Recommended torque for belt port in rear triangle?

Question

Last week while doing a bleed and rotor replacement on my Spot Acme, my bike accidentally fell from the stand with the wheels removed, causing a little damage to the pulley:

I didn't notice this warping at the time, but once I reinstalled the rear wheel and spun the pedals, I heard an intermittent, rhythmic strumming noise, like the teeth on the belt were straining to reseat. It's in sync with the revolutions of the crank. You can hear and see what I'm talking about in this video.

The pulley featured here is the brand-new replacement, not the one that got damaged in the photo above. You can also see the belt raising off the pulley a little bit each time it happens.

So if it's not the pulley, it's gotta be the belt still causing that noise, right? Is this just an annoyance that doesn't pose an eventual safety hazard?

Regardless, I'm inclined to replace it. At the same time I bought the replacement pulley, I also bought a new 113T CDX belt like the original.

What's the recommended torque for the belt port in the rear triangle?

And as long as it's the same 113T belt, I shouldn't have to mess with the tensioners, right? I'll check it with my Krikit gauge just in case, but can I expect it to be pretty plug-and-play?

Is there anything else I should know about replacing the belt?

To be safe, I've not really​ ridden the bike beyond the bedding-in process for the new pads and rotors, and most of that was coasting downhill.

Thanks for any help.

Answer 1

Something is wrong with the installation of the new front sprocket that's causing the belt to ride up like that. That's not normal and you shouldn't ride it. It looks like it's happening in the same place every time, and either the center track or the main teeth aren't meshing properly there. Given the circumstances, it's likely that the crank spider tabs were damaged along with whatever impact destroyed the old one.

Confirm that it's the spider tabs by taking the sprocket back off and creating a fixed reference point next to where the spider tabs pass by as you spin the cranks, and then check for "true." I usually do this by bolting a piece of slotted flat stock (like a rack stay) to the ST water bottle boss.

If it's found to be out of true, you have the choice of replacing it or truing it by bending the tabs back into place with an adjustable wrench. There is some risk of the crank failing now or later from this, but it usually works well.

There is not a circumstance where you can change out any of the parts of a belt drivetrain and reasonably expect it to work properly without resetting the tension. It's too sensitive.

I don't know the specific recommended torque for the belt access joint on your frame, and it's probably worth looking up. A lot of designs use something like 12-14 Nm, which is shorthand for saying about the highest reasonable torque input from a 5mm L-wrench.

Answer 2

This issue is fixed, and it required a replacement of the crank in addition to the front pulley. The belt wasn't the issue. My initial question was the wrong one.

I've accepted Nathan Knutson's answer above because it describes the troubleshooting approach that led me to the solution.

But for completeness for anyone maintaining their Acme, Mitchell, a tech at Spot Bikes, answered me via email saying that the belt port should be tightened to 9-10 Nm. This information isn't included in the user manual.

Here's his full email response, which also describes a technique for troubleshooting the issue and general specs for belt tension on the Acme. I've emphasized the key ideas:

I'm sorry to hear that you are running into some issues with the belt drive setup on your Acme! I do hear what you're talking about in the video, but the interesting thing to me is that it appears to happen at the same point in the crank rotation on the front ring, which wouldn't correlate to the belt since the drive ratio isn't 1:1. If you have a white or silver marker, I would suggest putting a little dot/line on the belt and the ring where it starts making noise, and then rotate the cranks and see which one it lands on consistently when it makes the sound.

Another thing to consider - remove the rear wheel and pop the belt off of the front ring. Once it is off, rotate the cranks and keep an eye on the front ring to see if it hops or wobbles at all. I would assume that you've inspected the belt for damage already and that when the bike fell, there were no signs of it landing on the belt correct?

The torque spec on the frame removal piece to replace the belt is 9-10Nm. Theoretically, replacing the belt in a like-with-like scenario wouldn't automatically require you to re-tension it, but I would absolutely check the tension to ensure it is in the proper range before riding to avoid any potential issues. We normally use the Gates Carbon Drive app to measure tension, which should be 35-50Hz for the Acme.

Thanks,

— Mitchell