e30 soft brake pedal solved?

#61

I know I should let this thread die a peaceful death but I saw this and LOL’d.

[attachment=1945]engineer.JPG[/attachment]

#62

To throw another wrench in the works, your small M/C will exacerbate any flex in your lines/system, and create a softer pedal during your so-called ‘granite’ phase.

As an observer looking at the output of the M/C, with the pads seated on the rotors, there will be a stiffness associated with the lines that could be expressed as the pressure [psi] required to push a certain volume [in^3] into the system. This deltaP/deltaV has units of [psi/in^3] = [lb/in], and so is directly analogous to a linear spring. This value is also independent of your master cylinder.

The work (energy) you put into flexing the lines/system has to be generated at the pedal. Smaller M/C --> smaller forces --> larger deflections.

Alternatively (equivalently), you have to make up for the extra volume you put into the lines/system. Smaller M/C --> more piston travel --> longer pedal travel.

I would define the firmness of a pedal as the linear stiffness [lb/in] of the foot pedal itself under a certain range of deceleration, say 0.2-0.5g, that ensures we aren’t including slack takeup.

I am also willing to do your experiment (my girlfriend won’t complain too much), but I am tearing down my car right now. I will certainly make a point to eyeball it when I get the car together.

#63

Bearing in mind we Aussies are RHD, we have an actuator bar across the firewall from the pedal box (RHS) to the MC (LHS), with right angle cranks at either end… There is slop in the linkages, and there is lateral flex. One of our 4 cars is a series II. BMW introduced a compression(?) brace from the MC bracket to the top of the transmission tunnel. This car also has the least pedal travel. I reckon Ranger is onto something. I am a fan of a MC brace, and will also add some sort of brace from either the MC to trans tunnel or MC to the roll cage.

I have adjusted the height of my pedal down to provide better ergonomics relative to accelerator pedal, and then bent the stop tab with the brake light switch down to reduce the free travel.

#64

IMHO, if you are pushing the brake pedal hard enough to bend the double layer firewall, you should be into anti lock!!! Get your pedal as firm as you can and forget about it!!

#65

We are not allowed ABS…and it not taking much pressure to see movement…

#66

Wow, just wow.

I ended up fastening the MC in place by using a turnbuckle between the front of the MC and a convenient 6mm bolt that comes out of the frame rail below.

I’ve tinkered around with a lot of projects on the car over the years. Sometimes the idea flat-ass doesn’t work, but usually the idea works “ok”. It’s pretty darn rare that one of my ideas works out to be truly spectacular.

Today, after almost 4 months, the e28 DD came home from the paint shops. It needed to go into the garage so I could work on putting all the trim and the bumpers back on. That meant that the SpecE30 had to go out on to the street. This was the first time I’d driven the SpecE30 since screwing around with it’s brakes a month ago.

Truly spectacular. The use of a turnbuckle to better fasten the MC into place has turned my mushy e30 brakes into Porsche brakes. Under hard braking it’s like pressing your foot on a rock. I never expected so much difference.

#67

Good news Ranger! Time for me to look at a brace for the end of my MC, and all the bits in between.

#68

How about a picture?

#69

Going to be some days before any kind of pic. Being under the MC it’s a hard place to take a clear pic. Too much clutter in the foreground. I’m currently focused on the e28 DD. It’s likely to be a couple weeks before I touch the E30. Right now it’s out of the garage and on the street.

The top of turnbuckle is fastened to the front of the MC by a hose clamp. The turnbuckle then drops down and goes slightly backwards to a bolt coming out of the frame. It’s around 13" from MC front to the bolt. I was concerned both about tightening the turnbuckle too much, and also it coming loose due to vibration. Therefore I was cautious about tightening it and I used blue locktite.

Like pressing your foot on to a brick, baby.

#70

[quote=“Ranger” post=71670]
The top of turnbuckle is fastened to the front of the MC by a hose clamp. The turnbuckle then drops down and goes slightly backwards to a bolt coming out of the frame.[/quote]
The immediate symptom (soft pedal) has been addressed. I’d be concerned about the fact that by holding the far end of the MC in a static position, the force that used to move the whole assembly is now borne by the weakest section of the MC-booster assembly. Maybe thousands of stress cycles WON’T weaken whatever part of the MC that might be? Sure would suck if they did.

#71

[quote=“Steve D” post=71678][quote=“Ranger” post=71670]
The top of turnbuckle is fastened to the front of the MC by a hose clamp. The turnbuckle then drops down and goes slightly backwards to a bolt coming out of the frame.[/quote]
The immediate symptom (soft pedal) has been addressed. I’d be concerned about the fact that by holding the far end of the MC in a static position, the force that used to move the whole assembly is now borne by the weakest section of the MC-booster assembly. Maybe thousands of stress cycles WON’T weaken whatever part of the MC that might be? Sure would suck if they did.[/quote]
Dang Steve, you ever going to say something optimistic or congratulatory?

The firewall and booster were deforming under the load. By fastening the end of the MC turnbuckle reduces, but not entirely prevents, the undesirable movement of the MC by taking some of the load. That is to say, the structure of the system has been reinforced.

I don’t really understand your suggestion that “the force that used to move the whole assembly is now borne by the weakest section of the MC-booster assembly.” Can you say that a different way.

#72

[quote=“Ranger” post=71679][quote=“Steve D” post=71678][quote=“Ranger” post=71670]
The top of turnbuckle is fastened to the front of the MC by a hose clamp. The turnbuckle then drops down and goes slightly backwards to a bolt coming out of the frame.[/quote]
The immediate symptom (soft pedal) has been addressed. I’d be concerned about the fact that by holding the far end of the MC in a static position, the force that used to move the whole assembly is now borne by the weakest section of the MC-booster assembly. Maybe thousands of stress cycles WON’T weaken whatever part of the MC that might be? Sure would suck if they did.[/quote]
Dang Steve, you ever going to say something optimistic or congratulatory?[/quote]
As soon as you fully solve a problem that truly exists. :silly:

Think of the pedal-booster-master assembly as a right angle, as shown in the crude drawing below. Push on the pedal, the assembly pivots at the firewall, causing the MC to move up.

[attachment=1948]mcnormal.png[/attachment]

What I am saying is that by restricting the movement of the MC upwards (see turnbuckle below), the force that moved the end of the MC up is now transferred somewhere else in the booster-MC assembly. Is that force enough to weaken that assembly over time? Dunno. Was it designed to be held rigid on the front end of the MC? Nope.

[attachment=1949]mcturnbuckle.png[/attachment]

Might it fail? Dunno. Is it introducing stresses that it wasn’t designed for? Yup.

#73

That’s not how force works but you’ve got good intuition.

Consider the dissimilar fastener. There is a basic problem whenever you use a fastener that has different properties then what ever it’s fastening. For example, using a steel bolt to fasten 2 pieces of AL.

A Stress vs. Strain graph shows how a material deforms due to stress. Think of Stress as just Force.

AL, because it is softer, shows more strain (deformation) than Steel. In the case of 2 sheets of AL fastened together by a steel bolt, the fact that the steel bolt isn’t deforming as much as the AL under load, means that a stress concentration occurs around the bolt that could result in failure. Under the right circumstances the stronger steel bolt will result in failure before a weaker AL bolt.

By fastening one thing more rigid you can potentially cause a stress concentration. But that’s as close as your “force transferred” example gets. This isn’t a case of using a dissimilar fastener.

Making something stronger does not, under normal conditions, transfer force and make something else fail. Stronger is stronger. Deformation can lead to work hardening and ultimately tearing. Deformation is bad. Making something stronger so it doesn’t deform is good.

Just you wait until you give my car a test drive around the paddock. You’re going to love it.

#74

[quote=“Ranger” post=71686]By fastening one thing more rigid you can potentially cause a stress concentration. But that’s as close as your “force transferred” example gets.[/quote]That “stress concentration” is exactly my point.

I’m not sure what the talk about fasteners was all about.

The weakness you are trying to fix is that the firewall (the pivot point in my See Spot Run drawing) flexes. It will continue to flex even if you secure the far end of the MC. Securing the far end of the MC will not prevent “stress concentration” - probably where the MC bolts to the MC? Anywho, your “fix” is not legal, is unnecessary, and may - however unlikely - cause failure in another part of the system.

That failure would be a much worse situation than a consistent, slightly soft brake pedal.

But yes, I will drive the sh!t out of your car in the paddock just for fun. It will be good to be able to trigger ABS 0.01 second faster. :evil:

#75

The talk about fasteners was an attempt to be nice. When nice mechanical engineers are discussing forces with finance types we be nice by telling them that altho they are entirely wrong, it was a good thought.

Your “may…cause failure” idea is baseless. In fact, by relieving stress on the booster shell, and to a lesser extent the firewall, it reduces the likelihood of failure by reducing stress.

Re. legal. Look up the rule on fasteners. In any event, I’ll write up a rule change this Fall so there’s no ambiguity. Once other folks try my car, everyone else will be doing it too.

#76

[quote=“Ranger” post=71688]Your “may…cause failure” idea is baseless. In fact, by relieving stress on the booster shell, and to a lesser extent the firewall, it reduces the likelihood of failure by reducing stress.
[/quote]
Here’s my little experiment. Pretend a fishing rod is the booster-master cylinder assembly. Hold it horizontal. Next, apply the same rotational force that the pedal would (i.e. rotate the rod 5 degrees above horizontal). That’s the movement you’re trying to restrict to achieve hard pedal feel, right?

Next, do the same thing with someone holding the previously free end of the rod. Does that end move? Nope. The force is transferred elsewhere in the rod, causing it to bend. The force does not disappear.

But you are right, I have no formal training in mechanical engineering. Which by now is the most obvious statement ever made on the interwebs.

#77

[quote=“Steve D” post=71690][quote=“Ranger” post=71688]Your “may…cause failure” idea is baseless. In fact, by relieving stress on the booster shell, and to a lesser extent the firewall, it reduces the likelihood of failure by reducing stress.
[/quote]
Here’s my little experiment. Pretend a fishing rod is the booster-master cylinder assembly. Hold it horizontal. Next, apply the same rotational force that the pedal would (i.e. rotate the rod 5 degrees above horizontal). That’s the movement you’re trying to restrict to achieve hard pedal feel, right?

Next, do the same thing with someone holding the previously free end of the rod. Does that end move? Nope. The force is transferred elsewhere in the rod, causing it to bend. The force does not disappear.

But you are right, I have no formal training in mechanical engineering. Which by now is the most obvious statement ever made on the interwebs.[/quote]

I don’t think a fishing rod is a fair comparison. I also don’t think Scott’s mod is going to cause premature failures… The idea of disappearing forces was worth a good chuckle!

#78

Generally anytime you reinforce something everything around it will break immediately and you need to reinforce everything. However, there is no added stress on the mc and pedal assembly just from reinforcing the fw.

Either way you need to apply the same amount of force to the pedal. If you think of it as a series of springs the pedal to piston is one spring and the firewall is another. The object is to apply sufficient force to the mc piston so all you are doing is decreasing the amount that the firewall deforms before pushing with sufficient force back.

I’m still not seeing where this turnbuckle goes to on the mc but if its pulling down on the nose of the mc it is putting stress on the mc that it wasn’t designed for. If you just reinforced the fw around the original mc mount by welding some extra sheet in you wouldn’t need to worry at all.

#79

[quote=“Steve D” post=71690][quote=“Ranger” post=71688]Your “may…cause failure” idea is baseless. In fact, by relieving stress on the booster shell, and to a lesser extent the firewall, it reduces the likelihood of failure by reducing stress.
[/quote]
Here’s my little experiment. Pretend a fishing rod is the booster-master cylinder assembly. Hold it horizontal. Next, apply the same rotational force that the pedal would (i.e. rotate the rod 5 degrees above horizontal). That’s the movement you’re trying to restrict to achieve hard pedal feel, right?

Next, do the same thing with someone holding the previously free end of the rod. Does that end move? Nope. The force is transferred elsewhere in the rod, causing it to bend. The force does not disappear.
[/quote]
Ah, I see where this is going wrong.

The fishing pole in the example is directed to rotate. Since the end is fixed, it can’t rotate so it bends. That’s fair.

The weakness of the example is that it is one of restrained rotation, but in the brake system it’s only the brake pedal that rotates. The shaft that goes to the booster is pinned to the brake pedal. The motion of that shaft, relative to the firewall, is purely horizontal.

The brake pedal arm has no idea that the end of the MC is restrained. All it knows is that a foot is pushing on it and it’s pushing the horiz booster shaft. That’s where the fishing pole example breaks down.

Then booster and firewall flex, neither of which is good for their health. The motion of the MC is forward and up probably because the firewall is stiffened by the cowl. Since the booster is pretty symmetric, one would expect it’s distortion to be purely horizontal.

Re. tightening the turnbuckle too much. The weakpoint isn’t the MC which is ferociously strong, but the booster. There’s no perfect answer to this. I tried to tighten the turnbuckle enough that there was barely perceptible deflection of the nose of the MC. So the booster is being stressed by the turnbuckle far less then it was being stressed by normal braking action.

It is pulling down on the nose of the MC. I used a hoseclamp to fasten the turnbuckle to the MC’s nose. The nose of the MC is the location of most movement because of the rotation up so placing the fastener was the most efficient way of doing it.

#80

While I can see pedal pressure moving the booster/MC around, The majority of the force on the master cylinder is produced by the booster. So preventing the motion of the assembly might give a firmer pedal, but I doubt it has any affect on braking force.