Rear Disk Brakes Pro & Con

[PhillipM]

A proportioning valve on the front circuit is usually a big no-no - they reduce the pressure as the line pressures build - so you get less front pressure the harder you brake - exactly the opposite of what you want.

[Ol'fogasaurus]

If I remember correctly, they just slow down the filling up of the slave cylinder. Pressure should be the same just fill the slave at a slower rate: e.g., the rears can start to apply a tad sooner than the fronts ... for example.

Proportioning valves are often found in a lot of dual chamber MCs or in the distribution block. In most, but all case,s the adjustable valve is not that bad of an idea as you can "tune" you system.

Don't forget to consider the need/use for residual pressure valves too.

Lee

[FJCamper]

Hi Phillip,

We are well aware of the effects on fluid volume proportioning valves have, as they are nothing but restrictors.

One of the lessor known aspects of "tuning" brake proportioning with them is reaching what seems to be the best adjustment, then going too far, and feeling your brakes just go away, starved for volume.

Truth be told, I have a deep distrust of brake proportioning valves in general and try never to use them. In this case, it was necessary. One of our drivers has had a front valve system on his autocross/track day Super Bug for years, and he installed the new one on the Blitzwagen. We have to adjust it by reaching in behind the left front tire.

We have a philosophy here at RetroRacing, and that is "Brakes only slow you down." We try to go easy on the brakes, or as easy as we can.

FJC

[Bruce.m]

Cheap proportioning valves are just a restriction, as you say but the proper versions do maintain a pressure differential rather than restrict volume.

[Bruce.m]

Here is an example :

https://tiltonracing.com/wp-content/upl ... -Valve.pdf

[PhillipM]

Cheap proportioning valves are just a restriction, as you say but the proper versions do maintain a pressure differential rather than restrict volume.

Exactly, they're not a restriction to flow that slows down filling, they have a seperate chamber and create a differential in pressure with a knee point according the spring rate and preload*
They directly reduce the pressure rise above that knee point, not just slow flow - that would be a terrible proportioning valve if you did, all it would do is make you lock one end of the car up when you hit the brakes and then bring the other end in whilst you were trying to sort it out - and then, just for fun, it'd hold the brakes on when you let go of the pedal

The reason you're feeling your brakes just 'go away' when you adjust too far is because you're fitting them to the front lines, and just as you're braking heavily and want the bias to shift up front to match, your valve is popping and reducing the pressure gain!
Why do you think all the OEM's who used them before electronic bias control ran them on the rear?

I can see why you have a distrust of them and try not to use your brakes much


*This is why you still run a bias box or differential in the calipers/MC's to set your intial bias for low grip situations, then use the brake bias valve to bleed off rear pressure and shift the bias forwards for situations where there's more traction available.

[Ol'fogasaurus]

Here is an example :

https://tiltonracing.com/wp-content/upl ... -Valve.pdf

Nice post Bruce. I am not familiar with the lever style, only the adjustable style.

As I intimated, newer braking systems have proportional valving as part of either the MC and others part of the distribution block.

Yes, it would be best to have the brake bias in the front to rear systems reasonably close to what you want allowing the adjustment (screw type) to finish the job. The lever style, as I see it, is more for racing as the article says (at a quick look at the bottom diagrams, I don't understand the street rod bit so I would have to spend some time on that thinking. My Hot Rods were still front end heavy and needed more rear braking partly because of the larger diameter rear tires ... 8.25s vs. 5.00 X 15). The balance bar (dual MCs) system I never thought was that accurate as angles to the bar can be involved.

In dual chamber MC system you can install the adjustable style of proportioning valve to either the chamber supplying the front or rear allowing you to tune full pedal pressure sooner or later. In a single chamber system you would probably want the valve after the split going to the front and rear. Both setups would be before the turning/cutting brakes.

Since I only run rear brakes I haven't spent a lot of time playing with a proportioning valve so the detail adjustment I haven't tried but I have talked to those who use them. The shut-off valve (when going on the sand) in the front, I would suspect, would be before the proportioning valve assuming you would controlling the front rather than the rear brakes which is an interesting dilemma if you are trying to control the rear brake system. The same with a hill holding front brakes when off-roading/hill climbing which I think would be sort of an oxymoron too (not talking about the Studebaker or Subaru style(s) either).

[PhillipM]

The lever style are the same internally as the knob ones, they just let you use more positive adjustments if you're changing balance on the fly.

[Piledriver]

So an easier to adjust setup would be to somewhat oversize the rears and use the proportioning valve.
Harder you brake the rear is reduced more vs. front.
(which is the conventional use)
Would work well in most situations.

The Tilton document shows reduced front pressure setup for sand/gravel/dirt rally etc. so you can steer while braking in loose stuff.

[PhillipM]

It does, but I'd say that was for very specific scenarios more like oval dirt where your turning whilst hard on the brakes as you want the brakes to bring the rear out, rather than slow the car down as such - you can still set that up with the valve in the rear line with your initial bias anyway - but there's other manufacturers will tell you never to fit one to a front line.

I tend towards setting up with the front locking first because it's more stable that way on the high speed rally/circuit stuff, and you can always shift the balance back the other way if you need to with the fiddle/cutting brakes.

[Piledriver]

I have read good arguments for the rear tires/brakes really only providing stability control when braking hard.

[PhillipM]

Well, if you overbrake the rear all you'll do is go around the corner facing backwards...

[Ol'fogasaurus]

Funny Phil but just after front brake problems with my "almost a truck" (Midis and I are back to not being friends again) I do not like the front locking up as you get a lot of "push" in the steering; almost like driving on sand in ways. Also, with the rear feeling like it wanted to lift off the ground... so to speak, I am not enthralled with front lock up either (Midis ).

(Exaggerated for effect) If you had a bug that had discs up front; say 360 pucks and 4 square feet of swept surface discs and a proportional MC and the rear was controlled by two overripe bananas controlled by a pair of tweezers actuated by goosing two over aged worms then I think you would have problem no matter.

If you considering in the weight balance shift rear to front for a bug but in a front engine car the rear is already light especially as the fuel tank lightens up.

Ya gotta get closer than that to have good braking. The positional style of bias valve sounds good for quick changes but I still think (and I am not locked in cement here) the adjustable unit has a bit more value to it. It also depends on just where you put the valve. I have seen pix of it set so the driver can change the balance. Not sure if it is the best way but sure does sound more comfortable to me.

Lee

Edited to clear up two run together thoughts and a spelling error.

[PhillipM]

I don't tend to use the lever types myself, I get the car setup close enough with the dial type ones and rarely change it from there, maybe some initial bias alteration on the balance bar for different pads or tight tracks where the rear pads are running hotter than the front, etc, but once setup right with a valve in the rear lines they should self compensate to an extent as grip levels change. The only time it will get tweaked more is if the ride height or wheelbase gets changed as obviously that will change the weight transfer at a given braking force.

Obviously it's no good having little front or rear braking - you still need to be using as much as you can, when it comes to lap time brakes are more powerful than any engine you could shoehorn in....but without electronic bias control or convoluted compensators you can never be perfect across the whole range of speeds and terrain, you'll always be front or rear biased rather than balanced at some point, personally I prefer to be slightly front biased where I have to be as locking the rears at 120mph into a sweeper on gravel or mud rather puckers the buttcheeks...