Nick Moore
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| posted on 19/5/07 at 11:22 PM |
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Caliper pistons - is bigger better?
Hi, like Fred WB, I'm thinking of fitting Midilites to my car, and have been trying to decide whether to import an entire kit with discs and
adapters from Rally Design, or try and get the calipers separately from the US.
Firstly, I presume that what are sold as Midilites in the UK are DynaPros in the US? They certainly look the same. Secondly, Wilwood have various
piston sizes (1.25, 1.38 & 1.75" - is biggest best, as a rule? Sierra rear calipers.
All this depends on whether Rally Design are willing to sell part of a kit. If I buy the whole thing in one hit the air freight to New Zealand will be
horrendous.
Any and all advice please!
It's not a bodge if no one finds out!
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RazMan
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| posted on 20/5/07 at 12:35 AM |
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Ahhhh glasshopper, you must first study the brack art of blaking to uncover the dark mysteries within 
Ask a hundred people and you will get a hundred different answers.
I have been going through a steep learning curve and tried many different combinations of master cylinders, caliper piston sizes and even disc sizes
....... and I am still confused
To get an accurate answer you will have to give more info -weight of car, present disc and master cylinder size, tyre size etc etc.
[Edited on 20-5-07 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Hammerhead
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| posted on 20/5/07 at 12:56 AM |
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well my system is completely experimental.
Caterham master cylinder, Lotus elise calipers, sierra cosworth discs.
Not tried them yet but I'm sure they will be fine  
Just suck it and see!
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Nick Moore
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| posted on 20/5/07 at 04:00 AM |
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Hi Raz,
The car isn't built yet, I'm trying to get it reasonably right first time! Weight will be around 950kg, and the Rally Design kit uses
285mmx22mm discs on Cortina uprights. I haven't decided on master cylinder size, and will fit either 15 or 16 inch wheels. Possibly overkill but
I'm fitting a Rover V8, and want the best brakes I can get.
As I understand it, larger caliper pistons require a larger bore master cylinder for the same pad movement. Am I right, though, that larger pistons
can provide greater control of braking force? If I have a choice of piston sizes I should get the biggest possible and design the rest of the system
to suit?
Cheers,
Nick
It's not a bodge if no one finds out!
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nitram38
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| posted on 20/5/07 at 04:48 AM |
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I look at it this way, if you can only afford single piston calipers, then go for that.
If you can afford the multi piston set up, do that.
Apart from the braking issue, there is one of unsprung weight.
For example, a single piston Rover 214 front caliper and carrier weighs 4kg and a wilwood 4 piston one weighs around 1Kg.
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RazMan
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| posted on 20/5/07 at 06:33 AM |
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Actually I was shocked at how little difference there appeared to be between the std rear Sierra setup and the Rally Design Powerlite kit with a 285mm
disc. I was expecting a dramatic difference, bracing myself for an overbraked effect ...... but no. 
During my entire build (mid engined btw) most things havent suprised me and I took to the road confident that I had designed everything within safety
limits, but the brakes were the only exception. The pedal effort required to bring my chariot to a halt is scary and I have had a drastic rethink to
rectify this area.
I first realised that the master cylinders were too large so I went down a size on both front and rear (0.7" rear 0.625" front) This
improved things slightly but I was still left with the feeling that I needed TWO FEET on the pedal to make an emergency stop . So then I substituted
the std Sierra single piston setup for 4 pot (25mm pistons) and a 285mm disc - STILL SCARY but getting better. Then I replaced the front 245mm discs
with 285mm jobbies - getting there now but I still feel underbraked for some reason. I can't get the brakes to fade, thats for sure, but they
still require a strong right leg.
It all comes down to the lack of a servo imo and you don't mention if you are using one - with a lumpy V8 I would recommend you design one in if
you have to (I most certainly will next time) but even more important is BALANCE. It is no good strapping on humungous 8 pot calipers and discs on the
front and having drums on the back is it
In short, if you do a search you will find a few different 'brake calculators' but the results are only as good as the data you feed into
them. Things like pedal ratio, brake pad surface area, centre of gravity, weight transfer etc etc will all affect your braking efficiency and it is
certainly a very very wriggly can of worms
Did that help at all?
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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nitram38
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| posted on 20/5/07 at 07:20 AM |
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Razman, how hard are your springs/shocks?
I recently change my front spring rates as the car was diving too much under brakes. Now the springs are much firmer, my brakes "feel"
harder under foot.
This might be part of your hard braking?
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907
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| posted on 20/5/07 at 07:58 AM |
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This may or may not be of use, but I read an article in a bike mag that explained
the advantages of multi pots over twin pots.
Lets assume that in the diagram below that the pot on the left is the same surface
area as the three added together on the right.
Lets also assume that a short fat caliper is the same weight as a long thin one,
and that the hydraulic forces applied to them are the same.
For the same size disc, and therefore the same weight, the six pot is effectively further out
than the twin pot, dim B versus dim A, so by the law of leavers, the six pot exerts more braking force.
An increase in disc size would achieve the same end, but at the expense of increased weight.
There is also the "law of the depth of your pocket" which has an even greater effect on what you fit.
I may of course be talking rubbish, I'm no expert, but it fooled me.
Paul G
 
Rescued attachment 2-pot-v-6-pot-s.jpg
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Mal
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| posted on 20/5/07 at 11:00 AM |
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Balance Bar Braking
Razman,
By the sound of your description you have a twin (not tandem) master cylinders. If you do, with the balace bar in the mid position you will only
develop HALF of the hydraulic pressure in the sytstem as you would with a single master cylinder with the same bore size.. Hence you needing such
heavy foot force. To compensate for this you need to change the geometry of the foot pedal pivots to double the force applied to the master cylinders.
In practice this is a juggling act with pedal travel, feel and force needed.
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britishtrident
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| posted on 20/5/07 at 03:43 PM |
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Today we tend to look at brakes particularly front brakes through eyes more accustomed to looking at brakes on FWD hot hatches. RWD cars don't
need the massive callipers and disc fitted to hot hatches.
The early Dunlop disc brakes used by Jaguar had pistons & pads roughly the size of a UK 50 pence piece.
A fwd car the size and weight distribution of an old Mini gets 95% braking from the front axle.
A Fwd Civic size super Mini hot hatch 90%
Larger FWD car Rover 75 or Avensis size 80 to 85%
Rwd BMW 320 or Sierra 75 to 80%
RWD Sports car with near 50/50 weight distribution Lotus Elan or Locost 70%
Mid engined MGT or Lotus Elise 60 to 65%
Rear Engine Porsche, Davrian Imp 48 to 55%
A Locost like car is 1/2 the kerb weight of a Sierra and 1/3 of the gross weight -- Sierra brakes are more than big enough.
So why are some builders running into problems ?
Mal has already pointed out the issues of pedal leverage and mastercylinder size ---- Leverage is always going to be a problem given the space
restriction but try to get a decent ratio (6 or 6.5:1) and design with consideration for the angularity of the pedal and pushrod.
With a dual circuit balance bar pedal box use 0.625" or 0.7" mastercylinders ---- most single seaters racing cars use 0.625"
Fit over size callipers and competition pads and the pad material won't get up to anywhere working temperature.
[Edited on 20/5/07 by britishtrident]
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RazMan
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| posted on 20/5/07 at 10:48 PM |
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quote:
Originally posted by nitram38
Razman, how hard are your springs/shocks?
I recently change my front spring rates as the car was diving too much under brakes. Now the springs are much firmer, my brakes "feel"
harder under foot.
This might be part of your hard braking?
Nope, my setup has always had almost no dive - it is the pedal that feels 'wooden' almost like the calipers are seized (but they're
not)
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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RazMan
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| posted on 20/5/07 at 10:51 PM |
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quote:
Originally posted by Mal
Razman,
By the sound of your description you have a twin (not tandem) master cylinders. If you do, with the balace bar in the mid position you will only
develop HALF of the hydraulic pressure in the sytstem as you would with a single master cylinder with the same bore size.. Hence you needing such
heavy foot force. To compensate for this you need to change the geometry of the foot pedal pivots to double the force applied to the master cylinders.
In practice this is a juggling act with pedal travel, feel and force needed.
I have a pedal ratio of 4.4:1 which is not ideal but apparently adequate according to many brake gurus. I can't increase the pedal length due to
space constraints and I don't want to lose the adjustability of the bias bar. A servo would certainly take a lot of the pedal pressure but that
has its own set of problems....
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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nitram38
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| posted on 21/5/07 at 06:54 AM |
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quote:
Originally posted by RazMan
quote:
Originally posted by nitram38
Razman, how hard are your springs/shocks?
I recently change my front spring rates as the car was diving too much under brakes. Now the springs are much firmer, my brakes "feel"
harder under foot.
This might be part of your hard braking?
Nope, my setup has always had almost no dive - it is the pedal that feels 'wooden' almost like the calipers are seized (but they're
not)
That is what I was suggesting. My car had compliant brakes when it did dive. Now I have removed a lot of the dive, my brakes feel solid under foot.
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