First off: Bolts/studs can handle large amounts of tension loads, that’s why two are fine for the master cylinder. The reason for four is, as already said, to withstand the leverage of the bounce loads imposed on the firewall by the booster + master cylinder that are now quite long (long lever arm). Using just two bolts would result in the fire wall flexing in and out, eventually bending or failing.
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The suspension is designed to damp both high (wheel frequency 10hz and above) plus low frequency (body motions 2 hz and below) effectively. The wheels must be damped so they stay on the ground and the body must be damped so you don’t toss your lunch. Both are important to ride… which as nothing to do with brake booster mounting.
The car is designed for the highest likely impacts as well as the stiffness of the the mounting. Crashing the bump stops multiple times should NOT cause your master or booster to fatigue off. Design impact loads vary between 5 and 10 Gs depending on how much upward (bump or compression) you have. Low slung sports cars have only about 3 inches to the stops so receive about a 10 G hit. normal cars about 4 in. so a 5 G hit. Normal rough-road G levels rarely exceed 1 G without hitting the stops but all add to reduce the fatigue life of the steel firewall.
If we hang that 10 lb master (old school iron!) about 10 inches further out and a 10 lb, 5 inch thick single booster about 4 inches out from the firewall, the bending moment at the firewall increases significantly.
Figure center of mass of a 10 lb, 5 inch long master 10 inches out with a 5 G hit applies a 50 lb-ft bending moment (W plus the 5G added). The booster at 6.5 inches adds 32.5 lb-ft so a total of 83.5 lb-ft compared to a master at 2.5 inches center of mass at 12.5 lb-ft. So 6.8 times more bending to a rather thin piece of sheetmetal. At 6 times the bending loads, it seems like those 2 extra studs spaced apart from the master’s mountings would be appreciated seeing as how we don’t want bent or broken firewalls.
That 50 lbs you think is being applied to the firewall is actually more like a 6 times that number from the brake pedal ratio of 6:1 so about 300 lbs. The bending torque to the firewall from that force is zero since the force is applied directly to the master or master and booster assembly with no offset. The bracket that supports the pedal assembly is mounted with those 2 studs normally used for the master cylinder alone so that no bending moment is applied to the firewall. All moments are contained in the pedal mounting bracket.
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There’s book smarts and then there’s street smarts. Shame on the professors. Could I get my ten minutes back?
The two I have recently dealt with are a 2004 Honda CR-V, and a 1988 Ford Ranger, and neither had a 10 LB booster or master, but I admit, I didn’t weigh them; I take that back, I just did the old ones. But nice analysis.
'88 Ranger
brake booster 9 lb (includes box) 5" depth
master 1.64 lb
'04 CR-V 6.4 lb 4" depth
master don’t have old one
scale: luggage type, accurate to +/- 0.01 lbs.
FYI: The brake ratio pedal pivot - pedal TO pedal pivot - brake rod is only about 4:1 in my '88 Ranger, not 6:1. So yes, the force on the firewall maybe ~ 200 lbs.
Does anyone know how much the brake booster amplifies the pedal force?
So it seems you’re a little loose with the numbers.
OK dokey, now we’re down to nit picking, not ‘oh, ok, I get it.’
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The brake booster is stamped steel, not very thick, with only two fasteners there would be too much flexing.
The master cylinder is cast aluminum or cast iron, much thicker. The housing is not going to flex from the force of the pushrod.
Some master cylinders are mounted with 4 fasteners.
NONE of which was in evidence… Sooo this is back-pedaling B.S. isn’t it?
Pivot ratios vary, booster sizes vary, masters are mostly aluminum these days. Some masters have 4 bolts as @Nevada_545 shows.
The booster is powered by vacuum (unless it isn’t). It is a function of diameter and the vacuum from the engine at idle. A 7 inch booster has roughly 11 square inches of actuation area (the 7 is not exact… just a reference) so figure the gain based on a value of vacuum somewhat less than idle readings. The booster could be a double… double 7s, 5s, or a 7/5 or any other assist size required to stop the car.
Master cylinder bore is chosen based on piston sizes, required brake torque, effective radius, coefficient of friction of the pads/shoes… Pedal ratio depends on all the stuff downstream. We all like short pedal travel, firm pedal feel, and low effort to stop our cars and trucks.
nope, in OP I mentioned last few cars I dealt with. I just specified what they were. Not back pedaling. Those are just examples. I was not trying to be general.
The mood and aggressiveness of this site is not for me. I am out of here. Good riddance!
When one walks into a room full of strangers, proclaims oneself the ‘smartest guy in the room’, and doesn’t make much of an effort to understand the information provided, this is what happens.
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Heh heh. The last booster I replaced was about 1969. I have no idea how many bolts or studs there were holding it on or the master. I just unbolted the stuff and bolted on the replacement. Everything was fine. What else is to know or care to know?
Well that was fun and interesting… I learned a lot, thanks to the engineering minds on here, but I will probably forget most of it cause I am not going to try to redesign the wheel as a simple humble grease monkey…
I looked back… no evidence of any cars mentioned by the OP anywhere in this thread.
@texases has a bang-on assesment!
We were just too “aggressively” correct for the OP! 