Cracked Torque Sensor plate on ST1

KidWok

Member
I had a flat tire a couple weeks ago on my ST1 and it happened to be across the street from a auto repair place. I used their 19mm wrench and air to change my flat tire. As I was tightening the nut back on, I heard a crack. Figured it was just those locking washers seating with each other but it turns out that I had overtightened and actually cracked the aluminum dropout plate that houses the TMM4 torque sensor. This resulted in the torque sensor not reading my output reliably and the power constantly cutting in and out, to the point where I just had to keep my thumb on the boost button to get where I was going.

MAKE SURE YOU DON'T OVERTIGHTEN THE REAR WHEEL NUTS! Bike is in the shop now waiting for the replacement part.

Tai
 
Wow, that’s too bad. I always change my wheels with hand tools, I never thought about breaking the torque sensor. Is your bike still under warranty?
 

KidWok

Member
Nope...and I did use a hand tool to tighten it so just be careful how tight you crank it down!

Tai
 
Mine is a 2013 ST1 Platinum that I’ve owned for over five years. I’ve changed the tires on it many times. I guess I’ve been lucky, but I will keep that in mind the next time I need new tires.
 

KidWok

Member
Doh...The torque sensor plate just cracked again. This time JRA. I think it has to do with two things. I use the rear rack to carry things a lot so there is a pretty decent amount of weight bearing down at the frame at the rear dropout. If you look at the TMM4 torque sensor design, you'll notice that the slit that allows it to move goes pretty much all the way around the top of the dropout. As such, the bike is kind of suspended over the rear hub axle right there and the little bit of aluminum between the slit and the dropout bears the brunt of any downward forces. I'm likely overloading my rear rack and causing fatigue in the plate, which eventually cracks when I hit a big bump. FWIW...Stromer does sell the TMM4 torque sensor as just the aluminum plate (part # 218472) and also as an entire assembly with the torque sensor (part # 213414). I'm ordering two plates, one to replace the broken one and one to keep on hand. Need to make sure I don't carry so much gear in the futures.

Tai
 

bluecat

Well-Known Member
I'm likely overloading my rear rack and causing fatigue in the plate, which eventually cracks when I hit a big bump.
You're a lucky guy :)

On another bike, the dropout would be broken - and the whole frame is lost!


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smitty

Active Member
You definitely need a torque wrench to tighten the rear wheel which should be set at 20Nm as I remember having to change rear tire a couple of times...
 

KidWok

Member
Experimenting a bit. I cut up some remnants from zip ties to fill in the full width of the slit just above the dropout. The thickness of a small zip tie is a press fit for the slit. The idea is to fill that gap with some fairly hard material to brace and dampen the vertical load without affecting the torque sensor measuring horizontal load. Installed it yesterday and rode it today. No impact on the torque sensor function. Fingers crossed that it will do what is intended and prevent the plate from cracking in the same spot. First pic is the new plate with the zip tie pieces out. Second pic is the new plate with the zip tie pieces in. Third pic is the old plate that was replaced with the crack.

Tai
 

Attachments

The zip tie is a clever solution.

As we both found out, torque sensors involve a trade-off between strength and maximizing the flex of the part being measured.

BionX measures the flex of the rear axle, which has been hollowed out to reduce the force needed to flex it.

I cracked a rear BionX motor axle last August, but it was an older used motor I had used for only one season, I'm not sure what kind of abuse it may have had prior to installing it on my bike. Newer BionX are said to be less likely to have the same problem.

BIonX patent expires in a year or two, I wonder if Stromer will continue to use this external sensor design once that occurs...