EMTB gear drive design and build

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
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Why helical, rather than straight cut?

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Great job breaking down the common variables and associating them with what they affect. I always appreciate an accurate, well laid out, simple explanation.

Looking forward to following your progress and wishing you all the luck.

Do you have access to the tools yourself to cut these and compare results?

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So basically you’re redesigning the Boardnamics gear drive from scratch to get basically the same product?

But in EU?

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Cool project, what parameters have you settled on?

E: for the gear geometry I mean

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And better if I understand correctly.

Helicals are quieter, and due having essentially larger teeth can carry more load on the same size. Only downside is the thrust load, so there has to be a thrust bearing in the design somewhere.

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I mean the only upgrade is water resistant I guess, not to take away from his endeavor it just seems funny that he’s trying to get to the same point that the BN AT is at

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Bruh…

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What? I’m not trying to be rude here lol

All I’m saying is that it sounds like the same path of reasoning to get to a full steel geardrive

Which is cool. Great minds think alike I guess

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I’ve been holding down the fort as boardnamics shill

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:eyes: :eyes: :eyes:

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@Linesflag summed it up nicely. Helical gears are quieter compared to straight cut gears, have you ever heard one of the original straight cut MOON Drives they are super loud. You can also put more torque through a helical drive as more teeth are in contact when meshing (it’s like 1.5 teeth carrying the load instead of 1). The downsides are you must deal with the thrust load, and they are less efficient compared to straight cut gears due to increased friction. .

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Thanks mate, I appreciate that.
I work in product design so I have a lot of contacts for machining aluminium and stainless steel, and have found some companies that can cut gears for me recently.

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Both my drive and Boardnamics use module 1 gears, but apart from that they are a completely different design, as far as a gear drive can go, they are just two meshing gears in a box at the end of the day.

I’ve also put a lot of thought into trying to make my gear drive watertight and corrosion resistant, and my gears will be made of hardened steel.

Yes mate , I’m based in the UK.

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Aside from that there are a few differences, why would that be a bad thing? More competition is better for everyone.

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Correct me if I’m wrong, but hardened steel gears are usually machined and then hardened before being finish ground, is that how your process will work? (At least once you’ve decided on a design)

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Cool cool

Will be interested to see how your sealing system works!

It’s not necessarily, probably came across wrong.

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Thanks mate

All the gears will have helix angle (β) = 15° and pressure angle (α) = 20

I’m planning one having 3 different pinion options:

  • 17 tooth pinion : 90 tooth drive gear (5.29 ratio)
  • 20 tooth pinion : 90 tooth drive gear (4.5 ratio)
  • 23 tooth pinion : 90 tooth drive gear (3.91 ratio)
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Yes, exactly! They are machined first and then the surface is hardened. Different hardening processes can affect the tolerance. I’m not an expert on this, but I found this webpage that explains the different options quite well:

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I 100% support more options, more vendors, and the diy spirit. Very excited to see what you come up with! Partially bc i think this would be a really fun exercise.

I’m most intrigued by whatever solution you come up with for this:

Also, Khk makes very high quality gears. You may contact them directly with questions - they’re a great resource.

That said-

Maybe I’m thick and this was a joke. But this sounds like a mech-e saying “I have a PhD in Mechanical Engineering, so i figured designing a bulletproof 20s speed controller with a built-in 10yr million kilometer antispark would be a quick and easy job! My goal is to correct the simple mistakes of ever other vendor”

:upside_down_face:

Let any of the very talented ppl here know if you need help with material selection, heat treat, drawings, gd&t, tolerances, surface finish, keys, broaching, set screws, lubricants, gaskets, sealants, fastener selection, thread depth, anodizing, prototyping, testing, etc etc

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