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Re: Steering shudder on braking

Posted: 10 Dec 2017 05:43
by Clem
So I tried the brake cleaner. Hasn't resolved the issue. But so here's the thing. It's remarkably inconsistent. Sometimes I can brake hard from high speed and its absolutely normal and smooth. Yesterday I even got onto the ABS at about 100km/h or a bit more and everything was fine. Other times its like a shudder that runs right through the whole machine. I just don't get it at all. I don't think its a warped rotor (disc) issue as that would be more consistent and would normally give rise to "pedal-pulse", of which there is none.

So the OME suspension lifted the vehicle about 20mm in the front and 30mm in the back. I am wondering about that spring linkage that is designed to adjust brake force according to load and whether there might be any effect there. Is it possible?

The shocks are the original Nissan shocks (done about 42000km) but springs are OME. Could it be a consequence of mismatch between spring and shock rates?

Re: Steering shudder on braking

Posted: 10 Dec 2017 05:54
by hugejp
:thumbdown: Sorry to hear this man.

The brake bias balancer arm is worth a look.

Going forward a proccess of eliminating components 1 by 1...

I would really like to know what the cause is.

Re: Steering shudder on braking

Posted: 10 Dec 2017 08:02
by David M
Copied straight from an Aussie forum and I did adjust mine after fitting OME but my lift at the back landed up being 65 mm with 400 kg springs. There is a bit of adjustment on the bracket connected to the diff as the top hole is slotted.

"Rear Brake Force Less Due To Lift Kit - Brake Proportioning Valve

If you have lifted your GU Nissan Patrol it is better to re-align the rear Brake Proportioning
Valve (BPV) control arm. Most people don’t do it.

What Is It - A Technical Explanation

A load sensing proportioning valve system for the hydraulic brake system of passenger cars and

the like for varying the amount of brake fluid pressure and thus the braking torque at the rear

brakes of a passenger car.

The system includes a load sensor oriented between a suspension component, such as a

supporting spring, and the vehicle frame or body and includes a cavity for incompressible fluid.

A variation in load will vary the volume of the cavity for providing increased pressure and

volumeric flow of an incompressible fluid from the cavity to a proportioning valve incorporated

into the hydraulic brake lines extending from the master cylinder, to the rear brakes of a

passenger car for regulating the proportioning valve to vary the braking pressure and thus

braking torque to the rear wheels. This reduces or eliminates premature rear wheel locking

when applying brakes of a lightly loaded vehicle and to maintain adequate braking force for the

same vehicle when heavily loaded.

A flexible line interconnects the load sensor and the proportioning valve to compensate for

relative movement between the vehicle frame and suspension system and the proportioning

valve includes an actuator in the form of an actuating piston, engageable with the proportioning

piston in the proportioning valve to regulate the movement of the proportioning piston and thus

regulate the proportioning valve in response to variations in load applied to the rear suspension

components of the vehicle.

What is it - A plain english explaination

To reduce hydraulic pressure to the rear brakes so the rear brakes don't lock up when the

brakes are applied, a "proportioning valve" is required. This valve helps compensate for the
differences in weight distribution front-to-rear as well as the forward weight shift that occurs
when the brakes are applied.
What we're really talking about here is "brake balance" or "brake bias", which is the difference
in the amount of hydraulic pressure channelled to the front and rear brakes. The front brakes
on most rear-wheel-drive vehicles normally handle about 60-70 percent of the brake load. But
on front-wheel-drive cars and minivans, as well as RWD and 4WD pickups and SUVs, the
percentage handled by the front brakes can be as much as 90 percent of the load.
Consequently, the front brakes need a higher percentage of the total hydraulic force that's
applied to keep all four brakes properly balanced.
If the front-to-rear brake force isn't balanced correctly by the proportioning valve, the rear
brakes will receive too much brake force, causing them to lock up and skid when the brakes are
applied. The other reason for using a proportioning valve to reduce hydraulic pressure to the
rear brakes has to do with the design of the brakes themselves. When hydraulic pressure is
applied to the wheel cylinder inside a drum brake, the shoes are pushed outward against the
drum. When the shoes make contact, the rotation of the drum tries to drag them along. But
since the shoes are anchored in place, the drum pulls the shoes up tighter only against itself.
Because of this, drum brakes that are "self-energising" require little additional pedal effort once
the brakes are applied. Disc brakes, on the other hand, are not self-energising. It takes
increased pedal effort to squeeze the pads against the rotor.
Some vehicles have load sensing proportioning valves that change rear brake metering to
compensate for changes in vehicle loading and weight shifts that occur during braking. This type
of proportioning valve has an adjustable linkage that connects to the rear suspension or axle.

As the vehicle is loaded, ride height decreases and pressure to the rear brakes is increased. This
type of proportioning valve can be found on many minivans, 4WDs and even some passenger
Load sensing proportioning valves usually are adjustable, and must be adjusted correctly if they
are to properly balance the rear brakes to the vehicle's load. The valve linkage is adjusted with
the suspension at its normal height (wheels on the ground) and the vehicle unloaded. The
adjustment bracket or linkage is then adjusted according to the vehicle manufacturer's
instructions, which typically involves adjusting the linkage to a certain position or height.
Load-sensing proportioning valves are also calibrated to work with stock springs. Any suspension
modifications that increase the load-carrying capability (installing helper springs, or overload or
air-assist shocks, for example) may adversely affect the operation of this type of proportioning
valve. Modifications that make the suspension stiffer reduce the amount of deflection in the
suspension when the vehicle is loaded, which prevents the proportioning valve from increasing
rear brake effort as much as it normally would. A defective proportioning valve, or one that is
not properly adjusted, can also upset brake balance. If the rear brakes on a vehicle seem to be
overly aggressive (too much pressure to the rear brakes), or the vehicle seems to take too long
to stop (not enough pressure to the rear brakes), the problem may be a bad proportioning valve.
Proportioning valves can be tested by installing a pair of hydraulic gauges (one on each side of
the valve) to see if the valve reduces pressure as it should.
On some late-model vehicles, the mechanical proportioning valve has been replaced by
"electronic" brake proportioning through the ABS system. By sensing wheel speeds, the ABS
system reduces pressure to the rear brakes as needed when the brakes are applied.

So putting the above simply

It makes sure that the right amount of brake fluid pressure is

being directed to the rear brakes when you need it most, like carrying a load. If you modify the
height of the vehicle then you will have to modify the BPV bracket to suit. If you do not have
the skills to make a bracket yourself, Snake Racing sell them, take a look at, click on the
button and search on “brake bracket”.
They are between $22 and $28 each.

How to do and examples

Always make the bracket the same height as your lift, I measured from the centre of the bottom
hole 2” (50 mm) as I have a 2” lift. The idea is to make sure that the spring re-aligns itself at an
almost 45 degree angle to the BPV lever. On a GU 4 this does not sit level as some may mention.
I checked a stock standard GU4 prior to adjusting mine. On other models it may be different so
find a standard un-lifted vehicle, climb in underneath and take a look to be doubly sure.
Once fitted you do not need as much force on the pedal. Under hard braking it is much more
controlled and has less nose dive as it is not just up to the front brakes to slow you down now.

DO NOT touch the allen head screw on the valve these are pre-set from the factory. The spring
needs to measure between 175mm and 178mm end to end of the spring not just the coils.
Some will tell you that the spring is self adjusting, meaning that when you lift the vehicle the
spring will stay at the correct length, this in fact it actually does with a 2” lift. What changes is
the ability of the spring to provide the correct tension at the correct angle. As for higher lifts
check out the picture in this section of a GU4 that has a 5” lift, the spring is almost vertical to
the BPV lever. "

Re: Steering shudder on braking

Posted: 10 Dec 2017 08:25
by Clem
I'll take a look at the bias. For what it's worth it's not steering wheel shake per se. That's not where one feels it. The passenger can also clearly feel it.

Re: Steering shudder on braking

Posted: 10 Dec 2017 08:27
by Clem
Thanks David - I just went and downloaded the manual a minute back. It's very useful. I'll suss it out a little later the morning. First go measure it up and check the angle out.

Re: Steering shudder on braking

Posted: 10 Dec 2017 08:49
by Clem
Spring is the right length and looks like the right angle.

Next thing is to balance the wheels again and check out the shock absorbers because I suspect the latter may not be adequate to the upgraded spring rate, bearing in mind that the original shock absorbers were retained when the Old Man Emu springs were fitted. I took the liberty of standing on the one corner of the bull bar and making the vehicle oscillate a bit. I note that when I stop jumping, the opposite corner continues to oscillate very slightly up and down for about five movements before the vehicle comes to a complete standstill. Very slight but very definitely there. So I'm thinking that when the surface is not perfectly smooth (level) and one breaks hard that there may be some factor relating to inadequate damping which causes a bit of "skipping" of the front axle from side to side given the massive unsprung weight concerned. This type of action could very well give rise to the type of symptom that I experience. How likely it is, I don't know but it seems to me to be a worthwhile factor considering.

Re: Steering shudder on braking

Posted: 10 Dec 2017 14:09
by hugejp
That "bounce" is very likely to be amplified at higher speed & cause tyre skip, which would bring the abs into play as you described previously.

Well done on spotting that!

Re: Steering shudder on braking

Posted: 10 Dec 2017 15:13
by Clem
Thanks JP. Especially with that massive unsprung weight. I still marvel at how massively constructed the underside of a GU is. I can just lie there and look at it.

Re: Steering shudder on braking

Posted: 10 Dec 2017 15:35
by hugejp
The shocks may be to blame as std aged dampening may not be inadequate for the taller uprated OME springs.

Re: Steering shudder on braking

Posted: 10 Dec 2017 16:23
by Clem
Agreed completely. Even though they are only 42,000 km old, I suspect that they cannot handle the spring rate of the newer springs.

Interestingly, I once had this problem on the sand tracks to and from Liuwa in a Land Rover. I had put air springs in the back and we had a very undulating track to travel along. Eventually the back of the Land Rover was bouncing so much that the guy behind me could see air/light between my tyres and the surface of the track. There was nothing wrong with the shock absorbers – they were brand-new Koni shock absorbers – but they were specified for the original springs in the vehicle and not for the springs with airbags in them. So… It is not completely inconceivable that this could be the problem here. In fact, given the age of the vehicle and the recent change to the new springs and the fact that the problem has only surfaced now after we put on the bull bar, winch and new springs, it is probably the most likely explanation.

I have sent an email to John van Onselen of OME asking for prices on the shock absorbers, which I had planned to fit anyway at some point.