelbase (length) reduces longitudinal load transfer while increasing the vehicle's track (width) reduces lateral load transfer. Most high performance automobiles are designed to sit as low as possible and usually have an extended wheelbase and track.

One way to calculate the effect of load transfer, keeping in mind that this article uses "load transfer" to mean the phenomenon commonly referred to as "weight transfer" in the automotive world, is with the so-called "weight transfer equation":

{\displaystyle \Delta Weight_{front}=a{\frac {h}{b}}m} {\displaystyle \Delta Weight_{front}=a{\frac {h}{b}}m} or {\displaystyle \Delta Weight_{front}={\frac {a}{g}}{\frac {h}{b}}w} {\displaystyle \Delta Weight_{front}={\frac {a}{g}}{\frac {h}{b}}w}
where {\displaystyle \Delta Weight_{front}} \Delta Weight_{{front}} is the change in load borne by the front wheels, {\displaystyle a} a is the longitudinal acceleration, {\displaystyle g} g is the acceleration of gravity, {\displaystyle h} h is the center of mass height, {\displaystyle b} b is the wheelbase, {\displaystyle m} m is the total vehicle mass, and {\displaystyle w} w is the total vehicle weight.[7][8]

Weight transfer involves the actual (relatively small) movement of the vehicle CoM relative to the wheel axes due to displacement of the chassis as the suspension complies, or of cargo or liquids within the vehicle, which results in a redistribution of the total vehicle load between the individual tires.

Center of mass
Weight transfer occurs as the vehicle's CoM shifts during automotive maneuvers. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoM. Front-back weight transfer is proportional to the change in the longitudinal location of the CoM to the vehicle's wheelbase, and side-to-side weight transfer (summed over front and rear) is proportional to the ratio of the change in the CoM's lateral location to the vehicle's track.

Liquids, such as fuel, readily flow within their containers, causing changes in the vehicle's CoM. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced.

By way of example, when a vehicle accelerates, a weight transfer toward the rear wheels can occur. An outside observer might witness this as the vehicle visibly leans to the back, or squats. Conversely, under braking, weight transfer toward the front of the car can occur. Under hard braking it might be clearly visible even from inside the vehicle as the nose dives toward the ground (most of this will be due to load transfer). Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn can occur.

Sorry for the long winded reply!
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Old 04-05-2018, 07:54 AM   #27
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Instant Custard/Gadget555
Heres some info that may explain it.
I know my car is 839 Kg (Driver + full Fuel) and can achieve a 7.21Ms^2 deceleration rate as tested on a brake dyno 3 months ago.
So I borrowed this equation from Wiki: I use a different one but the result is very similar.
a = deceleration rate m/s^2 (7.21Ms^2)
G = Gravity (9.81 m/s^2)
h= center of gravity above ground level (0.5 M)
W= Weight of Car. (839 kg)
B = Wheelbase (2.03 M)
Weight f = The change of weight over the front axles during braking at 7.21
= a/G * (h/B)* W
So I will use my numbers as an example.
Wf = (7.21/9.81) * (0.5/2.03)* 839
Wf = (0.7349) * (0.2463)*839
Wf = 0.7349 * (206.6)
Wf= 153 KG Increase in weight over the front wheels while braking at my maximum decal rate.
This means a decrease of theweight over the rear tires.
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Old 13-05-2018, 10:45 AM   #28
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Here's another sideline to this thread, I've just replace my rear slave cylinders and the only way that they fit on is with the bleed nipple downwards (I've checked the mini manual and had a look on google and they are the right way), does this not mean that air can get trapped in the top of the cylinder where the brake pipe goes in?
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Old 13-05-2018, 10:46 AM   #29
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Default Rear wheel cylinder sizes

You have to drill a hole in the backplates to fit the 1/2” cylinders so that they fit correctly.


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Old 13-05-2018, 11:06 AM   #30
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Yeah I've done that, but why (to me anyway) are they upside down shouldn't the nipple be at the top so all the air can be removed?
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