icaneat50eggs
icaneat50eggs HalfDork
10/3/13 4:05 p.m.

Project frankenfiat is having brake issues. The front brakes are nice slotted wilwood discs. Problem is there is only about 1/3 the weight of the car on them.

Back brakes are currently stock vw type 1 drums, with lots of weight.

I put in a proportioning valve, cranked it all the way down, and i get SOME braking out of the back now, whereas before the valve the front just locked up and slid.

The whole car with me in it weights 1650 lbs.

I'm trying to figure out exactly how much brake I need in the back. This winter I'm planning on upsizing from 13 to 15 inch rims, and most likely runs bfg rivals. There is a huge range in rear disc brake kits, from sub $300, to $1500. I'm stumped as far as finding out exactly how much brake I'll need in the rear to utilize all the grip of the new tires. Right now I'm VERY quick and competitive in every area except braking, which is just abysmal.

Is there anyway of figuring out how much brakes I'll need?

Dusterbd13
Dusterbd13 Dork
10/3/13 4:24 p.m.

theres such thing as too much brake.

just not enough tire.

honestly, though, id think that you could alter the brake bias further to hold iff the fronts until about half brake before they got any line pressure. that light, the stock vw rear drums should be fine. are they working properly? adjusted properly?

iceracer
iceracer UberDork
10/3/13 4:48 p.m.

Switch the brakes front to rear.

Keith Tanner
Keith Tanner MegaDork
10/3/13 5:30 p.m.

I'd put on rear discs with Wilwood calipers because you'll dump a huge amount of weight and you'll be able to swap piston sizes if needed. There's been some discussion of brake sizing here relatively recently, but overall I'd aim for too much rear brake when the prop valve is at max brake. Then you can balance it out.

BTW, with high-grip tires, you're going to find your ideal brake bias will move towards the front. You'll get more weight transfer. So your current rears may be acceptable then.

Kenny_McCormic
Kenny_McCormic SuperDork
10/3/13 6:03 p.m.

Trailing arms or swing axle? For some reason, I want to say there's a cheap way of going to rear disc on a trailing arm VW with junkyard front engine Porsche parts.

I also want to say sandrail guys figured out how to use larger front wheel cylinders in the back brakes to shift the bias rearward.

JThw8
JThw8 PowerDork
10/3/13 6:07 p.m.
Kenny_McCormic wrote: Trailing arms or swing axle? For some reason, I want to say there's a cheap way of going to rear disc on a trailing arm VW with junkyard front engine Porsche parts.

944 rear disks are a bolt on if you get all the bits from the 944. The only piece you'd need to buy/make is the adapter to convert the VW parking brake cable end to fit the porsche parking brake, its a pretty stupid simple little chunk of metal to join the 2 together.

AngryCorvair
AngryCorvair PowerDork
10/4/13 9:12 a.m.
icaneat50eggs wrote: Is there anyway of figuring out how much brakes I'll need?

yes there is. you need to know:
- CG height (reasonably close, within an inch or two)
- wheelbase
- number of master cylinders
- master cylinder piston diameter(s)
- front rotor diameter
- front caliper number of pistons and piston diameters
- front pad "advertised" friction level - corner weights as raced with driver plus ballast plus full fuel
- corner weights as raced with driver plus ballast plus empty fuel

from there we can walk through the math and come up with some combo of off-the-shelf parts that will provide you a well-balanced braking system.

icaneat50eggs
icaneat50eggs HalfDork
10/7/13 6:33 a.m.

okay, I'll get these numbers ASAP.

Flight Service
Flight Service MegaDork
10/7/13 7:10 a.m.

JoeyM
JoeyM Mod Squad
10/7/13 7:30 a.m.

watching this with interest

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