Stiffened suspension and brake balance
02-24-2003, 04:18 PM
#1
Instructor
Thread Starter
Join Date: Jan 2002
Location: boston and new york
Posts: 181
Likes: 0
Received 0 Likes
on
0 Posts
Stiffened suspension and brake balance
With the koni yellows and stiffer lowering springs I notice I have much less brake dive than most cars (my g-tech doesn't even register braking forces). However, I still have the stock brakes. I notice my front locks up pretty early and I have never locked up the back, even in the wet. Could it be that my modified suspension changes the weight distribution under braking? If this is happening should I change the f/r braking balance? I do mostly street driving with 4-5 DEs per year so I don't want to compromise street safety with a very agressive balance.
02-24-2003, 06:03 PM
#3
Racer
Join Date: May 2002
Location: menlo park
Posts: 460
Likes: 0
Received 0 Likes
on
0 Posts
</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by boostaholic:
<strong>With the koni yellows and stiffer lowering springs I notice I have much less brake dive than most cars (my g-tech doesn't even register braking forces). However, I still have the stock brakes. I notice my front locks up pretty early and I have never locked up the back, even in the wet. Could it be that my modified suspension changes the weight distribution under braking? If this is happening should I change the f/r braking balance? I do mostly street driving with 4-5 DEs per year so I don't want to compromise street safety with a very agressive balance.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">this is typical of these cars...they are underbraked in the rear from the factory.
id run the 5/33 or 5/55 bias valve, or better still an adjustable one.
as an aside...the suspension bits dont change total weight transfer..only rate of weight transfer...transfer is a function of anti-dive %, wheelbase, and cg height...
<strong>With the koni yellows and stiffer lowering springs I notice I have much less brake dive than most cars (my g-tech doesn't even register braking forces). However, I still have the stock brakes. I notice my front locks up pretty early and I have never locked up the back, even in the wet. Could it be that my modified suspension changes the weight distribution under braking? If this is happening should I change the f/r braking balance? I do mostly street driving with 4-5 DEs per year so I don't want to compromise street safety with a very agressive balance.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">this is typical of these cars...they are underbraked in the rear from the factory.
id run the 5/33 or 5/55 bias valve, or better still an adjustable one.
as an aside...the suspension bits dont change total weight transfer..only rate of weight transfer...transfer is a function of anti-dive %, wheelbase, and cg height...
02-24-2003, 07:13 PM
#4
Racer
Join Date: Sep 2002
Location: Victoria, BC, Canada
Posts: 313
Likes: 0
Received 0 Likes
on
0 Posts
Not sure what the deal is with the factory brake setup but my Turbo S has a 928 brake proportioning valve (distributes more braking force to cars a$$) and it stops one heck of allot faster than my previous Turbo S. Brake feel is also much better and no kidding I have never felt the ABS kick in at autocross - it stops in a hurry!
<img border="0" alt="[bigbye]" title="" src="graemlins/xyxwave.gif" />
<img border="0" alt="[bigbye]" title="" src="graemlins/xyxwave.gif" />
02-24-2003, 08:15 PM
#5
Three Wheelin'
951 comes with the 5/18 brake biasing valve. The 928 uses the 5/33 biasing valve iirc, as does the 944S. 5/55 may be too much for our cars without ABS; rear end lockup. Adjustable would probably be better.
I have only felt the ABS once since I bought my car, and that was hard braking downhill over frost heaves (Duffy Lake Road.) The standard 5/18 proportioning valve seems very conservative for these cars.
I have only felt the ABS once since I bought my car, and that was hard braking downhill over frost heaves (Duffy Lake Road.) The standard 5/18 proportioning valve seems very conservative for these cars.
Trending Topics
02-24-2003, 11:49 PM
#9
Rennlist Member
Lets see if I remember this correctly...
A 5/33 valve means that both front and rear brakes are fed identical pressure up to 33bar. Above that, only 5% of the additional pressure being applied is fed to the rears.
Someone correct me if I remembered this wrong.
A 5/33 valve means that both front and rear brakes are fed identical pressure up to 33bar. Above that, only 5% of the additional pressure being applied is fed to the rears.
Someone correct me if I remembered this wrong.
02-25-2003, 01:53 PM
#10
Instructor
Thread Starter
Join Date: Jan 2002
Location: boston and new york
Posts: 181
Likes: 0
Received 0 Likes
on
0 Posts
I went ahead and ordered the 5/33 valve. How hard is it to install this thing? I assume I will need to bleed the brakes while I'm at it. Can the stock valve be re-used if I don't like the new balance? Hello oversteer 
02-25-2003, 02:13 PM
#11
Rennlist Member
Since you have an '86 without ABS, it's quite a simple install. The prop valve hangs right off the master cylinder. Unscrew the old one, screw in the new one. Bleed brakes. Done!
ABS cars have the prop valve behind a wheel well lining which is just a few extra steps to get to.
ABS cars have the prop valve behind a wheel well lining which is just a few extra steps to get to.
02-25-2003, 04:49 PM
#13
Rennlist Member
The car stops flatter and nominally shorter with the 5/33 valve. The rears will tend to lock up first in a panic stop, however. I think it's a no-brainer mod if you've got ABS. In my specific circumstances, it was a good change.
02-28-2003, 07:27 PM
#14
Addict
Rennlist
Lifetime Member
Rennlist
Lifetime Member
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by ninefiveone:
<strong>A 5/33 valve means that both front and rear brakes are fed identical pressure up to 33bar. Above that, only 5% of the additional pressure being applied is fed to the rears.
Someone correct me if I remembered this wrong.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">OK, you're wrong The '5' refers to the size of an internal bleed orifice if I remember correctly. It doesn't really have anything to do with the pressure in the line.
You are correct about the 33. It's the pressure in bars where the shape of the pressure/force curve changes slope to a lower value. So, once you hit 33 bar pressure, more and more pressure on the brake pedal will result in less pressure in the rear line compared to the front line. It will still increase, but at a slower rate.
There are 5/18, 5/33, 5/45, and 5/55 valves available. I use the 5/33. Since I installed it at the same time as my big brake upgrade I can't really say how effective the change was. I can still activate the ABS, but I have to work at it.
<strong>A 5/33 valve means that both front and rear brakes are fed identical pressure up to 33bar. Above that, only 5% of the additional pressure being applied is fed to the rears.
Someone correct me if I remembered this wrong.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">OK, you're wrong
The '5' refers to the size of an internal bleed orifice if I remember correctly. It doesn't really have anything to do with the pressure in the line.You are correct about the 33. It's the pressure in bars where the shape of the pressure/force curve changes slope to a lower value. So, once you hit 33 bar pressure, more and more pressure on the brake pedal will result in less pressure in the rear line compared to the front line. It will still increase, but at a slower rate.
There are 5/18, 5/33, 5/45, and 5/55 valves available. I use the 5/33. Since I installed it at the same time as my big brake upgrade I can't really say how effective the change was. I can still activate the ABS, but I have to work at it.
03-01-2003, 04:23 AM
#15
Race Director
"What do the numbers in the #/## mean?"
"A 33/5 valve means that both front and rear brakes are fed identical pressure up to 33bar. Above that, only 5% of the additional pressure being applied is fed to the rears."
"The '5' refers to the size of an internal bleed orifice if I remember correctly. It doesn't really have anything to do with the pressure in the line."
Hmmm, actually I think the '5' relates to the alignment of the planets! The purpose of a brake-proportioning valve is to manage front/rear brake-balance under light to heavy/full braking. Under light braking, the 50/50 weight distribution allows both front and rear brakes to handle the load evenly. At low braking-forces and fluid-pressure, the prop-valve does nothing and both front & rear brakes will get even pressure.
However, as braking forces increase, there is a more and more weight transfered to the front wheels. Thus the front wheels have more traction while the rears have a lower percentage. So... a brake-proportioning valve is added to the rear brake line to reduce the rate of pressure-increase going to the rear brakes. An ideal valve would be continously variable, but impossible to make.
So a two-stage valve is used. Initially, the valve does nothing and passes through all pressure uninhindered. Then at a certain pressure, as represented by the '18' and '33' figures (in bars), this 'knee point' is where the valve starts to reduce pressure. The '5' number actually refers to the reduction ratio of how much output pressure is reduced compared to input pressure. This reduction is 0.46 of input-pressure or 46% (some German was probably being efficient and reduced it to just '5').
Check out this pressure chart and how brake-proportioning valves work:
<img src="http://www.gururacing.net/ImagesMisc/BrakeProportioningValves.gif" alt=" - " />
As braking forces increase (more pedal pressure) after the knee-point, the increase going to the rear brakes doesn't increase as much to account for the extra weight over the front wheels.
Compare how the stock 18/5 valve's pressure-curve compares to the 33/5 valve. For the same pedal-pressure increases, the 33/5 starts to reduce rear pressure later than stock. So with a 33/5 valve at the same maximum-braking force, say 1000psi in the hydraulics, you have 41% more pressure in the rear brakes compared to stock (650psi vs. 470psi).
Of course, I could be completely wrong because the planets aren't favorable tonight. <img src="http://www.gururacing.net/ImagesMisc/Graemlins/Graemlin-dunno.gif" alt=" - " />
Here's the list of available valves:
18/5 951.355.305.02
33/5 928.355.305.02
45/5 964.355.305.02
55/5 930.355.305.02
However, for the costs and effort, you might as well get an adjustable brake-proportioning valve so you can dial in the exact balance you want for your particular car (and track). Like this one:
<img src="https://secure.vbcomm.net/storephotos/A0707BLACK.jpg" alt=" - " />
<a href="https://secure.vbcomm.net/store/detail.asp?part=A0707+BLACK" target="_blank">Tilton A0707 Adjustable Brake Prop-valve</a>
"A 33/5 valve means that both front and rear brakes are fed identical pressure up to 33bar. Above that, only 5% of the additional pressure being applied is fed to the rears."
"The '5' refers to the size of an internal bleed orifice if I remember correctly. It doesn't really have anything to do with the pressure in the line."
Hmmm, actually I think the '5' relates to the alignment of the planets! The purpose of a brake-proportioning valve is to manage front/rear brake-balance under light to heavy/full braking. Under light braking, the 50/50 weight distribution allows both front and rear brakes to handle the load evenly. At low braking-forces and fluid-pressure, the prop-valve does nothing and both front & rear brakes will get even pressure.
However, as braking forces increase, there is a more and more weight transfered to the front wheels. Thus the front wheels have more traction while the rears have a lower percentage. So... a brake-proportioning valve is added to the rear brake line to reduce the rate of pressure-increase going to the rear brakes. An ideal valve would be continously variable, but impossible to make.
So a two-stage valve is used. Initially, the valve does nothing and passes through all pressure uninhindered. Then at a certain pressure, as represented by the '18' and '33' figures (in bars), this 'knee point' is where the valve starts to reduce pressure. The '5' number actually refers to the reduction ratio of how much output pressure is reduced compared to input pressure. This reduction is 0.46 of input-pressure or 46% (some German was probably being efficient and reduced it to just '5').
Check out this pressure chart and how brake-proportioning valves work:
<img src="http://www.gururacing.net/ImagesMisc/BrakeProportioningValves.gif" alt=" - " />
As braking forces increase (more pedal pressure) after the knee-point, the increase going to the rear brakes doesn't increase as much to account for the extra weight over the front wheels.
Compare how the stock 18/5 valve's pressure-curve compares to the 33/5 valve. For the same pedal-pressure increases, the 33/5 starts to reduce rear pressure later than stock. So with a 33/5 valve at the same maximum-braking force, say 1000psi in the hydraulics, you have 41% more pressure in the rear brakes compared to stock (650psi vs. 470psi).
Of course, I could be completely wrong because the planets aren't favorable tonight. <img src="http://www.gururacing.net/ImagesMisc/Graemlins/Graemlin-dunno.gif" alt=" - " />
Here's the list of available valves:
18/5 951.355.305.02
33/5 928.355.305.02
45/5 964.355.305.02
55/5 930.355.305.02
However, for the costs and effort, you might as well get an adjustable brake-proportioning valve so you can dial in the exact balance you want for your particular car (and track). Like this one:
<img src="https://secure.vbcomm.net/storephotos/A0707BLACK.jpg" alt=" - " />
<a href="https://secure.vbcomm.net/store/detail.asp?part=A0707+BLACK" target="_blank">Tilton A0707 Adjustable Brake Prop-valve</a>
