early 944 brakes, stock MC....dosent feel right

[speedy]

been thinking about this all day and it is sending me nuts , i thought i would give an example and somebody could correct it if i am incorrect .
say if we using wallys calculations (54mm caliper cup diameter / 23 mm front m/c cup diameter)²= 5.51
944 rears:
(36mm rear caliper cup / 19.06 mm rear m/c cup diameter)²= 3.57
F/R ratio would then be 1.54
i understand this this is the ratio of mechanical advantage but and this is the thing i cannot get my head round area is pi r squared so half 23squared *3.1472=419
and half19squared *3.1472=284
so for 1 mm of movement the 23mm piston displaces 419mm of fluide and the 19mm piston 284 mm of fluid if they are on the same shaft then the limiting factor is the amount of force that can be applied to the 23mm piston , as far as ican see increasing the size of the front mc piston will decrease the mechanical ratio but increase the force on the front caliper pistons therefore increasing the bias because the rear cannot displace as much fluid for the same movement please help
jon

[1303R]

ahh. i see what you mean. have you tried contacting a brake specialist by email and see what they say? maybe worth trying a few different ones.

[super vw]

Well i have been running my early N/A 944 setup (from and rear stock) with the stock M/C (19 front 23 rear) and have to say thay it is front bias (to much really, as i lock up to early due to the very little weight transfer of a 1303 under breaking). i have done 6 Autocross sessions with this set up, and have found adding weight (60 lbs) to the front of the car HELPs A LOT with breaking AND turning (no more scrunbbing in mid turn). anyways im going to swap the front to rear lines Ex: 23mm front and 19mm rear. this should give me more of the rear bias that im looking for. If i get time this week to do it i will post back. Im going to do another Autocross session this weekend.

[1303R]

i am right in thinking the 23mm front circuit is the end nearest the driver, 19mm rear is furthest?

[speedy]

i belive this is true, in relation to this thread i found this on a 944/924 tech website (sorry 1303r cannot be arsed to type it all

The final note that is relevant here is on brake biasing or proportioning. The front wheels must develop more brake torque (braking power, if you will) than the rears, due to weight transfer under braking, in order to prevent the rear wheels from locking first and spinning the car. This can be achieved many different ways; in modern cars it is now being handled electronically. However, in the 924's, 931's, and 944's, it's handled by hydraulic component sizing (to get the correct ratio front-to-rear). In the 924 and 931, the master cylinder has the same bore size for each circuit, and the front and rear calipers use different sizes to achieve the correct proportioning. The 944 uses a master cylinder with different bore sizes front and rear - since the fronts and rears are on distinct circuits. Therefore, for a given amount of pressure on the brake pedal, a 924/931 will develop identical pressure on all four corners, whereas the 944 will develop more pressure up front than at the rear. It then becomes clear why the brake corner components must be matched with the master cylinder when doing a brake conversion -
The most important thing to remember from this - use the correct brake master cylinder from the car from which the calipers came from when converting, and ensure that the brake lines are plumbed correctly.


this seems to back up my theory on larger overall volume of liquid displaced but lets not forget the weight distribution of a 944 was between 45/55 and later 50/50 ,although not 100 percent certain i think a bugs weight dist will be atleast 40/60 35/65 front to back therefore requiring more rear bias than a 944
any thoughts on this are welcome
cheers jon

[1303R]

ok but that still doesn't tell you anything.also where did it come from? grabbing stuff from random forums or the like is not real infomation. i'd like to here from the people who make brake systems. knowing what works for some people doesn't really tell me anything, i'd much rather find out why and how it works. sorry if this sounds off, i'm here to learn, just wanna make sure i'm learning the right info.

[speedy]

i agree, there is alot of BS**t written on forums that is why when i give an opinion or infomation that i am not 100% sure about i explain where it came from it is then up to the person recieving the info what they do with it anyway the above came fromhttp://www.924.org/techsection/9brak...%20description when investigating the stepped m/c i have come to realize that there are many factors involved in brake system design it is mind bending , one thing is for sure standard 944 biasing is not the ideal for a beetle as it was calculated for a front engined car(more weight over the front wheels) , on the road under normal conditions this is not an issue but on the track or under heavy use braking distances will be larger than for a car with a well setup bias because the rear brakes will not be working as hard as they could .In theory a bigger set of brakes with bad biasing will increase braking distance over a well biased smaller set of brakes . i think the thing that brought home the importance of the many considerations when designing a system is the fact that on a perfectly balanced braking system(front locks just before back) fitting softer compound tyres on the front alters the biasing because the tyre will then give more grip meaning the rears could lock up first hence the need for a biasing valve to correct this.
anyway enough ramblings
here is a list of some of the considerations when designing an optimal braking system
front to rear weight distribution
spring rate on suspension (effects weight transfer under braking )
ratio of tyre size front to back
weather the car is lowered
downown force of fitted spoilers etc
tyre compound
disk size
area of the pads and compound of the pads used
ratio of calipers and piston size etc
there are others but i cannot remember them
After studying a picture of m/c i now realize that Wally was right ,the Fand R m/c pistons can and must work independantly of each other , after you realize this then it is simple to see how the mechanical advantage formulea will hold true . If you apply 100kg of force to the front piston this will apply 100kg of force to the front caliper and then 100 kg to the rear piston , because the volume of liquid beetween the m/c pistons remains constant the front pistons resting position is dependent on the rear pistons position when it is at 100kg pressure. The system will balance out when both front and back pistons are at 100 kg at this point the mech advantage ratios hold true , sorry it has taken me this long to understand it (must be slow)
Hope this helps
jon