bonzobanana
Guru
If the Sirrus has a steel fork a front hub motor would be ideal for it. This might mean you don't need a torque arm and front hub motors can be relatively small and light. On a low power 250W front hub motor there aren't really issues with loss of traction and in fact you would have 2 wheel drive which means better traction on looser surfaces like sand and grit. It also keeps your comfort levels up as you have full length spokes on the rear wheel plus the fact you don't have all power through the back wheel would mean less spoke breakages at the rear. Either hub type you wouldn't need a huge capacity battery so can keep the bike lighter and geared hub motors have no drag when unpowered so the bike would be easy to cycle unpowered. The front hub motor also means you can easily swop it back to your original wheel if you want to re-use it as a standard bike thanks to the steel forks. That would be made more difficult with torque arm or arms on a aluminium frame. Also bear in mind an aluminium frame sometimes needs the dropouts filed down a bit to accomodate the rear hub motor axle, this can compromise the frame a bit depending on design. That isn't normally an issue with front steel forks although you may have to file a bit it won't normally compromise their strength and normally much less material needs to be removed with forks.
Also because hub motors are independent of the drivetrain they can massively extend the life of the drivetrain up to about 3x over a standard bicycle because on the hills the hub motor is doing more of the work which reduces the wear rate massively. It really depends on how you cycle though. If your the type of rider who enjoys more leisurely hill climbing then there can be a huge benefit but if you are the type of rider who will put in the same effort as a normal bicycle and just go up the hill 3x as fast then not as much benefit but overall there is always a huge gain in drivetrain lifespan hence why many hub motor ebikes just have basic tourney bottom end drivetrains even on high cost £2000 plus ebikes. The drivetrain quality becomes less important with a hub motor. The chances of chain snaps is massively reduced over a conventional bicycle.
Throttles tend to also help with drivetrain lifespan as they operate immediately where as cadence sensors create a delay in operating and if you are doing a standing start going up a hill there is a delay where the chain is under very high load until the hub motor kicks in.
The front hub motor also means the steering is slightly heavier but depending on bike you may prefer this. The Brompton ebike steering feels better than the more twitchy steering of a standard Brompton for most people.
