Introduction: Graber MAG Bike Trainer – Increased Resistance Modification

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

Only two of the settings of the Graber MAG unit that I have feel like they offer any additional resistance. Also, neither setting seems particularly challenging. The other three settings seem to be the flywheel effect only.

This makes sense based on the characteristics of magnets. Pull force decays exponentially with the increase in distance. Based on measuring the “steps” and calculating the pull force, the steps look like: 100%, 65%, 40%, 25%, and 17%. The modifications that follow will increase the pull force, but they won’t make steps linear.

The calculator used is at K&J Magnetics, Inc. (https://www.kjmagnetics.com/calculator.asp?calcType=disc) and allows you to compare the pull force of disc magnets. You can change type, grade, diameter, thickness and distance.

There are three ways to increase magnetic force to increase resistance; increase the strength (grade) of the magnet (N value), reduce the distance from the magnet to the flywheel, or increase the physical size (diameter x thickness) of the magnet.

Step 1: Strength (magnetic Flux Density)

The magnetic flux density for the OEM magnet (7.89 mm x 19.94 mm) was checked with a Hall-effect Gauss meter to determine the strength of the magnet. It has same magnetic flux density as a known N50 disc magnet so it can be assumed to be a N50. There is only one higher rating for disc magnets, N52, and it would only increase the force by about 4 percent. So this one is a “No go”.

Step 2: Distance

With the OEM magnet, reducing a nominal 2.5 mm distance to the flywheel to 2.0 mm has a surprisingly large effect, about 20 percent. You want to get the magnet as close to the attraction plate and still maintain reliable rotation. Placing non-ferrous shims under the OEM magnet would reduce the gap.

OEM at 2.0 mm - OEM at 2.5 mm

9.27 lbs -7.72 lbs = 1.55 lbs

1.55lbs/7.72lbs = 20 percent

Unfortunately, the flywheel is not precision made and mine came out of the box quite warped. You can see it “wiggle” as it spins. I had to file down the top of two of the three assembly studs that were each causing a bulge in the attraction plate. Reducing the distance by adjusting the magnet position is a “go”.

Step 3: Size

Because the magnet is housed in a thin walled tube, you are limited to 20 mm diameter. Adding thickness is the only option. Fortunately, there is enough space and play to add magnets.

Adding one “3 mm x 20 mm with a 5 mm hole” (2.66 mm x 19.69 mm actual) disc magnet(s) to the bottom of the OEM magnet (7.89 mm x 19.94 mm) to increase the size, adds an additional 23 percent pull force. You can reuse the M3.5 x 18mm Phillips, flat head, self tapping screw for one. For two need a M3.5 x 25mm Phillips, flat head, self tapping screw. Increasing the magnet size is also a “go”. Unfortunately, the calculations show that the pull force rises slowly after a third magnet.

One magnet

11.46 lbs - 9.27 lbs = 2.19lbs

2.19lbs / 9.27lbs = 24 percent

Two magnets

13.05 lbs - 9.27 lbs = 3.78lbs

3.78lbs / 9.27lbs = 41 percent

Did it work in practice? Yes, one magnet has shifted the resistance one full setting. Setting 1 is now 2 and setting 2 is now 3.

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2015-12-25 Update - While looking at the selector assembly, I realized that adding two magnets was the realistic limit. This is because the spring is approaching complete compression at the lowest resistance setting.

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Upgrade kits from the manufacturer would be really simple. Two kits, an intermediate and an advanced kit, each with a correspondingly larger magnet and shorter selector shaft would be nice. Also, a flywheel that isn’t warped on the unit when you buy it would be nice. The only problem, we can already do it for less than 5 bucks.

You can also change the rolling resistance by cranking down on the large knob. Cranking it down probably also increases tire and roller wear due to friction and heat.

Step 4: Disclaimer

I like this disclaimer from the author Samuel M. Goldwasser (http://www.repairfaq.org/sam/captest.htm)

“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 5: Parts and Tools

Parts

1 or 2 each N50 disc magnet 3 mm x 20 mm with a 5 mm hole

1 each M3.5 x 25 mm flat, Phillips, self tapping screw (for 2 magnets)

Tools

Adjustable wrench

Screwdriver, slot and Phillips

File

Steel rule in millimeters

Step 6: Disassemble Unit

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. Set the selector to the lowest resistance position. Undo and remove the axle lock nut, washer, and flywheel. The flywheel spacer is loose and can fall off if the unit is tipped. Just slip it back on if it does. The flywheel may have to be rocked a little to remove it.

Note: Selector setting instructions are either to keep the flywheel from clinging to the magnet or to reduce compression on the magnet spring.

Note: There is a loose washer that doesn’t belong in the upper right.

Step 7: Disassemble, Modify, Add Magnet(s), and Reassemble Selector

1. Set the selector to the highest resistance position. With a Phillips head screwdriver, remove the screw from center of the magnet. The spring is over compressed so the magnet will jump outward when the screw releases..

2. Remove the selector. Mark around the tip of the selector stem at 2 mm for one magnet or 4.5 mm for two magnets. You can use washers to help mark the selector. Measure washers against a metric ruler to find ones the right size. Lay the washer(s) flat with the knife (or scribe or other sharp object) lying on top. With the selector upright, carefully rotate it against the blade to mark a line completely around the selector. This will allow you gauge if you are maintaining a square end. If you file more than you expected, add non-ferrous shims at the bottom of the stack. Also, if you set up for two magnets and want to go to one, add non-ferrous shims to the bottom of the stack.

3. The stock screw (M3.5 x 18 mm Phillips, flat head, self tapping screw) extends 9 mm from the stock magnet and the hole is 15 mm deep. For one magnet, you can reuse the stock screw and you don’t need to make the hole deeper. If you are planning on adding two magnets, you need a longer screw (M3.5 x 25 mm Phillips, flat head, self tapping) and the hole needs to be about 4 mm deeper. I used a 3/32 inch bit in a pin vise to drill it out. Control is more important than speed so you don’t damage the existing threads.

4. Add the magnet(s) at the base of the OEM magnet and insert the screw. Assemble the selector assembly with it set to the highest resistance position (knob closest to the unit body) and hold it in place. Add the spring. Press down on the magnet stack with the screwdriver seated in the screw until the screw touches the shaft of the selector. Tighten the screw until it is finger tight. Do no over tighten or you could strip out the selector shaft.

Step 8: Reassemble Unit and Test Operation

1. Set to the selector to the lowest resistance position. Assemble the unit, except for the cover. Make sure you line up the axle and flywheel flats (D-shape).

2. Set to the selector to the highest resistance position. Hand-spin the flywheel to check for clearances. Some small adjustment can be made by bending the attraction plate but only between the three assembly studs and only a little. Two of my assembly studs were crowned and had to be filed down.

3. If you have interference, disassemble the unit and the selector. File the selector shaft to shorten it. Repeat as necessary until the flywheel spins freely.

4. When it spins freely, line up the three cover indexes with the flywheel and snap the cover on.

5. 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.