XF07 on 20" wheel

[sjpt]

We have a Woosh XF07 kit on our current Viking tandem (700C) and have been very pleased with it; just 3 years old this month.
Slightly underpowered (as we were warned before we bought it); on steeper hills we tend to slow down and the motor can't help too much at slow speeds (around 5-6mph) even if we increase the power setting. We rarely go much above 12mph. We mostly ride on setting 2/5.

Owing to stiff hips/back we are thinking of a step-through. One option is the Circe Helios. This has 20" wheels. If we put the XF07 on a 20" rim that would clearly give us better torque at lower speeds (which we would welcome), but lose our at higher speeds (which we mostly don't use).

Another possible option is the Van Raam Twinny, which has 26" wheels, which with slightly fatter tyres would not be that different from our

I'd be interested in any comments about the suitability of the XF07 on a 20" wheel tandem. Or any experience with step-through tandems.

 

[vfr400]

In a 20" wheel, you'll get good power and torque up to about 12 mph, then the power will ramp down to give a top speed of about 14 mph.

 

[Sturmey]

This is something that I have considered. I have a 20 inch folder and a high mileage xf07 clone with a KT 36v controller and battery that has a meaningful top speed of about 28 km/hr on a 700 wheel. On a 20 inch wheel, I would expect this to reduce to 20 km/hr. One suggested economical possibility is to increase voltage (36v) by about 20% using a buck converter which would increase the motor speed proportionally. The buck converter recommended (German forum) is in the link below.

1200W 20A Konverter Boost Step-up Stromversorgungsmodul DC-DC 10-60V to 12-83V 8027467916301 | eBay

Entdecken Sie 1200W 20A Konverter Boost Step-up Stromversorgungsmodul DC-DC 10-60V to 12-83V in der großen Auswahl bei eBay. Kostenlose Lieferung für viele Artikel!

www.ebay.de

It has been suggested that the Kt36V controllers will work safely at 12s .
I would welcome any comments or criticism or potential snags to this.

 

[vfr400]

Most 36v controllers will work with 12S voltage (44V), but you'll have the wrong low voltage cut-off, so you need to keep an eye on the voltage and control it yourself. You can't rely on the BMS to cut off.

I see the converter has it's own LVC so maybe OK - if you can figure out how to set it.

I wonder if it gives a constant voltage output or whether it gives a set difference between input and output.

 

[Sturmey]

Most 36v controllers will work with 12S voltage (44V), but you'll have the wrong low voltage cut-off, so you need to keep an eye on the voltage and control it yourself. You can't rely on the BMS to cut off.

I see the converter has it's own LVC so maybe OK - if you can figure out how to set it.

I wonder if it gives a constant voltage output or whether it gives a set difference between input and output.

I think it gives a constant voltage output as one complaint is that the lcd gives no indication of battery usage. But I think you can set the low voltage protection in the converter to say 35 volts to give some advanced notice of battery depletion. Once battery drops below this level, converter stops increasing voltage but battery still remains connected to controller via converter which is now passive. (A sort of DIY limp home mode) Controller lvc works in normal way and will shut off at say 31 volts.
It seems to be a popular solution in the German forum where a little extra cadence for mid drives or extra rpm is required rather than trying odd 11s battery arrangments.

 

[sjpt]

Thank you both for the comments; much as I thought but helpful to get confirmation. Sounds as if XF07 on 20" wheel will suit what we want quite well.

We might try out a Circe and see how it feels without the electric; there's one available to hire locally. The Circe supplied ebikes are a rather more expensive than we would want to pay; we might buy a new conversion kit but it would be nice to be able to reuse our current one.

 

[vfr400]

I think it gives a constant voltage output as one complaint is that the lcd gives no indication of battery usage.

You can get round that by powering the LCd from the battery side of the converter.

 

[harrys]

I've used the buck converter. Yes, you could set it to 48V, if you wished, which will still be within the range of a 50V electrolytic capacitors in the 36V KT controller.

The output is limited, I recall to 20A, but you will pull more than that out of the battery. What's nice is that I could set it at 44V, and have good performance as the
battery voltage went down, w/o the resultant slower speed under 36V. I did not try powering the LVC directly off battery, but that should work.

 

[vfr400]

I've used the buck converter. Yes, you could set it to 48V, if you wished, which will still be within the range of a 50V electrolytic capacitors in the 36V KT controller.

The output is limited, I recall to 20A, but you will pull more than that out of the battery. What's nice is that I could set it at 44V, and have good performance as the
battery voltage went down, w/o the resultant slower speed under 36V. I did not try powering the LVC directly off battery, but that should work.

Does it keep the output voltage constant as the input voltage goes down?

 

[dwvl]

I've used the buck converter. Yes, you could set it to 48V, if you wished, which will still be within the range of a 50V electrolytic capacitors in the 36V KT controller.

The output is limited, I recall to 20A, but you will pull more than that out of the battery. What's nice is that I could set it at 44V, and have good performance as the
battery voltage went down, w/o the resultant slower speed under 36V. I did not try powering the LVC directly off battery, but that should work.

This sounds great! Why aren't we all using these on our "36V" systems?!?

 

[vfr400]

This sounds great! Why aren't we all using these on our "36V" systems?!?

Efficiency and simplicity.

 

[harrys]

The buck converter will keep the voltage up as the battery drops, I was running it on a relatively big load for me, a fat bike motor. with a 20A controller. With a real time volt meter, I recall a 4V sag with throttle. It would probably would be less sag with my Q100 motors, but I'm already happy with their performance on 36V.

For $20USD, I thought it would be fun to try out. The tradeoff is it's limited to 20A output, and power out = power in, so you'll pull proportionally more than that from the battery with a voltage boost plus efficiency loss.