Increasing planetary gear reduction on my GMAC10T

[frk1206]

Is this possible to do? I have a cargo bike and am used to the torque of a BBSHD in very high gears. The GMAC is pretty anemic in comparison with its 5:1 gearing torque-wise.

Is it possible to open up the gmac and increasing gearing to say 10:1 or something like that? I understand that'll halve the top speed (which is fine by me). A front all axle secondary hub can do the job of operating at speed (ideally) - if my rear GMAC can tractor me up hills.

And yes I understand that the gmac CAN do 5-10% hills - but its pretty anemic specially if you don't approach these hills with speed. I live in the mountains and have 5-8% grade for miles sometimes.

 

[AHicks]

There's a really good reason people go for mid drives when looking for a bike with a lot of guts (for hills or cargo). When talking about power available from the motor, as good as the MAC's are, they'll never compete well with the likes of a BBSHD....

There's just the one gear set available for them, the 5:1 (or so) that they come with.

If talking an increase in climbing power with the rider's assistance, a smaller ring gear on the front can make a noticeable difference.

 

[frk1206]

Thanks guys - I already contacted grin but haven't heard back about this situation. They just said that their motor can never compete with a BBSHD in torque due to gearing - but maybe you can increase the gearing on the gmac? I usually would ride with my BBSHD in pretty high gear and was okay with the resulting loss in speed.

On paper/simulator seems like they SHOULD have the same amount of torque - atleast in the 1:1 gear ratio I was driving the BBSHD.
My biggest issue is starting torque - right now my bike pretty much falls if I try to start at like a 5% grade (happens a lot in the SF Bay area). Takes it like 5 seconds to get to 5mph then it blasts off like a rocket Cycle analyst reports pretty low amps and I see no voltage sag. Seems like the controller waits for the motor to spin up before it adds amps to it. I'm wondering if there's something just wrong with my setup where I can get it to 'torque up' faster at 0RPM

 

[spinningmagnets]

I dont know anyone who makes a different ratio set of gears for the GMAC.

 

[Chalo]

Keep in mind that your GMAC, like almost all brushless DC motors, makes maximum power at about 50% of its unloaded speed. If you lower the gear ratio enough to get confident climbing of steep hills, your speed on the flat will be choked to about 1.8 times whatever the max power speed is.

I advise playing with the Grin simulator and monkeying with the RPM/volt value until you match the max power RPM to the ruling gradient you want to climb. Then set the grade to zero and see how much cruising speed that gets you. It's likely to be slower than you want.

Whatever gear ratio you have in a hub motor is the one you use to deal with every riding situation. Optimizing it for the steepest hill you ride on will necessarily limit the bike's usefulness when you're not climbing hills. So far, I set up my own bikes to be RPM limited rather than power limited (for instance using about 1500 max watts to go about 20 mph). Still I use an RPM/volt that's not ideal for hill climbing, because I don't want to feel like I'm driving a Zamboni the rest of the time.

If you really want wall crawling torque-- plus reasonable cruising speed-- from a modest amount of power, you're going to need to use a mid drive. You can dramatically reduce maintenance and breakdowns of a mid drive by using a NuVinci or Enviolo hub and a chunky chain like 410H.

 

[frk1206]

Totally makes sense! My idea was to use the rear gmac for crawling and supplement it (with some custom throttle control) using a front hub that’s geared for speed. The idea being that that the front won’t be super useful for climbing since it gets unloaded - so the rear is the main powerhorse when climbing and accelerating and the front takes over for cruising.

 

[Chalo]

My idea was to use the rear gmac for crawling and supplement it (with some custom throttle control) using a front hub that’s geared for speed. The idea being that that the front won’t be super useful for climbing since it gets unloaded - so the rear is the main powerhorse when climbing and accelerating and the front takes over for cruising.

Ah I see, you will have an electric drive with two "gears". In that case your best bet with the GMAC is to give it all the amps it is rated to withstand. Grin can probably advise you in this regard, if they don't already have the information posted on their site.

 

[LewTwo]

Use a mid drive to drive the rear hub drive when extra torque is needed ???
OK .... it is a ridiculous idea but it is late and it does keep all the driving forces on the rear wheel

 

[frk1206]

Hahah so I've since put my chain back on and I am the mid drive with a really high gear on the back. Pedaling my bicycle like a plebian. Just as god intended.