dgk02

I had to replace a Bionx wheel for the second time in three years due to breaking an axle. My shop says that the problem is that hub motors use a threaded freewheel, not a cassette, and that means that part of the axle is unsupported and prone to breaking. They also can't just replace the axle since it seems to be integrated into the hub motor and would be a big deal. I trust them but that doesn't mean that they're right; but they have a lot of experience. They tell me that it is likely the axle will break again in a year or so. I was lucky that they had a wheel that Trek never took back from them so I got it for only $400. Normally it would be $800 and I would probably have to shoot the bike before I put $800 into it.

The bike is a Trek Valencia+, so it came with the Bionx assist mechanism integrated into the bike. It is a fairly upright riding position so a lot of weight is on that rear axle. I'm 61 y/o, 190 lbs, and I use that bike to commute 25 miles r/t daily in NYC. I don't jump curbs and try to stay on the smoothest pavement but it's a bike and we do tend to get pushed onto the road margins.

I originally posted in the EBike forum but someone suggested asking about it here and so I am. I guess that there is some reason that Bionx and the other manufacturers don't use a cassette, but it seems that this is pretty much a major problem with the implementation of ebikes, yet I don't hear much about it. I do put at least 3000 miles a year on this bike, but usually with the assist set at the lowest level so there really isn't a whole lot of torque hitting that axle.

The only solution I see is getting an ebike with the motor in the front wheel, but that won't work for Bionx because it uses the bend of the rear axle to determine how hard you're pedaling and thus how much assist to give. I like the concept but the implementation seems to have a pretty glaring problem.

FBinNY 

There are inherent design reasons why freewheel axles are prone to breakage, but OTOH it shouldn't be an annual thing. First of all what size wheel, what do you weigh, how are you about helping the bike over potholes (I see you're in NYC), what kind of baggage do you carry on a rear rack?

Then there are mechanical considerations, like whether your rear dropouts are truly parallel, and (something most don't think of) the size and shape of the right side lock nut.

Start by checking the frame and making sure the dropouts are parallel (a special tool is used), then try to find the largest diameter locknut for the right side, and make sure it meets the frame out near the rim. That broadens the support base and reduces axle flex. Depending on the hub, you might also be able to shore up the right side of the axle itself inside of the bearing cone.

The rest is up to you, learn to steer around the worst of NYC's potholes, use fatter tires if possible, and stand absorbing the bumps with your knees when you hit rough patches.

sreten

Hi,

How does the system ascertain the "bend" in the rear axle ?
Will trying to beef up the drive side to reduce bend affect it ?

rgds, sreten.

Fattest tyres that will fit are a way to go ....

FBinNY 

I haven't seen your hub, but on a typical freewheel hub, the bearing in inboard of the freewheel, under the mounting threads. That means that when you pedal hard with the chain on an inner sprocket, such as when climbing a hill, the chain will pull the axle forward near the bearing, flexing it. Likewise with hard bumps. Over time this flexing leads to metal fatigue and failure, usually at the first thread inboard of the right bearing.

Increasing the width of the right locknut face helps support the axle better against these side loads, just as a larger base keeps a floor fan from tipping. There are also other things you can do to reduce axle flex, or the likelihood of breakage, but these things depend on the design of the axle itself.

BTW- I'm surprised that the axle isn't a replaceable part. Did you check with Bionx? If it isn't easily replaced at the dealer, they might offer a factory rebuild service.

BTW- you shouldn't be breaking axles, but you never answered my initial questions trying to ascertain a possible cause.

Kimmo

If it's affected, it'll offer more assistance for a given amount of power...

dgk02

I'm 190, the rack carries the battery and panniers, nothing too major in the panniers for the most part. Tool kit, pump, rain pants, a magazine or two. Lots of spare gloves and things during the winter for layer changes but nothing heavy. Battery and motor is around 17 lbs I think, panniers add another 3 lbs at most. Not all that much weight compared to me but of course it's in addition to me. I'll have to check with Bicycle Habitat to see if they can determine if the dropouts are parallel.

The mechanic did comment that he straightened out one of the parts on the rear derailler so maybe that had something to do with the axle failure? He also replaced the freewheel since the old one was pretty shot - maybe that put extra strain on it.

I should add that Bionx requires the torque on the axle nuts to be, I think, 40 nm. I have a Park torque wrench so I can get it there after fixing a flat. I've only used it once or twice though since I don't get many flats. I did have a problem with nipples getting loose and dropping into the rim but that problem went away after I learned to tighten the spokes when they felt loose.

I really don't know why it is difficult to replace the axle, just that the mechanic said that it is a major job and might not be possible. Apparently the motor and axle forms sort of a joined mechanism, particularly since the system works by the bend of the axle. I know that if you don't torque the axle nuts to the proper torque, it won't work at all or will not work properly. Maybe a hub motor that just works at a steady rate rather than adjusting to the riders effort will not have that limitation. It won't be as nice as the Bionx but maybe it won't be such a pain.

I'll check to see if I can get a larger lock nut and also look into fatter tires. Currently it's a 32 and I'm sure a 35 will fit. I'm not sure about anything larger but it has discs rather than V-brakes so maybe. Hmm. How about if I drop tire pressure? I currently run it around 100 since I figured it's less rolling resistance but I can drop it to 85 or so. Might be easier on my butt as well.

I do what I can to reduce the bang going over rough pavement and will stand up and flex the knees when hitting it but there is only so much that can be done. There's one block where I take to the sidewalk because the pavement is so bad but there are no alternatives to that route.

There are many tweaks that can be made to how the power is applied so maybe reducing the bend of the axle can be compensated for in the software.

I'll forward the suggestions to the mechanic since he has to do much of it. I'm better with software than hardware.

FBinNY 

The motor and freewheel have little to do with your problem. The axle is flexed froward by chain tension, and upward by gravity, so only things that affect those can help or hurt.

As I said, a bigger face on the right locknut will help somewhat but the biggest improvement will come from changes in riding habits. For example, using a larger rear sprocket lowers chain tension for the same driving force.

Lowering tire pressure may also help slightly. At first glance 100psi in s 32mm tire seems high. Here's a link to information that may help you find a better pressure for your purposes.

Whatever you do, I'd call Bion-X and check into their warranty, and repair services. You can't keep buying motors every year.

dgk02

Thanks, I'll lower the pressure a bit, looks like 90 or so would be better than 100. And I just found that the new tire is filled with slime. Ugh. I probably am guilty of riding on a too large sprocket for the speed, I suppose a bit more revs will help. I'll look for a bigger face on the locknut but there isn't too much room there, maybe another mm or two.

I was just groaning on the route in this morning - the streets are pretty good but there are so many bangs and bumps that it's impossible to avoid them.