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  #91  
Old July 12th 2007, 16:53
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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
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  #92  
Old July 13th 2007, 15:51
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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.
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  #93  
Old July 14th 2007, 14:41
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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
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Last edited by speedy; July 17th 2007 at 21:56. Reason: realized i had dropped one
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  #94  
Old July 15th 2007, 17:59
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OK here is the thing if the above is true and brake pressure ratio is caliper to m/c piston ratio , you have to take into account the fact that the front 23mm piston is pushing a 19mm back piston hydralicaly in the master cylinder .In the calculation this gives a ratio of 1.9 on area guess what the equivelent areas are now 23.8 is 444 and the 19.05 is 443 so if the 23 is nearest the driver it will supply more fluide but not effect the bias at all ie harder pedel on 944 na calipers , maybe this is the reason some people swear 23 on the rear is better and some 23 on the front is better , it seems strange that porsche picked these two bore sizes that when area is calculated as a ratio of each other give the same surface area , could anybody chime in that has infact run the m/c both ways , i would be interested to hear your results and if they match with my calculations .I rang a brake specialist(allegedly) and due to the fact split bores are not that common he could not give me a definative answer , infact no answer
cheers jon
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Last edited by speedy; July 17th 2007 at 21:57.
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  #95  
Old July 17th 2007, 20:57
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Have a read of the 914 tech articles on the Pelican pages, that might help explain a few things.

This one in particular may answer some of your master cylinder questions

http://www.pelicanparts.com/techarti...rake_calc2.htm

I have 944 NA brakes front and rear on a 1302 and I find them okay on the street and once warm on the track I really like the way they feel and brake. They may not be perfect, but I suspect they offer better front to rear ratio than most people think. I use the 23.8mm side to the fronts as designed by Porsche. If you read the above tech article you'll see why it is done this way.

Friends with quicker cars than me would like to upgrade their 944 NA's (also on a 1302), but I really think it comes down to what you want and the use you give your car.

Out of curiousity have you compared the front to rear hydrallic ratio of this set up to early 911's, which are more tail heavy than our Beetles are.

Cheers
J
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  #96  
Old July 17th 2007, 21:56
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Jeza, when i did the calculations i did them converting everything into lbpsi, i read that artical earlier, it is what got me thinking , if you aplly the theory that 100lb from the driver is absolute pressure , this translates into 155psi from the front piston because the piston is less than 1inch squared in area and exerts more pressure because the input pressure is not in psi ,therefore the rear piston because it is less area but pressure is now applied as hydraulic pressure in psi ,it is dependent on the reduction ratioo of the the area,s 444 and 228 which is 1.9 so 79lb of pressure is aplied to the rear circuit ,say we had two pistons on the calipers that were 23mm front and rear the resultant pressure at the front piston would be 23-23 which isa 1-1 ratio so 155lbpsi or 100lb the rear would be 19-23 19mm area=228mm, 23mm area =444mm in area gives a ratio of 1.93 -1 multiply this buy 79 and the answer is 155lb or 100lb absolute it may sound sound confusing but if hydralic pressure is dependant on area then the ratio theory must hold true weather going up or down i put all sorts of caliper piston numbers in and the answers the same, connect the calipers front or back and apart from different pedal pressure it does not matter
what do you think
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Last edited by speedy; July 17th 2007 at 22:08.
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  #97  
Old July 20th 2007, 09:52
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Just to clarify?

I have been reading this with much interest as i am converting my 66 bug to early 944 na brakes all round. I have done all the mods and machining etc .So to clarify before i do this :1.Swapping the front connections with the rear will make no difference with the 23/19 MC.2.you must use the 23/19 MC if using NA brakes all round.3.No solution for bias issues as it depends on tyres/weight etc.I still have not read any reviews about peoples experiences which i find strange as so many people are doing it .I am looking forward to fitting the brakes but hope all this work will be worth it .
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  #98  
Old July 20th 2007, 09:53
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Just to clarify?

I have been reading this with much interest as i am converting my 66 bug to early 944 na brakes all round. I have done all the mods and machining etc .So to clarify before i do this :1.Swapping the front connections with the rear will make no difference with the 23/19 MC.2.you must use the 23/19 MC if using NA brakes all round.3.No solution for bias issues as it depends on tyres/weight etc.I still have not read any reviews about peoples experiences which i find strange as so many people are doing it .I am looking forward to fitting the brakes but hope all this work will be worth it .
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  #99  
Old July 23rd 2007, 14:59
Eliasson Eliasson is offline
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Hello
I m about to fill my brake system with brake fluid and start testing how the braking system is performing.
Whats important for me is two questions.
1. I dont want the M/C to bottom.
2. The rear brakes should never lock before the front ones.

The first ones is quite simple to calculate, if you know how much the pistons retard in the calipers, the amount of oil to puch them back should decide wath to use. And then I use some safty factor. I have 944 T calipers front and rear and I decided to use the original beetle M/C to start with.

The second question is more difficult. Every car on this forum is unique in some way. Ride hight, tire size, spring stiffnes, weight...... So my conclusion is what is right for one car is not right for every car. There could only be guidlines where to start testing.
If you continue the calculationes and find out how much possible braking torque that is produceed at brakedisc. If I have made the calculations right the 944 have a bit more braking torque at the font/rear (55/45%), 911 have around (60/40%) front/rear. That is what the manufacturers decide for thier modells, it could be some kind of guidline to follow?
Then its up to myself to set up the car so the maximum of force could be used for stopping my car. Inceasing or decreasing front and rear bias. Speedy and Super VW have mention some ways to do that in this tread. super vw for example put some weight in the front and then he increased the rear bias.

Anders
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  #100  
Old July 23rd 2007, 17:55
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putting more weight in the front does increase rear bias by default, this is a good solution to improve beetle braking by shifting a little weight over the front wheels improves grip therefore increasing the braking torque that can be applied ,many people place batterys in the front to accomplish this without adding weight to the car .
A bias valve will only work if the car is rear biased to start with !
Like you said each car is individual , wether you have a spoiler on the front or back, oilcooler up front , tyre size ratio and compound full tank of fuel etc
I think it is down to trial and error to get the balance you are happy with .
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  #101  
Old July 24th 2007, 22:44
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I already have a Optina Dry cell battery up from (right hand side of car in the corner by the fuel tank and above the spare tire well).
I really should have added a heavy battery (60 lbs or so) to facilitate weight shift better. but then i would have had to give up cargo space.

This week i al going to raise the rear end of the car up about 1/2- 3/4 of an inch to give me some more weight transfer to the front. at the moment i have a perfectly level car with NO rake to speak of, adding a rake will help with the front a bit. I alreay have the front end up on the adjust- a struts max setting (about 1/2 inch fender gap in the front). and i have no fender gap in the rear.
I raised the front to tame down the bump steering issue i had when it was "dumped", much better.... but still the steering tierods are not PERFECTLY level at a settled rest (best for no bump steer).

Although this kind of fine tuning is not going to make the brakes pefect, but it helps to get the car bananced so the brakes can work as well as they can.

The only time i have bias issues is if im not doing controled HARD braking at SPEED (60+) such as autocross events and sudden emergency stops at lower speeds.... and thats related to weight transfer and not having optinum bias). although if i transfer the weight smoothly and with more time i can really bite hard and stop no probelms with very litle tendency to lock up (although it will lock the front still, and thats fine)

So really i should be looking for ways to make the rear brakes work better, thats with hydro, friction changes like pads or mechanical changes such as tires..ect

Ramble....
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  #102  
Old July 25th 2007, 12:20
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what i'd really like is anti lock brakes. has anyone fitted them on here? can't be that hard can it? i think some later 944's used it
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  #103  
Old July 25th 2007, 15:59
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Sorry I'm too lazy to go look at what you want to do with your car but most "pro" drivers hate anti lock brakes, they say it takes the control away from the driver, but for the street no problem. Just a thought, take care and good luck. JR
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  #104  
Old July 25th 2007, 16:02
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what "pro driver's"? i wonder why most modern cars have them fitted?
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  #105  
Old July 26th 2007, 10:31
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All the GT3 Porsche Factory CUP cars have ABS. works fine... better really once you know how to use it. all you do is mash the brakes and you can never lock up .
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