Brake upgrade w/power delete

This has been a great thread. The master cylinder in the e30 is going out. I want to replace it with something much smaller to accommodate any swap I do later down the road.

Taiden, completely off-topic, but you did get my reply about Formula SAE, right?

You might want to have a look at your brake pedal.

In my T/A, it has both holes in the brake pedal.

Power brakes were optional on the early cars, the pedal never changed.

If you remove the booster and install the manual master cylinder, you simply move the rod to the upper hole.

Shawn

I dug out my copy of the June 2008 GRM with the brake article, and in the sidebar discussing hydroboost it lists one 'con' as having totally different pedal feel to vacuum boost.

Anybody have an impression of how the feel differs from vacuum boost?

ransom wrote: I dug out my copy of the June 2008 GRM with the brake article, and in the sidebar discussing hydroboost it lists one 'con' as having totally different pedal feel to vacuum boost. Anybody have an impression of *how* the feel differs from vacuum boost?

i love you man! (fwiw, i wrote that article!)

my experience is with diesel pickups and with the mustang cobra hydroboost from a few years ago. to me, the difference in feel is that the HB always felt over-boosted at low effort, so the overall decel vs pedal force didn't feel linear. i always got used to it quickly, but i was also always glad to get back to a vac booster.

HB makes sense in some circumstances though: low (or no) manifold vacuum, no packaging space for a vacuum canister, no "natural" vacuum source (diesel).

In reply to AngryCorvair:

Thanks for writing that article! (and to, IIRC, Curmudgeon for telling me to look it up. EDIT: Haha! Nope, it was you, so thanks on both counts!)

For my application, the compelling 'pro' is packaging, though I don't have (nor want) power steering, so something like the self-contained, electric-powered Saab (eek) unit would be neat.

I hate over-eager initial bite, though, especially on a fun, street-driven car where i'm likely to be heel-toeing but not actually braking that hard.

I suppose the nonlinearity is intrinsic. I suppose the best answer for overboosted brakes is enlarging the master cylinder bore to raise effort and shorten throw... Though I wonder whether if you were otherwise happy with the system whether you could restrict the hydraulic boost or change its assistance proportion. Don't they do that with power steering systems by changing orifices or the compliance of an element between input torque and hydraulic metering?

ransom wrote: Though I wonder whether if you were otherwise happy with the system whether you could restrict the hydraulic boost or change its assistance proportion. Don't they do that with power steering systems by changing orifices or the compliance of an element between input torque and hydraulic metering?

I have no idea how those components are tuned. :-(

Let's say I wanted to try manual brakes on my e30. Could I just cap the booster line and reposition the clevis on the pedal arm? Or does the booster being there (but disabled) introduce stiffness that would otherwise not be there.

The point being that I'd like to try manual brakes without tearing the whole thing out.

I doubt you'd notice very much resistance from the booster.

There will be some, but it will be minimal.

Shawn

Trans_Maro wrote: I doubt you'd notice very much resistance from the booster. There will be some, but it will be minimal. Shawn

that's correct. some inefficiency in force transmission (maybe lose a couple percent) plus some additional return spring force to overcome, but nothing insurmountable. keep in mind that pedal force required with no vacuum will be 5x to 7x the boosted pedal force to achieve the same decel.

mguar wrote: The idea of needing power brakes to stop is silly.. I'm an old fart with weak old legs who by simple hydraulic tricks can get fast powerful cars to stop hard enough so it feels like my eyeballs will pop out of my head.. As for non-linearity in stopping.. you can add or remove that by simple bellcranks.

Need? Sure, but I'm building a car I really want to enjoy, so it's worth the effort to make it respond like I want. I'm a 190-pound cyclist, so I'm not too concerned with my ability to generate sufficient pedal pressure I'm trying to build something that can be daily driven without having to mash on the pedal, but also not wanting to jerk the car around if I'm heel-toeing into a 25-mph corner on a city street where I'm only shedding 10 mph and applying just a little braking... Touchy brakes make it hard to do that. My 2002's actually got good feel for that sort of thing stone stock, but once I've got a big set of R-comps on it, it can't even lock them up when it's time for the Big Whoa...

I'm a bit concerned about introducing a bellcrank to mellow initial bite while ramping up with travel to maintain a linear feel (what feels linear may not actually be, I suppose), because one of the goals is minimal travel after initial application; no travel, no bellcrank leverage change...

While it comes with a rotating/unsprung mass penalty, seems like rotor diameter relative to tire diameter (increasing braking torque relative to inertial torque) is just about the only thing that increases braking power without a user-interface downside (that is, unlike increasing hydraulic advantage, it doesn't affect pedal travel).

AngryCorvair wrote:

Trans_Maro wrote: I doubt you'd notice very much resistance from the booster. There will be some, but it will be minimal. Shawn

that's correct. some inefficiency in force transmission (maybe lose a couple percent) plus some additional return spring force to overcome, but nothing insurmountable. keep in mind that pedal force required with no vacuum will be 5x to 7x the boosted pedal force to achieve the same decel.

The only possible problem I can see: I will use the Jensenator as an example. The stock pedal ratio was 4:5-1, to achieve the 5:8-1 I had to move the hole up the pedal by, IIRC, .780 inch. If the pushrod is short enough, weird things will happen which could possibly include bending the pushrod (meaning no brakes!) or damaging the booster. In my case, the angle of the pushrod entering the M/C was worrisome enough that I made an adapter plate out of 3/4" aluminum plate with the M/C mount holes redrilled to move it up the same amount as the hole moved. That way the pushrod entered the M/C at no angle.

In reply to Curmudgeon:

in my reply i was considering only removing the vacuum source, no change to the pedal.

you are correct about pushrod angle. in the OE world the typical guideline i've heard is max plus or minus 3 degrees off of the MC bore axis, to avoid premature wear of the piston seals.

In reply to Curmudgeon:

I'm slightly confused (or at least want to verify that I'm not more confused than I think I am) about your notation...

I'm familiar with a colon as an indicator of ratio. E.g. 3:1 for "three to one".

Am I correct that the "-1" is for your M/C bore (or maybe unit M/C bore, whatever bore that may actually be), and that the 4:5 and 5:8 are "pivot-to-pushrod:pivot-to-pedal" ratio? (Which would also be the pedal-force:pushrod-force ratio, correct?)

Or am I totally out in left field?

In reply to ransom:

i think Curmudgeon took the maths in SC public schools, lol. i'm pretty sure you're reading too much into what he wrote, and that he really meant 4.5:1 and 5.8:1 as his two pedal ratios.

Are we bashing on SC public schools in this thread? I am SO excited for this.

No floundering this thread! I'm getting too much good info!