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ransom
ransom Dork
12/15/11 9:30 a.m.

What are the realities of deleting power brakes, if you have the option to also do a ground-up replacement of most of the system to optimize the replacement?

I know that the booster on many/most cars provides a substantial amount of assistance, and just deleting it with the stock system would require a giant jump in pedal force for a given amount of braking.

OTOH, if you are going to larger rotors (greater leverage), upgraded calipers and brake lines (less flex in the system), and an aftermarket pedal and master cylinder (ability to choose hydraulic ratios/advantage), can you undo some of that increase in effort by taking advantage of the system's increased "firmness" to go with a greater mechanical advantage (e.g. smaller master cylinder bore) without having too much pedal travel?

Brake pads seem like another area, though I am talking about a car which is mostly street-driven and autocrossed, so if it needs heat to become more aggressive, that doesn't help.

What are people's experiences with this sort of thing? My particular application is a BMW 2002 this time, but I suppose the only really pertinent part is that it's a fairly light car.

mad_machine
mad_machine SuperDork
12/15/11 9:35 a.m.

not sure if I would do it. I had a ACVW and the manual brakes on that were.. hard, at best. That too was a fairly light car with no weight on the nose, but it would still take quite a bit of pressure to stop the car.

I imagine if you went with a tilton pedal set with duel MCs, you could fine tune the system to how you wanted it.. and on the track, I would probably want manual brakes for feel. Not so sure for the street.

An aside.. going from a manual braked car to one with power or power assist is a great way to give you passengers whiplash

ransom
ransom Dork
12/15/11 9:57 a.m.

In reply to mad_machine:

That's really the question; can it be made to work if all the hardware is a clean slate? I'm not talking about just removing the booster or using the same car's manual bits (did the 1600 or a sufficiently-similar BMW have manual brakes?)...

Everything is negotiable in this case; If you go to large enough caliper pistons and a small enough master cylinder bore, you could get the pedal pressure very low, but at some point the pedal travel's going to become excessive.

I'm just wondering about people's experience in trying to find the sweet spot, and whether it exists.

Don49
Don49 Reader
12/15/11 10:01 a.m.

Talk to the folks at Tilton. They will be able to direct you to the right combination. I've raced with both manual dual mc set-up ( F Prod Turner) and with factory power assist ( E Prod RX7). Both worked well with no real issues. It's all about the right set-up.

unevolved
unevolved Dork
12/15/11 10:20 a.m.

You can either go with what people have done in the past, or you can go with a clean slate and design from scratch. I just designed a clean-slate braking system for a Formula SAE car, and while some of the dimensions are smaller, the process is identical. If you're interested in what calculations I used to determine what MCs, I'd be happy to help you design a system from scratch.

Giant Purple Snorklewacker
Giant Purple Snorklewacker SuperDork
12/15/11 10:31 a.m.

My E30 race car has overhung Tiltons and the setup has quite a bit more effort than boosted, OE power brakes but it has a rock hard pedal that I can feel the bite and modulate the release really well. I am not sure I'd go to all the effort on a street car - nevermind that everytime the grip changes I have to do a few stops to setup the bias again or risk locking the rears. I even move it a little forward then back once the tires come up to temp on cold days.

On a buddies car we made a nice compromise (another E30). We used a smaller booster, a little fabrication, a resized fixed dual (3/4F, 7/8R) master (machine shop bronze sleeved it too) and an adjustable bias on the rear line. It feels great but I think it is not as good at release when trail braking because the valve in the rear seems to hang a little. I'm not certain it is even an issue for him - and you would not care on a street car but I want a manual adjustment rather than the fluid pressure limiter for the track.

ncjay
ncjay Reader
12/15/11 10:36 a.m.

A few years ago I removed the power booster from my brake system on my late 70s Monte Carlo. Everything else stayed stock. A little bit of fabrication on the brake pedal in the way of adding some new holes and rasing the mounting bolt a bit higher in the mounting assembly and presto, back to normal, or pretty close anyway. It took a bit of trial and error to see how far to move things and how much of an effect it had, but overall it was pretty basic fabrication, no rocket science involved. It helps to have a spare brake pedal to cut, weld on, or drill before deciding on a final setup.

ransom
ransom Dork
12/15/11 10:45 a.m.

This is good stuff, thanks guys!

I asked the question pertaining to my biggest concern, but I should've known folks would want to be more specific than that...

This is all a ways down the line. Still in the garage-building phase of the 2002 project, so lots of pondering and planning. This is significantly motivated by the desire to retrieve the space taken up by the booster and (rather bulky) brake linkage for use by intake hardware. Of course, the removal of any unnecessary hardware is always a bonus; the question is how unnecessary it is... I have also half-pondered things like other booster/master mounting locations or hydroboost, but all that seems to come at a high price in terms of complexity (especially given that the 2002 doesn't have power steering, so hydroboost would need its own pump).

GPS, It's true, the manual adjustment seems like overkill for the street, and I'd probably mostly just set it at a conservatively forward-biased never-lock-the-rears level and leave it, though perhaps I'd want to play with it at an autocross...

Unevolved, thanks very much! I'm probably too far from acting to take up your time on chewing through a design, though I'm curious about your order of operations. Did you have a targets for pedal pressure, pedal travel, pad face pressure? The math on the hydraulic and mechanical bits seem pretty straightforward, but I don't know what sort of numbers I'm really looking for, or how to guess at how much deflection the system is likely to occur and how it'll show up in pedal travel...

Don49, just checked out the Tilton site. Those floor-mount pedal sets are, as a Swedish friend would say, tasty like sausage.

AngryCorvair
AngryCorvair SuperDork
12/15/11 10:53 a.m.

In reply to ransom:

there's an excellent brake system article in the June 2008 GRM. it contains all the equations for the gain of every component from the pedal to the tire contact patch. you can type those equations into a spreadsheet and then play around with the values of any component to see the effect it has on the system.

there are practical limits, such as the increased pedal travel that comes with a smaller MC bore or a larger caliper bore, or the noise that comes with more aggressive pads, or the rotating unsprung weight and the expense of larger wheels and tires that comes with larger rotors.

if you are healthy and strong, then you can probably just push the pedal a lot harder than what a boosted system requires. but do you want to do that every stop, every day?

z31maniac
z31maniac SuperDork
12/15/11 11:08 a.m.

I'd also shoot Lee of www.massivebrakes.com an email and pick his brain as well. He usually responds the same day within a few hours, and is happy to help guide you in the right direction.

ransom
ransom Dork
12/15/11 11:13 a.m.

In reply to AngryCorvair:

Awesome, I'll go dig that issue off the shelf!

Re: your last paragraph: Yep, that's the complication. It may not be exactly a daily driver, but nearly, and I do want it to be liveable and fun to drive, not faster-at-any-cost. And it seems to me that as long as it's not so low-effort as to be hypersensitive, I'm more likely to keep my braking and other control inputs smooth and sensitive if I'm not having to heave on the pedal too hard.

mad_machine
mad_machine SuperDork
12/15/11 12:36 p.m.

hydroboost is not that hard. Some classic saab 900s have it with a self contained electric pump

ransom
ransom Dork
12/15/11 12:50 p.m.

In reply to mad_machine:

That's simultaneously great news and terrifying. If I can avoid involving parts from a discontinued Saab in this project, that's probably for the best

Curmudgeon
Curmudgeon SuperDork
12/15/11 1:03 p.m.

What, you don't trust Scandinavian electronics?

Angry's recommendation to grab that issue is a good one. From my personal experience: I deleted the power brake booster on the Jensenator. When I did that, I had to change the stock 4:5-1 brake pedal ratio to 5:8-1 to get the pedal effort in a reasonable range. Solid pedal, very confidence inspiring and linear.

bravenrace
bravenrace SuperDork
12/15/11 1:06 p.m.

I have manual discs on my Mustang. I prefer them greatly to power. It will stop just as fast, but you have to use a lot of pedal pressure. The plus side is much better feel and modulation ability.

ditchdigger
ditchdigger Dork
12/15/11 1:30 p.m.

Simply depowering a boosted setup isn't a good thing. I have driven VW rabbits that swapped the booster out for the spacer that came on early cars. With no other changes it was pretty terrible. They were like an on-off switch and took all you had to mash the pedal down. Other rabbits that changed the pedal ratio and master cyl diameter were awesome though.

ransom
ransom Dork
12/15/11 1:45 p.m.

As a personal preference thing, I definitely want to keep the pedal effort reasonable. If the brakes are either touchy or too high-effort it makes heel-toeing difficult. And again, the primary target for this car is that it should be fun to drive, apart from thinking that making the controls suit me will lead to better performance anyhow.

Giant Purple Snorklewacker
Giant Purple Snorklewacker SuperDork
12/15/11 1:54 p.m.
ransom wrote: As a personal preference thing, I definitely want to keep the pedal effort reasonable. If the brakes are either touchy or too high-effort it makes heel-toeing difficult. And again, the primary target for this car is that it should be fun to drive, apart from thinking that making the controls suit me will lead to better performance anyhow.

My Tiltons are 6:1 with 3/4 and 7/8 mcs. The pedal effort is firm but not "hard". It is just that if you are used to an experience where there is travel that progressively adds more brake - it is very different. The pedal is like stepping on a brick. It never moves once the initial slack is taken up. It comes more from the linkage than the fluid. You push harder - more brakes but even right on the verge of lock-up I wouldn't say I was struggling to press the pedal. Heel toe is no issue - since the pedal never travels, you adjust the go pedal to be within reach and its always the same except a little travel as the pads wear.

ransom
ransom Dork
12/15/11 1:59 p.m.

In reply to Giant Purple Snorklewacker:

That's a good point.

My reference is my 2002 with pretty much stock brakes with booster. Not amazing brakes by any stretch, but with braided lines the pedal is pretty firm, and doesn't move much once the slack is taken up (though I think there's still more compliance than in your system; the pedal feels like a rock with the engine off, but there is clearly compliance when the booster's kicking in).

After the 2002, cars that modulate more by pedal travel than effort tend to be really disconcerting, and of course having your heel/toe platform moving around with modulation doesn't help anything.

So, yep. Targets include reasonable/comfortable pedal effort and a rock solid pedal.

tuna55
tuna55 SuperDork
12/15/11 2:44 p.m.

I DD'ed my 72 GMC that came with manual steering and manual brakes for over a year. It was fine. Look into older master/pedal/caliper type setups, that truck stopped very well, you just had to mean it.

mad_machine
mad_machine SuperDork
12/15/11 2:54 p.m.

I have thought of doing this to my fiat.. as it would only be a weekend play toy, and like the 02, there is NOT a lot of room in the engine bay. My problem with doing it comes from getting the car inspected.. I can only guess what the testers would think of a pedal that doesn't move and feels like the booster is dead

unevolved
unevolved Dork
12/15/11 8:35 p.m.
ransom wrote: Unevolved, thanks very much! I'm probably too far from acting to take up your time on chewing through a design, though I'm curious about your order of operations. Did you have a targets for pedal pressure, pedal travel, pad face pressure? The math on the hydraulic and mechanical bits seem pretty straightforward, but I don't know what sort of numbers I'm really looking for, or how to guess at how much deflection the system is likely to occur and how it'll show up in pedal travel...

I set a target brake pedal gain of about 80-100 lbs/G. There are certain things you have to know about the car, like vertical CG Height, tire rolling coefficient of friction (a good guess can work), fore/aft weight distribution, etc. From there it's a matter of summation of moments and hydraulic ratios to figure out the proper master cylinder and caliper piston diameters. Granted, my method assumes that the tires aren't load sensitive and there's minimal compliance in the system, but it's more than accurate enough to get within range of a balance bar.

I've got an excel spreadsheet that could help you figure some things out if you're interested. Feel free to shoot me an email.

ditchdigger
ditchdigger Dork
12/15/11 8:54 p.m.
mad_machine wrote: I have thought of doing this to my fiat.. as it would only be a weekend play toy, and like the 02, there is NOT a lot of room in the engine bay. My problem with doing it comes from getting the car inspected.. I can only guess what the testers would think of a pedal that doesn't move and feels like the booster is dead

I use the 124 master cylinder in my 850 with no booster of course (that would be a hell of a vacuum line run) with the same rear wheel cylinders and slightly smaller front caliper piston volumes and it feels fantastic. I can't imagine it would be bad on your car.

Streetwiseguy
Streetwiseguy Dork
12/15/11 9:15 p.m.

I spent the past summer oval racing a super late model, which has dual masters, obviously not boosted. Fairly high effort, but excellent feel and modulation, plus a crank for front to rear bias. I then headed off to Calgary to roadrace my IT2 Neon, and spent most of my time on the track flatspotting tires, since the pedal was softer, shorter and harder to modulate.

Clean sheet, using aftermarket stuff, I'd have no hesitation about going manual. Trying to cobble up something out of street parts might work, but I'd bet it will take a lot of experimentation to get right.

unevolved
unevolved Dork
12/15/11 10:35 p.m.
Streetwiseguy wrote: Clean sheet, using aftermarket stuff, I'd have no hesitation about going manual. Trying to cobble up something out of street parts might work, but I'd bet it will take a lot of experimentation to get right.

I'm not so sure. I bet with the proper "database" of what car had what diameter master cylinders and what car had what diameter calipers you could make something work without a ton of experimentation. Same math applies to everything else.

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