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fiat uno bias valve
givemethebighammer - 3/6/05 at 12:18 AM

I fitted oneof the above and had no problems getting the car through the SVA (SVA man recorded a 70/30 split front/rear brakes if I remember).

To get this I just left the valve without an adjuster (most restrictive rear braking). Question is do I now need to adjust it to gain optimum braking performance ? Second question is what is the best front/rear split to go for.

thanks

theconrodkid - 3/6/05 at 06:27 AM

without adjuster screwed in there is very little restriction to rear brakes,70/30 is about ideal i recon

givemethebighammer - 3/6/05 at 08:17 AM

quote:
Originally posted by theconrodkid
without adjuster screwed in there is very little restriction to rear brakes,70/30 is about ideal i recon


I thought that as you screwed the adjuster in it gives you more rear braking. So mine is currently set to minimum rear braking ?

nick205 - 3/6/05 at 08:30 AM

I think Conrod is right!

My understanding is that as fluid goes through the valve it forces the plunger up. The more you restrict the movement of the plunger, the more you restrict the fluid flow therefore restricting the rear braking. So with the adjuster fully unscrewed, you already have maximum rear braking and 70/30 split sounds pretty good to me.

Nick

britishtrident - 3/6/05 at 09:38 AM

quote:
Originally posted by nick205
I think Conrod is right!

My understanding is that as fluid goes through the valve it forces the plunger up. The more you restrict the movement of the plunger, the more you restrict the fluid flow therefore restricting the rear braking. So with the adjuster fully unscrewed, you already have maximum rear braking and 70/30 split sounds pretty good to me.

Nick


It can't work that way --- as you push the plunger in it increases the preload on the spring this increases the presure that the fluid has to reach before it shuts off the port to rear brakes.

On any car with a load sensing valve I have seen loading up the rear suspension forces the plunger in -- increasing the pressure to the rear brakes. Used to get a lot of trouble with Chrysler Alpines with the plunger sticking fully in after a heavy load had been put in the back of a car that wasn't normally loaded up the result being when the car was unloaded the rear brakes would instantly lock up as soon as the brake pedal was given a reasonable hard prod.

[Edited on 3/6/05 by britishtrident]

theconrodkid - 3/6/05 at 09:57 AM

if you look under an uno,there is a spring attached to the trailing arms,as that is streched by the body going up(trailing arm cant go down)it pushes a lever on the piston in the valve thus restricting flow,as the body goes down when aunt ethel sits in the back the spring relaxes,the lever dosnt touch the piston and full braking is restored(thats my theory and i,m sticking to it)

DarrenW - 3/6/05 at 10:58 AM

If a good setting is to screw it right out (no restriction) then it begs the question why have one??

flak monkey - 3/6/05 at 11:07 AM

IIRC you only need one if you are using the disk braked rear end. If using drums you shouldnt need to use a bias valve...

craig1410 - 3/6/05 at 12:00 PM

I would also have expected that pushing the plunger in would result in greater rear braking due to the extra pre-load keeping the ports open for longer but Conrod's observations seem to contradict this.

There's bound to be someone out there who has adjusted the Uno bias valve while setting their car up during MOT or something who can comment more decisively. Whoever you are, don't be shy...

Cheers,
Craig.

givemethebighammer - 3/6/05 at 12:48 PM

Found this on an Alfa / Fiat forum. seems to confirm my original thinking.

"Fiat Uno Turbo where the brake proportioning valve actuator rod is connected
between the body and to the rear beam axle. As the nose of the car dips under braking the
rear of the body rises and pulls the valve shut so preventing rear lockup, and it
automatically adjusts for extra load/passenger weight in the rear"

so based on this pushing the plunger in increases the presure to the rear brakes.

however someone on here must have adjusted one of these things ?

[Edited on 3/6/05 by givemethebighammer]

theconrodkid - 3/6/05 at 02:28 PM

i have twiddled with mine but it didnt seem to make a great deal of difference,i,m going for a run morra if it,s not raining,ill try both extremes and report back

bob - 20/6/05 at 05:57 PM

So what was the final verdict ?

theconrodkid - 20/6/05 at 07:22 PM

i un-twiddled my adjusting bolt,the pedal moves a little further than it did and the car slows a little better,its difficult to say but going on how it is fitted to the uno,screwing out increases pressure to the rears

givemethebighammer - 20/6/05 at 09:53 PM

So unscrewing the bolt definitely increases the pressure to the rear wheels ? Time to head for the rollers to take a closer look. I screwed my bolt right in the other day, like you say the pedal travel decreased (no movement of bias valve piston) however I couldn't tell if there was any effect on the braking performance. I guess 20 mins at the local MOT station should see things right.

Peteff - 20/6/05 at 10:31 PM

I thought if you put more tension on the spring by screwing the plunger in more effort was needed to open the valve so less gets through to the rear cylinders. I'm sure that's how the converted mini valves used to work.

craig1410 - 31/3/09 at 12:14 AM

Hi Guys,

Sorry to drag up an old thread but I have my SVA test on Friday and would like to know how to adjust my Fiat Uno bias valve once and for all...

At the moment my brakes lack bite and take quite a lot of pedal pressure and I am wondering if I am lacking rear braking. I have a Sierra m/c without booster and have cortina front discs and sierra rear disks.

So, can anyone put their hand on their heart and tell me how the Fiat Uno bias valve operates? If I don't allow my stop screw to touch the plunger does this give me minimum or maximum braking force to the rear?

Thanks guys - starting to panic now....
Craig.