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Old 09-08-2017, 07:30 PM
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Question Platetronic Brake Tests - Bias and Imbalance

Anybody had a platetronic brake test done before? Have done an LS swap and a front disc conversion, but also gone with a non-stock MC/prop valve setup - so the engineer wants a brake test.

If you've had one done, and it has had significant left/right imbalance, what did you find to be the cause, and solution?

Here's my test results (pic should load).



As you can see, I am having 2 issues.

The front to back bias was 49% front, and 51% rear. This needs to be at least 60/40 front to back (up to 90/10 is acceptable). I'm not worried about this as is I do have a Wilwood adjustable valve hooked to the rear line, so a slight adjustment will help that.

The bigger issue is the rear left-to-right imbalance. The left-rear is supplying twice as much pressure than the right. Reducing the rear-left will also aid in correcting the front/rear bias. The rear-left is also supplying more pressure than the fronts.

I've adjusted the prop valve, slightly reduced the rear-left handbrake adjustment, checked the rear wheel cylinders (they're not sticking/seized), and then re-bled the brakes. Goes back for retesting tomorrow.

Just to clarify, my brake setup is:

Front: 330mm Disc
Rear: 11" drums (stock)
Booster: Dual 8"
MC: VT Commodore

I went with the VT MC as it has a built in prop valve - I was running low on space with the original setup that uses a separate MC and Prop Valve. However the VT MC is designed for a Disc/Disc setup, not Disc/Drum. This is why I added an adjustable prop valve to the rear line.

Can anyone make sense of any of this?
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Last edited by Russ; 09-08-2017 at 07:34 PM..
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  #2  
Old 09-08-2017, 07:42 PM
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Can't the phucker who did the test make sense of it and tell you what you need to do

Isn't that why you paid the phucker?
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Old 09-08-2017, 08:31 PM
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I had to brake test mine Russ but none of that crap. Hooked up a Gforce meter suction cupped to the windscreen. Drove it to 100k hit the brakes. Need to pull more than 1g of deceleration to pass and stop straight. Failed three times as engineer was a pussy. Took it to only just over 80 and let it coast before hitting the brakes. When he finally grew a pair it passed easy. 1.2g from memory. Can't help you on your questions as Jim says I'd be hitting up the engineer
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Old 09-08-2017, 09:02 PM
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Quote:
Originally Posted by rodent View Post
Can't the phucker who did the test make sense of it and tell you what you need to do

Isn't that why you paid the phucker?
Well I only paid $25 for the test (They also didn't set up the brake system. I did.)

They ran me through the basics, but said it could be a couple reasons. A seized wheel cylinder, air in the line, etc.

It's not being tested where the work is being done. I'm taking it back tomorrow for another test. I checked the wheel cylinders, and they are fine (also new). Lines were re-bled since I also adjusted the rear adjustable prop valve.

If it fails again, I'm going to ask to leave it there and let them sort it, as they are the experts.
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Old 12-08-2017, 09:02 PM
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Ok. More on this, as I'm sure you're all chomping at the bit for an update.

Sorted the difference between the left and right rear brakes. The left drum handbrake adjustment was wound back a bit. Apparently adjusting this does have an effect on general braking, not just the handbrake.

As for getting a better front-to-rear bias (engineer wanted 70% front / 30% rear), it turns out since my front disc conversion was new, I hadn't properly bedded in the pads. It was done months ago, and since it hadn't been driven, it completely slipped my mind.

To get the rears lower, they decided to chamfer the edges of the shoes, to give it a little less bite.

The Wilwood adjustable prop valve I'm using is completely maxed out, and is only just enough. Had I needed less pressure to the rear, I would have had to alter brake line sizes. Thankfully, I didn't.
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Old 13-08-2017, 07:35 AM
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Wow that's a hell of a lot of effort to get that outcome. I would have thought 60/40 with discs up front would have been more than fine
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Old 13-08-2017, 01:13 PM
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Originally Posted by cluxford View Post
Wow that's a hell of a lot of effort to get that outcome. I would have thought 60/40 with discs up front would have been more than fine
I think it was simply because of the cars size/weight. With no ABS, airbags, or even shoulder belts, I'm guessing the engineer just wants to be sure the rears won't lock up before the fronts can do their thing.
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Old 14-08-2017, 03:38 PM
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As recently said in another thread, the rears locking before the fronts, creates an over-steer panic stop situation which is to be avoided at all costs. It's deadly.

I was actually thinking to check the mechanical adjustment of your drums to fix the L-R bias of your rears Russ.
Shoes don't run anywhere nears as close to the drums as pads to the rotors in disc brakes (because they have built-in "servo action" that assists pedal effort during braking), so the clearance can make a big difference -which the park brake has to do with.

(That servo action is also why when drums fade, the driver has to make up for the loss of that servo action, as well as deal with the loss of pad coefficient. It's a double whammy. Discs have no such servo action/built-in mechanical assist.)
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Old 15-08-2017, 06:56 PM
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Cheers Steve. We figured out as much as tests continued on.

One thing I couldn't find out was if there was a required pressure min/max per pedal. Whilst the engineer wanted 70% for the front - there was no specific amount of how much force the front required (example: 3000N per side).

Wasn't sure if there was a power/weight ratio calculation to determine required numbers for each car, and I couldn't find anything to state such.
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Old 15-08-2017, 08:19 PM
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In a typical tandem m/c braked car, there is no real braking action until ALL the braking surfaces [ whether disc or drum ] have made contact with their respective friction surfaces [ rotor or brake drum ]. The usual layout of the tandem m/c is the chamber for the rear brakes is at the front of the m/c, front brake section is at rear of m/c. For line pressure to build up in the front brakes & apply the fr brakes, the rear piston has to to wait until the front piston [ rear brakes ] has travelled as far as it can, then front brake piston can build pressure.

If you have rear brake locking or excessive pressure & have drum brakes, quickest fix might be 1/16" smaller wheel cyls. Wheel cyl are easily sleeved to do this. Burt Bros in Sydney can do this, Ph 9681 1188
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