I'm getting ready to start my "hot rod" build.....basically I'm putting a big motor (a 4cyl turbo, not a V-8!) in a tiny little car. Said car currently has 8.4" discs in front and 7" drums in back as it's front wheel drive. Hot rod will have discs all around and the engine will be moved to the back. It will be very lightweight, probably around 1600lbs all in and run 13" wheels.
So.........I have several possible brake scenarios I can use utilizing parts that will fit inside the car's 13" wheels and bolt right up to the hubs. Car is intended to be a street car with a very occasional track day....but mostly a spirited, canyon carver, fun type build
Questions:
1) Any disadvantage to using the same sized discs front and rear?
2) Any advantage/disadvantage to using 4 piston calipers on one end and 2 piston on the other - car currently has 2 piston calipers on the front, those could be moved to the rear and new 4 pistons put on front. Or I can buy two sets of 4 pistons, or buy another set of two pistons and use those all around.
3) Bias - with the engine weight moved to the rear, will the front end still do the majority of the braking effort? Or will it balance out some? Does a 911 still have larger discs in the front? The overall balance should be pretty close to 50/50 as the new engine/gearbox is considerably lighter than the old front drive combo, and I'll be moving radiator, fuel tank and battery to the front.
4) the car currently has a single brake master with a booster - it would be easy to add another master and split the system front/rear, or I could continue to use the current system (split master) and add a manual brake balance valve.
Thoughts and experiences?
PS. OK, the car is a classic Mini...... :-) The engine is a Ford Ecoboost from a Focus ST with a 6 speed gearbox.
You've got a short wheelbase and a high-ish CG relative to wheelbase. So, even though you're tail-heavy at curb, you're probably going to be nose-heavy at max decel. I would run same brakes at all four corners, plumb the hydraulics front/rear split, with an adjustable prop in the rear circuit.
that's what I'm doing on MonZora.
1) not really, as rotor diameter is only part of brake torque calculations. Some Miatas actually have larger rotors on the rear than the front.
2) piston count is not important. Piston area is. Get the diameter of those pistons and then plug them into later calculations.
3) you'll still get forward weight transfer which will lead to the front doing more braking than the static weight distribution would suggest, but since you're starting with rear weight bias the rear brakes will do more work than on a normal car. This is one of the reasons 911s brake well, they can use their rear brakes more than, say, a classic Mini. For your project, you'll probably be able to work those rear brakes quite hard.
4) Dual masters and balance bars give you a fixed front/rear pressure distribution, which means it can be dialed in to be perfect at max decel but will not be perfect any other time. If this is a street car, better to go with a proportioning valve. You can do this with a single master, but ideally it'll be a dual circuit master - like basically every production car for the past 50+ years. If it's got outlets at two different points along the length of the master, that's what you're looking for.
If you can provide wheelbase, static weight distribution and CG height, I'll bet someone like Angry could calculate a good starting point for you. But I'd probably start the same place as him - make it symmetrical and adjustable, then work from there.
Srsly, 45 minutes from thread start to matching answers from independent industry experts. Yay GRM.
thank you all, as this build progresses I'm sure I'll have lots more questions.
One thing I'll need to know is whether I need a different master cylinder, since the area of the pistons will be larger with even two pistons per caliper compared to the calipers/drum set up.
In reply to MiniDave :
That's a good question, and the answer is "maybe".
Along the lines of what Keith said, take inspiration from cars with similar stock layouts. Generally 911s have similar size rotors front to back but bigger pads and more piston area up front. My cayman has slightly smaller rear rotors and much smaller rear pads, and even with that the front brakes generate much more heat which means they're doing much more work. There's very few vehicles in existence that aren't front heavy at max decel. I'd guess same size rotors front to back is fine, but you're going to want more front pad and caliper plus some sort of bias adjustment ability. If you do some research I believe it's not overly difficult to do some basic math to determine approximate master cylinder sizing, but I've never done it.
An amazing number of cars have larger diameter rear discs, generally the ones with a drum style handbrake. They have less swept area (not needed) and the piston ratios front to back are more critical than anything else.
That was the direction I was heading too......the 4 piston calipers have a much larger pad than the 2 piston, so I was planning to go with those on front and 2 piston on the rear, with the same size rotors. These 8.4" rotors are really inexpensive to buy tho it does cost to have them shipped (from England).
That's also why I was thinking of adding a master cylinder - the current one is plenty for the fronts and a standard one would work just fine for the rears, but in this scenario only the fronts would be boosted. Dunno, still sorting this one out. The easy button would be to just use the single boosted master if it's enough to do the job.
Now all I need to do is sort that out and I'll be good on brakes. This is all preliminary planning, but I'll probably order the brake kits this week as they're finally in stock again.
If you're worried about your master not having enough volume to feed whatever you have planned for caliper pistons, you can always mess with the pedal ratio. You'll definitely want to know what the piston sizes of those calipers are. I am not getting happy feels about the concept of one boosted master and one unboosted one, never mind what the actual mechanism might look like.
Based on the 8.4" rotors, you're using the Mini brakes with 12" or 13" wheels? You might be able to use them with a Wilwood Powerlite, they've got a fairly small minimum rotor diameter.
Yes, 13" wheels.....
Yeah, the extra master was just one of those wild weird ideas.....this version of Minis when LHD had a crossover linkage to keep the master and booster in the same location as on RHD cars (the booster wouldn't fit otherwise) so my idea was to put a master in the original LHD location in addition to the existing master/booster. That way the rears would not be boosted like the fronts, limiting the amount of pressure to them. Controling it would be one problem, the second would be the feel in the pedal......among other issues.
I think I'll just stick with the std setup.
I have both calipers on hand so I'll go measure the pistons a bit later and report back.
The master cylinder has a stepped bore giving two different sized bores for the front and rear
A stepped master makes things more interesting, that's for sure. You'll want to know what those diameters are.
Or just bolt it together and find out :)
I do tech inspections on builds like this regularly here in Sweden and I have also done a couple of builds close to what you're aiming for.
My experience basically mimics what has been mentioned above. My general recommendation for a mid-engine build is same size brake discs (rotors) front and back. Usually also a similar swept area but a high CoG car, as has been mentioned, can usually use less rear brakes.
What I have tried several times that "never" works is to use rear calipers from a normal front-heavy car. It is way to common to have standard VW (Golf, Passat...) rear calipers due to the mechanical parking brake - they are usually too small.
I would also, 1000% say you need a dual circuit system to be reasonably safe. A tandem master cylinder may be easier to fit but a bias box usually makes it easier to find a working front/rear balance. Care has to be taken to adjust it so you don't gear rear lockup in really heavy braking from high speeds (so combining with a deceleration sensitive proportioning valve can be wise).
Gustaf
The 4 piston calipers have 1 3/4" pistons, the two piston calipers have 2" pistons, so 4 X1 3/4 = 7" vs 4", although the brake pads don't look that much bigger. I can buy the 4 pistons with vented rotors too, although I just don't know that I'll be pushing the car that hard to need them over std rotors and say green stuff pads.
I may not have made it clear, but the standard brake system on this 94 model car is a dual system with a proportioning valve to the rear brakes, and only the fronts are boosted.
Edit: This is not the way this car is built, my bad.
That's I'm sure because a Mini is already very light in the back and it's very easy to lock up the rears. My situation will be a little different since I'll have the motor and gearbox in the back.
MiniSpares says the bore size on the master with the booster is not stepped, and is 20.6mm or about .811 "
Assuming they are both fixed calipers and not sliders, the 4 piston calipers have 4.8 square inches of area per side and the 2 piston have 3.14 square inches per side.
If the two piston are sliders, then it works out to 4.8 for the fours vs. 7.3 for the twos.
MiniDave said:
.., but the standard brake system on this 94 model car is a dual system with a proportioning valve to the rear brakes, and only the fronts are boosted...
Fascinating. How is that done?
All the calipers are fixed, not sliders......the pistons are on both sides of the disc.
I was wrong on the boosted comment I made before - my previous car had a separate booster fed from the main master cylinder. The way it worked is the main M/C fed the boosted M/C and the rear brakes, the boosted M/C fed the front discs only. Sounds weird but it worked well. E-Type Jags worked this way too.
This car has a single M/C mounted to the booster feeding both front discs and rear drums with a proportioning valve at the rear. My plan is to go with the 4 piston fronts and 2 piston rear calipers.
