Adjustment of proportioning valve (Exa and N/A models)

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Callumgw
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Adjustment of proportioning valve (Exa and N/A models)

Post by Callumgw » Sat Nov 06, 2010 5:49 pm

Exa Proportioning Valve


WARNING - This is a brake mod and you take responsibility for what ever happens if you choose to follow these instruction, I accept nothing for the action you take. Only attempt this is if your willing to take full responsibility for the outcome.


The Exa (and N/A pulsar) prop valve is a twin piston unit for a diagonally split system. It works by using the pressure to the fronts to activate the piston that changes the rear pressure response. So once the line pressure reaches a certain value the rear line is restricted, so that the pressure beyond the crack point increases only at 40% of the front increase. For the Exa (drum rear) the crack value is quoted as 2.6MPa, and interestingly the ET (disc) is less, at 1.5MPa. This would suggest that after a rear disc conversion you would get too much rear lock and need to reduce the crack point. However, most common experience suggests the opposite and the different may come from the weight distribution difference between Exa and ET.

Complete valve with spring untensioned:
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The following images show the Exa prop valve disassembled:
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You can see the twin pistons are held by the spring. By using a pair of scales the load displacement was measured as:
5mm for 7.4kg; 10mm for 14.8kg; 15mm for 23kg; 20mm for 31.1kg

Image

This would suggest a spring constant of 1.5kg/mm

Measuring the normal installed height of the spring gives 14.8mm, or 22.2kg starting force. The piston moves a total of 6.2mm, so the final force would be (14.8+6.2)*1.5 = 31.5kg. The crack point is somewhere between beginning and final forces.

So what effect will mod’s have?
1 mm spacer = 6.76%-4.76% higher crack point (2.8-2.7MPa)
2 mm spacer = 13.51%-9.52% higher crack point (3.0-2.8 MPa)
3 mm spacer = 20.27%-14.29% higher crack point (3.1-3.0 MPa)
4 mm spacer = 27.03%-19.05% higher crack point (3.3-3.1 MPa)
5 mm spacer = 33.78%-23.81% higher crack point (3.5-3.2 MPa)

This works the same for removing spring load by backing the spring off.

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Kimmo
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Post by Kimmo » Sun Nov 07, 2010 6:52 am

Nice research!

Although transport authorities wouldn't be too keen on this, am I right?

Looks like a potential insurance nullifier, at least

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Callumgw
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Post by Callumgw » Sun Nov 07, 2010 8:23 pm

yep, only really needed if your tracking it or done a brake mod.

C

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Kimmo
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Post by Kimmo » Mon Nov 08, 2010 2:53 pm

Just to clarify...

I know why you'd want to do this - but I'm under the impression it's not a legal mod for a registered car.

Is that right? I hope not... seems a bit fucked that it's legal to bolt discs on and screw up your brake balance, but it's not okay to fix it again... unless you can find a stock proportioning valve preset to just the right balance...

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n12sumfin
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kk

Post by n12sumfin » Mon Nov 08, 2010 4:48 pm

okay well now we know what the specs are needed to adjust the bias, you could easily just get a custom fabricated bolt to replace the original which would also change the bias and no one would ever know or be able to prove it. how many times have a insurance company removed the brake bias valve and measure the length of a bolt lol.

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Callumgw
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Post by Callumgw » Mon Nov 08, 2010 6:22 pm

Kimmo wrote:Just to clarify...

I know why you'd want to do this - but I'm under the impression it's not a legal mod for a registered car.

Is that right? I hope not... seems a bit fucked that it's legal to bolt discs on and screw up your brake balance, but it's not okay to fix it again... unless you can find a stock proportioning valve preset to just the right balance...

umm, big brake conversions should have an engineers certificate for road use. The engineer should require you to rebalance the car brake bias..... That said the difference in size of piston area for the twin spots wasn't much, so some engineers may consider that that change by itself is ok. But a disc rear with a drum master cylinder and prop valve might be a different matter.

So in short neither would be legal without and engineers certificate.

C

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Callumgw
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Re: kk

Post by Callumgw » Mon Nov 08, 2010 6:31 pm

n12sumfin wrote:okay well now we know what the specs are needed to adjust the bias, you could easily just get a custom fabricated bolt to replace the original which would also change the bias .
The problem here is going through the process of getting it right, you should test it for the worst case, which I would guess is with the boot completely empty, no fuel (just enough fumes for the test!) and worn rear tyres on a greasey/wet road. Set it for this and it'll be safe always.......

but

for track cars this wouldn't be ideal, and some may not like this balance set up. It'd be easier to adjust it with washers/shims and allow it to be reversed. Put the washers under the bolt head for more rear bias and between to bolt and valve body for less rear bias (this only works up to the preset height of the valve pistons - see above pictures). Once we get a good feel for the required working range for a track car, we can then put together an easy adjust mechanism/kit. This might even be adjustable from the cockpit....maybe, and just for those who are serious.

As I get round to it I'll put more up on this one, but if anyone has ideas first feel free to shout them out!

C

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