Carrera crossfire 2 bafang 750w

[97alex]

Hi all,

Sorry if this questions been answered in the past, I've tried googling but struggling to find any answers.

I have a 2018 carrera crossfire 2, I want to order a bafang 750w kit to use off road on private land, What chainwheel size? 40,42 or 44?
Will it fit nicely in the bottom bracket?

Would appreciate any help, thank you so much

 

[mr_ed]

How many teeth do the existing chain wheels have?

I did a spreadsheet for mine to work out the speed in each gear based on the rpm of the chain wheel. i geared mine for a max of 40mph flat out, I’ve software limited it to 25 so the top gears are a kind of low rpm/overdrive though.

mine has 29 inch wheels, a 11t smallest gear on the cassette and I went for a 40t chain wheel. I have a 11 speed cassette with 11-51 range but barely use the lowest few gears.

 

[mr_ed]

I’d probably for starters consider the same number of teeth as your largest chainwheel. You'll have the assistance of the motor to counter the higher gearing. The clearance should be ok as the motor will push it out to kinda the same position it is in now.

 

[Texy]

I've also now got the exact plan, although I've just bought a brand new crossfire 2. The largest existing chainwheel has 48 teeth, the rear cassette is 8 speed, 11-34 teeth, and the bike has 28" wheels. As I'd be doing more road/towpath cycling than any serious off road or hill rides, I was going to option the largest bafang chainwheel I could find within reason and in stock, so this looks like it'll also be 48T.
I'm also interested if anyone else has done similar, or if the OP was successful ?
Also @mr_ed you mentioned a spreadsheet to calculate the speeds for each gear - is this available, or perhaps another online calculator ?
Cheers,
Texy

 

[mr_ed]

I knocked up a spreadsheet in excel:

1. Based on knowing the number of teeth on the chain wheel and the RPM of the pedals you can calculate teeth per minute on the input to the chain
2. From that teeth per minute, with the number of teeth on the cassette you can calculate RPM of the rear wheel.
3. From the diameter of the rear wheel you can calculate MPH.

I knew the max RPM of the BBSHD was 120 (if I recall correctly) and I knew my wheel size and rear cassette size, so ended up deducing a 42 tooth chain wheel was right for me (max theoretical speed of 37mph). I'd have gone lower but 42 is the smallest available.

I think this will do the same thing:

Bicycle Gear Speed Cadence Table | BikeCalc

Calculate cycling speeds at a specified cadence given chainring, sprocket, tire and wheel sizes.

www.bikecalc.com

 

[Texy]

Thank you @mr_ed

 

[mmcc81]

Hi all, I'm a noob when it comes to bikes and I'm looking for a little advice.

I have a crossfire 2 and I want to fit the bafang bbs02b on it, is this possible?

Also, if it is possible, I'd like to understand if the gears will need to be adjusted in some way as most of the gears seem to change at the crank! I hope that makes sense as I'm not sure how else to explain it.

If anyone could help I'd appreciate it.

Many thanks in advance.

Mick.