Cheap rear disc brake conversion

Louie76 · Oct 27, 2011

Just thought id share what I went with for a cheap disc brake conversion on the rear of my 76. I read alot of guys buying front rotors and grinding down the axle for the rotor to fit properly. I went about this a different way (and may have been covered before for all I know) I wasnt happy with the way the rotors could still shift around on the studs and was not interested in making spacers that could easily wear out/get lost or re drilling rotors. after changing my front rotors out i realized you can simply use a set of the front studs and install the rotor on the INSIDE of the axle flange. This way the rotors are supported just as solid as they are on the front with NO movement on the studs. then simply notch a hole out in the rotor same as on the axle to remove/install and bolt er back up. order a set of ruff stuffs weld on brackets and a cheapo set of chevy calipers and thats it.

***prices in canadian***
front studs $2.49 each
front rotors from 76 bronco: $52 each
brackets from ruff stuff: $50 pair
calipers from 85 chev 3/4 ton: $22 each
custom stainless braided brake lines: $50 pair

so for under $275 CAD you have rear disc brakes with all NEW parts
substitute the caddy calipers if e-brake is required. CHEERS!!

Broncobowsher · Oct 27, 2011

Too much rear brake. Bias will be way off. Too far off for an adjustable proportioning valve to comfortably dial in. Need smaller rear caliper for better balance front to rear. Then an adjustable proportioning valve can be used to fine tune the balance, not try and recover from a gross imbalance.

Louie76 · Oct 27, 2011

Ya didnt post about that, my setup is a lil different. seems like a caliper from the early 80's monte carlo would be a better fit for a stock bronco. was mainly posting for the whole rotor mount business. pick your poison for caliper with 7.04" mounting holes

dieselbronco · Oct 27, 2011

Don't shun the big calipers yet. I had the same setup on a ranger with bronco axles and it worked good. I think the bigger calipers take more fluid which makes them not to strong for the rear. I now have the smaller s10 calipers on my EB and they seem to lock up more than the bigger ones.

fordtrucks4ever · Oct 27, 2011

What are you doing for emergency brake? I used the rear metric GM calipers Then machined the front rotors from a newer model Jeep Grand Cherokee the correct thickness, bolt pattern and to fit over the stock axle. The only reason for this direction is because I got the rotors for free. I have less than $50 in my conversion. That also includes the mounting brackets and brake cables from donor car the calipers were pulled from.

Some of the older conversions were done with rotors bolted on the back side as you did.

dieselbronco · Oct 28, 2011

Some of the older conversions were done with rotors bolted on the back side as you did.[/QUOTE]

Do your j**p rotors fit over the axle from the out side? What application are they from? I have mine mounted on the back side of the axle but would rather have them on the out side for my next truck.

Apogee · Oct 28, 2011

dieselbronco said:
Don't shun the big calipers yet. I had the same setup on a ranger with bronco axles and it worked good. I think the bigger calipers take more fluid which makes them not to strong for the rear. I now have the smaller s10 calipers on my EB and they seem to lock up more than the bigger ones.

I'm not arguing with your results, because I believe your old brakes may have worked better, but the reasoning behind it seems to be a bit flawed as that is not how simple hydraulic master/slave systems work.

A brake circuit requires two things, fluid volume followed by fluid pressure. When you first apply the brakes, you're just pushing fluid at low pressure in order to take-up the "slack" in the system. This "slack" includes the clearances between the caliper piston, pads and rotors. While the master is pushing fluid, the pressure generated is only enough to overcome friction in the system, that's it. When all of the clearances in the system have been taken-up, then the system begins to generate pressure and the master cylinder would hydraulically lock if the system were perfectly ideal in terms of the fluid being incompressible and everything being 100% rigid. At that point, more force at the pedal would equal more pressure at the calipers. Since nothing is ideal, you still get a little pedal travel, but the principles are the same.

It gets a bit more involved with a tandem master cylinder since the two circuits are somewhat independent of one another when things are working correctly, but do have mechanical relationships should one circuit fail or require more volume than the master cylinder can supply.

Last by not least, caliper body size can have very litle to do with piston size/area. The older GM calipers with the 7.04" pin spacing didn't have exceptionally larger pistons than the smaller 5.47 [139mm] metric calipers used on the 1978+ GM applications as both were ~2-15/16" for the front calipers and ~2-3/32" for the rear parking brake calipers.

Rustytruck · Oct 28, 2011

Are the back side of axles all machined? I thought they were still rough from forging?

Louie76 · Oct 28, 2011

I decided to mount them on the inside so i didnt have to pay for a machine shop to re-drill rotors. the back side is machined however you do have to touch up the inside where it starts to taper a bit with a grinder but very minimal. as for e-brake, im not running one. (trailer queen)

Louie76 · Oct 28, 2011

now you have me second guessing these calipers i have. havent got to try em out yet but that is what ruff stuff recomends (mind you they arent getting into vehicle specifics) i see jeff's recomends the monte carlo calipers with same mounting, then there are the hundreds of aftermarket calipers aswell??

blazinchuck · Oct 28, 2011

i heard the Suzuki sami front rotors fit the rear axle shafts perfect over the studs and axle flange. i dont like the idea of permanently mounting the rotor to an axle shaft. if shaft fails...you have to have a spare just like it or remove all the studs and swap stuff over...i like removable rotors personally

Louie76 · Oct 28, 2011

after tossing it around i would think that a larger piston would lock up later as you need more fluid to compress the pads?? however given that logic you would want it small enuff to be effective cuz you can always cut down pressure with a prop valve. boosting pressure if the calipers arent effective enuff could be more in depth. any thoughts?

Broncobowsher · Oct 28, 2011

blazinchuck said:
i heard the Suzuki sami front rotors fit the rear axle shafts perfect over the studs and axle flange. i dont like the idea of permanently mounting the rotor to an axle shaft. if shaft fails...you have to have a spare just like it or remove all the studs and swap stuff over...i like removable rotors personally

Don't know about Sami rotors, but I have a set of Geo Tracker (Suzuki) front rotors on the 9" under my utility trailer. Yea, disk surge brakes on a beater trailer. I scabbed together brackets for the Geo calipers. Fits under 15" wheels too.

As for the volume/pressure thing, you are forgetting that the pressure is balanced in the duel chamber master cylinder under normal use. Only during a failure will the bits inside put pressure to one end more then the other. Prop valves should be used for fine tuning, not gross adjustment.

Now a trailer queen, this gets even more fun. Balance front to rear is nullified when in 4WD. Braking action will balance itself through the drivetrain as both axles will spin at the same speed.

Louie76 · Oct 28, 2011

ya i realize its gonna work no matter what in 4wd but wanna try and get em close so they wear semi evenly between front and rear.

As for the volume/pressure thing, you are forgetting that the pressure is balanced in the duel chamber master cylinder under normal use. Only during a failure will the bits inside put pressure to one end more then the other. Prop valves should be used for fine tuning, not gross adjustment.

I would like to get it as close as possible without having to use a prop valve. but in my head a larger piston would require more pedal travel to compress?? meaning a larger caliper piston would create less braking power using the same volume of fluid as a smaller piston? make sense broncobowsher or am i out to lunch on this??

Broncobowsher · Oct 28, 2011

The volumes are so miniscule they are hardly a factor. But pressure/surface area are a big factor. You are treating volume and pressure like they are equeal in factors, they are not. Pressure highly trumps volume. And most of volume is accounted for with line stretch and caliper flex, not travel of the piston.

br0nc0xrapt0r · Oct 29, 2011

How about some pics? I would like to see the rotor mounted on the inside. do the studs still fit snug in the flange on the shaft? are they 31 spline? Thanks.

Louie76 · Oct 31, 2011

yes 31 spline, the studs fit perfectly but expect to use a press to install them, tight lil bastards. I stripped one trying to use a lug nut to install, then beat on it for a while, then went to the shop to use the press. sorry, dont know how to host pics on here. looks like a rotor mounted to the inside of an axle tho

cheers!

Louie76 · Oct 31, 2011

and thanks broncobowsher, i think ill return these and try the ones that jeff's recomends for the rear (monte carlo fronts) people seem to have good luck getting close with those. my master cylinder is only .93ish as compared to the factory 1" but from what i understand that just means i will have a softer pedal with a lil more stroke to lock em up.

br0nc0xrapt0r · Oct 31, 2011

Louie76 said:
yes 31 spline, the studs fit perfectly but expect to use a press to install them, tight lil bastards. I stripped one trying to use a lug nut to install, then beat on it for a while, then went to the shop to use the press. sorry, dont know how to host pics on here. looks like a rotor mounted to the inside of an axle tho cheers!

Nice! I think I will go this route then is it a pain to get to the nuts for the axle retaining plate though?

Louie76 · Nov 1, 2011

I just set the rotors on the axle, traced out the hole and had it milled. could prolly do the same with a small grinder, dosnt have to look pretty.

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