Introduction: Graber MAG Bike Trainer – Replacement Roller Modification

About: “There's always something.” - Violet Baudelaire:

The stock roller got creased in the first two minutes of operation. It wasn’t being used against a tire so I really can’t fault it. It needed to be softer and more elastic.

I tried beefy rubber bands on the stock roller. They crawled off to one side. I could have glued them but replacing them as they wore out would have been a pain.

I went looking for rollers or wheels with a maximum outside diameter of 54 mm. The best solution I found was “soft” skate wheels. Skate wheel are more dent resistant, softer, and abrasion resistant than the stock roller. They also come in a variety of diameters.

For the skate wheels, 54 mm was the maximum and a lower durometer like cruisers or Ricta Clouds (e.g. 78a) was preferred. Oddly, the 52 mm Ricta Clouds I selected are listed as 52 mm OD, called out at 52.75 mm OD, and the set I got measured 53.6 mm OD. They will work since they are less than 54 mm. I recommend checking the diameter before using them.

The next challenge was to adapt the skate wheel that normally uses free spinning 22 mm OD bearings to be a fixed roller on a 10 mm shaft. I found tattoo machine handles that were 22 mm OD by 8 mm ID, had a pair of set screws (Allen, Cup point, Stainless steel 18-8, 5 mm x 5 mm), and were cheap. They were 8 mm ID which could be overcome. I chose the solid handle with two set screws and uninterrupted knurling.

Step 1: Disclaimer

I like this disclaimer from the author Samuel M. Goldwasser (

“We will not be responsible for damage to equipment, your ego, blown parts, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury that may result from the use of this material.”

Add to that the usual admonitions to use safety equipment, be careful of hot, sharp, or rotating parts and equipment, and be cautious around electricity. Also, if you change it, and they can tell, it probably voids your warranty.

Step 2: Parts and Equipment


Ricta Clouds, 78a, 52 mm

Tattoo machine grip, Aluminum Alloy, 22 mm OD by 8 mm ID – aka the “hub”


Drill press and vise or reamers


8mm, 8.5 mm, 9 mm, and 10 mm split point drill bits


Dead blow hammer

2.5 mm and 3 mm hex wrenches

Note – I recommend using split point drill bits. Standard twist drill bit can easily go off center.

Step 3: Roller

1. Remove the set screws from the hub. Enlarge the ID to 10mm. This has to be done carefully to keep the shaft hole centered. You can’t hand drill this.

a. Reamers – using adjustable reamers start with 8mm and work up to 10mm. Clamp the part or the knurls will chew off your skin. Lube and clean your reamer as you go.

b. Drill press – chuck up a 8mm bit and slide the hub onto the bit. While still on the bit, clamp the hub in a vise and clamp the vise to the table. Retract the bit and replace it with an 8.5 mm bit. Drill out the shaft hole. Repeat for 9mm and 10mm. The bits always appreciate some lube. Be careful, the hub it gets hot.

c. Note – I recommend using split point drill bits. Standard twist drill bits can easily go off center.

2. Measure the hub and relieve the inner ring of the skate wheel to 3 mm (about 0.010 inch) less than the diameter of the hub (e.g. 21.7 mm). Work up to the dimension slowly and test fit it. When you go from not being able to tap it on to just being able to tap it on, you have reached the right dimension, You can always take a little more off but you can’t put a little more on. An adjustable reamer works best to maintain the center and roundness of the hole.

3. Replace the set screws and run them down below the surface of the hub.

4. Center the skate wheel on the drilled hub. Tap around edge of the skate wheel with a dead blow hammer to seat. Tap on skate wheel with a dead blow hammer to center it between the set screws. Re-drill with a 10 mm drill bit if you peen over the edges of the hole in the hub.

5. Back out the set screws so they don’t extend into the shaft hole.

Step 4: Assembly

Resist the urge to remove the 10MM external retaining ring that you see at the end of the shaft. It is under a lot of tension, is a cheap grade, it will become unusable (bent), and you will have to replace it. Been there, done that.

If you have honked up the ring, it is an external retaining ring for a 10 MM shaft diameter that fits a groove diameter of 9.60 mm and a groove width of 1.10 mm (e.g. Drillspot DSH10SA). The good news is they are cheap.

1. Unscrew the large adjustment knob. Undo the hinge nut, hold the unit, remove the hinge bolt, and remove the unit.

2. Place the slot screwdriver in one of the three indents in the flywheel, parallel to the edge. Twist the screwdriver to scoot the cover “forward”. Remove the cover.

3. Loosen the roller set screw with a 3 mm hex wrench.

4. Set the selector to the lowest resistance to keep the flywheel from clinging to the magnet. Undo and remove the axle lock nut, washer, flywheel and flywheel spacer. The flywheel may have to be rocked a little to remove it.

5. Thread on the axle nut and tap it to start the axle moving. The axle slides to the left when the roller set screw is loosened. Remove the axle nut, the axle, and the roller. The roller drops down to be removed.

6. Slide the axle back in, through the new roller. Replace the flywheel spacer, the flywheel, the washer, and the axle lock nut. Tighten the lock nut.

7. Line up the three cover indexes with the flywheel and snap the cover on.

8. Hold the unit and replace the hinge bolt. Make sure the square head on the hinge bolt is seated. Install and tighten the hinge nut. Lift and hold the unit. Align the “L” bolt (1/4-20) with the underside of the large adjustment knob. The large adjustment knob will need to be at an angle to the unit to catch the “L” bolt threads. Tighten the large adjustment knob.

9. Line up the new roller with your tire and one set screw with the flat on the axle and tighten both set screws with a 2.5 mm hex wrench.

Edit 2016-05-22 -----------------------------------------------------------------------------
Note: If you need to replace the bearings, they are 6200-2RS Bearing 10x30x9 Sealed Ball Bearings, NEK brand on mine.

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Step 5: Smoothing the “Ride”

This was installed on a flywheel exercise bike, so you may not have to do this with a bike with tires.

The skate wheel roller was about 0.005 inches out of round. With the roller off the axle, a 10mm drill bit was chucked in an electric drill and the roller axle secured to the part of the bit without cutting edges. The drill was clamped with the roller squared. A board and heavy paper spacers were used to get a Shinto saw rasp to the correct height. The drill was run until there was only a light pull on the Shinto saw rasp. Be careful where you do this. It can generate a lot of small particles.