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Need more Torque and longer Run Time for Razor MX650


I have 10 year old triplets (1 boy and 2 girls) that each got a Razor MX650 for Christmas.  The batteries on two of them have already given out and so the run time on flat roads is only a couple minutes each.  The kids weigh between 80 and 90 pounds and use the bikes on both hard flat surfaces and also a dirt track with hills and jumps.  The dirt track is an actual short motocross track with about a 20 foot elevation change from one end to the other, but most of the hills are just large to mid size jumps for a YZ250, but they are taller than the kids, so they just ride up them and down the other side.  

The first day out, the kids could ride around the track twice before there was not enough energy to get up the hills.  Even brand new, the bikes went very slow up the hills, so torque has always been an issue in how they would like to ride the bikes.  I've read over this forum and seen many recommendations, but would like to try and get all my questions answered in one place if possible?

My main goal is to get more torque and a lot more run time.  If I can pick up 5+ mph in the process then fine, but extra speed isn't the main goal.  Ideally I'd like to get at least an hour run time on the track if that's possible?  If not, then what kind of run time on the dirt track is practical to expect?  Of course, with three kids and three bikes, money is somewhat of an issue, but I would like to make these as useful and fun as possible for the kids.

As I see it I have a few options:

Option 1:  Larger Batteries and Sprocket Change.

If I were to just upgrade the batteries to a higher Ah rating and make a sprocket change, what kind of impact should I expect?  What battery would I need to get the hour run time in the current 36v setup?  How much speed would I lose, but how much extra torque would I gain?

Option 2:  Overvolting the motor to 48v.

If I were to overvolt the motor from 36v to 48v by buying 4 x 12v batteries, how much speed and torque increase should I expect?  Would I need to change the controller or anything else to 48v to do this overvolting or can I just change the batteries?  What about the reliability of overvolting when used on this dirt track with hills?  What would overvolting and a sprocket change do to help increase torque?

Option 3:  Upgrade to 48v 1600w Brushless motor.

If I upgraded to your 48v 1600w Brushless motor, what would the increase in torque, speed and run time be versus a standard MX650?  I noticed you include 12v 15Ah batteries with that kit, so it doesn't seem like the run time would be very long using a 1600w motor?  I saw in one response you used a run time calculator to find the proper Ah rating for a battery to last a certain you have a link to that?  What battery size would I need to get for the one hour run time on the dirt track?

Also, if I upgraded to Lithium batteries, are there certain types you would recommend or not recommend for safety reasons?   Thanks in advance for all your help.

Hello and thank you for starting this topic. 

Regarding Option 1: This is a safe and dependable way to increase torque and runtime. Lowering the gear ratio by installing a smaller motor sprocket and/or larger wheel sprocket will increase torque and hill climbing ability (at the expense of a slower top speed). And increasing the capacity (Ah rating) of the battery pack will increase runtime. Runtime depends so much on the terrain that an electric dirt bike is driven on and the speeds at which it is driven that estimating runtime on any terrain other than flat ground is difficult. We know that a stock MX650 with new batteries will go for around 40 minutes on flat ground. If the same bike was used on a course that was not flat the runtime would be significantly decreased, how much of a decrease in runtime though could not be determined without doing a real-world test of the bike on the course that it will be used on. One hour of runtime on a course that is not flat could potentially require a battery pack with a high capacity, so to meet this goal we recommend stuffing the largest size (highest capacity) battery pack that will fit in the frame to try to meet the one-hour runtime goal. We have never played around with fitting different size batteries in the MX650 frame so we are not giving advice on that but just speculating that maybe three 22Ah batteries would fit, or maybe six 10Ah batteries would fit, or something along those lines which will provide the highest capacity battery pack that can fit inside of the frame. 

Regarding Option 2: Overvolting from 36 to 48 Volts will increase speed and torque and usually the runtime will remain the same, at least on flat ground, we are not sure about runtime on a course that is not flat though. Motors and controller often overheat and fail when overvolted so it is definitely not a risk-free option. Combining hills with overvolting is probably a recipe for motor and controller failure so we recommend looking at other options under those type of riding conditions. 

Regarding Option 3: We have sold many 48 Volt 1600 Watt MX500, MX650, and SX500 upgrade modification kits and everyone that we have talked to and exchanged emails with has been very happy with them. We have received reports of MX 500/650 series bikes with our 1600 Watt kits reaching top speeds of 31 MPH and of good acceleration, however, we have not received any feedback comparing the acceleration with the 1600 Watt kit compared to the acceleration of a stock bike. We also have not received any feedback yet on the runtime of the 1600 Watt kits with 15 Ah batteries. Runtime is very mathematical to the motor size and battery capacity so a stock bike with three 12Ah batteries and 500 Watt motor is sure to have more runtime than a modified bike with four 15Ah batteries and a 1600 Watt motor, presuming that both bikes are being run at full throttle the entire time. However, if the modified 1600 Watt bike was running at the same 17 MPH speed that the stock bike can go then it would have a longer runtime than the stock bike. The wheel sprocket we currently include with the 1600 Watt kits has 63 teeth which is the largest size we have right now, however, in a few weeks we will be getting 72 tooth sprockets for the kits which will lower the gear ratio for even increased torque, faster acceleration, and possibly even a faster top speed since the motor will be allowed to run at a higher RPM and produce more power. 

Regarding lithium batteries, LiFePO4 is definitely the safest and best way to go these days. 

Since you want more torque and longer runtime we would lean toward recommending lowering the gear ratio with different size sprockets and installing the highest capacity battery pack that will fit inside of the frame. 

Please let us know if you have any questions.

Thank you very much for your detailed response.  I do have some additional questions that perhaps you can assist me with?

I think Option 1 will reduce the top speed too much to get the torque we need, so that might take a lot of the fun out of riding them.  I also agree with you that Option 2 is not viable since the dirt bikes will be ridden off road and on hills.  However, one question regarding overvolting is whether the motor or the controller are more likely to overheat and breakdown assuming neither are changed when upgraded to 48v?  If I change the controller, will that help, or still have the same potential for the motor to overheat?

I still think my best option, even considering the cost, is number 3.  I've seen several people comment on how much improvement they've received from an 800w or a 1000w motor upgrade, so my question is how much stronger is the 1600w brushless motor compared to those two?  Is the 1600w motor 2.5 times stronger than the 650w motor, or how does the comparison work on these motors?  Also, what size wheel sprocket comes with the standard MX650?  I believe you include a new wheel sprocket with the 1600w upgrade kit, so my question is why?  I'm not sure the kids should be going over 30 mph, but if there's plenty of torque along with that speed, then I guess it would be fine.  I'm just curious how much torque and hill climbing increase they should get with the 1600w motor?

You mentioned that the new 72 tooth sprocket will have increased torque and possibly an even faster top speed?  Can you please elaborate on this?  How will the motor be allowed to run at a higher RPM just by a sprocket change?  I thought speed and torque were inverse of each other?

Regarding batteries, do you have SLA's that are larger than 22ah?  If I were to get a LiFePO4 battery, what specifications would I be looking for besides volts and ah?  How many amps would it need to put out and what other specifications should it have?  Can I splice the wiring existing wiring harness onto one of these or would I have to buy a new harness to fit?  I like the idea of the upgraded 48v 1600w motor with the increased torque and speed, but it just seems like the run time will be very short unless I can find a suitable battery with a larger capacity?  

I saw in one response you used a run time calculator to find the proper Ah rating for a battery to last a certain you have a link to that?  Thank you.

From the feedback that we have received through this forum, it is usually the motor that burns out first when overvolting. Upgrading the controller will only supply more current to the motor and cause it run hotter than it would with the original controller. 

The power of the motor is mostly determined by the controller and most 800 and 1000 Watt brushed motor MX dirt bike modifications use 32 Amp controller. The controller supplied with our 48 Volt 1600 Watt kits is a 40 Amp controller. So the difference in maximum power supplied to these different motors is 25%. It is a little more complicated than this though because brushless motors are 85-90% efficient and brushed motors are 75-80% efficient, so the 1600 watt brushless motor running on 40 Amps should be around 35-40% more powerful than a 1000 Watt brushed motor. 

The OEM MX dirt bike sprocket is 80 teeth and for #25 chain. A 1000 or 1600 Watt motor will go through 25 chains like crazy so we only sell kits with 8mm chain for maximum reliability. 

We have only received reports of top speed with our 1600 Watt kits and not how much torque and hill climbing has increased. Maybe someone who has installed the 1600W kit can reply to let us know. 

With the 63 tooth sprocket the calculated top speed is 38 MPH, however, due to drag (wind resistance) the motor only has enough power to get the bike up to 31 MPH. So with a larger wheel sprocket such as a 72 tooth the calculated top speed would be 33 MPH and with this lower gear ratio the motor would spin faster and produce more power for a faster top speed. 

We carry batteries larger than 22Ah on our 26-200Ah Batteries page. When selecting a LiFePO4 battery pack the maximum continuous discharge current rating should be the same or higher than the controllers current limit (Amp) rating. Our kits include wiring harnesses that are suitable for the larger motor in the kit and a LiFePO4 battery pack could be spliced into the harness or preferably attached to it with a connector for easy removal. 

Our runtime calculator is on our Battery Pack Ride Time Calculator page. We are finding that it provides conservative ride time estimate as it recently calculated that a go-kart we made a kit for would run for 30 minutes when in fact it ran for 45 minutes so calculational changes are planned for it. 

Please let us know if there are any other questions and we will try our best to answer them. Thank you.

Thank you for all the detail in your answers.  When do you think the 48v 1600w kits with the 72 tooth sprocket be available?  Regarding the chain, it doesn't look like the 8mm chain is that much bigger than the #25, so do you think an even larger chain should be used, or has the 8mm proven to be reliable?  And I assume that the kit comes with a new motor sprocket as well as a wheel sprocket?

Regarding run time.  Thank you for including that link.  For some reason I just could not locate it on your website.  It looks like the run time with this 1600w motor on flat ground using the 4 x 12v 15ah batteries would only be about 30 minutes and even less if there are hills.  Since space is at a premium, I'm going to have to find a lithium battery that will fit, so my question is do you have adapters to fit?  Instead of 4 SLA batteries, it looks like most 48v lithium batteries will be just one or maybe two, so the wiring will be different.  Thank you.

We should have the 72 tooth sprockets in around three to four weeks. 8mm chain is quite a bit larger and heavier than #25 chain and it is reliable when used with a 1600 Watt motor. We do not see any reason to increase to a #35 chain and add the expense and weight of such a large wheel sprocket and chain. Our 1600 Watt MX dirt bike kits have both sprockets and a chain included. They have everything needed for the modification with no other parts needing to be purchased. 

We can work with you on the lithium battery pack adapter once you know which battery pack you will be getting. Once we know what type of power connector the battery pack has then we will be able to make the adapter for it. 

Please let us know if you have any questions. Thank you.

Thanks again for all the details.

I was thinking that since I have 3 Razor MX650's for my three kids that all weigh about the same, I would do a little comparison to see how much each different upgrade affects the performance of the dirt bikes.  I'm going to upgrade all three of the dirt bikes to the 48v 1600w kit and add Lithium batteries.  But since I have one stock MX650 with the standard 12v 12ah SLA batteries in great shape, I'll upgrade that one last to have a base line to judge the upgraded performance of the other two against.  One of the other MX650's I'll just upgrade at first with the 48v lithium batteries to Overvolt the motor and see how much performance increase I get just from doing that.  Then the third MX650 I'll put in the lithium batteries along with the 48v 1600w kit.  This way I can line them up next to each other and have a direct comparison to judge the increase in acceleration and speed, as well as the torque on the hills.

I think I would prefer to have the upgraded kit with the 72 tooth rear sprocket versus the 63 tooth rear sprocket, but perhaps I'll go ahead and order the 63 now.  Then when the 72 tooth sprocket comes in I can order that one and switch them out?  This will give me a chance to compare both sprockets against each other and provide those results.

I'm still uncertain about which type of lithium battery to buy for this project.  I've heard so many horror stories about the poor quality of the Chinese made batteries, but the American made lithium batteries are very expensive.  Also, I understand there are many safety concerns about certain lithium batteries?  The LiFePo4 seems to be the safest, but also the largest and heaviest.  Does anyone know if the newer Lithium NMC batteries are safe enough to be in one of these dirt bikes?  Does anyone have a link or links to any threads on this topic?  If anyone has recently upgraded to lithium, I would appreciate any help on this matter so I can go ahead and get the batteries to get this project started.  Thanks again.

Those all sound like a great ideas. We know that the LiFePO4 batteries are popular with MX650 upgrades, however, have not heard anything about Lithium NMC batteries yet. Maybe someone else reading this has more information about the NMC batteries though. 

I have a standard E300 and want more torque to climb hills. Speed is not a factor - 5 or 6 MPH would be fine.  Will an 80 tooth sprocket fit?

Earl Jackson 

I have a razor mx650 and I had a custom sprocket made for me by Top speed is 14mph and run times are super good. Im 200lbs and it pulls me up hills just fine. The MX650 is an awesome e bike and worth every dime. the sprocket is a 100t sprocket.

The MX650 will accept a larger rear wheel sprocket and a 100 tooth sprocket for #25 chain should fit, not so sure about a 100 tooth sprocket for 8mm chain though. I would check for clearance before ordering a 100 tooth sprocket just to be sure though.

We have a Sprocket Diameter Calculator which can be used to determine the outside diameter of a custom-made sprocket before you buy it. 

We also sell Custom-Made Sprockets for #25 chain, 8mm (T8F) chain, and all other chain sizes. The MX650 rear wheel freewheel has a F4 mounting pattern and when custom ordering a sprocket from us you can specify an F4 mounting pattern when requesting a price quote for the sprocket.

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