Low Pedal Ratio: What Master Cylinder

[CTX-SLPR]

Hello,

I've finally got far enough into the dash to measure my pedal ratio and it's low... 2.64:1 (14.5 pedal length, 5.5 pivot to pin). I was going to use a 96 Impala SS master cylinder (1.125in bore) but now I'm up in the air. Full stroke is 3.5in at the pedal but I'd like to cut that down some so the pedal is closer to the same plane as the gas pedal at the top.

Specs:
R32 Skyline front calipers (40.4mm 4 piston)
12in 71-76 Riviera front rotors
Jeep WK1 Grand Cherokee rear calipers (48mm 1 piston)
12.6in Jeep WK1 Grand Cherokee rear rotor

I do intend to upgrade to 350mm rotors front and rear with the same front calipers and same piston size and count on the rear.

Suggestions for what to bore I'm looking for?

Thanks!

[CTX-SLPR]

Need to add that I'm running a power brake system with Hydraboost. Current unit is out of an early 90's Astro van if that make any difference.

Thanks

[CTX-SLPR]

So I've been playing with the spreadsheet BobR (a67) provided and have some very serious questions about how the hydroboost applies assist. I read the "Brake Sizing and Selection Tutorial" thread and a few others around the web and I get a 2-7x assist ratio for a hydroboost system. Because the pedal ratio is so low on my system it's VERY sensitive to this assist to get an adequate line pressure and brake force when sizing the master cylinder.

If I apply a 3x assist to the pedal pressure I get 560lb (100lbs pedal to get 100*2.6+300) at the master. I can't find documentation to say if the assist ratio is calculated from the pedal force or the input rod force. It would make more sense for it to be on the input rod force so the same calculation gives 780lbs ((100+300)*2.6). The difference I can spot is to get the line pressure between 900 and 1200 the assist on the pedal force requires a 13/16in master cylinder which is going to be really marginal on system volume with those caliper bore sizes vs. the assist on the input rod gives a 15/16in master cylinder which is far more comfortable territory for volume.

[CTX-SLPR]

I'm surprised that there hasn't been any response to this... I've done some more reading on what the "standard" hydroboost + C5/LS1 4th Gen brake setup is using for a master and get Tobin's statement about 99% of all hydroboost users using a 1.125in master. My front calipers are a bit smaller and the rears a bit bigger than the C5/LS1 setup but it's roughly comparable I think.

Additionally I pulled the brake pedal out of the car and it's entirely possible for me to shorten the pedal 2.5in and move the pedal down the same amount for a 3.8:1 ratio. This seems like it would help make things work more in the general range of experience for folks but I'm not committed to taking that step quite yet.

Anyone want to weigh in? Thanks,

[kevmurray]

I'm surprised that there hasn't been any response to this... I've done some more reading on what the "standard" hydroboost + C5/LS1 4th Gen brake setup is using for a master and get Tobin's statement about 99% of all hydroboost users using a 1.125in master. My front calipers are a bit smaller and the rears a bit bigger than the C5/LS1 setup but it's roughly comparable I think.

Additionally I pulled the brake pedal out of the car and it's entirely possible for me to shorten the pedal 2.5in and move the pedal down the same amount for a 3.8:1 ratio. This seems like it would help make things work more in the general range of experience for folks but I'm not committed to taking that step quite yet.

Anyone want to weigh in? Thanks,

I'm surprised no one has responded either. Lets go through a simplified example scenario where we calculate the brake torque, as a physicist you will appreciate the math. We wont bother calculating past the torque on the brake rotor since I believe this is the number Ron uses in his recommendations for different vehicle types. I don't know your vehicle weight or tire diameter so I have to assume it's close to average and that Ron's recommendations apply.

We start with the reference 100lbs of leg force. The pedal ratio as you measured is 2.64 which gives us a force of 264lbs on the pedal rod. Using your researched number of 3.0 for the gain of the hydroboost we get 792lbs of force output from the hydroboost acting on the master cylinder piston. Assuming you use the 1.125" master we have a piston area of about 1 square inch which gives us a line pressure of force / piston area = 792psi (after any knock back, we are looking at static conditions here where the pads are against the rotor so we can build pressure).

Assuming the master front and rear pistons are the same size and result in identical line pressures we can go on to calculate the force generated by the calipers. The fronts are 40.4mm per piston which results in a total area of about 4 square inches per caliper. The resulting force from one front caliper's pistons is line pressure x piston area = 3152lbs. Multiply this by the brake pad's coefficient of friction to get the force acting to slow the rotor (force x CoF). Assuming a brake pad CoF of 0.47 we get 1482lbs. At this point lets double it to get the total front force (at the rotor face, not the tire contact patch) of 2964lbs. Repeating this for the rear results in 2085lbs. At this point we see your total of 5049lbs is well in excess of the ~3500lbs recommended by Ron in the brake sticky. You will only need a fraction of that 100lbs reference foot pressure to achieve good brake torque. If that suits you then the 1.125" master should pair up nicely with the hydroboost. Note though, your front bias is only 58%.

Edit: after rereading your first post I see you must be talking about the Riv. It's a bit longer and heavier than average so at the very least you will need to add an adjustable proportioning valve to control rear lockup. You may want to look at substituting another caliper for the front or rear to bring the bias to ~70%.

[CTX-SLPR]

Kevin,

Thanks for your response. The added information relevant to what your example are the car is ~4000lbs (best guess currently with the motor dry its topped out on the bump stops) and will run a 26.7in tall tire. 64 Riviera has a 117in wheel base and I do have a Wilwood adjustable prop valve for the car (as well as a Bosch 2U ABS system but implementation on that one is a separate variable). At some point I might be able to afford a set of Audi Q7 calipers which would probably be closer to bias and capacity. Because of the Riviera's weight I've been looking at Hi-Po SUV's for parts donations.

Your math puts the results a bit higher than the spreadsheet though rechecking his formulas (didn't do that completely before) I see he had a 24 in the brake torque equation which I can only assume from unit analysis should have been a 12 to convert from in-lbs to ft-lbs since the radius the force is applied at is in inches. That indeed does raise my rotor torque up to 5000lbs which is well in excess of the 3500-3800lbs recommended (which I seem to have been taking that number as in reference to the tire force which is only 2150lbs with the revised math). That sounds like I might want to step up the master bore to make it a bit less touchy down the road.

I've actually started machining the pedal down to reweld it together with 2.5in taken out of it but I'm only 0.100in into the ~1/2in thick pedal before Christmas guests came over and I stopped work. Now I wonder if I should keep doing that despite the rather large dissimilarity between the P/T "normal" ratio and what I have. Basic calculations look like not, just get more front bias to keep from having to take stuff out of the back.

Any further comments?

[a67]

[QUOTE=CTX-SLPR;1214913Your math puts the results a bit higher than the spreadsheet though rechecking his formulas (didn't do that completely before) I see he had a 24 in the brake torque equation which I can only assume from unit analysis should have been a 12 to convert from in-lbs to ft-lbs since the radius the force is applied at is in inches.

Any further comments?[/QUOTE]

The use of the value 24 is correct. The conversion uses the diameter of the tire to derive the tire patch force. Which as you mention is derived from the radius of the tire. If the tire radius was used directly, then dividing by 12 would be correct.

The same is done in the rotor clamping pressure to rotor torque calculation.

Bob.

[CTX-SLPR]

The use of the value 24 is correct. The conversion uses the diameter of the tire to derive the tire patch force. Which as you mention is derived from the radius of the tire. If the tire radius was used directly, then dividing by 12 would be correct.

The same is done in the rotor clamping pressure to rotor torque calculation.

Bob.Yeah, I realized that after posting it and went to bed but didn't get back up to post a correction.

Making sure that is correct now I still get a really low (2738lb-ft) rotor torque with a 7x booster assist with the stock pedal. Same boost on a modified pedal gives me 3112lb-ft. The "Control" case (4.0 pedal ratio, C5 front brakes, LS1 rear brakes, 1.125in master, 7x assist) gives 3390lb-ft at the rotor so I believe the math from a sanity check as well. Looks like the pedal mod is still the best way forward.

[kevmurray]

The use of the value 24 is correct. The conversion uses the diameter of the tire to derive the tire patch force. Which as you mention is derived from the radius of the tire. If the tire radius was used directly, then dividing by 12 would be correct.

The same is done in the rotor clamping pressure to rotor torque calculation.

Bob.

So the spreadsheet takes the calculations to the tire patch? Then conversion from linear force to torque and back to linear force is not necessary. Just calculate the force gain from the wheel which in ctx-slpr's case is 11.25"/26.2" = 0.43 (a loss of course). Using this to scale the rotor force will give you braking force at the tire patch. In the sticky Ron quotes "brake force" in pounds which I now assume is at the tire patch since that leaves us with no variables for comparison.

CTX: I agree that 3x boost from the hydroboost system is low and 7x is more likely. I just don't know what is typical or if they are adjustable in any way. Where does your control case come from, are others using 7x? Does anyone know for sure what gain is typical from hydroboost?

[a67]

Yeah, I realized that after posting it and went to bed but didn't get back up to post a correction.

Not a problem.


Making sure that is correct now I still get a really low (2738lb-ft) rotor torque with a 7x booster assist with the stock pedal. Same boost on a modified pedal gives me 3112lb-ft. The "Control" case (4.0 pedal ratio, C5 front brakes, LS1 rear brakes, 1.125in master, 7x assist) gives 3390lb-ft at the rotor so I believe the math from a sanity check as well. Looks like the pedal mod is still the best way forward.

While double checking the SS this morning I ran some scenarios though it. With the stock pedal ratio (2.6) the braking is poor. And is much better with the 3.8 ratio.

As for the assist force, if it is a multiplier as Kevin posted the force on the M/C would look like this (3.8 pedal ratio and a 3x boost):

f = 100 + 100 * 3.8 * 3

Enter 100 in the pedal pressure, then 1140 in the assist box.

I don't know enough about hydroboost to know if that is correct or not.

Note that the Total Tire Force is the one to look at as the tire diameter affects it. A smaller diameter tire puts more brake force onto the road.

Bob.

[a67]

So the spreadsheet takes the calculations to the tire patch? Then conversion from linear force to torque and back to linear force is not necessary. Just calculate the force gain from the wheel which in ctx-slpr's case is 11.25"/26.2" = 0.43 (a loss of course). Using this to scale the rotor force will give you braking force at the tire patch. In the sticky Ron quotes "brake force" in pounds which I now assume is at the tire patch since that leaves us with no variables for comparison.

Yes, to the tire patch force. You can grab the SS in that same Ron & Tobin sticky, page 7 or 8 IIRC.

Bob.

[kevmurray]

Yes, to the tire patch force. You can grab the SS in that same Ron & Tobin sticky, page 7 or 8 IIRC.

Bob.

Thanks Bob, did you create the spreadsheet? Also, what's SS?

Edit: never mind (insert embarrassed smilie here)

[kevmurray]

I did some searching but couldn't find any mention of the actual boost from a hydro-boost system. A document on the CPP site states an 11 inch vacuum booster provides ~2.3x gain with 23" vacuum. I've heard hydroboost is significantly higher so it's likely at least 3x, probably more. Anyone know for sure? The pump likely factors in.