"Theoretical" Brake System Comparison

[Charbel]

Hi Everyone,

At the start of the year I had the intention to run some back to back testing on various brake upgrade options to be able to provide some "real" world feedback to people that constantly ask "How much better will this kit be than stock?"

Unfortunately, we never got around to that testing (strange how a new born occupies so much time?) so I resorted to undertaking a theoretical comparison. This is purely based on hydraulics calcs that ignores things like caliper stiffness, heat dissipation/capacity & friction coefficients. So bear that in mind, this is more a guidance tool rather than an absolute comparison. We basically assumed a constant line pressure to arrive at a brake torque based on piston size, pad height & rotor diameter. The % are the % increase over the stock single piston caliper & rotor combo.

Ford 68-73 % Brake Increase % Pedal Travel Increase
Hopper Stoppers 3% / -3%
Ford KH 4P -5% / -7%
Falcon SS4 12% / 8%
Falcon SS4 12" 21%/ 8%


The interesting thing we found was a hopper stopper kit reduces pedal travel by 3% (based on piston area) but the increase in rotor diameter results in a 3% increase in brake torque.

Even more interesting the KH 4 piston calipers actually have a smaller piston area than the larger single piston caliper and because rotor diameter is the same it results in a 5% reduction in brake torque and the smaller piston area results in 7% less pedal travel.

Our Baer SS4 kit on the OE rotor uses larger pistons the KH, which results in a 12% increase in brake torque but at the cost of 8% increase in pedal travel. The 12" kit which we have almost finished developing is the kicker, the 0.7" larger diameter rotor yields a 21% increase over stock for the same 8% increase in pedal travel. And because Mustang/Falcon spindles will allow us to use tall rotor hats, this kit should fit most 15" wheels. So we'll keep you all posted when they are ready for sale.

Again, this is purely a hydraulic/static force calculation. One of the main factors that is ignored is caliper stiffness. So where the stock slider style calipers start to flex at higher pressures, stiffer calipers like the KH & Baer will continue to perform. Further, it ignores rotor thickness which is another significant factor in braking performance/fatigue.

That's me geeked out. Sorry no photos Kerry...

[ozbilt]

Thanks for that. Really interesting, I was thinking David (Boofhead) would have really been contributing to the discussion. So I guess we will have leave it to George to comment .....

[nassi]

Thanks for the facts, now if only I knew how to deal with them, but that would be another story.
Thank you Charlie.

[Charbel]

Hey Nassi the purpose is to try mathematically compare apples with apples as far as brake systems go. Once I get some more data I'll circulate the full table but I'll try to explain a bit now.

For a given line pressure, acting on the piston(s) in your brake system this generates a force or combined force (Force = Pressure x Area). That force is then transferred via your brake pads to act on your disc brake rotor. That force multiplied by the radius from the force to the axle centre provides us with a torque. The results above are indicating the percentage increase in that torque rather than the raw torque number as it is kind of a meaningless number. I've added the percentage in pedal travel because you could just find a caliper with massive pistons and this would yield a large torque increase, but the consequence of this is pedal travel as the larger pistons require more volume to be displaced.

You could go further and include the friction coefficient of brake pads mu (a greek letter that looks like u) but I've ignored friction coefficient because values for it vary massively from brand to brand, there a quite a few skeptics of the Mu number in the motorsport world and the purpose was to compare systems not pads. But it's worth noting, brake pads dont cost between $40 & $300 a set because they are all the same!

One thing we can't easily evaluate is the effects of uneven force distribution and flex in calipers. But the most interesting thing for me was the kelsey Hayes calipers not actually providing any improvement in brake force/torque. So if you are experiencing improvements with these calipers is it almost entirely due to the stiffness of the caliper and force distribution and potentially pad compound or your old calipers were really knackered! Otherwise you would really need to improve your pedal ratio or decrease the bore of your master cylinder to improve line pressure to get any brake force gains.

Charbel

[Husky65]

Interesting stuff Charbel. Would you be able to do your calculations with the large twin piston PBR calipers found on ford/holdens etc

[nassi]

Thanks Charnel, very well explained.
I thought the 68 - 73 KH were single pot though?

[ozbilt]

Thanks Charnel, very well explained.
I thought the 68 - 73 KH were single pot though?

Correct. I feel that was a cost thing rather than an improvement.

The way we "fixed" the 65/6 4 spot calipers on the race cars was to machine them where the O rings are fitted to allow Corvette O rings to go in there. Stopped the pistons twisting.

[Charbel]

Hey Nassi, I'm referring to the 4 Piston KH Calipers in comparison to the the single potters found both XW on Falcons & 68-73 Mustangs. I guess that comparison is only really applicable to the early cars considering a stub axle change over.

Interesting to note kerry, that the modern crop of calipers that use a "dust seal" rather than boot is partially to help stop the piston cocking as well as have a dust seal that doesn't melt under sustained abuse.

Husky, the Hopper Stopper result is using the twin piston PBR with 298mm BA rotor which is their base kit I think these days. Again the twin 42mm PBR calipers on Commodores & Falcons has slightly less piston area than the single 60mm piston caliper, so the gain in brake torque is from the larger rotor.

As mentioned previously once I get more data for other kits like 330mm kits etc I'll post a full table. By all means post/PM me what kits you have used with some of the details and I can add the info in.

[Ausjacko]

Thanks for that. Really interesting, I was thinking David (Boofhead) would have really been contributing to the discussion. So I guess we will have leave it to George to comment .....

The same thought crossed my mind a I started to read.

Lee, do you have any insight into such things considering you make your own calipers!

[Husky65]

Hey Nassi, I'm referring to the 4 Piston KH Calipers in comparison to the the single potters found both XW on Falcons & 68-73 Mustangs. I guess that comparison is only really applicable to the early cars considering a stub axle change over.

Interesting to note kerry, that the modern crop of calipers that use a "dust seal" rather than boot is partially to help stop the piston cocking as well as have a dust seal that doesn't melt under sustained abuse.

Husky, the Hopper Stopper result is using the twin piston PBR with 298mm BA rotor which is their base kit I think these days. Again the twin 42mm PBR calipers on Commodores & Falcons has slightly less piston area than the single 60mm piston caliper, so the gain in brake torque is from the larger rotor.

As mentioned previously once I get more data for other kits like 330mm kits etc I'll post a full table. By all means post/PM me what kits you have used with some of the details and I can add the info in.

Yeah cool. I have a Mustang Steve pedal box with a revised pedal ratio for power disc brakes also. I wonder if that attributes to my good feeling brakes. Still want to upgrade to your 4 pot track kit later on

[Shaunp]

Interesting to note kerry, that the modern crop of calipers that use a "dust seal" rather than boot is partially to help stop the piston cocking as well as have a dust seal that doesn't melt under sustained abuse.

.

Poms ie "Lockheed" have been using dust seals since the 60s, the twin piston MGB/Hillman hunter calipers are about the earliest I can recall, but Jags, Land & Ranger rovers did as well, in both 2 , 3 and 4 piston calipers, The 4 piston Jag ones work well but they are iron and very heavy. They ran them on the QLD T/A race cars for bit , then 6 piston willwoods from a Corvette, now locally made calipers from BHSS-TBS in Brisbane.

[LEEH69]

Thanks for that. Really interesting, I was thinking David (Boofhead) would have really been contributing to the discussion. So I guess we will have leave it to George to comment .....

The same thought crossed my mind a I started to read.

Lee, do you have any insight into such things considering you make your own calipers!

No Jacko, I didn't really do much in that aspect when I did my research before making them, But I copied the specs from a Brembo used on a Porsche race car. So my six piston Calipers use 28mm, 30mm and 32mm diameter pistons, and running a 350mm rotor. They seem to work fine pushing the car around the shed and Driveway..lol. will be interesting to see how they really perform.................hopefully I won't be starting another rebuild...

[Charbel]

Once you start approaching rotor diameters greater than 330mm just about any piston size will yield decent results based on the lever arm of the larger rotor.

Its worth noting Lee, porsches are a unique case, in that they are rear or mid engined in most cases and therefore run a lot more rear brake, i.e. less frnt piston area, more rear. Those piston sizes are probably an "intermediate" sizing in that they are a touch smaller than what we normally see up front and a bit bigger than what would go on the rear. I would bet the rear calipers off that car wouldn't be much smaller in piston size too. You just want be careful what you run on the rear. For arguements sake the porsche has 40/60 weight dist. you will have more like 60/40 and cant afford to run anywhere near the same rear braking.

I've seen those Vari calipers before Shaun and dismissed them as another brand getting the cheap die cast calipers out of China/Taiwan. Didn't realise they were being made locally, would be interesting to see how they go. Not surprised you ditched the Wilwoods I have only ever experienced flex with those calipers...

[Shaunp]

Once you start approaching rotor diameters greater than 330mm just about any piston size will yield decent results based on the lever arm of the larger rotor.

I've seen those Vari calipers before Shaun and dismissed them as another brand getting the cheap die cast calipers out of China/Taiwan. Didn't realise they were being made locally, would be interesting to see how they go. Not surprised you ditched the Wilwoods I have only ever experienced flex with those calipers...

Not sure on the Vari ones, They are doing some for Tran-am, TCM cars now as well, Ian Woodward's new TCM Camaro is running a set of their brakes, and most of the Trans-ams have been swapped over as well.

[LEEH69]

it will be interesting to see how mine perform when it hits the street, although the original brakes on mine were pretty shit so any upgrade would be a positive, but mine are front rotors 350mm dia, six piston calipers with 28,30 and 32mm pistons, and the rears have a 330mm rotor with four piston calipers with 28 and 30mm pistons, so hopefully the front to rear proportions will work, but I have also fitted an adjustable proportioning valve on the rears if I need to back them off a little, so I just need to get my finger out and get her back on the road this year and find out how they go.