Rear discs w/ Stock Fronts - OK?

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BlackWidow
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Rear discs w/ Stock Fronts - OK?

Postby BlackWidow » 16 Apr 2009 20:24

Hello all


I am in the process of installing energy suspension on my 72 510. While i have everything out and cleaning it i would like to upgrade the rear brakes to discs. I am considering using the 200sx discs and clipers. i want to leave the front discs in place until i can later on upgrade those to 280 discs. Any suggestion on what might happen if i use this combo (200sx rear discs and stock front discs)? Will i need to change the MC or add a poroptioning valve? Any info i can get before i go to the junk yard would be very helpfull. Thanks

Mike G
1972 4dr 510 (Turbo KA24E build in progress)

510Martinez
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Re: Rear discs w/ Stock Fronts - OK?

Postby 510Martinez » 16 Apr 2009 20:53

Exactly as you describe - need master cylinder and maybe a proportioning valve.

Here are the possibilities:

1. it works just fine

2. not enough flow to the rear brakes so you lose some braking force.

Solution is to use an MC with more volume (larger piston). That, in turn, *may* lead to the rear locking up before the front (since the new MC gives more flow). The subsequent solution is a proportioning valve to limit pressure to the back so they are balanced with the front.

3. If correcting #2, the new master cylinder increases volume all around potentially making the front still over-proportioned because of the small calipers and you still have low braking force to the rear.

At that point the only solution would be to change the front calipers to match the master cylinder (commonly the 280zx swap) or use individual master cylinders and modified brake pedal. Since you are on a budget, that last option is not ideal.

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okayfine
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Re: Rear discs w/ Stock Fronts - OK?

Postby okayfine » 17 Apr 2009 08:23

BlackWidow wrote:While i have everything out and cleaning it i would like to upgrade the rear brakes to discs.


There isn't really much involved with swapping to rear discs, at least from the point of view of your situation above. The only thing that's made significantly easier if you have the rear completely disassembled is the removal of the backing plates. However that can easily be done with them on the car if you don't care about them (and if you're going to rear discs, I doubt you do care about 'em). Just cut 'em off when you're ready.

510Martinez wrote:Here are the possibilities:


There's a #3 - too much force to the rear brakes.

It's all fluid dynamics. Calculate the difference between the rear wheel cylinders compared to the rear piston of the calipers you want to run. If there's a difference, there will be a difference in braking force. Which way the difference lies will tell you how your braking force will be affected.

510Martinez wrote:Solution is to use an MC with more volume (larger piston). That, in turn, *may* lead to the rear locking up before the front (since the new MC gives more flow).


Again, it depends on the relative piston sizes involved. What it will definitely do is lead to less braking at the front. A larger BMC bore pushes more fluid out, however it is at a lower pressure at the pad.

510Martinez wrote: The subsequent solution is a proportioning valve to limit pressure to the back so they are balanced with the front.


Seems like a lot of unnecessary work, given the OP is going to swap to 280ZX stuff up front eventually.

Mike, just hold off, or do the upgrade at the same time. IMO you're adding unnecessarily to your project load for what amounts to an afternoon of work once you're ready.
Because when you spend a silly amount of money on a silly, trivial thing that will help you not one jot, you are demonstrating that you have a soul and a heart and that you are the sort of person who has no time for Which? magazine. – Jeremy Clarkson

BlackWidow
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Joined: 10 May 2008 13:23
Location: Tehachapi Calif

Re: Rear discs w/ Stock Fronts - OK?

Postby BlackWidow » 17 Apr 2009 09:27

Thanks

I will take your advice and hold off and do the disc swap all at the same time. I have the entire rear end out of the car now and just called a local pick ur part. when i go to the salvage yard i will try to laocate the 280 front end stuff i need. Thanks for the info i really appreciate it.

Mike
1972 4dr 510 (Turbo KA24E build in progress)

510Martinez
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Re: Rear discs w/ Stock Fronts - OK?

Postby 510Martinez » 17 Apr 2009 11:36

okayfine wrote:

Again, it depends on the relative piston sizes involved. What it will definitely do is lead to less braking at the front. A larger BMC bore pushes more fluid out, however it is at a lower pressure at the pad.
...
Seems like a lot of unnecessary work, given the OP is going to swap to 280ZX stuff up front eventually.



It was more of a academic response. The solution to the brake force issue is a brake booster or longer pedal to mechanically make up for the lower pressure. I agree all around with you; he should wait until he is ready to do it all at once.

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Byron510
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Re: Rear discs w/ Stock Fronts - OK?

Postby Byron510 » 17 Apr 2009 14:15

Swapping the rear drums is an interesting "engineering" topic. Not only is the bore size of the drum wheel cylinder much smaller than a comparable disc (say 82 200SX), but the drum brake also has a self energized design, requiring even less pressure to activate the brake itself.

I have seen a 200SX rear disc in a 510 with no other mods including a M/C, but I can't tell you how the bias or balance was.

Has anyone out there on this site done this very mod (rear 200SX discs; stock or 280ZX front discs) on their cars and have anything to say about it?
Did you auto X or track the car where your baking becomes more extreme or the exact balance of the system becomes more of an issue?

Just curious.

Byron
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because the opposite never works.


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