Brake System - Pad Sizing Considerations

[markocosic]

Brakes:


My crude spreadsheet is here (http://www.cosic.org.uk/misc/BrakeCalcs.xls), but the meaningful data is that it reckons on 220mm front discs and a 205mm rear (effective diameters 201.5 and 186.5mm respectively), assuming those 32mm piston diameter P32 Brembo calipers. 4.5:1 or 5:1 pedal ratio and 15.875mm (5/8"�) bore master cylinders for front and rear give "balance"� adjustable from 0.7g to 1.9g deceleration assuming 63-37 to 37-63 adjustment available on the balance bar.

The line pressures run around 40 bar on the 50-50 balance bar setting and up to 70 bar at the extremes of the balance bar adjustment so I can (just) use either the P32F or the P32G caliper. (P32G has a maximum operating pressure of 70bar, where the P32F is 100bar)


Question: which one should I run? (there are drawings of the pads in each on a worksheet in the spreadsheet)

A couple of thoughts I'm having: For a given pad stiffness, the larger pad should deflect less/have less pedal travel. For a given thermal input, the larger pad should have a lower temperature rise. Then again, a smaller pad shouldn't get knocked about so much by disc runout etc and might sit nearer the disc. A pad closer matched to the piston area might have a more consistent feel than one with bendy edges. Pad stiffness is non-linear (stiffer at higher load) so the smaller pad might get up on the stiff part of it's curve for consistent feel earlier.

Price is almost identical and the difference in mass is trivial. Given time and any leftover funds we might try both and quantify the differences (mountings are identical), but I wondered what you lot thought and if there's something I'm missing that could prevent us using one or the other.


I'm admitting defeat on the laziness/thermal modelling side (this thread (http://fsae.com/eve/forums/a/tpc/f/125607348/m/28110523521/p/1) has useful links) and handing it to somebody else to do the final thicknessing of the discs but can't see it being much different more than 5mm (.65kg and .6kg per disc) or restricting caliper choice - given that most discs at comps seem about that thick, friction materials aren't quoted as giving a stuff so long as you're in the 200-700C range and we didn't see any glowage/telltale heat marks on the discs.

[markocosic]

Brakes:


My crude spreadsheet is here (http://www.cosic.org.uk/misc/BrakeCalcs.xls), but the meaningful data is that it reckons on 220mm front discs and a 205mm rear (effective diameters 201.5 and 186.5mm respectively), assuming those 32mm piston diameter P32 Brembo calipers. 4.5:1 or 5:1 pedal ratio and 15.875mm (5/8"�) bore master cylinders for front and rear give "balance"� adjustable from 0.7g to 1.9g deceleration assuming 63-37 to 37-63 adjustment available on the balance bar.

The line pressures run around 40 bar on the 50-50 balance bar setting and up to 70 bar at the extremes of the balance bar adjustment so I can (just) use either the P32F or the P32G caliper. (P32G has a maximum operating pressure of 70bar, where the P32F is 100bar)


Question: which one should I run? (there are drawings of the pads in each on a worksheet in the spreadsheet)

A couple of thoughts I'm having: For a given pad stiffness, the larger pad should deflect less/have less pedal travel. For a given thermal input, the larger pad should have a lower temperature rise. Then again, a smaller pad shouldn't get knocked about so much by disc runout etc and might sit nearer the disc. A pad closer matched to the piston area might have a more consistent feel than one with bendy edges. Pad stiffness is non-linear (stiffer at higher load) so the smaller pad might get up on the stiff part of it's curve for consistent feel earlier.

Price is almost identical and the difference in mass is trivial. Given time and any leftover funds we might try both and quantify the differences (mountings are identical), but I wondered what you lot thought and if there's something I'm missing that could prevent us using one or the other.


I'm admitting defeat on the laziness/thermal modelling side (this thread (http://fsae.com/eve/forums/a/tpc/f/125607348/m/28110523521/p/1) has useful links) and handing it to somebody else to do the final thicknessing of the discs but can't see it being much different more than 5mm (.65kg and .6kg per disc) or restricting caliper choice - given that most discs at comps seem about that thick, friction materials aren't quoted as giving a stuff so long as you're in the 200-700C range and we didn't see any glowage/telltale heat marks on the discs.

[markocosic]

75 people with nothing to say? http://fsae.com/groupee_common/emoticons/icon_frown.gif



Here's a picture for you from a crummy production car braking system:

http://www.cosic.org.uk/misc/brakepad.jpg

Master cylinder is 25mm diameter. Front calipers are naff single-piston floaters with a 50mm diameter piston. 'Lever ratio' of 1.6mm of master cylinder movement for every 0.1mm of piston movement.

The pedal feel in that car has always been horrendous whilst I owned it, and I'd always attributed it to a bulkhead (firewall), pedalbox and steering column that visibly flexes when you brake. (nothing broken - just naff design)

I would never have believed how big a diference a new set of pads could make had I not switched them. (it's diferent enough for even my mother to notice the difference...) Nor would I believe how significant pad bending is had I not had such a bad example to work with.

For the FS car anything at the pad gets multiplied by 18.2 hydraulically and 5 mechanically, so in order to keep it's contribution to pedal travel small (<10mm at 2g deceleration is what I'm working to, so <2.5mm at the pedal due to pad movement as an initial goal) we'll need to keep pad deflection under ~0.025mm (1 thou)

So pad bending could prove pretty significant after all?



Generic pad stiffness? Non-linear pad stiffness (http://meweb.ecn.purdue.edu/%7Emetrib/Research/Conklin/Lining%20Material/Pad%20Stress%20Strain/pad%20stress%20strain.htm)

AP call it 'pad compressability' and don't generally fuss too much over it, though say there is a huge difference between competition pads and road-pads (no doubt we'd be getting road-pads with the OE Brembos?). Pete Collen (AutoSport show) is trying to dig out some data.



Run-out and caliper movement also gets multiplied by 18.2 hydraulically and 5 mechanically, so in order to keep it's contribution to pedal travel small we'll need to keep runout/deflection with heating/deflection of the caliper mounts under ~0.025mm (1 thou) - mandatory floating discs?



Then there's the caliper itself and what AP term 'caliper spread' under load (currently a complete unknown) and the pedalbox itself.



So many things to bend and respectable multiplication ratios too - how are you folks working to get stiff braking systems/what installed stiffness are you working to?

Anybody have data they'd be willing to share?

[Buckingham]

Caliper spread:

To get from 'complete unkown' to 'something' put the car up on stands, take the wheel off, put dial indicators on both sides of the caliper and push the brake pedal.

If you want to get fancy plumb a pressure gage into the fluid line to get force/defl.

If you want to get really really fancy, do this immediately after the car comes in from some hot laps (or on your brake dyno).

If you want to know how much caliper spread makes a difference in pedal feel, C-clamp the caliper halves together to constrain them from spreading and push the pedal.

If you want to know how much caliper spread or pad stiffness matters to coefficient, you can do all of these things on your brake dyno.

[rjwoods77]

One thing you definitely have to consider is the push of the pad up against the side of the caliper/clip. When your pads contact the rotor the friction of the pad to the caliper literally drags the pad(obvious) but it unloads contact with the caliper on one side of the pad unless you are running a hooked pad(Teves style). This induced monumental loads into the shelf of the caliper that the pad rides on on a full passenger car. You are talking between 1000 to 2000 lbs of load with a street car that is then put into the area of contact the pad edge has with the caliper which is a very small area. Basically what I am saying is statically checking deflection is not anywhere near the whole picture and you need to realize this. How big is this force in fsae cars. Probably not too big but for sure a contributor. There is a section of brake mechanics(moment equations) in Rudolph Limperts Brake Handbook that just rocks the house. The whole book is sick. It has a really nice understandable thermal stress section too. I really equate it to the Millikan RVD book of braking. It looks very similar in the way it is written. Buy it because you'll never find a book that just straight up answers questions with examples and diagrams. If you have figured out your indiviual wheel brake torque then take that effective raidus and that will give you these loads I speak of. You can also use the radius of the brake mount versus this torque to determine input loads into you upright to figure out what bolt size and mount thickness.

[markocosic]

Absolutely DH - though difficult with no previous fsae cars to work from. http://fsae.com/groupee_common/emoticons/icon_wink.gif

Pete's again coming up trumps with a range of the AP caliper spreads and I'll base it 'aproximately off there' for now.


Any ideas what a typical 'duty cycle' on lathes is? (ie - how big a lathe would one need to heath-robinson a brake dyno up)


RW - I'm not seeing how that would particularly influence pedal feel. Might alter the longitudinal stiffness of the brake at the contact patch, but wrong plane for the piston/pedal travel, no? Currently spent-out on books to the tune of $500, but have asked the library to hunt out Puhn and Limpert. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[rjwoods77]

Deflection is deflection. You have to remember that those two forces are some distance off the caliper mounts which cause a torque. Also the relationship between that pad contact is why differetial bore calipers are made. The pads "hinge" off of the contact side of the caliper based on the mu between the pad and the caliper body. Theoretically if there was no friction at that interface then the contact edge of the pad to the caliper body would not drag and eliminate the moment that causes a need to use differential pistons to equalize brake pad pressure on the rotor in the first place. I cant really quantify the effect for your setup but I do know it exsists and the forces generated are large enough to cause problems. There was a large difference in FEA results with and without those forces. Our simple bench pressure tests were very accurate but we didnt do actual testing. Large car brake dynos are super expensive. So yeah I think it would have a bunch to do with brake feel if you have floppy calipers with lots of end load,bla,bla,bla i think you get the point. Just pointing out more of the picture you might not have seen before. This is a major reason for going with individual pad per piston calipers at higher levels of racing. There isnt much moment effect on a single piston and pad combo so over the length of the caliper you dont have to use a differential bore which allows you to get more piston area which generates more clamping force over a caliper that does have differential bores. Limperts book goes into depth on this as well as calculating the effect force of a differnetial bore setup. Just think. On common full size car race 6 piston calipers you have something like a 1.75,1.375,1.125 piston set up to equalize the force pressing on the rotor which keeps the rotor from torqing at it mounts. With an individual pad per piston caliper you could do a 3x1.75"

3x1.75"= 7.2158 in^2
1.75,1.375,1.125 = 4.874 in^2

Wholly wow thats alot of area difference. I know it isnt a direct relationship but i kinda just wanted to throw the numnbers at you. So check this. You could make the individual pad per piston caliper much smaller because you would be ablke to use 3 smaller pistons to equal the same area(calculated of course, not direct relationship) which would allow you to make the caliper thinner which would allow you to increase distance from wheel centerline which allows you to fit a larger rotor to get more effective radius which then in change allows you to reduce the piston area more which......

Alcon make a 12 piston 6 pad caliper that has an effective radius that is about .5 more than comparible size calipers I have seen. The Puhn book is for grade schoolers. Get the Limpert book. It is the best one hands down.

[Buckingham]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Any ideas what a typical 'duty cycle' on lathes is? (ie - how big a lathe would one need to heath-robinson a brake dyno up) </div></BLOCKQUOTE>

The hardest thing to find to build a brake dyno is a reliable motor that can provide both the HP and RPM that you are looking for. I wouldn't hesitate about taking any motor up to its HP rating. Besides, most motors come with thermal switchs. http://fsae.com/groupee_common/emoticons/icon_smile.gif