contaminated
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| posted on 17/4/11 at 10:39 PM |
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Bias bar reality check
Guys
If you've seen some of my previous posts you'll know I have had some serious problems with my brakes. The issue is a spongy pedal and then
when I get a pedal there is just not enough pressure to stop the car. They have been bled and bled with every trick in the book (calipers inverted,
master cylinders bled, easibleed, manual bleed, front and rears individually and together, brake light switch bled etc etc).
As a reality check today I disconnected the rear system from the cylinder, fitted a bleed nipple and bled them out. I did this because I thought the
rear system was the issue and I wanted to check. With just the fronts connected and the rear cylinder blocked of and bled I re-bled the fronts. No air
coming out, no leaks, everything checked. Then I took it down the road. Same issue - I get a pedal after an inch or so travel, the brakes come on, but
not with enough pressure to stop the car. The fronts just will not lock - at 30 mph if I slam on the brakes I will slowly grind to a halt. It's
like there is no friction.
As a second reality check I wanted to check disk/pad contamination - so I sanded off the disks and cleaned them with a solvent. I also changed the
front pads. No different.
Tonight is has dawned on me that the issue might be that I don't have enough piston travel in the front cylinder. I run 0.7 to the rear (VW/Audi
calipers - very similar to Sierra). I run 0.625 to the fronts (M16). As most of you will know you run a smaller diameter cylinder to the front because
you'll get more pressure - but you are also moving less fluid for the same pedal press to the rears. Since the volume of fluid in the fronts is
larger than the rears (bigger calipers up front) this also means you'll get more travel on the fronts. Now I'm thinking this is my issue.
My cylinders have matched length pistons and I have adjusted them so at rest the bias bar is level. I now suspect that as I push the pedal the rear
system prevents enough piston travel to the fronts.
To cure this I think I need to adjust the pistons so that at rest the bias bar is slightly tilted - the rear clevis nearer the bulkhead. This should
mean I get more piston travel on the front system.
Any comments? Are my assumptions correct? The below (from OBP) seems to bear my suspiscions out.
"With the pedal tube and clevises squared away, we now look at master cylinder push rod length. The key is to set up the bias adjuster so that
it is perpendicular to the master cylinder center lines with the brake pedal under compression. Typically, this means that the front master cylinder
push rod will be 3mm-5mm longer than the rear master cylinder push rod at rest. This is due to the fact that the front braking circuit has a larger
fluid volume, due to the larger piston diameters in the front calipers. As a result the front master cylinder requires a higher feed rate than does
the rear. If the push rod length is equal front and rear, than the feed rate of the rear master cylinder is too high relative to the front. The result
in this case is the rear circuit “hitting” before the front. With the push rod lengths adjusted properly, the bias bar will be square under
compression and the front and rear circuits will “hit” approximately at the same time."
[Edited on 17/4/11 by contaminated]
Tiger Super Six Independent
www.southernkitcars.com
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steve m
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| posted on 17/4/11 at 11:37 PM |
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Hi
I do not have a bias bar, nor feel the need to have one on my locost,
But i did have similar problems with braking,
My setup was
standard escort mk2 master cylinder, and rear drum brakes from the donar escort
front m16 calipers with green stuff pads
home made pedal box
When the 7 was first on the road, my brakes were not very inspiring, and i had to stand on the pedal with so much force, and my backside firmly
planted in the seat, that to be honest it hurt!!
Once all the pads and rear brakes were bedded in, the braking was marginly better, but i would still hold my distance back from anything infront of
me, as i could really see me rear ending someone
My first improvement was to fit 4 pot calipers on the front (princess units )
Braking was better, but still not GOOD
secondly, and it took about 3-4k mileage to work this out, was my pedal box, as i had modifyied the original MK2 escort unit, and cut down the pedal
to accomodate them in the tunnel,
So in my wisdom i have altered the leverage on the pedal, and needed to put in a lot more foot pressure to get the brakes working
So, i have now moved the m/c to pedal rod up about 3/4 of a inch on the brake peddle , and thus altered the fulcrum point considrably
i have also changed the fluid to dot5, so no longer hydroscopic
My brakes now are superb, and i can easily lock them up if over compensating
Hope this helps in some way !!!
regards
steve
[Edited on 18/4/11 by steve m]
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blakep82
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| posted on 17/4/11 at 11:46 PM |
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i'd swap the master cylinders round and see how it feels, if you can lock the fronts with the 0.7 on the front, then good times. now master
cylinders confuse me lots, so i don't know whether its a good idea to have a 0.7 front and back, or whether you should have 0.7 on the front,
and 0.75 on the back, but at worst you only need to buy 1 new cylinder.
what type of cylinders do you have? wilwood? integral or seperate resevior?
i might have one with out resevior in the garage for sale, unused, but i can't remember what size. can check if its any good for you?
edit: ok, its a 0.7" non integral resevoir type, its not made by wilwood though. not sure who made it. no manufacturer cast into it
unfortunately.
[Edited on 18/4/11 by blakep82]
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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contaminated
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| posted on 18/4/11 at 12:13 AM |
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Thanks chaps - could be useful. I have seperate girling master cylinders with remote resevoirs. I'll have another play tomorrrow night.
Dan
Tiger Super Six Independent
www.southernkitcars.com
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Bare
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| posted on 18/4/11 at 02:37 AM |
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Isn't that a clear example of WHY? God gave us brake proportioning valves and then.. even humbled us further by introducing
(gasp!) on the fly driver adjustable ones?
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Dusty
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| posted on 18/4/11 at 04:10 AM |
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You can get the rear m/c stopping the pedal and reducing the stroke on the front m/c. This has a springy feel on the pedal as one piston is
hydraulicaly locked, one end of the bias bar is solid but the pedal is twisting to push the other end of the bar forward. It doesnt feel solid as it
would if both cylinders were locked. It's also a different feel to the spongy air in system feel.
Two things to do. First is to loosen one of the sadles on the bias bar two or three turns so the bar can swivel about its bearing in the pedal tube.
It needs to be able to use all the possible angular motion. Have you got a large penny washer between saddle and bearing?
Setup balance bar to start position with the front saddle well back from a right angle and the rear well forward.
There used to be good info on setup of the rally design website. Under brakes - tech tips - setting up bias bar.
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lsdweb
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| posted on 18/4/11 at 07:17 AM |
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Some help on balance bar setups here
and
here
Regards
Wyn
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ashg
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| posted on 18/4/11 at 07:21 AM |
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sounds like a duff master cylinder to me. your brakes worked fine before you changed the single cylinder to doubles so that is where i would be
looking. my offer still stands if you want me to come down and have a look.
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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adithorp
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| posted on 18/4/11 at 07:35 AM |
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Before anybody else confuses the issue... You have your master cylinder sizes the right way around. Do not swap them.
Yes, adjust the balance bar to give a slight cock in favour of more front piston travel. Don't adjust out the free play or the brakes will
bind.
If you have new pads then braking will be weak untill they have bedded in. With performance pads this will take even longer. If there is any rust (or
even just remnants of rust) they'll be weak; It's not a good friction surface.
Are you used to none servo brakes? It takes a while to get used to the pressure required.
Is the play you feel spongy/springy or just free movement and then hard pedal? Some spongyness will be just new pads, unbedded-in. If you pump the
pedal does it go?
Can you get an expeirienced mechanic to have a feel? The differences between causes are often too subtle to describe but years of doing the job gives
you a feel for the cause.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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britishtrident
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| posted on 18/4/11 at 07:53 AM |
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If everything is set up and working properly the amount of fluid that is actually pumped by the master cylinder is tiny.
All the symptoms of seized piston in one of the front calipers , pull one pad out at at time and check each piston (and pad) is moving
freely. This is a very common problem with M16 and P16 callipers.
Jacking out the pushrods in a bias pedal box really not the way to go.
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contaminated
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| posted on 18/4/11 at 07:57 AM |
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quote:
Originally posted by Bare
Isn't that a clear example of WHY? God gave us brake proportioning valves and then.. even humbled us further by introducing
(gasp!) on the fly driver adjustable ones?
I think you are exactly right. I found the choice of pedal box a bit limiting really and I went for the bias box as a bit of a compromise. If I was
doing it again I'use the 3ge pedal box and a single master cylinder with a valve.
Tiger Super Six Independent
www.southernkitcars.com
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contaminated
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| posted on 18/4/11 at 07:58 AM |
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quote:
Originally posted by Dusty
You can get the rear m/c stopping the pedal and reducing the stroke on the front m/c. This has a springy feel on the pedal as one piston is
hydraulicaly locked, one end of the bias bar is solid but the pedal is twisting to push the other end of the bar forward. It doesnt feel solid as it
would if both cylinders were locked. It's also a different feel to the spongy air in system feel.
Two things to do. First is to loosen one of the sadles on the bias bar two or three turns so the bar can swivel about its bearing in the pedal tube.
It needs to be able to use all the possible angular motion. Have you got a large penny washer between saddle and bearing?
Setup balance bar to start position with the front saddle well back from a right angle and the rear well forward.
There used to be good info on setup of the rally design website. Under brakes - tech tips - setting up bias bar.
This makes sense to me. Kind of where my head is at now.
Tiger Super Six Independent
www.southernkitcars.com
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contaminated
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| posted on 18/4/11 at 08:01 AM |
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quote:
Originally posted by lsdweb
Some help on balance bar setups here
and
here
Regards
Wyn
Thanks Wyn
Tiger Super Six Independent
www.southernkitcars.com
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contaminated
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| posted on 18/4/11 at 08:02 AM |
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quote:
Originally posted by ashg
sounds like a duff master cylinder to me. your brakes worked fine before you changed the single cylinder to doubles so that is where i would be
looking. my offer still stands if you want me to come down and have a look.
The beer is always in the fridge mate! I may well give you a call towards the end of the week!
Tiger Super Six Independent
www.southernkitcars.com
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contaminated
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| posted on 18/4/11 at 08:09 AM |
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Thanks mate - see below.
Before anybody else confuses the issue... You have your master cylinder sizes the right way around. Do not swap them. Agreed
Yes, adjust the balance bar to give a slight cock in favour of more front piston travel. Don't adjust out the free play or the brakes will bind.
Agreed, although I don't quite know what you mean by the second sentence.
If you have new pads then braking will be weak untill they have bedded in. With performance pads this will take even longer. If there is any rust (or
even just remnants of rust) they'll be weak; It's not a good friction surface. I'm using a second, used set for that
reason.
Are you used to none servo brakes? It takes a while to get used to the pressure required. Actually I have always run it with a survey, although
lots of my old cars had none. This is way worse than just the effort required for non-servo brakes.
Is the play you feel spongy/springy or just free movement and then hard pedal? A bit of both - mostly free movement I think.Some spongyness
will be just new pads, unbedded-in. If you pump the pedal does it go? No - hence I think the issue is MS related.
Can you get an expeirienced mechanic to have a feel? The differences between causes are often too subtle to describe but years of doing the job gives
you a feel for the cause. Worth a try, although in my experience even experienced mechanics scratch their heads with kit cars!
[Edited on 18/4/11 by contaminated]
Tiger Super Six Independent
www.southernkitcars.com
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ashg
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| posted on 18/4/11 at 08:11 AM |
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well im on holiday from wed so let me know. i have got to build an exocet from the ground up for the ckc stand at stoneleigh but other than that i
have loads of time  are you doing the skcc roadtrip? if you are we need to get you sorted out asap.
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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contaminated
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| posted on 18/4/11 at 08:11 AM |
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quote:
Originally posted by britishtrident
If everything is set up and working properly the amount of fluid that is actually pumped by the master cylinder is tiny.
All the symptoms of seized piston in one of the front calipers , pull one pad out at at time and check each piston (and pad) is moving
freely. This is a very common problem with M16 and P16 callipers.
Jacking out the pushrods in a bias pedal box really not the way to go.
Thanks BT - I saw a similar post on my other thread. Since then I've changed the calipers. They are deffo OK.
Tiger Super Six Independent
www.southernkitcars.com
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adithorp
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| posted on 18/4/11 at 08:39 AM |
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quote:
Originally posted by contaminated
Thanks mate - see below.
Before anybody else confuses the issue... You have your master cylinder sizes the right way around. Do not swap them. Agreed
Yes, adjust the balance bar to give a slight cock in favour of more front piston travel. Don't adjust out the free play or the brakes will bind.
Agreed, although I don't quite know what you mean by the second sentence.
If you have new pads then braking will be weak untill they have bedded in. With performance pads this will take even longer. If there is any rust (or
even just remnants of rust) they'll be weak; It's not a good friction surface. I'm using a second, used set for that
reason.
Are you used to none servo brakes? It takes a while to get used to the pressure required. Actually I have always run it with a survey, although
lots of my old cars had none. This is way worse than just the effort required for non-servo brakes.
Is the play you feel spongy/springy or just free movement and then hard pedal? A bit of both - mostly free movement I think.Some spongyness
will be just new pads, unbedded-in. If you pump the pedal does it go? No - hence I think the issue is MS related.
Can you get an expeirienced mechanic to have a feel? The differences between causes are often too subtle to describe but years of doing the job gives
you a feel for the cause. Worth a try, although in my experience even experienced mechanics scratch their heads with kit cars!
[Edited on 18/4/11 by contaminated]
If you adjust the push rods so there's no play in them, then the master cylinder pistons don't fully return and they hold residual
pressure. Not the problem you currently have but it comes up on here often so I just warned against it.
Are the second used set from use on the same discs? Still suspect the lack of bite will be related to rusty brakes. It s amazing how ppor the friction
is on rust even just residue on the pads.
Just mentioned non-servo brakes as again that comes up regularly.
Yep, we mechanics like things we recognise. but remind them they are just brakes... although you won't find many that are familiar with bias
bars...
What angle are your master cylinders at. They couldn't be holding a pocket of air in them that doesn't bleed out could they? Unlikely in a
bias set-up. Are ther any loops in the lines that could do the same?
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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contaminated
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| posted on 18/4/11 at 10:47 AM |
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quote:
Originally posted by ashg
well im on holiday from wed so let me know. i have got to build an exocet from the ground up for the ckc stand at stoneleigh but other than that i
have loads of time are you doing the skcc roadtrip? if you are we need to get you sorted out asap.
Thanks Ash - I shall be in touch.
Tiger Super Six Independent
www.southernkitcars.com
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