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Question about dual master cylinders...


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I read something on Miata forums about the length of the hard lines being different, and this contributing to their tendency to lock up the LF tire, so I tried to put the Ts in the lines right in the middle. But yeah, T the front lines and go to the front master, and T the rears and go to the rear master.

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On 6/7/2019 at 8:18 PM, grannyknot said:

Ahhh, makes sense, all I had to do was read the title of the thread,  apologies.

No worries, it makes sense in any other application that this wouldn't be a good idea to do.

 

17 hours ago, JMortensen said:

I read something on Miata forums about the length of the hard lines being different, and this contributing to their tendency to lock up the LF tire, so I tried to put the Ts in the lines right in the middle. But yeah, T the front lines and go to the front master, and T the rears and go to the rear master.

Interesting, you would think the pressure would be uniform throughout the plumbing regardless, but that is really good insight! I will probably have to run these lines for a bit and buy a new bit of brake line soon so I will keep a careful eye on that happening!

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A longer line will always have more resistance to move material through it then a shorter one.  You don't have to move nearly as much fluid, and with less bends, there's less places for fluid flow to "drag" against the walls of the tube. With a properly functioning proportioning valve, you can get around this, or you can just try and and get the lengths equal where you make your "T".

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On 6/10/2019 at 5:37 PM, Neverdone said:

...and with less bends, there's less places for fluid flow to "drag" against the walls of the tube.

 

Where is the "fluid flow", and how much 'flow' is really happening?

 

By definition there will be some movement of fluid at the Master Cylinder end of the equation, but it should be minimal at the business end if everything is working as it should. What you're doing is putting pressure into a fluid.

 

I tend to think of it as like people joining the end of a queue and pushing. Everyone squashes up, and the people at the front of the queue feel the most pain. 

 

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Both concepts (one being the concept of drag while the fluid is being pushed through. The other being the concept that there isn't much drag in the lines as the fluid doesn't move enough.) make sense that either could be correct. I'll test it out as I already bent them all into place without focusing on a center point and I'll share my own personal results!

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There are SO MANY other things that could affect it, that I think your results (and mine) aren't really meaningful for other people. The thing about the Miatas is that they have a tendency to lock up the LF tire under braking. Not sure that I've heard anyone say that about their Z.

FWIW I learned a new trick the other day: when corner weighting, set the front weights equal, rather than setting the diagonals even. This is supposed to load the front tires more evenly under braking. 

Between setting the fronts equal and making the lines equal length, you MIGHT be able to tell the difference. Probably not... ;) To me these are things that you do when you're building because it's theoretically better. The actual difference it makes is probably pretty minute.

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12 hours ago, JMortensen said:

......................................

FWIW I learned a new trick the other day: when corner weighting, set the front weights equal, rather than setting the diagonals even. This is supposed to load the front tires more evenly under braking. 

...................................

 

Have never bothered with corner weighting, something about it did not make sense in a vague sort of way. But what you have posted does make definite sense, same with the rear too even when accelerating. Say if when accelerating the L tyre has less grip due to track conditions and that side just happens to have an effective higher spring rate due to corner weighting.

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11 hours ago, 260DET said:

 

Have never bothered with corner weighting, something about it did not make sense in a vague sort of way. But what you have posted does make definite sense, same with the rear too even when accelerating. Say if when accelerating the L tyre has less grip due to track conditions and that side just happens to have an effective higher spring rate due to corner weighting.

I think you're missing out here, regardless of whether you do it diagonally or setting fronts even. Mine was really far out. I'd be shocked if fixing this didn't help.

before.jpg

after.JPG

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On 6/15/2019 at 3:39 AM, JMortensen said:

I think you're missing out here, regardless of whether you do it diagonally or setting fronts even. Mine was really far out. I'd be shocked if fixing this didn't help.

before.jpg

after.JPG

 

 Yeah, I've never noticed a problem that could be related to corner weighting but that could be me, getting the ride heights equal on each corner F to F and R to R is as far as I go

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I think the strongest argument for it is the way circle track racers will adjust their crossweights to make the car turn left better, because they don't give a shit about turning right; what improves one hurts the other, so getting them balanced or close should make the car handle turns in opposite directions more equally. Also see this on rovals, where they'll set road race cars up to handle on the oval portion because it's higher speed than the infield and results in better lap times overall.
https://grassrootsmotorsports.com/articles/understanding-corner-weights/

Would be interested to hear how far off yours were and how big a difference it made after fixing it. 

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Getting the individual ride heights even is my priority, if the corners are too much different then there is a car weight distribution problem that should be fixed first. IMHO :)  As far as suspension generally goes I've always found that getting the spring rates right and even more importantly having the best 3 way dampers possible to be most important. A circuit ace car is rarely never in a state of constant dynamic so particulars like individual wheel loadings are constantly changing anyway. Attending to factors that govern that is where my priority lies. 

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True. But car weight distribution is not always an easy thing to correct. We are kind of lucky with the S30 chassis that it is close to 50/50 F/R.  But getting 50/50 L/R is not quite as easy. 

Anyway, I always start with front ride height even where I want to get my desired roll center and clearance. Then rear even although slightly higher. I then try to get the cross weights correct by adjusting the rear, and only tweak the front if needed. I dont know if this is right, but it always results in a tame handling car for me. Then at the track you can add/subtract more rake by adjusting the rear equal turns. I do this to balance aero issues.

All adjustments with sway bar off, weight simulated for track, etc, etc.

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And I think 1 tire always locks up before the other one. But it should only be slight difference and not cause bad issues like changing lanes or something. I have never driven a car where both front tires lock at exactly the same time. I think corner weighting mostly balances L/R transitional and steady state cornering.

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On 6/9/2019 at 6:55 AM, Jboogsthethug said:

No worries, it makes sense in any other application that this wouldn't be a good idea to do.

 

Interesting, you would think the pressure would be uniform throughout the plumbing regardless, but that is really good insight! I will probably have to run these lines for a bit and buy a new bit of brake line soon so I will keep a careful eye on that happening!

Technically unequal lengths will cause the pressure on the calipers to be different for a short period of time.  The amount of fluid that moves is very small.  What mostly happens is a pressure wave that travels along the fluid.  If you want to get all geeky it can be calculated but you need to know details of the fluid and temperature, etc.  Think of it similar to how a sonic boom happens.  I've had lines that started out not equal and it worked fine.  I only ended up changing this so to equal length when I did work to make changing the engine/tranny easier.  

 

From previous posts it looks like you were asking about 50/50 on braking.  If the question is how much braking should be on the front versus rear this needs to be more like 70% front and 30 percent rear.  50F/50R will cause premature rear lockup.  Perhaps I'm not following along.  One of the best ways to test I've found in Neil Robert's book Think Fast. He recommends finding a medium speed corner and setting the brake balance so that when you brake hard you stay on line.  Too much front will cause you to understeer (car moves away from corner) and too much rear will cause you to oversteer (car tightens corner).  You don't need to come to a stop when doing this and you only need medium effort on the pedal.  A cloverleaf highway ramp would work if you can find one without a lot of traffic.  I was fortunate in having a track with the perfect corner to do this.

 

Keep in mind pedal feel is subjective.  This can lead to liking smaller master cylinders if you prefer more travel or larger masters if you like a pedal that doesn't move as much.  I'm in the latter camp and a number of people that drove my car said it was like standing on a 2x4 to make it stop.  There's also a lot to keep in mind if you change the mounting of the calipers.  They need to be parallel to the disk and you need to make sure the brake disks have minimal run out.  I've seen some that have mounts flex and engage one side a fraction before the other.  You really need to have someone stand on the pedal and inspect all four corners as well as the masters to make sure flex is minimal.  If you can see movement it's probably too much.

 

I'll try and find the pictures of my pedal box setup.  It appears google deleted the images I had posted at the beginning of this thread.

 

Hope that helps,

Cary

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@tube80z thanks for all that info! I decided on 1” front MC and 7/8” rear, I’ll decide how much I like it but I think it will feel nice! I actually just finished my setup today, I just need to attach the pedal to the MCs now! I’ll post pics of my setup, I’m happy with it so far and I’m hoping I got the clearances all correct

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Couple points: the smaller master usually goes on the front since a smaller master will give higher pressure, and you generally need that in the front where most of the braking is done. Might make sense if you have gigantic front brakes and tiny rear brakes though. The stock master is 7/8, so by going larger you're making it even harder than just getting rid of the power assist. About a month ago I ran across a guy on FB who was saying how dangerous manual brakes were, because his friend had installed larger than stock masters and then couldn't stop and rear-ended another car. Simple answer: he did it wrong by going with larger masters. 

Just be careful when you get it together and take that first drive, and be aware that there is a good likelihood you'll need to swap them around or go smaller.

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On 7/5/2019 at 7:02 AM, JMortensen said:

Couple points: the smaller master usually goes on the front since a smaller master will give higher pressure, and you generally need that in the front where most of the braking is done. Might make sense if you have gigantic front brakes and tiny rear brakes though. The stock master is 7/8, so by going larger you're making it even harder than just getting rid of the power assist. About a month ago I ran across a guy on FB who was saying how dangerous manual brakes were, because his friend had installed larger than stock masters and then couldn't stop and rear-ended another car. Simple answer: he did it wrong by going with larger masters. 

Just be careful when you get it together and take that first drive, and be aware that there is a good likelihood you'll need to swap them around or go smaller.

Thanks for the points! That's smart, I plan on eventually getting larger brakes for both front and back, maybe I'll grab a smaller temp MC and take the 1" off and keep the two smaller MCs. Either that or I'll just do 3 months of leg days ha.

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