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PostPosted: December 5, 2021, 5:33 pm 
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Im calculating the front caliper piston size I need for my DAX rush MC.
The setup is so far:
- VW Golf IV rear calipers with 38mm 1.496" piston.
- Ford sierra 253mm, 9,96" rotors at the rear.
- Wilwood pedal box top mounted. 6.25:1 ratio.

Master pumps I've got is 3/4", 5/8" and 1". but theres no problem buying some new ones.
i have a set of 20x260mm 0,787"x10,25" rotors for the front (might be honda discs with new pcd to fit Opel)

I'm afraid the car will be rear heavy with the driver inside as it only got a 65kg 143pound engine in the front.
But the goal must be 50/50 weight distribution. I know I can move a lot of the weight to the front when braking by adjusting spring rate, shocks, etc. But this will be the final adjustments done at the track I think. But the basic must be in the right range first.

I read a lot about the 70lbs pedal force.
but there is little described about which values ​​are the upper and lower limit.
And what the effect of the differens will be on the brake feeling.
The car will be light. Will I have problems with controlling wheel locking when braking hard into corners if the total brake force is sub 60lbs? Or is the ultimat goal to get as close to 70lbs total brake force?


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PostPosted: December 5, 2021, 6:50 pm 
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In older publications, the goal was to modify for 75 lbs for much heavier production chassis and heavier vehicles approaching 100 lbs. I think 60-70 is a good place to be for such a light vehicle as long as you don’t use full pedal travel (leave some margin/extra travel). I’d probably start with the ¾. Pedal feel improves with higher effort and travel decreases.

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Ford IFS viewtopic.php?f=5&t=13225&p=134742
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Tiger Avon:114x40x13.3-12.6
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PostPosted: December 6, 2021, 8:42 am 
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You will probably need a front caliper that is around a 2" Dia. I started out with a 10.25" Dia. front disc and 2.25" Dia. caliper, 13/16"Dia. master cylinder and more wt on the front wheels. Pedal ratio just over 5 to1. I found it was pretty easy to lock up the front under heavy braking. After a year I switched to a smaller 2" Dia. front caliper, which is much easier to control under heavy braking.
Davew


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PostPosted: December 6, 2021, 12:18 pm 
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Joined: August 31, 2015, 2:24 pm
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Location: Delaware
Attachment is a modified version of Mike's Brake calculator from the old 7builder site. I've found it very useful on a couple cars for getting the initial choices close. We've added a plot for a rear proportioning valve. It's also useful to consider the type of seating arrangement you have. If you are in race seats and harnesses a heavier pedal is easier to manage but if you have a bench consider going on the lighter side of effort.


Edit: change extension to .xls


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PostPosted: December 6, 2021, 5:56 pm 
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I've made some test using a excel file, when changing the brakepad coeff of friction from 0.3 to 0.38 (Mintex 1144 a prefered pad for lightweight track/street use) the brakeforce will be reduced with alot.

I was thinking something like this:
Quote:
1.5" 4pot calipers and 0.813" master at front, rear master 0.7", gives a pedal force 74,4lbs with 0,3 coeff of friction. And 58,7lbs with 0,38 coeff of friction.
pedal movment will be somthing like 2,33". And the masters movment will be closest of the calculations I've made. 0.373" front, and 0.247" rear.


Is there a goal to have the same movment in the two brake master cylinders? :?
And Is there any chance for a locost to achieve a max deceleration higher than 1G on street tyres?
Becase then I will also need a proportioning valve to stop the rear from locking first.


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PostPosted: December 7, 2021, 9:10 am 
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No need for a prop valve with split, adjustable masters.
Ideally, with the right bore sizes, the balance bar could apply evenly to both.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: December 8, 2021, 12:49 pm 
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Location: Cornelius OR
If you are placing this much concern over braking performance, you definitely want dual master cylinders and a balance bar.

Also it is easier and cheaper to change a MC than to change 2 calipers (for resizing). Different size masters are easy, different size calipers are not so easy.

Brake pedal ratio also comes into play (6:1 to 7 :1 pedal ratios are most common)

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PostPosted: December 8, 2021, 3:52 pm 
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Joined: January 27, 2021, 6:46 am
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Miatav8,MstrASE,A&P,F wrote:
No need for a prop valve with split, adjustable masters.
Ideally, with the right bore sizes, the balance bar could apply evenly to both.


The problem is that I already have bought the rear calipers. So I'm a little stuck with those.
The files says that the blue line is the ideal pressure curve to follow, If I cross that pressur the rear brakes will lock before the front brakes. (given the same tyres and tyre temp)

Image
But I think I'm at the limit here. adjusted the bar to 54%.
The gren line is how the brakepressure would have kept going without a prop valve.
The red line is wihout a balance bar. (balance bar at 50%)
The purple line is with balance bar and prop valve.
The X-axis is deceleration measured in G-force
The Y-axis is brake line pressure

If I had been better in excel I would have changed the files so you can use seperat types of pads front and rear. (Pad coeff of friction)
I will try some more, but it changes fast if the weight is changed, and also the weight distribution.
So some small changes will be done when the car is completed.


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PostPosted: December 8, 2021, 5:34 pm 
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Revisiting this. You might try a larger front bore that puts you sightly above the ideal curve at lower G, then pull the curve down below ideal with the balance bar, followed by a prop adjustment. Should stay closer to ideal through 1G that way but from what I can tell, you are close already. Probably closer than most builds that have either a tandem with prop or a dual and balance bar.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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