What are the effects on car of SMALLER Rear wheel Clylinders?

[PATZ]

Hi everyone!

I finally got a pair of REAR wheel cylinders, but just noticed afterwards that it was sized 13/16 inch against the correct and LARGER 7/8 inch! now, i'm not an engineer or anything but i do know that using SMALLER Rear wheel cylinders will weaken the rear brakes and thus affect the front/rear brake bias. being that my only choice is to order from abroad (more time and money) i'd like to ask your feedback on how this would affect the CONTROL and POWER of my brakes/car during everyday driving (of course occasionally i'd like to stretch my cars legs a bit, even on the twisties! ) thanks a lot!

patz

p.s. can you recommend a website for more info?

[JimmyZ]

Your profile doesnt say which type of Z you own. I was thinking that perhaps you got the "right" parts for your car but we'd have to know what kind of Z you had first.

The 1972 240Z FSM had some detailed descriptions and formulae regarding the proportioning valve. (Don't know if this was true for other FSM's because this was the only one I ever had) It truly is amazing how much thought goes into designing any system on humble passenger cars. The simple choice of using a resistor in a plug can cut plug erosion to a fraction for instance. The refinement of these sciences makes modern car's 100K tune-ups a reality.

If the cylinders are dimensionally the same as the originals (except bore ID) then you could get an adjustable proportioning valve and adjust the rear brakes function. This is what you'd do for any custom brake system. There are actually books on brake system design that the average Joe can buy. I've only thumbed through them but they were interesting.

2c Hope this helps.

Jim

[PATZ]

hey thanks Jimmyz! Oh sorry, its a 1973 240Z. I originally bought these wheel cylinders just a few days ago coz i took the neighborhood salesmans word for it that they were "ok" for 240Z`s, but discovered afterwards that they were smaller than the correct 7/8(although the exterior dimensions are the same as the OEM). But for the sake of convenience and PRICE(roughly US$30/pc.) we already connected it, so i guess i get to try it out. Being that i`m sorta on a budget i definitely dont intend to buy an adjustable proportioning valve, otherwise i woulda just ordered the correct wheel cylinders from the U.S. By the way, whats an FSM? Anymore comments anyone?

[Z train]

A smaller bore will INCREASE the pressure to the shoes,thus decreasing the amount of pedal pressure needed.

[240Z-Fan]

A smaller bore will INCREASE the pressure to the shoes,thus decreasing the amount of pedal pressure needed.

I humbly disagree. The smaller bore will give you less rear brake force. In other words, for a given pedal pressure the rear braking will be less due to the smaller cylinder area, a net increase in forward brake bias.

This is better than using larger cylinders; increased rear brake bias could cause the rear end to lock up on heavier braking and loss of control.

In this case, there will be a slight reduction in ultimate braking capacity, but the car should stop straight.

Al

PS. I am not a brake engineer. This is just my opinion. Any changes you make to your braking system are at your own.

[sblake01]

A smaller bore will INCREASE the pressure to the shoes,thus decreasing the amount of pedal pressure needed.

Everything else remaining the same, wouldn't that cause a tendency for the rear brakes to lock up easier than using the correct sized wheel cylinder? 13/16 inch were used 10/69-6/72 and 7/8 inch were used 7/72-6/78. FSM means Factory Service Manual.

EDIT: Al, besides answering my question, you're a faster typist than I!

[240ZX]

I humbly disagree. The smaller bore will give you less rear brake force. In other words, for a given pedal pressure the rear braking will be less due to the smaller cylinder area, a net increase in forward brake bias.

This is better than using larger cylinders; increased rear brake bias could cause the rear end to lock up on heavier braking and loss of control.

In this case, there will be a slight reduction in ultimate braking capacity, but the car should stop straight.

Al

Yep......240Z-Fan has it correct! It's a simple matter of piston area. Smaller = less force / larger = more force

Tom

[jmortensen]

Yep......240Z-Fan has it correct! It's a simple matter of piston area. Smaller = less force / larger = more force

Yes, this is correct.

I went round and round and round with Dan Baldwin and others on the IZCC list maybe 5 or 6 years ago, convinced that smaller wheel cylinders would have more travel, and that would mean more rear braking. In actual practice, once the brake shoes touch the drums, the pressure is more important because the travel is minimal. The larger wheel cylinder has more pressure. Took them a while to beat it into my thick skull, but it is true.

Smaller master = more pressure

Larger wheel cylinder or piston in a disk setup = more pressure

Confusing but true.

[Six_Shooter]

I doubt you'll notice any difference in braking ability, with such a small difference in piston diameter.

If you get the book "Auto Math" it goes into detail on how to calculate bias and changes in braking ability, based on mesurements. Another book I have deals solely with suspension chassis and braking, but don't recall the name currently.

[PATZ]

I`d have to agree with you guys who say that a larger wheel cylinder piston increases breaking force on the drums(or clamping force on a disc brake rotor), thus a smaller rear wheel cylinder has even less potential to lock up earlier.

sblake01- thanks for "deciphering" FSM for me(i can be really ignorant)!

jmortensen- well thanks to the CLASSIC ZCAR CLUB and all your responses my head wont hopefully be going around that long(no offense meant).

sixshooter- thanks for the recommended reading! i`l surf the net for now.

All that said, i decided to just refund my money for the wrong 13/16 wheel cylinder for the 240z and will order the proper 7/8 one from Autoparts Warehouse. Oh, if anybody`s interested you might check this website out : www.teamscr.com and check out their tech articles on brakes and braking performance.