Introduction

With respect to data collection using the Motion Instruments (MI) system, there are two types of bikes: 1) Bikes that you can attach the sensor in line with the rear shock, 2) Bikes that you cannot attach the sensor in line with the rear shock. When quantifying how the bike interacts with the ground, we need to be able to calculate axle speed in the y (up/down) direction. This is easy with the fork, but for the rear axle, we need to measure anything that moves and use a lookup table to calculate the instantaneous position of the rear axle. 

Most of the bikes we have on our vendor list, we get the exact kinematic data directly from the manufacturer. In our MIBD (motion instruments bike definition) file, we describe the bike geometry in a very simple way. The file contains information like head tube angle, fork travel, rear axle travel, and a table that describes the curve. The information that we need is simply data points that describe the relative motion between the shock and rear axle vertical position. As the rear axle moves up, the shock is compressed. In short, we are left with a simple table that looks like this:

Shock Position (mm), Rear Axle Vertical Position (mm)

0, 0

0.1, 1

0.2, 2

.

.

.

.

Fully Compressed ShockPosition (mm), Final Rear Axle Vertical Position

Please note that we are only interested in the relative motion. We don’t care that the shock eye to eye is going down, only that it moves x mm and the corresponding rear axle movement in mm. 

Measuring the bike to make a file for MotionIQ Pro

As an example, we are going to pick a difficult bike to mount a sensor to the shock and measure it to use the MI system. Let’s pick the Propain Rage as an example. 

It will be difficult to get a sensor mounted on this shock. It’s literally buried in the frame. However, there is plenty of room under the top tube to mount a sensor. In this position, we can just measure the upper rotating link. 

Using a couple of simple stick ons we provide, you can easily mount a sensor in this location. Once the sensor is mounted, you want to make sure that the sensor will measure all of the motion without bottoming out. You may want to take the spring off and just install the damper and look to see that the sensor has enough clearance to measure the entire stroke range without bottoming out the sensor. If this is verified, you can follow these simple steps.

1. With the shock mounted with the spring compressed, turn the bike upside down.
2. Remove the rear wheel, put the rear axle back in the frame. You will measure from the axle to the floor.
3. Measure the distance from the rear axle to the floor.
4. Measure the shock eye to eye

Now this is where things can get difficult based on the tools you have in the garage. We use a floor jack to lower the rear axle in 5-10mm increments until we capture the range of motion for the rear axle. 

1. Remove the shock from the frame
2. Place a floor jack under the rear axle and pump up the jack > than the rear axle motion. IE for a 150mm bike, raise the jack 160 mm or so. 
3. Place a piece of wood between the jack and rear axle. 
4. Now adjust the jack so the rear axle is exactly the distance to the floor in Step 3
5. Drop the jack 5-10 mm
6. Measure the distance between rear axle and floor, shock eye to eye
7. Repeat steps 9 & 10 until you hit the rear travel distance as specified by the manufacturer

You should have a list of values that looks like this:

Measurement	Rear Axle to Floor	Sensor Eye to Eye		Relative Rear Axle	Relative Sensor
M0	279	87.0		0	0
M1	269	86.0		10	1.0
M2	259	84.0		20	3.0
M3	249	82.1		30	4.9
M4	239	80.0		40	7.0
M5	229	77.9		50	9.1
M6	219	75.6		60	11.4
M7	209	73.3		70	13.7
M8	199	70.4		80	16.6
M9	189	68.8		90	18.2
M10	179	65.8		100	21.2
M11	169	63.0		110	24.0
M12	159	60.0		120	27.0
M13	149	57.0		130	30.0
M14	139	53.0		140	34.0
M15	129	49.8		150	37.2
M16	119	44.8		160	42.2
M17	114	42.5		165	44.5
M18	119	41.0		170	46.0
M19	124	39.5		175	47.5
M20	129	38.0		180	49.0
M21	134	36.5		185	50.5
M22	139	35.0		190	52.0
M23	144	33.5		195	53.5
M24	149	32.0		200	55.0
M25	154	30.5		205	56.5
M26	159	29.0		210	58.0
M27	164	27.5		215	59.5

The table on the left is the measured values, the table on the right is the relative motion of the rear axle and sensor starting from 0 and going up to max movement. Once all of the measuring is completed, now you can create an MIBD file. This is just a simple text file that you can make with any text editor. All you have to do is change the file extension to .mibd. This will make it easier for the MotionIQ app to pull in the file.

Your final Propain Rage MIBD file should look like this:

<-------------------------------------------------------- File Start ------------------------------------------>

section, description

manufacture, Propain

model, Rage

year, 2022

suspension, softail

fork travel, 200

HA, 63

section, linear sensor

0 , 0

1.0 , 10

3.0 , 20

4.9 , 30

7.0 , 40

9.1 , 50

11.4 , 60

13.7 , 70

16.6 , 80

18.2 , 90

21.2 , 100

24.0 , 110

27.0 , 120

30.0 , 130

34.0 , 140

37.2 , 150

42.2 , 160

44.5 , 165

46.0 , 170

47.5 , 175

49.0 , 180

50.5 , 185

52.0 , 190

53.5 , 195

55.0 , 200

56.5 , 205

58.0 , 210

59.5 , 215

<-------------------------------------------------------- File End------------------------------------------>

To import the file into your pro app, just follow the instructions laid out in this youtube video:

(88) How to Import a Recording - YouTube

Measuring a bike using the free MotionIQ app

If you will not be needing a full motion curve for the bike, it’s best to use the Generic Soft Tail bike. For this, all you need to know is the final sensor position when the bike is fully compressed. 

1. Measure the sensor eye to eye
2. Remove the spring on the shock and install it back on the bike, or if you have an air shock, remove all the air
3. Have a rider stand on the pedals with all of their weight
4. Measure the sensor eye to eye
5. Subtract the measurement in step 4 from step 1.

In the app, when creating the bike, put in all of the information about the bike including fork travel, head tube angle, etc. For rear axle travel, put in 215 for the Propain Rage. For the shock value, put in 59.5 (bike fully compressed). 

Conclusion

It is important to use data analysis when optimizing your bike suspension setup. Without data you are just guessing. Best practice is to use the bike leverage ratio from the manufacturer. When this data is not available, you can just mount the sensor to capture motion anywhere on the bike that gives you a lot of motion relative to the sensor stroke range. 

Pick something that moves close to the upper range of motion of the sensor without bottoming it out. If you only get a small amount of motion, there will be errors introduced into your measurement, so it’s best to avoid this. 

After that, just measure the range of motion of the bike and make a simple file you can import. If you are using generic soft tail, you just need 1 measurement.

Note: when using this method to measure your bike, you need to ignore the shock data in the app. In this case, what you are looking at is the sensor data, not the shock.