Disc Size Issue

[2Bad4Ya]

I put the ssbc 123a disc kit on the front of my 68 last yr and then ordered the a125-26 ssbc rear disc kit from Summit and we pulled the rear and installe the kit they sent us with no problems. Both kits went on with no problem at all.

However we did not look at the size of rear disc at all in relation to the front disc setup, it just was not on our mind. We did notice the difference though once we putthe rear wheel on and stepped back to look at all we did in 1 saturdays work.

They sent us the 13' rear disc kit not the 11' kit. So now the rear brakes look HUGE compaired to the front.

I am not that knowledgable in this area at all, will this be a problem to run 13 disc in rear and 11 in the front on a manual disc brake setup? I know its the norm to run bigger/same size in the front but at the moment it is opposite thanks to summit sending the bigger kit by accident.

[Sieg]

You need to get the 11" rotors coming. For multitude of reasons, car control being number one, since front brakes, especially on front engine cars provide the majority of your stopping power. Summit made an error on the order but it is ultimately the mechanics responsibility to verify the parts are correct before installation.

Besides.........it looks backasserds!

[2Bad4Ya]

yeah i know it does not LOOK right... but i am not asking visual oppinions thanks. I am looking for technical reasons. A proportioning valve handles the balance of stopping power from front to rear.

[Sieg]

Originally Posted by 2Bad4Ya

yeah i know it does not LOOK right... but i am not asking visual oppinions thanks. I am looking for technical reasons. A proportioning valve handles the balance of stopping power from front to rear.

All the prop valve does is regulate line pressure from the master cylinder to the calipers. Simply put stopping power comes from surface area, caliper design, rotor and pad material.

Consult the component manufacturer for accurate technical info.

[2Bad4Ya]

hmm i thought this was a discussion board with a section for brakes... i must be in the wrong place... thanks for the input though.

Since the above poster clearly does not have any input other than pumping his post count...

I am working under the following beliefs...
Front to rear bias only really matters when you reach braking threshhold and one end or the other locks up. To maximize the braking power, you start with more rear and adjust proportioning valve so the rears don't lock up before the fronts.

Is that a correct assumption, and that my prop valve will equalize out the diffrence until the front is replaced?

I will upgrade the front rotors/calipers later but at the moment the $$ needs to be allocated for other areas.

[Sieg]

Originally Posted by 2Bad4Ya

I put the ssbc 123a disc kit on the front of my 68 last yr and then ordered the a125-26 ssbc rear disc kit from Summit and we pulled the rear and installe the kit they sent us with no problems. Both kits went on with no problem at all.

However we did not look at the size of rear disc at all in relation to the front disc setup, it just was not on our mind. We did notice the difference though once we putthe rear wheel on and stepped back to look at all we did in 1 saturdays work.

They sent us the 13' rear disc kit not the 11' kit. So now the rear brakes look HUGE compaired to the front.

I am not that knowledgable in this area at all, will this be a problem to run 13 disc in rear and 11 in the front on a manual disc brake setup? I know its the norm to run bigger/same size in the front but at the moment it is opposite thanks to summit sending the bigger kit by accident.

Originally Posted by 2Bad4Ya

yeah i know it does not LOOK right... but i am not asking visual oppinions thanks. I am looking for technical reasons. A proportioning valve handles the balance of stopping power from front to rear.

Originally Posted by 2Bad4Ya

hmm i thought this was a discussion board with a section for brakes... i must be in the wrong place... thanks for the input though.

Since the above poster clearly does not have any input other than pumping his post count...

I am working under the following beliefs...
Front to rear bias only really matters when you reach braking threshhold and one end or the other locks up. To maximize the braking power, you start with more rear and adjust proportioning valve so the rears don't lock up before the fronts.

Is that a correct assumption, and that my prop valve will equalize out the diffrence until the front is replaced?

I will upgrade the front rotors/calipers later but at the moment the $$ needs to be allocated for other areas.

You win.

[Apogee]

Originally Posted by 2Bad4Ya

...I am working under the following beliefs...
Front to rear bias only really matters when you reach braking threshhold and one end or the other locks up. To maximize the braking power, you start with more rear and adjust proportioning valve so the rears don't lock up before the fronts.

Is that a correct assumption, and that my prop valve will equalize out the diffrence until the front is replaced?...

I think you answered your own question, however you won't know until you tune your proportioning valve. You are correct that a prop valve can be used to tune the rear brake bias, however it is meant to fine-tune a system, not fix major component mismatches as these are limited to a maximum pressure reduction to the rear of 57% the full line pressure by design. If you can achieve lockup at all four wheels with the rear locking just before the fronts, then you're good to go. If not, then you may need to make other changes to get there.

The larger rear rotor is increasing your rear brake torque by about 20%...you can make more of a difference than that in brake torque on either end by swapping out the pads. Lower CoF in the rear or higher in the front can and will shift your bias around back in your favor for not a lot of money.

Tobin
KORE3