Discs - Why in front?

[RanX0R0X]

The tranny is just being completed by David Crow here in Austin, the short axles are on and the engine is completely rebuilt. By the end of the week I should be motivating.

One thing I've never quite understood is why the '68 has the disc brakes up front and the drums in back. This was obviously before anti-lock brakes and when they lock you are stuck with no ability to turn. With the wide axles, narrow tires and the disc brakes up front I had a heck of a time with understeer.

Wouldn't it have made more sense to have the drums in front where complete lock up wouldn't be an issue?

[donmurray]

I would guess that even with a rear engine, under hard braking the weight transfer makes the front heavier than the rear. So, the front needs more brake power to balance the system.

[70coupyel]

Disc brake have more surface area than drums so it has more grip and cool faster.
Brakes do not cause under steer. Some reading for a balanced car.
http://www.aircooled.net/vw-handling-suspension-tuning/
The thread Here is one of the best about type I set ups.
What tire pressure are you running in the front tires? Are your rear brakes adjusted properly? A properly tuned stock brake system has enough power to control the weight of a Ghia.
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[NOVA Airhead]

I would think cost would be a factor as well. Cheaper to put them up front than rear.
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1971 Ghia Convertible Autostick
1974 Thing

[Derek Cobb]

It's not the brakes that are the problem, it's the narrow tires. Smart Car tires would go a long way towards increasing your brake performance and the under-steer without rubbing.

[mandraks]

the real concern is not locking up the front wheels, that just means you can't steer.... if your rear wheels lock up BEFORE the front wheels lock up, then your rear wheels will immediately overtake the front wheels, in short: you will spin out.

ever watch a NASCAR race? watch them try to make it on pit road with too much rear break bias

all the other stuff about weight transfer and stuff is also important, of course

[mlhsquared]

[retrowagen]

The front brakes on a car do about 80% of the work, so you want the better brakes up front.

Also, back when these were new, it wasn't particularly easy to rig up a parking brake system with rear discs.
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Dave Ruby

[RanX0R0X]

You're right about brakes not causing understeer, per se, I just didn't have a better word for when you're going around a turn, put on the brakes, turn the wheel and have your car continue to slide directly forward because the wheels are locked. Obviously pumping the brakes works for that purpose as we did before the anti-lock braking systems of today started automatically doing it for us.

[RanX0R0X]

Spin out but not called oversteer in that case? Around '68 is when Nader had the car world in a tizzy about the Corvair and the swing axles jacking, which, in turn I believe, cause the car to go ass end forward. Lift oversteer was a problem.

I'd wondered if that might have something to do with the discs being up front. Obviously the wide axles on the '68 go along way toward increasing the fulcrum point to prevent the jacking effect and wheel tuck.

[RanX0R0X]

That makes a heck of a lot sense.

[RanX0R0X]

The new tranny is switching to short axles with wider wheels and tires. I'm changing from 165/80R15 to 195/65R15 Potenza RE960AS Pole Position. Not just wider tires, better quality tires in general.

I have a camber compensator to be installed when things are back together.

[retrowagen]

Also, you might want to eliminate some variables in the equation by making sure your rear brakes have been properly bled, that the lines are in good condition (can't remember when the soft lines were last replaced? Then install new ones NOW!)... Put the car on jackstands, spin the rear wheel, and have a helper stomp on the brake pedal...

Have you adjusted the rear brakes per factory spec and procedure?

Cars also have a wonderful device called a brake bias valve (a.k.a. proportioning valve), which, properly working, keeps one side from locking independently from the other (given all other factors are working and in proper adjustment). Many racing cars have bias valves adjustable from the driver's seat, to make slight changes to the braking behavior during a race!

Also, I'd advise you against putting tires that are too grippy (i.e., the Pole Positions) on an old KG. It'll cause undue wear on the ball joints, ride really harshly, and grip like mad until they break away at a disasterously high speed, with no warning. I like a tire that gives me a little warning.
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Dave Ruby

[RanX0R0X]

Wow! Great advice. I really appreciate it.

[RanX0R0X]

[nikita]

If the front brakes are locking up regularly, there is some probem with the system on the car. Master cylinder, caliper, and wheel cylinders are all sized by VW to provide the right proportion of braking power front/rear. If the rear brakes are not right, that could be the problem. However, early master cylinders were DIFFERENT for disc front than all-drum cars. Replacement master cylinders sold by vendors today dont seem to note the difference. The Bentley manual has the details. In addition to the '69 Ghia we have now, I once had a '67 German model Deluxe beetle. It had disc fronts with a single piston master that was specially marked. I never had a front wheel lockup issue with either car.

[RanX0R0X]

Nikita,

My choice of words was poor I think. I don't mean that they lock up and won't let go even when I take my foot off the grake. I mean that on hard braking one's turning the wheel results in no turning whatsoever. The front brakes don't give. The skinny tires don't help the matter. Since the brakes don't slip at all, one manually pumps the brakes to get the turning.

As I'm sure you're aware, if you hit the brakes hard on a modern car they do the pumping for you.

Wide axles, discs up front, and skinny tires do combine in such away as to make turning under braking conditions harder.

[analengineguy]

To me, its pretty simple. Disc brakes fit the nature of the front wheels better than the back. Disc brakes, due to the physics of there design stop "straighter" with the grip of the caliper. Drum brakes are a bit less sure, since they push out against the drum. Since the discs hang out in the air, if you will, they cool faster...fighting fade (In the '50s and '60s they were often called "anti-fade" or "fade-free" brakes). With the dynamic geometry of a front end, discs are the obvious choice since control under braking is maintained better. The rear end is "fixed" with fewer variables and no steering, so drums are up to the task there.

Wider tires will help with stopping. They have a lot of drag due to higher rolling resistance, which is also why they corner better. A fatter tire has it drawbacks, though. That drag cuts MPG and also creates the "snowshoe effect". The wide tire is a bit floatier with less bite in snow and rain, so hydroplaning, etc. is more likely. I recently put a set of Vredensteins radials on with the stock size for my '64 of 155-15. They're narrower than the the Nankang/Tiger 165-80s they replaced (damn skinny, in fact!) but being a better design and quality they perform better. Anyway the '64 is 40 HP stock, so with it topping out at 75, there's less to be concerned about.

[Bruce]

[analengineguy]