RazMan
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| posted on 22/7/09 at 11:23 AM |
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Brake options needed guys
Recently I have been tweaking the brakes on my middy and I have a bit of a problem - basically I am under-braked on the rear. The front brakes lock up
waaay too easily compared to the rears so I need to increase braking efficiency to the rears (remember this is a middy)
First of all, here's my original setup - Front: 285mm vented discs with Dynalite 4 pots.
Rear: 273mm solid discs with Powerlite 4 pots w/handbrake.
Bias bar connecting to 2 x 0.625" m/c
My first thought was to look at the bias bar - all is set up correctly with full bias to the rear circuit. The pushrods are equal in length when all
slack is taken up.
Next I changed the Smart Pads (which I found rather wooden at slow speeds) to Mintex 1144 - big improvement in feel and braking efficiency at
'road' speeds (yet to try on the track)
So now I'm a bit stumped and need some ideas that don't involve spending hundreds of ££  I did try and speak to Rally Design (I
originally bought the kit from them) but Lee just went into 'pendantic arse' mode and was extemely unhelpful, spouting lots of rubbish
about pedal ratios and hydraulic problems (which I had already been through in the design stage)
As I see it, one option is to use a bigger disc to make the most out of the existing caliper, moving it further away from the axle. Does anyone know
what discs will fit my Sierra rear hubs?
I could try and play with the pushrod lengths, making more bias to the rear but I am reluctant to go against RD's setup instructions.
Any help will be appreciated as always 
[Edited on 22-7-09 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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nitram38
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| posted on 22/7/09 at 11:27 AM |
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From my limited experience put a bigger master to the rears. Either a 0.7 or 0.75
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RazMan
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| posted on 22/7/09 at 11:31 AM |
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quote:
Originally posted by nitram38
From my limited experience put a bigger master to the rears. Either a 0.7 or 0.75
That will surely make things worse, reducing the mechanic/hydraulic advantage won't it? A smaller m/c will move less fluid but exert greater
pressure.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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cd.thomson
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| posted on 22/7/09 at 11:33 AM |
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that should be the other way around nitram, more brakes to the rear needs a bigger cylinder on the front brakes.
it should make a big difference to balance.
Craig
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will121
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| posted on 22/7/09 at 11:33 AM |
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initial thought wouldnt think there would be a problem with the size of the rear disks being to small or pad friction area size but hate to say it not
enought hydraulic force going to the rear calipers, (or to much to the front) is the ballance bar working properly and not jamed, have you wound it
fully the other way to see?
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nitram38
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| posted on 22/7/09 at 11:38 AM |
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I been here before and got a right earful for my trouble.
Both my rear engine cars had larger rear master cylinders, both passed sva.
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britishtrident
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| posted on 22/7/09 at 11:39 AM |
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Are you sure you are winding the balance bar the correct way ?
For more rear bias the bias bar should be closer to the cylinder conected to the rear brakes .
Also the bias bar should be set up so the trunnions at either end have just enough free side clearance to allow the bar to pivot.
Any Bias bar should work properly within about 4 turn of the central position --- one turn towards the rear is normally enough to make the
difference between dry and wet settings (you need a greater % of braking from the rear for track use in the wet).
If the that doen't work think about putting a 0.7" dia master cylinder on the front circuit. However this will
increase pedal pressure.
[Edited on 22/7/09 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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wicket
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| posted on 22/7/09 at 11:39 AM |
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I think it may be worth having another look at how the bias bar is set up. I found the attached very useful when doing mine. The push rods are set at
different lengths.
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wicket
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| posted on 22/7/09 at 11:39 AM |
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and another doc
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nitram38
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| posted on 22/7/09 at 11:42 AM |
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Easy way to find out.
Buy a bigger master and fit it.
Try the brakes.
If I'm wrong then just switch the circuits.
Either way, the cost is the same in parts.
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RazMan
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| posted on 22/7/09 at 11:47 AM |
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Yep, I set up my bias bar exactly how the Wilwood link states so I am confident that things are ok there - the rods are level when moderate pressure
is applied.
I used to have a larger (0.7" ) m/c on the front circuit but that required eye-popping amounts of pedal pressure to stop even at slow speeds so
I went back to the 0.625". I realise this is not ideal and could have a lot to do with my imbalance, but I am hoping to find another solution /
compromise. Ideally I would like to fit an even smaller m/c on the rear circuit but as far as I know there is no such animal. 
[Edited on 22-7-09 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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britishtrident
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| posted on 22/7/09 at 11:48 AM |
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quote:
Originally posted by wicket
I think it may be worth having another look at how the bias bar is set up. I found the attached very useful when doing mine. The push rods are set at
different lengths.
Push rods at different lengths --- oo missus !
Great care needed with that one ----- should only be done if you have a nasty problem with excessive piston travel on one circuit, get it wrong and
you end up with a hydraulic lock and the brakes stuck full on when the fluid warms up and expands.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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nitram38
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| posted on 22/7/09 at 11:50 AM |
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quote:
Originally posted by RazMan
Yep, I set up my bias bar exactly how the Wilwood link states so I am confident that things are ok there - the rods are level when moderate pressure
is applied.
I used to have a larger (0.7" ) m/c on the front circuit but that required eye-popping amounts of pedal pressure to stop even at slow speeds.
[Edited on 22-7-09 by RazMan]
My point exactly, put the 0.7 on the rears and try it..............cost zero
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britishtrident
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| posted on 22/7/09 at 11:56 AM |
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I just noticed the words middy -- with a middy you should have the front calipers with only a slightly larger total piston area than the rears.
You can allow for it by fitting a bigger master cylinder on the front but you will still another problem the the rear brakes will warm up at a
different rate from the fronts ie you will have cold front brakes but hot rears -- more so if they are vented discs and the rears aren't.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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RazMan
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| posted on 22/7/09 at 12:02 PM |
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quote:
Originally posted by nitram38
My point exactly, put the 0.7 on the rears and try it..............cost zero
But that will achieve the opposite effect - requiring more pedal pressure to exert the same braking force.
I suppose that logic suggests that a 0.7" front m/c is called for ...... if I can live with the pedal pressure.
[Edited on 22-7-09 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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britishtrident
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| posted on 22/7/09 at 12:03 PM |
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quote:
Originally posted by nitram38
My point exactly, put the 0.7 on the rears and try it..............cost zero
Why that will give much less effort rear brakes basic hydraulics
see http://www.physlink.com/Education/askExperts/ae526.cfm
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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nitram38
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| posted on 22/7/09 at 12:04 PM |
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Humour me.....try it.
Sometimes trying things work.
You have the cylinder already. All it will cost you is some time.
As I said, it worked on both my rear engined cars.
As to your example (britishtrident) of pressure, that is all well and good accept when you run out of master cylinder volume before the pistons on
your calipers are fully out.
You may find that the .625" is bottoming out, which would require a double pump to shift the correct volume of fluid.
Something you don't want to do is pump the pedal twice!
Fitting a bigger master will require more effort but maybe it won't be exhausted of fluid by the time the pads are against the disc.
[Edited on 22/7/2009 by nitram38]
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hughpinder
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| posted on 22/7/09 at 12:15 PM |
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Its not a problem with the volume of fluid in the master cylinders compared to your rear calipers is it? e.g can't displace enough fluid to fill
the pistons of the rear calipers properly to push them against the disks? If you have the bias bars adjusted to give maximum pressure to the rears as
you say, does that limit the travel of the piston in that master cylinder more than in the front circuit?
When you replaced the brake pads for ones with higher friction material - you did just do the rears (as these are the ones you want to increase the
power of) didn't you, otherwise you increase both circuits equally, so the imbalance remains.
If you change to bigger rear disks I would have thought the major problem would be moving the caliper mounts to match!
Don't know if any of that is any use to you
Good luck
Hugh
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nitram38
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| posted on 22/7/09 at 12:19 PM |
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quote:
Originally posted by hughpinder
Its not a problem with the volume of fluid in the master cylinders compared to your rear calipers is it? e.g can't displace enough fluid to fill
the pistons of the rear calipers properly to push them against the disks? If you have the bias bars adjusted to give maximum pressure to the rears as
you say, does that limit the travel of the piston in that master cylinder more than in the front circuit?
When you replaced the brake pads for ones with higher friction material - you did just do the rears (as these are the ones you want to increase the
power of) didn't you, otherwise you increase both circuits equally, so the imbalance remains.
If you change to bigger rear disks I would have thought the major problem would be moving the caliper mounts to match!
Don't know if any of that is any use to you
Good luck
Hugh
Just wot I said! 
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RazMan
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| posted on 22/7/09 at 12:29 PM |
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All good stuff so far guys - thanks.
The volume of the m/c shouldn't really be an issue because both front and rear setups are similar in size (fronts are marginally larger pistons)
so they both move roughly the same amount of fluid and have similar pushrod travel once all the slack is taken up.
I am presently waiting for the rear pads to arrive so at the moment I only have Mintex 1144s in the fronts and Smart pads in the rears which compounds
the problem a little more, although if I am to believe Rally Design's advice today that 'won't make any difference to braking
efficiency' despite being told the exact opposite last week  I ordered the pads on their own recommendation! It seems advice changes
according to your salesman.
Changing the subject slightly, I had a close look at the rear calipers and it appears that the pads are not in 100% contact with the disc. There is a
crescent shaped 'overlap' on the inside of the disc, so a small percentage of the pad area is not in contact. Quite a shocking bit of
mismatch from Rally Design imo. If the swept area of the disc was increased by 10mm to the inside radius, that would improve things a little.
I will try to illustrate .... dammit I can't upload a pic
[Edited on 22-7-09 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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matt_gsxr
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| posted on 22/7/09 at 12:46 PM |
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You can check whether it is volume of fluid that is causing the problems by using the handbrake.
Light application of the handbrake takes any "slack" out of the circuit. This statement is incompatible with the "eye-popping
force requirement" unless the pedal stroke is tiny.
I think you have already answered your question:
"I used to have a larger (0.7" ) m/c on the front circuit but that required eye-popping amounts of pedal pressure to stop even at slow
speeds so I went back to the 0.625".
You shouldn't need stupid force, unless there is something wrong. Loads of people run 0.7 and 0.625 masters and Sierra rear calipers are a
burden many of us live with. Either something is wrong with some of your components (i.e. glazing, binding etc.), or the pedal ratio needs altering,
or the pedal is too short.
Good luck,
Matt
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RazMan
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| posted on 22/7/09 at 01:49 PM |
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The handbrake on the Powerlites is very crude in design and it simply pulls the pads onto the disc with a cable - pretty crap for anything other than
stopping the car rolling on a gentle hill. As a result there is next to no slack in the caliper - I was referring to the slack in the bias bar and m/c
stroke before fluid gets pushed.
As for pedal ratio, it certainly was an issue in the early days but it still passed SVA - they just put it down to brand new pads & discs needing
bedding in. I remade the pedal to improve this situation but the pedal is fairly short (not excessively though) due to the design limitation of my
footwell.
TBH I have never had confidence in the Wilwood pads. They were originally intended as a cheap 'get me started' pad so I guess the Mintex
can only improve matters - it certainly did in the front brakes with a much better feel. I guess I will wait and see what the rear pads do before I
start doing anything too drastic. I have got a spare 0.7" m/c which I can bung in there as a trial along with the new pads.
Meanwhile I am going to have a look at some alternative discs to solve the pad overlap problem. I am sure I saw a 'disc database'
somewhere, giving all dimensional stuff of various oem discs as a kind of 'mix & match' guide. Anyone?
[Edited on 22-7-09 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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britishtrident
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| posted on 22/7/09 at 02:14 PM |
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quote:
Originally posted by hughpinder
Its not a problem with the volume of fluid in the master cylinders compared to your rear calipers is it? e.g can't displace enough fluid to fill
the pistons of the rear calipers properly to push them against the disks? If you have the bias bars adjusted to give maximum pressure to the rears as
you say, does that limit the travel of the piston in that master cylinder more than in the front circuit?
When you replaced the brake pads for ones with higher friction material - you did just do the rears (as these are the ones you want to increase the
power of) didn't you, otherwise you increase both circuits equally, so the imbalance remains.
If you change to bigger rear disks I would have thought the major problem would be moving the caliper mounts to match!
Don't know if any of that is any use to you
Good luck
Hugh
The volume of fluid thing always comes up --- the short answer is no, the actual fluid volume pumped is tiny it won't any effect
unless something else is badly out of kilter.
A properly setup and bled disc brake should need only a truly tiny amount brake fluid to move the pistons into contact with the disc. No fluid
actually moves or is compressed in transfer force to the disc.
Most people around the forum are to young to have driven tintops with disc brakes but no servo. A good example would be a 1960s Hillman Super Minx or
very ealy Hillman Hiunter/Minx -- I service hundreds of these after servicing and setting the self-adjuster on the rear drum you could press the
brake pedal and it was rock solid it didn''t move at all -- (incidentally this disproves all the hype surrounding braided brake flex
hoses).
Wih a disc brake caliper if it is sitting properly square to the disc surface and the disc is running true the brakes pads skim the surface of the
disc with a fraction of a mm clearance. Because the clearance is so small only a tiny amount of fluid has to be pumped into the caliper to bring it
in contact with the disc.
On forums around the inter-web you will find many racers and builders having problems problems with Wilwood and some other multi-pot light alloy
causing spongey pedals because of caliper flexing and perhaps difficulty bleeding but this is nothing to do with braking effort only pedal travel.
In the real world most of the problems with brakes on home built sports cars hinge around using hydraulic components designed for much larger vehicles
fitted with brake servos. I bang ou about this a lot but a big problem particularly with locost style chassis is getting enough pedal leverage to
avoid the need for "eye popping" pedal pressures.
[Edited on 22/7/09 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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cloudy
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| posted on 22/7/09 at 02:26 PM |
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Trident is right - you definately want a smaller MC for the rear to give a bigger braking effort for the same input force.
I've never seen insufficient fluid volume the cause of the problem, I am using the smallest 0.625 MC's with willwood 4 pots all round and
the cylinder moves barely 1/3 of a stroke. If it moves more your pads are backing off or you have excessive runout on the discs which is a seperate
issue... I suggest dropping a size on the rear if you can.
www.warnercars.com
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RazMan
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| posted on 22/7/09 at 08:44 PM |
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quote:
Originally posted by cloudy
I suggest dropping a size on the rear if you can.
Which is where I am at the moment - If I could find a m/c smaller than 0.625" then that would be my first choice, however it looks like I may
have to go larger (0.7" ) on the front m/c to achieve some sort of balance ..... and then strengthen my right leg muscles to get the neccessary
pedal force
Actually I may be exaggerating a little and I have a feeling that the Mintex pads will improve things a little.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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