HOW TO Convert From Drum Brakes to Disc Multi Part Video Series

[Schurkey]

I'm not in a position to confirm or deny your math, in particular the correction factors for drum cylinder "effective area". That is ALL NEW to me.

The missing info for your braking-power calculations is the friction-material lining area, as well as the swept area (diameter AND WIDTH) of the drums. 2.5" wide, 13" diameter drums have less stopping power than 3.5" wide, 13" drums. Disc pads have less lining area than one drum brake shoe.

Disc brakes have much less lining area; they may or may not have less swept area on the rotors because the rotors are double-sided. They make up for less lining area via higher hydraulic force (brake fluid pressure X area of the caliper piston(s).) so disc brakes need much more piston area than drums. Disc brakes are also not really self-energizing like typical drums. There may be some very mild self-energizing that reduces even more as the pads wear.

More math needed. My W-A-G is that a stock Eldo caliper pair, on 12.8" rotors, is going to be CREAMED in actual usage by 2.5" wide, 13" diameter Duo-Servo drums. The 3.5" wide version will have still more stopping power.

Things may be different with the big-piston "Eldo" calipers. There may also be an improvement in heat-dissipation.

 

[95 Silverado]

I made lots of simplifying assumptions especially for the drums, which may not be entirely accurate given the complex geometries, but the two corrections I applied were 1) the piston treats the shoes as a lever with force at one end and a pivot at the other which effectively doubles the force and 2) a correction factor to account for the fact that the friction surface on the drum is further from the centre of the axle than the disk which creates more stopping torque i.e. if they were the same size this wouldn’t be needed.

In simple friction models the friction equals the force times the coefficient of friction. Surface area has no effect because as the surface area increases the PSI decreases by a corresponding amount so it all cancels out as described in this article. Larger pads are definitely still beneficial due to longer wear and better heat dissipation i.e. smaller pads overheat and stop working much earlier Also brakes may not precisely follow a simple friction model so these are really just ballpark calculations.

Another way of looking at it is the larger Eldo brakes with 12.8 inch disks should provide a similar amount of stopping power in the rear as the stock disks provide up front which I’d be pretty happy with. If my current 13 X 3.5 drums had substantially more stopping power than the front disks I’d be constantly locking up the rears with the truck empty and I’m not doing that even with non-functional ABS.

I’ll report back on how it actually works…or not!!!

 

[Schurkey]

Okee-dokee, doggy daddy.

 

[Erik the Awful]

I made lots of simplifying assumptions especially for the drums, which may not be entirely accurate given the complex geometries

There's your flaw. You can't compare discs to drums on pressure without a lot more geometry and math. Discs require significantly more pressure to operate. Before discs, brake boosters weren't really a thing. Go look at those old 4-wheel drum cars from the '50s and '60s. They didn't have brake boosters, and they didn't need them. What they did need was a brake adjustment at every oil change. That's the superiority of discs, they're more reliable over a longer term without as much maintenance.

 

[95 Silverado]

The drum calculations aren’t really necessary if you consider that the Silverado 3500 currently in production uses pretty much the same sized brakes on the rear as on the front which is pretty much what I’ll get with the 03 Express 3500 disks and Eldo HD Calipers on my truck.

 

[Schurkey]

The drum calculations aren’t really necessary if you consider that the Silverado 3500 currently in production uses pretty much the same sized brakes on the rear as on the front which is pretty much what I’ll get with the 03 Express 3500 disks and Eldo HD Calipers on my truck.

Following OEM sizing is wise.

Might be useful to also examine what they're using for master cylinder bore size, and how they've got the valving--proportioning valve, holdoff valve, etc., set up.

 

[95 Silverado]

I made lots of simplifying assumptions especially for the drums, which may not be entirely accurate given the complex geometries

Yes my drum brake calculations did not properly capture the "self energizing" component and were therefore low. In retrospect I could have saved myself some time and energy and gotten better results by using one of these fancy brake calculators.

Following OEM sizing is wise.

Might be useful to also examine what they're using for master cylinder bore size, and how they've got the valving--proportioning valve, holdoff valve, etc., set up.

Thankfully disks are much easier to reason about and here are some interesting comparisons on OEM disk setups versus my truck (top line) with rear disks from an Express 3500 DRW and Eldo HD calipers:

Vehicle

GVWR

Master Cylinder Diameter

Front Rotor Diameter

Front Piston Diameter

Number of Pistons

Front Piston Area

Front Power Ratio

Rear Rotor Diameter

Rear Piston Diameter

Number of Pistons

Rear Piston Area

Rear Power Ratio

95 Chev K3500 DRW

9000​

1.25​

12.5​

3.285​

1​

8.475​

52%​

12.8​

3.15​

1​

7.793​

48%​

03 Chev Express 3500 DRW

10000​

1.45​

12.8​

2​

2​

6.283​

50%​

12.8​

2​

2​

6.283​

50%​

23 Chev 3500 DRW

14000​

1.5​

13.98​

2.36​

2​

8.749​

53%​

13.98​

2.24​

2​

7.882​

47%​

This doesn't take into account valving, boost power, but at least the hardware sizing doesn't seem too far out of whack...

 

[bretcopsey]

Yes my drum brake calculations did not properly capture the "self energizing" component and were therefore low. In retrospect I could have saved myself some time and energy and gotten better results by using one of these fancy brake calculators.


Thankfully disks are much easier to reason about and here are some interesting comparisons on OEM disk setups versus my truck (top line) with rear disks from an Express 3500 DRW and Eldo HD calipers:

Vehicle

GVWR

Master Cylinder Diameter

Front Rotor Diameter

Front Piston Diameter

Number of Pistons

Front Piston Area

Front Power Ratio

Rear Rotor Diameter

Rear Piston Diameter

Number of Pistons

Rear Piston Area

Rear Power Ratio

95 Chev K3500 DRW

9000​

1.25​

12.5​

3.285​

1​

8.475​

52%​

12.8​

3.15​

1​

7.793​

48%​

03 Chev Express 3500 DRW

10000​

1.45​

12.8​

2​

2​

6.283​

50%​

12.8​

2​

2​

6.283​

50%​

23 Chev 3500 DRW

14000​

1.5​

13.98​

2.36​

2​

8.749​

53%​

13.98​

2.24​

2​

7.882​

47%​

This doesn't take into account valving, boost power, but at least the hardware sizing doesn't seem too far out of whack...

@95silverado, are you 100% going the disc conversion route and no longer in need of replacement drum backing plates? A trip to the yard may happen this weekend but I’d rather not mess with trying to score those if they are no longer needed.

Also, since I have some interest in a rear disc swap I’m really interested in seeing this play out if you go this direction. Are you planning to purchase the lugnut brackets then modify to provide better caliper support?

From what I’ve found, the 3500 DRW Express/Silverado/Sierra use the same rotor and the backing plate crosses between them. Your chart shows the calipers with dual 2” pistons but when you look under the trucks there are also dual 2.4” piston calipers available.

To me, assuming the math works out with the caliper bore sizes, this is a better option. You would have a more robust caliper mount and a better designed parking brake with the internal drum vs the ratchet/screw style from the Caddy. Welding the replacement axle flange onto the axle tube should be pretty straightforward using the brake assembly to set proper spacing and getting everything square.

This would be my preferred method of making the swap, so really curious if you already considered and found a reason to rule it out?

 

[Caman96]

@95silverado, are you 100% going the disc conversion route and no longer in need of replacement drum backing plates? A trip to the yard may happen this weekend but I’d rather not mess with trying to score those if they are no longer needed.

Also, since I have some interest in a rear disc swap I’m really interested in seeing this play out if you go this direction. Are you planning to purchase the lugnut brackets then modify to provide better caliper support?

From what I’ve found, the 3500 DRW Express/Silverado/Sierra use the same rotor and the backing plate crosses between them. Your chart shows the calipers with dual 2” pistons but when you look under the trucks there are also dual 2.4” piston calipers available.

To me, assuming the math works out with the caliper bore sizes, this is a better option. You would have a more robust caliper mount and a better designed parking brake with the internal drum vs the ratchet/screw style from the Caddy. Welding the replacement axle flange onto the axle tube should be pretty straightforward using the brake assembly to set proper spacing and getting everything square.

This would be my preferred method of making the swap, so really curious if you already considered and found a reason to rule it out?

Where did you find that axle flange?

https://www.gmt400.com/attachments/img_4190-jpeg.362008/

 

[Keeper]

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