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Author: Subject: Rock hard brake pedal
jon200

posted on 30/10/21 at 04:31 PM Reply With Quote
Rock hard brake pedal

Hi peeps, Its been a very long time since i last worked on the Locost let alone driven it! Im slowly working through some issues hence why I'm here.

The brakes have always been really hard and a few years ago i bought a 3/4 BMC to replace the sierra unit and was going to fit it soon, but.....

I just checked the pedal ratio and its miles away from where it should be! Its currently as follows
190mm pedal length with and 110mm pivot to push rod length giving a ratio of 1.75:1 !!

Can i just remove the servo(unused) and mount the new master cylinder on the bulkhead lower down (floor mounted pedal) with a new hole in the pedal? Or is there more to it? Not sure what drawings were used for the pedal?

Thanks in advance 🤞

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theconrodkid

posted on 30/10/21 at 04:42 PM Reply With Quote
having a servo will make it hard work as you have to overcome that first, the bigger dia the master, the harder the pedal so removing the servo, going for a smaller dia master and a longer pedal with a better ratio will all help.
when i built mine, i used a Fiat 124 master, 17 or 19 mm dia ? and the brakes were spot on





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jon200

posted on 30/10/21 at 05:28 PM Reply With Quote
Is there a minimum distance between the pedal and master? For pushrod angles etc?
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adithorp

posted on 30/10/21 at 06:05 PM Reply With Quote
Why have you got the servo if it's not in use? It's not needed on a light car so remove.

What's the original master cyl diameter? You need to go smaller to give a longer (softer) pedal

Distance wise, you need enough throw on the push rod to get nearly full stroke on the piston but not quite bottom out and the push rod not to hit/bind against the mouth of the cylinder at full stroke.





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jon200

posted on 30/10/21 at 06:10 PM Reply With Quote
Ive no idea why its not connected etc. It never has been. I guess the best thing is to have the master at 90 degrees to the pedal at rest.

Looks like its a case or trial and error now then or lots of calculations! I thing the original is a sierra 22mm?

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Dingz

posted on 30/10/21 at 07:08 PM Reply With Quote
Why not just connect up the servo?





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adithorp

posted on 30/10/21 at 07:08 PM Reply With Quote
I'd probably go a little past 90 so it's 90 with pressure on... but yes trial and error. Take some measurements and play about with a cardboard model before you start drilling holes.





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jon200

posted on 30/10/21 at 07:33 PM Reply With Quote
quote:
Originally posted by Dingz
Why not just connect up the servo?


I did think that but i think it will still be hard. Maybe i should just try it. Car isnt running at the mo though.

[Edited on 30/10/21 by jon200]

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jon200

posted on 30/10/21 at 07:55 PM Reply With Quote
I just cant understand why the ratio is so far out! Even with the servo it should be a higher ratio searching on forums/Google. It looks like even with the servo attached it wouldn't be enough still.
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hellbent345

posted on 30/10/21 at 08:39 PM Reply With Quote
How difficult is it to connect it (servo) up and try it? Better to try an easy thing first, if it’s just a vacuum feed from your manifold that’s needed. If it’s easier to change the ratio then I guess start with that.
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jon200

posted on 30/10/21 at 10:06 PM Reply With Quote
Even with the servo connected i feel its still miles out. Im going to have a look tomorrow hopefully and double check what Mc it is. I know there is not vacuum takeoff on the manifold currently. (Plus its not currently running either!)
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jon200

posted on 31/10/21 at 10:24 AM Reply With Quote
So the one on the car is a Girling74660220. It looks to be smaller! but im struggling to find what size it is.

:edit: its 3/4! Not much point swapping it although it needs new seals.

[Edited on 31/10/21 by jon200]

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davew823

posted on 31/10/21 at 11:39 AM Reply With Quote
brake pedal

The brake lever ratio for a manual system should be around 5 or 6 to 1 by relocating the push rod clevis hole on the brake pedal lever. If you have concerns about the physical strength needed for a manual brake system with the your M/C, you may want to increase the lever ratio to 6 to 1. The 6 to 1 ratio will require longer pedal travel to engage the system. But the ratio can easily be reduced later if it is not to your liking. Try to adjust your pedal hanger bracket so the actual pedal lever is approximately +12” in overall length. When locating the push rod hole on the brake lever, try to keep the push rod within a 7-degree angle when setting up the master cylinder. When fabricating the pedal, carefully design the fore/aft positioning of the brake pedal hanger bracket i.e., pivot point, to avoid an over center condition at the end of the stroke, which would decrease the applied pressure.
davew823

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jon200

posted on 31/10/21 at 02:51 PM Reply With Quote
quote:
Originally posted by davew823
The brake lever ratio for a manual system should be around 5 or 6 to 1 by relocating the push rod clevis hole on the brake pedal lever. If you have concerns about the physical strength needed for a manual brake system with the your M/C, you may want to increase the lever ratio to 6 to 1. The 6 to 1 ratio will require longer pedal travel to engage the system. But the ratio can easily be reduced later if it is not to your liking. Try to adjust your pedal hanger bracket so the actual pedal lever is approximately +12” in overall length. When locating the push rod hole on the brake lever, try to keep the push rod within a 7-degree angle when setting up the master cylinder. When fabricating the pedal, carefully design the fore/aft positioning of the brake pedal hanger bracket i.e., pivot point, to avoid an over center condition at the end of the stroke, which would decrease the applied pressure.
davew823


Hi Dave, thanka for the Reply. I plan on modifying to pedal to get the right ratio. I think i will Aim for the correct angle at 5:1 ratio and adjust either way if need be. The only worry i have is that the pedal hole needs to be quite big so more hole will weaken it.

Hole is 12mm and pedal and 25mm ish thick. Unless i make a smaller clevis.

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jon200

posted on 31/10/21 at 02:52 PM Reply With Quote
How do people keep the pushrod ball in the cylinder? Spring above to hold it against it?
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adithorp

posted on 31/10/21 at 03:21 PM Reply With Quote
quote:
Originally posted by jon200
How do people keep the pushrod ball in the cylinder? Spring above to hold it against it?


You can't use a spring like that as the cylinder requires the pushrod to have slight play in it when the brake is released to ensure the piston returns fully. Without that the brakes will hold slight pressure and gradually start to bind.

The push rod is usually retained by a circlip and on cylinders without that, by the dust seal rubber and can't escape as the free play is far less than what's needed for it to fall out.

[Edited on 31/10/21 by adithorp]





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jon200

posted on 31/10/21 at 07:11 PM Reply With Quote
Ah i see thank you. Found a boot now i think. Makes sense really.

Would an 8mm high tensile hex bolt be ok for the clevis pin? The original is a 12mm plastic bush with a 10mm pin through and if i drill a few holes in the pedal for adjustment i think it will be too weak! Thinking 8mm. The pivots are 8mm anyway.

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adithorp

posted on 31/10/21 at 07:47 PM Reply With Quote
I should think 8mm will be adequate. Use a bolt (plain shank) rather than a screw as the thread would wear and give play. That is if you can't get an 8mm clevis pin.





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jon200

posted on 31/10/21 at 08:20 PM Reply With Quote
Thats what i was going to do yeah. Would 3 8mm holes intersecting(for finer ratio adjustments) weaken the pedal by much do we think) Its a 1/4 x 1” plate for the pedal. ()()() hopefully you know what i mean!

EDIT: scrap that i can get 4.45:1 or 5.72:1 with two holes. Hopefully this will suffice. 2mm gap between the holes too then.

[Edited on 31/10/21 by jon200]

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davew823

posted on 1/11/21 at 11:12 AM Reply With Quote
brake pedal

I would try the 5 to 1 ratio before drilling additional holes. My final adjustment was only about 4mm. I welded up the first hole before drilling the 2nd one. As mentioned you should have a "C" clip to retain the push rod in the cylinder. Once completely set up, your push rod should have about 1mm free play before it contacts the piston in the M/C. Put a lite return spring on the brake pedal lever. Must M/Cs and push rods, will not completely return without one.
Davew823

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jon200

posted on 1/11/21 at 08:03 PM Reply With Quote
I have no clips on my master. It was a servo’d master originally though so that will be why i guess. Not really sure what to do with it now!
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jon200

posted on 1/11/21 at 08:38 PM Reply With Quote
No idea how to post a pic but my push rod doesnt have a washer on the shaft so it cant be retained. Im going to cut a washer then weld it back on past the ball then use an internal circlip to hold it all in hopefully. Thats basically all it is looking at images on google
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snapper

posted on 2/11/21 at 07:16 AM Reply With Quote
quote:
Originally posted by jon200
Ive no idea why its not connected etc. It never has been. I guess the best thing is to have the master at 90 degrees to the pedal at rest


The pivot at the pedal will go through a slight arc so 90 degrees at half arc travel however as long as your close it’s no big deal





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