Reply With QuoteCorrect, still crap and I made the pushrod inside the booster longer to remove and free play/movement.Originally Posted by sambb
From that I would think the standard booster is to big/provides to much assistance.
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Volkswagenist
hmm yeah thats a decent jump up. So the pedal didnt take up under pressure earlier than with the smaller master at all? weird.
Volkswagenist
Correct, still crap and I made the pushrod inside the booster longer to remove and free play/movement.
From that I would think the standard booster is to big/provides to much assistance.
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Volkswagen Enthusiast
looks very similar... https://www.ebay.com.au/itm/233244330019?chn=ps&mkevt=1&mkcid=28
makes me wonder about jumping right up to a 26mm MC - what did that come off Hayden?
do you have any old stuff laying around?
Volkswagenist
26mm came off a VE commodore.
Yes I have my standard R32 master in a box somewhere.
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Volkswagenist
Yeah I'm not sure on how much displacement change you need to get a certain change in feel at the pedal. It could be that a jump from 20.64 all the way up to 26mm is what would give a result, and a swap to the Boras isnt far enough. dunno.
I'm just going to egg you on Simon so that you can be the guinea pig. I'll text you some pics of the master cyl brace installed.
Volkswagen Enthusiast
Shouldn't make any difference to the pedal feel however it will change the amount of effort required, a "larger" power booster = less effort. Cars with excessive assistance are touchy on the brake pedal, easy to lock up and hard to modulate. Doesn't seem like the case here.
Seems to me, based on the related experiences, that it's not one thing causing the long pedal, it may well be a combination of pedal free play, pedal ratio (mechanical leverage), firewall flex, too small a bore master cylinder (hydraulic leverage), brake hose expansion and calliper flex.
Cheers
Gary
Golf Mk7.5 R, Volvo S60 Polestar, Skyline R32GTST
Volkswagenist
Perhaps I don’t understand the difference between pedal feel and pedal effort.
To me, pedal feel is pedal effort. If the pedal feel is spongey/soft the pedal effort is also spongey/soft/light.
This is where I still think the booster is playing a part.
In my case. I was using Wilwoods and brembo’s so minimal caliper flex compared to oem calipers. I removed any free play from the system via a longer rod. I did not change the pedal ratio and did not reduce firewall flex. (Minimal but may play a part and I’m working on a fix for this) i have increased bore size. I have solid/braided lines so shouldn’t be any flex there either
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Volkswagen Enthusiast
Through the leg muscles an average person can exert about 2500N on a brake pedal, with a power booster the required effort is around 1/2 what would be the case unboosted. Assuming all else is equal the same system unboosted would require twice as much effort (how hard we would have to push) but would have the same feel (how far we would have to push). Changing the power booster affects the effort but doesn't affect the feel. Brake fluid is incompressible (normally) so, assuming zero "flex", the movement (how far) is determined by the ratio of the master cylinder size to the brake calliper piston size/s.
An unboosted brake pedal typically has a leverage/movement ratio of 5 to 1 to 7 to 1, whereas a boosted brake pedal typically has a 2.5 to 3.5 to 1 ratio. To reduce the amount of pedal travel an unboosted car typically uses a master cylinder that has a larger piston bore size (higher hydraulic ratio), but that is offset by the pedal's higher leverage ratio.In my case. I was using Wilwoods and brembo’s so minimal caliper flex compared to oem calipers. I removed any free play from the system via a longer rod. I did not change the pedal ratio and did not reduce firewall flex. (Minimal but may play a part and I’m working on a fix for this) i have increased bore size. I have solid/braided lines so shouldn’t be any flex there either
The list of what causes a spongy (long) pedal is pretty simple;
Firewall flex
Pedal and pedal box flex
Calliper flex
Expanding brake hoses
Flex in the calliper mounts to the uprights
Air/water in the hydraulics
Leaking hydraulics
Too low a hydraulic ratio (between the master cylinder and the calliper pistons)
Flex in the brake pad backers (race pads have high tensile backers)
When diagnosing a "long brake pedal" in a race car we just go through the check list and tick them off one by one. Most often it's not one thing, it's a combination, then we fix one thing and that causes another to be worse. In the Skyline for example I fixed the firewall flex and it promptly broke the pedal box because it was then absorbing all of the push on the pedal.
Cheers
Gary
Last edited by Sydneykid; 07-04-2021 at 02:25 PM.
Golf Mk7.5 R, Volvo S60 Polestar, Skyline R32GTST
Volkswagen Enthusiast
very interesting Gary... other than having having a long pedal, I find the lockup point hard to judge. I'll push hard on the brake pedal to pull it up at the end of a straight and it's like sometimes I'll get a lockup and other times not. Given what you said I'm still keen changing the MC, a 23.8mm MC is 25% more volume/surface area than the stock 20.6mm, Hayden went up 15% with his change so maybe the amount of change is worth having a go.
Before pressure bleeding a couple of weeks ago we clamped off the front flexible lines and the the pedal didn't change (not substantially anyway).
I took my engine mount off and added a master cylinder brace to it (it may not be stiff enough at the engine mount end though) I had my mate jump on the brake pedal again and it appears to make a difference to how much the MC moves, I can't say it made a huge difference to pedal feel but I did have my rotors machined and have gone back to the Remsa pads (nothing like changing multiple variables at once)
I am back at Winton on the 17th for a sprint so will see how it feels.
Volkswagenist
Simon how did it go at Winton?
I was at SMSP on brand new RPF600 fluid and ferodo DS3000's etc and my pedal was long and vague too. I've got a suspicion that the Audi Mk1 TT rears have bigger bore calipers which might even be making the pedal longer for me. I think the biggest problem I found was the ABS. It just intervenes way too early when you are on semis. After sailing around corporate hill I'd climb on the brakes for the hairpin with braking in hand and the damn thing would would start ABS pulsing and as soon as it did that the car would predictably loose braking potential and then suddenly I WOULD be outbraking myself. Bloody awful. I also had an Alpine A110 (the new one not the classic) that was holding me up. I was closing up on him preparing to pass him going up the hill out of turn 5 when he had a big moment on entry and got a big slapper happening. Again without ABS I wouldnt have been anywhere near a problem but suddenly the ABS decided I was about to die, pulsed away to the extent that the pedal went almost straight to the floor and suddenly I was in a situation where if he'd stayed on the racing line I could have punched him from behind.
I'm seriously considering just pulling the ABS fuse and finding a way to proportion to the rears if it turns out that the brakes do become overly rear biased. In hindsight as I became more comfortable out there I should have disabled the ESP. Its possible I guess that when a couple of things are happening eg a bit of lateral sliding as you are braking and if the ESP gets triggered while you are braking then it goes into full wig out mode. You just cant threshold brake with the ABS active. What I'm trying to say is maybe next time out, pull the ESP fuse at minimum and see if it isnt causing the ABS to kick in prematurely, because its pretty rare to be always braking in a dead straight line. Next time I go out I will kill the ABS and try to get a read on what that does to the rear brake bias.
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