Jim bailey - 928 International

Yes Adam ideal weight distribution is NOT 50/50 for braking or acceleration !

dr bob

Just to make sure that there's clarity on a few issues discussed here--

The clamping force available in a caliper is DIRECTLY related to the total area of the piston face. Bigger pistons, more pistons, don't care. With any give MC diameter, total area under pressure is what counts.

"Anti-dive geometry" plays a big part in the way the front of the car dips under braking. Correctly designed, front suspension will wind up at about the same rate that weight transfers on to it during braking. Weight transfer is very important obviously, but the relationships among the control arm pickup points determine how much that weight transfer actually compresses the front suspension.


There's a reason the early 930 cars were flaming quick to 100 and back to 0. With all the weight saddled over the rear wheels during both acceleration and braking, there was plenty of loading on the drive wheels, and planty of braking available from those same wheels. It only got exciting when lateral forces were added. Rockets in a straight line though. 50-50 Is great for steady-speed cornering. Any other time I think a bit of rear weight bias is desirable, up to the traction limits anyway.

Jim bailey - 928 International

There is probably a reason why F-1 Cars are about 60/40 with the most weight on the rear wheels You can also get into discussions of anti-squat built into the rear suspension and how that gets screwed up when you lower a car with trailing arms ...My old 911 race car was about 70/30 before I got inside then it evened out a bit !

mark kibort

This almost gets into a phlysophical topic. is weight transfer good or bad. generally, a car with most of the weight up front, will brake better, and in the rear will acceleratate with more grip. too much weight up front, and the rear end will get light under braking, and burn rubber under acceleration. too much in the rear and it will be pushy under acceleration, especially on turn exits, and slidy under braking, where rear brake bias becomes critical.

lots of factors. If i remember correctly, i think in Tune to Win, there is a couple of chapters dedicated to handling issues and weight distribution.

generally, 50 50 weight disty, is good for race cars as it helps braking and accleration as well as cornering. 911s with more rear weight or vets with more nose weight can have advantages in certain areas of a track, doing different things (i.e. accel, decel, trail brake, straight brake, power exit, etc)

Mk

ROG100

Mark,
The rear does not require any modification apart from the machining of the caliper for the S4.
My assumption is the S4 rear is the same as the GTS.
Calipers are $525 each plus machining.
Rotors are cast hole and $125 each.
Jim is still working on the dust shields but if the 993 ones work they are less than $25 each if you need them.
Plus pads and sensors.
Roger

Jim M.

Mark, as Roger has stated above, the S4 and GTS rear brakes are the same. My pictures are of the rear wheel in a before and after shot.

Jim Mayzurk
93 GTS 5-spd

FlyingDog

Why is machining necessary?Are they spaced out too far radially?

mark kibort

oh i see, i thought you were showing front and rear. thats rear before (still red) and with 993TT new REARs (bigger red) right??

So, those reds look like the same size as the front S4 calipers. are they the same? maybe not due to the offsets and things. those rotors look like S4 as well (fronts)

what did you do up front?

Mk

Dennis K

Are you sure this is how it works? I agree w/ the first two sentences but not the third. For a given deceleration, there is going to be a corresponding amount of weight transferred. It's not dependent on whether the braking force comes from the front or rear. There's a moment arm from the plane of the contact patches to the center of gravity & that determines the weight transfer.

Ispeed

On a motorcycle, grabbing front or rear brake is dramatically different in terms of nose dive.
Front brake=max dive
rear brake= even squat.
That's the motorcycle world, where you grab each brake separately...

FlyingDog

Jim's second a third sentences contradict each other and neither is necessarily true. There are cars with equal calipers and rotors (usually just diameter, not thickness) front and rear. Rear braking is then limited by the proportioning (correction) valve. Rear brakes are usually smaller just because they don't need to be as capable. Weight transfers forward and manufacturers want the fronts to lock first, so there is no point in having extra rotating mas and large expensive components in the rear.

dr bob

What Matt said... Except the part about the bias valve. You are thinking about a proportioning valve I suspect. The bias valve is not a linear device. It allows full force up to it's limit/rating, beyond which a reducing percentage of input is passed through.

----

The rear brakes on a scooter are perhaps a perfect example of the way the geometry helps, where the weight is transfered off the rear suspenion at the same time the torque reaction from the brakes is transfered to the swing arm, causing the rear to want to squat a little. Moving the torque arm from the swing arm to the frame reduces this a lot, and is desirable in some race situations when you really want the rear to stay planted.

IcemanG17

It seems brake fade is highly dependant on the driver......I've seen cars at the track with HUGE brakes...& the drivers are complaining of fade.....then I see heavy cars with marginal brakes that seem just fine and the drivers don't complain.......just remember to use your brakes like a light switch, its either all or nothing & you should be fine! (now if I could just follow my own advice!)

On weight transfer I have an interesting story about how I learned it.....I took a Mountain Bike class in Tahoe years ago......up to this point I thought only the front brake caused the front to dive.....we were doing wheelie drills and using the rear brake to bring the front tire down if it was too high.....I was stunned at how easy it was to bring the front end down (& fast) with just the rear brake.....just a little touch and the front end came slamming down hard!! The principles are the same for cars, just less intense due to the much lower center of gravity compared to a bicycle

Lizard928

But trail braking is soo much fun, esp with a massive rear bias valve

mark kibort

trail braking is still all part of the light switch. (just a light switch with a little curve at the end)

Take that massive bias setting, do some agressive trail braking and will be finding yourself with a sloppy mess of a car.

mk