When I set out to design my own electric mountain board gears, I though it would be a quick and easy job I have a PhD in Electronic Engineering, how hard could it be to design some helical gears to mesh together, right!
Sadly, I wrong, it’s easy to design 3D models of gears in CAD there are plugins to help you do it, but designing helical gears that are quiet, efficient, and hardwearing takes some in depth knowledge of the maths behind gear design. I won’t go into the details in this build diary as I’m not an expert myself, but some of key parameters to consider are:
- Gear module (m): comparative size of the teeth, larger the module larger the teeth.
- Pressure angle (α): is the leaning angle of the gear tooth, normally set to 20°.
- Helix angle (β): the angle between the gears bore axis and the line tangent of the tooth, normally between 5° and 20° for single helix gears.
- Number of teeth (z): sets the gear ratio (drive gear No. teeth: motor pion gear No. teeth)
This web page goes into the math behind helical gear drive design and helped me out a lot during the design process:
One thing I have notice with existing EMTB gear drive manufacturers is that they are using module 2 gears, I assume this is because the teeth are quite big thus making them easier to machine and possible to make out of plastics like POM. One issue I see with this is that the minimum number of teeth you can have with a pressure angle of 20° is 17.
z = 2/sin^2(α) = 2/sin^2(20) = 17
And all these module 2 gear drives have less than 13 teeth some as low as 8 teeth. The reason they are doing this is because if use module 2 gears, and for example wanted a gear ratio of 5 and followed the min No. of teeth rule of 17, then your drive gear (85 teeth) would be too big (⌀180mm) diameter compared to your 8” (⌀203mm) tires, especially when you consider the gear drive casing.
It is possible to get away with reducing the number of teeth if some correction is made to the teeth by shifting the profile, but this makes them non-standard, so I doubt anyone is actually doing this. I’ve heard some really noisy gear drives on group rides, and this might be one of the reasons. Gears that are not properly designed will also wear out fast and not have poor free roll capability.
Design aims of this gear drive build
- Mathematical design approach to produce quality gears that are quiet, efficient, and hardwearing.
- Hardened steel gears: no plastic POM, I want these gears to outlast your board.
- Rugged: all metal construction, no 3D printed or plastic parts to ensure the gear drive can survive harsh riding conditions such as offroad riding and jumps.
- Work with common trucks: MBS Matrix II, Trampa, Apex Air.
- Work with common hubs: MBS Rockstar and Fivestar; Trampa Superstar, Megastar and Phatladz.
- Watertight: One of the worst things for gear drive is wear is when water and contaminants such as mud get into the gears, they need to be properly enclosed.
- Rapid wheel changes: No one wants to spend ages trying to undo multiple bolts to remove their wheel if the have a flat tire.
- Corrosion resistant: no black screws or exposed metals than can corrode after you’ve ridden you board through the mud and hosed it down.
- Affordable: RRP ~£300-£350
