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PORSCHEV · Posted: Tue Jul 03, 2007 12:00 am Post subject: Rear brake set up

Just wondering how much rear brake I should be running on car.

The rear brakes were way out of ajustment so I had little to no rear brakes at the last race.

I assume that I don't want the rear brakes to come on before the fronts otherwise I will spin out??

Thanks
_________________
1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

!tom · Posted: Tue Jul 03, 2007 1:24 am Post subject:

Adjusting the rear brakes will adjust the pedal height where you start to develop significant stopping power. You can't adjust which brakes come on first, nor can you adjust the bias without modifying the original braking system.

Normally you want the front to lock up just before the rear so that you can maximize the braking force without spinning out, but again, without doing something special to the car (and adjusting the brakes is not special), you'll have to rely on the bias settings Porsche came up with, which for my car has sufficient bias so that the front locks up first on the dry with the tires I have.

Changing the surface you drive on, and changing tires will both change the ideal bias. The higher the coefficient of friction between your tires and the road, the more front bias you need. Too much front bias and you might as well not have rear brakes.
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78 924 NA
5-lug

924RACR · Posted: Tue Jul 03, 2007 1:56 am Post subject:

Well, things do get a little different for racecars.

First off, you won't necessarily spin if your rear brakes start biting before the fronts. Just ask any go-kart racer - most have rear brakes only! Those are great fun to drive, just don't pound on the brakes mid-corner or you'll be spinning like a top. But it is very possible to trail-brake them - you just have to know what you're doing.

But typically the question is one of balance front/rear, not actuation point - and this is very commonly adjusted to get the cars to turn better on the brakes.

As Tom's noted, on the disc/drum setup adjusting the drums only adjusts how quickly the rear drums start biting; until they do, and all the slack is taken up, the fronts WON'T bite at all. This is due to the nature of the system design, in part that they're diagonal split cars (each brake hydraulic circuit has one front and one rear, diagonal opposites).

I can't help but wonder, if you changed it over to a "TT" or front/rear split (which would be done by changing plumbing at the master cylinder, you could potentially start to build pressure at the fronts entirely before the rears. Equally, you could then install a driver-adjustable proportioning valve, allowing you to reduce pressure to the rears.

This could prove helpful if you're having problems with balancing the fronts to the rears; by reducing the pressure at the rears and relying more on the fronts, the rears might not wear out so quickly (though you'd get less braking from them), potentially keeping the linings alive longer and keeping you from having the pedal fall away from you.

What kind of lining (brake friction material) are you using in the rear? Do you have race shoes or no? Some friction manufacturers will re-line your old shoes - John Brown may have dealt with this. A true race compound will last longer under race conditions than a street compound.

As for a guideline of how much you want - it's a driver preference thing, really. I don't really have much on my car, and so while I can't really use the brake to rotate the car so much, it's very stable on the brakes (making it easier to out-brake other drivers), and I can get the car to rotate well enough by other means. Some drivers like a lot more rear brake. You are somewhat hampered by the design of the system, since all the rear brakes must touch the drums before you can even develop front pressure and start slowing down...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

PORSCHEV · Posted: Tue Jul 03, 2007 3:56 am Post subject:

Thanks guys..gives me a great starting point!
_________________
1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

PORSCHEV · Posted: Tue Jul 03, 2007 11:43 am Post subject:

The car would just have stock brake shoes and pads ATM but I would like to upgrade asap to a better set.

I found the car wasn't braking much at all in the rear.... I am a bit confused when you say that the front won't lock ubntil the rear take up the slack....weel somewhat..lol.

I couldn't remove my axel nuts via impact or by having the car on it's wheels...so i tried having someone step on the brakes while I tried the powerbar on the axel nut and I STILL just spun the tire on the garage floor. The brakes didn't hold at all.

So I was assuming I could get some bias by adjusting the shoes tighter or looser. ATM I have them adjusted up tight as possible and will try that on the track.

Over the weekend I also dropped the rear suspension and indexed the torsion bars (twice..first time had it slammed onto the snubbers

0) The car is now 6 inches all the way around when measured from the rocker panel line (not the flange), I also installed a set of HEAVY front springs from a mustang.

The car was 9 inches at the rocker in the rear...24 inches at the bumper!! now it is 6@ the rocker and 17 3/4" at the bumper.

I'll report in with how the car works....hope I didn't make too many changes at once

_________________
1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

924RACR · Posted: Tue Jul 03, 2007 9:56 pm Post subject:

Sounds much better. Don't forget to cut the rear bump stops in half - or you'll hit them when cornering and the rear end will snap loose.

For the fronts, I've been very happy with KFP's - very durable. Paragon sells them. I don't recall what your options are for rear shoes - gotta get input from John Brown.

Adjusting the rear shoes won't change bias - it'll only change the pedal position at which you develop pressure in the brakes.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

hey nice to hear that you've done the indexing job succesfully ! do you want to do it again soon ?

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Supercharger and EFI kits
https://www.the924.com

PORSCHEV · Posted: Wed Jul 04, 2007 5:24 am Post subject:

It wasn't as bad as I thought it would be. I am freakin amazed that this car sat for 8 years and all the bolts cam out easily and were still shiny and not siezed in the bushings

Even the brake lines just unbolted from the flex lines.

Progress report from the track.... no more sliding in the rear any more at all. The car was running a 23 second lap time and now it is down to 21.63. Thats a GREAT improvement. Most of the stock class cars are only running 23 or just under. I think the mustang springs up front are a bit too heavy because now I am pushing the front-right tire through the corner alot more. But I still haven't played with the air pressures all that much.

The brakes feel much better as well...not touching a thing there.... 6 sleeps until race day!!
_________________
1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

924RACR · Posted: Wed Jul 04, 2007 6:24 am Post subject:

Excellent! Keep it up, and you may get the 924 banned from your track!

Front springs - you still running a stock-style 5" spring, or 2.5" coilover style? Seems like the circle-track places sell other rates in the 5" style, might be an option to swap in...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

!tom · Posted: Wed Jul 04, 2007 6:47 am Post subject:

PORSCHEV · Posted: Wed Jul 04, 2007 8:21 am Post subject:

Your getting over my head here Tom.... honestly the alingment is just eyeballed..although I think my RR has some toe which will work in my advantage on a banked oval.

I am sure I have too stiff of a spring running upfront. What is a good aftermarket sorce ..mucah harder then stock..but something that has some give. The springs I am running are pretty rugid.

I know your talking about wedge and what not..but i am pushing the envelope of the stock class.

Hers my class rules on suspension..I am already breaking the rules

- All steering components, shocks, struts, etc must remain stock for vehicle. Sway bars allowed but must be welded in place for race day.
- Wheelbase must remain equal on both sides, No tolerances.
- Camber on all four wheels will be zero.
- Vehicles must have working brakes on all four wheels at any given time, and will be subject to spot testing.
- Air ducts are allowed
- NO performance rotors or pads
- Steel braided brake lines can be used.
_________________
1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

PORSCHEV · Posted: Thu Jul 05, 2007 9:50 pm Post subject:

I recieved some tire pressure advice from THE best driver at our track. Glad I tried it. It was LF 15 LR 20 RF 28 and RR 26. The car still has some push but I can live with it. I am runnning 21 seconds flat and hitting a top speed of 140 km down the back stretch. According to some of the other 4cyl guys...most can't get 130.

I get to start in the pole this week where I am last in points..so I am hoping as long as I don't get piled up in turn 1 of the first lap I should do well.
_________________
1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

good luck ! lets hope you dont get banned for having a fast car

_________________
Supercharger and EFI kits
https://www.the924.com

!tom · Posted: Fri Jul 06, 2007 9:41 am Post subject:

John Brown · Joined: 07 Nov 2002 Posts: 903 Location: Leesburg VA

Sounds like you are having a ball with the car! Keep it up.

In another thread you will read that we have dropped the rear drum brake setup completely. Took it as far as we could stand it (constant fiddling). Had a lot of fun, learned a ton. But enough. However, I suspect we are MUCH harder on the brakes road racing than you are on a tight oval?

You can get custom brakes shoes either Porterfield or Carbotech. Possibly others.

Ultimately, just the general design limitations of the drums are too much especially coupled with the inevitable slop of this rear axle and bearing design. As you discussed drums tend to have noticeable slack that must take up before anything happens. There really is no solution to this. As Vaughan suggested re plumbing so the front discs work independently would be about all.
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82