Balance bar, for rear disk brake?

[Lyngeled]

Hi
Is a balance bar usually used with disk brake at the rear (or always)?

[andy harwood]

This is (to me) a great question, as I'm doing the Mazda rear caliper conversion. So, Watching.... I will say I have read the with this conversion, a balance bar is not necessary. YMMV.

[jbcollier]

If you are constantly braking at the limit, at the track for instance, you need a way to adjust the brake balance due to changing conditions (fuel load, etc).  On the street, assuming things are sized correctly, it is not necessary.  That said, practicing at the limit braking is a good idea.  Proper technique makes all the difference.

[EuropaTC]

I think it depends on exactly what conversion you are going for and if you retain the standard front brakes or uprate those as well.  If it's the standard front with a rear caliper piston of 34mm and disc/rotor diameter of around 240mm then no, you won't need to balance the system. You'll get improved rear braking but not enough to lock the rears before the fronts.

If you use larger rear discs or caliper pistons then you might need to do so. I don't know where the critical point would be but something around 255mm rears with 232mm fronts starts to lock the rear brakes before the fronts on my spreadsheet. 

Brian

[Grumblebuns]

Balance bar or proportioning valve? I believe separate master cylinders for front and rear uses a balance bar, a single master cylinder will use a proportioning valve.

What rear disc system/ master cylinder are you planning on using?

[Lyngeled]

I have not decided to change or upgrade brake, but I just playing with ideas.

And I am not sure how much braking comes from the drumbrakes or if it is too little.

[EuropaTC]

I have not decided to change or upgrade brake, but I just playing with ideas.

And I am not sure how much braking comes from the drumbrakes or if it is too little.

Richard on the Yahoo group list produced a spreadsheet for his S2 brake system and modeled both with rear drums and discs.  With drums the contribution from the rear is quite low, on my TC version of his spreadsheet it comes in around 25%.   Even moving to relatively small 240mm rear discs with 34mm single piston caliper is going to push that significantly higher, 36-37%.

A rear disc conversion can be a bolt-on modification but it's not a cheap thing to do if you want to buy a ready-made kit. Mine was very cheap in comparison at only £125 for version #1, but then again I like the challenge of making things myself.

Brian

[Lotuswins]

I have replicated the Banks rear disk conversion on my TCS, and installed a proportioning valve in series with the rear brakes, just in case the rears started locking up.  I then took the car to a track day at PIR and measured rear vs front disk temps after each 15 minute run.  I slowly backed off the proportioning valve until it was fully at zero proportioning, and measured temps within 30F front vs rear.  Braking with the disks is significantly improved over the drum system on the track.  I have driven it in the rain, and so far no rear lockup so I'm very happy with the results.  I use Mintex 1144 pads front and rear and a single Tilton 7/8 M/C, which gives a firm pedal, without too much travel.  The rear pads are only available in the UK when I was looking, the calipers being from a Ford Sierra/Mekur Scorpio.  I understand the Miata calipers are more readily available.

YMMV, 

Jerry Rude
4005R

[Lotuswins]

Oh, and you might want to note that with the rear disk conversion your pedal travel will increase since the disk calipers will take more fluid volume.  I had a 0.700 S2 M/C on the car when I converted, and the pedal travel really started to concern me so I finally had to up the cylinder back to the original 7/8 bore to have the confidence in the system.  Pedal pressure is firm, but the Mintex pads do give a better friction factor than the stock pads did.  FWIW.

To answer your original question though, no, a balance bar system is not needed nor really recommended unless you track the car.  A proportioning valve actually has better braking performance on the street, giving higher rear pressures to the rears at normal stopping pressures.  When on the limit, like during track/race events, the balance bar is better since it gives adjust-ability that the proportioning valve does not. 

Jerry Rude
4005R

[gideon]

But if you go with a balance bar then you can use two independent master cylinders which gives you the safety factor of two hydraulic circuits and plenty of choice in master cylinders.  Wilwood and Tilton both provide 6 different sizes in the 5/8" to 1" range.  So then you have the option to make coarse adjustments to front/rear brake balance using different sized master cylinders.  You can do that and use a proportioning valve.  It may be a good idea to have both since they do different things, and neither one alone gives you all the control you might want. 

Well that's what I'm thinking for my own S2 restoration.  When I get to that point.