48v battery/controller on 72v rated BLDC motor?

[fatmarley]

I know it works because I have it set up on my kitchen table. What I'd like to know, is what gear ratio to choose? This is actually for a go-kart, but this forum seems to be full of very knowledgeable people, so I thought I may have the best answer here.

I have a Kunray KR5V 72v BLDC motor with a Fardriver ND72450 controller that spins at 5000RPM with 72v (according to the specs). With my 48v battery (150 amp continuous if I want to use it) and no load, it spins around 3400rpm (confirmed in the Fardriver app on my kitchen table). I'm planning on giving it 130 line amps, and probably 300 phase amps. I've heard people say they've put 150 amps into it no problem, so 130 amps should be ok.

What confuses me, is I'm told to keep the gear ratio above 4.9 because It puts stress on the bearings. My old MY1020 had a gear ratio of 5.8, but it's only a 2000w motor, and was pulling 41 amps at its highest load according to the battery app on my phone. So with 11" wheels, 64t rear sprocket and 11t front, It had a theoretical top speed of 23.6mph with the motor at 4200rmp (the rated rpm).

With the Kunray, if I keep the same gear ratio (5.8), with a 53t rear, and 9t front sprocket, and the 3400rpm - I now have a reduced top speed of 18.8mph because the motor will only spin up to 3400rpm with a 48v battery - BUT I'm now giving it 130 line amps over the 41 line amps on the MY1020!

Surely, if I'm pumping that many amps into it, I can reduce the gear ratio?

I have a 40t sprocket here that will then produce a theoretical 25mph, BUT the gear ratio will be below the recommended (with the lower 48v battery, does the recommended gear ratio still apply?). The MY1020 powered up on the kitchen table feels gutless compared the KR5V. The KR5v takes some effort to hold it down, and stop it spinning out of your hand when you pull the throttle back. You can tell it's got a lot more torque.

Gear ratio calculator here - Go Kart Gear Ratio Speed Calculator

Taking a quote from this thread here - Motor basics, overvoltIng and undervolting tricks
Quote "Running low volts is not a problem it can actually help in some situations like hubs with no gearing, as the efficency drops down the chart, I find with my mxus 3k dd hub i run it at 10s with 80amps phase so the rpm I need to reach for efficiency is lower and i still have good launch and climbing torque to maintain 20mph."

I'm just not sure if I should go for the 53t rear sprocket or 40t. I only have a 9t front, but can order an 11t from China (it will take ages to arrive though)

 

[amberwolf]

What confuses me, is I'm told to keep the gear ratio above 4.9 because It puts stress on the bearings.

If you can setup a pillow bearing on the end of the shaft past the sprocket, so that the shaft is supported by bearings on either side of the sprocket, there will be less torsion on the shaft as it rotates and less stress on the bearings. Many shafts are not long enough for this, but if yours is, it would help if you have to pull harder than the shaft is sized for or the bearings are built for.

You could also replace the stock bearings with better ones; if you find out which are in there you can ask Grainger, Mcmaster-carr, bearing shops, etc., if they have any that would handle the stress better and still fit in the same place.

If the shaft is machined with any shoulders or c-clip rings, though, it's probably the limiting factor as those will be cyclicly stressed every rotation and eventually shear thru if limits are exceeded.

I know it works because I have it set up on my kitchen table. What I'd like to know, is what gear ratio to choose? This is actually for a go-kart, but this forum seems to be full of very knowledgeable people, so I thought I may have the best answer here.
<snip>
Surely, if I'm pumping that many amps into it, I can reduce the gear ratio?

You can experiment with the simulators at ebikes.ca to figure out gear ratios for middrives vs power / torque / voltage / kV etc. If the parts you have aren't listed you can use the custom options to input the parameters; if you don't know all the parameters you can leave the defaults for any you don't know, as long as you have the motor kV (or kT which is inverse of kV).

I'm just not sure if I should go for the 53t rear sprocket or 40t. I only have a 9t front, but can order an 11t from China (it will take ages to arrive though)

You can manufacture your own sprockets if you want; it's easier to do if you can weld (so you can install them on a collar) but depending on the shaft you may be able to drill and then file-to-shape a hole to precisely fit the shaft (with no play that wil damage the shaft or sprocket). If you can weld then steel is an easy option, otherwise aluminum works (though for such a small sprocket it's likely you'll need to make spares as it'll wear a lot faster than a large sprocket).

 

[fatmarley]

If you can setup a pillow bearing on the end of the shaft past the sprocket, so that the shaft is supported by bearings on either side of the sprocket, there will be less torsion on the shaft as it rotates and less stress on the bearings. Many shafts are not long enough for this, but if yours is, it would help if you have to pull harder than the shaft is sized for or the bearings are built for.

You could also replace the stock bearings with better ones; if you find out which are in there you can ask Grainger, Mcmaster-carr, bearing shops, etc., if they have any that would handle the stress better and still fit in the same place.

If the shaft is machined with any shoulders or c-clip rings, though, it's probably the limiting factor as those will be cyclicly stressed every rotation and eventually shear thru if limits are exceeded.



You can experiment with the simulators at ebikes.ca to figure out gear ratios for middrives vs power / torque / voltage / kV etc. If the parts you have aren't listed you can use the custom options to input the parameters; if you don't know all the parameters you can leave the defaults for any you don't know, as long as you have the motor kV (or kT which is inverse of kV).




You can manufacture your own sprockets if you want; it's easier to do if you can weld (so you can install them on a collar) but depending on the shaft you may be able to drill and then file-to-shape a hole to precisely fit the shaft (with no play that wil damage the shaft or sprocket). If you can weld then steel is an easy option, otherwise aluminum works (though for such a small sprocket it's likely you'll need to make spares as it'll wear a lot faster than a large sprocket).

Good idea! Unfortunately, there's not enough room for an extra bearing on the end.

I'll see if anyone knows the bearing size.

Had a look at the simulator over at ebikes.ca, but I don't really understand it.

How do you manufacture your own sprockets? The KR5v uses the #35 chain (or 420, but that's too big), and I can only find 40t 53t, 56t and 60t in #35 (anything else is too big).

I'll probably end up playing it safe, and use the 53t sprocket. 18.9mph will probably be fast enough for my bodged up, off-road go-kart... If I upgrade the battery to 72v, it will be capable of around 27mph, and that would be scary on this thing!

 

[amberwolf]

Had a look at the simulator over at ebikes.ca, but I don't really understand it.

If it helps, it has instructions on how ot use it below the chart/etc. There's also at least one thread here that talks about it, and a bunch of posts where people use it to work out which system woudl do the job they need it to.

How do you manufacture your own sprockets?

Well....https://www.google.com/search?q=How+do+you+manufacture+your+own+sprockets

The actual process varies depending on the tools you have available, and your skils with them, as well as the materials you are going to make them from.

I've only done one, made out of aluminum panel from some piece of old test equipment, more than a decade ago, and it "worked" but was pretty rough. For mine i used a simple protractor for making the circle, clamped that as a divider for marking the centers of each tooth spot, then a hand drill to drill holes between each tooth on those marks, then a hand file to "finish" the tooth profiles; was for a regular bicycle chain. The measurements to do it all with ere caluculated by one fo the various online sprocket calculators. Probalby took me a week or two to finihs making, a few minutes at a time here and there; I don't really recall exactly, just the general process and that I managed to not give anyh blood sacrifices to it.

I was inspired by someone doing a lefthand middrive on an ebike; they used the bottom of an aluminum pan to start with as it was nice and thick and already round.