fizzybrain

Ah, that's more like it - a proper first derivative! Maybe bike-roll had a facility for a proper plot of gradient, but I couldn't find it. That route (and back, obvs.) is pretty much all I expect to be using this bike - whichever one it ends up being - for. Will inspect other donor bikes over the next few days.

Thanks for the responses everyone. My main concern was that 250W wasn't going to be enough without some extreme gearing, but it sounds like it won't be a problem (for a mid-drive or a hub motor), so that's good. The "48% gradient" is just the maximum gradient of that route from bike-roll.net. Like I said, it's probably an exaggeration - perhaps even caused by a mapping error - and it's hardly representative. I know the bit it means and it is indeed really steep, but it's very short - a decent run-up could maybe do it. Agreed - Escape Route is a good shop; they helped me out with some tyre sealant for a mobility scooter. I'll see if I can get a quick lend ;-) So the "walk assist" limits current, not just speed? Oh well, it was a nice thought. Good to know that a hub motor should be an option if I can't find a donor bike to which I can fit a mid-drive motor, which still seems greatly preferable. I have several potential donor bikes in my cellar, but it looks like I'm going to have to find out about what kind of bottom bracket would be suitable for a mid-drive - the last time I opened up a bottom bracket it was on a bike with cotter pins! The bikes in question are newer than that - but older than 2012, which seems to be in their favour - with these new-fangled cotterless cranks. Back in the nineties, when I was in my twenties, I was riding up hills like these on a bike from the sixties - with forward-sloping dropouts. But yes, good advice, the hefty pull is something to bear in mind (as some of my donor bikes are likely to have them too) - some sort of chain tug (Tuggnut, Simson, home-made, etc.) would be a good idea. I see no reason why a responsibly-ridden ebike needs disc brakes any more than a regular bike. As always, I will ride no faster than my brakes can handle at any time, but yes, disc brake mounts are a good thing. At least one of the (other) donor bikes will accept that bike's back wheel with the SRAM S7 IHG, so I'll try to use it if I can. I was forgetting that a mid-drive motor will be naturally give the best operation at a human cadence, so I have done the sums that I should have done in the first place: Based on 60rpm cadence, with 42T chainring, 700c, and 18T cog, I get a speed range of 6.6 - 20 mph. With a 23T cog I'd get 5.2 - 15.7 mph. That seems pretty good - nice and slow :-) There is indeed a coaster/backpedal brake, which goes against my lifetime cycling habits. If I can backpedal without engaging it - because of the motor - then great. If backpedalling engages it, then I'll get used to it (or disable it - internet says "possibly"). Finally, considering that particular bike: I think that that bottom bracket is pretty standard, though it is not obvious from that photo and I admit I don't know much about what is considered "standard" these days. It is 68mm wide, anyway. I will know more when my new crank puller arrives.... (The suspension hinge is on the left, the rest is fully welded) Yes, over-thinking and too much research - but until the weather improves.... Thanks again for all your insights. Lots of options, it feels like progress :-)

Hello all, lovely to be here. I have this big heavy bike which I think came from Germany and seems to be very well-suited to riding around in comfort on mostly flat plains. However, I live in Pitlochry, which is far from flat. This, specifically, is the route that I wish to use to take my aged, 72kg body up the hill to my even more aged parents: (the 48% gradient is the steepest bit and is probably an exaggeration, but it is pretty hellish in places) I figure, though, that that bike would make a nice comfy pedelec, having front and rear suspension and a 7-speed internally geared hub. I will state right now that I have no interest in going faster than 25km/h - or, frankly, going anywhere else - I'll be happy just to get up that hill without getting off and pushing, and without arriving at my parents' house looking like a bit of sweaty lettuce. In fact, if the thing had to be geared to give peak efficiency at only-just-fast-enough-so-that-I-can-stay-upright, that would be sufficient. The way down is easy - if I'm feeling brave I can pretty much get all the way down without pedalling. From reading this forum and applying a bit of engineering common sense, a hub motor is not going to be very happy with those hills, so it'll be a mid-drive motor, with the hub gears helping the motor. The SRAM-S7 IHG effectively has between 10.3 and 31.4 teeth on the rear sprocket, with its current 18t cog - I might consider getting a larger one for higher maximum torque if it appears necessary, and sacrifice my top speed (both motorised and leg-powered). So the idea is that I will be in a super-low gear pretty much all the way up, with the motor spinning the chainring pretty fast. Questions - with a mid-drive motor, does the chainring only ever spin at the speed of the cranks? Even though the motor will be doing the pushing, pedalling with a high cadence will still be far from ideal. I can imagine that it would be possible to have a torque sensor system that didn't "tie" the cranks to the chainring, but I can also imagine that that would be complex and would not really be how torque sensors are intended to work anyway. A system that tried to spin the chainring at a multiple of the speed of the cranks would be nice; or gave current as a function of cadence (or even applied torque), that sort of thing. Can mid-drive motors work like this - like the "primitive" cadence-sensing system of hub motors? Questions - I have never ridden with a speedo, but I can cycle pretty slowly (sorry if that sounds like bragging ;-) ). Is it possible that the "walk throttle" - moving the bike at 4mph(?) - will get me up that hill without pedalling - do you think one can cycle that slowly and stay upright? What's your *minimum* cycling speed up a hill? Is a walk throttle a true throttle, or just an on/off switch? Question - I am aware that the 250W power limit is about rated continuous power, and that some motors are ... helpfully conservative in their rating. If there was a mid-drive kit that could handle significantly more than that at peak while still being legal in their rating, then that would be very helpful. Suggestions? (IMHO, speed limit = good, power limit = bad, but I'm sure that that discussion has been done to death in these forums) I suppose at the end of the day I will just have to put up with whatever I can get (at the moment I'm thinking of the cheapest tongsheng unit), but I'd be interested in hearing any thoughts or advice. The keen-eyed reader will see that it has a backpedal brake. AIUI, the mid-drive motor will render this useless. This pleases me greatly, as I hate it. Will I actually be able to pedal backwards without the chainring rotating backwards and activating the brake? It might also be felt that the crossbar is rather high for that seat position. It is. I will consider getting myself a steel codpiece or a pair of platform shoes, or both.... Thanks for reading my not-so-little story, looking forward to joining your community and getting my cycling (and tinkering) confidence back, Ian