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Thread: Pedal Box Strangeness!!

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Pedal Box Strangeness!!

    I use a GRP4Fabrications hydraulic clutch pedal box. Originally I set it up with the clutch master on the left, front brakes (0.75) in the middle and rear brakes (0.625) to the right. I was never very happy with the power of the big AP calipers I have up front and could never lock them up. The pedal was very hard with little travel. The balance bar was straightish at all times when using 50:50 bias. I knew something was wrong.

    After a heap of research I developed a calculation spreadsheet to determine what size master cylinders I needs on the car and it turned out that I needed to replace the front 0.75 with a 0.625. This makes sense as a smaller bore means a longer throw but more pressure. Calculations below;




    Now I have a pair of 0.625 M/C in there and set to 50:50 bias. I can lock the front brakes with a big pedal push and they feel much more powerful on the front. But, when applied the bias bar is cocked over at a big angle. Also, when bleeding the brakes I get plenty of fluid out of the front but little out the rears. Having used a pressure bleeder I am confident this is not an issue with air in the M/C (the M/C are angled such that the inlet is high so the air would bleed back into the reservoir anyway). I am confident there is no air in the hydraulic handbrake M/C also as the handbrakes feels breat (HB plumbed into the rear brake line). This picture shows what I am getting;




    What I am seeing with the bias bar cocking over does not make sense. Both M/C rods are currently set to the same length (I realise this needs to be changed but until I get to the bottom of this I wont touch them). The front caliper pistons have an area 217% that of the rear. This tells me that the front M/C will need a bigger throw than the rears in order to reach pressure. The bias bar shows the complete opposite!!!! In order for the bias bar to cock towards the rear M/C, the rear M/C must be showing greater travel than the front!! I cannot comprehend this.....

    Further, if the rear M/C has greater travel then I would expect it to be flowing fluid when bleeding the brakes. (The pressure bleeder pushes fluid out no worries so there is not a blockage.)

    The easy answer is to run the bigger (0.75) M/C on the front again but the maths and physical evidence shows this not to be correct even though 99% of people automatically do this. I won't get full braking though.

    Are there any brake engineers on here who know the answer to this conundrum????????

    Out of interest these are the calculations using the 0.75 front M/C showing only 74% front braking effort;

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    Re: Pedal Box Strangeness!!

    Put the larger cylinder on the rear, smaller on front and all should be happy again.

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Pedal Box Strangeness!!

    .....that would result in poor front performance again.
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    Re: Pedal Box Strangeness!!

    Quote Originally Posted by MK1_Oz View Post
    .....that would result in poor front performance again.
    Why? - you haven't indicated that you've tried it that way so you don't know! Also - on my pedal box at least - there is a degree of built in bias between pedal and master cylinder centres ie they are offset compared to pedal so building in a degree of bias whatever the linkage is set to so will affect if LHS cylinder goes to front or rear and vice versa!

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    World Champion Decade Plus User alladdin's Avatar

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    Pedal Box Strangeness!!

    I only skim read above but sounds logical as rear is almost solid with little movement or force due to larger cyl. Use longer rod on front so it starts back but once it's pushed bar is level


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    World Champion Decade Plus User alladdin's Avatar

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    Re: Pedal Box Strangeness!!

    thats the problem with skim reading. i didnt spot you have now swapped F / R cylinders left right in box ? you didnt mention it.

    first up its normal in practice to use a .7 on rear, your right though it doesnt compute except that its possible that rears need a lot more mechanical slack taken up before pressure can be built hence needing a larger cyl to match the tight front. this compliments the rears needing less pressure anyway.

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Pedal Box Strangeness!!

    Katana - As I said in OP, I started with a 0.75 front and was not impressed with the power of the brakes.

    Alladdin - having more mechanical and hydraulic slack in the rear system is the only reason I could come up with too. The rears are Nissan R32 GTST 2 pots so should be of reasonable quality but are known to have pistons 217% smaller than the front. All things being equal the front M/C should travel over twice as far as the rear but mine is reversed!!! The hard lines to the rears are much longer than the fronts and even bundy line swells a slight amount. Also the hydraulic handbrake is plumbed in series in the rear line so maybe that is adding more slack somehow (pretty sure there is no air in the handbrake M/C). There is surely no way rear slack is causing the rears to move twice that of the front. To get the bar level when the brakes are applied I would need to make the REAR rod longer not the front. My rear M/C is to the right on the pedal box (when looking from seat) as this is just what happened when thrown together. Is it normal to have the rear on left and front on right? Does it make any difference? Maybe that is what Katana was eluding to when he talks about built in bias???

    Maybe the rear M/C is bottoming out but that would not explain why I do not get much flow when manually bleeding the rears. The model I developed does not incorporate any allowances for slack on the basis that the slack in the front system is equal to that in the rear system.

    I found another calculator on the locost forum and that model independently calculated that I needed a pair of 0.625 M/Cs too.

    Time to pull the pedal box again and check for binding etc
    Last edited by MK1_Oz; 29-10-2016 at 20:13.
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    World Champion Decade Plus User alladdin's Avatar

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    Pedal Box Strangeness!!

    You say rear mc is on rh but you drew it on lh ? Doesn't matter except it makes it very hard to discuss.
    Fluid doesn't compress as such so stroke is down to play or air

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    Last edited by alladdin; 29-10-2016 at 20:52.

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Pedal Box Strangeness!!

    I drew it correctly. The GRP4 Fab pedal box has the pedals on the same side as the M/Cs.

    I just came up with a test. The handbrake M/C is also a 0.625. If I measure the travel of the handbrake M/C it SHOULD be the same as the travel of the rear M/C at the pedal box. If the pedal box is longer then there must be air in the system somewhere between the reservoir and the handbrake M/C (lines in between are all hard so should not be any measureable slack).
    1970 Mk1 Escort Tarmac Rally Car

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Pedal Box Strangeness!!

    OK hold the phone.......the answer is so obvious when you know it!!!

    The rear M/C rear seal is knackered so when the cylinder is presurised the rear seal is not stopping the fluid flowing back into the reservoir = zero system pressure.

    Using a pressure bleeder gets around the issue and makes it look like all is well.

    Don't feel so annoyed at taking the PB back out now!!!!

    Cheers for the input fellas - hopefully this thread can be closed.....hopefully!
    1970 Mk1 Escort Tarmac Rally Car

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    Re: Pedal Box Strangeness!!

    Good to know Glen, I'll be running the same rwar setup with you (Hilux Diff) so I've been reading this with interest.

    What size pistons do your AP's run?

    Great to see you have found a solution

    Cheers

    Matt

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Pedal Box Strangeness!!

    38.1 and 41.3 in the front APs

    38.1 in the r33 gtst rears
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    Re: Pedal Box Strangeness!!

    Legend,

    My Brembos are 38 & 42mm

    Cheers

    Matt

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