Converting this bike with a mid-drive motor for climbing hills - I have questions!

[fizzybrain]

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

 

[saneagle]

I hate to have to say this, but just about everything above is wrong.

Let's take the bike first. It would be towards the bottom of my list for a crank-drive conversion.
1. It has rim brakes, which are never good for an ebike unless you're very light. 75 kg is about on the limit, but those brakes will always be shite by comparison with hydraulic discs. It looks like there's a disc mount on the fork, so you could upgrade at least the front.
2. It has those diagonal drop-outs for the chain tension. You'll be lucky to get enough tension on the axle nuts to prevent the axle slipping under full power.
3. It has a weird bottom bracket. How wide is it and is it the standard Shimano/BSA type? There is nowhere to anchor a TSDZ crank motor, so you'd need to fabricate a torque arm somehow.
4. I can't see any bottle screws for the battery fixing. Does it have any?
5. I'm quite keen eyed, but I don't see a coaster brake. Instead I see a conventional brake with a lever on the handlebars and a cable running along the crossbar to the rear caliper brake. If it did have a coaster brake, that would be another unnecessary problem to solve.

Basically, if that were the only donor bike in the world, you could do it, otherwise it makes no sense.

If I understand it right, your idea about making power related to cadence is terrible. Our Chinese friends had the same idea. I tried it, and it's diabolical. Every time you change to a higher gear, you lose power and have to change down again, then the higher power spins up your feet so you need to change up again. You can never get comfortable pedalling. The crank spins at pedal speed and is designed for a fairly low cadence of around 60 to 100. Crank-motors are not good for spinners.

There are bikes with 250w hub-motors that would have no trouble going up that hill. You can find ready made ebike with them that can do it, but I haven't seen anybody offering them in conversion kits, though you can get them if you buy them as individual items from Aliexpress and put the right bike (20" wheels" with them.

I'll summarise the rest by saying you're over-thinking everything, probably because of too much research and not enough actual riding.

If you want a bike to get up that hill without much effort, you should get a standard MTB with disc brakes and a triangle frame, then fit a 48v TSDZ8 motor kit from Woosh. I use the TSDZ02 quite a lot, which is the same , but less powerful, and I regularly ride up such hills without any effort, though I would be going slowly in a low gear. The only difference with higher power is that you can use a higher gear to go faster; however, the ride and gear-shifting become much harsher when you have more power on a crank-drive bike, though you can always turn the power down if you don't want it, but if you don't have the higher power, you wouldn't be able to turn it up if you needed it.

The walk assist doesn't have enough power to get you up a steep hill, but you don't need it. You can just pedal without effort. The TSDZ motors do not give power in proportion to pedal force, so you can get good power without much effort. Mine has two levels of power for each of 4 power settings. You get about half the power of that setting just by turning the pedals then the full power when you pedal a bit harder. I'm guessing all TSDZs are like that. The Bafang BBS01/02s have many power levels, but they only give a fixed amount of power fro each one regardless of how hard you pedal. Some ready-made bikes have control systems that give sort of power proportional to how hard you pedal, though they're not as simple as that, and if you did have it, it would be terrible, so they have to do smoothing and add in other factors to make it more convenient.

 

[Peter.Bridge]

I know people get attached to their bike, but it doesn't mean it's a good candidate for conversion. As @saneagle says buy a bike off Facebook marketplace with disc brakes, a normal square taper bottom bracket and a decent size frame triangle. I converted an old Gary fisher MTB for my sister who is heavier than you using the woosh TSDZ8 conversion kit. It has a twist throttle to give maximum power up a hill if you need it and she has gone up a hill that climbs 700ft in a mile, so comparable to your hill. Just be careful with flared chain stays and use the tsdz8 template to make sure it fits the donor bike before you buy it. A 68mm bottom bracket gives you a bit more room to play with than a 73mm bottom bracket

You can see the gap between the motor and the chainstay

 

[matthewslack]

Start with a straightforward donor bike and try again with the gradient estimation. The average is only 11.4%, which any ebike should manage. 48% is out of range for all but extreme MTBs.

BBS01 with settings tweaked will give you way more than 250W for long enough to get up most hills, and is simple cadence sensing. You need to check which versions, if any, currently allow settings tweaking before buying.

The TSDZ8 is usually a 750W motor, but Woosh have a version labelled 250W, so kit with torque sensor, that is your route.

When you know the max gradient more accurately, hub motor folk can provide reassurance. But 48%.... not going to work.

The Escape Route bike shop seemed helpful when I needed a charge a couple of years ago. They might be able to show you what an off the shelf torque sensored mid-drive is like on that hill as a comparator.

 

[Nealh]

Fwiw walk assist is just that it doesn't deliver high current to be used for hill climbing , enough current is gently supplied to enable to walk/giude abike at 4mph walking pace.

 

[fizzybrain]

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

 

[Nealh]

Walk assist current will depend on the controller used but suspect 2a or 3a isn't far off.

 

[saneagle]

I will know more when my new crank puller arrives....
View attachment 62298

That's certainly not a normal BB. There's no lockring for a cup and cone type and there's no spline for a normal BB tool.

 

[guerney]

Converting this bike with a mid-drive motor for climbing hills - I have questions!

My bike's aluminium frame bends a little when high torque is applied by my cadence sensing legal 36V X 20A = 720W BBS01B mid-drive motor. The entire front of the OP's bike frame ahead of the BB pivots up and down? I really wouldn't install a powerful mid-drive on this bike, because the there would be too much force acting on these plates either side of the BB. Over time, I fear they'd bend or snap, or both.

I strongly suggest you find a more suitable bike to mid-drive convert.

I wish to use to take my aged, 72kg body up the hill to my even more aged parents

You don't want to die before your parents.

 

[Peter.Bridge]

Would you be able to map your route on mapometer and tick the show elevation and show gradient tick boxes to get another estimate of the gradient ?

Yes , the right hub motor will very probably get you up that hill (especially as you are relatively light). Woosh do a
DWG22C 48V 60NM rear hub.

I generally slightly prefer hub motored bikes for road use as I find them more relaxing to ride, but if the vast majority of your use is up and down that hill then a mid drive kit may be more suitable

 

[gw8izr]

In my experience the old TSDZ2 mid drive will drag me and my heavy bike up much steeper hills than you might expect .. you may have to go a bit slowly and wind your way along but the motor is plenty powerful enough. The new TSDZ8 appears to be even better so ... The torque sensing assist is excellent (to me) and you just press a tiny bit harder when you need to.

However in my experience this next bit is important.. please don't start off with an unsuitable bike for your conversion. You should factor in for decent brakes for descending steep hills. Bear in mind that a heavy bike with rim brakes is exciting enough in the dry.. never mind the wet.

Starting with a better bike should make the whole experience more pleasant from build to use.

HTH Paul