MBK AX-Ion 2.2 - transmission problem

[oliverjames]

I've acquired one of these, because it has poorly designed transmission system.

The Yamaha motor and battery are good but the chain drive is via a 9 tooth cog, behind the crown wheel that I assume incorporates a type of torque sensor, and when the bike reaches close to 25 km/hr it appears that the drive cog either continues at the 25km/hr speed of rotation or fails to disengage and pass into freewheel mode. This means that as the crown wheel is now pushing the chain at a higher speed of rotation, whereby it bunches between the crown wheel and drive cog. This allows the chain to drag and rub on the chain guard and results eventually in the chain coming off.

I hate to see an old bike die and hence I'm trying to find a solution: In addition, apart from this issue the bike is well equipped and a pleasure to ride.

Has anyone experience of this problem, and can suggest a solution?

 

[fechter]

Can you post a picture of the drive setup? Somehow I'm not picturing what the problem is.

 

[oliverjames]

Here is a photo of the transmission showing where the chain loops down. What I do at present is to stop pedalling as soon as I hear the first sound of chain dragging on the chainguard (removed in the photo). I then freewheel until the bike slows to below 23 km/hr at which point I give the pedal a good kick to activate the motor. Then with luck the motor will take up the chain slack and drive will be re-established.

 

[fechter]

Seems like it needs more chain tension. Could also be your chain is gunky and needs cleaning/replacement. Some derailleurs have an adjustment for spring tension. If not, you might see if you can shorten the chain a bit and still be long enough for the largest cogs. Also make sure the drive cog is perfectly aligned with the chain ring.

 

[oliverjames]

I've tried shortening the chain and increasing the chain tension but it doesn't cure the problem. The chain is new and well lubed.

As I pedal the bike past the 25km/hr assistance limit the chain is passing the crown wheel faster than it passes the drive cog. When I start with the motor cold I can get pass this limit, but as soon as it warms up (happens quickly) the problem returns.

 

[Voltron]

There must be something sticking in either the freewheeling mechanism of the motor drive sprocket or the chain tensioner idler roller to have the chain bunch up at that spot. Is the drive sprocket a freewheel mounted on the motor shaft where you can get at it to lube it? If not, it might have an internal clutch inside the unit that takes disassembly to get at....the fact it changes that fast as it warms up suggests that its internal to the unit, but could be something like the bushing for the idler roller being dry and getting hotter and binding more after rotating for a while....

 

[Voltron]

Am I seeing that right in that there is no freewheeling crank, i.e the pedals go around when the motor is on?

 

[oliverjames]

Am I seeing that right in that there is no freewheeling crank, i.e the pedals go around when the motor is on?

No there is a freewheeling crank.

 

[Voltron]

That's good

If that is an external freewheel for the drive gear, the solution could be as simple as laying the bike on its left side and dripping some lube into it...

 

[oliverjames]

That's good

If that is an external freewheel for the drive gear, the solution could be as simple as laying the bike on its left side and dripping some lube into it...

That might work but its freewheel mechanism is inside the motor casing There is generally an oil seal to prevent oil escaping and dust getting in so I'm not optimistic. I think the problem is that the the drive gear is not getting the signal to stop driving and to freewheel. It seems to work when cold but as soon as warm the problem appears.

I guess there’s some type of torque sensor on the crank drive to start the motor's drive wheel and logically the speed sensor on the front wheel tells the control electronics to cut power to the drive gear as it speed approaches 25 km/hr. The fact that things go wrong when the transmission gets hot suggests electronics to me but I just don't know much about the control circuits of electric bikes.

 

[oliverjames]

That's good

If that is an external freewheel for the drive gear, the solution could be as simple as laying the bike on its left side and dripping some lube into it...

Hello Voltron, following your suggestion, I did add lube to the drive gear, not with oil but with silicon spray (thinking to reduce dirt pick-up) and can report a major improvement, thank you. Next stop, remove the drive gear and try to introduce more lubricant to its axle.

I'll see if it looks possible to remove the motor casing. Unfortunately I've been unable to find information regarding how to open up the motor for this particular Yamaha motor.