View Full Version : Help with Motor sizing for a motocross bike conversion

26 June 2013, 2137
Hey Guys,
I have been reading through the different topics on here but I am getting all sorts of confused on what motor to uses!!!!
I am wanting to do a conversion on a Honda CRF150R and donít 111 want to lose any performance on hp but gain on Torque. The CRF150r are around the 23hp range. This will be fairly similar to a conversion of a 125 2 banger!
I was looking at the PMG-132. What are your thoughts on this? Weight will be an issue i am trying to keep it low. I am pretty happy with getting the gearing sorted out but need to lock in the motor side of things.
Once I get this sorted I can get the CAD model done then start mocking it up.
Any help would be great thanks guys
Cheers Simon

26 June 2013, 2339
Hi Simon. Welcome!

Sounds like fun project! What cells are you planning on using? As one who is converting a 350 dual sport, I can say space will be at a premium in a 150 frame. Aside from the straight up power of the motor choice, the cells will have to support that motor's current discharge rate to achieve that power. Will this be a distance runner or a track sprinter?

27 June 2013, 0038
Hi Robo, haven't got into the battery side yet, it is going to be a track rider so looking at around 30min per battery pack. Hoping to get max voltage around 72v which might be achievable with a shorter run time. Any good recommendations for battery's that I can stack

27 June 2013, 0203
I'm working on a Suzuki RM125 right now, and I'm using an original Briggs ETEK. Should be perfect - really light, really strong. The battery module is the same as on my cafe bike but you could easily set it up for 72 (80)V but these are 40V and 20ah with the Headways. I can also fit one of the CALB modules in there - 40V at 40ah. I'm not going to worry about higher voltage because I'm old and I ride trails. Slow. :cool:


Oh. ...where are my manners? WELcome! :D

27 June 2013, 1017

I played with the idea of converting the same machine and picked up a bare frame to start trying to fit stuff into. I came to the conclusion that there was not enough space to fit the pack I wanted and a motor in the space.

I was using Headways and the Perm at first and then thought about a hub motor to get the battery space I wanted. I ended up abandoning the project because I was not satisfied with how the hub motors performed and could not stuff both the Headways and Perm into the space.

I think it could be done using a different battery and perhaps having less range than I wanted.
I will try to dig up some of the pictures I took during the process.

27 June 2013, 1114
With my conversion, I had no choice but to think of the motor and cell pack together as a system with the controller to make everything fit. But even then, I'm using two rear pannier cell packs in addition to the pack under the tank. I'll be running 60v (64v nominal) with 20 60ah CALB cells. My application doesn't require the extra motor rpm from 72v. Using 40ah cells would have been a lot easier and the result would probably be prettier, but I preferred the higher 60ah current discharge rate over the 40s. I also wanted a bit of range. You'll be more of a sprinter, so your needs are a bit different than mine.

Deciding the performance criteria up front leads to what's needed to get there...can save big money and effort up front!

I'm using a Motenergy ME1003 motor which is a bit larger (and a bit cheaper) than the PMG-132. There are a lot of nice conversions using the PMG-132. Here are two (very) in-progress blurry pictures showing the space in my Suzuki DR 350's frame. This was when I was just fitting everything together:


That front pack only holds twelve 60ah cells...the other eight cells will be split into the two 4-cell pannier packs.

I'm using CALB large-format LiFePO4 prismatic cells (you could use GBS or other LiFePO4 cells as well). The prismatic cells are kinda big and their C rating is lower than a LIPO pack, but for my needs, they are easier (for me) to work with and they meet my performance criteria.

Like you said, a track bike is preferably light with a responsive and torquey throttle. Being responsive means potentially high current needed popping out of turns and whatnot. Those current demands can challenge the discharge rates of the "brick" LiFePo4 cells depending on the Ah size used. Pulling too much current too fast can heat them up and reduce their life. For most, they're generally not considered cheap enough to expend that way. But, there are many, many very nice conversions with the LiFePO4 prismatics, if not the majority.

My point is to simply make the numbers work for your performance criteria up front.

A LIPO pack could also work really well for your application (higher C ratings than LiFePO4, lightweight, smaller). However, these cells require some care and feeding beyond the LiFePO4 prismatic bricks. I don't know your background, but if you know the complete absolute ins and outs of lipo pack charging to mitigate fire risk, you could make a snappy track bike with these as well. But, you gotta know them. Most first e-moto conversions have used LiFePO4 prismatics.

28 June 2013, 0819
Welcome Simon_Smith,

You found a great project to work on. Use the CAD (cardboard aided design :)), it gives you a good idea on what will fit. Also look up the guys in your area who might be building Ev bikes or autos, they can give you some ideas.

Los Angeles

28 June 2013, 0840
Agnimotor and Lipo! Gear it for 50 mph.