Brake pedal feel

[Sydneykid]

From that I would think the standard booster is to big/provides to much assistance.

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

[HaydEn]

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

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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[Sydneykid]

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.

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.

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

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.

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

[simon k]

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.

[sambb]

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.

[simon k]

yeah..... so Winton was ok-ish, I'm definitely still not happy with the brakes, I don't think the change of pads or pedal brace made a huge difference. What did happen though was half way through the day the pedal just went to sh*t, I suspect the fluid boiled, I bled it the day after and it came out darker than it went in. I also checked the brace and one of the locknuts had backed itself off and the bolt wasn't as tight as it was. I've done it up and maybe the pedal is better, but nothing to write home about

I did a rally school day once and they talked about road braking (the called it "taxi braking") v's race car braking. On the road, people usually start with light pressure on the brake pedal and increase pressure the closer they get to the stopping point. For example, coming up to a give way sign you brake gently as you approach, looking at the intersection (hoping to glide through) and checking for cars, as you get closer you brake more while you keep checking, then a car approaches and you brake harder and come to a stop. On a racetrack you do the opposite, coming in to a braking zone you brake as hard as you can, then release pressure as the car approaches turn-in speed, then you start to turn in and release the brakes. The two methods are opposite...

Driving the Polo on the road with a vague brake pedal is OK, push the pedal gently until the car starts to slow down, then push a bit harder as you need to brake more and come to a stop.

Pressing the brake pedal a small amount does slow the car down, but not much compared to the other cars we have. It's like you push the pedal and watch the speedo needle go to the left, rather than push the pedal and feel negative G's. My daughters Focus feels the way I want the Polo to feel. When you touch the brake pedal on that car it grabs straight away... even getting in my 50 year old mini I feel like the brakes are actually doing something when I push the pedal

Braking hard at the end of a straight feels like a gamble, I know that the car is going to pull up, but I don't know how hard to push the pedal to make it slow down the way I want.. I mash the pedal and regulate once the car reacts

I'm a kinesthetic person, I sense and feel rather than analyse so I can't describe it the way Sam does

[sambb]

I'm pulling fuse 23 tonight to kill the ABS. I want that pump/module unit acting as nothing but a manifold for a while to see what effect its having.
This is what I dont understand. There's a finite amount of fluid in the system once the pedal is pushed and the master passes the reservoir port. The pads can only move so far. If a good non-abs push of the brakes can haul you down from from 160kph and the pedal only goes halfway, how is that a crash stop with full abs at a slow speed will see the pedal go to the floor. Where's the fluid going? The brake pads didnt move further than the first instance. The fluid didnt fall on the ground. The master cylinder didnt suddenly blow a seal. To me there has to be some dynamic internal accumulator (picture a balloon expanding or contracting if a solenoid gives fluid a pathway there?) inside the abs module that might even be active all the time, not just in a full abs intervention. Or could it be that a bypass circuit is opened up in the module that actually pulls fluid from out in front of the master cylinder when the abs becomes active so that you cant add any additional force with your foot to the pre programmed abs routine thats being pressured by its pump. Dunno. Why else would the pedal literally become redundant in an ABS stop. I'm thinking they dont want you having any input at that stage to how the braking is performed. Where I'm going with all this is that it really could be some sort of active line pressure management thats governed by the ABS module, which is making our pedals feel so long, vague and variable. If its a dynamic system thats active all the time to varying degrees ie not just on when a full ABS stop happens, then that could be what is killing the pedal feel, because you are never actually feeling feedback through the pedal of the fluid pressure directly against the pads - its damped and tampered with through the module.
I've seen it written here and there that the pedal gets better immediately after a crash stop. Maybe after said accumulator or bypass circuit is utilised the system then evacuates it again which restores some pedal feel. Well that would be the right time to pull the ABS fuse in my opinion.
Simon does your daughters car have ABS? If you do a crash stop does the pedal head for the floor or stay up where it normally should be?

[simon k]

Here's a nice simple explanation of how the ABS pump works.. Understanding ABS Modulator Problems | Know Your Parts

Not sure about the Focus, it definitely has ABS (2015 model) but I haven't done an ABS stop with it. The Subaru maintains the pedal height when doing an ABS stop, the pedal just vibrates/pulses underfoot, it doesn't fall away.

[Sydneykid]

I'm no expert on ABS systems, but from long time ago training on them I recall that they commonly have 2 valves, per wheel. The inlet valve (sometimes called an isolation valve) isolates its brake from the ABS pump, accumulator and master cylinder. The outlet valve (sometimes called a dump valve) allows the brake pressure to be released, for example when a locking of that wheel is detected. When you brake the inlet valves are open, the outlet valves are closed and the brakes operate as "normal", it's only when the sensors detect lock up that the valves work together to release the pressure and unlock the wheel.

Maybe if the outlet valve is leaking (worn seat, sticky solenoid etc), when it should be closed, that causes the spongy pedal.

I found the the schematic helpful (same link)
https://www.knowyourparts.com/technical-resources/brakes-and-brake-components/understanding-abs-modulator-problems/

Cheers
Gary

[sambb]

Ok pulling the cabin fuse 23 kills ABS/ESP/traction but also takes out the power steering and speedo. The way to do it is to pull fuse 6, the normal blade fuse ( I think 25 amp) in the engine bay. It'll kill ABS without affecting the power steering or the speedo. Problem is I can confirm:
1. it doesnt improve the pedal
2. The rear biasing is very real to the extent that if you were to even do an abrupt stop because you were a bit caught out by a red light it will lock the rears. Braking hard in a straight line to get the car to pitch and its literally like a hand brakey. Would be sweet for autoX or dirt khanacross though!

So there's not any ABS module isolation solution that I can see RE pedal feel improvement.

The ESP killing fuse is cabin fuse panel, furthest right column, 2nd down from the top 5A. It'll take your dash clock with it and kill the chime to tell you you've left your lights on. But ESP is rendered inactive and the car does feel very different when hustled through corners. Maybe that'll help keep ABS from jumping in to help when any brake application is happening during cornering eg trail braking into a corner and the inside rear is in the air.

Its got to be the MC or booster

[Sydneykid]

If the outlet valves/s are leaking then pulling the fuse won't help.

Cheers
Gary