Yet another CRF 250 Build but its my first so... ;)

[fmxxx666]

Hi All,

totally new to all of this.
I left the MX hobby about 10 years ago.
Recently visited the local track to smell some 2 Stroke smoke but then a dude on a Husqvarna E MX bike came across and the performance of the bike blew me a way.
I havent really looked into bikes the past years so I have not noticed any new trends in that regards the last thing I knew was KTM came out with the Freeride E but I guess that has been a few years now as well.

So I started googling around and watching some YT vids and finally thought oh great an ALTA sounds about right for me... found one close by for a reasonable price but then a day or so before I wanted to go there and check it out I read about ALTA is out of Business and you don´t get parts anymore etc. so I made a step away from the ALTA of cause.
Stark Varg is just a little outside my Budget right now and it still looks a little too futuristic to me.
I like the looks of a "classic" 2002-2021 MX Bike.

after deep diving into the DIY E Dirtbike Facebook groups I think I can do it myself
A friend had a perfectly working CRF 250 R 2009 Honda standing around and he was looking to sell it so I jumped on it.


Plans for the Build:

Motor:
QS138 70h V3 (the new improved 2024 version)

Controller:
EBMX X-9000 V2

Battery:
no idea yet probaply custom made Amorge one when I cannot find anything from the shelf that would fit in but in general 72V with near 600a peak and near 400a constant and as much AH as I can fit

Throttle:
unsure yet whats gonna work... bought a surron but the twist way is super short... bought a E-cable box but ppl say everywhere they wont trust these... maybe also getting a "Hirschmann dual Hall throttle" we will see what works out.

Rear Brake:
keep it original for the beginning but definalty wanna try left hand rear brake.


Progress so far:
engine out


QS palced with Lithium Kings Universal Mount:



now waiting for delivery of some parts...
Bolts, Nuts, Shrink Tubes, Spacers, EBMX Controller to continue but I will keep updating this thread as the build progresses any comments, hints, feedback welcome

Greetings from northern Germany

 

[Philip]

QS palced with Lithium Kings Universal Mount:

These mounts appear awfully flimsy! Do they work well?

@DonCox

 

[fmxxx666]

@Philip
they are acutally very sturdy.
They look thinner than they are but once in place with the right screws nothing moves even a milimeter.
I have no fear letting 33kw dragging on it

 

[DonCox]

These mounts appear awfully flimsy! Do they work well?

@DonCox

Yes they are made of steel, and work well. Two things I see are the third bolt for the motor and spacers in the front and rear mounts. You said you are waiting for the long bolts for the front motor mount with spacers, ( I use the stock bottom motor mount bolt from the gas motor ), and the long 8mm bolt for the rear motor bolt. You will need spacers in the swing arm bolt also between the brackets.

 

[DonCox]

I have built a lot of these type bikes, and have just finished a 2005 450CRF. I used Lithium-king for mounts and sprockets, QS138 70H V3 motor and the Votol EM260S ( the EBMX X9000 is a very good controller ) and a Amorge battery. ( 20S12P Molicell P42A cells, ANT smart BMS in a metal box that tightly fits into the CRF frame) I had to notch the frame in the 4 corners of the battery. The Battery was $1801 to my door from China. A friend had a same size box battery with 20S13P Samsung cells 50S built by Amorge also, he likes to Trail ride and wanted more distance and lower power. I can send you lots of picture of different parts of the build, if you want. Here is a picture of the bike and the battery

 

[fmxxx666]

Thanks for the input @DonCox
I choose the EBMX controller mainly for its easy to use userinterface with bluetooth and the app, to not have to carry a laptop to the track to quickly change something although I think once you find the right settings you no longer have to touch it anyways.
I am still a bit lost when it comes to this Battery details like "20S12P" is that 72V 50ah? or less? whats the nominal and max amps its able to discharge?
My 250 CRF frame might be a little different from your 450 Frame so I anyway have to tell Amorge whats the max size I can put in and then they should calculate whats the Max AH they can put in at 72v with a Metal Casing and QS10 Connectors.
I am not afraid of cutting / grinding a little on the frame to make more space for the battery (as long as it doesn´t affect the sturdyness of the frame) as I just sold the combustion Engine yesterday there is no plan to ever put a gas engine in it again.

today I recieved the missing parts:

the controller should be with me tomorrow so I can continue on the weekend.
Fix the motor in place and find the right place for the controller.
then I will briefly check how much space I can create for the Battery.

 

[fmxxx666]

Mounting for the Battery I am playing with the idea of doing it out of 8-10mm Carbon sheets cut to size with large bolts going across to the other side with spacers in between.
Saw it on a bike that a company here in Germany (https://www.zweiradsport-brunner.de/) build for a customer: (its a CRF450 2021+ model)

they used a 55ah 72v Battery (I sadly have no more detials on the battery than that)

if anyone wants to see more of that bike beeing build here is a Vlog of the build thats shows at least some details (but its of cause all in german) :

they placed the EBMX controller in the very front of the Frame which I absolutely don´t want as I have seen so many big Rocks knocking on the front fo my eninge in the past.

my plan is to mount it inside the Airbox as I have seen most ppl do it that way.

 

[Theo]

Very cool and interesting project.

If I were you, in order to decide in which position to install the battery, I would install it provisionally in different positions and then I would check the weight distribution of the bike in those positions, choosing if possible the best one.

I like the idea of using carbon fiber sheets, they also look very cool. AFAIK, before machining or cutting them, you should consider the orientation of their fibers. 8-10 mm sounds like something made up of a lot of layers; if their fibers are all oriented in the same direction, then the stiffness should vary a lot depending on the directions of the force applied to them, or there are also sheets with layers having fibers oriented in different directions in order to have similar stiffness with forces applied in different directions.

Considering that both Altas and Starks can have thermal limits, have you thought about something to avoid that on your bike?

 

[fmxxx666]

@Theo good points

I cannot really check the weight distribution upfront as I am not aware of how much the battery will wight in the end and I have to decide for its place before I can find out about the weight... but in general they are something beween 22-35kg I can maybe place something heavy inside a wooden box of the battery size and check.
But relatively middle of the frame where the OEM Engine was sitting should be fine.

have to check on the carbon sheets haven´t had any in my hands yet.
but definatly will look into the details there.
I may build everything out of 8mm wood first to get it sorted and once I have the right design I copy paste it to the carbon.
(have access to a CNC router and laser)

regarding thermal limits I havent spent a single thought yet since so many ppl build their DIY full size MX bikes in that way.
I mean QS138 motor, controller in the airbox and armorge battery middle of the frame if thats a problem I would have read about it before I guess.
But thats the kink of DIY, I guess we will see xD

 

[DonCox]

As far as thermal limits, the QS138 70H V3 is quite good. We ( probably 20+ DIY Bikes with pretty much the same QS138 70H V3, EM260S, 72V 50AH with an ANT smart BMS ) have not overheated the motor, We have overheated the controller without water cooling, and we have overheated the battery. The best counter for the battery is to get the BMS out of the battery box, go to Open Discharge (no BMS control of discharge), and /or make the battery so you can quick change it, and cool it during charge. The battery warms up while being used. If the BMS is inside the Battery box, it induces heat into the battery, your first moto or practice is fine. Come in to charge the warm battery, add more heat. Run second moto or practice, add more heat. Come in to charge a hotter battery and it will allow charge or not because it is already hot, or it will stop in the middle of a charge. It is slow to get the heat out of the battery box.
One friend has made it easy to change the battery, in a 2003 CRF450 . He has the whole build here on this forum Starting my bike build.

 

[Patrick Verner]

I'm considering converting a KX85 for my short wife. Is there any reason the Stark throttle won't work on projects like these?

Throttle

A standard ride-by-wire electronic throttle supplied by specialist German company Hirschmann. The cables are held by a small, handy rubber tie to the handlebar.

starkfuture.com

 

[DonCox]

I believe the throttle used on the Stark is the same one used on the ALTA, it is the Hirschman Throttle. You could use it, but is has a large diameter and it makes it hard to actuate the front brake on the smaller bikes, and a lot of front brake master cylinders. There are ways around it and it does the job, but there are others that are used in the Surrons.