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I'm looking convert a mountain bike and convert it to electric with a rear hub kit. I am wondering what features I should look out for to make this easier:

  • Is there any way to keep the cassette that comes with the bike or does this have to be replaced? Does it matter if it's a "freewheel" or "freehub"?
  • Will the front crankset need to be replaced with a larger gear for the higher speeds? Currently looking at a bike with a 40t large gear.
  • Does it matter which brakes and shocks come with it?
  • Do mountain bike tires work well at higher speeds or does it make sense to get something less "stubby"?
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    Am I reading it right that you're planning to first acquire a bike and then convert it to an e-bike? If so, is there a particular reason to not just get an e-bike in the first place?
    – Wsal
    Jul 21, 2019 at 11:43
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    Because, it's my bike. I enjoy custom projects and I enjoy customizing it the way I wish. I posted this thread looking for technical pointers, not looking for a everyone's personal opinion. This is a question and answer site, not a forum.
    – SurpriseDog
    Jul 22, 2019 at 5:20
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    Benjamin, that comment sounds rather harsh. @WaltoSalonen asked a relevant question to put your question into a context. If the answer were 'because I want an e-mtb' answers to your question will be different than that to 'I like difficult hobby projects'. What is more, since a converted MTB will perform much worse in all aspects than a purpose built e-mtb one it is only prudent to ensure you are aware of that.
    – gschenk
    Jul 22, 2019 at 8:21
  • Indeed, while you might have plenty of experience regarding custom projects and aI good understanding of their feasibility compared to purchasing ready-made solutions, this question would be more useful to other people if it had some context, like an explanation of why you want to go this way. Nothing wrong with customizing tho, I occasionally enjoy that too.
    – Wsal
    Jul 22, 2019 at 11:40

3 Answers 3

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The cassette and hub will depend entirely on the conversion kit - is it a front wheel, rear wheel or center drive? If rear wheel, you probably will replace the whole wheel with a choice of swaping the cassette over or buying a new cassette.

It might not be possible to install a 40 tooth chain ring on some modern mountain bikes. While the 3x bikes of yesteryear regularly ran 44, its rare for a MTB today to run bigger than 38, and 32 or 34 is most common. The frame designers are probably not considering large chain rings as an important feature in the difficult and crowded area around the BB (on Soft tails). You would need to check the frame to be sure.

To me, the main consideration is the ability to put an interrupter on the brakes, that cuts the engines when the brakes are applied. This means you will need E-Bike specific hydraulic brakes (Most E-Bike kits have levers for cable brakes with a switch built in). There are after market adapters for hydraulic brakes, but they reply on a stick on magnet and switch.

Knobbly MTB tires work best on soft conditions, slicks on smooth pavement. The E-Bike aspect changes nothing. (Keep in mind an elite rider can probably output more power than a weekend warrior with a 500W motor). Install the tires that suit the trails you ride.

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  • Updated above. It's a rear wheel kit. -- About the brakes, are you saying that a bike with hydraulic disc brakes won't work well with a conversion kit?
    – SurpriseDog
    Jul 21, 2019 at 16:29
  • When the brakes are applied, the electric motor needs to be shut down. They need a sensor on the brake handle. Most kits come with a brake handle for cable brakes. With Hydraulic brakes, you have buy a E-Bike brake with the sensor, or stick on a switch and magnet to the brake lever, which IMHO is (being polite, its a family show here) not ideal.
    – mattnz
    Jul 21, 2019 at 20:19
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Frankly, this question is too broad. You need to research what's out there and what other people have done, and this is not the place to do it. Read up in places such as endless-sphere.com And be realistic with yourself about how you will use it.

To attempt to answer your specific questions:

  • it depends on whether you go with a rear hub motor, front hub motor, or one that mounts on the bottom bracket.

  • same as above, and also depends on how fast you are planning on going / will the bike be pedal assist? Throttle only? A combination? Also note that most jurisdictions have laws regarding the maximum speed of ebikes.

  • brakes and shocks? Depends on the type of riding you will be doing. Is it all going to be on nicely surfaced roads? (do you need shocks at all?) On mixed terrain? Will you be riding in wet conditions? (Maybe disc brakes would be a good idea...) or fair weather flat roads? (V-brakes are probably fine?) Don't waste money on cheap suspension. You'll be better off with none in most cases.

  • Not sure what you mean by "stubby"... large blocky tread? Are you going to ride on pavement? Gravel? Mud? Get tires that are appropriate for the riding you will do.

No one off the shelf MTB is going to fit the mold for whatever it is you want to do. You will need to research what fits your needs.

Do you want a steel frame? 8-/ aluminum, carbon :-(

There's a lot more you will need to research to come to an initial conclusion (speaking as someone who has done it... good luck)

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New point - added weight.

No matter what you do, adding a motor will add the weight of the motor, wiring, batteries, and controller. In a modern lithium-something battery this will probably be below 10 kilograms, but that's very roughly doubling the standing weight of the bike. It will feel different with that change, more-so if you mount the batteries high up.

I suggest you buy the bike you want, don't look to mod a brand new bike. A new ebike would have the batteries placed inside the downtube, or aft of the seat tube. This helps overall weight distribution.

A retrofit kit might use up a bottle cage mount which isn't good because as noted, batteries are heavy and two little M5 bolts are barely up to the task.

Other retrofit kits include a carrier/rack and locate the batteries high over the rear axle. This is particularly sub-optimal because the added weight is 100% on the rear, which is moving the balance away from the 60/40 ideal.

TL:DR; get the ebike you want - buying a bike to retrofit an electrification kit will result in a poorer bike than one built to be electrified.

Rephrasing that on the question - look for a bike that lets you mount the batteries inside the frame, or in a properly secured way that is low on the frame. Not a bottle cage mount, and not a rack mount. Downside - they don't exist, so if you want an ebike, buy an ebike.

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  • Yes we all know that batteries make a bike heavier. However this opinion doesn't answer any of the questions in the OP and would be better suited as a comment.
    – SurpriseDog
    Jul 22, 2019 at 6:09
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    @Benjamin fair point - answer modified to suit. Leaving the flag for another mod to decide.
    – Criggie ♦
    Jul 22, 2019 at 7:24

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