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specialized sirrus

Posted: 2 Jun 2020, 12:18pm
by theballboy
Anyone converted a Specialized sirrus hybrid?
I have that as an option for donor bike (but not sure of bottom bracket size if anyone can signpost)

Im looking at a bafang mid motor 1000w with 52v 17ah - frame is steel but forks carbon
Would have preferred suspension in the forks but maybe could upgrade at some point

Hard to get hold of seemingly at the minute unless anyone can signpost who is selling/ shipping quickly

Thanks in advance all

Re: specialized sirrus

Posted: 2 Jun 2020, 12:59pm
by stodd
Don't forget that any motor over a nominal 250w makes a bike illegal to ride on road without plates, insurance etc, and illegal to ride in any off-road public place such as a bridleway. Good 250w motors can actually generate up to 750w for short bursts.

Re: specialized sirrus

Posted: 2 Jun 2020, 2:01pm
by theballboy
Yeah, thanks . I was aware of that
I know they are switchable though so can operate at legal limit when on road and at full limit off

Re: specialized sirrus

Posted: 2 Jun 2020, 5:35pm
by stodd
It remains illegal on the road if switchable, even with the switch on low. And remains illegal off road in public places. But you are very unlikely to be caught if you ride sensibly.

Re: specialized sirrus

Posted: 2 Jun 2020, 9:40pm
by philg
I know zilch about mid-motors, but I understood that the 500+ Watt hub motors are pretty useless apart from high speed burn ups due to lack of low speed torque, i.e. great for speed but useless for hills. Is this correct?
Does the same apply to mid-motors?
And does being able to use the bike's gears mitigate this to some extent?

Re: specialized sirrus

Posted: 2 Jun 2020, 9:57pm
by hemo
philg wrote:I know zilch about mid-motors, but I understood that the 500+ Watt hub motors are pretty useless apart from high speed burn ups due to lack of low speed torque, i.e. great for speed but useless for hills. Is this correct?
Does the same apply to mid-motors?
And does being able to use the bike's gears mitigate this to some extent?


As wrong as you can be.
Any geared hub motor can be high torque if the rpm winding is low enough, typically 160 - 201 rpm.

Re: specialized sirrus

Posted: 2 Jun 2020, 10:41pm
by stodd
I think you are talking a little at cross purposes.
Many (?most, especially the cheap ones) 500+ watt hub motors are direct drive, and have the issues philg mentioned.
But, as hemo points out, the geared hub motors do not generally have those issues, especially if suitably wound and geared.

I'm pretty sure any 500w mid motors would be geared.

You'll get more specialized advice at https://www.pedelecs.co.uk/forum/forums ... ussion.42/

Re: specialized sirrus

Posted: 3 Jun 2020, 12:33am
by theballboy
Obviously a total noob question but if I purchase a bafang - and opt for a 42t, I lose use of my 32t chainring? and thus the left microshift correct?

Re: specialized sirrus

Posted: 3 Jun 2020, 9:04am
by stodd
theballboy wrote:Obviously a total noob question but if I purchase a bafang - and opt for a 42t, I lose use of my 32t chainring? and thus the left microshift correct?
Some mid drives will accept double chainrings, varies with details of the spider and chainline. I've not heard of one that takes a triple.

I'll repeat my comment above: You'll get more specialized advice at https://www.pedelecs.co.uk/forum/forums ... ussion.42/

Re: specialized sirrus

Posted: 3 Jun 2020, 8:13pm
by hemo
theballboy wrote:Obviously a total noob question but if I purchase a bafang - and opt for a 42t, I lose use of my 32t chainring? and thus the left microshift correct?


The Bafang and Tongsheng both will accept 2x chainrings with a mid drive 5 hole spider adapter chain line will be compromised for some gears, 32t is the smallest you can get away with.