you might be running into a HP limit. What voltage are you using?
I know I decreased my sprocket a bit and it helped me get the torque needed to get to speed.
I thought I knew what I wanted to do for chain and sprockets, but got some surprising data on a ride yesterday that has left me unsure, so I will share here, and hopefully some of you will have some thoughts that help nudge me in the right direction.Welcome to Sprockets, I am Deiter your host.
I haven't used one of the uber spreadsheets to predict my top speed, since I figured I'd be taking a shot in the dark so much at variables like wind resistance and rolling resistance, I expected real-world experience from similar bike builds would probably put me on target. I'm running an ME0709 (Etek-RT) powered by 24 cells of CALB 40Ah, through a 400 amp Kelly controller.
When DRZ400 was running the same motor and frame with 35Ah lead, he got a top speed of 52 Mph with 15/72 gearing (1:4.8) and 55 Mph with 16/72 gearing (1:4.5 ratio).
Right now for testing I'm running a 530 chain with an 11 tooth 530 sprocket up front and stock 520 sized 45 tooth sprocket in the back for a ratio of 1:4.1. I plan to buy a new rear sprocket in 530 sized to an appropriate ratio with a 12 tooth or larger 530 up front.
Although I've got a 400 amp controller, I've set it at a 200 amp limit while test riding.
As it is, the bike accelerates comfortable riding around town. I'd like a little more torque on the low end and have figured I could achieve that both by stepping up my amp limit and changing the gear ratio a bit.
My target top speed is about 55, as I'd like a bit of zip to get out of someone's way, and all the streets I'm riding on normally have speed limits from 30 to 45 mph.
All seemed well and good until I found a spot where I could open up the throttle yesterday, and found myself with good acceleration from 40 to 50mph, but going no faster than 50. At 50Mph I was pulling 100A (2.5C for my batts) and I know I've delivered 200 battery draw momentarily during acceleration, so I don't think that's the limit.
So I'm thinking, if I gear down more I will lose even more top speed and be below the top speed I want, or is there some paradox here that gearing down will give me the torque I need to get to a higher speed? Or is it the 11 tooth I'm running up front binding and slowing me down?
Thoughts and input appreciated, especially from those of you (and you know who you are) who have really played the gear ratio game, running several different sprockets.
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Last edited by billmi; 20 April 2011 at 1018.
How much does your bike weigh and how much torque does your motor put out? This is very important to determine what ratio to use.
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100A at 72V is about 7.2 kW (9.7 HP). Depending on your motor efficiency and drag, that's in the right ballpark for 50 mph. I think what's probably happening is that you're at the max power at that rpm. If you look at the HP curve for a typical motor, it will go up until the torque knee, then go down again. With that gearing, you're in a high RPM range (for that motor), which means you're down in a lower HP range.
If you use a higher gear ratio, say 1:3.5 or 1:3.7, you will get the motor back down into a lower RPM where you have more HP. That will give you a higher top speed, at the price of less acceleration from a stop. You might actually notice acceleration from 30-50 get better.
And yeah, increasing the current limit will help acceleration, but 200A is already 5C for your 40Ah cells. You should be able to go above that, but watch your voltage sag and make sure you're not going below LVC, which is about 2.8V / cell, or 68V pack voltage.
Also, the ME0709 is rated to 3050 RPM unloaded. Under load, it will be less. On my bike, 3050 RPM with 11:45 would put me at 55 mph, but since the RPM under load would be less, it makes sense that you're maxing out around 50 mph.
That motor is rated for 8 HP continuous. That's just enough to hit 50 mph on a flat road with no wind. If you want to go faster, it will put you above the continuous rating - that might be OK if you have sufficient cooling.