meniosgko

Hello there i have a sirrus specialized 2020model 700c. it runs 3x8 speeds. i don't know how to translate it further. i will buy a bafang 750w mid drive motor but it has the option to use 44T,46T,48T,52T. Here where i live it has many ups and downs it has many hills. which i should buy? should i change the rear cassete also? i know that 44is more for hills right? and 52 eg is more for speed. correct meif i am wrong.

KPREN

You can convert as long as your bike is not a carbon frame. I would recommend and aftermarket chainring like Liekie or Luna . The Bafang chainrings are chain losers and therefore are tossers. Your top speed will be determined by voltage and watts as well as the chainring you choose up front. No matter what you choose you will top out around 28-29 mph. With a 52 tooth up front, you will likely overload/over heat the motor or top out from watts. To sustain high speed with the Bafang you need a high cadence. Bafang mid drives are very good units but have their own learning curve.

You can play around with the e bike simulator over on Grin Technologies They have some wonderful resources and tools. Good company to deal with.
https://ebikes.ca/tools/simulator.ht...axis=mph&tf=46

andychrist

Yes a 44T chainring is certainly best for hills but there are aftermarket kits for the Bafang that will allow two chainrings (though they may not be suitable for every model of bike). Also the new Tongshen TSDZ2 is available from EcoCycles with up to three chainrings. Alternatively, a hub drive such as this 500W front wheel Bafang will allow you to retain all your current gearing. They also carry a rear hub drive but the front one would be easier to install and less expensive. There are other companies such as theebike offering higher wattage hub drives, not sure whether their kits are as nice as Bafang’s — their P850C display is tops.

KPREN

What you will likely find with playing with the simulator is that a 42 tooth ring is probably your best bet. The 42 tooth aftermarket rings are the smallest rings that give a correct chainline.

You will likely find out that climbing and into the wind is more important than a mile or two an hour faster at peak. You can't sustain peak for long either.

andychrist

An 11-42T 8-speed cassette will go a long way toward leveling out the hills on an e-bike even with a chainring as large as 52T. But a just a 48T will still allow you to pedal down hill at 30mph, and easily climb the steepest gradients with the chain shifted to the 42T inner sprocket. Might have to install a derailleur hanger extender to handle the wide range, no big deal as they’re available for cheap.

Other thing to consider is wheel build. Can your current Specialized rear wheel handle the increased torque a 700W mid-drive would apply? Nice thing about hub drives, they are built to withstand the torque they put out.

2old

If you have a 3 X 8 it must certainly have an aluminum frame (carbon models wouldn't have 8-speed rear IMO). Check the bottom bracket width; if 68 mm, you can proceed with confidence, but if 73 mm you'll need to (at least) use LockTite on the bottom bracket fastener since there won't be enough space for the "jam" nut; preferably use a retainer arm like Luna and others provide. I went up the steepest off road hills in socal with a 52 front and 11-32 rear, but for optimal shifting performance, you should consider a 42 tooth Lekkie front chainring. This should provide about 30 mph with 11-34 gearing (pedal speed = 90 rpm) and should get you up any hill even if you need to up the degree of assist temporarily. Also, I think Sirrus employs 14 gauge spokes and 38 mm tires, so the wheels should be very robust IMO.

Pop N Wood

According to the place that sold me mine 42 tooth chainrings are the most popular option. Have it on an 8 speed rear and have to say it is an excellent choice. Bike can cruise at 38 mph on the 11 tooth rear cog with me pedaling. That is plenty fast.

Going with a 52 tooth ring won't overheat the motor as long as you keep the pedal speed up and don't bog the motor.

2old

You must enjoy pedaling at 130 rpm; might get the speedometer checked unless you're going downhill.

gsa103

On a bike like a Sirrus, I'd go for a 48t, unless you have lots of really hellacious hills (>8%). A 48/28 combination is doing 8mph at 60rpm. Most people riding an ebike are going to want to go faster than that uphill. Meanwhile a 48x14 gear is a nice sweet spot at 22mph at 80 rpm, which is a good cruise speed and rpm for flat ground.