Brake master cylinder sizing.

I could use some advise here on brake master cylinder sizing.
My KVA B type has Jaguar XJ series 2 brakes all around (as its part of the B type development).
Google tells me:
Series 2 XJ has four pot callipers up front, 4 pistons 48mm/1.89in each side.
11.22 sq/in piston area.

Rear has two pot callipers, 2 pistons 43mm/1.69in each side.
4.48 sq/in piston area.

The series two has a Tandem master cylinder size, 24mm/0.9450in.

I have a feeling my current Bias brake box has undersized master cylinders for the brake fluid demand as pedal travel is all the way down before serious braking happens. Pedal travel is full travel of the masters.
OBP bias brake box. Bias set as per instructions. 0.7 front master cilinder and 0.625 rear master cylinder.

Can someone shine a light on how to determine the correct master cylinders, do I need two 0.875 or 1" masters cyclinders to get equal to the Jaguar XJ tandem system?

Thanks in advance.
 

Terry Oxandale

Skinny Man
Boy, what a question, but good background info. I have the larger (aftermarket) 4-pot calipers with 1.88/1.75 staggered pistons, so slightly smaller than yours. My reference has always been the total piston area for one side only (that way floating versus non-floating caliper piston sizes are comparable). Using your method, my area is 10.36^2 inches. I run 3/4" master cylinders (all four corners are identical set-ups), which seem to work well in amount of travel, and modulation (using Tilton pedal assembly) with about 55% front bias on the adjustment (about one turn from centered). IMHO, if I went larger, then I would have a firmer pedal, but would really need to lay some weight to the pedal in order to stop hard (no assist). Being on the track more than on the street, I really want the leverage, which brings along with that, more pedal travel. I guess like anything else in motorsports, compromise is the first rule.

So it would appear to me that yes, your .7" MC size for the front is a tiny bit small, perhaps the .875" would help you (that's a big jump), but I would consider the 1" a non-starter as being much too large. I see a lot of mid-engine projects (e.g. GT40, etc) with significantly different set-ups front to rear, which puzzles me, BUT, my CG and weight distribution may be what allows me to make all four corners the same in terms of calipers and rotors. I'm guessing your car may be relying a lot more on the front brakes, where it would probably stop better distributing that braking power to the rear tires, depending on how your car is set up, weight distribution, CG, etc.

Before going too far, make sure any rotor run-out is recognized or addressed, and confidence that no air is in the system. Lastly, the XJ set-up was designed around a significantly different configuration of chassis. That said, I would look into a future upgrade of the rear calipers. The MC setup seems to me to be mismatched to the large variance in calipers sizes front/rear. I see a very large difference between the front and rear MC/SC ratios, and not sure if that has an impact using a bias-bar type of pedal assembly. My assumption is that the XJ also had a fixed proportioning valve somewhere in the rear plumbing, thus limiting the pressure to the rear, AND, that it had a power-assist to the brakes, which may explain how they could use a 1" MC for the OEM set-up.
 
Last edited:

Howard Jones

Supporter
I have used this calculator several times. It does work well as a comparison tool when trying different pistons sizes. Enter your best estimate for the other parameters such as axel weights, roll center, track, and others as well as actual tire sizes and rotor diameters. Leave them unchanged and then you can try different piston sizes to arrive at a general idea as to how master sizes and caliper sizes compare to each other. I like to use net dynamic front and rear percentages as a good indication of overall balance.

Set friction coefficient for brake pads at .5 and leave it the same thereafter for comparison purposes.

Go for about the same brake torque from bias table front to rear percentages as the front to rear axle weight distribution. This has seemed to get me close enough to allow for the bias bar adjustment.

A good target would be 58% front and 42% rear for a mid-engined car like a GT40 Assuming wider tires on the rear. Something like 8-inch tread width on the front and 10-inch tread width on the rear.

To be clear you have 4 pistons in each caliper at the front and 2 pistons in each caliper at the rear. The fronts are all 1.89" in diameter and the rears are 1.69" in diameter.

I think I have my GTD calculator data somewhere. If I can find it I will run these numbers through it and let you know what comes out. I also have my SLC calculator data and it should provide some useful information also. Give me a day or two.

I ran my SLC data with your calipers and 12-inch rotors all the way around and came out with a brake torques balance of 63%F and 37%R using 1-inch front master and a .7 rear. Using your .7F and .625R it will swing to 75% on the front and about 25% on the rear. That won't work very well and you will get a lot of peddle travel. The small front master has to stroke a lot to fill those big F caliper pistons.

Play with the calculator for a while and see what you think. Be prepared to swap around masters cylinder a bit and keep in mind that pad friction C can make a huge difference in balance.
 

Malcolm

Supporter
That seems quite a high % for rear braking Howard. I was under the impression the target % for the rears should around the 25% to 30% range. Another factor is that different bore sizes will effect the pedal pressure. Not too light and not too heavy!
 
Thanks so far, I have to do some math to verify if the sg/in numbers Google showed match with the pistonsizes as I doubt this figur.
Then again I live in the Metric side of the world so I have to recalculate sq/mm into sg/in.
My front corner weight is 500lb, dont know my rear number yet with an alloy RV8 engine.

I would like a suggestion for a good simple online calculator as I found several which take downforce numbers also.
 

Howard Jones

Supporter
A front-engine car like a mustang will have a very high percentage of vehicle weight on the front wheels. They are also relatively softly spring and therefore will transfer even more weight forward under braking forces as well. They need very little brake torque on the rear as a result. 75/25 would probably work well on a front-engined car.

Both of my mid-engined cars, my GT40, and my SLC have about 60% of the weight on the rear axel statically. They are both stiffly sprung and the shocks are tuned for high forces levels. The fronts have quite a bit of compression damping and the rears have a bit more rebound damping than the fronts do. These things tend to keep the car more evenly pitched when the brakes come on hard. Interestingly they both do not transfer weight onto the front wheels anything like a front-engined car does, They just sort of squat. Especially the SLC at high speed due to the added downforce at the rear with its big wing. My GT40 even seems to do this a bit also with the added spoiler.

Also, I am running wide tires on the rear of both cars. The GT40 has 315.30.17s and the SLC has 345.30.19s This is a LOT more tire than the fronts. 245.40.17 and 285.30.18. The SLC also has larger diameter rotors on the front. 13.06F and 12.88R The GT40 has 12's on all four corners.

I have spent a lot of time with the brakes on both cars. My performance targets have been track orientated but I believe the results will transfer to the street to the extent that street tires can provide grip.

Lastly brake pad coefficient of friction can make a HUGE difference front to rear. Both my cars are running Wilwood B's on the front and C's on the rear. I have also wound out brake pressure to the rears with the balance bar on the SLC and the proportioning valve on the GT40.

Overall my best guess is about 66/33 - 60/40 F/R with the last bit of balance tuning being the pads.

Note: the balance bar total adjustment is quite limited. I have found that I don't like to change it more than 1 or two complete turns AT THE MOST from being centered. Ballance bars can be problems if you try to use them to alter brake balance too much. They can jam mechanically if turned too far in either direction.

So.......................... brakes can take some experimenting to get right. The calculator I posted can help you see the differences between different pistons, rotors, pad sizes, and compounds.

Heres a pad compound chart below for the pads I run so you can see the differences.

As a final note: the difference between .45 and .65 friction coefficient is HUGE. Pad material can completely change the way the car brakes just by itself.
 

Attachments

Last edited:
"They are also relatively softly spring and therefore will transfer even more weight forward under braking forces as well. "

The only factor that influences steady-state weight transfer is the height of the CG divided by the wheelbase, although you can get some strange things happening during the transition.
 

Neil

Supporter
Thank you for posting a link to that calculator. It shows that I can easily increase the bore diameter of my master cylinder to 0.875" and still achieve a 1G stop with 50 lbs of pedal force.
 
Hi JP,

I raced an XJ saloon with factory calipers and once had a failure of the power brake booster in competition - I could barely slow the thing down, so it goes without saying that the stock jag master size of 24mm is miles too big for an unassisted set-up. I now run a McLaren M1 canam replica with pretty large 6 pot wilwoods on all four corners. It is basically a 600hp chev powered GT40 with the roof missing. After advice I got from this forum I run 50/50 brake bias with 0.75 master cylinders supplying both ends, identical brake hardware on each corner, and it stops well. I have no rear lock-up issues, the pedal feel is good - some depth but also firm. As stated above the big rubber on the back, the weight distribution, and the low height of the CG all make this possible. I suspect 0.875 is too big. Hope this helps.

Cheers, Andrew
 

Terry Oxandale

Skinny Man
Hi JP,

I raced an XJ saloon with factory calipers and once had a failure of the power brake booster in competition - I could barely slow the thing down, so it goes without saying that the stock jag master size of 24mm is miles too big for an unassisted set-up. I now run a McLaren M1 canam replica with pretty large 6 pot wilwoods on all four corners. It is basically a 600hp chev powered GT40 with the roof missing. After advice I got from this forum I run 50/50 brake bias with 0.75 master cylinders supplying both ends, identical brake hardware on each corner, and it stops well. I have no rear lock-up issues, the pedal feel is good - some depth but also firm. As stated above the big rubber on the back, the weight distribution, and the low height of the CG all make this possible. I suspect 0.875 is too big. Hope this helps.

Cheers, Andrew
Wow Andrew...pretty much exactly as I did on my M8 replica, with same results. I used a similar calculator when I set mine up, and then just for giggles, ran my set-up again with this calculator, with pretty much the same result.
 

Howard Jones

Supporter
Bob, I believe you are correct in that the CG does not change as the car pivots around the CG. The nose goes down and the tail goes up more in a front-engine car because it has the CG farther forward and thus starts out with more mass ahead of the center of the wheelbase Thus the greater mass towards the front of the car accelerates at the same rate as the lesser mass behind the center of the wheelbase causes the nose to drop and the rear to lift.

This all feels like mass is transferring but it is not. Force is transferring as measured as weight (acceleration X mass) on each end of the car. Conversely, a rear-engined car starts out with more mass aft of the axle centerline so less force is transferred to the front wheels.

This all feels like a weight is moving around in the car, but of course, it is not, Varying amounts of mass sitting on each tire are being accelerated equally to greater force levels as measured by weight as the car slows. Oh hell.................something like that.

Your right Bob.

Back to brakes. The goal in all this is to apply the maximum amount of torque to each end of the car so that the tires can transfer it to the road and not stop turning. Bigger tires on the rear can take more torque and so the brakes on the rear can be designed to apply more torque. BUT the thing to remember is that if one end is to stop the tires from turning (skid) under maximum braking it must be the fronts first. If the fronts skid slightly before the rears at max braking effort and will do so at all speeds then you have plenty of brakes. Then the question becomes how to get rid of all that heat.

My GTD has the original single tandem 5/8" master power-assisted to make up for a low peddle ratio, four-pot 1.75 fronts, and four-pot 1.38 rears on 12 inch X 21/4 rotor on all four corners. The rears have a proportioning valve in the line and I estimate it is attenuating 10% of the brake pressure.

The SLC has two .7 (not 3/4) masters, 13.06 X 21/4 F and 12.88 X 21/4 rear rotors and a balance bar type Tilton peddles set pretty close to centered.

Both cars stop a ton.
 
Last edited:
Howard, its probably taken us both some 25 years of experimentation and development to reach this point in our knowledge, but I still think there is a lot more out there we dont know, and the more of this joint sharing of what we have found, and some dismissed, is very valuable indeed.
I have become interested in the human part of the set up. I have had some ( interesting ) drivers to build these cars for, and have realised that while we have for long time had to include the calculations of the pedal ratio in our engineering, the human ratio is just as significant. I have recently been adjusting the floor level under the brake pedal pad to allow the best leverage in the distance from the ankle joint to the foot position acting on the pedal. Most drivers are fairly similar in this measurement, but many have larger, and some smaller, shoe size, and this makes a difference, particularly as we cannot assess how much human power the individual can exert onto the pedal. Older man with size 11 boots is not a teenager with size 8s .
Perhaps this is nonsense, but it has been a line of thought and active development I have been doing recently. Any thoughts ?
 

Terry Oxandale

Skinny Man
Wholly agree Frank. The Tilton pedal has a pad about 3" tall. Depending on where, on that pad, you push, impacts the pedal ratio. I have a size 8.5 shoe, and placed a riser under the pedal to ensure a better ratio, and then rotated the pedal around the axis to ensure the greatest leverage was being applied to the pedal when the pedal binds under breaking (a point in which the pedal lever-line is perpendicular with my hip to pedal-lever line.
 

Howard Jones

Supporter
Very good point about your foot being in essence the lever that is being used to depress the brake peddle, My "lever" is a size 6 1/2 and I have built my system around that length of foot. However, I have driven my SLC with different shoes and when I did it made quite a lot of difference. The difference between my diver's race shoes' effective length and a pair of tennis/workout shoes was an estimated 1/2 inch. That 1/2 longer shoe made the car different enough that I simply stopped, went back to the trailer, and put on my race booties. This was a morning warmup and I didn't expect to do much other than warm things up a bit and check out the car. I haven't done that again since. I would suggest dedicated shoes for driving the car should be considered. Especially for track work.

Placing a "shim" under your heel can be an effective way to final-tune the complete system. Just be sure that the shim is totally fixed in place with strong hardware. IMHO this is not the place for a piece of wood. Anything that comes loose in that location can result in really tragic consequences.

By the way, something must be said about the type of driving we are talking about and its effect on the brakes system you are comfortable with. Streetcar normal legal speed driving is NOTHING like a hard as you can go lap with 150MPH straightaway speeds. I like my peddle to have a bit more travel and be softer in general in a streetcar, whereas my SLC is quite hard with just enough travel to get some modulation.

That takes us to leg pressure. My guess is around a 100 pounds in the SLC but it could be 150 in anger. My GT40 is at least a third less than that. Put a bathroom scale against the wall and see what 100 pounds feels like.

What you do not want in a track car IMHO is a lot of travel. I made mine so that I would achieve near max brake effort within the 1/2 half of the piston travel in the master at a maximum, and really, I like it better in the first 3/4 inch of peddle travel, and that translated to the first 25% of piston travel in the masters. Why? If it's going to go soft I want to know it as soon as I press the brake peddle.

As I think we are mostly saying, the brakes will take some playing with to get just right. I have at least three or four different size masters in my shop. everything from 5/8 to 1-inch assorted piston sizes. They all came from just trying a different setup until I found the goldilocks spot. I have also gone through a few caliper sizes as well over the years.

Again, don't neglect the pad material differences, They are really dramatic.
 
Last edited:

Neil

Supporter
Here is some information on PFC brake pads.

 
When I stated the I often adjust the floor level to achieve better ratio, this is by lowering the floor foot pad under the pedals, not raising it with a bit of wood ! It make a substantial difference on some bulkhead mounted pedals. Frank
 
Top