Apocolipse

Anyone run manual brakes on their c5/6? I'm running C6Z51 brakes and am looking at ordering wilwood pedals 6.25:1 ratio, just unsure what m/c to use. I've read from 7/8"-1". Should I base it on 100lbs foot force for 1G?

Thanks!

0Todd TCE

100lbs of leg effort is quite heavy. Try propping yourself against something and pushing on a scale to 100lbs. I think you'll find a more manageable value at roughly 60-70lbs.

The 'exact' bores chosen will be based on your effort as stated but also the requirements of the calipers on both ends. And how long/short, soft/hard a feel one prefers. One size vs the other won't "brake better" it will simply be an alternate hydraulic equation achieving the same net results- maximum brake torque required for 1G braking.

With the stock calipers a .875 and 1.00 may seem at first glance a bit much area making for a hard pedal. You may need that 100lbs...Given the front area of the calipers is not that large you may need more pressure- a smaller bore. Also the spits will be based on the front and rear piston areas so given the rear has less area you could actually need a smaller rear mc.

Spend some time on my TWIN CALCULATOR trying some alternative plans. Good luck.

crimlwC6

Todd's the best. Listen well!

Apocolipse

Hey Todd,

I have used your calculator a bunch, thank you very much for providing access to a great tool. I will try the scale trick. I was addressing it by standing on one leg and going on my toes...210lbs there. Yes I know it's different but same same. As for front / rear caliper, I put same size bore m/c in your Calc with all other values and it crosses the threshold at 1.15G (where rear brake bias is exactly providing maximum required torque). I know a Z can brake higher than 1.15G with sticky tires but I am running a 285/315 stagger so that should raise that threshold value over 1.4G+ and not have to worry about a spin due to rear lock.

I do not like soft pedals or long travel but I also don't want a rock hard pedal. I've been trying to calculate the amount of assistance a stock c5 booster gives (9"?) with its approx 3.5:1 pedals.

RX-Ben

As touched on, you should swap to larger front piston area for a non boosted setup. The larger bore Wilwood calipers are a good place to start.

0Todd TCE

Boosted values and a sliding scale proportioning valve are items I've yet to be able to put into that sheet..be fun but maybe a bit too complex.

Bigger bore calipers will almost be a requirement for manual brakes. Just look at the bore sizes from the old D8 vs the six pot today. The D8 has far more piston area whereas the BPR works at higher line pressure. So without a small bore mc to boost the pressure up you'll be getting a leg work out.

I will confess that I am usually close with what I do on my own work but at the end of the day mc's are not very expensive. Expect to change one. I'd say you'll want a split set but not more than 1/8". You can then jump up or down a step and swap things.

I'm not looking for an argument here but I think your deceleration goals are a bit optimistic. I'm not a Corvette owner or driver but well over 1G seems pretty high to me. I would not work my data off numbers that high.

Apocolipse

The data was pulled from this quick discussion:

https://www.corvetteforum.com/forums/autocrossing-and-roadracing/2858451-g-force-when-braking.html

Let me know if I'm on the right track here...

9" booster, 10psi Vac engine idle/ engine braking, pi*r(sq)= ~64sq inch * 10psi = ~640lbf

So this means the booster adds 640lbf to the mc when applied. Factor in 3.5:1 pedal ratio at 100lbs foot force and that's about 1000lbf (640+350) pushing on the m/c (1" bore=0.785sq in) which will give a line pressure of 1275psi.

Now let's go manual brakes...100lbs leg force 6.25:1 pedal ratio = 625lbf.

1" (0.785) = 796psi
15/16" (0.69) = 906psi
7/8" (0.60) = 1042psi
13/16" (0.518) = 1206psi
3/4" (0.441) = 1417psi

So to get approximately the same braking effort manual I'm looking at a 13/16" m/c if all else holds equal.

Am I on the right track here?

0Todd TCE

I won't argue with that math, you seem to have it done well and I agree with your comparison.

The three thing that jump out at me are;

1. That pressure is only as relative as the piston area you have in the caliper on the other end. If you don't change the piston area then I seems logical.

2. I personally still believe that 100lbs of effort is a ridiculous value of required leg effort. If it's only to compare that makes sense I suppose but you won't last ten minutes on track with that much force required.

3. With 2 above in mind the projected line pressure of 1200psi is equally insane. That kind of line pressure to me suggests that there some poor engineering going on. Hard braking upwards of perhaps 800psi, maybe a short spike to 900+ would be my maximum comfort zone. More that that you're over pressuring the caliper for no good reason.

I'll stand by my own thoughts here: use 65 of leg max, target 1G of braking and bump your piston area up by roughly 25% and shoot for about 400-800psi to vary the final G. If you feel you can pull 1.4g, well; push harder on the pedal for you 100lb leg effort at that time. My feeling is you won't do it often.

Like a lot of things; there's more than one way here. Some of it will always be trial and error based on some testing. MC's are cheap to change. Calipers are not. I'd start at the corners and work inward.

Apocolipse

Agreed Todd, 100lbs would eventually get tiring. With that being said I also would not want to have 6" of pedal travel either. Will need to make sure I watch that.

Using your calculator at 1G decel, 2600lb car, 13.4/13" rotors, c6z51 calipers, 0.45mu avg pads, stopping takes around 3000lbf which (when using 13/16" mc) is about 100lb of applied foot force.

We know a stock vette can do 1G decel so that isn't a crazy assumption. We also know that people on slicks are hitting 1.4-1.6G so the car CAN do it (how often / even if i'll be doing that is questionable). I am just weary of going down to a 3/4" mc and having way too much pedal travel...

If I assume each caliper has about 0.005" pad/rotor clearance each time brakes are Reapplied I calculated 1.16" brake pedal travel using 7/8", 1.36" with 13/16" and 1.59" with 3/4".

Would you agree 5thou is sufficient? Sliding calipers so will be less than fixed.

0Todd TCE

That's probably a good number. I don't have any firm data to back it up but yes far less than a slider. And you could run anti knockback springs to maintain some light contact.

If you're looking at track use I'd also bump my Mu up to about .60 which is not out of the question either. The lower value is more in line with a good street pad.

I'm looking at the numbers you put up and the 'issue' remains your use of the stock calipers. So long as you use the lower area parts you're going to need Boost from something somewhere. Be it the car or you leg. Another angle is going to the larger 14/13/4 rotor set up for more leverage.

Apocolipse

What torque advantage would larger calipers have over stockers if I maintain the m/c ratio? Sure going larger area caliper with the same m/c piston would increase pressure, but is that not the same as a standard caliper area and smaller m/c? I realize the additional benefits associated with larger calipers but I'd like to focus solely on the brake torque generated in this example.

0Todd TCE

L/L vs S/S....we'd have to run the exact numbers, I get that. Functionally there may not be a huge difference except for what I believe you'll see as very highly elevated line pressures which I'd personally not want.