I purchased some hydraulic disk brakes to give my electric mountain bike better braking. These would replace the cable ones I had on the bike.

This project compares the stopping distances of the two.

The test track was a downward sloping private road with little traffic. This allowed for the tests to be at a constant speed by rolling down the hill and applying the brakes at the same place each time. Chalk marks measured out braking distances.

Step 1: Cable Brakes.

Three warm up runs were done before measuring braking distances with cable brakes. A GPS app on my phone gave the maximum speed prior to to braking for each run.

Results:
8.6 m 31 km/h
9.0 m 31 km/h
9.0 m 30 km/h
8.8 m 29 km/h

Average distance = 8.85 meters

Step 2: Cable Brakes Unadjusted

Cable brakes need adjusting as they wear. Otherwise there will be 'play' when the handle bar leavers are pulled. I also tested braking distances when the brakes were unadjusted and there was a lot of 'play' in the leavers.

9.0m 30km/h
9.2m 29 km/h
9.3m 30 km/h
Average distance = 9.16 m

Step 3: Fitting Hydraulic Disk Brakes

Follow a you tube video to see how to replace the cable brakes with hydraulic ones. I watched this one:
Fitting Hydraulic Disk Brakes
Or better still, get a bike shop to fit them for you.

A couple of tips:

The handle grips can be made easier to remove by adding some rubbing alcohol between the grip and the bar. This makes it easier to slide off the hand grips, the alcohol quickly evaporates afterwards.
The disks purchased will probably have a spacer between the pads, remove this by sliding it out just before mounting the wheel. Be careful not to squeeze the brake leaver when the spacer has been removed prior to fitting the wheel.
When ordering disk brakes specify which leaver is to control each brake. Often the Right hand leaver controls the rear brake. But I am use to having the right hand leaver operate the front brake, so I requested this when I placed my order.

Step 4: Hydraulic Disk Brakes

The first 4 runs were disappointing - taking longer to stop than with the cable brakes, then I realised the new disk pads were still 'bedding in' to the disk, so I tested a few more runs, these were better.

Bedding in runs:

31km/h 11m
31 km/h 9.8m
33 km/h 10.9m
31 km/h 10.m
32 km/h 9.1 m
32 km/h 9.1 m

Average: 9.98m

Runs after pads had been 'bedded in'

32 km/h 8.3 m
32 km/h 8.2 m
32 km/h 8.0 m
31 km/h 8.6 m
31 km/h 8.5 m

Average distance = 8.32m

Step 5: New Brake Leavers

Unexpectantly, the new brake leavers took more space on the handlebars. This effected the ability to change gears somewhat and is something to consider when choosing which hydraulic brakes to retro fit.

Step 6: Summary

Average braking results

Cable brakes (unadjusted) = 9.16 meters
Cable brakes (adjusted) = 8.85 meters
Hydraulic brakes = 8.32 meters

There doesn't appear to be much difference between braking distance of cable and hydraulic disk brakes. However, I found the hydraulic brakes were better to use as they had much better modulation.

Modulation is the control you have when braking. Hydraulic brakes can be used with just one of two fingers and have a better 'feel'. They felt less likely to lock up and skid as they can slowly be squeezed for the right amount of braking control.