BBS01 250W gears and max gradients

[notfatjustbigboned]

Hi,


I have had the BBS01 250W for a couple of years and have done a couple of thousand miles so far. I sometimes commute to work (52 mile round trip) and according to this website (https://www.doogal.co.uk/RouteElevation.php) the maximum gradient I encounter is 9%. I am not a light person (21stone - think a taller Bob Hoskins, not John Candy) and wondered what the effect of losing some weight might be on my gearing, and what would be the maximum incline I could manage if I took the bike on holiday.

This website gives all the equations needed (https://www.gribble.org/cycling/power_v_speed.html) and with the input data below I produced the power versus gradient versus gear graph below it. I ride a trek 7500 hybrid, 700c wheels with Schwalbe 35mm tyres. The dark green region is provided by the BBS01 (35V*15A*0.75 = 405 Watts, i.e. 75% efficiency at 67rpm). The light green, amber and red regions are my own power input (yours will no doubt be different). Note the data is valid for the BBS01 crank rpm which gives maximum power output for the BBS01, and the speed is only valid for required powers above 405W. If the intersection of gear and gradient is below 405W, you will end up going a bit faster. Going slower reduces power required, but the power output of the BBS01 drops off as below. Essentially the data is for max power output of the BBS01.






Sorry - I don't know where I got this from - I think it is for the 18A/350W BBS01, but I assumed the efficiency curves v rpm were the same for the 250W BBS01.

The results are in line with my experience for the 9% hill (I use 42x32 which feels easy, 42x28 is harder but very doable, but I wouldn't/can't use 42x24).


I have added graphs for different total weights so you can use/interpolate to suit. I have learned so much here and over at endless sphere, these are just superb forums - I hope it might help someone who is considering the BBS01 for its hill climbing ability.

 

[Deleted member 4366]

That's really useful info. thanks a lot.

I looked at that elevation finder, but it's wildly off. I have three routes that i regularly do up Ironbridge Gourge. One is a long steady climb, that is fairly comfortable. The second is shorter but steeper. The third is impossible on a normal bike and needs a really high-torque motor to get me up it. That tool gives them as 15%, 13% and 16%.

It seems to be approximately right on some long sections of road, but I think if you select a particular hill, it's just pot luck where it gets its reference points from. Google Maps is the same. It doesn't seem to pick up the height from the actual point you specify, but it gets it from some nearby reference point instead, which may be further down the hill or up the hill. Surprisingly, the picture of the hill/road profile looks about right.

 

[anotherkiwi]

Great stuff!

The GSM stalled on a 17-18% hill 900 metres long about 1/3 the way up. Gearing was 46:32 and 28" wheel.

32:32 it will climb 15% with me seated and in level 4 so about 250 W.

 

[notfatjustbigboned]

That's really useful info. thanks a lot.

I looked at that elevation finder, but it's wildly off. I have three routes that i regularly do up Ironbridge Gourge. One is a long steady climb, that is fairly comfortable. The second is shorter but steeper. The third is impossible on a normal bike and needs a really high-torque motor to get me up it. That tool gives them as 15%, 13% and 16%.

It seems to be approximately right on some long sections of road, but I think if you select a particular hill, it's just pot luck where it gets its reference points from. Google Maps is the same. It doesn't seem to pick up the height from the actual point you specify, but it gets it from some nearby reference point instead, which may be further down the hill or up the hill. Surprisingly, the picture of the hill/road profile looks about right.

Hi d8veh,
Thanks for the info on the elevation finder. I guess you can't beat local knowledge.

 

[notfatjustbigboned]

Great stuff!

The GSM stalled on a 17-18% hill 900 metres long about 1/3 the way up. Gearing was 46:32 and 28" wheel.

32:32 it will climb 15% with me seated and in level 4 so about 250 W.

Hi Anotherkiwi,
Thanks for the response. I would be interested to know what your total weight is. I made an estimate based on what I thought a general human power contribution might be:


I am not sure whether the GSM is the same as the BBS01. The max torque for the BBS01 is at ~ 55 crank rpm, which would make the gear v speed v power as below.

24742[/ATTACH]

Note the speed versus power required for a given weight and gradient don't change, just the position of the gearing lines change as the as the crank rpm changes. The contribution of the motor changes with crank speed though.

 

[anotherkiwi]

Me at that time about 78 kg probably 79-80 kg today. The bike weighs about 25 kg at a guess.

The GSM is a clone but Woosh would have to say where the power is. The controller is a KT 36v 15 Amp which I will upgrade to a 17 or 20 Amp @ 44.4v to have a faster cadence - 110 in theory.

The motor will be going on a trike given for 17 kg plus equipment so I would hope about 25 kg too. Gearing will be different - 48 and 32 tooth chain wheels and at the beginning the Sunrace 8 speed 11-40.

Aerodynamics will be about the same as a tucked rider I guess, not sitting upright but two wide front tyres.

I have been crunching the numbers for a while now and your graphs are very encouraging because we have similar results. Yours are prettier

 

[Retyred1]

A wee bit of trivia -

https://en.wikipedia.org/wiki/Baldwin_Street

In New Zealand a couple hundred kms from home. I've walked it and it really is steep!

 

[Nealh]

At a cadence of 110, AK must be on the same drugs as Lance .

 

[anotherkiwi]

I have my blood washed twice weekly! In red wine I discovered spinning fast when I built my first e-bike, I used to be a strong lad pulling big ratios at slow cadence. Spinning just under two revs in less than a second isn't that fast when you analyse. I calculated my cadence by counting in my head "one and two and..." as one does for mentally counting seconds. I don't have a cadence meter... It is a lot easier on a recumbent than on the upright too I might add, even without a motor.

The BBS01 has a loaded peak rpm of 78/83 according to the Bafang site. The GSM is given for 85/90 rpm on their site so at 44.4v that gives a loaded rpm of 105/110. The motor is quieter than at 36v and I don't get back EMF like I was having at 36v so I deduced that I was spinning the motor at my natural rhythm at that voltage.

High cadence does wonders for the knees! The only problem I have is after I get bogged down and spin slower I get cramp in my right calf muscle once I get back to speed again. No, never the left so it looks like a physiological problem, I'll talk to the doctor.

 

[Nealh]

An easy accurate cadence measurement is via your phone.
Set the alarm/clock count down to 60 secs then start it and peddling at the same time.